Light, carbon dioxide and temperature: the rate is set by whichever is scarce, and raising one factor helps only until another becomes limiting.
Step 1 of 6 · The weakest link
Step 1 of 6
The weakest link
The Scene has a third factor now: Temperature. Enzymes build the sugar, and they work fast or slow depending on the warmth, as you saw in the Unit on the molecules of life.
When several factors affect the rate, it is set by the one in shortest supply: the limiting factor. Like a chain that breaks at its weakest link, however strong the others are.
Step 2 of 6
More light
Light is free, and the rest are fixed. Start almost in the dark, and turn it up slowly.
Step 3 of 6
A cold morning
At low temperatures, for every 10 °C of cooling the enzymes work about half as fast. Between 25 °C and 30 °C they work fastest.
Step 4 of 6
A hot day
In a shut greenhouse on a summer day the temperature quickly climbs past 35 °C. Light and carbon dioxide are still plentiful.
Step 5 of 6
All three
All three factors are free. Start from a cold, cloudy morning, and remember: at each moment only the limiting factor changes anything.
Step 6 of 6
In a greenhouse
Out in a field a farmer has no say over the light and the temperature. In a greenhouse they do: they can heat, light and add carbon dioxide, and each of those costs money. The trick is to know what is limiting now.
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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.
Practice · Predict, then look
Light is at 30%, Carbon dioxide at 80%, and Temperature at 25 °C, comfortable for the enzymes. What is the limiting factor?
Practice · Predict, then look
Light is at 30%, Carbon dioxide at 80%, and Temperature at 25 °C, comfortable for the enzymes. What is the limiting factor?
Practice · Reach the target
Carbon dioxide is fixed at 40%, and the temperature is comfortable. Raise Light until photosynthesis reaches the highest rate these conditions allow, then press Check.
Practice · Reach the target
Carbon dioxide is fixed at 40%, and the temperature is comfortable. Raise Light until photosynthesis reaches the highest rate these conditions allow, then press Check.
Practice · Predict, then look
Full light and plenty of carbon dioxide, at 25 °C: the rate is 100%. We will cool it to 15 °C. At what percentage of the top rate will photosynthesis run, if the enzymes slow by half for every 10 °C of cooling?
Practice · Predict, then look
Full light and plenty of carbon dioxide, at 25 °C: the rate is 100%. We will cool it to 15 °C. At what percentage of the top rate will photosynthesis run, if the enzymes slow by half for every 10 °C of cooling?
Practice · Predict, then look
Full light and plenty of carbon dioxide, at 25 °C. A hot day comes, 40 °C. What will happen to the rate?
Practice · Predict, then look
Full light and plenty of carbon dioxide, at 25 °C. A hot day comes, 40 °C. What will happen to the rate?
Practice · Reach the target
A cold, cloudy morning: light 50%, carbon dioxide 30%, 10 °C. Change all three until photosynthesis runs at 80% of its top rate, then press Check.
Practice · Reach the target
A cold, cloudy morning: light 50%, carbon dioxide 30%, 10 °C. Change all three until photosynthesis runs at 80% of its top rate, then press Check.
Practice · Choose
A winter tomato greenhouse has heating and strong lamps, and the windows shut. The grower adds carbon dioxide to the air, and the crop grows. Why?
Practice · Choose
A winter tomato greenhouse has heating and strong lamps, and the windows shut. The grower adds carbon dioxide to the air, and the crop grows. Why?
Lesson complete. Every practice question here is done.