Respiration: Quick Revision

A brief note on Respiration. Read it before a test. Try each question at the end before you open its answer.

Respiration in every cell

• Respiration releases the energy of food, mostly glucose, inside every living cell.
• It goes on in plants and animals, by day and at night. Breathing only moves the air.
• The energy is stored as ATP, the energy currency of the cell.

Three ways to break down glucose

• First step, in the cytoplasm: glucose (6 carbon atoms) becomes two pyruvate (3 carbon atoms each).
• No oxygen, in yeast: pyruvate becomes ethanol and carbon dioxide. This is fermentation.
• Lack of oxygen, in our muscles: pyruvate becomes lactic acid, which causes cramps.
• Oxygen present, in the mitochondria: pyruvate becomes carbon dioxide and water.
• Energy: about 2 ATP from one glucose without oxygen, about 38 ATP with oxygen.

Getting oxygen in

• A good surface is large, thin and moist, with a rich blood supply.
• Water holds far less oxygen than air, so fish breathe fast, over their gills.
• Insects use tracheae that open at spiracles. Land vertebrates use lungs.
• Plants use stomata in leaves and lenticels in woody stems, by diffusion.
• By day a leaf takes in carbon dioxide; at night it gives it out. Respiration never stops.

The human respiratory system

• Nostrils, pharynx, larynx, trachea (rings of cartilage), bronchi, bronchioles, alveoli.
• The alveoli are thin, moist sacs with blood capillaries, about $80$ m² in all.
• Breathing in: the ribs lift and the diaphragm flattens, so the chest gets bigger.
• Residual air always stays in the lungs.
• Haemoglobin carries oxygen. Most carbon dioxide travels dissolved in the plasma.

A worked example

During a sprint, a muscle breaks down $9$ glucose molecules. Only $4$ of them meet oxygen. How many ATP are made, and what builds up in the muscle?

1. With oxygen. $4 \times 38 = 152$ ATP.
2. Without oxygen. The other $5$ molecules give $5 \times 2 = 10$ ATP.
3. In all. $152 + 10 = 162$ ATP.
4. What builds up. The $5$ molecules without oxygen leave lactic acid in the muscle.

If all $9$ had met oxygen, the muscle would have made $9 \times 38 = 342$ ATP, more than twice as much.

Practice with new numbers

A program to try

The program finds the oxygen that your body keeps from the air in one day at rest.

Mistakes to avoid

• Respiration and breathing are not the same. Respiration happens inside the cells.
• Muscles do not make alcohol. They make lactic acid; yeast makes ethanol and carbon dioxide.
• Plants respire by day too. In light, photosynthesis only hides it.
• The lungs do not pull air in. The ribs and the diaphragm make the chest bigger.
• Haemoglobin is not the main carrier of carbon dioxide. Most of it is dissolved in the plasma.

Check yourself

1. Name the part of the cell where pyruvate is broken down with oxygen.

2. Why does a runner keep breathing hard for a while after the race ends?

3. Put these in order, starting from the nose: bronchioles, trachea, alveoli, bronchi.

4. Most of the carbon dioxide in your blood does not ride on haemoglobin. How does it travel?

5. Give one way in which the gills of a fish are like the alveoli of your lungs.