Flashcards in 3.3 - Transport in Plants Deck (41):
Why do plants need transport systems?
Plants are multicellular and so have a small SA:Vol ratio and a relatively big metabolic rate.
Exchanging substances by direct diffusion (from the outer surface to the cells) would be too slow to meet their metabolic needs.
How is the xylem adapted to its function?
It contains lignified cell walls, which are waterproof, provides support and prevents collapse under tension.
Xylem vessels are empty and form a continuous column for ease of flow.
Continuous column also allows tension to pull water up.
Bordered pits in the cell wall allow movement of water sideways (lateral movement).
Compare and contrast vascular tissues.
Xylem: Dead; lignified walls; cells arranged end to end to form continuous vessel; no cross walls in vessels; bordered pits between vessels; no contents in vessels.
Phloem: Alive; walls not lignified; sieve tubes elements arranged end to end; sieve tube elements separated by sieve plates; plasmodesmata between companion cells and sieve tube elements; sieve tube elements contain cytoplasm only - kept alive by companion cell.
Outline the process by which water enters the cells of the root from the soil.
Root hair cells take up ions by active transport.
This lowers the water potential inside root hair cells.
Water moves down a water potential gradient from high ψ to low ψ by osmosis through channel proteins or aquaporins.
Once water has been taken up by the root hair cells how does it get across to the xylem vessels?
The apoplastic pathway goes via the cell walls but is blocked at the endodermis by the Casparian strip. Water is then forced into the endodermal cells.
The symplastic pathway goes through the cytoplasm via the plasmodesmata.
How does water move in the two different pathways?
Apoplast pathway – by mass flow.
Symplast pathway – by osmosis.
How does water move from the endodermal cells to the xylem?
Endodermal cells actively transport ions into the xylem.
This lowers the water potential of the xylem.
Water then moves from the endodermal cells into the xylem by osmosis.
What is transpiration?
The loss of water vapour from the leaves of a plant (via stomata) by diffusion down a water potential gradient.
Why is transpiration described as by diffusion and not osmosis?
As the movement is not across a partially permeable membrane.
Explain how water moves from the xylem and out of the leaves.
At the leaves, water leaves the xylem and moves into the cells, mainly by the apoplast (cell wall) pathway.
Water evaporates from the cell walls into the spaces in the leaf (spongy mesophyll cells).
When stomata are open this water diffuses out of the leaf, down a water potential gradient, into the surrounding air.
What is the transpiration stream?
The flow of water through a plant.
What are the mechanisms that move water?
Cohesion and tension, adhesion and root pressure.
What is the cohesion-tension theory?
As water molecules are cohesive (they stick together due to H bonding) when some are pulled into the leaf others follow.
Water evaporating from the leaves creates tension (suction) which pulls more water into the leaf.
Describe how adhesion moves water up the stem.
Water rises in the narrow vessels partly because water molecules are attracted to the walls of the vessels (capillary action).
Water also moves up the stem by mass flow. How is root pressure generated?
Active transport of ions into the xylem of root draws water into the stem by osmosis resulting in a high hydrostatic root pressure.
A pressure gradient is generated and will result in mass flow of water in the xylem.
What does a potometer measure?
Describe how to set up a potometer.
Cut healthy shoot at a slant, under water.
Check potometer is full of water with no air locks.
Insert shoot into potometer under water.
Use petroleum jelly (Vaseline) to ensure seal between shoot and potometer is airtight.
Allow time for shoot to acclimatise.
Shut screw clip.
Keep ruler fixed and record position of air bubble on scale.
Start timing and record distance moved per unit of time.
Why is the shoot cut under water and at a slant?
To stop air entering xylem vessels and to increase surface area.
Why must the leaves be dry?
Water will reduce the water potential gradient between the air spaces in the leaf and the air outside and reduce the rate of transpiration.
What features of a plant may affect the rate of transpiration?
Leaf surface area – larger area - more surface for evaporation and diffusion.
Number of leaves - larger area - more surface for evaporation and diffusion.
Number of stomata – more available allow greater loss of water vapour by diffusion.
Thickness of cuticle – thinner cuticle allow more evaporation.
If you wanted to compare the rates of transpiration of two species of plant using a potometer, how would you ensure a valid comparison?
Set up must be the same for both species – same environmental conditions.
Calculate the rate per unit area of leaf - do this by drawing round all leaves on graph paper.
Pick both species at the same time, run for the same amount of time.
Describe and explain the effect of increased temperature and sunlight on the rate of transpiration.
Water molecules will have KE and so will diffuse faster.
Sunlight will result in the stomata opening wider, allowing more water vapour out.
What effect will reduced humidity and increased wind have on the rate of transpiration?
Reduced humidity – steeper water potential gradient and increased rate of diffusion.
Wind - moves saturated air resulting in steeper water potential gradient and increased rate of diffusion.
What is translocation?
The movement of assimilates (dissolved substances) to where they are needed in a plant.
Movement is from source to sink by mass flow.
Give 2 examples of plant assimilates.
Sucrose (assimilation of inorganic carbon as a result of photosynthesis).
Amino acids (assimilation of inorganic nitrogen actively taken up by root hair cells).
What are the two sugars that make the disaccharide sucrose?
Glucose and fructose.
Why are sugars transported as sucrose?
Sucrose is soluble and metabolically inactive – so does not get used up during transport.
What is a source?
Part of a plant that loads (adds) assimilates into the phloem sieve tubes.
What is a sink?
Part of a plant that unloads (removes) assimilates into the phloem sieve tubes.
How do substances enter the phloem at the source?
By active loading.
What is active loading?
H+ ions are actively transported (ATP) out of companion cells. This generates a concentration gradient (high H+ concentration outside cell, low H+ concentration inside).
H + ions diffuse back in with sucrose molecules through special cotransport proteins (moving sucrose against its sucrose gradient).
Sucrose then diffuses through plasmodesmata into the sieve tube.
How does mass flow of materials between the source and the sink occur?
Water enters the source by osmosis - down a water potential gradient. This causes an increase in hydrostatic pressure at the source.
At the sink, sucrose is removed, resulting in an increase in the water potential.
Water moves out of the sink by osmosis into the surrounding cells, reducing the hydrostatic pressure.
Sap flows down pressure gradient from source to sink by mass flow.
Explain how, at different times, the same plant root may be a source or a sink.
Source – when root converts starch into sugars which are transported around plant.
Sink – when root stores sugars as starch.
When loading it is a source and when unloading it is a sink.
Explain why transpiration cannot be avoided in plants.
Stomata have to be open for gas exchange.
Gas exchange is necessary for photosynthesis and respiration.
The cell walls of the mesophyll are relatively moist compared to the air and result in a water potential gradient.
What are xerophytes?
Plants that have adapted to live in dry conditions.
Explain how a covering of leaf epidermal hairs helps xerophytes survive in their habitat.
Hairs trap water vapour.
This reduces the water potential gradient (between the moist mesophyll cells and outside air).
So less transpiration.
Why are the leaves of some xerophytes reduced to spines or needles?
Provides a small surface area and so will reduce transpiration.
Also will mean there are less stomata and so less transpiration.
What benefit is there of having sunken stomata or curled up leaves to xerophytes?
Water vapour is trapped – reducing the water potential gradient and so reducing transpiration.
What are hydrophytes?
Plants that are adapted to live in water.
What is the main problem for the roots of hydrophytes and how is the plant adapted to overcome this?
Many of the cells are underwater where oxygen levels will be very low.
Without oxygen the cells cannot respire and will die.
Hydrophytes have air spaces in the roots and stems to allow oxygen to move from the floating leaves down to the parts of the plant that are underwater.