Incomplete 9 - Transport in plants Flashcards
Why do plants need transport systems? (5)
- low SA:V ratio (can’t rely on diffusion alone
- mineral ions need transporting from roots to area of growth
- hormones
- size - big so substances need transporting great distances
- some areas don’t photosynthesise - sucrose needs transporting from source to sink
What does the root structure (in terms of xylem) look like in dicots and why?
xylem arranged in x shape in center (withstand forces such as pulling)
What does the stem structure (in terms of vascular bundles) look like in dicots and why?
Xylem is located on the inside to provide support and flexibility to the stem
Phloem is found on the outside of the vascular bundle
What does the leaf structure (in terms of vascular bundles) look like in dicots and why?
vascular bundles form the veins of a leaf, dicots veins branch off from center to improve transport and give support
What is the role of the xylem?
Transport water and minerals and give structural support
What is the structure of the xylem? (4)
- Long cylinders made of dead tissue with open ends, form a continuous column.
- Lignified walls (waterproof, structure, keep open)
- Unlignified pits (water can flow sideways in case of blockages)
- One way flow
What is the structure of the phloem? (4)
- They are tubes made of living cells
- Sieve tube elements joined by sieve plates
- companion cells joined by plasmodesmata (lots of mitochondria for active transport)
- two-way flow
What is the role of the phloem?
Transport sucrose and assimilates from source to sink
Why do plants need a water transport system?
Water is absorbed in the roots but needed in the leaves for photosynthesis
Water is used to transport sucrose, minerals and hormones
How do plants loose water?
Stoma open to let co2 in for photosynthesis. Consequently, water diffusés out of plant
What is transpiration
The loss of water from the leaves of a plant
Name 4 factors that affect the rate of transpiration
Temperature
Light intensity
Humidity
Air movement
How does temperature affect the rate of transpiration
Greater KE, more frequent collisions, greater rate of diffusion
How does light intensity affect the rate of transpiration
Greater rate of photosynthesis, greater co2 demand, stoma open more, more transpiration
How does humidity affect the rate of transpiration
More water vapour in air, less steep conc. gradient, lower rate of diffusion
How does air movement affect the rate of transpiration
Windy = moisture in air moved away from stomata, steeper conc. gradient, greater rate of diffusion
How are the roots adapted to their function? (2)
- greater sa, greater area for osmosis
- single cell thick, short diffusion pathway
How does water get from soil to inside a plant?
High water potential outside roots, water moves in through osmosis (through cell wall and then plasma membrane)
What is the apoplast pathway?
Water moving through connecting cell walls (adhesive and cohesive properties)
What is the symplast pathway?
Water moves through cytoplasms of cells and through plasmodesmata (diffusion)
What pathway does the majority of the water travelling through a plant take and why?
Apoplast pathway (faster)
When does the water travelling in the apoplast pathway join the symplast pathway?
When it reaches the endodermis (casparian strip of waxy suberin)
What is the role of the casparian strip?
Made of waxy suberin, prevents water from progressing further in apoplast pathway so it join the symplast pathway
Pathogens in water will not be able to join and therefore unable to spread via xylem
How does water get from the roots to the xylem?
Mineral ions actively transported into xylem
Water potential in xylem decreases, water potential in cells increases
Water joins xylem via osmosis
Where does the water in the roots join the xylem?
Pericycle
What does a potometer measure and why is this a good measure of the rate of transpiration?
Measures rate of water uptake in a plant
Majority (95%) of water uptake is lost via transpiration so is a good estimate
Describe what measurements you need to take for a potometer experiment
How far an air bubble moves in a time period (therefore how much volume has be taken up plant - pi r2 h)
Rate of uptake =v/t
What is a xerophyte?
Plant adapted to dry environment
What is a hydrophyte?
Plant adapted to a very wet environment
Name 3 adaptions of “normal” plants to their water availablity
Waxy cuticle (reduces transpiration through leaves)
Stomata (prevent water loss if co2 is not needed)
Deep roots in soil (allows roots to reach as much water as possible)
State the layers of leaf structures (from top waxy cuticle to bottom waxy cuticle)
Waxy cuticle Upper epidermis palisademedophyll spongy mesophyll lowe epidermis (with stomata and guard cells) waxy cuticle
Name at least 5 adaptions of xerophytes
- very thick waxy cuticle
- sunken stomata
- reduced no of stomata
- reduced leaves
- hairy leaves
- curled leaves
- succulents
- leaf loss
- root adaptions eg. tap roots
- avoiding mechanisms eg. survive as bulbs
Why do xerophytes have a thick waxy cuticle?
prevent water loss through transpiration through their cuticles
Why do xerophytes have sunken stomata?
reduces air movement, creates areas of humidity, reduces water potential gradient, therefore, decreased rates of transpiration
Why do xerophytes have reduced numbers of leaves?
reducing sa:v ratio, reduces transpiration
less leaves = less photosynthesis = lower co2 demand = stomata less open = lower rate of diffusion
Why do xerophytes have hairy leaves?
creates a microclimate of humid air, reduces water potential gradient, therefore, decreased rates of transpiration
Why do xerophytes have curled leaves?
confines stomata to humid microclimate, reduces water potential gradient, therefore, decreased rates of transpiration
State some root adaptions for xerophytes
long tap roots, get water from far into soil
widespread shallow roots - max absorption when rainfall occurs
State some avoiding the problem mechanisms for xerophytes
become dormant or die when dry conditions, can spread/leave seeds behind for rapid growth when next wet
lose leaves and plant structures, service as organs eg. bulbs
Why do hydrophytes need adaption?
need to be as light as possible float on water surface, gas exchange channels need to be in air (not underwater)
State at least 5 adaptions of hydrophytes
- very thin/no waxy cuticle
- lots of permanently open stomata
- reduced structural support
- wide flat leaves
- small roots
- air sacs
- aerenchyma
- pneumatophores
Why do hydrophytes have a very thin/no waxy cuticle?
loss of water via transpiration not as issue (water in plentiful supply), maximum rate of photosynthesis (max co2)
How are hydrophytes stomata adapted to their environment?
lots of permanently open stomata on the surface of leaves
How are hydrophytes roots adapted to their environment?
very small roots, water can diffuse directly into stem and leaf tissue
What are pneumatophores?
in case of water logging in hydrophytes (not enough air for water circulation) aerial roots grow up into air to increase air and therefore water flow
What are aerenchyma?
many large air spaces in hydrophytes, increases buoyancy and creates a low resistance pathway for gas to areas underwater
What is translocation?
The movement of glucose from source to sink in a plant
What is the source in translocation?
Where the glucose/sucrose is being produced - photosynthesising areas
What is the sink in translocation?
destination of the sucrose/glucose - respiring areas
What are the assimilates in translocation?
The substances being moved (mostly sucrose)
Why is sucrose transported in translocation rather than glucose?
Less metabolically active - less likely to be used up on the way
Sucrose is ready to be transported via translocation. What is the first step of translocation?
Assimilates are transported via symplast and apoplast pathway (through cytoplasm - diffusion or through cell wall as water containing sucrose)
Sucrose travelling along the apoplast pathway is transported back into the symplast pathway. Where does this happen and what processes are involved?
at companion cells
requires ATP
via cotransport (proton pump sends H+ ions out and they bring back sucrose molecules)
Sucrose is in the companion cells ready for the next stage of translocation. What happens next?
Diffuse into sieve tube elements
Sucrose is chilling in the sieve tube element, what is the next stage of translocation?
STE has a very negative ψ, water from xylem travels into STE by osmosis
increases hydrostatic pressure- water carrying assimilates is forced up and down STE
Sucrose in the sieve tube element reaches its sink, what happens next?
Sucrose diffuses out of STE into sink
How do cells ensure they have a plentiful supply of sucrose (don’t mess up diffusion in translocation)?
convert sucrose into glucose for respiration
convert into starch and store it
move into the vacuole and store it
What evidence do we have that supports the mass flow hypothesis?
- poisoning mitochondria in companion cells stops translocation ∴ active transport is involved
- rate of trasnlocation is quicker than if it were relying on diffusion alone
- electron microscopes can be sued to see the adaptions of companion cells for active transport and translocation
- aphid studies show pressure in phloem is greater at the source than the sink
What is meant by vascular bundle?
Transport system in plants made from xylem and phloem
