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.
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
PMG Biology 
Hello thank you
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.
Could you explain this in simpler words? I don’t understand why the air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells? and why does the layer of water speed up the process of gas exchange? and what is the gas exchange between the air space and mesophyll? I thought it was between oxygen and carbon dioxide?
Haha you can’t rush me as I am on my summer holidays and rushing is not possible at the moment! Leaf cells need carbon dioxide for the process of photosynthesis. Carbon dioxide is a gas found in air. So having air spaces in the leaf means that the air is closer to the leaf cells than if it had to move in from outside the leaf. Gases like carbon dioxide move by a process called diffusion and diffusion is affected by distance – the greater the distance, the slower the diffusion. So air spaces in the leaf mean that carbon dioxide moves into leaf cells (mesophyll cells) faster than if there were no air spaces. Diffusion of carbon dioxide is also speeded up if the gas exchange surface is moist (ie lined with a film of water) The reason for this is complicated but to keep it simple, diffusion can happen faster if the gas is dissolved in water. And finally you are right that gas exchange involves carbon dioxide moving one way (into the leaf cells) and oxygen moves the opposite way (out of the leaf cells) Hope this helps!
Thank you very much! Now I understand. This is SUPER HELPFUL! THANK YOU!
One happy customer! Have a great day.
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.
Could you explain this in simpler words? I don’t understand why the air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells? and why does the layer of water speed up the process of gas exchange? and what is the gas exchange between the air space and mesophyll? I thought it was between oxygen and carbon dioxide?
I will reply later. Good questions though!
Umm I dont want to rush you but can u reply asap? u see im a 7th grader and I dont understand this and I have to explain this to other students soooooo instead of answering my questions can u just explain i simpler words? I mean I dont want to rush you or offend u in anyway
Haha I’ve just replied to your earlier question! Good luck with explaining this topic to other students – it is a tricky one….
This site is very useful and very helpful to me and i would recommend to anyone who finds biology hard. fantastic explanation!
Thank you for your positive feedback – much appreciated!
Thanks for your positive feedback – much appreciated!
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.
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.
interesting and Satisfying
Wow, my teacher used this for science yesterday and I get why as well, it’s very useful and has detailed explaining
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.
This page is really helpful
Thank you for your feedback.?
hellow paul you are great man which made biology easy for us thanks
Haha I don’t know about that, but I’m glad you found my site useful. Good luck with all your future studies!
whats wrong if you say light is —transported— through the upper epidermis
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….
its great
what is specialised cell
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.
its helps me for my studies
thax its helpful for me
which vascular tissues ( parenchyma, fiber, sclerenchyma, collenchyma ) contain greatest number of chloroplast ?
Pretty sure it is parenchyma… Sclerenchyma are almost always dead and collechyma have a structural function for the most part…
thnks the answer helped me alot
Really good
HOW IS THE ARRANGEMENT OF THE LEAVES IN THE STEM RELATED TO ITS EFFICIENCY IN ABSORBING FOR PHOTOSYNTHESIS
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.)
Thanks you for this information it is very useful
Thank you. I’m pleased you find the website useful. Spread the word!
Thanks Paul. May you shed more light on how we describe the light independent of photosynthesis
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!
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
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.
thanks you helped me get some knowledge.
The site has been useful,i’d like to ask about the carbon (iv) oxide theory of photosynthesis.
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…..
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?
The cell wall is rigid so able to withstand the turgidity generated in the cell. The nucleus controls all the chemical activities inside every eukaryotic cell.
Thank you so much.
I will be appearing foe my biology paper in this may/june so probably i will need more help…THANKS
Good informational source
Thank you – I’m pleased you find my site useful!
how is the structure of a palisade mesophyll cell specialised to support the process of photosynthesis?
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!
Infact I cannot explain how helpful this site is!
Thanks for the feedback. I’m pleased you find the site useful.
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.
I’m really pleased some of these posts are useful to you. Please ask if anything is unclear or could be better explained.
How are the guard cells specialised for photosynthesis?
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!!
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
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!
Thankyouuuu
I’m pleased you find the site useful – keep working hard!
This site is really helpful, It has helped me answer my assignment without difficulties. Thanks.
Thank you. I’m pleased you find it useful. Keep studying and recommend it to your friends!
this information was useful for my C.I.Es .I m a student in St.paul’s pakistan.thnx
Thanks for the feedback – glad you find the site useful….
Thank u
You are welcome. I hope you find the website useful.
the epidermal cells are transparent but is it considered as an adaptation for photosynthesis?
Yes because it allows light to pass unimpeded to the palisade mesophyll beneath which is where most of the photosynthesis occurs.
Unimpeded?What of refraction of the rays?Does that affect it?
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!