Leaves and Photosynthesis 

Leaf Structure

Learning Intentions - Leaves and Photosynthesis

3. Identify and explain the structure and functions of a leaf

Leaf Structure 

Why are leaves green

Leaves are green because the chlorophyll reflects green light

Chlorophyll  absorbs Blue and Red light  (Infra-red is also reflected)

Chlorophyll is kept small organelles called chloroplasts

There are more chloroplasts, and thus chlorophyll, at the top of a leave than the bottom, this is why the top is greener.

The absorbed  blue and red light energy is used by the chloroplasts to split water and carbon dioxide molecules up, they then rejoin to make oxygen and glucose

Have a play with the Virtual Photosynthesis simulator

Palisade Mesophyll Cells - Why so upright?

There are 2 types of mesophyll cells - Palisade and Spongy

The Palisade Mesophyll Cells are were most Photosynthesis occurs

They are the top layer of cells after the epidermis

They are at the top of the leaf to get the most light

They are upright to pack as much chloroplast into the space as possible

They are also upright to maximize the surface area for photosynthesis by absorbing most of the light, and reflecting a small amount

In the diagram you can see the advantage to being upright. Any unabsorbed light can bounce (reflect) to the neighbouring cell so it can then be absorbed. A small amount might not be, it will the bounce off to the next cell to be absorbed by the chlorophyll. In the diagram below you will see the change in colour for each reflection - this is to show that only a small amount is reflected with most of the energy being absorbed 

Any light that gets past the Palisade Mesophyll Cells will be absorbed by the Spongy Mesophyll Cells which also photosynthesis (just not as much as the Palisade Mesophyll Cells)

Spongy Mesophyll Cells - Where's the Air?

The haybales are like the mesophyll cells, the space to walk and crawl is like the 'air spaces' in the spongy mesophyll

Leaves are green because the chlorophyll in the chloroplasts absorb Blue and Red light, but green is reflected

This blue and red light energy is used to split water and carbon dioxide molecules up, they then rejoin to make oxygen and glucose

To get the carbon dioxide molecules in to the leave they move through stomata.

 To get carbon dioxide from the stomata to the photosynthesizing palisade cells, the carbon dioxide must move through air spaces in the leaf. The area that contains the air spaces is called the spongy mesophyll.

Oxygen created by photosynthesis will also move through the spongy mesophyll and out through the stomata. 

With oxygen and carbon dioxide moving through the stomata this is the same, but if reverse to our own breathing. Thus,  the stomata is like the nostrils of the plant. 

The air spaces are kept open by the network of cells, acting like haybales in a haybale maze - these cells are called the Spongy mesophyll cells

With  the leaf getting warm due to the sun, water could evaporate from it, to minimize this, water can only leave through the stomata. Evaporation of the water from the stomata, causes water to then be sucked into the leaf through the straw like xylem, from the roots. This process is called transpiration (like the transport of water through breathing)

To prevent water from leaving through the leave, from any other area, the top of the leaf, where there is the most heat energy, has a waxy layer that water can't get through. This is called the waxy cuticle. 

Just like our skin, the outside layer of cells is called the Epidermis

Water gets to the leaf through the xylem. 

Sugary water leaves the leaf like fanta, through the phloem. 

Leaves are green 


Structure of the leaf from the top down

Cuticle - waxy to prevent water getting out

Epidermis - outside most layer of cells

Mesophyll cells - these are the cells of the leaf

All mesophyll cells contain chloroplasts

Palisade mesophyll - these are at the top of the leaf to capture as much of the sun light as possible, and are the reason that the top of the leaf is more green than the bottom of the leaf. These cells do the photosynthesizing

Spongy mesophyll - these cells do some photosynthesizing. However, their main role is to provide structure to the leaf to allow water and air to move within the leave

Air channels - the spongy mesophyll cells act like the walls of a channel that allows carbon dioxide to diffuse from the air to the palisade mesophyll for photosynthesis

Stomata - these are the nostrils of the leaf. They are underneath the leaf. They are holes. They are made up of 2 Guard cells. The guard cells start to close the stomata if there is not enough water - so they regulate water loss from the plant

Vascular Bundle - contains the Vascular Tissues - Xylem and Phloem




In the Vascular Bundle - Vascular Tissue

Like our own Veins and Arteries. The plant has Liquid filled Vessels. 

The vessels taking water from the roots up to the leaves are called Xylem

The vessels taking sugar from the leaves to the rest of the plant are called Phloem

Transpiration

Two Vessels: Xylem and Phloem

Xylem = Just water = water xylophone

Phloem = Sugar = Fanta = Flow'em

Transpiration  process

Transpiration its like a giant straw

Transpiration - like combining perspiration and respiration - its the transport of WATER


Translocation is the opposite 

Translocation is how sugar moves from the leaves, through the Phloem, and out to the rest of the plant, to its very tip, and down to its smallest root hair cells

Recap of Learning Intentions - Check you can do all of this

My Drawing of how the structure of the leaf facilitates photosynthesis

Helpful Notes

Germination = Development of a seedling from a seed following a period of dormancy

Another version of my drawing