6.2) Leaf structure
Tip: allows the water to drip off and not block light or damage leaf.
Mid-rib: contains the xylem and phloem.
Vein: contains the xylem and phloem.
Lamina: the site of photosynthesis and production of useful substances.
Cuticle: Made of wax, waterproofing the leaf. It is secreted by cells of the upper epidermis.
Upper epidermis: These cells are thin and transparent to allow light to pass through. No chloroplasts are present. They act as a barrier to disease organisms.
Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis
Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out photosynthesis and store nutrients.
Vascular Bundle: it is a group of phloem and xylem vessels that transport water and minerals to and from the leaves. (called translocation)
Lower epidermis: This acts as a protective layer. Stomata are present to regulate the loss of water vapour (called transpiration). It is the site of gaseous exchange into and out of the leaf.
Stomata: Each stomata is surrounded by a pair of guard cells. These can control whether the stoma is open or closed. Water vapour passes out during transpiration. Carbon dioxide diffuses in and oxygen diffuses out during photosynthesis.
Adaptation of leaves for photosynthesis:
- Their broad, flat shape offers a large surface area for absorption of sunlight and carbon dioxide.
- Most leaves are thin and the carbon dioxide only has to diffuse across short distances to reach the inner cells.
- The large spaces between cells inside the leaf provide an easy passage through which carbon dioxide can diffuse
- There are many stomata in the lower surface of the leaf. These allow the exchange of carbon dioxide and oxygen with the air outside.
- There are more chloroplasts in the upper (palisade) cells than in the lower (spongy) cells. The palisade cells, being on the upper surface, will receive most sunlight and this will reach the chloroplast without being absorbed by too many cell walls
- The branching network of veins provides a good water supply to the photosynthesising cells. No cell is very far from a water-conducting vessel on one of these veins.