2.4 Plant tissues, organs and systems Flashcards
Upper epidermis
Protective barrier against damage and pathogens.
Translucent so it can let light go through to the palisade mesophyll.
Spongy mesophyll
Less highly packed irregular shaped cells
Contains less than 20% of the chloroplast
Air spaces for gas exchange
Palisade mesophyll
Regular box-like structure to maximise photosynthesis
Contains over 80% of the chloroplast
Vascular bundle
Contains xylem cells for transport of water and mineral ions
Contain phloem cells for transport of amino acids and dissolved sugars (sucrose)
Waxy cuticle
It is waterproof to stop water evaporating out of the leaf
Xylem
Function – transport tissue for water and mineral ions
Xylem cells line up in tubes and the individual cell walls break down to make xylem vessels which are long and hollow
Cells are dead (have no organelles or cytoplasm) – less resistance for easier flow of water
Xylem outer walls contain lignin – lignin strengthens the vessel, providing support to the plant
Phloem
Function – transport of dissolved sugars and amino acids
Surrounded by companion cells which are found along the outside of the phloem vessels
Contain many mitochondria to provide energy for translocation
The energy released by companion cells are used to help transport the sugars and amino acids (translocation)
Cells have limited amount of cytoplasm and no vacuole to reduce the resistance of flow
Phloem cells are joined end – to – end to make a tube/vessel
They are 2-way which allows substances to be transported all around the phloem
Contain sieve plates – the cell walls between adjacent phloem cells have pores in them to allow material through. The modified cells wall with the pores are known as sieve plates
Palisade cells
Function – carry out photosynthesis in the leaves
Regular shape to pack tightly together. Lots of palisade cells can fit into one layer at the top of the leaf too maximise photosynthesis
Contains many chloroplasts containing chlorophyll
Most chloroplasts are found near the top of the cell where there is the most light
Guard cells
Function – open and close the stomata to allow gas exchange in plants
Banana – shaped cells that are found in pairs
They contain chloroplasts so they can do some photosynthesis
They change shape to open and close the stomata (gap between the guard cell) to allow gas exchange in the leaves
Root hair cell
Function – absorption of water and mineral ions from soil
Has a projection to increase surface area so the rate of water uptake is greater
Thinner walls than other plant cells so that the water can move through easily (due to shorter diffusion distance)
Mitochondria for active transport of mineral ions
No chloroplast – there’s no light for photosynthesis underground
Transpiration stream
The flow of water through a plant, from the roots to the leaves, via the xylem vessels. Water will eventually be lost by evaporation out of the stomata.
Transpiration
The loss of water from leaves by evaporation through the stomata.
Limiting factors of transpiration
Increasing the light intensity, increases photosynthesis and plants need more CO2. The stomata open to let the gas in but this leads to water loss by transpiration.
Increasing the temperature, increases the kinetic energy of the water molecules in the leaf and this leads to loss of water by evaporation (transpiration)
Increasing the windspeed carries the water molecules away from the leaf faster and this leads to a high concentration as there is more water in the leaf than outside so more water leaves through transpiration.
Increasing the humidity means that there is lots of water in the air and lots of water in the leaf this means it is close to an equilibrium so a lower concentration gradient, so it slows down the rate of transpiration.
Translocation
The movement of amino acids and dissolved sugars around the plant mainly from the leaves to the roots.
