Section 4

Cells & Photosynthesis

Carbon dioxide + Water Glucose + OxygenLight

Sunlight & Starch Production

• Cells in green plants make their own food in a process known as……….

Carbohydrates• Plants make glucose. Some of this is used immediately

as an energy source and the rest is converted to starch for storage.

• Starch is a large molecule so it cannot leave the plant cell.

• Glucose and starch are carbohydrates made up of the following elements:

• These are:1) Carbon (C)2) Hydrogen (H)3) Oxygen (O)

Carbohydrate

Carbohydrates

• You now know that glucose is converted to starch for storage

• Presence of starch shows that plants have first made glucose by photosynthesis

How is starch made?

Small, soluble glucose molecules Large, insoluble

starch molecule

Chloroplasts• Light energy from the sun is trapped by the green

pigment chlorophyll.

• Chlorophyll is found in disc-shaped structures called chloroplasts in green leaves.

chloroplasts

Variegated leaf

White area:

No chlorophyll

Green area: Contains

chlorophyll

Raw Materials

sunlight

water absorbed

from soil

carbon dioxide

absorbed by the air

Carbon Dioxide + Water Glucose + Oxygen

Light energy

Raw Materials Products

Oxygen is a waste product and given off

as a gas

Word equation for photosynthesis

The Two Stages of Photosynthesis

Photosynthesis occurs in two stages

1. Photolysis - The splitting of water

2. Carbon Fixation

Carbon Dioxide + Water Glucose + Oxygen

Light energy

Light energy from the sun trapped by chlorophyll

Light energy converted to

chemical energy

Passed to second stage

Released to the air as

oxygen gas

Carbon Dioxide + Water Glucose + Oxygen

Light energy

CO2 + H2O C6H12O6 + O2

Light energy

Stage 1: Photolysis

• Water is split up into the two elements that it is made from – hydrogen and oxygen

• Oxygen is a by-product of plants and is given off as a gas

• Hydrogen is carried to the second stage by a hydrogen carrier

• Light energy from the sun used to build up ADP + Pi into ATP for use in the second stage

• Photolysis is controlled by a series of enzymes

Stage 1: Photolysis

1. Water is split into hydrogen and oxygen.

2. Hydrogen is carried to the second stage (carbon fixation) by a hydrogen carrier and is used in this stage.

3. Oxygen is a by-product that is given off as a gas.

4. ATP is built up using light energy. This ATP is used in carbon fixation.

Stage 2: Carbon Fixation

• Hydrogen that is produced in photolysis combines with carbon dioxide to form the carbohydrate, glucose

• Hydrogen is carried to the second stage by a hydrogen acceptor molecule

Carbon Dioxide + Water Glucose + Oxygen

Light energy

CO2 + H2O C6H12O6 + O2

Light energy

Stage 2: Carbon Fixation

• Carbon dioxide and hydrogen are able to combine using the energy from the ATP made in the first stage photolysis

Hydrogen (from stage

1)

ATP

(from stage 1)

ADP +Pi

Glucose

(C + H + O)

Carbon Dioxide

(enters by diffusion from the

air)

Stage 2: Carbon Fixation. This reaction is controlled by a series

of enzymes

Diffusion

• Carbon dioxide gas diffuses into the leaf from the air

• Oxygen gas that is produced inside the leaf diffuses out Carbon

dioxide

Oxygen

Storage of Carbohydrates

• Glucose molecules can link to each other in different ways

• This results in different types of carbohydrates being formed

• E.g. Starch and cellulose

Starch

• How plants store excess glucose– Large and insoluble

• Chains of glucose units that form are coiled up and becomes tangled to form a dense starch grain

• Easily broken down to be used as a source of energy

Cellulose

• Used as building materials – Cell walls in plants– Not soluble in water

• Forms parallel chains

• Not easily broken down (tough fibres!)

Cellulose Fibres

• Fibres arranged in sheets that run at right angles to each other

• Criss-cross arrangement adds strength to cell wall

• Small molecules are soluble, e.g. glucose, water and carbon dioxide and can pass into cell

Measuring rate of photosynthesis

The rate of photosynthesis can be measured in the following ways:

1. Measuring the increase in dry mass of a plant over a period of time

2. Measuring the volume of oxygen given off over a period of time

3. Measuring the volume of carbon dioxide taken in over a period of time

Measuring rate of photosynthesis with Elodea (pondweed)

elodea

Solution of sodium carbonate (a source of CO2)

Thermometer

lamp

funnel

Upside down test tube(to collect oxygen gas)

How could the light intensity be altered?How could the temperature be altered (and kept constant)?How could the CO2 concentration be altered?How can the rate of photosynthesis be measured?

Elodea• http://www.kscience.co.uk/animations

/photolab.htm

Limiting FactorsA limiting factor is a factor which slows down the rate of photosynthesis when it is in short supply. There are three limiting factors that affect the rate of photosynthesis:

1. Light intensity

2. Carbon dioxide concentration

3. Temperature (because photosynthesis is an enzyme-controlled reaction and enzymes are affected by temperatures)

Light intensity as a limiting factor

Light intensity

A

B

At A, low light intensity is limiting the rate of photosynthesis.

At B, light intensity is high, so another factor must be in short supply (carbon dioxide concentration or temperature).

Carbon dioxide concentration as a limiting factor

Carbon dioxide concentration

A

B

At A, low carbon dioxide concentration is limiting the rate of photosynthesis.

At B, carbon dioxide concentration is high, so another factor must be in short supply (light intensity or temperature).

Temperature as a limiting factor

Temperature (°C)

A

B

At A, low temperature is limiting the rate of photosynthesis.

At B, temperature is high, so another factor must be in short supply (carbon dioxide concentration or light intensity).400 20

Limiting Factors

Light Intensity

At this point, light intensity is the limiting factor

At this point, something other than light intensity is the limiting factor

As a general rule,

whatever is on the x-axis is the limiting factor on the slope of the

graph

What’s the limiting factor?

A

B

At A, low ________________ is the limiting factor.

At B, ___________ or ______________ is the limiting factor.

Making the most of photosynthesis – Reducing

limiting factors• By reducing the limiting factors for photosynthesis, plant

growers can grow larger plants with higher yield of fruit, flowers or grain.

• By covering plants with polythene tunnels, or growing them in greenhouses, the temperature is raised and the plants are protected from damaging wind and pests

• If a paraffin stove is burned, this increases the temperature and has the added benefit of providing them with extra carbon dioxide.

• Artificial lights provide extra light and warmth for plants