Photosynthesis Notes

Cambridge IGCSE Biology: Revision notes

Key facts

Photosynthesis: carbon dioxide and water make glucose and oxygen, using light energy absorbed by chlorophyll.
Glucose is used for respiration, stored as starch, made into cellulose for cell walls, and moved as sucrose.
Plants need nitrate to make amino acids (proteins) and magnesium to make chlorophyll.
Chlorophyll, light and carbon dioxide are all required for photosynthesis.
Light intensity, carbon dioxide concentration and temperature can each be the limiting factor.

The process

Photosynthesis uses light energy to turn carbon dioxide and water into glucose, releasing oxygen.

Chlorophyll in the chloroplasts absorbs light energy. The raw materials are carbon dioxide and water; the products are glucose (a carbohydrate) and oxygen as a by-product.

Photosynthesis $\ce{6CO2 + 6H2O ->[light] C6H12O6 + 6O2}$

1
Light absorbed
By chlorophyll in the chloroplasts
2
Raw materials
Carbon dioxide and water
3
Glucose made
A carbohydrate
4
Oxygen released
As a by-product

Photosynthesis in outline

Which of these is NOT needed for photosynthesis?

Uses of glucose

Glucose is used for respiration, converted to starch for storage, built into cellulose and moved around as sucrose.

When asked about the fate of glucose, give at least three uses: respiration, starch for storage, cellulose for cell walls and, if space allows, sucrose transport in the phloem.

Function Notes Glucose Immediate energy Used in respiration Starch Energy storage Stored in leaves and roots Cellulose Structural support Strengthens cell walls Sucrose Transport Moved through the phloem Nectar Attraction Attracts insects for pollination

	Function	Notes
Glucose	Immediate energy	Used in respiration
Starch	Energy storage	Stored in leaves and roots
Cellulose	Structural support	Strengthens cell walls
Sucrose	Transport	Moved through the phloem
Nectar	Attraction	Attracts insects for pollination

Function

Glucose:: Immediate energy
Starch:: Energy storage
Cellulose:: Structural support
Sucrose:: Transport
Nectar:: Attraction

Notes

Glucose:: Used in respiration
Starch:: Stored in leaves and roots
Cellulose:: Strengthens cell walls
Sucrose:: Moved through the phloem
Nectar:: Attracts insects for pollination

Which product of photosynthesis is used for the plant's structure?

Mineral ions

Nitrate is needed for amino acids and proteins, and magnesium for chlorophyll.

Both ions can become limiting factors. Nitrate leads to proteins and enzymes, which speed up reactions. Magnesium is at the centre of each chlorophyll molecule, so without it less light is absorbed. Yellow leaves with green veins suggest magnesium deficiency; stunted, pale growth suggests nitrate deficiency.

\ce{NO3-}

	Nitrate, $\ce{NO3-}$	Magnesium, $\ce{Mg^2+}$
Used for	Making amino acids, then proteins and enzymes	Making chlorophyll
If lacking	Stunted, pale growth	Yellow leaves

Nitrate, $\ce{NO3-}$

Used for:: Making amino acids, then proteins and enzymes
If lacking:: Stunted, pale growth

Magnesium, $\ce{Mg^2+}$

Used for:: Making chlorophyll
If lacking:: Yellow leaves

A plant has yellow leaves with green veins. Which ion is it probably lacking?

Requirements

Remove one requirement at a time and test the leaves for starch.

Starch is the evidence that photosynthesis took place. Hydrogencarbonate indicator detects gas exchange: red in normal air, yellow when carbon dioxide is added and purple when it is removed. In light a plant removes carbon dioxide by photosynthesis, so the indicator turns purple; in the dark it only respires, so the indicator turns yellow.

Test setup Result if required Chlorophyll Variegated or albino plant Non-green parts show no starch Light Plant kept in the dark, compared with one in light Dark plant makes no starch Carbon dioxide Plant sealed with soda lime, compared with one in normal air Plant without carbon dioxide makes no starch

	Test setup	Result if required
Chlorophyll	Variegated or albino plant	Non-green parts show no starch
Light	Plant kept in the dark, compared with one in light	Dark plant makes no starch
Carbon dioxide	Plant sealed with soda lime, compared with one in normal air	Plant without carbon dioxide makes no starch

Test setup

Chlorophyll:: Variegated or albino plant
Light:: Plant kept in the dark, compared with one in light
Carbon dioxide:: Plant sealed with soda lime, compared with one in normal air

Result if required

Chlorophyll:: Non-green parts show no starch
Light:: Dark plant makes no starch
Carbon dioxide:: Plant without carbon dioxide makes no starch

Worked example

Two identical aquatic plants are put in hydrogencarbonate indicator, one in light and one in the dark, for 1 hour. What do you see, and what does it show?

1
The light plant removes $\ce{CO2}$ by photosynthesis, so its indicator turns purple.
2
The dark plant releases $\ce{CO2}$ by respiration, so its indicator turns yellow.
3
Only the variable light differs.

What colour does hydrogencarbonate indicator turn when carbon dioxide is removed?

Limiting factors

The rate is held back by whichever factor is in shortest supply: light, carbon dioxide or temperature.

At low light, more light increases the rate; at high light the rate levels off because something else limits it. Air has about 0.04% carbon dioxide, which is often limiting. Photosynthesis is enzyme-catalysed, so the rate rises with temperature to an optimum (typically 25 to 35 °C), then falls as enzymes denature.

Rate of photosynthesis against light intensity (typical shape)

Dense forest floor Winter (temperate) Sealed greenhouse Main limiting factor Light Temperature and light Often carbon dioxide Why The canopy blocks sunlight Low light and cold Used faster than it is replaced

	Dense forest floor	Winter (temperate)	Sealed greenhouse
Main limiting factor	Light	Temperature and light	Often carbon dioxide
Why	The canopy blocks sunlight	Low light and cold	Used faster than it is replaced

Dense forest floor

Main limiting factor:: Light
Why:: The canopy blocks sunlight

Winter (temperate)

Main limiting factor:: Temperature and light
Why:: Low light and cold

Sealed greenhouse

Main limiting factor:: Often carbon dioxide
Why:: Used faster than it is replaced

Worked example

On a cold, cloudy winter day a farmer adds extra $\ce{CO2}$ to a greenhouse and sees little change. Why?

1
On a cold, cloudy day, temperature and light are low.
2
They, not $\ce{CO2}$ , are the limiting factors.
3
Adding more of a factor that is not limiting has little effect.

What is the limiting factor if increasing light intensity no longer increases the rate?

Try an exam question

Explain why a plant growing in soil lacking magnesium has yellow leaves and a lower rate of photosynthesis.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.