IGCSE Biology · ⁨Biology IGCSE⁩

Every living thing shares the same seven characteristics 特征 — seven life processes 生命过程 that all organisms 生物体 carry out. You can remember them with the letters MRS GREN. The examiner gives marks for the exact wording, so learn each definition.

• Movement 运动 — an action by an organism, or part of an organism, that changes its position or place.
• Respiration 呼吸作用 — the chemical reactions in cells 细胞 that break down nutrient molecules 营养物质 and release energy 能量 for metabolism 新陈代谢.
• Sensitivity 应激性 — the ability to detect and respond to changes in the internal or external environment 环境.
• Growth 生长 — a permanent increase in size and dry mass 干重.
• Reproduction 生殖 — the processes that make more of the same kind of organism.
• Excretion 排泄 — the removal of the waste products 废物 of metabolism, and of substances the body has in excess (more than it needs).
• Nutrition 营养 — the taking in of materials for energy, growth and development.

Metabolism means all the chemical reactions that happen inside an organism's cells.

There are millions of kinds of living thing. To study them, scientists classify 分类 them — they sort them into groups by the features they share. Organisms in the same group share more features with each other than with organisms in other groups.

Species and the binomial system

The smallest common group is the species 物种. A species is a group of organisms that can reproduce to make fertile offspring 可育后代 ("fertile" means the offspring can themselves go on to have young).

Each species has a two-part scientific name 学名. This way of naming is the binomial system 双名法 — a system agreed all over the world, so every scientist uses the same name.

• The first part is the genus 属 (a larger group that the species belongs to). It starts with a capital letter.
• The second part is the species. It starts with a small letter.

The whole name is written in italics, for example Homo sapiens (humans). Two rats named Rattus norvegicus and Rattus rattus belong to the same genus but are different species.

Dichotomous keys

A dichotomous key 二歧检索表 is a tool that helps you identify an unknown organism. "Dichotomous" means "splitting into two". At each step the key gives you two choices about a feature. You pick the one that fits your organism, and that choice sends you on to the next pair of choices. You repeat this until you reach the organism's name.

When you build a key, choose clear features you can see — for example "has wings / has no wings" — not features that change, such as size.

Worked example. A key reads: step 1, has wings, go to 2; no wings, go to 3. Step 2, one pair of wings, it is a housefly; two pairs of wings, it is a butterfly. An unknown insect has two pairs of wings. Step 1 only asks "wings or no wings?", so having wings sends you to step 2. At step 2, "two pairs" gives butterfly. Follow the route the key gives you, one step at a time - never jump straight to the name that looks right.

Classification and DNA (Supplement)

Modern classification tries to show evolutionary relationships 进化关系 — how closely different organisms are related. Organisms that are more closely related share a more recent ancestor 祖先.

To measure this, scientists compare the base sequence 碱基序列 of DNA (the order of the chemical "letters" that make up an organism's genes 基因). The rule is simple:

• Organisms that share a more recent ancestor (more closely related) have more similar DNA base sequences.
• Organisms that share only a distant ancestor have less similar base sequences.

So a DNA base sequence can be used both to classify organisms and to work out how closely two species are related.

The biggest groups that organisms are sorted into are called kingdoms 界. At Core level you place organisms into the first two kingdoms below; for Supplement you need all five kingdoms. You decide where an organism belongs by looking at its main features.

The fungus, prokaryote and protoctist kingdoms are Supplement only.

Grouping the animal kingdom

The animal kingdom is split first into two: animals with a backbone and animals without one.

Vertebrates 脊椎动物 are animals with a backbone 脊柱. There are five main groups.

Arthropods 节肢动物 are animals with no backbone. They have a hard outer skeleton (an exoskeleton 外骨骼) and legs that bend at joints 关节. There are four main groups.

Grouping the plant kingdom (Supplement)

Plants are placed into groups too. You need two of them.

• Ferns 蕨类植物 — have roots, stems and leaves, but make no flowers or seeds. They reproduce using tiny spores 孢子.
• Flowering plants 开花植物 — make flowers, and form seeds 种子 inside the flower. They split into two groups by their seed-leaves (the first leaves inside a seed, called cotyledons 子叶):

Viruses (Supplement)

Viruses 病毒 are not placed in any kingdom. They are not made of cells, so many scientists do not count them as living. A virus is very simple, with only two parts:

• an outer protein coat 蛋白质外壳, and
• genetic material 遗传物质 (its genes) inside.

A virus cannot carry out the life processes on its own. It can only reproduce inside the living cells of a host 宿主 — the organism it infects.

• Learn the seven characteristics by heart, with their exact definitions. Write them as clear, full sentences.
• Digestion 消化 is not one of the seven characteristics — it is just one part of nutrition. "Breathing" is not one of the seven either.
• In a scientific name, the genus starts with a capital letter and the species with a small letter, and both are in italics.
• To use a dichotomous key, start at the first pair of choices and follow the route step by step. Do not jump ahead.
• More similar DNA base sequence → the organisms are more closely related (they share a more recent ancestor).

Every living thing is made of cells 细胞. A cell is the smallest part that can carry out the life processes. In this topic you compare a plant 植物 cell, an animal 动物 cell and the cell of a bacterium 细菌. All new cells are made by the division 分裂 of cells that already exist.

Which structures are in each cell?

So plant and animal cells share five structures: cell membrane, cytoplasm, ribosomes, nucleus and mitochondria. A plant cell has three extra structures: a cell wall, a large vacuole and (in green parts) chloroplasts.

What each structure does

• Cell membrane — controls which substances enter and leave the cell.
• Cytoplasm — a jelly-like liquid where many chemical reactions happen.
• Nucleus — controls the cell's activities and holds the genetic material 遗传物质.
• Ribosomes — where proteins 蛋白质 are made.
• Mitochondria — where respiration 呼吸作用 happens to release energy 能量.
• Cell wall — made of cellulose 纤维素; it gives a plant cell strength and a fixed shape.
• Chloroplasts — contain chlorophyll 叶绿素, a green substance that traps light for photosynthesis 光合作用.
• Large vacuole — filled with cell sap 细胞液; it helps keep the plant cell firm.

Bacterial cells

A bacterium is a prokaryote 原核生物 — its cell has no nucleus. Instead its DNA is a single loop (a ring), loose in the cytoplasm. Bacteria also have small extra rings of DNA called plasmids 质粒. A bacterial cell has a cell wall and cell membrane, cytoplasm and ribosomes, but no mitochondria and no chloroplasts.

Most cells are specialised cells 特化细胞 — their shape and parts suit one special job.

Sperm and egg cells are the sex cells, called gametes 配子.

An organism 生物体 made of many cells is built up in levels, from small to large:

• cell — the smallest unit of life (for example a red blood cell).
• tissue 组织 — a group of similar cells that work together (for example muscle 肌肉).
• organ 器官 — several different tissues that work together to do a job (for example the heart, or a leaf).
• organ system 器官系统 — several organs that work together (for example the digestive system 消化系统, which breaks down food).
• organism — all the organ systems together make one whole living thing.

Cells are tiny, so you look at them under a microscope 显微镜, which makes them appear much larger. How many times larger the image is, is called the magnification 放大倍数.

• image size = the size in the drawing or photo.
• actual size = the real size of the specimen 标本.

Magnification has no unit — it is just a number, for example ×250. You can rearrange the formula:

Worked example. A structure is shown at a magnification of 250. Its image size is 5.00 mm. So the actual size = 5.00 ÷ 250 = 0.02 mm. Always put both sizes in the same unit before you divide.

Changing units (Supplement)

Cells are often measured in micrometres 微米 (μm), which are smaller than millimetres (mm):

So 0.02 mm = 20 μm. To change mm into μm, multiply by 1000. To change μm into mm, divide by 1000.

Worked example. A cell is drawn 60 mm wide, and its real width is 30 μm. Find the magnification. First make the units match: image size = 60 mm = 60 × 1000 = 60,000 μm. Then magnification = image ÷ actual = 60,000 ÷ 30 = ×2000. If you forget to convert and divide 60 ÷ 30, you get ×2 — a thousand times too small. This is the most common mistake in the whole topic.

• A plant cell has a cell wall, a large vacuole and chloroplasts; an animal cell has none of these. Both have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
• A cell with no nucleus but with plasmids and a loop of DNA is a bacterium.
• For magnification, cover the quantity you want: magnification = image ÷ actual; actual = image ÷ magnification.
• Always convert to the same unit before you calculate. Remember 1 mm = 1000 μm.
• For a specialised cell, link its job to the one feature that suits it (for example a red blood cell has no nucleus, so it can hold more oxygen).

Diffusion 扩散 is the net movement 净移动 of particles 粒子 from a region of their higher concentration 浓度 to a region of their lower concentration. We say the particles move down a concentration gradient 浓度梯度.

The particles spread out because of their own random movement 随机运动. The energy 能量 for diffusion comes from the kinetic energy 动能 of the random movement of molecules 分子 and ions 离子 — so diffusion needs no extra energy.

Some substances pass into and out of cells by diffusion through the cell membrane 细胞膜.

Why diffusion matters. Many gases 气体 and dissolved substances (solutes 溶质) move by diffusion in living things. For example, oxygen 氧气 diffuses into cells for respiration, and carbon dioxide 二氧化碳 diffuses out.

The rate of diffusion changes with four factors:

Water as a solvent

Water is a very good solvent 溶剂 — many substances dissolve 溶解 in it to make a solution 溶液. This matters for digestion 消化 (food must dissolve before it can be used), excretion 排泄 (wastes are carried away dissolved in water) and transport 运输 (substances travel around the body dissolved in blood and sap).

What osmosis is

Osmosis 渗透 is a special kind of diffusion: the net movement of water across a partially permeable membrane 半透膜. This kind of membrane has tiny holes that let small water molecules through but hold back larger solute particles. Water moves into and out of cells by osmosis through the cell membrane.

A concentrated solution has a lot of dissolved solute and little water. A dilute solution has little solute and a lot of water.

(Supplement) Osmosis is the net movement of water molecules from a region of higher water potential 水势 (a dilute solution — more water) to a region of lower water potential (a concentrated solution — less water), through a partially permeable membrane.

Osmosis and plant cells

When you put plant tissue into different solutions, water moves by osmosis:

• In pure water or a dilute solution, water moves into the cells. The cells swell and become firm, or turgid 膨胀. The water presses outward on the cell wall; this outward push is the turgor pressure 膨压. Turgid cells make a plant stand up straight — this is how plants are supported.
• In a concentrated solution, water moves out of the cells. The cells lose their firmness and become soft, or flaccid 松软.
• (Supplement) If even more water leaves, the cell membrane pulls away from the cell wall. This is plasmolysis 质壁分离.

You can investigate osmosis using dialysis tubing 透析管 (an artificial partially permeable membrane), or using cylinders cut from a potato. Measure their length or mass before and after soaking. Cylinders in pure water or a dilute solution gain length and mass; cylinders in a concentrated solution lose length and mass; in a solution of equal concentration there is no change.

Worked example. A potato cylinder has a mass of 5.0 g before soaking and 5.6 g after soaking in a dilute solution. Find the percentage change in mass.

• change in mass = 5.6 - 5.0 = 0.6 g (a gain).
• percentage change = change ÷ starting mass × 100 = 0.6 ÷ 5.0 × 100 = +12%.

A positive answer means the cylinder gained water, so the solution was more dilute than the cell contents. Always work out the percentage change, not just the change in grams — it lets you compare cylinders that started at different masses fairly.

Active transport 主动运输 is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — that is, against the concentration gradient. Because this is "uphill", it needs energy from respiration 呼吸作用.

(Supplement) Active transport lets a cell take in useful molecules or ions even when they are already more concentrated inside the cell. For example, root hairs 根毛 take up mineral ions from the soil by active transport. Protein carriers 载体蛋白 in the membrane pick up the molecules or ions and carry them across, using energy.

• Always write net movement: particles move both ways, but the overall flow is down the gradient (for diffusion and osmosis).
• Osmosis moves water only, and only through a partially permeable membrane. Diffusion can move many kinds of particle.
• Only active transport uses energy from respiration, and only it goes against the gradient.
• For the potato or plant experiment: water in → turgid, longer and heavier; water out → flaccid, shorter and lighter; equal concentration → no change.
• A larger surface area, higher temperature, steeper concentration gradient and shorter distance all make diffusion faster.

A catalyst 催化剂 is a substance that speeds up the rate 速率 of a chemical reaction 化学反应 but is not used up or changed by the reaction. The same catalyst can be used again and again.

Enzymes 酶 are biological catalysts — catalysts made by living cells. They are proteins 蛋白质. Enzymes take part in all the reactions of metabolism 新陈代谢, so they control almost every chemical reaction in a living thing.

Enzymes are vital. Without them, the reactions that keep you alive would be far too slow. Enzymes raise the reaction rate enough to keep the organism alive.

Each enzyme has a special pocket called its active site 活性位点. The molecule 分子 it works on is called the substrate 底物.

• The shape of the active site is complementary 互补 to the shape of the substrate — they fit together like a key in a lock.
• The substrate slots into the active site. (Supplement) This makes an enzyme-substrate complex 酶底物复合物.
• The reaction takes place, and the substrate is changed into one or more products 产物.
• The products leave the active site, which is then free to be used again.

Why enzymes are specific (Supplement)

Each enzyme is specific 专一 — it works on only one kind of substrate. This is because only that substrate has the right shape to fit the active site. A substrate of a different shape will not fit, just as the wrong key will not open a lock.

The activity of an enzyme depends on temperature 温度 and pH. You can investigate this by measuring how fast the reaction goes at different temperatures or pH values.

Temperature

• As the temperature rises from cold, enzyme activity speeds up. The enzyme and substrate molecules have more kinetic energy 动能, so they move faster and meet more often. (Supplement) There are more useful collisions 碰撞 each second.
• Activity is highest at the optimum temperature 最适温度 (about 37 °C in the human body).
• Above the optimum, activity falls fast. The heat changes the shape of the active site, so the substrate no longer fits. The enzyme is denatured 变性. Denaturation is permanent — the enzyme cannot recover.

Worked example. A reaction finishes in 50 s at 20 °C, and in 20 s at 37 °C. Which temperature is faster, and by how much? A shorter time means a faster rate, so work out a rate with rate = 1000 ÷ time. At 20 °C the rate is 1000 ÷ 50 = 20 units; at 37 °C it is 1000 ÷ 20 = 50 units. So 37 °C is 50 ÷ 20 = 2.5 times faster. Do not say the rate "went up by 30 s" - a time is not a rate. Turn the time into a rate first, and remember the bigger rate always goes with the smaller time.

pH

• Each enzyme works best at one particular pH.
• If the pH is too high or too low, the shape of the active site changes, the substrate stops fitting, and the enzyme is denatured.

• An enzyme is both a protein and a biological catalyst: it speeds up a reaction and is not used up.
• Describe the shape: the active site is complementary to the substrate (the lock-and-key idea). Each enzyme is specific to one substrate.
• Low temperature → slow (too little kinetic energy). Optimum → fastest. Too hot → denatured (the active site loses its shape, permanently).
• The wrong pH also denatures the enzyme by changing the shape of the active site.
• "Denatured" does not mean "killed" — enzymes are not alive. Say that the shape of the active site has changed so the substrate no longer fits.

Photosynthesis 光合作用 is the process by which plants make carbohydrates 碳水化合物 from simple raw materials 原料, using energy 能量 from light. It is how plants feed themselves.

The word equation

In words: plants take in carbon dioxide 二氧化碳 and water, and make glucose 葡萄糖 and oxygen 氧气. This reaction can only happen in the light, and only where there is chlorophyll 叶绿素.

(Supplement) The balanced chemical equation is:

What chlorophyll does

Chlorophyll is a green pigment 色素 found in the chloroplasts 叶绿体. It traps light and transfers the energy from light into energy stored in chemicals. That chemical energy is then used to build carbohydrates.

What the plant does with the glucose

Minerals from the soil

Plants also take up mineral ions 离子 from the soil through their roots:

• nitrate ions 硝酸根离子 — needed to make amino acids 氨基酸 (and so proteins).
• magnesium ions 镁离子 — needed to make chlorophyll.

A plant short of nitrate grows poorly with weak stems; a plant short of magnesium has yellow leaves (it cannot make enough chlorophyll).

The rate of photosynthesis

The rate 速率 of photosynthesis depends on three things:

• light intensity 光照强度 — more light gives a faster rate, up to a point.
• carbon dioxide concentration 浓度 — more carbon dioxide gives a faster rate.
• temperature 温度 — a warmer temperature is faster, until it gets so hot that the enzymes are denatured.

(Supplement) At any moment, the one factor that is in shortest supply holds back the rate. This is called the limiting factor 限制因素. For example, on a dull day light intensity is usually the limiting factor; on a bright day carbon dioxide may be.

Worked example. On a graph of rate against light intensity, the line rises steeply at first and then levels off flat. Which factor is limiting on each part? On the rising part, adding light raises the rate, so light intensity is the limiting factor there. On the flat part, adding more light changes nothing, so light is no longer limiting - something else is, usually carbon dioxide concentration or temperature. The test is always the same: if adding more of a factor speeds the reaction up, that factor was the one holding it back.

Investigating photosynthesis

To show that a plant needs light, chlorophyll and carbon dioxide, you test a leaf for starch after taking one of them away. A control 对照 keeps every other condition the same, so the test is fair. (First leave the plant in the dark to use up its starch.) A leaf with green and white parts shows starch only in the green parts, which have chlorophyll.

You can also watch gas exchange 气体交换 in a water plant using hydrogencarbonate indicator 指示剂, which changes colour as the carbon dioxide level changes. In the light the plant takes in carbon dioxide for photosynthesis; in the dark it gives out carbon dioxide from respiration.

A leaf is well adapted 适应 for photosynthesis. Most leaves are broad, with a large surface area 表面积 to catch plenty of light, and thin, so that gases and light quickly reach all the cells.

The middle of the leaf, the mesophyll 叶肉, is where most photosynthesis happens. These are the main parts of a leaf:

• Learn the word equation exactly: carbon dioxide + water → glucose + oxygen (in light, with chlorophyll).
• Chlorophyll is in the chloroplasts; it does not get used up — it transfers light energy.
• Link each factor to the rate: more light, more carbon dioxide, warmer (but not too hot) → faster, until a limiting factor stops further increase.
• For leaf structure, always link the part to its job: broad and thin to catch light; palisade cells full of chloroplasts; stomata and air spaces for gas exchange.
• Nitrate ions → amino acids; magnesium ions → chlorophyll.

A balanced diet 均衡饮食 gives you all the substances you need, in the right amounts. There are seven parts.

Two diseases come from missing a vitamin:

• too little vitamin C → scurvy 坏血病 (bleeding gums, slow healing).
• too little vitamin D → rickets 佝偻病 (soft, bent bones), because the body cannot take in enough calcium.

Worked example. A child eats almost no fresh fruit or vegetables. Their gums bleed and small cuts heal slowly. Which part of the diet is missing, and why does that explain the symptoms? Fresh fruit and vegetables are the main source of vitamin C, and too little vitamin C causes scurvy - whose signs are exactly bleeding gums and slow healing. So the missing part is vitamin C. Name the substance and the disease separately: an answer that says only "a vitamin deficiency" does not earn the mark.

The digestive system 消化系统 breaks food down. Food passes along one long tube, the alimentary canal 消化道:

mouth → oesophagus 食道 → stomach 胃 → small intestine 小肠 (the duodenum 十二指肠 then the ileum 回肠) → large intestine 大肠 (the colon 结肠, rectum 直肠 and anus 肛门).

Some organs help with digestion but food does not pass through them: the salivary glands 唾液腺 in the mouth, the pancreas 胰腺, the liver 肝脏 and the gall bladder 胆囊.

Five things happen to food, in order:

Physical digestion breaks food into smaller pieces without changing the food molecules. This gives a larger surface area 表面积 for the enzymes 酶 to act on later.

Teeth

You have four kinds of teeth 牙齿:

A tooth is built from these parts:

Teeth are set into the bone of the jaw and held firm by the gums 牙龈.

The stomach

The wall of the stomach is made of muscle 肌肉 that squeezes and mixes the food, breaking it into smaller pieces.

Bile (Supplement)

Bile 胆汁 is made in the liver and stored in the gall bladder. It emulsifies 乳化 fats and oils — it breaks large drops of fat into many tiny droplets. This gives a much larger surface area for the enzyme lipase 脂肪酶 to digest the fat. Bile itself contains no enzymes.

Chemical digestion breaks large insoluble 不溶性 molecules into small soluble 可溶性 molecules. Only small soluble molecules can be absorbed into the blood. Enzymes carry out this work.

Acid in the stomach

The stomach makes hydrochloric acid 盐酸, which is part of the gastric juice 胃液. This acid:

• kills harmful microorganisms 微生物 in the food, and
• gives an acidic 酸性 pH, the best pH for the stomach's protease to work.

Digesting starch and protein (Supplement)

Starch is digested in two steps:

• amylase breaks starch into maltose 麦芽糖.
• maltase 麦芽糖酶, on the membranes of the epithelium 上皮 lining the small intestine, breaks maltose into glucose 葡萄糖.

Protein is digested by two proteases:

• pepsin 胃蛋白酶 breaks protein down in the acidic stomach.
• trypsin 胰蛋白酶 breaks protein down in the alkaline 碱性 small intestine.

Bile is alkaline. It neutralises 中和 the acidic food and gastric juice as they enter the duodenum, giving a suitable pH for the enzymes there.

Digested nutrients are absorbed into the blood in the small intestine. Most water is absorbed here too; the colon absorbs the rest, leaving solid faeces.

Villi (Supplement)

The inside of the small intestine is covered with millions of tiny finger-shaped villi 绒毛. Each villus is itself covered in much smaller microvilli 微绒毛. Together these give a huge surface area, so absorption is fast.

A single villus has:

• a wall just one cell thick, so molecules have only a short distance to travel.
• a network of capillaries 毛细血管 that carry away absorbed glucose and amino acids.
• a lacteal 乳糜管 in the centre that carries away absorbed fatty acids and glycerol.

• A balanced diet has the right amount of each part. Learn one source and one use for each.
• Vitamin C ↔ scurvy; vitamin D ↔ rickets.
• The five processes in order: ingestion → digestion → absorption → assimilation → egestion. Egestion (faeces) is not excretion.
• Physical digestion makes pieces smaller (more surface area); chemical digestion uses enzymes to make molecules smaller and soluble.
• amylase → sugars, protease → amino acids, lipase → fatty acids + glycerol. Stomach acid kills microorganisms and sets a low pH.
• Villi and microvilli give a large surface area, and a thin wall gives a short distance — both make absorption faster.

Plants have two transport tissues that run through the roots, stems and leaves:

• xylem 木质部 — carries water and mineral ions 矿物质离子 up from the roots, and gives the plant support 支撑.
• phloem 韧皮部 — carries sucrose 蔗糖 and amino acids 氨基酸 to wherever they are needed.

In a root, stem or leaf, the xylem and phloem lie side by side in vascular bundles 维管束.

Xylem structure (Supplement)

Xylem is made of long tubes called vessels 导管, well suited to carrying water:

• their walls are thick and strengthened with lignin 木质素, which also helps support the plant.
• the cells are dead and empty (they have no cell contents), so water flows through freely.
• the cells are joined end to end with no walls across the tube, making one long, continuous pipe.

Water and mineral ions enter the plant through root hair cells 根毛细胞. The huge number of root hairs 根毛 gives a very large surface area 表面积, which speeds up the uptake of water and mineral ions.

The water then follows this pathway:

root hair cells → root cortex 皮层 cells → xylem → mesophyll 叶肉 cells in the leaf.

You can show this pathway by standing a plant or a white flower in water that contains a coloured stain 染色剂. The stain is carried up the xylem and colours the veins.

Transpiration 蒸腾作用 is the loss of water vapour 水蒸气 from the leaves.

• Water evaporates 蒸发 from the wet surfaces of the mesophyll cells into the air spaces 气腔 inside the leaf.
• The water vapour then diffuses 扩散 out of the leaf through the stomata 气孔.

What changes the rate of transpiration

Worked example. In a potometer the air bubble moves 60 mm in 5 minutes. Find the rate of water uptake, and say what happens when a fan is switched on. Rate = distance ÷ time = 60 ÷ 5 = 12 mm per minute. Always give a rate its full unit, "mm per minute", not just "12". A fan raises the wind speed, which carries water vapour away from the leaf, so transpiration speeds up and the bubble moves further in the same 5 minutes.

How water rises up the xylem (Supplement)

As water vapour leaves the leaf, it pulls more water up behind it. This pull is called the transpiration pull 蒸腾拉力. The water molecules stick to one another by forces of attraction 吸引力, so they are drawn up the xylem together as one long column 水柱 of water.

Wilting (Supplement)

If a plant loses water faster than it can take it up, its cells become soft and the plant wilts 萎蔫 — the leaves and stem droop. This often happens on a hot, dry, windy day.