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Science 10 ­ Biology
Leaf Tissues and Gas Exchange
Pg. 309 ­ 314
There are specialized cells and tissues in the leaf of a plant that function in providing the gases needed for photosynthesis and eliminating the gases produced by cellular respiration.
A: Mesophyll tissue (ground)
B: vein (vascular)
C: Guard cells (dermal)
D: Cuticle (dermal)
E: Stoma
F: Palisade layer (ground)
G: Xylem (vascular)
H: Spongy layer (ground)
I: Phloem (vascular)
J: Air Spaces
K: Epidermis (dermal)
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There are three types of cellular tissues that make up the leaf of a plant:
1. Dermal Tissue: (pg. 309)
o Lines the top and underside of the leaf
o Specialized cells called guard cells form tiny openings called stomata that allow gas exchange to happen easily.
o Guard cells that open can allow water vapour and O2 in the leaf out and CO2 from the air into the cell
§ Why does it make sense that light would stimulate the guard cells to open up and let gases in/out.
o It would not be efficient to have the stomata open all the time. Guard cells control whether the stomata is open or closed.
http://www.tutorvista.com/content/biology/biology­iv/plant­water­relations/opening­closing­stoma.php
o When light hits the leaf, the guard cells are stimulated to transport potassium ions (K+) into the cell by active transport. o When potassium enters the guard cells, water follows into the cell which makes the guard cells enlarge and open up.
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o In dry conditions, when water is not available, K ions exit the guard cell and the water follows. The guard cells become limp and the stomata close. This prevents the plant from dehydration.
o Water vapor leaving the plant is called transpiration.
o Stomata are found on the underside of leaves. Do you think that more stomata would be found in a desert plant, or in a herbaceous plant like the geranium? Why?
o Gas exchange in larger plants, like the birch tree can also occur in the stem or trunk. § Lenticels are slashes on the trunks of trees that are natural openings for gas exchange. That is why stripping the bark from birch trees all the way around the trunk will kill the tree.
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2. Ground Tissue: (pg. 311 ­ 313)
o Between the upper and lower dermal tissue of the leaf is the ground tissue. These tissues are called mesophyll.
o There are two types of mesophyll tissue:
i. Spongy mesophyll tissue ­ § less rigid cells that allow gas exchange throughout the leaf
§ most gas exchange occurs by diffusion
ii. Palisade mesophyll tissue ­ § cells that are tightly packed together and are responsible for photosynthesis.
3. Vascular Tissue: (pg. 313)
o Provides the leaf with water needed for photosynthesis. It also removes sugar formed in photosynthesis.
o The leaf “veins” contain the xylem and phloem tissues that are bunched together to form the vascular bundle.
o The xylem transports water and dissolved minerals from the roots to the leaves.
o The phloem transports glucose, made during photosynthesis, from the leaves down to the rest of the plant. Sometimes, extra sugar is stored in the roots. (ex. Potatoes, carrots, beets)
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Water Transport ­ From Root to Leaf (pg. 316 ­ 320)
http://www.pearsoned.ca/school/science11/biology11/sugartransport.html
As you know, water moves from the roots of the plant up into the leaves where it is used for photosynthesis. This movement occurs against the downward force of gravity.
There are a number of ways the plant makes this happen:
1. Cohesion of water molecules ­ o Water is a polar molecule, this means it has positive and negative ends.
o Because opposite charges attract each other, water molecules are attracted to each other. This attraction binds them together and creates an inward pull.
o This is the reason for the surface tension that water has.
2. Adhesion ­ o This occurs when molecules of one substance are attracted to molecules of another substance.
o Water molecules adhere to xylem cells which prevents them from falling back down the xylem cells.
3. Root Pressure ­ o When minerals are brought into the plant roots through active transport a higher solute concentration is present in the roots. Water will now enter the root to balance the solute concentration.
o This creates increased pressure in the roots. Water will now move from the area of high pressure (root) to an area of low pressure (leaf).
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