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  • Created by: Jess
  • Created on: 14-03-13 19:06
Where do palisade cells occur and why are they often used for plant cell diagrams?
They occur in the top layer of mesophyll and have typical features of a plant cell.
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Which structures are specific to plant cells?
Chloroplasts, cell wall and vacuole
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What are the vacuolar functions?
To create pressure when turgid to push the plasma membrane against the cell wall and to provide a storage area for food
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How is the ultrastructure of a plant cell similar to that of an animal cell?
They both contain: mitochondria, rough and smooth endoplasmic reticulum, Golgi apparatus and peroxisomes.
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What does the cell wall of a plant cell consist of?
Cellulose microfibrils in a matrix of other complex polysaccharides.
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What are plasmodesmata and what is their function?
They are cytoplasmic strands that connect neighbouring cells for easy transport of substances and communication via chemical signals.
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What are chloroplasts and what is their function?
They are double-membraned organelles where photosynthesis occurs.
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What is stroma and thylakoid?
The stroma is the matrix in chloroplasts and the thylakoids are the internal membranes in chloroplasts where chlorophyll is stored. They can be stacked to form grana.
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What is meristematic tissue?
Unspecialized tissue in plants which all cells are derived from. The main plant meristems are the apex of the shoot and the tip of the root.
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What are the non-meristematic parts of a plant?
Protective tissue: epidermis on leaves and stems/cork on surfaces or trees and shrubs. Packing tissue: parenchyma filling spaces between other tissues which can be adapted for storage, support or photosynthesis.
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cont...
Mechanical tissue: Collenchyma which have cell walls thickened with cellulose at the corners to provide strength and flexibility, sclerenchyma which are lignified fibres. Vascular tissue: xylem for water transport and phloem for the transport of
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cont...
dissolved substances such as sucrose.
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What does totipotent mean?
Each plant cell has the potential to give rise to a whole new plant.
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What are stomata?
Microscopic pores in the epidermis that, when open, enable the exchange of atmospheric gases between the plant and the air.
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How does carbon dioxide diffuse into plant cells?
CO2 has a low atmospheric concentration and so stomata provide an unobstructed diffusion route. Photosynthesis and use of carbon dioxide create a concentration gradient for diffusion of CO2.
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Why is it an advantage to have mesophyll cells in close contact with air spaces?
Carbon dioxide diffuses much faster in air.
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What is the disadvantage of a large surface area for CO2 absorption?
It is also a large surface area for water loss.
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How is water lost from stomata?
Heat evaporates water on surface of mesophyll cells which diffuses into substomatal air spaces and out the stomata. Diffusion shells form on the surface of the leaf and water is carried away by the air in mass flow.
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What factors change the thickness of the diffusion shell?
Size, shape and hairiness of the leaf as well as wind speed.
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How does the rate of diffusion accelerate?
Increase in temperature, wind speed or a decrease in humidity which increases the water potential gradient and diffusion shell thickness.
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What are guard cells?
They are specialized epidermal cells that flank the stomata and control the stomatal aperture by changes in turgidity.
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How do guard cells open/close stomata?
When stomata open, the guard cells have had an increase in water pressure and turgidity and the walls furthest from the opening are most flexible and expand, forming a semicircular shape and opening the stomata. To close, they decrease in turgidity
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What role do potassium ions play in stomatal aperture?
Active transport of potassium ions into guard cells creates a water potential gradient whereby water moves in by osmosis, making them turgid and open the stomata. When they diffuse out, water leaves the cell by osmosis and the stomata close.
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What are root hairs and what is their function?
They are cellular extensions of special epidermal cells behind the root tip. They come into intimate contact with soil particles and water and increase the surface area of the roots enormously. They absorb water.
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What is cohesion tension theory?
When transpiration occurs, water molecules 'pull' other water molecules up because they are attached by hydrogen bonds. This is transmitted all the way down the xylem to the roots and pulls water into the roots passively.
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What is root pressure?
It depends on an osmotic gradient being created in the roots. Solutes accumulate in epidermal cells by diffusion and active transport and decreases the water potential, meaning water moves in by osmosis.
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cont...
Endodermal cells secrete solutes into the xylem which creates a concentration gradient high enough for water to be drawn up the stem against gravity. This is root stem pressure.
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What is the apoplast route of water movement from the root to xylem?
Water passes freely through the cellulose cell walls of plant cells by cohesion tension generated by transpiration. As water is pulled up the xylem, these cohesive forces pull water into adjacent cells.
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What is the symplast route of water movement from the root to xylem?
Water diffuses along a water potential gradient through the cytoplasm. Plasmodesmata interconnects the cytoplasm of adjoining cells and form a continuous pathway for water transport.
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What is the vacuolar route of water movement from the root to xylem?
Water moves along the same water potential gradient as in the symplast route but through the vacuoles as well as the cytoplasm.
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What is a transpiration stream?
It is a continuous stream of flowing water from the highest part of a plants shoot to its deepest root.
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How can transpiration be measured?
Using a potometer which measures water uptake, with the assumption that water lost by transpiration equals water uptake.
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What affects transpiration rate?
Temperature increase, decrease in humidity, increase in wind speed and light intensity (because stomata open in response to increased light intensity for diffusion of CO2)
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What are the functions of transpiration?
It cools the plant, some is used for photosynthesis (less than 1%).
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Where does transpiration occur?
90% through stomata, 10% through cuticle. After leaf fall, lenticles (small pores in the stem) are the main site for water loss in deciduous plants.
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How does water move up the transpiration stream?
Water on mesophyll cells evaporate, this decreases water potential in the outermost mesophyll cell and water moves in by osmosis. This osmotic gradient is established causing a flow of water from the xylem to the outermost mesophyll cell.
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How does adhesion work?
It prevents the water molecules slipping down the stem as there are forces of attraction between the water molecules and walls of the water-conducting vessels.
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