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THE MAJOR PHOTOSYNTHETIC ORGAN OF GREEN PLANTS Leaf Structure THE MAJOR PHOTOSYNTHETIC ORGAN OF GREEN PLANTS This typical dicotyledonous sycamore leaf is the major photosynthetic organ for the sycamore tree Dicotyledonous leaves are structurally adapted for their photosynthetic role

THE DICOTYLEDON LEAF The palisade layer of cells located close A section through the leaf from upper to lower surface reveals a characteristic arrangement of tissues The palisade layer of cells located close to the upper epidermis are the principal photosynthetic cells of the leaf These palisade cells are packed with chloroplasts chloroplasts are the site of photosynthesis

STRUCTURE OF THE DICOTYLEDON LEAF waxy cuticle upper epidermis Layer of palisade mesophyll cells – main photosynthetic layer xylem vessel Spongy mesophyll layer; layer of irregular shaped cells with numerous air spaces substomatal air space lower epidermis stoma guard cell thin cuticle

STRUCTURE OF THE DICOTYLEDON LEAF UPPER EPIDERMIS PALISADE CELLS STOMA AND GUARD CELLS SPONGY MESOPHYLL CELLS XYLEM INTERCELLULAR AIR SPACES PHLOEM MAGNIFY PARENCHYMA LOWER EPIDERMIS TS lamina and midrib of a privet leaf (Ligustrum) – a typical dicotyledon STRUCTURE OF THE DICOTYLEDON LEAF

STRUCTURE OF THE DICOTYLEDON LEAF UPPER EPIDERMIS PALISADE CELLS LEAF VEIN INTERCELLULAR AIR SPACES LOWER EPIDERMIS SPONGY MESOPHYLL CELLS GUARD CELL STOMA

ADAPTATIONS OF THE DICOTYLEDON LEAF FOR PHOTOSYNTHESIS The colourless flattened cells of the epidermis and the colourless protective waxy cuticle readily allow light to pass through the leaf surface to the photosynthetic tissue The closely packed palisade cells with their numerous chloroplasts maximise light absorption for photosynthesis Numerous transport tissues permeate the leaf structure allowing water to be efficiently delivered to the photosynthetic cells The extensive network of air spaces in the spongy mesophyll layer provides for an easy passage of gases to and from the palisade cells and an efficient gas exchange system via the stomata Stomata are the sites of gas exchange and their opening and closing is controlled by specialised epidermal cells called guard cells – such regulation allows for efficient gas exchange while, at the same time, reducing water losses through transpiration

According to Fick’s Law: ADAPTATIONS OF THE DICOTYLEDON LEAF FOR PHOTOSYNTHESIS The leaf is very thin and presents a large surface area for maximising light absorption and gas exchange According to Fick’s Law: Rate of diffusion = surface area x difference in concentration thickness of the diffusion barrier The large surface area presented by dicotyledon leaves and the large differences in concentration for oxygen and carbon dioxide between the leaf interior and the external environment, produce a high value for the top line of this equation The thinness of the leaf, providing short diffusion paths for gases from the environment to the photosynthesising cells of the palisade layer, produces a low value for the bottom line of the equation The consequences of these leaf features allow for a rapid rate of diffusion of CO2 gas to the photosynthesising cells – a feature essential for efficient photosynthesis

MORE ABOUT PALISADE CELLS Palisade cells are well suited to their role as the principal photosynthesising cells of the leaf A PALISADE CELL vacuole containing cell sap nucleus cell wall cell surface membrane chloroplasts The column-shaped nature of the cells allows for close packing of the palisade layer close to the surface of the leaf, and each cell possesses numerous chloroplasts The chloroplasts are able to move freely around the cytoplasm The chloroplasts have the ability to orientate themselves in positions that maximise light absorption in dim light and reduce potential damage when light intensities are very high

MORE ABOUT PALISADE CELLS IN DIM LIGHT IN INTENSE LIGHT In dim light, chloroplasts tend to aggregate at the top surface of the cell and to orientate themselves so as to display a large proportion of their surface area to the incoming light rays In intense light, chloroplasts aggregate at the lower end of the cell and orientate themselves in a vertical position. This behaviour reduces the chances of damage to the chloroplasts through bleaching

CLOSED STOMA OPEN STOMA STOMATA AND GUARD CELLS Stomata are the sites of gas exchange between the leaf and the environment Stomata are the pores that allow for exchange of gases and their size is regulated by specialised epidermal cells called guard cells CLOSED STOMA OPEN STOMA epidermal cells guard cells containing chloroplasts Numerous stomata are found in the lower epidermis of leaves although they may be present, in fewer numbers, in the upper epidermis The large number of stomata, together with the regulation of their size by guard cells, allows for efficient gas exchange at the interface between leaf and environment; this facilitates rapid uptake of CO2 for use by the photosynthetic cells during daylight hours

thick, inelastic inner wall of guard cell stomatal pore thin, elastic outer wall of guard cell epidermal cell

The leaves of many typical sun and shade plants display adaptations that enable them to thrive in their respective environments thick cuticle to reduce transpiration losses in the more intense sunlight thin cuticle more than one palisade layer with smaller chloroplasts single palisade layer with larger chloroplasts to maximise light absorption thicker leaf; greater distance between upper and lower epidermis thinner leaf; shorter distance between upper and lower epidermis

Autotrophs manufacture their own food using simple inorganic sources and, in doing so, provide the resources on which heterotrophs depend Approximately 600 billion tonnes of carbon dioxide is converted into organic food by the autotrophs each year, with 400 billion tonnes of oxygen being released into the environment All life is centred around the activities of the autotrophs, with photosynthesis being the central process for powering our existence