Transport In Plants Flashcards
What are the 3 main reasons a plant needs a transport system?
1- Metabolic demands
2- Size
3- SA:V ratio
What is a dicot plant?
Make seeds that contain 2 cotyledons - organs that acts as food stores for the developing embryo and form the first leaves when the seed first germinates
In the stem of a herbaceous dicot, where are the vascular bundles?
Around the edge for strength and support
What is a vascular bundle?
The xylem and phloem grouped together
In a herbaceous dicot root, where is the vascular bundle found?
In the middle to help the plant withstand the tugging strain from wind
In a herbaceous dicot leaf, where are the vascular bundles found?
In midrib which also helps to structure the leaf
What is a midrib?
The main vein in a leaf that carries the vascular bundle
What are the 2 main functions of the xylem?
- Transport water / mineral ions
- Support
What are xylem vessels?
- The main structures in the xylem
- They are long, hollow structures of dead cells fused end to end
How is lignin relevant to the xylem?
- Lines the walls of xylem vessels in ring, spiral or tube form
- Contain many bordered pits (unlignified areas) to allow water to leave
What is the phloem?
Living tissue that transports organic solutes around the plant
In what direction can organic solutes in the phloem travel?
Both up and down the plant
What are sieve tube elements?
Unlignified hollow structures that are the main transport vessels of the phloem
What are sieve plates?
Thin pores between sieve tube cells to allow phloem contents out
How are companion cells linked to the sieve tube elements?
Via many plasmodesmata
What are plasmodesmata?
Microscopic channels through cell wall linking the cytoplasm of adjacent cells
What do companion cells do?
- Supports sieve tube elements
- Provides ATP (sieve tube cells have no mitochondria)
What are root hair cells?
Exchange surface in plants where water is taken in from the soil
How are root hair cells adapted to perform their function?
- Microscopic size to easily penetrate the soil
- Large SA:V ratio
- Thin surface layer allows for a short diffusion and osmosis pathway
- High water potential gradient between cell and soil water
What are the 2 possible pathways water can take through a root hair cell?
Apoplast and symplast
Describe the symplast pathway
- Water moves across plasma membrane by osmosis
- Moves between cytoplasm of one cell to the next via plasmodesmata
Describe the apoplast pathway
- Water and minerals move through the cell walls
What is the endodermis?
The layer of cells surrounding the vascular tissue in the roots
What is the Casparian Strip?
A band of waxy suberin that runs around each of the endodermal cells forming a waterproof layer
What is the function of the Casparian Strip?
- Impermeable to water so forces water from apoplast pathway through the cell membrane (symplast pathway)
- Membrane repels charged particles
Why do xylem cells have a much lower water potential than the endodermal cells?
Solute concentration in endodermal cells is relatively dilute
Once inside the vascular bundle, water takes which pathway up the xylem?
Apoplast
What helps to give water a push up the xylem?
Root pressure
How is root pressure generated?
The pumping of minerals into the xylem producing movement of water by osmosis
What is some evidence for the role of active transport in root pressure?
- Adding cyanide to root cells
- Active transport requires energy released by ATP and cyanide affects mitochondria, where ATP is held. When cyanide is added to root cells, root pressure stops
- Root pressure rises with temperature increases and falls with a fall in temperature
- This suggests chemical reactions are involved, i.e the breaking of the phosphates in ATP
- If oxygen levels fall, root pressure falls
- Oxygen is needed for respiration to release energy
Why is water required in a leaf?
A leaf is the site of photosynthesis and water is required for photosynthesis
Why is carbon dioxide required in a leaf?
A leaf is the site of photosynthesis and carbon dioxide is required for photosynthesis
What is transpiration?
The loss of water vapour from the leaves and stems of plants via stomata
What are 3 adaptations of the leaf to aid transpiration?
1- Large SA - captures sunlight to carry out PHS
2- Waxy cuticle - makes leaf waterproof preventing too much water loss by evaporation
3- Stomata - can be opened or closed by guard cells to allow gases to diffuse in or out
How does water move in the transpiration stream?
By mass flow
What is mass flow?
All water molecules move together as one body, helped by cohesion and tension
What is capillary action?
- Water molecules attached to lignin in xylem walls
- Adhesion and cohesion move water upwards
What is the benefit of narrow vessel diameter in terms of capillary action?
Prevents the formation of air bubbles
Describe the process of transpiration
- Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and then out via stomata to the surrounding air by diffusion
- This lowers the water potential of the mesophyll cell, so water moves into this cell by osmosis
- This is repeated across the leaf to the xylem
- Capillary action occurs in the xylem as water moves up to replace the water lost in the leaf
What is the cohesion-tension theory?
- Transpiration of water through stomata
- Creates low pressure at top of xylem
- Water is pulled up, creating tension
- Water molecules stick to each other (cohesion)
- Water molecules are sucked up to the leaves (like a straw)
What is the evidence to support cohesion-tension theory?
- Changes in the Diameter of Trees
- When transpiration is at its highest during the day, xylem tension is at its highest so the diameter shrinks
- When transpiration is at its lowest, the tension in the xylem is at its lowest and the tree diameter increases
- Breaking a Xylem Vessel
- Breaking a xylem vessel causes air to be drawn into the xylem and this stops the continuous flow of water molecules
Why may high light intensity be an issue for the transpiration of plants?
- High light intensity means photosynthesis will be occuring at a rapid rate
- Therefore a lot of gaseous exchange will be occuring and the stomata will be open frequently
- This could mean that the plant loses too much water via the stomata
What piece of equipment measures transpiration?
Potometer
Why may measuring transpiration not be 100% accurate?
Measuring water loss is difficult so water uptake is measured and we make the assumption that water uptake = water loss
Draw and label a potometer
-
Why is the stem of the leaf used in a potometer cut at an angle?
To increase its surface area
What factors affect transpiration?
- Light - more light increases rate (as it opens stomata)
- Temperature - higher temperature increases rate (as increases KE)
- Humidity - less humidity increases rate(as increases water vapour gradient)
- Wind - more wind increases rate (as increases water vapour gradient)
What is translocation?
The process of transporting organic solutes from source to sink (requires energy)
In translocation, what are the products of photosynthesis to be transported called?
Assimilates
What is the main assimilate transported in translocation?
Sucrose
Explain the process of translocation of assimilates from source to sink
(Mass flow hypothesis)
Source:
- High conc of solutes, e.g. sucrose
- Active transport of solute from companion cell into sieve tube element
- Decreases water potential
- Water moves in by osmosis from companion cell and xylem
- Creates high hydrostatic pressure in phloem
Sink:
- Uses, breaks down or converts the solute into something else
- Creates a low concentration of solute (e.g. sucrose into starch)
- Increases water potential
- Water moves out by osmosis
- Decreases pressure in phloem
Explain the process of active loading into companion cells
- Active transport of H+
- Out of companion cells
- Creates H ion concentration gradient
- Facilitated diffusion if H+ back into companion cells
- Assimilates (sucrose) move in with H+
- By co-transport
What is the evidence to support translocation?
- Radioactive labelling
- Use radioactive C14 label
- Grow plants in C14 atmosphere
- Measure C14 as it moves down stem - Ringing experiments
- Remove bark in a ring from the tree trunk (contains phloem but not xylem)
- Solutes can’t move up or down
- Bulge forms about the ring
- Fluid above the ring has more solutes, e.g. sugars than below
- Evidence the solutes are moving down
What are xerophytes?
Plants that are adapted to living in dry environments
What are the adaptations of xerophytes?
- Curled leaves
- Stomata sunken in pits
- Hairs on epidermis
(Water vapour builds up and decreases water vapour gradient) - Thick waxy cuticle (reduces evaporation)
- Fewer stomata (less opportunities for the water vapour to come out)
What are hydrophytes?
Plants that are adapted to living in/on water
E.g. water lilies
What are the adaptations of hydrophytes?
- Air spaces in leaves (leaves float on surface of water so more light for PHS)
- Long, flexible leaf stalks (leaves don’t break off in current so don’t need to support any weight)
- Stomata on upper surface (allows gas exchange and prevents air spaces filling with water)
- Large, thin leaves (increase SA for gas exchange)
Name the 7 structures of a leaf in order from top to bottom
1- Waxy cuticle 2- Upper epidermis 3- Palisade cells 4- Spongy mesophyll 5- Lower epidermis 6- Guard cells 7- Stomata
