Transport in Plants Flashcards
In terms of metabolic demands why do plants need transport systems
- The cells of the green parts of the plant make their own glucose and oxygen by photosynthesis – but many internal and underground parts of the plant do not photosynthesis but still need these nutrients
- Hormones made in one part of a plant need transporting to the areas where they have an effect
- Mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell
In terms of size why do plants need transport systems
- Some plants are small however, because plants continue to grow throughout their lives, many perennial plants are large
- Thus, meaning the plants need very effective transport systems to move substances both up and down from the tip of the roots to the topmost leaves and stems
In terms of SA:V ratio why do plants need transport systems
- Aren’t simple in plants
- Leaves are adapted to increase the ratio for exchange of gases with the air
- However, the size and complexity of multicellular plants means that when the stems, trunks, and roots are taken into account they still have a relatively small SA:V ratio
- Means they cannot rely simply on their cells for everything they need
What are dicotyledonous plants
- Make seeds with two leaves
- Have broad leaves
- Branching veins
- No endosperm in seed
What are xylem vessels
• Largely non-living tissue that has two main functions:
o The transport of water and mineral ions
o Support
• The flow of materials in the xylem is up from the roots to the shoots and leaves
• Long hollow structures made by several columns of cells fusing together end to end
• Two types of tissue associated with xylem
o Thick-walled parenchyma packs: around the vessel, storing food, and containing tannin deposits (protects tissue from attack from herbivores with bitter taste)
o Xylem fibres: Long cells with lignified secondary walls that provide extra mechanical strength
• Lignin can be laid down the walls of xylem vessels in many ways
o Forms rings
o Forms spirals
o Relatively solid tubes with lots of small unlignified areas called bordered pits ( where water leaves xylem)
What are phloem vessels
- Living tissue
- Function: Transports food in the form of organic solutes around plants from leaves where they are made. It supplies the cell with sugars and amino acids needed for cellular respiration and synthesis of all other useful molecules
- Phloem tissues aren’t lignified
- In areas between cells the walls become perforated to form sieve plates. As the large pores appear in the cell wall, the tonoplasts and nucleus and other organelles break down so the phloem becomes a tube filled with phloem sap and tissues without a nucleus (mature cells
- Companion cells form with sieve tube elements by many plasmodesmata (maintain their nucleus and organelles). These cells are very active (life support system) for sieve tube cells
- Tissue also contains supporting tissues including fibres and sclereids (cells with extremely thick cell walls)
How does water move into the root?
- Water moves from high to low water potential
- Water enters root hair cell my osmosis
- Water moves from cell to cell via the plasmodesmata across the cortex of the root by osmosis. Symplastic pathway
- Water also moves across the cortex of the root along the cell walls of the parenchyma cells by cohesion. Apoplastic pathway
- Water molecules move along the apoplastic pathway cannot pass through the casparian strip of the endodermal cells so they enter cytoplasm of cell and join the symplastic pathway
What forces act to move water up xylem, and describe these forces in depth?
- Root pressure – created by movement of water into xylem in roots
- Capilliarity – created by forces of adhesion between water and lignin
- Transpiration pull – created by forces of cohesion between water molecules
• As water molecules leave the leaf through stomata (transpiration) they cause other water molecules behind them to be drawn through the leaf tissue and up the xylem (transpiration stream)
• This occurs because each water molecule is hydrogen bonded to all of its neighbours. The transpiration pull is large enough force to raise water to the very top of the plant. It requires a continuous column of water (no air bubbles)
• If air enters xylem then transpiration stream is broken and water below the air lock can’t move upwards
• If neighbouring xylem vessels are undamaged water may instead move into them through pits (unlignified holes in xylem walls) and then continue to move up
• The lignification strengthens the xylem and prevents it from collapsing under the tension created by the transpiration pull
How does what move out of the leaf
- Water vapour leaves the leaf via the stomata which are concentrated mainly n the underside of the leaf
- Each stoma is surrounded by a pair of guard cells which are the only cells in the epidermis to contain chloroplasts
- Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a conc gradient
- This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf
What are xerophytes and how are they adapted
Plants in dry habitats that have evolved a wide range of adaptions that enable them to live and reproduce in places where water availability is very low
E.g. Marram grass and cacti
Adaptions:
• Thick waxy cuticle: minimise water loss
• Sunken stomata: reduce air movement producing a microclimate of still, humid air which reduces water vapour potential gradient which reduces transpiration
• Reduced no. of stomata and leaves
• Hairy leaves: create microclimate of still, humid air reducing water potential gradient
• Curled leaves: Confines all the stomata within a microenvironment
• Succulents: store water in specialised parenchyma tissue in their stems
• Leaf loss: lose leaves when water isn’t available
• Root adaptions: e.g. long tap roots growing deep into the ground, widespread, shallow roots with large SA
What are hydrophytes and how are they adapted
Plants that live in water (submerged, on the surface or on the edges of bodies of water)
E.g. water lilies
Adaptions:
• Thin or no waxy cuticle
• Many, always-open stomata
• Reduced structure to the plant: water supports the leaves and flowers so no need
• Wide flat leaves: to capture as much water as possible
• Large SA of stems and roots underwater: maximises area for photosynthesis and for oxygen diffusion
• Air sacs: enable the leaves or flower to float on water surface
• Aerenchyma: specialised parenchyma (packing) tissue forms in leaves which has large air spaces which has several functions:
o Making the leaves and stem more buoyant
o Forming low resistance internal pathway for movement of substances to tissues below water
Define translocation
Translocation is an active process transporting assimilates (mainly sucrose)
What are the sources of assimilates
- Green leaves and stems
- Storage organs such as tubers, tap roots that are unloading their stores at the beginning of a growth period
- Food stores in seed when they germinate
What are the main sinks in plants
- Roots that are growing and/or actively absorbing mineral ions
- Meristems that are actively dividing
- Any parts of the plant that are laying down food stores, e.g. developing seeds, fruits or storage organs
Describe mass flow hypothesis
- Active transport is used to actively load the assimilates into the sieve tubes of the phloem at the source
- This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
- This creates high pressure inside the sieve tubes at the source end of the phloem
- At the sink end, solutes are removed from the phloem to be used up
- This increases the water potential inside the sieve tubes, so water also leaves tubes by osmosis
- This lowers the pressure inside the sieve tubes
- The result is a pressure gradient from the source to the sink so this pushes solutes along the sieve tubes to where they’re needed
