Biology, 4a.


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Plant cells have chloroplasts (for photosynthesis), vacuole and a cell wall.

Chloroplasts absorb light energy for photosynthesis: water + carbon dioxide = oxygen + glucose. The water is taken in through the roots, and the carbon dioxide through the stomata pores. These pores also let oxygen escape, and even water, (transpiration).

Photosynthesis occurs mainly in the leaves. They are adapted for efficiency: 1. They have broad leaves, so there is a larger surface area to absorb light. 2. They are thin, so CO2, and H2O only have a short distance to travel, in order to reach the photosynthesising cells. 3. contain chlorophyll, the pigment that absorbs light. 4. network of veins for support, deliveringwater and nutrients to every part of the leaf, and taking away food. 5. they have stomata on the bottom, for gas exchange.

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Leaf structure.

The main parts of a leaf are; upper and lower epidermis; palisade and spongy mesophyll layers; vein; waxy cuticle;  stomata; air space; guards cells; chloroplasts. (make sure you can locate these parts).

The structure of a leaf is adapted for photosynthesis: 1. upper epidermis is transparent, so the light can pass to the palisade layer. 2. thepalisade layer contains most of the chlorophyll, so they are near the top, to get more light. 3. air spaces in the spongy mesophyll allow diffusion, between stomata and photosynthesising cells. 4. they have a big internal surface area to volume ratio.

The structure of a leaf palisade cell is related to its function. 1. packed with chloroplasts for photosynthesis. 2. tall shape for big surface areaexposed down, to absorb CO2 from air in leaf.

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Diffusion in Leaves.

Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration.

Diffusion happens during photosynthesis. CO2 diffuses into the leaf, and oxygen and water vapour diffuse out of the leaf. Oxygen is a waste product of respiration, and so diffuses out through the stomata.

At night, there is no light for photosynthesis, so plants have to respire. lots of carbon dioxide is made and oxygen is used up.

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