Respiration

Respiration is the set of chemical reactions in cells that transfer energy from glucose so the organism can use it for life processes.

Three key facts the AQA spec wants you to memorise:

1. Continuous — happens in every living cell 24/7, even when you are asleep.
2. All organisms — animals, plants, fungi, bacteria. Not just animals!
3. Exothermic — energy is transferred FROM the chemicals TO the surroundings (and to other useful processes in the cell). Note: this is the OPPOSITE of photosynthesis, which is endothermic.

What the organism uses the energy for (AQA list):

• Building larger molecules from smaller ones (e.g. amino acids → proteins; glucose → starch/cellulose; glycerol + fatty acids → lipids).
• Movement — muscle contraction in animals (and beating cilia in some single-celled organisms).
• Keeping warm — mammals and birds maintain a constant body temperature using heat released from respiration.
• Active transport — moving substances against concentration gradients (e.g. mineral ion uptake in root hair cells, glucose absorption in the small intestine).

Where it happens. Most respiration takes place in the mitochondria of cells — the powerhouses. Cells that need lots of energy (muscle, liver, sperm) contain many mitochondria; cells with low energy needs have fewer.

Aerobic means 'with oxygen'. This is the most efficient form of respiration and is used whenever enough oxygen is available — which is most of the time.

Word equation:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$

Symbol equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$

Notice this is the exact reverse of photosynthesis. That's not a coincidence — photosynthesis stores energy in glucose; respiration releases it again.

Where it happens. In the mitochondria of eukaryotic cells (animals, plants, fungi).

Why it releases so much energy. Glucose is FULLY oxidised — every carbon ends up as CO₂, every hydrogen as part of water. That complete oxidation extracts almost all the energy stored in glucose's bonds (around 16 kJ per gram of glucose).

Where does each input come from?

• Glucose — from food digestion (animals) or photosynthesis (plants).
• Oxygen — from the air. In humans, breathed into lungs and carried by haemoglobin in red blood cells (4.2.2 The heart and blood vessels).

Where do the products go?

• Carbon dioxide — diffused out (exhaled by animals; out through stomata in plants).
• Water — used by the cell, sweated/breathed out, or excreted in urine.

When oxygen supply can't keep up with demand (e.g. sprinting, lifting heavy weights), muscle cells switch to anaerobic respiration so they can keep producing energy.

Word equation in animals:

$\text{glucose} \rightarrow \text{lactic acid}$

Key points:

• No oxygen used.
• Glucose is only partially oxidised — only a small fraction of the stored energy is released.
• The product is lactic acid, which builds up in muscles and contributes to muscle fatigue (and the burning feeling during hard exercise).
• After exercise, the body has an oxygen debt — extra oxygen must be breathed in to convert the lactic acid back to glucose and CO₂/H₂O (covered in spec 4.4.2.2).

Why is less energy released? Because glucose isn't broken all the way down — most of its energy stays trapped in the lactic acid molecule. Aerobic respiration releases roughly 15-19× more energy per glucose than anaerobic respiration in animals.

Why bother, then? Speed. Anaerobic respiration releases energy QUICKLY without waiting for oxygen delivery. A 100 m sprinter relies on it for most of the race; a marathon runner uses mostly aerobic respiration.

In plant cells (e.g. roots in waterlogged soil) and especially in yeast (a microscopic fungus), anaerobic respiration follows a different pathway:

Word equation in plants/yeast:

$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$

When carried out by yeast for human use, this process has a special name: fermentation.

Why is fermentation economically important?

• Bread-making. Yeast added to dough ferments the sugars and releases CO₂ bubbles, which expand and make the dough rise. The ethanol evaporates during baking.
• Brewing beer and cider, and making wine. Yeast ferments sugars in malted barley (beer) or grape juice (wine) to make ethanol — the alcohol in alcoholic drinks. The CO₂ either escapes (still wine) or is kept dissolved under pressure (sparkling wine, beer fizz).
• Biofuel production. Yeast fermenting sugar cane gives bioethanol, used as a petrol additive in some countries.

Energy-wise, fermentation also releases LESS energy than aerobic respiration — glucose is again only partially oxidised.

When you start exercising, your skeletal muscles contract more frequently and more forcefully. Every muscle contraction needs energy — and that energy is supplied by respiration.

The faster the muscles work, the faster they respire, so they need:

• More glucose — for the fuel.
• More oxygen — for aerobic respiration (efficient energy release).
• Faster removal of waste CO₂ — otherwise it builds up in blood and lowers pH.
• Faster removal of heat — respiration is exothermic, and your body needs to stop overheating.

The circulatory and respiratory systems work together to meet this demand. The body's three main responses are: increased heart rate, increased breathing rate, and increased breath volume (tidal volume — how much air per breath). These changes happen quickly — within seconds of starting to exercise — and are controlled mostly by the medulla in the brain detecting changing CO₂ levels in the blood.

1. Heart rate increases. The heart beats more times per minute, so blood circulates faster around the body. Each pump now delivers oxygen-rich blood and glucose to muscle cells more quickly, and brings CO₂-rich blood back to the lungs faster for removal. Resting heart rate is roughly 60–80 bpm; during vigorous exercise it can reach 150–200 bpm.

2. Breathing rate increases. The number of breaths per minute rises from about 12–18 at rest to 40+ during hard exercise. Each breath happens faster.

3. Breath volume increases. Each breath is also DEEPER (sometimes called increased 'tidal volume'). At rest, you might breathe in about 0.5 L per breath; during exercise that can rise to 2–3 L per breath.

The combined effect: total air moved into the lungs per minute (ventilation rate = breathing rate × tidal volume) can increase from around 6 L/min at rest to over 100 L/min in athletes. This brings far more oxygen into the bloodstream and expels far more CO₂.

Even with heart rate at 180 bpm and ventilation through the roof, the cardiovascular system has limits. During short bursts of very high-intensity exercise — sprinting, lifting weights, doing rapid press-ups — oxygen delivery cannot match what the muscles need.

So muscles supplement aerobic respiration with anaerobic respiration:

$\text{glucose} \rightarrow \text{lactic acid}$

Two important consequences:

1. Less energy per glucose. Anaerobic respiration is incomplete oxidation, so each glucose molecule releases far less energy than aerobic respiration would. That's why you can't sprint for an hour — your glucose reserves drain quickly.

2. Lactic acid builds up. Lactic acid (a mild acid) accumulates in muscle tissue. It lowers the pH locally, interferes with enzyme action, and contributes to muscle fatigue — the heavy, burning sensation that forces you to slow down or stop. AQA exam answers should describe muscle fatigue as a consequence of lactic acid build-up.

Why doesn't the body just always go anaerobic? Because aerobic respiration is roughly 15–19× more efficient per molecule of glucose. Anaerobic is a short-term emergency mode — fast but wasteful.

When you stop exercising, you don't immediately stop panting. You carry on breathing fast and deep for several minutes. This is the body repaying the oxygen debt.

Definition (learn this for the exam): Oxygen debt is the extra amount of oxygen the body must take in after exercise to react with the accumulated lactic acid and remove it from the cells.

What happens to the lactic acid?

• Blood transports lactic acid from muscle cells to the liver.
• In the liver, lactic acid reacts with oxygen to be converted back into glucose (which can be reused or stored as glycogen) or oxidised completely to CO₂ + water.
• The longer or harder you exercised, the more lactic acid built up, the bigger the oxygen debt, and the longer it takes for breathing to return to normal.

Graph clue. On a heart-rate vs time graph, the heart rate doesn't drop straight back to resting after exercise — it falls gradually over a few minutes. That gradual fall corresponds to oxygen-debt repayment.

Fitness factor. Fitter people:

• Have lower resting heart rates (their stroke volume is bigger, so fewer beats are needed).
• Reach a lower peak heart rate for the same effort.
• Return to resting heart rate FASTER after exercise (shorter recovery time). This is why 'recovery time' is used as a measure of cardiovascular fitness in PE.

Metabolism is the AQA-specification term for the sum of all the chemical reactions in a cell or in the body. Some of these reactions build big molecules from small ones (anabolic reactions, e.g. making proteins). Others break big molecules down into small ones (catabolic reactions, e.g. digesting starch).

Energy for metabolism comes from respiration. That's why respiration (covered in 4.4.2.1) is called the 'powerhouse' reaction — without the energy it releases from glucose, none of the cell's building and breaking reactions could happen.

Why metabolism matters.

• It builds the molecules cells need to grow (cellulose for cell walls, proteins for enzymes and muscle, lipids for membranes).
• It builds stores for later use (starch in plants, glycogen in animals, lipids in adipose tissue).
• It breaks down excess molecules so they don't poison the body (e.g. excess amino acids → urea).
• It maintains body temperature in mammals and birds — the heat released by metabolic reactions keeps us warm.

Speed of metabolism (metabolic rate) varies between people and species. It depends on body size, age, sex, activity level and genetics. A higher metabolic rate uses more energy and therefore needs more food.

Glucose is a small, soluble sugar that can't be stored as it is — it would mess with osmosis. So organisms convert it into other forms.

In plants, glucose is converted to:

• Starch — large insoluble storage molecule, kept in seeds, tubers (e.g. potatoes), roots.
• Cellulose — the tough, fibrous polymer that builds plant cell walls (gives plants structural strength).
• Sucrose — soluble sugar transported around the plant in the phloem.

In animals (and fungi), glucose is converted to:

• Glycogen — storage carbohydrate, kept in liver and muscle cells. When blood glucose drops, glycogen is broken back down to glucose for respiration.

All these conversions are anabolic — they use energy from respiration to bond small glucose units together into larger polymers.

Lipid (fat/oil) synthesis. Glucose can also be used to make lipids. A lipid molecule (a triglyceride) is built from:

• 1 glycerol molecule
• 3 fatty acid molecules

These join in a single anabolic reaction (energy from respiration powers it). Lipids form energy stores (in seeds, in animal adipose tissue) and structural components like cell membranes.

Building proteins. Proteins are built from amino acids. Amino acids contain nitrogen, which comes (in plants) from nitrate ions (NO₃⁻) absorbed by roots, and (in animals) from eating plant or animal protein.

The AQA-specification sequence is:

$\text{glucose} + \text{nitrate ions} \rightarrow \text{amino acids} \rightarrow \text{proteins}$

Glucose supplies the carbon-hydrogen-oxygen backbone; nitrate supplies the nitrogen. The amino acids are then joined together in long chains to form proteins (enzymes, antibodies, structural proteins like collagen, muscle protein, etc.).

Breaking down excess proteins. Unlike carbohydrates and lipids, proteins cannot be stored in the body. If you eat more protein than you need, the excess amino acids must be broken down. This happens in the liver:

1. The amino group (–NH₂) is removed from the amino acid (a process called deamination).
2. The amino group is converted to ammonia, which is toxic.
3. Ammonia is quickly converted to a less toxic substance: urea.
4. Urea passes from the liver into the blood.
5. The blood carries urea to the kidneys.
6. The kidneys filter urea out of the blood and excrete it in urine.

This is why the kidneys form part of the excretory system — urea is a waste product of protein metabolism that has to be removed.

Bringing it all together:

Metabolic process	Builds / breaks	Energy from
Glucose → starch / glycogen / cellulose	Builds	Respiration
Glycerol + 3 fatty acids → lipid	Builds	Respiration
Glucose + nitrate → amino acids → proteins	Builds	Respiration
Excess protein → amino acids → urea	Breaks down	Respiration
Glucose → CO₂ + H₂O (respiration itself)	Breaks down	— (releases energy)

Key insight: respiration releases the energy, and that energy then drives the OTHER metabolic reactions. It's why every living cell respires non-stop — without respiration, every other reaction in the body grinds to a halt.

Why is body temperature linked to metabolism? Metabolic reactions release heat as a by-product (because respiration is exothermic). Birds and mammals are 'endothermic' — they use this heat to keep their body temperature roughly constant, no matter the outside temperature. That's why active animals with high metabolic rates (small mammals, hummingbirds) need to eat almost continuously.