3.1.3 - Transport in Plants Flashcards
Why do plants need transport systems?
- size
- high metabolic rate
- small surface area to volume rato (SA:V)
What are vascular plants?
plants that have specialised transport systems
-have vascular tissue
Why do some plants not need a specialised transport system?
small plants don’t
- can absorb materials directly from environment
eg. mosses
What is vascular tissue?
- made up of xylem and phloem
- involved in transport and structure
What is the structure of the vascular bundle like in the roots of a plant?
- drill like structure
- X of xylem (in centre as it is the strongest)
- sections of phloem around xylem cross
What is the structure of the vascular bundle like in the leaf of a dicotyledonous plant?
- xylem located on top of phloem
- sections of vascular bundle throughout
What is the structure of the vascular bundle like in the stem of a plant?
- several vascular bundles in circle
- xylem vessels in inside of vascular bundles
- phloem in middle of vascular bundles
- cambium layers on outside of vascular bundles
What is the structure of the xylem vessel? (And why is this beneficial?)
- non-living
- lignified walls (lignin laid down in walls as rings, spirals or solid tubes which strengthens xylem and stops it from collapsing)
- thick walls (prevents bursting/collapsing
- no contents of cells (less resistance to flow)
- pits in cell walls (allows water to leave xylem vessel to supply it to cells and tissues)
- thick-walled xylem parenchyma cells pack around xylem vessels (contain tannin deposits -tannin is a bitter chemical that protects plants from being attacked by herbivores)
What does the xylem transport?
water and mineral ions up the plant (from roots to shoots and leaves)
What is the structure of the phloem vessel? (And why is this beneficial?)
- living tissue
- vessel made up of sieve tube elements (cells joined end-to-end -organelles broken down)
- areas between cells called sieve plates
- companion cells are linked to sieve tube by many plasmodesmata (companion cells have lots of mitochondria are active and provide energy for phloem)
What does the phloem transport?
food in form of organic solvents (eg. sucrose, amino acids, etc) up and down the plant
Why do plants need water?
- it creates hydrostatic/turgor pressure (provides hydrostatic skeleton for support and drives cell expansion)
- keeps plants cool (when it evaporates)
- transports substances
- needed in photosynthesis
How does water move from the soil to the xylem vessel?
-taken into roots via osmosis (down water potential gradient)
two options:
1) symplast pathway (moves through cytoplasm and plasmodesmata via osmosis)
2) apoplast pathway (moves through cell walls and intercellular places via diffusion -however at casparian strip in endodermis, it leaves apoplast pathway, passes through cell membrane and joins symplast pathway)
What is the symplast pathway ?
movement of water through cytoplasm and plasmodesmata
-each cell further from roots has lower water potential so water is drawn up plant
What is the apoplast pathway?
movement of water through cell walls and intracellular spaces
-fastest movement of water
What is the Casparian strip and what happens there?
- waxy strip which is impermeable to water located in the endodermis
- water in apoplast pathway is forced into symplast pathway
- this prevents toxic solutes from the soil entering tissues and means plants use all the minerals
How does water move up the plant?
- through the xylem
- uses tension and cohesion -transpiration pull
- as water evaporates of leaves, tension causes water to be pulled up because the water molecules cohere to each other (they are attracted to eachother due to hydrogen bonds) -capillary action
- adhesion (water molecules are attracted to xylem walls
What is the evidence for the cohesion-tension theory?
- changes in tree diameter (at higher transpiration rates, diameter decreases due to tension)
- cut flowers (draw water in rather than out -transpiration pull continues)
- broken xylems (water stops being drawn up as air breaks transpiration stream)
What is transpiration?
the loss of water vapour by evaporation from leaves (and stems) via stomata
-by diffusion out of leaf down water potential gradient
What factors affect the transpiration rate?
- light
- relative humidity
- temperature
- air movement
- soil water availability
How does light affect the transpiration rate?
increased light intensity = increase no. stomata open => increased rate of transpiration
How does relative humidity affect the transpiration rate?
high relative humidity = lower water vapour potential gradient => decreased rate of transpiration
How does temperature affect the transpiration rate?
increased temp = increased kinetic energy of H2O molecules + decreased relative humidity => increased rate of transpiration
How does air movement affect the transpiration rate?
windier conditions = increased water potential gradient => increased the rate of transpiration
How does soil water availability affect the transpiration rate?
damp soil => increased the rate of transpiration
dry soil => decreased the rate of transpiration
What is a potometer?
apparatus used to measure the water uptake of a plant (estimates transpiration rates)
How do you set up a potometer?
- cut healthy shoot under water (to stop air entering xylem)
- cut shoot at slant (increases SA)
- check apparatus is full of water (no air bubbles)
- insert shoot into apparatus under water
- remove potometer from water and ensure airtight/watertight joints around shoot
- keep leaves dry (to keep conditions constant and not affect water potential gradient)
- allow time for shoot to acclimatise
- shut screw clip
- keep ruler fixed + record position of air bubble on scale
- start timing + measure/calculate distance moved per unit time
What are mesophytes?
plants that can take up sufficient water to replace transpiration
What are hydrophytes?
plants that live partially or completely submerged in water
eg. water lilies
What are xerophytes?
plants that live in areas where water loss via transpiration is greater than water taken in by plants
eg. cacti and marram grass
How are xerophytes adapted to their environment?
- thick waxy cuticle (prevents water loss by transpiration)
- sunken stomata (reduces air movement -creates a microclimate of still, humid air => reduces water potential gradient + transpiration)
- less stomata (reduces gas exchange + transpiration)
- reduced no. leaves (lower SA:V => reduces water loss)
- hairy leaves (creates a microclimate of still, humid air => reduces water potential gradient + transpiration)
- curled leaves (confines stomata to a microclimate of still, humid air => reduces water potential gradient + transpiration)
- succulents (swelled/fleshy -water stored in parenchyma tissue which can be used in drought)
- leaf loss (loose leaves when water isn’t available)
- root adaptions (surface roots -collect rain water after short rain shower +long roots -go deep into ground to deep water sources
How are hydrophytes adapted to their environment?
- very thin/no waxy cuticle (water doesn’t need to be conserved)
- many always-open stomata on upper surfaces (maximises gas exchange)
- reduced structure to plant (water supports leaves/flowers -no strong supporting structures needed)
- small roots (water can diffuse directly into stems/leaves so roots aren’t needed)
- air sacs (enable leaves/flowers to float)
- arenchyma (large air spaces -makes plant more buoyant)
- wide, flat leaves (spread across the surface of water -capture as much sunlight as possible for photosynthesis)
- large SA of stems/leaves under water (maximises area for photosynthesis)
What is translocation?
the movement of assimilates (dissolved substances, eg. sucrose and amino acids) from parts of the plant where the substances are made to other parts of the plant where they’re needed
- active process
- occurs in phloem in both directions
What are sources?
places in plant where assimilates (eg. sucrose) are loaded into phloem
- high hydrostatic pressure
eg. leaves, storage organs
What are sinks?
places in plant where assimilates are unloaded from phloem (ie. where they are used for growth or respiration)
- low hydrostatic pressure
eg. meristem, growing roots, flowers, fruit, seeds
What is mass flow?
how solutes are transported from source cells into sinks through the phloem
What happens in mass flow?
- assimilates enter the phloem (decreases water potential near source end)
- water enters by osmosis (down water potential gradient)
- this causes an increase in hydrostatic pressure (because source/sink can’t expand)
- assimilates move down the pressure gradient towards the sink end of the phloem.
- assimilates move into sink cells (are converted into other molecules eg. starch)
- removal of assimilates increases the water potential at the sink end -causes water to move out of the phloem by osmosis (this maintains the hydrostatic pressure gradient between the source and the sink)
How are assimilates loaded into sources?
- companion cell actively transports hydrogen ions into the surrounding cells
- creates a hydrogen ion gradient between the surrounding cells and the companion cell
- hydrogen ions diffuse back into the companion cell with a sucrose molecule through a co-transporter protein
How are assimilates unloaded into sinks?
- sucrose is unloaded from phloem at any point it is needed
- occurs via diffusion
- sometimes it is converted into substances (eg. glucose, starch) to maintain sucrose concentration gradient
What is the evidence for translocation?
- microscopes allow us to see adaptions for active transport
- if mitochondria (in companion cells) are poisoned, translocation stops
- flow of sugars is faster than by diffusion alone
- aphids can be used to demonstrate the translocation of organic solutes in phloem
