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Topic last updated: 09 Nov 2023

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    • Biology Biology
    • Ico Science Science
    • 14-16
    • 60

Cell biology, plants and photosynthesis

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Plant tissues and organs

Like animal cells, plant cells also form tissues that, in turn, form organs. In the case of plants, these organs include the stem, roots and leaves, each comprised of various tissues.

Plant tissues include:

  • Mesophyll tissue: either palisade or spongy. Both types of mesophyll tissue have a role in photosynthesis.
  • Xylem and phloem: both are involved in transport. The xylem transports water and minerals from the roots up the stem and into the leaves, and the phloem transports sucrose and other sugars from photosynthesis to sites where they are needed (such as parts of the plant which are growing).
  • Epidermal tissue: provides a protective barrier from the environment.

The role of the xylem and phloem in transport: 

Phloem: 

Phloem

The phloem facilitates the movement of cell sap (water and sugars) in any direction, to areas where it is needed. The sugars (which are the products of photosynthesis) are either stored or used as energy. This process is known as translocation, and it is an active process (meaning that it requires energy).

The phloem is made up of columns of living cells which have sieve plates (which contain pores) at the ends.

 

Xylem:

Xylem

Stomata, found on the underside of the leaf, control the movement of water and gases into and out of the leaf. This is orchestrated by the guard cells, which control whether the stomata are open or closed depending on the conditions.

When water exits through the stomata, this is known as transpiration. As transpiration occurs, more water is drawn up the transpiration stream (through the xylem). This process does not require energy.

The rate of transpiration is controlled by:

  • The temperature: the warmer the temperature the more transpiration occurs. This is because water molecules have more energy and so are more likely to evaporate and diffuse out of the stomata.
  • The light intensity: a stronger light intensity means that there is more photosynthesis. As there is more photosynthesis, the stomata are open more to allow CO2 to enter (this is controlled by the guard cells). At night (when it is dark) the stomata are closed, and so there is little transpiration.
  • The air flow: the stronger the air flow (wind) the more transpiration there is. Transpiration is controlled by the concentration gradient of water – when there is a higher concentration in the leaf compared to the outside, the water exits down a concentration gradient. When there is strong air flow, the water which exits the leaf is quickly blown away, maintaining the low concentration on the outside and allowing transpiration to take place.
  • Humidity: When it is humid transpiration is reduced as there is a high concentration of water on the outside of the leaf, and there is no concentration gradient.
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Photosynthesis