AQA Biology 4.4.1.2
Rate of photosynthesis
Move the lamp, count the bubbles, work out the rate. Then find what stops it going any faster.
Combined Science · Higher tier
Start here
Windowsill or corner?
Two identical basil plants in the same kitchen: one on a sunny windowsill, one in a dim corner.
If you had to guess, will more light speed up the plant’s food-making, or make no difference?
Required practical 5
Light intensity and the rate of photosynthesis.
Put pondweed in a tube of water with a lamp at a measured distance. A large beaker of water between them acts as a heat shield, and a thermometer checks the temperature stays the same. Count the bubbles of oxygen given off in one minute.
distance · bubbles in 1 min
Pick a distance.
Repeat at each distance and find a mean. Measuring the volume of oxygen, for example with a gas syringe, is more accurate than counting bubbles.
Limiting factors
What holds the rate back?
A limiting factor is the one in short supply that stops the rate going any higher. Four factors affect the rate of photosynthesis. Pick each one.
As light intensity increases, the rate increases: more energy is available. Then the line levels off. Light is no longer the limiting factor; another factor is.
Interacting factors · greenhouses
These factors interact, and any one of them may be the factor that limits photosynthesis. When a graph has more than one line, decide which factor is limiting on each part.
Growers add heat, light or carbon dioxide to greenhouses, but only while that factor is limiting, and only while the extra crop earns more than it costs. Is each step worth it?
Pick a step.
Equation · rate of photosynthesis
The amount is bubbles of oxygen, or a volume of oxygen in cm3. Time must be in the unit the rate asks for: seconds ÷ 60 gives minutes, so 90 s = 1.5 min. Tap the quantity you want to find.
Equation
Rate of photosynthesis
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
rate = amount of oxygen ÷ time
- A
- amount of oxygen
- bubbles or cm³
- R
- rate
- per min
- t
- time
- min
Light intensity and distance
Light intensity is inversely proportional to the square of the distance from the lamp. Put both distances in the same unit before you square them. Work out the light intensity at each distance on its own line, then compare.
Equation
Relative light intensity
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
relative light intensity (L) = 1 ÷ d²
- 1
- one, a fixed number
- L
- relative light intensity
- no unit
- d
- distance from the lamp
- cm
Think again · why it levels off
“It levels off because the plant runs out of chlorophyll.”
Your call
ChooseA graph of rate against light intensity levels off. Why?
Sort
Change, measure or keep the same?
Place each variable in the practical.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Calculate
- Amount ÷ time, with the time in the right unit.
- Explain
- A graph’s shape, using limiting factors.
- Describe
- A method, with controls and repeats.
Key fact
Temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll all affect the rate. Rate of photosynthesis = amount of oxygen ÷ time.
Key note · AQA 4.4.1.2
Rate of photosynthesis
- A limiting factor is the one in short supply that stops the rate rising: light intensity, carbon dioxide, temperature or the amount of chlorophyll.
- Light and carbon dioxide graphs rise, then level off when another factor limits. Above the optimum temperature, enzymes are denatured and the rate falls.
- Rate = amount of oxygen ÷ time. Convert seconds to minutes first if the rate is per minute.
- Required practical 5: lamp distance changes light intensity; heat shield and thermometer keep temperature the same; repeat and find a mean.
- Higher: factors interact. Relative light intensity = 1 ÷ d², both distances in the same unit. Add heat, light or carbon dioxide only while it is limiting and pays.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Ask MrBadmus
Not sure which factor is limiting?
Bubble counts, curves and greenhouse figures are made-up teaching data.