A-Level Biology · ⁨A-Level 生物⁩

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.

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.

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.

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 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.

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.

Worked example. A solution gives a negative Benedict's test. It is then boiled with dilute hydrochloric acid, neutralised with sodium hydrogencarbonate, and re-tested with Benedict's - now it turns brick-red. What was present, and why is each step needed? The first negative result rules out a reducing sugar. Boiling with acid hydrolyses the glycosidic bond, splitting a non-reducing sugar such as sucrose into its reducing monosaccharides. The neutralising step is essential because Benedict's only works in alkaline conditions - skip it and the test fails even when sugar is present. The positive re-test therefore shows a non-reducing sugar was there all along. Quote the first, negative test as part of the answer: without it, the final red colour cannot tell a non-reducing sugar from a reducing one.

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.

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.

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.

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.

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.

Four levels of protein 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.

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.

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.

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.

Worked example. In the first $20$ seconds of a catalase reaction, $16\ \text{cm}^3$ of oxygen is collected. Estimate the rate of reaction.

Because the reaction is fastest at the start, measuring over this short early interval gives a value close to the initial rate; averaging over a longer time would include the slower later stages and underestimate it.

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.

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.

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.

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

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.

• Explain enzyme action with the induced-fit model (the active site moulds around the substrate) — now preferred to lock-and-key.
• Above the optimum the enzyme denatures (hydrogen bonds and tertiary structure break) — write "denatures", never "dies" or "is killed".
• Distinguish inhibitors: competitive binds the active site (overcome by more substrate, same $V_{max}$); non-competitive binds elsewhere (lower $V_{max}$).
• Compare rates using the initial rate (tangent at time zero) — the fairest measure, before substrate becomes limiting.

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.

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

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.

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.

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.

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.

Worked example. Compare the surface area : volume ratio of a cube-shaped cell of side $4$ with one of side $10$.

For side $4$: surface area $= 6 \times 4^2 = 96$ and volume $= 4^3 = 64$, so the ratio is $96 : 64 = 1.5 : 1$. For side $10$: surface area $= 6 \times 10^2 = 600$ and volume $= 10^3 = 1000$, so the ratio is $600 : 1000 = 0.6 : 1$. The larger cell has the smaller ratio, so it exchanges materials across its surface more slowly for its size — which is why large organisms need specialised exchange surfaces such as lungs and gills.

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.

• Describe the membrane as a fluid mosaic: a phospholipid bilayer with proteins, cholesterol and glycoproteins.
• Sort each process: diffusion, facilitated diffusion and osmosis are passive (down a gradient); active transport and endo/exocytosis need ATP and can go against it.
• For osmosis always use water potential ($\Psi$): water moves from high (less negative) to low (more negative) $\Psi$; pure water is $0$, the highest.
• State the outcome by cell type: plant cell turgid/plasmolysed; animal cell lyses/crenates — link it to the water-potential gradient.

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.

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.

The mitotic index

The mitotic index is the fraction of cells in a sample that are in mitosis. You find it by counting cells under a microscope:

Worked example. In a root-tip sample, $8$ of the $50$ cells seen are in mitosis. Find the mitotic index.

A high mitotic index means many cells are dividing quickly — normal in a growing root tip, but in adult animal tissue it can be a warning sign of uncontrolled growth.

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.

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.

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.

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:

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.

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.

Worked example. A DNA template strand reads TAC GGA CTT. Give the mRNA codons, and say how long the polypeptide is. Transcribe by complementary base pairing, remembering that RNA uses uracil in place of thymine: TAC gives AUG, GGA gives CCU, CTT gives GAA. So the mRNA reads AUG CCU GAA. AUG is the start codon (methionine), so the polypeptide is three amino acids long - two once the start methionine is removed. Two errors cost most of the marks here: pairing A with T instead of U when writing mRNA, and transcribing the coding strand instead of the template strand. The mRNA is complementary to the template, and identical to the coding strand apart from U replacing T.

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.

• DNA is antiparallel, held by hydrogen bonds (A=T two, C≡G three); replication is semi-conservative (each product keeps one old strand).
• State where and what: transcription (nucleus → mRNA), translation (ribosome, mRNA + tRNA → polypeptide).
• Describe the code with all four terms: triplet, non-overlapping, degenerate, universal.
• For a gene mutation (substitution, insertion, deletion) explain the effect on the protein — and why a substitution can be silent (degeneracy).

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.

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.

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

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.

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.

Worked example. A plant cell with a water potential of $-800\ \text{kPa}$ is placed in a solution of water potential $-400\ \text{kPa}$. Which way does water move, and what happens to the cell? Water always moves down a water potential gradient - from the less negative (higher) value to the more negative (lower) one. The solution at $-400$ is higher than the cell at $-800$, so water moves into the cell. The cell swells, the protoplast presses on the cell wall, the pressure potential rises, and the cell becomes turgid. Two traps: $-400$ is greater than $-800$, which is the sign error that reverses half of all answers; and pure water is the maximum at 0, so every solution is negative and a water potential can never rise above zero.