IGCSE Biology

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.

A dichotomous key: each step gives two choices until you reach the organism

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.

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.

Living organisms are sorted into five kingdoms by their main features

The fungus, prokaryote and protoctist kingdoms are Supplement only.

A fungus: the fly agaric, a mushroom

A prokaryote (bacterium): E. coli seen under an electron microscope

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):

Monocot leaves have parallel veins; dicot leaves have a net of branching veins

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 is just a protein coat around its genetic material

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.

An animal cell: membrane, cytoplasm, nucleus, mitochondria and ribosomes

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

Onion epidermis cells under a light microscope — each cell has a clear cell wall

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.

A bacterial cell has no nucleus — its DNA is a single loop, plus small plasmids

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.

From a single cell up to the whole organism

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.

A light microscope, used to look at cells

Cover the quantity you want in the triangle to read off its formula

• 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.

• 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.

Diffusion: particles move from higher to lower concentration, down the gradient

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:

Red blood cells: water moves in and out of cells by osmosis.

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.

Osmosis: water crosses the partially permeable membrane; the solute cannot

(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.

Water entering or leaving a plant cell makes it turgid, flaccid or plasmolysed

Plasmolysed red onion cells: the purple contents have pulled away from the cell walls

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.

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.

Active transport uses energy and a carrier protein to move particles against the gradient

(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.

Bread is rich in carbohydrates, one of the main food groups.

Living things are built from a few kinds of molecule 分子. The most important are carbohydrates, fats and proteins. Each is made from a small number of chemical elements 元素.

• Carbohydrates 碳水化合物 and fats 脂肪 contain three elements: carbon 碳 (C), hydrogen 氢 (H) and oxygen (O).
• Proteins 蛋白质 contain carbon, hydrogen and oxygen too, plus nitrogen 氮 (N). (Some proteins also contain sulfur, S.)

Large molecules are made by joining many small molecules together, like beads on a string.

Starch and glycogen are energy stores; cellulose makes plant cell walls. Proteins are built from about 20 different kinds of amino acid joined in a chain. One fat molecule is made from three fatty acids joined to one glycerol.

Large molecules are built by joining many small units, like beads on a string

Starch grains from a potato, seen under a microscope — each grain is built from many glucose units

You can test a piece of food to find out which substances it contains. Add the test chemical and watch for a colour change.

Two things to remember: only the Benedict's test needs heat; and DCPIP loses its colour (goes colourless), while the other tests gain a new colour.

Each food test gives its own colour change for a positive result

DNA is the genetic material 遗传物质 — it carries the instructions for building and running an organism. Its structure has four key points:

• two strands 链 are coiled together to form a double helix 双螺旋 (like a twisted ladder).
• each strand carries chemicals called bases 碱基.
• bonds 化学键 between pairs of bases hold the two strands together.
• the bases always pair in the same way: A with T, and C with G.

DNA is a double helix; the bases always pair A–T and C–G

• Carbohydrates and fats are made of C, H and O. Proteins also contain N (nitrogen).
• Learn each food test and its colour change. For reducing sugars you must heat the Benedict's solution.
• DCPIP goes from blue to colourless; the other four tests each give a new colour.
• In DNA, the bases pair only A–T and C–G. So if you know the bases on one strand, you can work out the other.

Biological washing powders contain enzymes that break down stains.

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.

Kiwi fruit contains a protease enzyme that breaks down protein.

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.

The substrate fits the active site like a key in a lock, then leaves as products

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.

Activity peaks at the optimum temperature, then falls sharply as the enzyme denatures

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.

Each enzyme has one optimum pH; far from it 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.

Green leaves capture sunlight for photosynthesis.

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:

Photosynthesis takes in carbon dioxide and water and makes glucose and oxygen, using light

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.

More light speeds up photosynthesis until another factor becomes limiting

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:

A leaf in cross-section: each tissue is adapted for photosynthesis and gas exchange

Real stomata in the lower surface of a leaf — each pore has two guard cells (scale bar 20 μm)

• 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 includes plenty of fruit and vegetables.

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.

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.

The alimentary canal that food passes through, plus the organs that add digestive juices

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:

The four kinds of teeth: incisors and canines at the front, premolars and molars at the back

A tooth is built from these parts:

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

A tooth in section: hard enamel over dentine, a soft pulp, and a root set in the jaw

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.

Digestive enzymes break large insoluble molecules into small soluble ones

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 villus has a thin wall, a capillary network and a lacteal, for fast absorption

• 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.

A tree moves water up from its roots through xylem vessels.

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.

Xylem carries water up in dead lignified tubes; phloem carries sucrose in living cells

A stem in cross-section: the red-stained xylem sits in vascular bundles near the edge

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.

Water is taken up by root hairs, carried up the xylem, and lost from the leaves

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.

Water evaporates from the mesophyll and diffuses out of the stomata as water vapour

What changes the rate of transpiration

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.

Translocation 转运 is the movement of sucrose and amino acids in the phloem, from sources 源 to sinks 库.

• sources are the parts that release sucrose or amino acids — for example the leaves, which make sugar by photosynthesis.
• sinks are the parts that use or store them — for example growing roots, or a storage organ.

The phloem moves sucrose from a source to a sink; the direction can change

• Xylem: water and mineral ions, upward only, plus support — its cells are dead, with thick lignin walls. Phloem: sucrose and amino acids, in living cells.
• Learn the water pathway: root hair cell → cortex → xylem → mesophyll.
• Transpiration = evaporation from the mesophyll, then diffusion out of the stomata. It is faster when hotter, windier or drier.
• Water rises by the transpiration pull; the water molecules hold together as a column.
• Translocation goes from a source (where sugar is made or stored) to a sink (where it is used). The direction can change with the season.

The circulatory system 循环系统 carries blood around the body. It has three parts: a pump 泵 (the heart 心脏), a network of blood vessels 血管, and valves 瓣膜 that keep the blood flowing one way only.

Single and double circulation (Supplement)

• A fish has a single circulation 单循环: the blood passes through the heart once on each trip around the body.
• A mammal 哺乳动物 has a double circulation 双循环: the blood passes through the heart twice on each full trip — once on the way to the lungs, and once on the way to the rest of the body.
• The advantage of a double circulation: the blood can be pumped again at high pressure 压力 before going to the body, so it travels faster and delivers oxygen 氧气 quickly.

In a double circulation, blood passes through the heart twice on each trip round the body

The heart is made of muscle 肌肉. A wall called the septum 隔膜 divides it into a left side and a right side. On each side there is an upper chamber, the atrium 心房, and a lower chamber, the ventricle 心室. Valves between the chambers stop the blood flowing backwards. The heart muscle is fed with blood by the coronary arteries 冠状动脉.

(Supplement) The wall of the left ventricle is thicker than the right, because it must pump blood all the way around the body; the right side only pumps blood to the nearby lungs. The atria have thin walls, as they only push blood into the ventricles just below them.

The septum keeps oxygenated blood 含氧血 on the left side apart from deoxygenated blood 缺氧血 on the right side, so the two never mix.

(Supplement) The valves between each atrium and ventricle are the atrioventricular valves 房室瓣. The valves at the exit of each ventricle are the semilunar valves 半月瓣.

The heart's four chambers; the left ventricle has the thickest wall, to pump to the whole body

How the heart beats (Supplement)

1. The atria contract 收缩 and push blood down into the ventricles.
2. The ventricles contract and force blood out into the arteries; the valves snap shut so blood cannot flow back.
3. The muscle relaxes and the heart fills with blood again.

Blood is pumped away from the heart in arteries and returns to the heart in veins.

Heart rate and exercise

You can check the heart's activity with an ECG, by feeling the pulse 脉搏 in an artery, or by listening to the valves closing. During exercise the heart rate 心率 goes up, so blood reaches the muscles faster, bringing the extra oxygen and glucose they need.

Coronary heart disease

Coronary heart disease 冠心病 happens when the coronary arteries become narrow or blocked, so the heart muscle cannot get enough oxygen. Risk factors 风险因素 include a diet high in fat, lack of exercise, stress, smoking, family history (genetics), older age, and being male. A healthy diet and regular exercise lower the risk.

There are three kinds of blood vessel. The space inside a vessel is its lumen 管腔.

(Supplement) Arteries have thick elastic walls to cope with the high pressure from the heart. Veins have valves because their blood is at low pressure and could otherwise flow backwards. Capillaries are very thin (one cell thick) and very narrow, giving a large surface area 表面积 and a short distance for fast exchange.

Arteries, veins and capillaries are each built for their job

Main blood vessels

(Supplement) The liver is served by the hepatic artery 肝动脉, the hepatic vein 肝静脉, and the hepatic portal vein 肝门静脉, which brings blood from the gut to the liver.

Blood has four parts:

Blood is red cells, white cells and platelets carried in liquid plasma

Real blood cells under an electron microscope: red cells (discs), white cells and platelets

Defending the body, and clotting

White blood cells fight pathogens 病原体 (the microbes that cause disease) in two ways:

• phagocytes 吞噬细胞 carry out phagocytosis 吞噬作用 — they surround and digest the pathogens.
• lymphocytes 淋巴细胞 make antibodies 抗体, which stick to the pathogens and destroy them.

When you cut yourself, the blood clots to seal the wound. This stops blood loss and stops pathogens getting in. (Supplement) During clotting, a soluble protein called fibrinogen 纤维蛋白原 is changed into threads of fibrin 纤维蛋白. The threads form a net that traps blood cells and makes a solid clot.

• Circulatory system = pump (heart) + vessels + valves for one-way flow.
• Double circulation (mammals): blood passes through the heart twice. The left ventricle wall is thickest because it pumps to the whole body.
• Arteries carry blood away (thick, high pressure); veins carry it back (valves, low pressure); capillaries are for exchange (one cell thick).
• Blood = red cells (oxygen, haemoglobin) + white cells (defence) + platelets (clotting) + plasma (transport).
• Phagocytes engulf pathogens; lymphocytes make antibodies.

A pathogen 病原体 is an organism that causes disease 疾病. A transmissible disease 传染病 is one in which the pathogen can pass from one host 宿主 to another.

A mosquito can spread pathogens from person to person when it bites.

A pathogen is transmitted 传播 in two main ways:

• by direct contact 直接接触 — for example through blood and other body fluids 体液.
• indirectly — for example from contaminated 污染 surfaces, food, animals, or the air.

Your body has several defences 防御 that keep pathogens out or destroy them:

• skin — a barrier that covers and protects the body.
• hairs in the nose — trap dust and pathogens in the air you breathe in.
• mucus 黏液 — sticky liquid in the airways that traps pathogens.
• stomach acid 胃酸 — kills most pathogens in your food.
• white blood cells 白细胞 — find and destroy any pathogens that get inside.

The body keeps most pathogens out; white blood cells deal with any that get in

A white blood cell (green) engulfing round bacteria (purple), a key body defence

Good public health stops disease spreading:

• a clean water supply, so drinking water carries no pathogens.
• hygienic 卫生 food preparation and good personal hygiene, such as washing your hands.
• proper waste disposal and sewage treatment 污水处理, so waste does not contaminate water or food.

Antigens and antibodies

Every pathogen carries antigens 抗原 on its surface, and each kind of antigen has its own special shape.

Antibodies 抗体 are proteins 蛋白质 made by lymphocytes 淋巴细胞 (a kind of white blood cell). An antibody has a shape that is complementary 互补 to one antigen, so it fits only that antigen. When antibodies bind to a pathogen's antigens, they either destroy the pathogen directly or mark it so that phagocytes 吞噬细胞 destroy it.

An antibody's shape is complementary to one antigen, so it fits only that pathogen

Active immunity and vaccination

Active immunity 主动免疫 is protection made by your own body producing antibodies. You gain it after an infection, or after vaccination 疫苗接种.

Vaccination works like this:

1. weakened pathogens, or just their antigens, are put into the body; they cannot make you ill.
2. the antigens make your lymphocytes produce antibodies.
3. the body also makes memory cells 记忆细胞 that stay for years.

If the real pathogen enters later, the memory cells make antibodies very fast, so you do not become ill. This gives long-term protection. If most people in a group are vaccinated, the disease cannot spread easily.

After vaccination, memory cells give a fast, large response if the real pathogen arrives

Passive immunity

Passive immunity 被动免疫 is protection from antibodies made by another body, not your own. For example, a baby receives antibodies from its mother across the placenta 胎盘 and in breast milk 母乳. This is why breast-feeding 母乳喂养 helps protect infants 婴儿. Passive immunity is only short-term, because no memory cells are made.

Cholera 霍乱 is a disease caused by a bacterium 细菌 that spreads in contaminated water. The cholera bacterium makes a toxin 毒素. This toxin causes chloride ions 氯离子 to be pumped into the small intestine 小肠. Water then moves into the intestine by osmosis 渗透. The result is diarrhoea 腹泻, dehydration 脱水 (loss of water) and a loss of ions 离子 from the blood.

Cholera toxin pulls water into the gut by osmosis, causing diarrhoea and dehydration

• Pathogen = organism that causes disease. Transmissible = can pass between hosts, by direct contact or indirectly.
• Learn the five body defences: skin, nose hairs, mucus, stomach acid, white blood cells.
• An antibody's shape is complementary to one antigen (like an enzyme and its substrate).
• Active immunity (from infection or vaccination) makes memory cells → long-term. Passive immunity (placenta, breast milk) makes no memory cells → short-term.
• Cholera: toxin → chloride ions into the gut → water in by osmosis → diarrhoea and dehydration.

Gas exchange happens across the huge surface of the lungs.

Gas exchange 气体交换 is how oxygen 氧气 gets into the blood and carbon dioxide 二氧化碳 gets out, by diffusion 扩散. A good gas exchange surface has four features:

• a large surface area 表面积 — so more gas can cross at once.
• a thin surface — so gases have only a short distance to diffuse.
• a good blood supply — to keep a steep difference in concentration.
• good ventilation 通气 with air — fresh air keeps the difference steep.

The lungs are the gas-exchange organs of the human body.

Air travels in and out through these parts:

The alveoli make excellent gas exchange surfaces: there are millions of them (a huge surface area), each has a wall only one cell thick (a short distance), and each is surrounded by capillaries (a good blood supply).

At an alveolus, oxygen diffuses into the blood and carbon dioxide diffuses out

The lungs sit in the chest, protected by the ribs 肋骨. Below them is a sheet of muscle, the diaphragm 膈肌. Between the ribs are the intercostal muscles 肋间肌. (Supplement) The trachea is held open by rings of cartilage 软骨, so it cannot collapse when you breathe in.

Air travels from the trachea through the bronchi and bronchioles to the alveoli

Breathing changes the volume and pressure 压力 inside the chest (the thorax 胸腔).

Breathing in (inhaling 吸气):

• the external intercostal muscles contract, pulling the ribs up and out.
• the diaphragm contracts and flattens (moves down).
• the thorax becomes bigger, so the pressure inside drops below the outside air pressure.
• air is pushed in.

Breathing out (exhaling 呼气) is the opposite: the muscles relax (and the internal intercostal muscles may contract), the thorax becomes smaller, the pressure rises, and air is pushed out.

Breathing in makes the thorax bigger (lower pressure); breathing out makes it smaller

Expired air has less oxygen and much more carbon dioxide than inspired air

You can test for carbon dioxide with limewater 石灰水, which turns cloudy. Expired air turns limewater cloudy much faster than inspired air, showing it contains more carbon dioxide.

During exercise your muscles respire faster and make more carbon dioxide. This raises the concentration 浓度 of carbon dioxide in the blood. (Supplement) Your brain 大脑 detects the rise and makes you breathe at a faster rate 速率 and a greater depth. This brings in more oxygen and removes the extra carbon dioxide quickly.

The airways are lined with two kinds of cell that trap and remove dirt:

• goblet cells 杯状细胞 make mucus 黏液, which traps pathogens 病原体 and dust particles 粒子.
• ciliated cells 纤毛细胞 have tiny hairs that sweep the mucus, with the trapped dirt, up to the throat, where it is swallowed.

• A gas exchange surface is large, thin, with a good blood supply and good ventilation.
• Air path: larynx → trachea → bronchi → bronchioles → alveoli.
• Breathing in: external intercostal muscles and diaphragm contract → thorax bigger → pressure lower → air in. Breathing out is the opposite.
• Expired air has less oxygen, more carbon dioxide and more water vapour. Limewater tests for carbon dioxide.
• Exercise → more carbon dioxide in the blood → brain → faster, deeper breathing.

Respiration 呼吸作用 is the release of energy 能量 from food, and it happens in every living cell, all the time. (It is not the same as breathing.) Cells use this energy for many jobs:

• muscle 肌肉 contraction (movement).
• protein synthesis 蛋白质合成 (building proteins).
• cell division 分裂 (making new cells).
• active transport 主动运输.
• growth.
• the passage of nerve impulses 神经冲动.
• keeping a constant body temperature 温度.

Cells use the energy from respiration for many different jobs

You can investigate respiration in yeast 酵母: warmer yeast respires faster (up to its best temperature), giving off bubbles of carbon dioxide 二氧化碳.

Aerobic respiration releases energy steadily for sustained exercise.

Aerobic respiration 有氧呼吸 uses oxygen 氧气 to break down nutrient molecules 营养物质 (mainly glucose 葡萄糖) and release energy.

Word equation:

(Supplement) Balanced chemical equation:

Aerobic respiration releases a lot of energy from each glucose molecule.

A hard sprint relies on anaerobic respiration, building up lactic acid.

Anaerobic respiration 无氧呼吸 breaks down glucose to release energy without oxygen. It releases much less energy from each glucose molecule than aerobic respiration, because the glucose is not fully broken down.

Aerobic respiration needs oxygen and gives a lot of energy; anaerobic gives much less

In yeast

The alcohol 酒精 made by yeast is used to make bread rise and to brew drinks.

In muscles (Supplement)

During hard exercise your muscles cannot get enough oxygen, so they respire anaerobically:

The lactic acid 乳酸 builds up in the muscles and the blood. This creates an oxygen debt 氧债 — the extra oxygen the body will need later to break that lactic acid down.

After you stop exercising, the oxygen debt is repaid:

• your heart rate 心率 stays high, carrying the lactic acid in the blood from the muscles to the liver 肝脏.
• you keep breathing deeply and quickly, taking in extra oxygen.
• the liver uses this oxygen to break the lactic acid down by aerobic respiration.

Lactic acid from anaerobic respiration is broken down later, using extra oxygen

• Respiration releases energy in all living cells, all the time — it is not breathing.
• Aerobic: glucose + oxygen → carbon dioxide + water, and it gives a lot of energy. Learn the balanced equation too.
• Anaerobic (no oxygen) gives much less energy. In yeast: glucose → alcohol + carbon dioxide. In muscles: glucose → lactic acid.
• Lactic acid causes an oxygen debt, repaid by a fast heart rate and deep breathing, with the liver breaking the lactic acid down.

Excretion 排泄 is the removal of the waste products of metabolism, and of substances the body has in excess. Two main wastes are removed:

• carbon dioxide 二氧化碳 — made during respiration, and breathed out through the lungs.
• urea 尿素 — made in the liver 肝脏, and removed by the kidneys together with excess water and ions 离子.

The kidneys filter the blood and make urine.

The kidneys 肾脏 clean the blood and make urine 尿液. The urine then passes through:

• the ureters 输尿管 — tubes from the kidneys to the bladder.
• the bladder 膀胱 — stores the urine.
• the urethra 尿道 — carries the urine out of the body.

The kidneys make urine, which flows down the ureters to the bladder and out of the urethra

If the kidneys stop working, a dialysis 透析 machine can do their job. The patient's blood flows through the machine, where waste and excess water pass out across a thin membrane, and the clean blood returns to the body.

A dialysis machine cleans the blood when the kidneys stop working

A kidney has two regions: an outer cortex 皮质 and an inner medulla 髓质.

The nephron

Each kidney holds millions of tiny tubes called nephrons 肾单位. A nephron cleans the blood in two steps:

1. Filtration 过滤: at the glomerulus 肾小球 (a knot of capillaries), high pressure forces water, glucose 葡萄糖, urea and ions out of the blood and into the nephron.
2. Reabsorption 重吸收: as the liquid flows along the nephron, the blood takes back all of the glucose, some of the ions, and most of the water.

What is left behind — urea, excess water and excess ions — becomes the urine.

A nephron filters the blood, then reabsorbs the useful substances; what is left is urine

The liver carries out the assimilation 同化 of amino acids 氨基酸: it joins them together into the proteins 蛋白质 that the body needs.

The body cannot store extra amino acids. The liver breaks the excess down by deamination 脱氨基作用 — it removes the nitrogen 氮-containing part of each amino acid. This part is then turned into urea.

The liver makes urea from excess amino acids; the kidneys excrete it

Urea must be excreted because it is toxic 有毒 (poisonous) if it is allowed to build up in the blood.

• Excretion removes metabolic wastes. Carbon dioxide leaves through the lungs; urea, excess water and excess ions leave through the kidneys.
• Urine path: kidney → ureter → bladder → urethra.
• Nephron: the glomerulus filters the blood; the nephron reabsorbs all the glucose, some ions and most of the water. What is left is urine.
• Urea is made in the liver from excess amino acids by deamination.
• We must excrete urea because it is toxic to the body.

The brain coordinates the body through the nervous system.

Your body must coordinate 协调 all its parts and react to changes. The nervous system 神经系统 does this using fast electrical impulses 电脉冲 that travel along neurones 神经元 (nerve cells). It has two parts:

• the central nervous system 中枢神经系统 (CNS) — the brain 大脑 and the spinal cord 脊髓.
• the peripheral nervous system 周围神经系统 (PNS) — all the nerves outside the brain and spinal cord.

The nervous system coordinates and regulates 调节 the body's functions.

A neurone (nerve cell) carries electrical impulses around the body.

There are three kinds of neurone:

• sensory neurones 感觉神经元 — carry impulses from receptors to the CNS.
• relay neurones 中间神经元 — pass impulses on inside the CNS.
• motor neurones 运动神经元 — carry impulses from the CNS to effectors.

A reflex 反射 action is a fast, automatic response 反应 to a stimulus 刺激 (a change). It follows a fixed path, the reflex arc 反射弧:

receptor 感受器 → sensory neurone → relay neurone → motor neurone → effector 效应器 (a muscle 肌肉 or gland 腺体)

In a reflex arc the impulse passes from receptor → sensory → relay → motor neurone → effector

A synapse 突触 is a junction (a tiny gap) between two neurones. At the end of one neurone are vesicles 囊泡 holding neurotransmitter 神经递质 molecules. Across the synaptic gap 突触间隙, the next neurone has receptor proteins 受体蛋白.

Events at a synapse:

1. an impulse arrives and makes the vesicles release neurotransmitter into the synaptic gap.
2. the neurotransmitter molecules diffuse 扩散 across the gap.
3. they bind to the receptor proteins on the next neurone.
4. this starts a new impulse in the next neurone.

Synapses make impulses travel in one direction only.

At a synapse, neurotransmitter diffuses across the gap, so impulses travel one way only

Sense organs 感觉器官 are groups of receptor cells that respond to one kind of stimulus — light, sound, touch, temperature or chemicals. The eye responds to light.

Parts of the eye

Light is focused by the cornea and lens onto the retina; impulses leave by the optic nerve

The pupil reflex

In bright light the pupil gets smaller, to protect the retina. In dim light it gets wider, to let more light in. (Supplement) This is done by two sets of antagonistic 拮抗 muscles in the iris: the circular muscles 环肌 and the radial muscles 辐射肌. In bright light the circular muscles contract and the pupil narrows; in dim light the radial muscles contract and the pupil widens.

Accommodation (Supplement)

Accommodation 视觉调节 is changing the shape of the lens to focus on near or far objects, using the ciliary muscles 睫状肌 and the suspensory ligaments 悬韧带:

• near object: the ciliary muscles contract, the ligaments slacken, and the lens becomes fatter — more refraction.
• far object: the ciliary muscles relax, the ligaments pull tight, and the lens becomes thinner — less refraction.

Rods and cones (Supplement)

The retina has two kinds of light receptor:

• rods 视杆细胞 — very sensitive, good for vision 视觉 in dim light (night vision); they do not detect colour.
• cones 视锥细胞 — three kinds, each absorbing a different colour, giving colour vision; they work best in bright light.

Cones are packed most densely at the fovea 中央凹, the part of the retina that gives the sharpest image.

A hormone 激素 is a chemical, made by a gland and carried in the blood, that changes the activity of one or more target organs 靶器官. Hormones are made by endocrine glands 内分泌腺, which release them straight into the blood.

Adrenaline is released in 'fight or flight' situations, such as fear or danger. It increases the breathing rate, the heart rate 心率, the blood glucose concentration 浓度 and the pupil diameter, getting the body ready to act.

Homeostasis 稳态 is keeping a constant internal environment 环境 inside the body — for example a steady temperature and a steady blood glucose level.

(Supplement) Homeostasis works by negative feedback 负反馈: when a value moves away from its set point 设定点, the body acts to bring it back. The change itself triggers the correction.

Controlling blood glucose

The liver 肝脏 and the pancreas keep blood glucose steady:

• insulin lowers blood glucose after a meal — the liver stores the glucose as glycogen 糖原.
• (Supplement) glucagon raises blood glucose between meals — the liver releases glucose.

Insulin lowers blood glucose and glucagon raises it, keeping it near a normal level

In Type 1 diabetes 糖尿病 the pancreas cannot make insulin, so blood glucose rises too high. It is treated with insulin injections and a carefully planned diet.

Controlling body temperature (Supplement)

The brain detects the body temperature 温度 and keeps it steady. The skin helps:

When you are too hot: you sweat, and the arterioles 小动脉 widen (vasodilation 血管舒张) so more blood flows to the skin capillaries 毛细血管 and loses heat.

When you are too cold: you shiver 颤抖 (the muscles make heat), and the arterioles narrow (vasoconstriction 血管收缩) so less blood reaches the surface, keeping heat in.

A tropism 向性 is a plant's growth response to a direction:

• gravitropism 向地性 — growth in response to gravity 重力 (roots grow down, shoots grow up).
• phototropism 向光性 — growth in response to the direction of light (shoots grow towards the light).

(Supplement) These responses are controlled by a chemical called auxin 生长素:

• auxin is made in the shoot 嫩枝 tip.
• it diffuses down through the plant.
• light or gravity makes the auxin spread unequally to one side.
• auxin makes cells elongate 伸长 (grow longer). The side with more auxin grows faster, so the shoot bends.

Auxin gathers on the shaded side and makes those cells grow longer, bending the shoot to the light