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Bioenergetics: Class 9 Biology
Energy flow in living things, ATP, photosynthesis with its equation, light and dark reactions and limiting factors, aerobic and anaerobic respiration, their equations and the sites of respiration in the cell.
Key points
Bioenergetics is the study of how living things obtain, change and use energy. The main source of energy for life is sunlight. Plants change light energy into chemical energy in food by photosynthesis. All organisms release this energy by respiration and store it in a usable form called ATP.
ATP (adenosine triphosphate) is the energy currency of the cell. It is made of adenine, the sugar ribose and three phosphate groups. When the last phosphate is removed by hydrolysis, ATP becomes ADP (adenosine diphosphate) and energy is released for cell work. ATP is made again from ADP and phosphate using energy from food. It is used in muscle contraction, active transport, making proteins and nerve impulses.
Photosynthesis is the process in which green plants, algae and some bacteria make glucose from carbon dioxide and water using light energy absorbed by chlorophyll. The word equation is: carbon dioxide + water → glucose + oxygen (in light, with chlorophyll). The balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Photosynthesis takes place mainly in the leaves. The leaf is thin and flat to catch light. The upper epidermis is covered by a waxy cuticle, the palisade cells contain many chloroplasts, the spongy mesophyll has air spaces for gas exchange, and stomata in the lower epidermis allow carbon dioxide in and oxygen out. Veins bring water and take away sugar. Chloroplasts contain a stack of membranes (grana) holding chlorophyll and a fluid called stroma.
Two stages of photosynthesis:
- Light dependent reactions (in the grana): chlorophyll absorbs light, water is split to give oxygen (photolysis), and energy is stored as ATP and a hydrogen carrier (NADPH).
- Light independent reactions or Calvin cycle (in the stroma): carbon dioxide is fixed and, using ATP and hydrogen from the first stage, is reduced to glucose.
Factors that limit the rate of photosynthesis:
- Light intensity: the rate rises with light until another factor becomes limiting.
- Carbon dioxide concentration: air has about 0.04 percent, so it is often limiting.
- Temperature: the rate rises with temperature up to an optimum, then enzymes are denatured and the rate falls.
- Water and chlorophyll content also affect the rate.
The principle of limiting factors says that when a process depends on several factors, its rate is controlled by the one that is in shortest supply. Greenhouse growers use this idea by adding light, warmth and carbon dioxide. Plants are the producers of every food chain, and they replace oxygen in the atmosphere.
Respiration is the chemical breakdown of food, mainly glucose, to release energy. It is not the same as breathing, which is only the movement of air in and out of the lungs. Respiration goes on in every living cell all the time.
Aerobic respiration needs oxygen. Glucose is completely oxidised to carbon dioxide and water, and a large amount of energy is released (about 38 ATP per glucose molecule). The equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. It starts in the cytoplasm with glycolysis, in which glucose is split into two molecules of pyruvate with a small yield of ATP. In the mitochondria pyruvate is fully broken down in the Krebs cycle and the electron transport chain, which makes most of the ATP.
Anaerobic respiration (without oxygen), which gives much less energy (2 ATP per glucose):
- In yeast and some plant tissues (alcoholic fermentation): glucose → ethanol + carbon dioxide + energy. It is used in baking and brewing.
- In animal muscle (lactic acid fermentation): glucose → lactic acid + energy. During hard exercise the lactic acid builds up and causes muscle fatigue and cramps. An oxygen debt is repaid afterwards by deep breathing, which removes lactic acid.
- Some bacteria also use fermentation, for example to make yoghurt from milk.
Photosynthesis and respiration are opposite processes. Photosynthesis stores energy, uses carbon dioxide and water, produces glucose and oxygen, occurs only in cells with chlorophyll and in light. Respiration releases energy, uses glucose and oxygen, produces carbon dioxide and water, and occurs in all living cells all the time. Together they keep oxygen and carbon dioxide in balance.
Study tip: Learn both equations in symbols and in words, and be ready to name the place in the cell where each stage happens. Questions about graphs of limiting factors need you to say which factor is limiting at each part of the curve, and why the curve becomes flat.
Definitions and flashcards
Say the meaning first, then open the card to check.
Bioenergetics
The study of energy changes in living organisms.
ATP
Adenosine triphosphate, the molecule that carries usable energy in cells.
Photosynthesis
The making of glucose from carbon dioxide and water using light energy and chlorophyll.
Chlorophyll
The green pigment that absorbs light for photosynthesis.
Limiting factor
The factor in shortest supply that controls the rate of a process.
Respiration
The release of energy from food in the cells.
Glycolysis
The breakdown of glucose to pyruvate in the cytoplasm.
Fermentation
Anaerobic respiration that makes alcohol or lactic acid.
Oxygen debt
The extra oxygen needed after 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.
Write the balanced equation of photosynthesis. (2 marks)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and chlorophyll.
Why is ATP called the energy currency of the cell? (2 marks)
ATP carries energy released in respiration to places where it is needed. When it changes to ADP and phosphate, it gives out energy for cell work, and it is then made again, so it is used again and again like money.
What is a limiting factor? Name two for photosynthesis. (3 marks)
A limiting factor is the factor which is in shortest supply and so controls the rate of a process, even if other factors are plentiful. For photosynthesis, light intensity and carbon dioxide concentration are two limiting factors. Temperature can also limit the rate.
Compare aerobic and anaerobic respiration. (3 marks)
Aerobic respiration needs oxygen, takes place in the cytoplasm and mitochondria, breaks glucose down completely to carbon dioxide and water, and gives about 38 ATP. Anaerobic respiration takes place without oxygen in the cytoplasm, gives alcohol and carbon dioxide or lactic acid, and gives only 2 ATP per glucose.
Why do muscles get tired and painful during hard exercise? (3 marks)
During hard exercise the oxygen supply to muscles is not enough, so they respire anaerobically. Glucose is changed to lactic acid, which builds up in the muscle and causes fatigue and cramps. After exercise, extra oxygen is taken in to remove the lactic acid, which is called repaying the oxygen debt.
Describe the two stages of photosynthesis. (5 marks)
The first stage is the light dependent reactions in the grana of the chloroplast. Chlorophyll absorbs light energy, water is split into hydrogen and oxygen, and oxygen is released. The energy is stored in ATP and NADPH. The second stage is the light independent reactions in the stroma, also called the Calvin cycle. Carbon dioxide is taken in and, using the ATP and hydrogen from the first stage, it is changed into glucose. The glucose is then used for respiration, changed to starch for storage or used to make other substances.
Common mistakes
- Using respiration and breathing as the same thing.
- Saying that plants only photosynthesise and do not respire. Plants respire day and night.
- Writing the equation of respiration as the exact reverse of photosynthesis without showing that the conditions are different (light and chlorophyll versus enzymes in mitochondria).
- Saying that oxygen released in photosynthesis comes from carbon dioxide. It comes from water.
- Leaving out the conditions (light, chlorophyll) from the equation of photosynthesis.