OCR B4 Revision Notes

B4 OCR Revision Guide – Its Green World

·      Epidermis – A thin transparent layer allowing light through, but blocks water loss. There is an upper and lower epidermis on the top and bottom of the plant appropriately.

·      Palisade Cells – Contains large amounts of chloroplasts in order to absorb light.

·      Chloroplasts – Found down the sides of the palisade cells; contain the chlorophyll pigments for photosynthesis.

·      Spongy Mesophyll – Has large air spaces around in for gaseous exchange.

·      Guard Cells – These surround the Stomata on the bottom of the leaf and are effected by turgor pressure.

·      Stomata – Found mainly at the bottom of the leaf (though sometime at the top) in scarce quantities. They open when the leaf has a high concentration of water as the guard cells become turgid and force the gap to become wide. If there is a lack of water, the guard cells become flaccid and the stomata close. They can’t stay closed for too long though as they are also the hole used for gaseous exchange.

·      Waxy Cuticles – Some plants have these on top of the upper epidermis, again to stop water loss. It is an impermeable surface so that water runs straight off from outside, and cant escape from the inside.

·      Xylem  - These are one of the “veins” in a plant, which are used to transport both water and minerals. The xylem only flows one way (from bottom to top). It is made up of a chain of dead cells with a hollow lumen (central cavity) so that the water can run straight through. Any minerals a plant needs will have dissolved into…