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Topic 6 – Plant structure and their Function CGP Topic 6
Structure of a Leaf and Adaptations of Leaves
1. Leaves are broad, so there's a large surface area exposed to light which is needed for
photosynthesis.
2. The palisade layer has lots of chloroplasts. This means that they're near the top of the leaf where
they can get the most light.
3. The upper epidermis is transparent so that light can pass through it to the palisade layer.
4. The xylem and phloem form a network of vascular bundles, which provide the leaf with water for
photosynthesis and take away the glucose produced. They also help support the structure.
5. The epidermal tissues are covered with a waxy cuticle, which helps to reduce water loss by
evaporation.
6. The lower epidermis has lots of stomata, which let CO2 diffuse directly into the leaf.
7. The spongy mesophyll tissue contains air spaces which increase the rate of diffusion of gases into
and out of the leaf's cells.
Why do some plants lose their leaves in winter? Why don’t some plants lose their leaves?
• Water loss is dangerous to plants in cold climates – when the ground is frozen they can’t get water,
but still lose it from their leaves (and stem etc.)
• So they lose leaves to prevent disastrous water loss
• Growth would be slower in winter due to low temperature anyway
• Plants from warmer countries don’t need to lose their leaves
Photosynthesis
The process by which green plants and some other organisms use sunlight to synthesize nutrients from
carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and
generates oxygen as a by-product.
, Glucose
• Some is used to make complex longer molecules that plants need to grow –cell walls, proteins, DNA
• Some is stored as starch – an energy reserve
• The plant's biomass is the mass of living material in an organism
• Some is used in respiration to transfer energy for biological processes
Oxygen
• A waste product used by plants (and humans) in respiration
Photosynthesis takes place in chloroplasts in green plant cells – they contain pigments like chlorophyll that
absorb light. Energy is transferred to the chloroplasts by light. Photosynthesis is endothermic – this means
energy is transferred from the environment in the process. Photosynthesis happens in the leaves of all
green plants – this is largely what the leaves are for.
Test for the presence of starch in leaves via an iodine test – it will turn blue black:
1. Half fill a boiling tube with hot water. Place leaves in boiling tubes
2. Add boiling tube with leaves to water bath for 5 mins
3. Pour away water from boiling tube and add ethanol
4. Add boiling tube back to water bath for 5 mins
5. Pour away ethanol and add hot water. Leaves 1 min per leaf to soften
6. Place leaf on white tile and add iodine. Note the colour change.
The leaf that has been in the dark will have no starch compared to the one that is in the light.
What is a limiting factor?
A limiting factor is something that stops the reaction from happening any faster.
Effect of Light Intensity on Photosynthesis
Light provides the energy needed for photosynthesis.
As light intensity increases, photosynthesis gets faster.
After a certain point, if you increase the light intensity, it won't
make any difference. Something else is a limiting factor.
There’s enough light, but there might not be enough CO2 or
it’s too cold.
Effect of Carbon Dioxide Concentration on Photosynthesis
CO2 is one of the raw materials needed for photosynthesis. So the
rate of photosynthesis increases as the carbon dioxide
concentration increases.
But after a certain point, the plant has more than enough carbon
dioxide and something else is a limiting factor – perhaps the
temperature is too cold (or hot) or there’s not enough light.
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