B8: Photosynthesis

What do plants use as a source of energy?

Light

Reaction plants use to trap light energy

Photosynthesis

Photosynthesis

Process by which plants make glucose from sunlight

What type of reaction is PS and why?

Endothermic

• Energy taken in from the environment to the chloroplasts by light

Where does PS occur?

Leaves of the plant

What green chemical do leaves contain?

Chlorophyll

Chlorophyll

Absorbs light energy needed for PS

Stages of photosynthesis

1. Plant takens CO2 + H2O into the leaf

2. Light energy absorbed by chlorophyll

3. Light energy used to convert CO2 + H2O into glucose + O2

Word equation for photosynthesis

Symbol equation for photosynthesis

6CO2 + 6H2O → C6H12O6 + 6O2

What is needed for PS to take place?

• CO2

• Light

Factors affecting rate of PS

1. Temp

2. Light intensity

3. CO2 conc

4. Amount of chlorophyll

How to measure effect of light intensity on rate of PS?

1. Take plant, increase LI but keep everything else constant

2. Measure rate of PS at each level of LI

Rate of PS when LI is 0 and why

0

• Plants need light to carry out PS

Describe this graph about the level of light intensity on rate of PS

1. As LI increases, rate of PS increases

As LI increases, why does rate of PS increase?

Plant has more light energy to carry out PS → reaction gets faster

On this graph, how do you know light was initially a limiting factor?

If increase LI + PS rate increases → LI = limiting

What does it mean if light was a limiting factor of PS?

• PS not as fast as it could have been

• As there wasn’t enough light

Limiting factor

Factors that limit the rate of a reaction

What is shown at the point where LI increases but the rate of PS doesn’t (it levels of)?

LI isn’t a limiting factor anymore- something else is

Describe this graph about effect of CO2 conc on rate of PS

1. Initially, increase CO2 conc → PS rate increases

2. At a certain point, graph levels off → CO2 isn’t a LF anymore

How can low levels of chlorophyll in the leaf affect the rate of PS?

Low chlorophyll levels → leaves trap less light energy → lower PS rate

What is PS controlled by?

Enzymes

Explain this graph about the effect of temperature on rate of PS

1. Increase temp (before optimum), enzymes in PS work faster → rate increases

2. Increase temp (beyond optimum), enzymes denature → PS rate falls

Uses of glucose produced in PS

• 5

1. For respiration

2. Converted into insoluble starch for storage

3. To produce fat or oil for storage of energy

4. To produce cellulose, which strengthens the cell wall

5. To produce amino acids for protein synthesis

Why is glucose used for respiration?

To release energy from respiration

Where does respiration occur?

In mitochondria

When does photosynthesis produce glucose?

Only in the day (when there is light)

When do plant cells respire?

All the time

What does the plant do with stored starch?

Converts it back to glucose when needed (eg at night)

What can fats and oils store in plants?

Energy

What does a plant cell wall contain and why?

Cellulose → gives it strength

What do plants need to do to make amino acids from glucose?

Absorb nitrate ions from soil

Is glucose and starch soluble in water?

1. Glucose- yes

2. Starch- no

Why is glucose not stored in plant cells?

1. Soluble in water

2. So affects way water moves into + out of cells by osmosis

Why is glucose stored as starch in plant cells?

1. Starch insoluble in water

2. Doesn’t affect water balance of plants

3. So large amounts of starch can be stored

What does this graph show about LI as a limiting factor?

1. Initially, LI was an LF

2. Then LI isn’t an LF- something else is

When LI stops being a limiting factor, what else could be a LF as this point?

1. CO2 conc

2. Temp

3. Amount of chlorophyll in leaves

How to find out which factor is the limiting factor when LI stops being one?

1. Repeat experiment

2. Change CO2 conc / temp

3. Plot graph

What does this graph show?

When CO2 conc increases, PS rate increases

• CO2 conc was a LF

At a higher conc of CO2, why isn’t CO2 conc a LF anymore?

Plenty of CO2 available at higher concs

Why are limiting factors important in farming?

To increase rate of PS using a greenhouse

Why do farmers want to increase PS rates?

Increases yield of crops they produce

Why is a greenhouse used?

To grow plants in the most suitable conditions

How do farmers increase the rate of PS?

• They light + heat greenhouses

• + Add extra CO2

Cons of increasing rate of PS using a greenhouse

Expensive

Extra cost of increasing rate of PS must be?

Justified by the increase in yield

Why do gardeners use oil burners?

Release heat + CO2 at the same time

Why are LF important in the economics of enhancing the conditions in greenhouses?

To gain max rate of PS while still maintaining profit

How does a greenhouse ensure temp isn’t a LF for PS?

Traps suns radiation as heat inside the greenhouse

How does a greenhouse ensure LI isn’t a LF for PS?

Artificial lighting provides constant light energy

What do the enclosed conditions of a greenhouse prevent?

Pests

How can you show a plant is photosynthesising?

1. Use aquatic plant

2. Place in beaker with water + collect bubbles of gas given off when photosynthesising

3. Gas will relight a glowing splint

Issue with showing a plant is photosynthesising using a land plant?

O2 = colourless gas

Evidence a plant is photosynthesing

1. O2 produced

2. Presence of starch in leaves