Photosynthesis - TCE Biology (Tasmania)
“Photosynthesis: the role of chloroplasts (thylakoids, stroma and grana); inputs and outputs as a balanced net chemical equation; factors affecting rate of reaction (temperature, carbon dioxide concentration and light intensity); a general understanding of the light-dependent and light-independent stages”
Photosynthesis takes place in chloroplasts and converts light energy into chemical energy in glucose: 6CO2 + 6H2O → C6H12O6 + 6O2. In the light-dependent stage, in the thylakoid membranes (stacked into grana), light energy is absorbed by chlorophyll, water is split and oxygen is released, and ATP is produced. In the light-independent stage, in the stroma, the energy from that stage is used to reduce carbon dioxide to glucose. The rate depends on light intensity, carbon dioxide concentration and temperature, and whichever is in shortest supply limits it.
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What this dot point is asking
Photosynthesis is examined under criterion 5 (Section B of the TASC exam). The course asks for a general understanding only: where the stages happen, what goes in and out, and what affects the rate. Photosystems I and II, specific enzymes and coenzyme action are excluded.
The chloroplast
| Part | Description | Role |
|---|---|---|
| Thylakoids | Flattened membrane sacs containing chlorophyll | Site of the light-dependent stage |
| Grana | Stacks of thylakoids | Increase the membrane area for absorbing light |
| Stroma | Fluid surrounding the thylakoids | Site of the light-independent stage |
The balanced net equation
- Inputs: carbon dioxide (from the air, through stomata) and water (from the soil, through the xylem), plus light energy.
- Outputs: glucose (used in respiration, or stored as starch and built into cellulose) and oxygen (released through stomata or used in respiration).
The two stages (general understanding)
Light-dependent stage (thylakoid membranes, grana).
- Chlorophyll absorbs light energy.
- Water is split (photolysis) into hydrogen and oxygen; the oxygen is released as a by-product.
- The energy is used to make ATP, and the hydrogen is passed on to the next stage.
Light-independent stage (stroma).
- Carbon dioxide is taken in and reduced (combined with hydrogen) using the ATP and hydrogen from the light-dependent stage.
- This produces glucose.
- It is a series of enzyme-controlled reactions, so it is sensitive to temperature.
Water is used and oxygen is made in the light-dependent stage (thylakoids). Carbon dioxide is used and glucose is made in the light-independent stage (stroma). ATP from the light-dependent stage can limit the light-independent stage.
Factors affecting the rate
- Light intensity: more light increases the rate until another factor becomes limiting. Light drives the light-dependent stage.
- Carbon dioxide concentration: more carbon dioxide increases the rate of the light-independent stage until another factor becomes limiting.
- Temperature: the light-independent stage is enzyme-controlled, so the rate rises with temperature up to an optimum, then falls as enzymes denature.
A limiting factor is the factor in shortest supply, which stops the rate from increasing. On a graph of rate against one factor, the curve rises and then levels off (plateaus) when a different factor becomes limiting.
Rate of photosynthesis against light intensity at two carbon dioxide levels
Two curves rise together at low light, but the curve for high carbon dioxide levels off at a higher rate than the curve for low carbon dioxide.
- At low light: both curves are the same, so light is the limiting factor; carbon dioxide makes no difference.
- On the plateaus: increasing light has no effect, so light is no longer limiting. The higher plateau with more carbon dioxide shows that carbon dioxide was limiting the lower curve.
- Optimum versus maximum: the plateau is the maximum rate under those conditions; it is not an optimum. Use "optimum" for temperature and pH, where the rate falls on either side.
Marker's note: name the limiting factor for each section of the graph and justify it from the shape.
- Putting the stages in the wrong place
- Light-dependent: thylakoids and grana. Light-independent: stroma.
- Saying the oxygen comes from carbon dioxide
- It comes from splitting water in the light-dependent stage.
- Calling the plateau an optimum
- A plateau shows another factor has become limiting; an optimum is the peak of a curve that falls on either side.
- Adding excluded detail
- Photosystems I and II and coenzymes are not required and cost time.
Photosynthesis and aerobic respiration have reverse net equations. Plants do both: respiration happens all the time in their mitochondria, and photosynthesis only in the light. See cellular respiration.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original5 marksWrite the balanced net equation for photosynthesis and state where in the chloroplast (a) water is split and oxygen released, and (b) carbon dioxide is reduced to glucose.Show worked answer →
- Equation (3 marks)
- 6CO2 + 6H2O → C6H12O6 + 6O2, with light energy absorbed by chlorophyll. One mark for correct reactants, one for correct products, one for balancing.
- (a) 1 mark
- In the thylakoid membranes (stacked into grana), during the light-dependent stage.
- (b) 1 mark
- In the stroma, during the light-independent stage.
Original6 marksA student measured the rate of oxygen production by pondweed at increasing light intensities at 20 degrees Celsius. The rate increased steeply, then levelled off. Explain the shape of the graph and predict, with reasons, the effect of repeating the experiment at 30 degrees Celsius with extra carbon dioxide.Show worked answer →
- Rising section (2 marks)
- At low light intensity, light is the limiting factor: more light provides more energy for the light-dependent stage, producing more ATP and splitting more water, so the rate increases.
- Plateau (2 marks)
- At high light intensity another factor, such as carbon dioxide concentration or temperature, becomes limiting, so adding more light no longer increases the rate.
- Prediction (2 marks)
- The plateau would be higher, because more carbon dioxide and a warmer temperature (closer to the optimum for the enzymes of the light-independent stage) allow the stroma reactions to run faster, so light becomes limiting up to a higher intensity. If the temperature were well above the optimum, enzymes would denature and the rate would fall instead.