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Bioenergetics: Class 11 Biology
ATP as the energy currency, photosynthesis with its light and dark reactions and limiting factors, and aerobic and anaerobic respiration including glycolysis, Krebs cycle, electron transport and fermentation.
Key points
Bioenergetics is the study of how living things obtain, change and use energy. The main energy source for life is sunlight, which plants change into chemical energy in food by photosynthesis. All organisms release this energy by respiration and store it in ATP.
ATP (adenosine triphosphate) is made of adenine, ribose sugar and three phosphate groups. When the last phosphate bond is broken by hydrolysis, ATP becomes ADP plus inorganic phosphate and releases about 30.6 kJ per mole. ATP is made again from ADP and phosphate using energy from respiration or light. It is called the energy currency because cells use it for active transport, muscle contraction and making molecules.
Photosynthesis is the process in which green plants, algae and some bacteria make glucose from carbon dioxide and water using light energy. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of light and chlorophyll. It takes place in chloroplasts.
A chloroplast has an outer and inner membrane. Inside are stacks of thylakoid discs called grana, where the light reactions happen, and a fluid called stroma, where the dark reactions happen. Pigments are chlorophyll a and b, which absorb mainly red and blue light and reflect green, and carotenoids, which are yellow to orange accessory pigments.
Light dependent reactions (in the thylakoid membranes):
- Chlorophyll absorbs light, and excited electrons leave the photosystem.
- Water is split by photolysis into hydrogen ions, electrons and oxygen. The oxygen is released as a by-product.
- Electrons pass along an electron transport chain, and the energy released makes ATP. This is photophosphorylation.
- Hydrogen ions and electrons reduce NADP to NADPH.
Light independent reactions, also called the Calvin cycle, happen in the stroma. Carbon dioxide combines with a five carbon sugar, ribulose bisphosphate (RuBP), using the enzyme rubisco. The product splits into two molecules of a three carbon compound, glycerate 3-phosphate (GP). ATP and NADPH from the light reactions convert GP into triose phosphate. Most triose phosphate is used to regenerate RuBP, and some is used to make glucose, starch, lipids and amino acids.
Limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. If one factor is in short supply, it limits the rate whatever the level of the others. At low light the rate rises with light, then levels off when another factor limits. Very high temperature denatures the enzymes and reduces the rate.
Respiration is the release of energy from food molecules. In aerobic respiration glucose is fully oxidised using oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. It yields about 30 to 32 ATP per glucose in the textbook figure range, and the exact number depends on the source.
Stages of aerobic respiration:
- Glycolysis (cytoplasm): one glucose (6C) is split into two pyruvate (3C). Net gain is 2 ATP and 2 NADH. It does not need oxygen.
- Link reaction (mitochondrial matrix): pyruvate loses carbon dioxide and becomes an acetyl group attached to coenzyme A. NADH is formed.
- Krebs cycle (matrix): the acetyl group joins oxaloacetate to form citrate. Through a series of steps, CO₂ is released and NADH, FADH₂ and a little ATP are made.
- Electron transport chain (inner membrane, cristae): NADH and FADH₂ give up electrons, a proton gradient forms and ATP is made by ATP synthase. Oxygen is the final electron acceptor and forms water.
Anaerobic respiration happens without oxygen and gives only 2 ATP per glucose from glycolysis. In yeast it is alcoholic fermentation: glucose gives ethanol and carbon dioxide. In muscles it is lactic acid fermentation: pyruvate becomes lactate. Lactate builds up in hard exercise, and the oxygen debt is repaid afterward by deep breathing. Bakers and brewers use yeast fermentation.
Study tip: Make a table with the location, inputs and outputs of each stage of photosynthesis and respiration, and revise it daily. Examiners like balanced equations and correct sites, so state the organelle and part. For graph questions on limiting factors, name the factor that limits at each stage of the curve.
Definitions and flashcards
Say the meaning first, then open the card to check.
ATP
The molecule that carries usable chemical energy in cells.
Photolysis
Splitting of water by light energy in the light reaction.
Stroma
The fluid of the chloroplast where the Calvin cycle occurs.
Grana
Stacks of thylakoids in the chloroplast that hold pigments.
Glycolysis
Breakdown of glucose to pyruvate in the cytoplasm.
Krebs cycle
A cycle in the mitochondrial matrix that oxidises acetyl groups to carbon dioxide.
Fermentation
Anaerobic breakdown of glucose giving ethanol or lactic acid.
Limiting factor
The factor in shortest supply, which controls the rate of a process.
Oxygen debt
The extra oxygen needed after hard exercise to remove lactic acid.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why is ATP called the energy currency of the cell? (2 marks)
ATP is made in energy releasing reactions and broken down in energy requiring reactions. It carries energy from respiration to all parts of the cell in a usable form, like money is spent where it is needed.
Write the overall equation of photosynthesis. (2 marks)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light energy and chlorophyll.
Where do the light and dark reactions occur, and what are their products? (3 marks)
The light reactions occur in the thylakoid membranes of the grana. They produce ATP, NADPH and oxygen. The dark reactions occur in the stroma. They use ATP, NADPH and CO₂ to make triose phosphate, which is used to build glucose.
Compare aerobic and anaerobic respiration. (3 marks)
Aerobic respiration needs oxygen, takes place in the cytoplasm and mitochondria, oxidises glucose fully to CO₂ and water, and gives a high yield of ATP. Anaerobic respiration needs no oxygen, takes place in the cytoplasm only, gives ethanol and CO₂ or lactic acid, and gives only 2 ATP per glucose.
Describe glycolysis and the Krebs cycle. (5 marks)
Glycolysis occurs in the cytoplasm. A glucose molecule is phosphorylated using ATP and split into two three carbon units, which are oxidised to two pyruvate. The net gain is 2 ATP and 2 NADH, and no oxygen is needed. Pyruvate enters the mitochondrion and is converted to acetyl CoA by the link reaction. In the Krebs cycle in the matrix, the acetyl group combines with a four carbon compound to make a six carbon compound. This is oxidised in steps, releasing CO₂ and forming NADH, FADH₂ and some ATP. The reduced carriers then supply electrons to the electron transport chain.
Name three limiting factors of photosynthesis and explain one. (3 marks)
The factors are light intensity, carbon dioxide concentration and temperature. At low light intensity, the light reactions make little ATP and NADPH, so the Calvin cycle is slow. Increasing light raises the rate until another factor becomes limiting.
Common mistakes
- Saying oxygen released in photosynthesis comes from carbon dioxide. It comes from the splitting of water.
- Saying that the dark reactions occur only at night. They need ATP and NADPH from the light reactions and occur during the day.
- Writing that glycolysis needs oxygen or occurs in the mitochondria.
- Confusing the final electron acceptor. It is oxygen in aerobic respiration, and NADP receives electrons in photosynthesis.
- Saying plants only photosynthesise and do not respire. Plants respire all the time.