Structure of leaves

External structures of a leaf Veins Lamina Mid rib Show them a big leaves Briefy identify major parts of a leaf. Ask: what are the 2 major components of a vascular bundle. petiole Stem

External structure of a leaf Thin and flat lamina: - Allows rapid diffusion of carbon dioxide into the leaves. - Capture maximum amount of sunlight. Branching network of veins: - Supply water and mineral salts. - Remove sugar products of photosynthesis. Leaves held out at an angle by the petiole, arranged to prevent overlapping: - Able to achieve maximum exposure to light. show them 2 different kinds of branches

Internal structure of the lamina 1) Upper epidermis 2) Mesophyll 3) Lower epidermis

Internal structure of lamina Upper and lower epidermis Single layer of cells Covered with waxy cuticle Prevent excessive evaporation of water Transparent to allow sunlight to pass through

Internal structure of lamina Tell the difference between the spongy and the palisade layer Use white board

Compare and Constrast Palisade mesophyll Spongy Mesophyll Shape of cells Long, cylindrical Irregular in shape Packing Closely packed Loosely packed Air spaces Absent Present Chloroplasts Numerous Less Vascular bundles

Mesophyll layer More chloroplasts found in the palisade layer more light energy can be absorbed nearer to the leaf surface. Interconnecting air spaces Rapid diffusion of carbon dioxide into the mesophyll cells. Vascular bundles contain of xylem and phloem. Xylem transport water and mineral salts from roots to leaf. Water move out of veins and move through the cells by osmosis. Phloem transport sugar away from leaves.

Stomata (s: stoma) Minute pores found at the lower epidermis. Size of the pores controlled by guard cells. Contains chloroplasts for photosynthesis

Opening and closing of stomata Stomata found on the underside of the leaves functions: http://www.youtube.com/watch?v=cFX4JrsPaUs&feature=related

Functions of stomata and guard cells Stomata allows diffusion of CO2 , O2 and water vapour into and out of the cells. Guard cells regulates the size of the stomata. Eg: on a very hot day, guard cells become flaccid, stomata closes to reduce water loss.

Opening of stomata Day time Photosynthesis generates chemical energy. Energy pumps potassium (K+) ions from the epidermal cells into the guard cells. Water potential in the guard cell decrease. Water enters by osmosis. Guard cells swells, becomes turgid and pull the stomata open.

Closing of stomata Night time Potassium ions diffuse out of the guard cells. Water potential in the guard cells increases. Water leaves by osmosis. Guard cells becomes flaccid and stomatal pore closes.

night time day time Guard cells turgid Stoma Opens Guard cells flaccid Stoma Closes Stoma

How does CO2 enter the leaf? Lower concentration of CO2 higher concentration of CO2 CO2 diffuses in