A-Level Biology

Cells 细胞 are very small, so you cannot see them with your eyes alone. You use a microscope 显微镜 to make a bigger picture of them. The first kind you meet is the light microscope 光学显微镜. It shines light through a thin specimen 标本 (the material you look at) and uses glass lenses to enlarge the view.

Making a slide and drawing what you see

To look at living material, you make a temporary preparation 临时装片. You put a small, thin piece of material on a glass slide 载玻片, add a drop of stain 染色剂 (a coloured liquid that makes parts easier to see), then lower a thin cover slip 盖玻片 on top to flatten it and keep out air.

When you draw cells from a slide or a photograph, follow simple rules:

• use a sharp pencil and clear, single lines (no shading).
• draw only what you can really see, with the parts in the correct sizes.
• label the parts with straight lines that do not cross.

Magnification and actual size

Magnification 放大倍数 tells you how many times bigger the image is than the real object. It has no unit. You find it with one equation:

You can rearrange this to find any one value from the other two:

The top and the bottom of the fraction must use the same unit. Cells are tiny, so you work in small units:

• $1\ \text{mm} = 1000\ \text{micrometre}$ 微米 (µm)
• $1\ \text{µm} = 1000\ \text{nanometre}$ 纳米 (nm)
• so $1\ \text{mm} = 1\,000\,000\ \text{nm}$.

Worked example. In a photomicrograph at magnification $5000$, a chloroplast 叶绿体 measures $25\ \text{mm}$ across. Its actual size is

The same equation works for drawings, photomicrographs 显微照片 (photos taken through a light microscope) and electron micrographs 电子显微照片 (the most detailed photos, explained below). Always convert to the same unit first, then divide.

Eyepiece graticule and stage micrometer

To measure a real cell under the microscope, you use an eyepiece graticule 目镜测微尺 — a tiny scale inside the eyepiece. Its divisions have no fixed size, so first you must calibrate 校准 them (work out what one division is worth).

You calibrate using a stage micrometer 载物台测微尺 — a special slide with an accurate scale on it (often $1\ \text{mm}$ split into $100$ parts, so each part is $10\ \text{µm}$). You line up the two scales, count how many graticule divisions fit a known length, and divide. Once calibrated, you can swap in your specimen and measure it with the graticule.

Calibrate the graticule by lining it up with the stage micrometer's known scale 标尺

Resolution and magnification

These two words are easy to mix up. The examiner gives marks for the difference.

• Magnification is how many times bigger the image is than the object.
• Resolution 分辨率 is the smallest distance between two points that still lets you see them as two separate points.

Making an image bigger does not always show more detail. Past a certain point you just get a bigger, blurry image. Resolution sets the real limit on detail.

A light microscope has lower resolution because light has a fairly long wavelength 波长. An electron microscope 电子显微镜 uses beams of electrons 电子 instead of light. Electrons have a much shorter wavelength, so the resolution is far higher and you can see very small structures inside the cell. There are two kinds: scanning 扫描 (shows the surface in 3D) and transmission 透射 (passes electrons through a thin slice to show inside detail).

Plant and animal cells are eukaryotic cells 真核细胞: their DNA is kept inside a nucleus 细胞核. Inside the cell are many small parts called organelles 细胞器, each with its own job. The jelly-like fluid around them is the cytoplasm 细胞质.

A generalised animal cell and its organelles 细胞器

A plant cell also has a cell wall 细胞壁, chloroplasts 叶绿体 and a large vacuole 液泡

In a photomicrograph or electron micrograph you identify organelles by their shape, size and position; in a drawing you show their outlines and label them.

Cells use ATP made in respiration as their energy supply for every job that needs energy, such as making proteins, moving things and dividing.

Comparing plant and animal cells

Both have a cell surface membrane, cytoplasm, a nucleus, mitochondria, ribosomes, ER and a Golgi body.

A prokaryotic cell 原核细胞, such as a bacterium 细菌, is much smaller and simpler than a eukaryotic cell. Its key features are:

• unicellular 单细胞 — it is a single cell.
• generally $1$–$5\ \text{µm}$ across.
• a cell wall made of peptidoglycan 肽聚糖 (not cellulose).
• circular DNA lying free in the cytoplasm — there is no nucleus.
• $70\text{S}$ ribosomes (smaller than the $80\text{S}$ ones in the cytoplasm of eukaryotes).
• no organelles surrounded by a double membrane — so no nucleus, no mitochondria and no chloroplasts.

A prokaryotic cell: the circular DNA (nucleoid 拟核) lies free, with no nucleus and no double-membrane organelles

Comparing prokaryotic and eukaryotic cells

All viruses 病毒 are non-cellular 非细胞 — they are not made of cells at all. Each virus is built from just two or three parts:

• a core of nucleic acid 核酸, which is either DNA or RNA (never both).
• a protein coat around the core called a capsid 衣壳.
• in some viruses, an outer envelope 包膜 made of phospholipids 磷脂.

A generalised virus: nucleic acid 核酸 inside a protein capsid 衣壳, with a lipid envelope 包膜 in some viruses

A virus has no cytoplasm, no organelles and no ribosomes. It cannot respire or make its own proteins. It can only copy itself inside a living host 宿主 cell, so it sits at the edge of what we call "living".

Living things are built from four main kinds of large molecule 分子: carbohydrates 碳水化合物, lipids 脂质, proteins 蛋白质 and nucleic acids 核酸. You can use simple chemical tests to find out which kinds are present in a sample.

Benedict's test: blue turns green, then orange, then brick-red as more reducing sugar is present

Semi-quantitative Benedict's test

The normal Benedict's test only tells you "yes or no". A semi-quantitative 半定量 test gives a rough amount. First you standardise 标准化 the test: you run it on solutions of known concentration 浓度 and record the result for each. Then you can estimate an unknown by either:

• the time to the first colour change (more sugar changes colour faster), or
• comparing the final colour to your set of colour standards.

Testing for non-reducing sugars

Some sugars, such as sucrose, are non-reducing sugar 非还原糖: they give a negative Benedict's test. To detect them:

1. Do a normal Benedict's test first. It stays blue (no reducing sugar).
2. Take a fresh sample and add dilute hydrochloric acid 盐酸, then heat. This acid hydrolysis breaks the sugar into smaller reducing sugars.
3. Cool, then neutralise 中和 the acid with sodium hydrogencarbonate.
4. Now do the Benedict's test again. A brick-red colour shows a non-reducing sugar was present.

Monomers, polymers and macromolecules

• a monomer 单体 is a small molecule that is a single unit.
• a polymer 聚合物 is a long molecule made of many monomers joined together.
• a macromolecule 大分子 is any very large molecule.

Sugars come in three sizes:

• a monosaccharide 单糖 is a single sugar unit, such as glucose 葡萄糖 and fructose.
• a disaccharide 二糖 is two units joined, such as maltose and sucrose.
• a polysaccharide 多糖 is many units joined into a polymer.

Glucose, fructose 果糖 and maltose 麦芽糖 are reducing sugars. Sucrose 蔗糖 is a non-reducing sugar.

Two ring forms of glucose

Glucose has six carbon atoms and forms a ring. There are two ring forms. They differ only at carbon 1:

• in α-glucose, the –OH group on carbon 1 points down, below the ring.
• in β-glucose, the –OH group on carbon 1 points up, above the ring.

This small difference decides which polysaccharide the glucose can build.

The two ring forms differ only at carbon 1: the –OH points down 向下 in α, up 向上 in β

Joining and breaking sugars

Monomers are joined by strong covalent bonds 共价键. When two sugars join, a glycosidic bond 糖苷键 forms between them. This happens by condensation 缩合: a molecule of water is removed each time a bond forms.

The reverse is hydrolysis 水解: a water molecule is added to break a glycosidic bond. This is why the non-reducing sugar test needs acid and heat — they hydrolyse sucrose into glucose and fructose.

Condensation 缩合 removes water to join monomers; hydrolysis 水解 adds water to split them

Storage polysaccharides: starch and glycogen

Starch is the energy 能量 store in plants. It is made of two polymers of α-glucose:

• amylose 直链淀粉 — a long, unbranched chain that coils into a spiral.
• amylopectin 支链淀粉 — a chain with many side branches.

Glycogen 糖原 is the energy store in animals. It is like amylopectin but has even more branches, so it can be broken down quickly when energy is needed.

These stores suit their job well: they are compact, they are insoluble 不溶 (so they do not leave the cell), and they do not change the water potential 水势 of the cell (so they do not pull water in by osmosis 渗透). The many branches give many ends, so glucose can be added or removed fast.

Cellulose

Cellulose 纤维素 is made of β-glucose. Because of the β form, every other glucose is flipped over, so the chains are long and straight. Many straight chains lie side by side and are held together by hydrogen bonds 氢键 into strong bundles called microfibrils 微纤丝. These give the plant cell wall 细胞壁 its strength and stop the cell bursting.

Triglycerides

A triglyceride 甘油三酯 is the main fat or oil. It is non-polar 非极性 and hydrophobic 疏水 (it does not mix with water). It is made from one glycerol 甘油 molecule joined to three fatty acids 脂肪酸 by ester bonds 酯键. Each ester bond forms by condensation, so three water molecules are removed.

One glycerol 甘油 plus three fatty acid 脂肪酸 tails; a straight tail is saturated 饱和, a kinked one unsaturated 不饱和

Fatty acids are of two kinds:

• saturated 饱和 — no carbon–carbon double bonds 双键; these fats are usually solid.
• unsaturated 不饱和 — one or more double bonds; these oils are usually liquid.

Triglycerides make a good long-term energy store: they release about twice as much energy per gram as carbohydrates, they are insoluble, and they store little extra mass because they hold no water. Under the skin they also give insulation 隔热 and protect the organs.

Phospholipids

A phospholipid 磷脂 is like a triglyceride, but one fatty acid is replaced by a phosphate 磷酸 group. This gives the molecule two ends with different behaviour:

• a hydrophilic 亲水 ("water-loving") polar 极性 phosphate head.
• two hydrophobic ("water-fearing") fatty acid tails.

This split personality is why phospholipids form the membranes around cells.

The hydrophilic 亲水 heads face the water; the hydrophobic 疏水 tails hide inside, forming a bilayer 双层

Amino acids and the peptide bond

Proteins are polymers of amino acids 氨基酸. Every amino acid has the same general structure around a central carbon atom: an amino group 氨基 (–NH₂), a carboxyl group 羧基 (–COOH), a hydrogen atom, and a variable side chain 侧链 (the R group). The R group is different in each amino acid.

Two amino acids join by condensation. The bond formed between the amino group of one and the carboxyl group of the next is a peptide bond 肽键, and a water molecule is removed. Many amino acids joined this way make a polypeptide 多肽. Adding water (hydrolysis) breaks a peptide bond.

Every amino acid has an amino group 氨基, a carboxyl group 羧基 and an R group; two join by a peptide bond 肽键

Four levels of protein structure

The four levels: primary 一级 → secondary 二级 → tertiary 三级 → quaternary 四级 structure

The folded shape is held together by four kinds of interaction between R groups:

• hydrophobic interactions 疏水作用 (non-polar R groups cluster away from water).
• hydrogen bonding.
• ionic bonds 离子键 (between charged R groups).
• covalent bonding, including strong disulfide bonds 二硫键.

Globular and fibrous proteins

• globular proteins 球状蛋白质 fold into a rounded shape, are usually soluble 可溶, and do jobs in the body (for example enzymes and haemoglobin).
• fibrous proteins 纤维状蛋白质 form long strands, are usually insoluble, and give structure and support (for example collagen).

Haemoglobin — a globular protein

Haemoglobin 血红蛋白 carries oxygen 氧气 in red blood cells. It has a quaternary structure made of four polypeptide chains: two alpha (α-globin) chains and two beta (β-globin) chains. Each chain holds a haem group 血红素. At the centre of each haem group is an iron 铁 atom, and this is where one oxygen molecule binds. Four chains mean one haemoglobin molecule can carry four oxygen molecules.

Collagen — a fibrous protein

Collagen 胶原蛋白 gives strength to skin, tendons 肌腱, bone and blood vessel walls. One collagen molecule is three polypeptide chains wound tightly around each other in a triple strand, held by hydrogen bonds. Many of these molecules lie side by side, slightly staggered, and are cross-linked into thick fibres 纤维. The staggered, cross-linked arrangement makes collagen very strong when pulled.

Water is a small molecule, but its two O–H bonds are polar: the oxygen end is slightly negative and the hydrogen ends are slightly positive. So one water molecule attracts its neighbours, forming weak hydrogen bonds between them. These hydrogen bonds explain water's useful properties:

• solvent action — water is a good solvent 溶剂, so many substances dissolve in it. This lets reactions happen and lets substances be carried around the body.
• high specific heat capacity 比热容 — water needs a lot of energy to warm up, so its temperature stays steady. This protects living things from quick temperature changes.
• latent heat of vaporisation 汽化潜热 — water needs a lot of energy to evaporate 蒸发. So when water evaporates (for example as sweat dries), it carries away a lot of heat and cools the body.

Enzymes 酶 are globular proteins 球状蛋白质 — a type of protein 蛋白质 with a rounded, soluble shape. They are biological catalysts: they catalyse 催化 (speed up) the chemical reactions in living things, and they are not used up, so each enzyme works again and again.

Enzymes work in two places:

• intracellular 细胞内 enzymes work inside the cell 细胞 that made them. An example is catalase 过氧化氢酶, which breaks down harmful hydrogen peroxide.
• extracellular 细胞外 enzymes are secreted 分泌 (sent out) to work outside the cell. An example is amylase 淀粉酶, which is released into the gut to digest 消化 starch 淀粉.

Yeast enzymes ferment sugar, giving off bubbles of carbon dioxide

Each enzyme has a special pocket called the active site 活性位点. The molecule it acts on is its substrate 底物. The substrate fits into the active site to form an enzyme–substrate complex 酶底物复合物. The reaction then happens, and the products 产物 leave, freeing the active site for the next substrate.

Specificity

An enzyme is specific: it usually works on only one substrate. This is because the shape of the active site is complementary 互补 to (fits) the shape of that substrate and no other. We call this specificity 专一性.

Two ideas explain how the substrate fits:

• the lock-and-key hypothesis 锁钥学说 — the active site is a fixed shape, and only a substrate with the matching shape fits, like a key in a lock.
• the induced-fit hypothesis 诱导契合学说 — the active site is not quite the right shape at first. When the substrate binds, the active site changes shape a little to wrap around it tightly. This idea fits the evidence better.

Lock-and-key 锁钥: a fixed active site. Induced fit 诱导契合: the active site changes shape to grip the substrate

Lowering activation energy

Every reaction needs a small "push" of energy to start, called the activation energy 活化能. An enzyme lowers the activation energy. This lets the reaction go quickly at the cell's normal temperature 温度, instead of needing high heat.

The enzyme route has a lower activation energy 活化能 ($E_A$), so more molecules can react

You can follow an enzyme reaction in two ways:

• measure how fast product is made. With catalase, oxygen gas is a product, so you collect the gas and measure its volume over time.
• measure how fast substrate disappears. With amylase, you remove samples and use the iodine test; the blue-black colour fades as the starch is used up.

A colorimeter 比色计 makes this exact. It shines light through the tube and measures how much light is absorbed, so a colour change becomes a number you can plot.

The rate of reaction 反应速率 is steepest at the start (most substrate present), so the initial rate (the slope at time zero) is the fairest value to compare.

Temperature

As temperature rises, molecules gain more kinetic energy 动能 and collide 碰撞 more often, so the rate rises. But above the optimum temperature 最适温度 the enzyme begins to denature 变性: the heat breaks the bonds holding its shape, so the active site changes and no longer fits the substrate. The rate then falls quickly.

Rate rises to the optimum 最适, then falls fast as the enzyme denatures 变性

pH

Each enzyme has an optimum pH. If the pH moves too far from it, the enzyme denatures and the rate drops. To study pH fairly, you keep it steady with a buffer solution 缓冲液.

The rate peaks at the optimum pH 最适pH and falls away on either side

Enzyme concentration

With plenty of substrate, more enzyme means more active sites, so the rate goes up in proportion to enzyme concentration.

Substrate concentration

At first, adding more substrate speeds the reaction. But once every active site is busy, adding more makes no difference — the rate levels off at a maximum.

Inhibitor concentration

An inhibitor 抑制剂 is a molecule that slows an enzyme. The more inhibitor present, the lower the rate.

The levelling-off rate, when all active sites are full, is the maximum rate, written $V_{\text{max}}$.

The Michaelis–Menten constant 米氏常数 ($K_{\text{m}}$) is the substrate concentration that gives half of $V_{\text{max}}$. It tells you about the enzyme's affinity 亲和力 (pulling power) for its substrate:

• a low $K_{\text{m}}$ means the enzyme reaches half-speed at a low substrate concentration, so it has a high affinity.
• a high $K_{\text{m}}$ means a low affinity.

So $K_{\text{m}}$ lets you compare how strongly different enzymes hold their substrates.

Rate climbs to $V_{\text{max}}$ when all active sites are full; $K_{\text{m}}$ is the substrate concentration giving half $V_{\text{max}}$

Some inhibitors are reversible 可逆: they can leave the enzyme again. There are two types.

A competitive 竞争性 inhibitor raises $K_{\text{m}}$ (more substrate overcomes it); a non-competitive 非竞争性 one lowers $V_{\text{max}}$

An immobilised enzyme 固定化酶 is fixed in place — for example, trapped inside small beads of alginate 海藻酸盐 — instead of floating free in solution. The substrate solution flows past the beads.

A free enzyme usually works a little faster, because the substrate can reach it easily. But immobilised enzymes have big practical advantages:

• the enzyme is not washed away, so it can be used again and again.
• the product is pure — it is not mixed with enzyme.
• the enzyme is more stable, so it survives changes in temperature and pH better.
• the process can run continuously, with substrate flowing in and product flowing out.

Biological washing powders contain enzymes that digest food and blood stains at low temperatures

Every cell is wrapped in a cell surface membrane 细胞膜. We describe its structure with the fluid mosaic model 流动镶嵌模型.

Red blood cells under a scanning electron microscope — each is wrapped in a cell surface membrane

The phospholipid bilayer

The membrane is built mainly from phospholipids 磷脂. Each phospholipid has a hydrophilic 亲水 ("water-loving") head and two hydrophobic 疏水 ("water-fearing") tails. There is water on both sides of the membrane, so the phospholipids line up in two layers — a bilayer 双层 — with the heads facing the water outside and inside, and the tails hidden in the middle, away from water. This arrangement forms by itself because of those hydrophilic and hydrophobic interactions.

The model is called "fluid" because the phospholipids are not fixed: they slide past each other, so the membrane can move and bend. It is called a "mosaic" because many proteins 蛋白质 are dotted through it, like tiles in a picture.

What floats in the membrane

The fluid mosaic model 流动镶嵌模型: proteins, cholesterol 胆固醇 and carbohydrate chains sit in a fluid phospholipid bilayer

So the membrane molecules together give the membrane its stability 稳定性, its fluidity, its permeability 通透性 (control over what gets through), its transport jobs, its signalling jobs, and its cell recognition.

The membrane is partially permeable 半透膜: it lets some substances through easily but blocks others.

Cells talk to each other by cell signalling 细胞信号传递. The main stages are:

1. a cell secretes a signal chemical called a ligand 配体 (for example a hormone 激素).
2. the ligand is carried (often in the blood) to a target cell 靶细胞.
3. the ligand binds to a specific receptor 受体 on the target cell's surface membrane. The shape of the receptor matches that ligand only. Binding then triggers a particular response inside the target cell.

There are six processes. Some are passive 被动 (they need no energy 能量), and some are active (they use energy from ATP).

Simple diffusion

Diffusion 扩散 is the net movement of particles from where they are at a high concentration 浓度 to where they are at a low concentration, until they are spread evenly. Simple diffusion 简单扩散 is when particles pass straight through the bilayer, down the concentration gradient 浓度梯度. Only small or non-polar molecules can do this — such as oxygen 氧气 and carbon dioxide 二氧化碳. It is passive.

Facilitated diffusion

Charged ions 离子 and large polar molecules (such as glucose 葡萄糖) cannot cross the oily bilayer by themselves. In facilitated diffusion 易化扩散 they cross through channel proteins or carrier proteins, still moving down the concentration gradient. It is also passive.

Osmosis

Osmosis 渗透 is the diffusion of water across a partially permeable membrane, from a higher water potential 水势 to a lower water potential. It is passive.

A potato osmometer: the sugar solution rises up the tube as water enters the potato by osmosis

Water crosses to the lower water potential 水势; the solute 溶质 is too big to cross the partially permeable membrane 半透膜

Active transport

Active transport 主动运输 moves a substance against its concentration gradient — from low to high concentration. This needs carrier proteins and energy from ATP.

Diffusion 扩散 and facilitated diffusion 易化扩散 are passive (down the gradient); active transport 主动运输 goes against it and needs ATP

Endocytosis and exocytosis

These move large amounts of material in bulk, using ATP.

• in endocytosis 胞吞作用, the membrane folds inwards around material and pinches off a vesicle 囊泡 to bring it into the cell.
• in exocytosis 胞吐作用, a vesicle fuses with the membrane and releases its contents outside the cell.

Endocytosis 胞吞 brings material in by forming a vesicle 囊泡; exocytosis 胞吐 fuses a vesicle to release its contents

A cell takes in and removes substances across its surface. As an object gets bigger, its volume 体积 grows faster than its surface area 表面积. So the surface area to volume ratio gets smaller as size increases.

For a cube of side $L$:

A large $L$ gives a small ratio. This is why small cells (and thin, flat shapes) exchange materials quickly, while large cells cannot rely on diffusion alone.

As a cube (or cell) grows, its surface area : volume ratio 表面积体积比 gets smaller

You can show this with agar 琼脂 blocks of different sizes soaked in dye or acid: the smallest block, with the largest surface area to volume ratio, changes colour all the way through fastest. Diffusion across non-living materials can also be studied with dialysis tubing 透析袋 (Visking tubing).

Water potential measures how likely water is to leave a solution. Pure water has the highest water potential. Adding a solute 溶质 (a dissolved substance) lowers it. Water always moves by osmosis from a higher to a lower water potential.

To estimate the water potential of plant tissue, you place pieces in sucrose solutions of different water potentials. The solution that causes no change in mass or length has about the same water potential as the tissue.

Effect on plant cells

• in a solution of higher water potential (for example distilled water 蒸馏水), water enters the cell. The cell swells and becomes turgid 膨胀, but the strong cell wall stops it bursting.
• in a solution of lower water potential, water leaves. The cell contents shrink and the membrane pulls away from the cell wall — this is plasmolysis 质壁分离.

Effect on animal cells

Animal cells have no cell wall to protect them.

• in a solution of higher water potential, water enters and the cell may burst. In a red blood cell this bursting is called haemolysis 溶血.
• in a solution of lower water potential, water leaves and the cell shrinks.

A plant cell becomes turgid 膨胀 or plasmolysed 质壁分离; an animal cell may burst (haemolysis 溶血) or shrink

A chromosome 染色体 is one very long molecule of DNA wound tightly around special proteins 蛋白质 called histones 组蛋白. Winding the DNA like this lets a huge length fit inside the nucleus and keeps it tidy.

Before a cell divides, its DNA is copied (this copying is called replication 复制). After copying, each chromosome is made of two identical copies joined together. These two copies are the sister chromatids 姐妹染色单体, and they are held together at a point called the centromere 着丝粒. The tips of each chromosome are capped by telomeres 端粒, which protect the ends.

After replication a chromosome is two sister chromatids 姐妹染色单体 joined at the centromere 着丝粒, with telomeres 端粒 at the tips

Mitosis 有丝分裂 is a type of nuclear division that makes two daughter cells 子细胞 that are genetically identical — they carry exactly the same genes 基因 as the parent cell and as each other.

This matters for:

• growth of multicellular 多细胞 organisms 生物体 (making more cells).
• replacement of damaged or dead cells.
• repair of tissues 组织 by making new cells.
• asexual reproduction 无性生殖 (one parent makes identical offspring).

The cell cycle 细胞周期 is the full life of a cell from one division to the next. It has three parts:

1. interphase 间期 — the longest part. The cell grows in the G₁ phase, copies its DNA in the S phase (replication), and grows again and prepares to divide in the G₂ phase.
2. mitosis — the nucleus divides into two identical nuclei.
3. cytokinesis 胞质分裂 — the rest of the cell splits, giving two separate daughter cells.

Most of the cycle is interphase 间期 (G₁, S, G₂); mitosis (M) and cytokinesis 胞质分裂 are a short part

When DNA is replicated, the copying cannot reach the very end of the molecule, so a little is lost each time. Telomeres are short, repeated lengths of DNA at the ends that carry no genes. Because the telomeres are shortened instead, no important genes are lost during replication.

A stem cell 干细胞 is an unspecialised cell that can keep dividing by mitosis and can differentiate 分化 (change) into different specialised cell types. Stem cells are the source of new cells for replacing lost cells and repairing tissues.

The cell cycle is normally tightly controlled, so cells divide only when needed. If this control is lost, a cell may divide again and again without stopping. This uncontrolled division produces a lump of cells called a tumour 肿瘤.

Mitosis runs through four stages. You should be able to recognise them in photomicrographs and slides.

The four stages: prophase 前期, metaphase 中期, anaphase 后期, telophase 末期

A real onion root tip 洋葱根尖: in a growing tip many cells are caught dividing

Cytokinesis then follows. In an animal cell the cell surface membrane pinches inwards to split the cell; in a plant cell a new wall forms across the middle. The result is two genetically identical daughter cells.

Nucleic acids 核酸 (DNA and RNA) are polymers of small units called nucleotides 核苷酸. Each nucleotide is made of three parts joined together:

• a phosphate 磷酸 group,
• a sugar (a 5-carbon sugar),
• a nitrogen-containing base 碱基.

ATP is a special nucleotide. It has the base adenine 腺嘌呤, the sugar ribose 核糖, and three phosphate groups. Breaking off the last phosphate releases energy 能量 for the cell.

A nucleotide 核苷酸 is a phosphate 磷酸, a sugar and a base; ATP is a nucleotide with three phosphates

DNA extracted from cells appears as pale, stringy strands of these nucleotide polymers

There are five bases, in two groups:

• purines 嘌呤 have a double ring (two rings): adenine and guanine 鸟嘌呤.
• pyrimidines 嘧啶 have a single ring (one ring): cytosine 胞嘧啶, thymine 胸腺嘧啶 and uracil 尿嘧啶.

A DNA molecule is two strands twisted together into a double helix 双螺旋.

Each strand 链 has a backbone of alternating sugar (here the sugar is deoxyribose 脱氧核糖) and phosphate. The sugar of one nucleotide is joined to the phosphate of the next by a phosphodiester bond 磷酸二酯键.

The two strands are held together by their bases, which meet in the middle. The pairing is exact — this is complementary base pairing 碱基互补配对:

• A always pairs with T, held by two hydrogen bonds 氢键.
• C always pairs with G, held by three hydrogen bonds (so a C–G base pair 碱基对 is harder to separate).

The two strands run in opposite directions: one goes 5′ to 3′ while the other goes 3′ to 5′. We say they are antiparallel 反平行.

Complementary base pairing 碱基互补配对: A pairs with T (2 hydrogen bonds 氢键), C with G (3); the strands are antiparallel 反平行

The flat ladder above is twisted into a spiral. This space-filling model, where every atom is a ball, shows the real shape of the double helix:

A space-filling model of DNA: the two strands twist around each other into the double helix 双螺旋 — the ladder of the diagram, coiled up

DNA replication 复制 (copying) happens during the S phase of the cell cycle. It is semi-conservative 半保留复制: each new molecule keeps one old strand and one new strand. The steps are:

1. the double helix unwinds and the hydrogen bonds break, so the two strands separate.
2. each old strand acts as a template. Free nucleotides pair with the exposed bases by complementary base pairing.
3. the enzyme 酶 DNA polymerase 聚合酶 joins the new nucleotides into a strand. It can only add nucleotides in the 5′ to 3′ direction.

Because of that 5′ to 3′ rule, the two new strands are made differently:

• the leading strand 前导链 is built continuously, following the unwinding.
• the lagging strand 后随链 is built in short pieces, working away from the unwinding point. The enzyme DNA ligase 连接酶 then joins these pieces together.

Replication is semi-conservative 半保留: each new molecule keeps one old strand (blue) and one new strand (orange)

RNA is also made of nucleotides, but it is a single strand, its sugar is ribose, and it uses uracil in place of thymine. The most important type here is messenger RNA (mRNA), which carries a copy of a gene's instructions out of the nucleus to be used.

A gene 基因 is a sequence of DNA nucleotides that codes for one polypeptide 多肽.

The code is read in triplets 三联体 — groups of three bases. Each triplet either codes for one specific amino acid 氨基酸, or acts as a start or stop signal. The code is universal: nearly all living things use the same triplets for the same amino acids.

There are 64 possible triplets but only 20 common amino acids, so most amino acids are coded by more than one triplet.

Transcription (in the nucleus)

The DNA gene is copied into mRNA. This is transcription 转录.

• the strand of DNA that is copied is the template strand 模板链; the partner strand is the non-transcribed strand 非转录链.
• the enzyme RNA polymerase joins RNA nucleotides that pair with the template bases (with uracil pairing to adenine).
• in eukaryotes the first RNA made (the primary transcript 初级转录本) contains coding parts called exons 外显子 and non-coding parts called introns 内含子. The introns are cut out and the exons joined together to form the finished mRNA.

Translation (at the ribosome)

The mRNA leaves the nucleus and attaches to a ribosome 核糖体. Building the polypeptide from the mRNA code is translation 翻译.

• the mRNA is read in codons 密码子 (each codon is one triplet of mRNA bases).
• molecules of transfer RNA (tRNA) bring amino acids to the ribosome. Each tRNA has an anticodon 反密码子 that pairs with a matching codon.
• as the codons are read in order, the ribosome joins the amino acids with peptide bonds 肽键, building the polypeptide.

Transcription 转录 copies DNA into mRNA in the nucleus; translation 翻译 at the ribosome 核糖体 builds the polypeptide

A gene mutation 突变 is a change in the base sequence of a DNA molecule. It may change the polypeptide made. There are three types:

• substitution 替换 — one base is swapped for a different base. This changes at most one amino acid, and sometimes none (because most amino acids have more than one triplet).
• deletion 缺失 — a base is removed.
• insertion 插入 — an extra base is added.

A deletion or insertion shifts how every later triplet is read, so it usually changes many amino acids after that point and has a large effect on the polypeptide.

A substitution 替换 changes one triplet; a deletion 缺失 or insertion 插入 shifts every later triplet (a frameshift)

Plants move substances through two transport tissues 组织:

• xylem 木质部 carries water and dissolved mineral ions 矿物离子 up from the roots.
• phloem 韧皮部 carries dissolved foods (called assimilates 同化物, mainly sugars) to wherever they are needed.

In a transverse section 横切面 (a cut straight across) of a dicotyledonous plant 双子叶植物:

• in the stem, xylem and phloem sit together in bundles near the outside, with xylem on the inside of each bundle.
• in the root, the xylem is in the centre, often in a star shape, with phloem between the arms.
• in the leaf, both are found in the veins.

When you draw a plan diagram, you draw only the outlines of the tissues, not the single cells.

The wood of a tree trunk is xylem; each ring is one year of growth

Xylem vessels

Xylem water-carrying tubes are called vessels 导管. They are made of dead, empty cells joined end to end, with the end walls gone, so they form one long open pipe. Their walls are thickened and waterproofed with lignin 木质素. So the structure suits the job: the hollow, open tube with no contents lets water flow fast, and the lignin gives strength and support.

Phloem sieve tubes and companion cells

Phloem food-carrying tubes are called sieve tubes 筛管. They are living cells joined end to end, but their end walls are not gone — they become sieve plates 筛板 with many holes that sap flows through. To leave room for flow, a sieve tube cell loses most of its contents and has no nucleus.

Beside each sieve tube is a companion cell 伴胞. It keeps its nucleus 细胞核 and has many mitochondria 线粒体. It does the living work for the sieve tube and loads sugars into it.

Xylem 木质部 is a dead, open pipe; phloem 韧皮部 is living sieve tubes 筛管 with sieve plates 筛板 and companion cells 伴胞

This is what a real vascular bundle 维管束 looks like under the microscope, in a stained section of a young sunflower stem:

A real vascular bundle 维管束 in section: the big red cells are xylem 木质部 vessels (thick lignified 木质化 walls); the smaller cells above are phloem 韧皮部

Water enters a root hair cell 根毛细胞 by osmosis 渗透, because the root hair has a lower water potential than the soil water. Water then crosses the root to the xylem by two pathways:

• the apoplast pathway 质外体途径 — water moves through the cell walls 细胞壁 (made of cellulose 纤维素) and the spaces between cells, without entering the cytoplasm. This is fast.
• the symplast pathway 共质体途径 — water moves through the cytoplasm of cells, passing from cell to cell through the plasmodesmata 胞间连丝.

At a ring of cells called the endodermis 内皮层, the apoplast pathway is blocked by the Casparian strip 凯氏带, a waterproof band of suberin 木栓质. This forces all the water through the cell membranes, which lets the plant control what enters the xylem.

The apoplast 质外体 goes through the walls, the symplast 共质体 through the cytoplasm; the Casparian strip 凯氏带 blocks the apoplast at the endodermis

Transpiration 蒸腾作用 is the loss of water vapour from a plant. Water evaporates (turns to vapour) from the wet cell surfaces inside the leaf — this is evaporation 蒸发. The water vapour 水蒸气 then diffuses 扩散 out through the stomata 气孔 into the atmosphere 大气.

This loss at the top pulls water up the xylem in a continuous column. It works because of hydrogen bonding between water molecules:

• water molecules attract each other through hydrogen bonds 氢键, so they stick together. This sticking is cohesion 内聚力, and it lets the whole column be pulled up under tension 张力 (the cohesion–tension idea).
• water molecules also stick to the cellulose of the cell walls. This is adhesion 附着力, which helps hold the column in place.

Transpiration 蒸腾 at the leaf pulls the whole water column up the xylem; cohesion 内聚力 (hydrogen bonds) keeps it together

A xerophyte 旱生植物 is a plant adapted 适应 to live where water is scarce. Its leaves reduce water loss by transpiration in several ways: a thick waxy cuticle 角质层, stomata sunk in pits, hairs that trap moist air, and leaves that can roll up. You should be able to draw a labelled leaf section showing these features.

Xerophyte 旱生植物 leaves cut water loss: a thick cuticle 角质层, sunken stomata 气孔, trapped moist air and rolling up

Assimilates such as sucrose 蔗糖 and amino acids 氨基酸 are carried in the phloem from a source 源 to a sink 库.

• a source is where the assimilate is made or released (for example a photosynthesising leaf).
• a sink is where it is used or stored (for example a growing root).

Loading at the source

Companion cells load sucrose into the sieve tubes against its concentration gradient. They use proton pumps 质子泵 to pump hydrogen ions out, then cotransporter proteins 协同运输蛋白 bring sucrose back in together with those ions. This is a form of active transport 主动运输.

Loading sucrose lowers the water potential 水势 inside the sieve tube, so water follows by osmosis. This raises the hydrostatic pressure 静水压 there.

Mass flow

At the sink, sucrose is removed, so the water potential rises, water leaves, and the pressure falls. The result is a pressure difference between source (high) and sink (low). Sap flows from high to low pressure down this gradient. This pressure-driven flow is called mass flow 集流.

Loading sucrose at the source 源 raises the pressure; sap then flows by mass flow 集流 to the sink 库

Mammals have a closed double circulation. "Closed" means the blood stays inside blood vessels 血管 the whole time. "Double" means the blood passes through the heart 心脏 twice for each full trip around the body. This gives two linked loops, so we call it a double circulation 双循环 and the whole thing a circulatory system 循环系统:

• the pulmonary circulation 肺循环 carries blood from the heart to the lungs and back.
• the systemic circulation 体循环 carries blood from the heart to the rest of the body and back.

Blood travels in this order: arteries 动脉 → arterioles 小动脉 → capillaries 毛细血管 → venules 小静脉 → veins 静脉.

Double circulation 双循环: the pulmonary loop goes to the lungs, the systemic loop to the body; blood passes through the heart twice

The main vessels are:

• the pulmonary artery 肺动脉 — carries blood low in oxygen 氧气 from the heart to the lungs.
• the pulmonary vein 肺静脉 — carries oxygen-rich blood from the lungs back to the heart.
• the aorta 主动脉 — the big artery that carries oxygen-rich blood from the heart to the body.
• the vena cava 腔静脉 — the big vein that returns oxygen-poor blood from the body to the heart.

How the vessels suit their jobs

An artery 动脉 has a thick wall and narrow lumen 管腔; a vein 静脉 a thin wall, wide lumen and valves 瓣膜; a capillary 毛细血管 is one cell thick

Blood cells

You should recognise: red blood cells 红细胞 (which carry oxygen), and three white blood cells — monocytes 单核细胞, neutrophils 中性粒细胞 and lymphocytes 淋巴细胞.

A stained blood smear 血涂片: many small red cells, plus a lymphocyte (centre) and neutrophils (lobed nucleus 分叶核) — the white cells are larger and have a nucleus

Water, plasma and tissue fluid

Water is the main part of blood. It is a good solvent 溶剂, so it carries dissolved substances, and it has a high specific heat capacity 比热容, so the blood's temperature stays steady.

At the start of a capillary, the high blood pressure pushes liquid (but not the cells or large proteins) out of the plasma 血浆 and through the capillary wall. This liquid around the cells is tissue fluid 组织液. It supplies the cells with oxygen and glucose and carries waste away. Most of it returns to the capillary at the far end, where the pressure is lower.

High pressure at the arterial end pushes fluid out to form tissue fluid 组织液; most returns at the venous end

Carrying oxygen

Oxygen is carried by haemoglobin 血红蛋白 in the red blood cells. We show how well haemoglobin holds oxygen with the oxygen dissociation curve 氧解离曲线. This S-shaped graph plots the saturation 饱和度 (how full of oxygen the haemoglobin is) against the partial pressure 分压 of oxygen:

• where the partial pressure of oxygen is high (in the lungs), haemoglobin loads up and becomes almost fully saturated.
• where it is low (in respiring tissues), haemoglobin unloads its oxygen for the cells to use.

The Bohr shift

When tissues are very active, they release more carbon dioxide 二氧化碳, which lowers the pH. This makes haemoglobin release oxygen more easily, so the curve moves to the right. This helpful change is the Bohr shift 波尔位移: oxygen is given up exactly where it is most needed.

Haemoglobin loads oxygen in the lungs and unloads it in the tissues; the Bohr shift 波尔位移 moves the curve right so more is released

Carrying carbon dioxide

A little carbon dioxide dissolves straight into the plasma, but most is carried after a reaction inside the red blood cells:

1. the enzyme 酶 carbonic anhydrase 碳酸酐酶 speeds up the reaction of carbon dioxide with water to make carbonic acid.
2. the carbonic acid splits into hydrogen ions and hydrogencarbonate ions 碳酸氢根离子.
3. the hydrogencarbonate ions move out into the plasma. This is the main way carbon dioxide is carried.
4. to keep the charge balanced, chloride ions 氯离子 move into the red blood cells. This movement is the chloride shift 氯转移.
5. the hydrogen ions join haemoglobin to form haemoglobinic acid 血红蛋白酸; this mops up the hydrogen ions and keeps the pH steady.

Some carbon dioxide also joins haemoglobin directly to form carbaminohaemoglobin 氨甲酰血红蛋白.

Structure

The heart has four chambers. The two upper chambers are the atria 心房 (singular: atrium); they have thin walls because they only push blood down into the chambers below. The two lower chambers are the ventricles 心室; they have thick muscular walls because they pump blood out of the heart.

The left ventricle wall is thicker than the right ventricle wall, because the left side must pump blood all the way round the body, while the right side only pumps to the nearby lungs.

The four chambers, the valves 瓣膜 and the main vessels; the left ventricle 左心室 wall is the thickest

The cardiac cycle

One heartbeat is the cardiac cycle 心动周期. It has two parts: systole 收缩期 (when heart muscle contracts) and diastole 舒张期 (when it relaxes and fills). When a chamber contracts, the pressure inside rises; this pressure change opens and closes the valves so that blood flows one way only.

Controlling the heartbeat

The heart sets its own rhythm:

• the sinoatrial node 窦房结 in the right atrium is the pacemaker. It sends out a wave of electrical excitation that spreads across the atria and makes them contract.
• the atrioventricular node 房室结 picks up the wave, holds it back for a moment (so the atria empty first), then passes it on.
• the Purkyne tissue 浦肯野组织 carries the wave down and through the ventricle walls, so the ventricles contract from the bottom upwards and push blood out.

The SAN 窦房结 sets the rhythm; the wave passes to the AVN 房室结, then the Purkyne tissue 浦肯野组织 makes the ventricles contract bottom-up

Your body needs to take in oxygen and get rid of carbon dioxide. This swap happens in the gas exchange 气体交换 system. Air follows this path into the body:

• down the trachea 气管 (the windpipe),
• into two bronchi 支气管 (one to each lung),
• into many smaller bronchioles 细支气管,
• and finally into tiny air sacs called alveoli 肺泡, deep in the lungs 肺.

Each alveolus is wrapped in a network of capillaries 毛细血管, so air and blood are brought very close together.

The human lungs, where gas exchange takes place across millions of alveoli

Air passes down the trachea 气管, into the bronchi 支气管 and bronchioles 细支气管, to the alveoli 肺泡

The cilia, goblet cells and mucous glands work together to keep the lungs clean and healthy: the mucus traps dirt and microbes, and the cilia carry it away to be swallowed.

Goblet cells 杯状细胞 make mucus 黏液 that traps dust and microbes; the cilia 纤毛 sweep it up to the throat

The alveoli are excellent surfaces for exchanging gases, because they have:

• a very large total surface area (millions of tiny sacs),
• very thin walls — the squamous epithelium of the alveolus and the capillary wall are each only one cell thick, so the distance to cross is tiny,
• a rich blood supply from the capillary network,
• a moist lining, so gases dissolve before crossing.

Gases move by diffusion 扩散 down their concentration gradients 浓度梯度:

• oxygen 氧气 is at a high concentration in the alveolar air and a low concentration in the blood, so it diffuses from the air into the blood.
• carbon dioxide 二氧化碳 is at a high concentration in the blood and a low concentration in the alveolar air, so it diffuses from the blood into the air to be breathed out.

Across the thin, moist wall, oxygen 氧气 diffuses into the blood and carbon dioxide 二氧化碳 diffuses out

Under the microscope, real lung tissue looks like a fine pink lace. The many open spaces are the alveoli, and the thin pink lines between them are the walls where gas exchange happens:

Real lung tissue 肺组织 stained for the microscope: the open spaces are alveoli 肺泡 and the thin pink walls are where gases cross; a small airway sits in the centre

Breathing keeps fresh air in the alveoli, and the flowing blood keeps carrying gases away. Both of these keep the concentration gradients steep, so gas exchange stays fast.

An infectious disease 传染病 is caused by a pathogen 病原体 — an organism that lives in or on a host and causes harm. Infectious diseases are transmissible: the pathogen can be transmitted 传播 (passed) from one person to another.

You need to know four diseases, the pathogen that causes each, and how each spreads.

HIV infects and destroys certain white blood cells, so it slowly weakens the body's immune system 免疫系统.

The bacterium 细菌 Mycobacterium (red rods) that causes tuberculosis, stained in a sputum sample

The protoctist 原生生物 Plasmodium (small dark rings) that causes malaria, living inside red blood cells

Control has biological, social and economic sides — the science of the pathogen, people's behaviour and education, and the money and resources available. Examples:

• cholera: provide clean water and proper sewage treatment; good hygiene; vaccines 疫苗 in some areas.
• malaria: sleep under nets; remove pools of still water where mosquitoes breed; spray insecticides 杀虫剂; take anti-malarial drugs.
• TB: find and treat infected people with a long course of antibiotics; give the BCG vaccine; reduce overcrowding; trace contacts of patients.
• HIV: use condoms; use clean needles; test donated blood; educate people. There is no cure and no vaccine yet, but drugs can slow the virus down.

In every case, cost (economic), people's willingness to change behaviour (social) and the supply of drugs or vaccines (biological) all affect how well a disease can be controlled.

An antibiotic 抗生素 is a drug that kills bacteria or stops them growing. For example, penicillin 青霉素 stops bacteria from building their cell walls 细胞壁. As the bacterium grows, its weak wall cannot hold it, so the cell takes in water and bursts.

Penicillin 青霉素 stops new cell wall 细胞壁 forming, so the bacterium takes in water and bursts

Antibiotics do not work against viruses. A virus has no cell wall and no chemical reactions of its own to attack — it simply uses the machinery of the host cell. So there is no antibiotic target in a virus.

Sometimes a mutation 突变 makes a bacterium resistant to an antibiotic, which means the antibiotic no longer kills it. This resistance 耐药性 is a serious problem:

• when an antibiotic is used, the non-resistant bacteria die, but any resistant ones survive and multiply. Over time, more and more bacteria carry the resistance.
• some infections then become very hard, or impossible, to treat.

Antibiotic resistance 耐药性 spreads by natural selection: the antibiotic kills the rest, so the resistant survivors take over

Steps to slow resistance down:

• only use antibiotics when they are really needed (not for viral illnesses such as colds).
• always finish the full course, so no bacteria are left alive.
• use the correct antibiotic for the infection.
• reduce the heavy use of antibiotics in farming.

Your immune system 免疫系统 is the set of cells that defends the body against pathogens 病原体. The first cells to act are phagocytes 吞噬细胞, a group of white blood cells that includes macrophages 巨噬细胞 and neutrophils 中性粒细胞.

A blood smear: the large stained cells are white blood cells, scattered among the many red cells

A phagocyte destroys a pathogen by phagocytosis 吞噬作用: it surrounds the pathogen, takes it inside in a vesicle, and digests it with enzymes. After this, a macrophage displays parts of the pathogen on its own surface, ready to alert other immune cells.

Phagocytosis 吞噬作用: the phagocyte 吞噬细胞 engulfs the pathogen, digests it, then displays its antigen 抗原

This false-coloured electron micrograph captures the moment in real life: a phagocyte reaching out to grab and engulf rod-shaped bacteria.

A real neutrophil 中性粒细胞 (yellow), one kind of phagocyte 吞噬细胞, reaching out to engulf rod-shaped bacteria 细菌 (orange) by phagocytosis 吞噬作用

An antigen 抗原 is a molecule (usually a protein) on a cell surface that the immune system can recognise.

• self antigens 自身抗原 are the body's own markers. The immune system learns to ignore them.
• non-self antigens 非自身抗原 are foreign, for example the antigens on a pathogen. These trigger an immune response.

The first time a new pathogen enters, the body makes a slow immune response 免疫反应. The main steps are:

1. a macrophage engulfs the pathogen and displays its antigen.
2. T-helper cells 辅助性T细胞 recognise that antigen and become active. They release chemicals that switch on other cells.
3. B-lymphocytes 淋巴细胞 with a matching shape are selected. They divide to form plasma cells 浆细胞, which pour out antibodies 抗体, and memory cells 记忆细胞.
4. T-killer cells 杀伤性T细胞 destroy the body's own cells that have been infected.

Memory cells stay in the body for years after the infection is over. If the same pathogen enters again, the memory cells start a secondary immune response that is much faster and larger than the first. The pathogen is destroyed before it can make you ill. This is what we mean by long-term immunity.

The secondary response 二次免疫反应 is faster and larger, because memory cells 记忆细胞 are ready

An antibody is a Y-shaped protein made by plasma cells. The two tips of the Y are antigen-binding sites 抗原结合位点. Each site has a special shape, called the variable region 可变区, that fits one antigen only, like a lock and key.

An antibody 抗体 is Y-shaped; the two tips are the variable regions 可变区 that bind one specific antigen 抗原

Antibodies help in several ways: they stick to antigens, clump pathogens together so they are easier to deal with, mark pathogens so phagocytes find them, and block harmful toxins.

A monoclonal antibody 单克隆抗体 is a single type of antibody, all identical. They are made by the hybridoma 杂交瘤 method:

1. an animal is given an antigen, so it makes B-lymphocytes that produce the wanted antibody.
2. these B-lymphocytes are fused with tumour cells, which divide endlessly.
3. the fused cell (the hybridoma) both makes the antibody and divides without stopping, producing large amounts of one identical antibody.

Fusing a B-lymphocyte with a tumour cell makes a hybridoma 杂交瘤 that pours out one identical (monoclonal 单克隆) antibody

Monoclonal antibodies are used in the diagnosis 诊断 of disease (to detect a specific molecule, as in a pregnancy test) and in treatment (to carry drugs to specific target cells, such as cancer cells).

Immunity can be active or passive, and natural or artificial.

• active immunity 主动免疫 — your own body meets an antigen and makes its own antibodies and memory cells. It is slow to start but long-lasting.
• passive immunity 被动免疫 — ready-made antibodies are given to you from outside. It works at once but does not last, because there are no memory cells.
• natural immunity 天然免疫 — gained in a natural way (active: after an infection; passive: antibodies passed from mother to baby).
• artificial immunity 人工免疫 — gained on purpose (active: your body is made to respond to a safe dose of antigen; passive: you are injected with ready-made antibodies).

Active immunity 主动免疫 (your body responds) lasts; passive immunity 被动免疫 (ready-made antibodies) is fast but short

A vaccine 疫苗 contains antigens — often a dead or weakened pathogen, or part of one. The antigens trigger a primary immune response and make memory cells, so you gain long-term immunity without becoming ill.

Vaccination 疫苗接种 programmes can control the spread of a disease across a population. If enough people are vaccinated, the pathogen cannot pass easily from person to person. This protects even the people who are not vaccinated, an effect called herd immunity 群体免疫.

Cells need a steady supply of energy 能量, which they get from respiration 呼吸作用. Energy is needed for:

• active transport 主动运输 (moving substances against a gradient),
• movement (for example muscle contraction),
• anabolic 合成代谢 reactions — the building of large molecules, such as in DNA replication and protein synthesis.

During exercise, muscles need a constant supply of energy released by respiration

ATP is the molecule that carries energy to where it is needed. It is made from ADP and a phosphate group; when it loses that phosphate again, it releases a small, usable burst of energy. ATP suits this job well, so we call it the universal energy currency:

• it releases energy quickly, in small amounts that match a cell's needs.
• it is easily made and re-made, again and again.
• it is small and soluble, so it moves easily around the cell.

Respiration 呼吸作用 adds a phosphate to make ATP; the cell breaks it off again to release energy 能量

ATP is made in two ways: by direct transfer of a phosphate group in phosphorylation 磷酸化 reactions, and by chemiosmosis 化学渗透 across the membranes of mitochondria 线粒体 and chloroplasts.

A respiratory substrate 呼吸底物 is a molecule that is broken down to release energy. Per gram, lipids release the most energy (they have the most hydrogen), proteins are next, and carbohydrates the least.

The respiratory quotient 呼吸商 (RQ) compares the gases exchanged:

You can work out the RQ from a respiration equation. Carbohydrates give an RQ of about 1.0, lipids about 0.7 and proteins about 0.9.

A respirometer 呼吸计 measures the oxygen 氧气 taken in by living things, such as germinating 萌发 seeds or small invertebrates 无脊椎动物, and is used to find their RQ.

Aerobic 有氧 respiration (with oxygen) has four stages, each in a set place in the cell:

The four stages and where each happens; reduced NAD and FAD carry hydrogen to the inner membrane where most ATP is made

Glycolysis

Glucose 葡萄糖 (6 carbons) is first phosphorylated, using 2 ATP, to form fructose bisphosphate (6C). This 6C molecule is split into two triose phosphate molecules (3C each). These are then oxidised 氧化 to pyruvate 丙酮酸 (3C). Glycolysis makes a net gain of 2 ATP and some reduced 还原 NAD (NAD is a coenzyme 辅酶, a helper molecule).

The link reaction

When oxygen is available, pyruvate enters the mitochondria. There each pyruvate loses a carbon dioxide 二氧化碳 and is turned into a 2-carbon acetyl 乙酰基 group. This group is carried by coenzyme A 辅酶A to form acetyl coenzyme A. Some carbon dioxide is released and NAD is reduced.

The Krebs cycle

The 2C acetyl group joins a 4-carbon molecule, oxaloacetate 草酰乙酸, to make a 6-carbon molecule, citrate 柠檬酸. Citrate is then changed back to oxaloacetate in a series of small steps, ready to accept the next acetyl group. During these steps:

• decarboxylation 脱羧 removes carbon as carbon dioxide.
• dehydrogenation 脱氢 removes hydrogen, which reduces the coenzymes NAD and FAD.

The reduced NAD and FAD then carry the hydrogen to the carriers in the inner mitochondrial membrane.

Each turn releases carbon dioxide 二氧化碳 (decarboxylation 脱羧) and reduced NAD and FAD (dehydrogenation 脱氢)

Oxidative phosphorylation

This stage makes most of the ATP:

1. the hydrogen atoms split into protons 质子 and energetic electrons 电子.
2. the electrons pass along the electron transport chain 电子传递链, releasing energy as they go.
3. this energy is used to pump protons across the inner membrane.
4. the protons flow back into the matrix through a channel called ATP synthase 合酶. This flow provides the energy to make ATP (this is chemiosmosis).
5. oxygen is the final electron acceptor: it joins with electrons and protons to form water.

Electrons pump protons (H⁺) into the intermembrane space; they flow back through ATP synthase 合酶 to make ATP (chemiosmosis 化学渗透)

The structure of mitochondria

The inner membrane is folded into cristae 嵴, giving a large surface for the electron transport chain and ATP synthase. The matrix inside holds the substances and helpers for the link reaction and the Krebs cycle.

When there is no oxygen, only glycolysis can run. To keep glycolysis going, the cell must use up the reduced NAD. This happens by fermentation 发酵:

• in mammals, pyruvate is turned into lactate 乳酸 (lactate fermentation). The lactate is later broken down when oxygen returns.
• in yeast 酵母, pyruvate is turned into ethanol 乙醇 and carbon dioxide (ethanol fermentation).

Without oxygen, pyruvate 丙酮酸 becomes lactate 乳酸 (mammals) or ethanol 乙醇 (yeast); this regenerates NAD for glycolysis

Anaerobic 无氧 respiration gives far less energy than aerobic respiration. Aerobic respiration also runs the Krebs cycle and oxidative phosphorylation, which release a lot more ATP, while anaerobic respiration gains only the small amount from glycolysis.

Rice can grow with its roots under water, where there is little oxygen. It is adapted in three ways: it develops aerenchyma 通气组织 (air-filled spaces) in the roots to carry air down; the roots use ethanol fermentation to keep making some ATP; and the stems grow faster to reach the air above the water.

A redox indicator 指示剂 such as DCPIP or methylene blue loses its colour when it gains hydrogen from respiring cells. The faster the colour is lost, the faster the yeast is respiring, so you can test the effect of temperature or substrate concentration. A respirometer can also be used to measure how temperature changes the rate of oxygen uptake.

Photosynthesis 光合作用 happens inside the chloroplast 叶绿体. Its structure suits its two stages:

• inside are stacks of flat sacs called thylakoids 类囊体. A stack of thylakoids is a granum (plural grana 基粒). The thylakoid membranes hold the light-trapping pigments and are the site of the first stage.
• the fluid around the thylakoids is the stroma 基质, the site of the second stage.

Leaves are green because their chloroplasts are full of the pigment chlorophyll

Thylakoids 类囊体 stack into grana 基粒 (the first stage happens here); the stroma 基质 around them is where the second stage happens

Chloroplasts (the small green discs) inside the cells of an Elodea (pondweed) leaf, seen under a microscope

Photosynthesis has two linked stages:

1. the light-dependent stage 光反应阶段 happens in the thylakoids. It uses light energy to make ATP and reduced 还原 NADP.
2. the light-independent stage 暗反应阶段, also called the Calvin cycle 卡尔文循环, happens in the stroma. It uses the ATP and reduced NADP from the first stage to build complex organic molecules from carbon dioxide 二氧化碳.

A pigment 色素 is a coloured substance that absorbs light. The thylakoids hold several pigments that work together to trap as much light as possible:

• chlorophyll 叶绿素 a and chlorophyll b (which absorb mainly red and blue light),
• carotene 胡萝卜素 and xanthophyll 叶黄素 (which absorb other colours and pass the energy on).

We study them with two graphs. An absorption spectrum 吸收光谱 shows how much light each pigment absorbs at each wavelength. An action spectrum 作用光谱 shows how fast photosynthesis goes at each wavelength. The two graphs match closely, which shows the pigments drive photosynthesis.

The absorption spectrum 吸收光谱 and action spectrum 作用光谱 match: blue and red light are used most, green light least

You can separate the pigments by chromatography 色谱法: the pigments travel different distances up the paper. Each pigment is identified by its Rf value 比移值 (the distance the pigment moved divided by the distance the solvent moved).

In the thylakoids, light is used to make ATP. This is photophosphorylation 光合磷酸化, and it comes in two forms.

In cyclic photophosphorylation 循环光合磷酸化:

• only photosystem 光系统 I (PSI) is used.
• photoactivation 光激活 of chlorophyll occurs (light boosts its electrons 电子 to a higher energy).
• only ATP is made.

In non-cyclic photophosphorylation 非循环光合磷酸化:

• both photosystem I (PSI) and photosystem II (PSII) are used.
• photoactivation of chlorophyll occurs.
• the oxygen-evolving complex 放氧复合体 carries out the photolysis 光解 (splitting by light) of water, which releases oxygen 氧气.
• both ATP and reduced NADP are made.

In non-cyclic photophosphorylation 非循环光合磷酸化, photolysis 光解 splits water; electrons flow PSII → PSI, making ATP and reduced NADP

In both forms, energy is released in the same way:

1. energetic electrons pass along an electron transport chain 电子传递链, releasing energy as they go.
2. this energy is used to pump protons 质子 across the thylakoid membrane.
3. the protons flow back into the stroma through ATP synthase 合酶, and this flow provides the energy to make ATP.

The light-independent stage builds sugars in three main steps:

1. fixation 固定 — the enzyme 酶 rubisco joins carbon dioxide to a 5-carbon molecule, RuBP (ribulose bisphosphate). This makes two molecules of a 3-carbon compound, GP (glycerate 3-phosphate).
2. reduction — GP is reduced to TP (triose phosphate) using reduced NADP and ATP from the light-dependent stage.
3. regeneration — most of the TP is used to regenerate 再生 the RuBP (using more ATP), so the cycle can keep running.

Fixation 固定 adds CO₂ to RuBP; reduction makes TP using ATP and reduced NADP; regeneration 再生 remakes RuBP; some TP becomes sugars

Some TP leaves the cycle to make useful molecules. GP can be used to make some amino acids 氨基酸, and TP can be used to make carbohydrates 碳水化合物, lipids 脂质 and amino acids.

A limiting factor 限制因素 is the one in shortest supply that holds back the rate of photosynthesis. The three main ones are light intensity 光照强度, carbon dioxide concentration 浓度, and temperature 温度.

• raising light intensity speeds up photosynthesis, until some other factor becomes limiting.
• raising carbon dioxide concentration speeds it up, until some other factor becomes limiting.
• raising temperature speeds it up, but only to an optimum; too high and the enzymes denature.

While the rate rises, light is the limiting factor 限制因素; where it levels off, another factor (CO₂ or temperature) limits it

You can measure the rate of the light-dependent stage with a redox indicator 指示剂 such as DCPIP or methylene blue and a suspension of chloroplasts: the dye loses its colour as the chloroplasts work, and you can test different light intensities or light wavelengths 波长. With a whole aquatic plant 水生植物 you can count the bubbles of oxygen given off to compare rates under different conditions.

Homeostasis 稳态 means keeping the conditions inside the body steady, even when the outside changes. Keeping things like temperature, water and blood glucose steady lets enzymes and cells work properly all the time.

A blood glucose meter: homeostasis keeps blood glucose within narrow limits

The principles of homeostasis

Most homeostasis follows the same plan:

• a change (an internal or external stimulus 刺激) is detected by a receptor 受体.
• a coordination system carries the message — either the nervous system 神经系统 (using nerve signals) or the endocrine system 内分泌系统 (using hormones 激素).
• an effector 效应器 (a muscle or a gland 腺体) makes a response that corrects the change.

Sweating cools the body — part of temperature homeostasis, with the skin as the effector

This works by negative feedback 负反馈: a change away from the normal level triggers a response that pushes it back towards normal.

Negative feedback 负反馈: a receptor 受体 detects a change and an effector 效应器 corrects it, returning to normal

The body cannot store extra amino acids. In the liver 肝脏, the process of deamination 脱氨基作用 removes the amino group from excess amino acids 氨基酸, and this is turned into urea 尿素. The urea is carried in the blood to the kidneys to be removed.

The kidney 肾脏 cleans the blood and controls its water content. Its parts are:

• an outer fibrous capsule,
• an outer region, the cortex 皮质,
• an inner region, the medulla 髓质,
• a central space, the renal pelvis 肾盂, which collects urine,
• the ureter 输尿管, which carries urine to the bladder,
• branches of the renal artery (bringing blood in) and renal vein (taking blood out).

The nephron

Each kidney holds about a million tiny tubes called nephrons 肾单位. Along a nephron are: the glomerulus 肾小球 (a knot of capillaries), the Bowman's capsule 鲍曼囊 around it, the proximal convoluted tubule 近曲小管, the loop of Henle 亨利环, the distal convoluted tubule 远曲小管, and the collecting duct 集合管.

Ultrafiltration 超滤 happens in the Bowman's capsule 鲍曼囊; selective reabsorption 重吸收 happens in the proximal convoluted tubule

Making urine

1. Ultrafiltration 超滤 happens in the Bowman's capsule. The blood in the glomerulus is under high pressure, so water and small molecules (glucose 葡萄糖, ions 离子, urea) are pushed out into the capsule, forming the filtrate 滤液. Blood cells and large proteins are too big to pass, so they stay in the blood.
2. Selective reabsorption 选择性重吸收 happens in the proximal convoluted tubule. Useful substances are taken back into the blood. All the glucose and much of the water and ions are reabsorbed here. The tubule wall is well suited to this: its cells have microvilli 微绒毛 to give a large surface area, and many mitochondria 线粒体 to power active transport 主动运输.

Osmoregulation 渗透调节 controls the water content of the blood. It is run by the brain:

• the hypothalamus 下丘脑 detects the water potential 水势 of the blood.
• when the blood is too concentrated, the pituitary gland 垂体 releases antidiuretic hormone 抗利尿激素 (ADH).
• ADH makes the collecting ducts more permeable to water by adding water channels called aquaporins 水通道蛋白.
• more water is then reabsorbed back into the blood, so less, more concentrated urine is made. This is negative feedback.

ADH 抗利尿激素 makes the collecting ducts reabsorb more water, restoring the blood's water content by negative feedback

When blood glucose falls, the hormone glucagon 胰高血糖素 is released. It shows how a hormone passes its message into a cell — cell signalling 细胞信号传递:

1. glucagon binds to a receptor on the liver cell surface, causing a conformational change 构象变化 (a change in the receptor's shape).
2. this activates a G-protein G蛋白, which switches on the enzyme adenylyl cyclase 腺苷酸环化酶.
3. adenylyl cyclase makes a second messenger 第二信使 inside the cell, called cyclic AMP 环腺苷酸 (cAMP).
4. cAMP activates protein kinase A 蛋白激酶A, which starts an enzyme cascade 酶级联反应 — one enzyme 酶 switches on the next, by phosphorylation 磷酸化.
5. because each enzyme switches on many of the next, the signal is greatly amplified 放大.
6. the final enzyme breaks down glycogen 糖原 into glucose, which raises the blood glucose level.

The signal passes through a second messenger 第二信使 (cAMP) and an enzyme cascade 酶级联反应 — each step activates many, so the signal is amplified 放大

Negative feedback and blood glucose

Blood glucose is held steady by two hormones working against each other:

• when glucose is high, insulin 胰岛素 makes muscle and liver cells take in glucose and store it as glycogen, lowering the level.
• when glucose is low, glucagon makes liver cells break glycogen back into glucose, raising the level.

Insulin 胰岛素 and glucagon 胰高血糖素 work against each other to keep blood glucose steady

Measuring glucose

Test strips 试纸 and biosensors 生物传感器 measure glucose in blood or urine. They use the enzymes glucose oxidase 葡萄糖氧化酶 and peroxidase 过氧化物酶, which react with glucose to give a colour change (or an electric signal in a biosensor) that shows how much glucose is present.

Stomata 气孔 are pores in a leaf. The plant opens and closes them to balance two needs: letting in carbon dioxide 二氧化碳 for photosynthesis 光合作用, and limiting water loss by transpiration 蒸腾作用. Stomata usually open by day and close at night, following a daily rhythm.

Each stoma is opened and closed by two guard cells 保卫细胞 around it:

• to open: the guard cells pump in ions, so their water potential falls and water enters by osmosis 渗透. They swell and bend apart, opening the pore.
• to close: ions leave, water follows out, the guard cells go floppy, and the pore closes.

Guard cells 保卫细胞 open the stoma 气孔 by taking in water and turgid; they close it by losing water and going floppy

When the plant is short of water (water stress 水分胁迫), the hormone abscisic acid 脱落酸 is released. It makes the guard cells lose ions and water so the stomata close, saving water. In this signalling, calcium ions 钙离子 act as a second messenger inside the guard cells.