Bioenergetics
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Photosynthesis basics
Photosynthesis is an endothermic reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen.
This is how plants (and algae) make their own food — they are producers.
Photosynthesis basics — Key Knowledge
- Endothermic reaction takes in energy from the environment as light
- Happens in chloroplasts which contain chlorophyll to absorb light energy
- Reactants are carbon dioxide and water, Products are glucose and oxygen
Photosynthesis equations
You need to know both the word equation and the balanced symbol equation for photosynthesis.
The symbol equation must balance — six of each reactant molecule, one glucose and six oxygen molecules.
Photosynthesis equations — Key Knowledge
- Word equation carbon dioxide + water → glucose + oxygen
- Symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Light energy and chlorophyll are needed but are not reactants
Limiting factors of photosynthesis
The rate of photosynthesis is limited by whichever factor is in shortest supply at any given time.
Greenhouse farmers control all three factors to maximise crop growth.
Limiting factors of photosynthesis — Key Knowledge
- Light intensity more light = faster rate, up to a point
- Carbon dioxide concentration more CO₂ = faster rate, up to a point
- Temperature too low slows reactions; too high denatures enzymes
Inverse square law
Light intensity decreases rapidly as the distance from the light source increases.
Light intensity ∝ 1/d²
where d is the distance from the light source
Doubling the distance from a lamp reduces light intensity to a quarter — used in required practicals.
Inverse square law — Key Knowledge
- Light intensity is inversely proportional to the square of the distance from the light source
Uses of glucose
Plants use the glucose produced by photosynthesis in several ways — not just for energy.
Starch is the storage molecule because it is insoluble — glucose would dissolve and draw in water by osmosis.
Uses of glucose — Key Knowledge
- Respiration releases energy for life processes
- Converted to starch for storage insoluble so doesn't affect osmosis
- Made into cellulose for cell walls
- Combined with nitrate ions to make amino acids and proteins, Made into lipids fats and oils for energy storage
Aerobic respiration
Aerobic respiration uses oxygen to break down glucose, releasing energy. It occurs continuously in all living cells.
All living organisms respire — plants respire all the time, not just at night.
Aerobic respiration — Key Knowledge
- Takes place in the mitochondria, Reactants are glucose and oxygen, Products are carbon dioxide and water, Word equation glucose + oxygen → carbon dioxide + water
- Symbol equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Uses of energy from respiration
The energy released by respiration is used for essential life processes in all organisms.
Without respiration, none of these processes could happen — that is why it is continuous.
Uses of energy from respiration — Key Knowledge
- Chemical reactions to build larger molecules e.g. proteins from amino acids
- Muscle contraction movement
- Maintaining body temperature in mammals and birds
- Active transport moving substances against the concentration gradient
Anaerobic respiration
Anaerobic respiration occurs without oxygen and releases much less energy than aerobic respiration because glucose is only partially broken down.
Anaerobic respiration is a temporary solution when oxygen supply cannot meet demand.
Anaerobic respiration — Key Knowledge
- In animals glucose → lactic acid
- In plants and yeast glucose → ethanol + carbon dioxide — this is fermentation
- Much less energy released per glucose molecule than aerobic respiration
Response to exercise
During exercise, the body increases heart rate and breathing rate to supply muscles with more oxygen and glucose, and to remove carbon dioxide faster.
These changes are automatic responses to increased energy demand during physical activity.
Response to exercise — Key Knowledge
- Heart rate increases pumps blood faster to muscles
- Breathing rate and depth increase more gas exchange in lungs
- If oxygen supply cannot keep up, muscles switch to anaerobic respiration, Lactic acid builds up in muscles causing fatigue
Oxygen debt
After vigorous exercise, you continue to breathe heavily to repay the oxygen debt — the extra oxygen needed to break down the lactic acid that accumulated.
This is why you keep panting after a sprint — your body is clearing the lactic acid, not just "catching its breath."
Oxygen debt — Key Knowledge
- Oxygen debt the amount of extra oxygen needed after exercise to react with the accumulated lactic acid
- Lactic acid is transported in the blood to the liver where it is broken down, Heavy breathing and fast heart rate continue until the lactic acid is fully dealt with
Metabolism
Metabolism is the sum of all the chemical reactions in a cell or the body.
Metabolic rate varies between organisms and is affected by factors like activity level and body size.
Metabolism — Key Knowledge
- Includes synthesis reactions building larger molecules from smaller ones — e.g. amino acids into proteins, glucose into starch or glycogen, fatty acids and glycerol into lipids
- Includes breakdown reactions breaking down excess proteins to form urea for excretion
- Respiration itself is a metabolic reaction