Respiration, Glycolysis and Fermentation: Quick Revision

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

Respiration in plants

• Cellular respiration: stepwise breakdown of food in the cell, with the energy trapped in ATP. The usual substrate is glucose.
• Gas exchange: stomata in leaves, lenticels in woody stems, the surface of roots. Each part looks after itself, the distance is short and the need is low.
• One mole of glucose holds 2870 kJ. One mole of ATP holds about 30.5 kJ.
• Stages: glycolysis in the cytoplasm. Then fermentation without oxygen, or aerobic respiration in the mitochondria.

Glycolysis

• Ten reactions, in the cytoplasm of every living cell. Glucose gives 2 pyruvic acid. Also called the EMP pathway.
• Invertase splits sucrose into glucose and fructose. Hexokinase adds a phosphate to each, with the phosphate from ATP.
• ATP used: glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose-1,6-bisphosphate.
• Fructose-1,6-bisphosphate splits into DHAP and PGAL. DHAP changes to PGAL, so the steps below run twice.
• NADH forms when PGAL becomes BPGA. ATP is made when BPGA becomes PGA and when PEP becomes pyruvic acid.
• Net gain for one glucose: 4 made, 2 used, so 2 ATP, and 2 NADH.

Fermentation

• Alcoholic (yeast): pyruvic acid decarboxylase, then alcohol dehydrogenase. Products: ethanol and carbon dioxide.
• Lactic acid (some bacteria, muscle): lactate dehydrogenase. Product: lactic acid, no carbon dioxide.

• NADH is changed back to NAD$^+$, so glycolysis can go on.
• Net gain of 2 ATP for each glucose. Less than 7 per cent of the energy of glucose is released. Yeast dies at about 13 per cent alcohol.

A worked example

A cell takes in 4 sucrose molecules. What is the net gain of ATP in glycolysis?

1. Invertase gives glucose and fructose, so there are 8 hexose molecules.
2. Each hexose gives a net gain of 2 ATP, so the gain is $8 \times 2 = 16$ ATP.

Practice with new numbers

A program to try

The program works out the net ATP, the NADH and the pyruvic acid from some glucose.

Mistakes to avoid

• Saying that glycolysis makes 4 ATP. The net gain is 2.
• Saying that glycolysis needs oxygen. It does not.
• Saying that lactic acid fermentation gives carbon dioxide. It does not.
• Saying that respiration is breathing. Breathing is gas exchange.

Check yourself

1. At which step is NADH formed in glycolysis?

2. Why must NADH be changed back to NAD$^+$ without oxygen?

3. Name the products of alcoholic fermentation.

4. What is the net gain of ATP in fermentation for each glucose?