Cell structure

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.

A light microscope: light passes up through the specimen, then two lenses (the objective and the eyepiece) magnify it

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.

Making a temporary wet mount — lower the cover slip at an angle so no air bubbles are trapped

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:

Magnification = image size ÷ actual size

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

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).

Resolution: a light microscope blurs two very close points into one; an electron microscope, with its shorter wavelength, resolves them as two

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

A plant cell also has a cell wall, chloroplasts and a large vacuole

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) 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 real virus, magnified hugely. The rounded head is the protein capsid wrapped around the nucleic acid core; the tail injects that nucleic acid into a bacterium. Note the scale bar — the whole virus is about 100 nm, far smaller than any cell

A generalised virus: nucleic acid inside a protein capsid, with a lipid envelope 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".

• Magnification $=$ image size $\div$ actual size — convert units first ($\text{mm} \leftrightarrow \mu\text{m} \leftrightarrow \text{nm}$), then rearrange for whichever is unknown.
• Distinguish magnification (how many times larger) from resolution (smallest distance still seen as two points); electron microscopes resolve more because electrons have a shorter wavelength.
• Give each organelle a structure + function pair (e.g. mitochondrion: folded inner membrane → aerobic respiration).
• State the prokaryote vs eukaryote differences exactly: no nucleus, smaller (70S) ribosomes, no membrane-bound organelles, circular DNA.
• Viruses are non-living — describe them only by capsid, genetic material and (sometimes) an envelope.