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Photosynthesis: 3. Plant structure and leaf structure adaptations that help!

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(3) Plant structure and photosynthesis - leaf structure adaptations that help!

  • Photosynthesis in the context of plant organs including stems, roots and leaves.

    • Water and minerals are absorbed from the soil through the roots and moved up through the plant by transpiration.

    • Wherever a plant is green, photosynthesis is taking place, at least in daylight!

    • One essential green molecule for photosynthesis is chlorophyll found in the organelles called chloroplasts.

    • Compared to the underside of leaves, you find more chloroplasts in the cells in the cells of the upper surface where the light intensity is greater.

    • The broad flat green leaves of plants exposed to light provide a large surface area for the light absorbing sites of photosynthesis - more than the thinner stem.

    • The leaves are thin so the absorbed carbon dioxide has only a short distance to diffuse to photosynthesising cells.

    • Leaves have veins (vascular bundles of xylem and phloem cells) that support the leaf and transport water and minerals to the leaf and glucose away from the leaf.

    • Epidermal tissues are the outer layers which cover the whole plant.

    • The mesophyll, between two epidermis layers, is where most photosynthesis happens in the chloroplasts - it all looks green due to the green chlorophyll molecules needed for photosynthesis (they don't absorb green light).

      • Palisade cells in the mesophyll contains lots of chloroplasts containing chlorophyll - so palisade cells are well adapted for photosynthesis.

      • The palisade cells are near the top of the leaf and exposed to the most light.

      • 'Physics note': Plants look green because the chlorophyll absorbs the blue and red wavelengths of visible light, but not the green. The green light is either reflected or transmitted so the plant tissue looks green which ever angle you view it.

  • The large surface area is provided by having broad leaves, beneath the apparently flat surface of a leaf is quite a porous layer of air spaces between the outer layers of cells - particularly on the underside of leaves - quite often the lower surface of a leaves feel rougher and 'roughness' means a more disrupted surface of a larger surface area.

The upper side of a leaf is smoother and greener - richer in chloroplasts to capture the sunlight The under side of a leaf is rougher - more 'porous' for efficient gas exchange and the veins more prominent
  • The palisade cells contain the chloroplasts containing the green pigment chlorophyll to absorb light.

  • Plants look so green because they contain a relatively high concentration of chlorophyll.

  • The upper epidermis is transparent to visible light so it can get through to the palisade layer.

  • The stomata (tiny holes in the leaf surface) facilitate gas exchange (carbon dioxide, oxygen and water vapour)

    • The tiny stoma holes in the leaves which can open and close to let oxygen and carbon dioxide in and out (both associated with photosynthesis and respiration) and water vapour out (transpiration).

  • Xylem and phloem networks of cells, transport substances around the plant e.g. sugars like sucrose and glucose from photosynthesis, and through the roots minerals (e.g. magnesium) and water for photosynthesis.

  • The tissues of leaves are adapted for gas exchange.

    • The lower epidermis contains lots of stomata (plural of stoma, pores) which let carbon dioxide directly diffuse into the leaf for photosynthesis and oxygen to diffuse out of the leaves - the gas exchange system.

    • Leaves also have a network of veins which convey water to the leaf cells and in return transport the glucose made in photosynthesis.

  • The spongy mesophyll tissue also contains air spaces that help increase the rate of diffusion of gases in and out of the leaves.

  • In the outer epidermis layer guard cells are adapted to open and close the pores of the stomata (stomatal pores) which allows gas exchange and water evaporation eg for photosynthesis carbon dioxide in and oxygen out.

    • This helps regulate transpiration and respiration and all connected with photosynthesis.

    • See transport in plants more on plant structure, function including gas exchange and leaf adaptations

  • The epidermal tissues are covered with a waxy cuticle which helps reduce the loss of water by evaporation.

  • All of the above structures mentioned must be 'connected' for the 'system to function' in a healthy plant.

  • It should be mentioned that a large percentage of the Earth's photosynthesis occurs in oceans in phytoplankton.

A summary of some particular adaptations of leaves that aid photosynthesis

Layer Adaptation Function
Upper epidermal layer thin and transparent waxy cuticle allows light through for photosynthesis
Palisade mesophyll regular shaped cells arranged in end-on, near the upper surface, maximises chloroplasts at the top of the cells enables the maximum amount of light to be absorbed for photosynthesis
Spongy mesophyll irregular shaped cells creating air spaces increases the surface are for diffusion: CO2 in, O2 out - increases efficiency of gas exchange

Leaf structure, diffusion and photosynthesis

Carbon dioxide diffuses into the leaves through the stomata and is depleted through photosynthesis.

Therefore as photosynthesis proceeds, the internal carbon dioxide concentration in the leaf is much lower than in the surrounding air, so carbon dioxide will diffuse into the leaf down this concentration gradient.

The rate of diffusion of the carbon dioxide (and any other gas) is increased by:

Increasing the surface area of the leaf - always the broadest part of any plant.

The smaller the distance the molecules have to travel as they diffuse - thin leaves with an even thinner mesophyll layer.

Keywords, phrases and learning objectives for this part on photosynthesis

In the context of photosynthesis and plant structure be able to describe the evolutionary leaf adaptations that help photosynthesis e.g. broad flat thin leaves, palisade cells containing chlorophyll, stomata allowing diffusion gas exchange of oxygen and carbon dioxide through the pores.



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