Photosynthesis Notes

AQA GCSE Biology: Revision notes

Key facts

Photosynthesis is endothermic: it takes in light energy and stores it in glucose.
Equation: $\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$ , with light and chlorophyll.
The rate is controlled by the limiting factor: light intensity, carbon dioxide concentration or temperature.
Light intensity is inversely proportional to the square of the distance: $I \propto \frac{1}{d^2}$ (Higher Tier).
Plants use glucose for respiration, cellulose, starch and oils for storage, and amino acids.

Labelled cross-section of a leaf — A leaf in cross-section, where photosynthesis takes place.

The photosynthesis equation

Carbon dioxide and water make glucose and oxygen, using light energy absorbed by chlorophyll.

Photosynthesis is endothermic: it takes in energy from light. Light and chlorophyll go above or below the arrow because they are conditions, not reactants.

It is the reverse of aerobic respiration in terms of inputs and outputs.

Word equationcarbon dioxide + water → glucose + oxygen
Symbol equation $\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$

Worked example

Show that $\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$ is balanced.

1
Carbon: left $6 \times 1 = 6$ ; right 6 in $\ce{C6H12O6}$ .
2
Hydrogen: left $6 \times 2 = 12$ ; right 12 in $\ce{C6H12O6}$ .
3
Oxygen: left $6 \times 2 + 6 \times 1 = 18$ ; right $6 + 6 \times 2 = 18$ .

Which statement about photosynthesis is correct?

Limiting factors

The factor in shortest supply, the limiting factor, sets the rate. Raise it and the rate rises until another factor limits.

Three factors can limit the rate: light intensity, carbon dioxide concentration and temperature.

More light or more carbon dioxide raises the rate until the graph levels off. Another factor is then limiting.
Temperature raises the rate up to an optimum (about 25 to 35 °C). Above it enzymes denature and the rate falls sharply.

A steep, rising line means that factor is limiting. A flat line means something else is.

Rate against light intensity: it rises, then plateaus.

Rate against temperature: it peaks, then falls as enzymes denature.

A graph of rate against light intensity has levelled off. What does this show?

Light and the inverse square law

Light intensity is inversely proportional to the square of the distance from the lamp: double the distance, a quarter of the intensity (Higher Tier).

At low to moderate intensities, doubling the light intensity doubles the rate.

For a lamp, intensity $I$ falls with distance $d$ as $I \propto \frac{1}{d^2}$ , the inverse square law.

Double the distance: $\frac{1}{2^2} = \frac{1}{4}$ of the intensity.
Triple the distance: $\frac{1}{3^2} = \frac{1}{9}$ .

Small changes in distance make large changes in intensity, so control the distance carefully.

Relative intensity against distance from the lamp (I = 1 at d = 1).

Worked example

A lamp is 10 cm from a plant, giving a rate of 100 units. It is moved to 20 cm. Estimate the new rate, assuming light is limiting.

1
The distance has doubled: $20 \div 10 = 2$ .
2
Intensity changes by $\frac{1}{2^2} = \frac{1}{4}$ .
3
New rate $= 100 \times \frac{1}{4}$ .

A lamp is moved three times further away. The intensity becomes

What plants do with glucose

Plants use glucose for respiration, to build cellulose, to store starch and oils, and to make amino acids.

Glucose is used straight away or stored. The split depends on what the plant needs at the time.

By day, some glucose is stored. At night, stored starch and glucose supply respiration.
Cellulose is needed constantly for growth and cell walls.
Amino acids need nitrogen from nitrate ions.
Starch is the main store.

1
Glucose is made
by photosynthesis in the leaf
2
Used straight away
for respiration, to make cellulose, and with nitrate ions to make amino acids
3
Stored
as starch (the main store) or as fats and oils, especially in seeds

What plants do with the glucose they make.

Respiration

Releases energy for all plant activities

Cellulose

Glucose joined together
Makes cell walls

Storage

Starch in leaves, roots and seeds
Fats and oils, especially in seeds

Amino acids

Glucose plus nitrate ions
Then proteins

Which substance do plants make from glucose and nitrate ions?

The two stages

Light reactions in the thylakoid membranes make ATP and reduced NADP. The Calvin cycle in the stroma uses them to make glucose (Higher Tier).

Light-dependent reactions need light. They split water, release oxygen, and make ATP and reduced NADP.

The Calvin cycle is the light-independent stage. It needs no direct light but depends on ATP and reduced NADP, and fixes carbon dioxide into glucose. For every three CO₂ fixed, one G3P leaves the cycle, and two G3P make one glucose.

1
Light absorbed
chlorophyll absorbs light energy
2
Photolysis
water is split into hydrogen ions, electrons and oxygen
3
Electron transport
energy from the electrons is used to make ATP
4
NADP reduced
$\ce{NADP+}$ becomes reduced NADP
5
Oxygen released
as a by-product

Light-dependent reactions (thylakoid membranes)

1
CO₂ fixed
RuBisCO combines CO₂ with RuBP
2
Breakdown
forms 3-phosphoglycerate (3-PG)
3
Reduction
ATP and reduced NADP make G3P
4
RuBP regenerated
most G3P is recycled, using ATP

Then back to step 1

The Calvin cycle (stroma)

Where does the Calvin cycle take place?

Investigating photosynthesis

Change one factor, measure oxygen made by an aquatic plant, and keep every other variable constant.

Use an aquatic plant such as Elodea. Measure the rate by counting oxygen bubbles per minute, or collect the gas in an inverted test tube.

Light: change the lamp distance (10, 20, 30 cm).
CO₂: change the concentration of sodium hydrogen carbonate solution.
Control: temperature (water bath), light source, plant size, time and pH.

Keep lamps away from water spills and wear eye protection.

1
Set up
aquatic plant in a sealed container
2
Vary CO₂
different sodium hydrogen carbonate concentrations
3
Control
light intensity and temperature constant
4
Measure
count bubbles or gas volume
5
Plot
CO₂ on the x-axis, rate on the y-axis

Investigating carbon dioxide

In a light intensity experiment, which is the independent variable?

Try an exam question

A student investigated the effect of light intensity on the rate of photosynthesis in pondweed. Describe how the student could do this, and explain why the rate stops increasing at high light intensity.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.