Transport in plants Flashcards
1
Q
Why do plants need a transport system
A
- All living things need certain nutrients and water and a way to get rid of waste
- This could happen by diffusion for cells on the edge of plants, but cells in the centre of plants aren’t close enough to this supply, and so would die if they couldn’t get these things from elsewhere, as their SA:Vol ratio isn’t sufficient
2
Q
Distribution of tissues in a dicotyledonous plant root
A
- Xylem in centre in a distinctive star/x shaped bundle
- Phloem inbetween the arms of the xylem in bundles
- Surrounded by the endodermis, just outside a layer of meristem cells called the pericycle
- Epidermis at edge of plant
3
Q
Distribution of tissues in a dicotyledonous plant stem
A
- Vascular bundles at edge of the stem.
- Distinct in non-woody plants
- Continuous in older stems of woody plants
- Xylem on inside of vascular bundle
- Then the cambium - layer of meristem cells to make new xylem and phloem
- Then the phloem
- Then some supporting tissues
- Cortex on outside of vascular bundles
- Medulla in centre of vascular bundles
- Epidermis at edge
4
Q
Distribution of tissues in a dicotyledonous plant leaf
A
- Vascular bundles in vein in midrib
- Xylem above phloem
- Branching veins - may see other vascular bundles
5
Q
Structure of xylem vessels
A
- Thick cell walls, impregnated with lignin (waterproofing, strengthening cell walls so no collapse, even when little water)
- No end walls and contents - large lumen for water to flow through (decayed when cell died due to waterproofing)
- Lignin in spiral/rings/broken rings pattern to allow vessel to stretch
- Pits (gaps in lignin) to allow water to pass from one vessel to another/out into the living tissues of the plant
6
Q
Function related to structure of xylem vessels
A
Carries water and minerals from roots to tips of plants
-dead cells aligned end to end to form a continuous column
-narrow tubes for capillary action and water column doesn’t break easily
-pits to allow transverse movement of water
-spiralling lignin so can grow and stretch and bend
Flow not impeded because:
-no end walls
-no cell contents (e.g. nucleus or cytoplasm)
-lignin thickening so no collapsing
7
Q
Structure related to function of sieve tube elemens
A
Transport sugars (usually sucrose) in the form of sap
- Very little cytoplasm and no nucleus - allow sap to flow
- lined up end to end
- cross walls at intervals, perforated to allow sap to flow
- thin walls
- 5/6 sided
8
Q
Structure related to function of companion cells
A
- large nucleus
- dense cytoplasm
- small, fitting inbetween sieve tube elements
- many mitochondria to provide ATP for active processes needed by sieve tube elements (e.g. loading sucrose)
- many plasmodesmata to allow communication and minerals to flow
9
Q
Transpiration
A
The loss of water vapour from the aerial parts of a plant due to evaporation via stomata
10
Q
Transpiration as a consequence of gaseous exchange
A
- Water leaves the xylem and passes to the mesophyll cells via osmosis
- evaporates off surface of mesophyll cells into intercellular spaces
- water potential rises
- water diffuses out of leaf via open stomata
- stomata open for gaseous exchange for photosynthesis
11
Q
Factors effecting transpiration rate (8)
A
- number of leaves (more SA to lose water from)
- wind (removes water vapour from around leaf, maintaining a high water vapour potential gradient)
- temperature (high temperature means faster diffusion, more evaporation so higher water potential in leaf and less water vapour in air)
- humidity (higher humidity means lower potential gradient)
- light (photosynthesis, so stomata are open)
- cuticle (thick cuticle reduces evaporation from surface)
- number, size and position of stomata (lots of stomata means more water lost; if on lower surface, less sunlight so less exposure)
- water availability (not much water means plant can’t replace what it has lost. water loss reduced when stomata shut and leaves are shed in winter)
12
Q
Potometers
A
- estimate transpiration rate (only an estimate, as actually measuring water uptake, though as 99% is then lost, it is a good measure)
- water drawn up capillary tube
- air bubble/dye to measure movement of water
- capillary scale to get figures
13
Q
Water uptake from soil
A
- root hair cells in epidermis increase SA for diffusion
- they absorb mineral ions from soil by active transport with ATP
- lowers water potential of cytoplasm, so water diffuses in via osmosis, down a water potential gradient
14
Q
Movement of water across the root into the xylem
A
- moves from epidermis to endodermis by either symplast, apoplast or vacuolar pathways
- when reaches endodermis, apoplast pathway is blocked by casparian strip, made of waxy suberin (water proof)
- endodermis actively transports minerals into the xylem, lowering the water potential there
- water moves in via osmosis
- this creates a water potential gradient across the whole cortex, as there is a low water potential at the endodermis and high water potential at the epidemis, moving water via the symplast pathway
- water can also move by the apoplast pathway, which joins the symplast pathway at the endodermis because of the casparian strip
15
Q
Water movement up the xylem
A
- root pressure (of water being forced into the xylem) pushes water up the first bit of the stem
- transpiration pull: water being lost from the leaves must be replaced. As water molecules are cohesive, the water is pulled up the plant in a column. This creates tension in the xylem - yay for lignin! Called the cohesion-tension theory, there is a transpiration stream. Relies on a continuous column of water - if broken, water can leave via pits
- capillary action: as xylem vessels are narrow and water molecules are adhesive, the forces of attraction can pull water up the sides of the vessel
- pressure gradient: loss of water from top of plant causes low hydrostatic pressure, water moves from high hydrostatic pressure in roots to low hydrostatic pressure in leaves by mass flow
