Biology 4.1 Photosynthesis

4.1.1 Photosynthetic reaction

FSL: GCSE Biology Revision "Photosynthesis"

- Plants are autotrophs, meaning they produce their own food through photosynthesis. (we are heterotrophs)
- Photosynthesis is an endothermic reaction (energy is transferred from the environment to the chloroplasts by light to make glucose and oxygen).
- Leaves are where most photosynthesis takes place, in specialised mesophyll cells that contain chlorophyll.
- There is a large surface area to absorb light, and leaves are thin for a short diffusion distance.
- There are many chloroplasts, veins to carry water from the xylem and glucose to the phloem, and guard cells which open and close the stomata and regulate gas exchange.
- Photosynthesis is represented by the following reaction: Photosynthesis equation _{stolen from spec}
- This can also be written as: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- This is the opposite of the respiration reaction.

4.1.2 Rate of photosynthesis

- The law of limiting factors: The rate of that process is limited by the factor which is in shortest supply.
- Factors that affect the rate of photosynthesis are:

Light intensity: Higher light intensity = faster photosynthesis, to a point
Temperature: Most plants have an optimum temperature for photosynthesis, when enzymes work best (usually 25°C to 35°C)
Carbon dioxide concentration: More carbon dioxide = faster photosynthesis, to a point
Amount of chlorophyll*: More chlorophyll = more photosynthesis (obviously)

_{* only works as a factor if you are testing different types of plants}
- Water is NOT a limiting factor.
- These factors interact and any one of them may be the factor that limits photosynthesis.

The inverse square law

_{i hope you've done inverse proportion in maths by now}

- As light intensity increases, the rate of photosynthesis increases.
- Increasing distance = decreases light intensity.
- Light twice as far away has to cover four times as much area to be the same intensity.
- Light three times away has to cover nine times as much area to be the same intensity.
- light intensity ∝ 1 ÷ distance²

Required practical

- Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.

Method

Place a piece of pondweed in a beaker of water
Shine a lamp, at a set distance away from the pondweed, at the pondweed
Record the number of bubbles observed in three minutes
Repeat for different lamp distances

Results table

Distance between the lamp and the pondweed (cm)	Light intensity (1/d² * 1000)	Bubbles collected 1	Bubbles collected 2	Bubbles collected 3	Bubbles collected mean
5	40	49	50	52	50.3
10	10	40	35	39	38
20	2.5	21	19	20	20
30	1	10	26	11	15.7
40	0.6	2	4	5	3.7

4.1.3 Uses of glucose from photosynthesis

FSL: GCSE Biology Revision "Uses of glucose from photosynthesis"

Glucose is used in the following ways:

Respiration
Conversion into insoluble starch for storage (stored in leaves, tubers, and blubs)
Conversion into fat or oil for storage (especially useful in seeds)
Production of cellulose, which strengthens the cell wall
Production of amino acids (along with nitrate ions absorbed from the soil), for protein synthesis

Testing leaves for starch

Method

Heat leaf in boiling ethanol (to remove waxy cuticle)
Wash with water and spread onto a white tile (rehydrate sample and remove excess ethanol)
Add iodine solution from a dropping pipette
Blue-black if starch is present

Safety

Ethanol is flammable: use a beaker of hot water not a bunsen burner.

Exam Question

Without damaging the plant, suggest how starch can be removed from the leaf, and how the method allows this to happen. [5 marks]
- Place in the dark
- For a minimum of 48 hours
- No photosynthesis to produce glucose
- Existing starch stores broken down into glucose
- Used in aerobic respiration
- To release energy

Maximising Photosynthesis

- Greenhouses and labs allow growing conditions to be controlled.
- This guarantees optimum quality and yield.
- Waste CO₂ is directed into greenhouses.
- Lighting is controlled and temperature is controlled via vents and heaters.
- Blinds (for shade) [or tinted solar panels but no the point].
- Paraffin heater for carbon dioxide.
- Hydroponic systems.

Advantages and disadvantages of greenhouses

Advantages	Disadvantages
Control of light and temperature Control of CO₂ levels Control of water levels Control of mineral content of soil Control of air quality No need to plough or prepare land Turnover is fast so profits are high	Costs of electricty and gas Increased maintenance Increased energy consumption COst of expensive monitoring equipment Limited availability of space

Hydroponics

- Hydroponics is a method of growing plants in mineral water.
- The water should have a perfect balance of mineral ions instead of soil.
- It happens in the lab.
- Produces clean soil free plants away from disease and pests.
- However, it is expensive.