tissues & organs

is an amalgamation of cells all of which perform the same function a tissue is an amalgamation of cells all of which perform the same function

an organ is a body part composed of several tissues grouped together to perform an overall function

Understand the structure and function of a mesophytic leaf (plant organ) Draw diagram page 50 Jones & Jones

upper epidermis A layer of flattened closely fitting cells at the top of the leaf. Protects the inner layers of cells. The cells have no chloroplasts to allow light through to the photosynthesising cells below. The cells secrete a waxy cuticle which helps to conserve water.

palisade mesophyll A layer of cells below the upper epidermis which are tightly packed together. Main area in the leaf for photosynthesis. Cells have many chloroplasts which can migrate to top of the cell to trap as much light as possible for PS.

spongy mesophyll A layer of cells below the palisade mesophyll which are loosely arranged with many air spaces. Air spaces facilitate the diffusion of gases to and from the palisade cells.

vascular bundles These are found in the leaf veins and are responsible for carrying substances to and from the leaf. Xylem is found above the phloem and brings water and mineral ions up to the leaf from the roots. Phloem sieve tubes translocate sugars produced in photosynthesis away form the leaf.

lower epidermis A layer of cells at the bottom of the leaf similar to the upper epidermis. It secretes a waxy cuticle but this is generally thinner than that on the upper epidermis. The lower epidermis contains many stomata which are surrounded by guard cells. The guard cells change shape to open and close the stomata and so control the movement of gases into and out of the leaf and the loss of water by transpiration.

The stomata shut at night when PS does not occur. The leaf is an organ which is adapted to maximise photosynthesis and minimise transpirational water loss. During daylight the stomata are open to allow CO2 to enter the leaf for PS. The open stomata allow water to evaporate from the mesophyll cells and to diffuse out of the leaf. There must be a compromise between allowing CO2 to enter and water to leave. In very warm conditions the stomata will close and PS stop to prevent the plant from wilting. The stomata shut at night when PS does not occur.

Increasing exposure to light Function Adaptation Increasing exposure to light Large surface area Leaf supported by stem & petiole Thin No chloroplasts in epidermis Palisade cells arranged end on reducing no. cell walls light passes through Chloroplasts in palisade cells arranged broadside on Efficient gas exchange Stomata in lower epidermis Air spaces in spongy mesophyll Transport of materials Xylem within short distance of every mesophyll cell supplies water for PS Phloem within short distance of all mesophyll cells to carry away products of PS