能量与呼吸作用

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

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

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

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

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

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

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

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

Worked example. Find the RQ for the aerobic respiration of glucose: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$.

This is why an RQ near $1.0$ suggests the organism is respiring carbohydrate; a lower RQ (about $0.7$) suggests it is using lipid, which needs more oxygen per molecule of $\text{CO}_2$.

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

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

Glycolysis

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

The link reaction

When oxygen is available, pyruvate enters the mitochondria. There each pyruvate loses a carbon dioxide 二氧化碳 and is turned into a 2-carbon acetyl 乙酰基 group. This group is carried by coenzyme A 辅酶A to form acetyl coenzyme A. Some carbon dioxide is released and NAD is reduced. Coenzyme A only carries the acetyl group: it hands it to the Krebs cycle and is then free to collect another one.

The Krebs cycle

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

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

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

Each turn of the cycle releases 2 $\text{CO}_2$, 3 reduced NAD, 1 reduced FAD and 1 ATP. One glucose gives two pyruvates, so the link reaction and the cycle each run twice per glucose. Notice how little ATP the cycle makes directly. Its main job is to load NAD and FAD with hydrogen; that hydrogen is what makes most of the ATP in the next stage.

Oxidative phosphorylation

This stage makes most of the ATP:

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

Common mistakes.

• Saying oxygen is needed to make carbon dioxide. The $\text{CO}_2$ comes from decarboxylation in the link reaction and the Krebs cycle. Oxygen is used only at the end of the chain, where it accepts electrons and forms water.
• Saying ATP synthase pumps protons. The carriers pump protons out of the matrix using the electrons' energy. Protons then flow back through ATP synthase by facilitated diffusion, down their concentration gradient.
• Saying the Krebs cycle makes "lots of ATP". It makes only one ATP per turn; most ATP is made by oxidative phosphorylation.
• Forgetting why no oxygen stops everything. Without oxygen to accept electrons, the chain backs up, so reduced NAD cannot be turned back into NAD, and the link reaction and Krebs cycle stop too.

The structure of mitochondria

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

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

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

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

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

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

• State where each stage happens and its ATP yield: glycolysis (cytoplasm), link reaction + Krebs (matrix), oxidative phosphorylation (inner membrane, most ATP).
• Explain the roles of NAD/FAD (carry hydrogen to the electron transport chain) and of oxygen (the final electron acceptor).
• RQ $=$ CO2 produced $\div$ O2 used ($\approx 1.0$ carbohydrate, $0.7$ lipid); know how a respirometer measures it.
• Anaerobic respiration gives lactate (animals) or ethanol + CO2 (yeast/plants) and far less ATP (glycolysis only).