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What is photosynthesis?
The reaction that uses light to react carbon dioxide and water to produce glucose and oxygen. It is an endothermic reaction
What factors affect the rate of photosynthesis?
Temperature, light intensity and carbon dioxide concentration
Explain how temperature affects the rate of photosynthesis
A higher temperature means particles have more kinetic energy so the rate increases as temperature increases. But if temperature becomes too high, enzymes become denatured and the rate of photosynthesis decreases
Explain how light intensity affects the rate of photosynthesis
As light intensity increases, the rate of photosynthesis increases, but it eventually plateaus as another factor becomes the limiting factor
Explain how carbon dioxide concentration affects the rate of photosynthesis
As carbon dioxide concentration increases, the rate of photosynthesis increases, which eventually plateaus because another factor has become the limiting factor
Why is the rate of photosynthesis inversely proportional to the distance from the light source?
Light intensity is inversely proportional to the distance from the light source, so the rate of photosynthesis is directly proportional to the light intensity
What is the inverse square law?
Light intensity=1/distance²
How are root hair cells adapted to carry out their function?
They have long hair projections to increase surface area for absorption and many mitochondria to produce atp (energy) for active transport
How is the xylem adapted to carry out its function?
It has end-to-end dead cells which forms a hollow column, with no end walls which provides little resistance to passage of water. The lignified cell wall provides support
How is the phloem adapted to carry out its function?
It has sieve tube elements (with no nucleus and little cytoplasm to allow sugars to flow easily), compassion cells which contains dense cytoplasm and lots of mitochondria to provide energy for processes in both cell types
Describe the process of transpiration
Wtaer evaporates from the mesophyll and diffuses out of the stomata. Water molecules are drawn up the xylem to replace water that has been lost, causing more water to be absorbed from the soil by the root hair cells
What factors affect the rate of transpiration?
Light intensity, temperature, air movement and humidity
How does light intensity affect the rate of transpiration?
More light means more stomata are open for gas exchange and photosynthesis. An increase in photosynthesis causes more water to be needed from the soil, moving water from xylem. More water diffuses out of the stomata, increasing the rate of transpiration
How does temperature affect the rate of transpiration?
An increase in temperature means water particles have more kinetic energy. Photosynthesis increases so more water is taken from the soil, pushing water up the xylem. More water vapour diffuses out of the stomata, increasing the rate of transpiration
How does air movement affect the rate of transpiration?
An increase in air movement means a steeper water concentration gradient is maintained between air spaces in the leaf and the atmosphere. Increased rate of diffusion of water molecules from the stomata increases the rate of transpiration
How does humidity affect the rate of transpiration?
The lower the humidity, the faster the rate of transpiration because there are fewer water molecules in the atmosphere, so there is a larger concentration gradient. An increased rate of diffusion of water molecules from the stomata increases the rate of transpiration
What is translocation?
The movement of sugars up and down the plant from the source to the sink by the phloem. It requires energy
How are leaves adapted for photosynthesis?
Broad: large surface area for light absorption
Thin: short diffusion distance for gases
Vascular bundles: form a network to deliver water and remove glucose
Chlorophyll: absorbs light
How are plants adapted to live in hot environments?
Small leaves: reduce surface area so there is less water loss
Waxy cuticle: reduces evaporation, conserving water
Thick stem: more water storage
Widespread roots: large surface area to absorb water
Sunken stomata: reduces air flow and reduces water loss by transpiration
How do auxins affect plant growth?
Stimulate growth in shoots but inhibit growth in roots
Describe phototropism in a plant
Shoots grow towards the light and use positive phototropism. If the plant is in the shade, auxins accumulate and there is elongation of cells on shaded side so the shoot bends towards light
Roots grow away from light as they are negatively phototropic.
Describe gravitropism in plants
Roots grow towards gravity, so use positive gravitropism. If a plant is horizontal, auxins accumulate on the lower side of the root due to gravity and inhibit growth so the root bends downwards
Shoots grow away from gravity so use negative gravitropism (also called geotropism)
Describe the commercial uses of giberellins
Initiate germination at times of the year when they naturally wouldn’t grow, triggers flowering in plants under irregular conditions, and produce seedless fruit
Describe the commercial uses of auxins
In rooting powders to enable rapid plant cloning, weed killers to alter growth patterns in broad leaved plants, killing them
Describe the commercial uses of ethane
Stimulates fruit ripening
