Respiration

3. Anaerobic respiration

Anaerobic respiration releases energy from glucose without using oxygen. It is an incomplete breakdown of glucose, so it produces much less ATP per glucose molecule than aerobic respiration. The waste products are also different from the aerobic ones, and they vary between organisms.

Anaerobic respiration in animals (including human muscle)

In a hard sprint, your leg muscles need so much energy that the blood cannot deliver oxygen fast enough to keep up with aerobic respiration. The muscle cells switch to anaerobic respiration to top up their ATP supply.

Word equation for anaerobic respiration in animals:

glucose → lactic acid (+ a little energy)

Notice that no oxygen is needed, and the waste product is lactic acid, not carbon dioxide. Glucose is not fully broken down, so most of the energy that was in the glucose ends up locked inside the lactic acid molecules.

Two consequences for the body:

• Lactic acid builds up in the muscle, lowering its pH. Acidic conditions slow down the muscle's enzymes and cause the familiar burning and aching feeling.
• After the exercise, the body must break down the lactic acid using oxygen, in the liver. This takes time and is why you continue to breathe heavily for several minutes after stopping. Your body is still supplying the extra oxygen needed to clear the lactic acid. This extra oxygen demand is called the oxygen debt (or "excess post-exercise oxygen consumption").

Anaerobic respiration in yeast and plants (fermentation)

In yeast cells and in some plant cells, anaerobic respiration produces different waste products:

glucose → ethanol + carbon dioxide (+ a little energy)

This special case of anaerobic respiration in yeast is called fermentation and is industrially important:

• Brewing: yeast ferments the sugars in fruit or grain into ethanol (alcohol), giving beer, wine and spirits.
• Baking: yeast ferments sugars in bread dough. The carbon dioxide produced makes the dough rise; the small amount of ethanol evaporates during baking.
• Biofuels: ethanol from fermented crops is used as a renewable fuel additive.

Side-by-side comparison

4. Core practical: investigating respiration

Showing that respiration produces carbon dioxide

Principle: living cells release carbon dioxide during respiration. Limewater (calcium hydroxide solution) turns cloudy white when carbon dioxide is bubbled through it, so it can be used to detect the CO₂ given off by a respiring organism.

Apparatus: three sets of boiling tubes, each containing a delivery tube that bubbles into a second boiling tube of limewater. The three test tubes hold:

• Tube A: 10 germinating peas on damp cotton wool (the test)
• Tube B: 10 boiled (dead) peas on damp cotton wool (control for whether the organism is alive)
• Tube C: 10 glass beads on damp cotton wool (control for whether the seed material itself matters)

Method:

1. Set up the three identical test tubes as above, each sealed with a bung connected by a delivery tube to a fresh tube of limewater. Make sure each delivery tube dips below the limewater so any gas bubbles through it.
2. Leave the whole apparatus in a warm place (25–30 °C) for several hours or overnight.
3. Check each limewater tube for cloudiness.

Expected results:

The boiled-seed control rules out the possibility that the seed material (rather than living respiration) is releasing CO₂. The glass-bead control rules out the apparatus or air itself being responsible.

Showing that respiration releases heat

Principle: aerobic respiration releases energy, and not all of that energy is captured as ATP. A noticeable fraction is released as heat, which can be detected with a thermometer in a well-insulated container.

Apparatus: two vacuum flasks, each containing a thermometer and damp cotton wool. The flasks are inverted (mouth-down) so warm air does not simply float out. One flask holds germinating seeds, the other holds boiled (dead) seeds as a control.

Both the seeds and the flasks are first sterilised with weak bleach to kill any microbes. This step is crucial: without it, microbes growing on the seeds will respire themselves and produce heat, giving misleading results.

Method:

1. Sterilise both flasks and both sets of seeds with weak bleach.
2. Place boiled seeds in flask A and germinating seeds in flask B.
3. Insert a thermometer into each flask with the bulb buried in the seeds.
4. Plug the mouth of each flask with cotton wool (holds the thermometer in place and reduces heat loss).
5. Invert the flasks and leave them in a place with constant room temperature for 2–3 days.
6. Record the temperature inside each flask at the start and at the end.

Expected results: the temperature in the flask with germinating seeds rises by several degrees (e.g. 20 °C → 25 °C). The temperature in the flask with dead seeds stays the same as the room.

5. Gas exchange: getting oxygen to the cells

Gas exchange and respiration are easy to confuse. To keep them separate:

• Respiration is the chemical reaction inside cells that uses oxygen and produces CO₂ (covered above).
• Gas exchange is the physical movement of oxygen into the body and CO₂ out of the body, by diffusion across an exchange surface.

Why multicellular organisms need a gas exchange surface

A single-celled organism like Amoeba has a huge surface-area-to-volume ratio. Oxygen from the water can simply diffuse through its cell membrane straight into the cytoplasm fast enough to keep every part of the cell supplied. No special apparatus needed.

For a multicellular animal like a human, the inner cells of the body are far too deep for diffusion alone to supply them with oxygen at a useful rate. The body solves this by having:

• A specialised gas exchange surface (the lungs) with a huge surface area and very thin walls
• A transport system (the blood) to carry oxygen from the exchange surface to every cell

Features of a good gas exchange surface

Any good gas exchange surface, in any animal, has the same four adaptations:

1. Large surface area so a lot of oxygen can cross at once
2. Thin walls (one cell thick where possible) so the diffusion distance is short
3. Steep concentration gradient maintained by good ventilation on the outside and good blood supply on the inside
4. Moist surface, because the gases must dissolve before they can diffuse through the cells

6. The human respiratory system

The human gas-exchange organ is the lungs, housed inside the thorax (the chest cavity above the diaphragm). The thorax contains:

Order of air flow during inhalation:

mouth/nose → trachea → bronchus → bronchioles → alveoli

7. Breathing: how air gets in and out

Muscles can only pull, never push. So breathing relies on two sets of muscles working as antagonistic pairs, plus the diaphragm, to change the volume of the thorax. Because gas pressure depends on volume, changing the volume forces air to flow in or out.

Inhalation (breathing in)

1. The external intercostal muscles contract, pulling the rib cage up and out.
2. The diaphragm contracts, flattening from its dome shape downwards.
3. Both of these increase the volume of the thorax.
4. The pressure inside the lungs falls below atmospheric pressure.
5. Air rushes in from the outside, down the trachea, through the bronchi and bronchioles, until it fills the alveoli.

Normal exhalation (breathing out)

1. The external intercostal muscles relax, so the rib cage drops down and inwards under its own weight.
2. The diaphragm relaxes, returning to its dome shape pushing up into the thorax.
3. The volume of the thorax decreases.
4. The pressure inside the lungs rises above atmospheric pressure.
5. Air is pushed out of the lungs along the same path in reverse.

Normal exhalation is passive, relying on the elastic recoil of the lungs and rib cage. The internal intercostal muscles do not contract during normal breathing.

Forced exhalation (during exercise)

During hard exercise, the body needs to clear CO₂ much faster. The internal intercostal muscles contract, actively pulling the rib cage further down and in. This reduces the thorax volume even more, forcing air out faster. The two intercostal muscle layers are working as antagonistic pairs: when one contracts the other relaxes.

8. The alveoli: where gas exchange happens

Each lung contains roughly 300 million alveoli, the tiny air sacs at the very end of each bronchiole. The alveoli are where oxygen actually crosses from the air into the blood, and where CO₂ goes the other way.

How the alveoli are adapted for gas exchange

Each adaptation matches one of the four general features of a good exchange surface:

• Large surface area: there are hundreds of millions of alveoli per lung. If you flattened all the alveolar walls of one human's lungs into a single sheet, the area would be around 70 m², about half a tennis court.
• Thin walls: each alveolus is just one cell thick, and so are the capillaries wrapped around it. The total diffusion distance from air to blood is therefore only about 0.5 μm.
• Steep concentration gradient: maintained by two things working together. The lungs are constantly ventilated with fresh air (rich in oxygen, low in CO₂), and the capillaries are constantly carrying blood away (taking the absorbed oxygen with it, replacing it with deoxygenated blood).
• Moist surface: each alveolus is lined with a thin film of fluid. Oxygen and CO₂ both dissolve in this fluid before crossing the cell wall.

Direction of diffusion

• Oxygen moves into the blood, from a high concentration in the alveolar air to a lower concentration in the deoxygenated blood arriving in the capillaries.
• Carbon dioxide moves out of the blood, from a high concentration in the arriving blood to a lower concentration in the alveolar air, then is breathed out.

9. Exercise and breathing

During exercise the muscles use far more ATP than at rest, so they need far more glucose and oxygen and produce far more CO₂. The body responds in several ways:

• Breathing rate increases, with more breaths per minute drawing in fresh oxygen faster.
• Depth of breathing increases, so each breath moves a larger volume of air (the "tidal volume" rises).
• Heart rate increases to pump the oxygen-rich blood to the muscles faster.

The combined effect is that more oxygen reaches the muscles per minute and more CO₂ is cleared per minute.

Oxygen debt

If the exercise is so intense that even maximal breathing cannot keep up with the muscles' oxygen demand, the muscles switch to anaerobic respiration and produce lactic acid. After the exercise is over, the body needs extra oxygen to break the lactic acid back down. This is the oxygen debt (sometimes called excess post-exercise oxygen consumption, EPOC).

You "pay back" the oxygen debt by continuing to breathe hard for several minutes after stopping, even though the muscles are no longer working. The extra oxygen is being used by your liver to convert lactic acid back into harmless products (glucose, CO₂ and water).

Investigating the effect of exercise on breathing rate

A simple practical to demonstrate this:

1. Have each volunteer sit quietly for 1 minute, then count their number of breaths per minute at rest.
2. The volunteer then performs a controlled exercise (e.g. running on the spot, or step-ups at a set pace) for 1 minute.
3. Immediately after exercise, count the breaths per minute again.
4. Calculate the change in breathing rate: change = breathing rate after − breathing rate before.
5. Repeat with several volunteers and take a mean to reduce variation.

Expected result: breathing rate is markedly higher after exercise. The increase ranges from a few breaths per minute (for light exercise) to more than double the resting rate (for vigorous exercise).

Limitations:

• Different people have different fitness levels. Control this by using a similar age and fitness range, or report results for each person separately.
• Exercise intensity is hard to standardise. Use a fixed exercise (step-ups at a metronome pace) rather than free running.
• Counting breaths immediately after exercise is fiddly. Count for 15 seconds then multiply by 4 to catch the result before the rate falls back.

10. The effects of smoking on the gas exchange system

Cigarette smoke contains several chemicals that damage the gas exchange system and the rest of the body.

Three of the key chemicals

Three smoking-related lung diseases

Chronic bronchitis

• Tar inflames the lining of the airways. In response, the goblet cells and mucus glands grow larger and pour out far more mucus than they would in a healthy lung.
• The mucus builds up in the smallest bronchioles, blocking them and creating an ideal place for bacteria to grow. Frequent chest infections follow.
• Tar also damages the cilia on the airway lining, so the natural mechanism that sweeps mucus up out of the lungs no longer works.
• The smoker has to cough to clear the mucus, which is the familiar "smoker's cough".

Emphysema

• Repeated infections attract phagocytes (a type of white blood cell) into the lungs. The phagocytes release an enzyme called elastase, which destroys the elastic fibres that normally hold the alveolar walls together.
• Without elastic fibres, the alveoli cannot recoil properly. Many alveoli burst, merging into larger air spaces.
• The total surface area for gas exchange falls, sometimes to a tenth of the normal value.
• The patient becomes breathless and wheezy with even gentle activity, and in late stages may need a constant oxygen supply.

Lung cancer

• Many of the chemicals in tar are carcinogens that can damage the DNA of cells lining the airways.
• A damaged cell can become a cancer cell that divides uncontrollably, forming a tumour.
• Smoking is the single biggest cause of lung cancer; the risk falls steeply if the smoker quits.

Coronary heart disease

Smoking is also a major risk factor for coronary heart disease. Nicotine raises blood pressure, carbon monoxide reduces oxygen delivery, and both put strain on the coronary arteries that supply the heart muscle. Fatty deposits called atheromas are more likely to build up inside these arteries, eventually leading to a heart attack if a coronary artery becomes blocked.