Chapter 9: Leaves – plant organs for photosynthesis
Chapter 9: Leaves – plant organs for photosynthesis
- Many types: Bud scales, Cotelydons, petals of flowers
- Internal structure and functions of the green foliage leaves
Functions of Green leaves:
- Green leaves are the main place for photosynthesis (well adapted)
- Stomata on leaves, responsible for gaseous exchange – release of oxygen as product of photosynthesis and carbon dioxide –
- Important function of leaves is transpiration
- Monocot and Dicotyledonous leaves and be distinguished by the pattern of the veins.
- Vascular bundles transport water and mineral salts to leaves for photosynthesis/ other metabolic reactions
- Vascular bundles transport organic food from the leaves to other parts of the plants
- The venation of leaves, a characteristics network pattern in dicotyledonous leaves
- Monocot leaves; parallel venation
Anatomy of the leaves:
- Anatomy of leaves differ depending on the type of leaf studied
- Dorsiventral dicotyledonous leaves are the only ones studied now
Tissues of the leaf:
- Dorsiventral flattened
- The leaf is flat from bottom to top, definite upper and lower surface
- Upper surface is usually smooth
- Lower surface is usually rough, veins are clearly visible
- Adaxial for the upper/ top surface
- Abaxial for the lower/ bottom side
- Leaves are covered with a layer of epidermis cells
- A waxy cuticles decreases transpiration through the epidermis
- Epidermis cells do not contain chloroplast
- Guard cells of the stoma contain chloroplast
- Stomata are found on the adaxial/ adaxial surfaces
- In Mesophytes the stomata are generally found on the abaxial surface
- The epidermis cells can be modified à unicellular or multicellular (trichomes)
- Below the epidermis on the adaxial side there are layers of palisade parenchyma cells found
- These cells are cylindrical/ contain large numbers of chloroplast
- Few or no intercellular spaces
- Maximal light penetration in the tissue
- Large number of chloroplast
- Main photosynthesising tissue in the leaf
- Rest of the leaf is filled by spongy parenchyma tissue
- Cells are loosely arranged
- Large intercellular spaces
- Tissue has a spongy appearance
- Spongy parenchyma cells contain chloroplast – less than parenchyma cells –
- Light penetration is not effective in spongy parenchyma cells
- Large intercellular spaces allow gaseous exchange, important àrespiration, photosynthesis, transpiration
- Tissue between the adaxial/ abaxial epidermal layers are collectively known as the mesophyll – consisting of the palisade parenchyma/ spongy parenchyma
- Leaves have a large number of vascular bundles/ veins – extend through the leaf in a specific pattern –
- Main vein/ lateral veins strengthen the leaf, main function is transport
- Veins transport water/ dissolve mineral salts
The main function of the leaf is photosynthesis:
- During photosynthesis radiant energy is absorbed by chlorophyll – this is then converted to chemical potential energy; then bound into a carbohydrates molecule –
- The chloroplasts use carbon dioxide and water
- Carbon dioxide enters the leaf via the stomata – diffuses to the photosynthesising cells
- Water reaches the photosynthesising cells via the vascular bundles
- Oxygen leaves the leaf because of diffusion via the stomata
Adaption of leaves for the process of photosynthesis:
- Leaf of a plant is well adapted
- Tissue types have their own structure/ function
- Together they function as a unit in the organ of photosynthesis
- Leaf is flattened, offers a large surface area for absorption
- Leaf is thin, helps with sunlight penetration
- Leaf is borne in such a way on petiole/ stem – this exposes maximum surface to sunlight –
- Epidermal cells are transparent; this allows light to penetrate to deeper layers; large numbers of chloroplast
- Vascular bundles form a network through the leaf – this helps water from the roots reach all the photosynthesising tissues. The product of photosynthesis can be taken away –
- Stomata on the leaf surface make gaseous exchange possible
- Palisade parenchyma – large numbers of chloroplast, situated on the adaxial side of the leaf for maximum light absorption
- Palisade parenchyma cells; densely packed, few intercellular spaces to facilitate the diffusion
- Spongy parenchyma cells; loosely packed, this makes gaseous exchange easier