Leaf structure and Adaptations for Photosynthesis: Grade 9 Understanding for IGCSE Biology 2.21

The leaf is the organ in a plant specially adapted for photosynthesis.  You need to understand the structure of the tissues in a leaf together with their functions.

leaf  struture  cuticle mesophyll stoma

Upper Epidermis:  this is the tissue on the upper surface of the leaf.  It produces a waxy layer, called the cuticle, which is not made of cells but is a waterproof barrier to prevent excessive evaporation through the hot upper surface of the leaf.  The upper epidermis cells have no chloroplasts so light passes through them easily.

Palisade Mesophyll:  this tissue is where 80% of the photosynthesis takes place in the leaf.  The palisade cells have many chloroplasts in their cytoplasm and the box-like shape and arrangement of these cells ensures they are packed tightly together.

Spongy Mesophyll: this tissue contains large air spaces which are linked to the atmosphere outside the leaf through microscopic pores called stomata on the lower surface.  Spongy mesophyll cells also contain chloroplasts and photosynthesis occurs here too.  The air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells and the fact that these cells have fairly thin cell walls which are coated with a film of water together means that gas exchange between air space and mesophyll is speeded up.

Lower Epidermis is the most dull tissue in the leaf.  The only interesting thing about it is that it contains specialised cells called guard cells which enclose a pore called a stoma.  Carbon dioxide can diffuse into the leaf through the stomata when they are open (usually at day time) and water evaporates out of the stomata in a process called transpiration.

Adaptations of a Leaf for Photosynthesis

  • Large Surface Area – to maximise light harvesting
  • Thin – to reduce distance for carbon dioxide to diffuse through the leaf and to ensure light penetrates into the middle of the leaf
  • Air Spaces – to reduce distance for carbon dioxide to diffuse and to increase the surface area of the gas exchange surface inside the leaf
  • Stomata – pores to allow carbon dioxide to diffuse into the leaf and water to evaporate out (transpiration)
  • Presence of Veins – veins contain xylem tissue (carries water and minerals to the leaf from the roots) and phloem (transports sugars and amino acids away from the leaf)
  • Chloroplasts – mesophyll cells and guard cells contain many chloroplasts.  These organelles contain the light harvesting pigment chlorophyll and are where all the reactions of photosynthesis occur


  1. Pingback: Start of 2015-16 school year – welcome back! | PMG Biology
      • Paul Gillam

        I imagine there must be a small amount of reflection of the rays at the surface but because there are no chloroplasts in the upper epidermis, very little of the incident light is absorbed. Refraction of the rays must happen as well but cannot be significant as any refracted light will still pass through the upper epidermis into the palisade mesophyll. Hope this helps!

    • Paul Gillam

      Great question. Guard cells’ role in photosynthesis is an indirect one – photosynthesis does not happen to a significant extent in a guard cell. But guard cells do allow stomata to open and close and open stomata allow carbon dioxide to diffuse into the air spaces in the leaf during the day. How are guard cells adapted to allow stomata to open or close? Well they are the only epidermis cells in the leaf that possess chloroplasts and they have a sausage-shape with an unusual cell wall such that when they become turgid, they bend and the stoma opens. Hope this short answer helps!!

      • Maryam

        I thought it had to do with absorption of water by the guard cell and the elasticity difference of the walls of the guard cell

      • Paul Gillam

        Yes you are right but that is also what I said in the answer to the question. A plant cell becomes turgid when it takes in water by osmosis and the way cellulose fibres are laid down in the cell wall of these sausage-shaped cells causes the stoma to open up when a guard cells takes in water. Hope this helps!

  2. Prince

    Hi Paul huge fan and this really helped me in my biology assignment. You have really helped me.I thought it would be hard to understand coz am 13 and my vocabulary aint that good. Thanks again.

    • Paul Gillam

      Well the main thing is that palisade mesophyll cells are packed full of chloroplasts. They also show “cytoplasmic streaming” which is a process in which the cytoplasm rotates around the cell so every chloroplast is exposed to the same high light intensity at the top of the cell. Hope this helps!

  3. Rocks

    Thankyou,this site is really heloful,
    Well which part of the epidermal cells resists the turgor pressure of the cell and control the activities of the cell?

      • Rocks

        Thank you so much.
        I will be appearing foe my biology paper in this may/june so probably i will need more help…THANKS

    • Paul Gillam

      Good question. I’m no expert on this but my understanding is that it has recently been shown that CO2, as well as being reduced in photosynthesis to carbohydrate, as a hydrogencarbonate ion also plays a role in the electron transport chain in the light dependent reactions as an acceptor of protons from the splitting of water.

      But this is definitely way more complex than might ever be needed for IGCSE Biology…..

    • Paul Gillam

      That is outside the scope of this blog I’m afraid. I want to keep this material to IGCSE Biology content and light independent stages of photosynthesis are only needed at A level and beyond…. Apologies!

    • Swaroop

      Any anabolic reaction (synthetic) requires energy. So, does the photosynthesis. In the light dependent phase of photosynthesis, energy from the sunlight is converted into energy currency , ATP and stored. This energy is used to produce chemical energy ( Glucose ) during the light independent phase of photosynthesis

      • Paul Gillam

        I agree with this. The only slight correction is that ATP is not stored in the chloroplast – it is used up as soon as it is made! But otherwise I think you are correct.

    • Paul Gillam

      Leaves are arranged in a position to maximise absorption of sunlight. This means there is as little overlap between leaves as possible so that as many photons of light as possible hit a leaf. (Incidentally this is why it is so dark at ground level in woods/forests even on a sunny day: the tree has ensured that as much light as possible has been absorbed in the canopy by the leaves.)

  4. Elias Bekele

    which vascular tissues ( parenchyma, fiber, sclerenchyma, collenchyma ) contain greatest number of chloroplast ?

    • Paul Gillam

      A specialised cell is a cell in a multicellular organism that has a specific function for example a nerve cell (neurone), liver cell, skin cell etc. Cells become specialised in the process of development.

    • Paul Gillam

      The only thing that’s wrong is that “transported” makes it sound like the plant is actively transporting the light through the epidermis. “Transmitted” is a better word but it’s a small point I would say….

    • Paul Gillam

      I am very pleased you and your teacher find this site useful. Please spread the word amongst your classmates as I want as many people as possible to be able to use my posts to help with their Biology.

  5. Maaz Asim

    Sorry, I was on the CORMS tag and I found that there was no comment section there. Sorry for intruding I am a year 9 pupil and was wondering that you indirectly praised DORIC, is that a general technique used in the Biology Curriculum for a) GCSE b) IGCSE. As well as that please can you tell me if DORIC is used frequently in other schools near you because, in my school, we have more traditionalists than liberalists.

    • Paul Gillam

      Thanks for your comment. I don’t know much about other exam boards other than IGCSE so I think that the DORIC shortcut for answering “design an experiment” questions only really applies to this one exam board. But the principles of how to carry out a valid and reproducible experiment are the same in every context. I hope this answers helps.

  6. Anonymous

    This site is very useful and very helpful to me and i would recommend to anyone who finds biology hard. fantastic explanation!

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