Transport in plants

Question 1

Why do plants need a transport system?

Answer 1

Large multicellular organisms with low SA:V.
Photosynthesis and gas exchange at leaves but can’t absorb water from the air.
Roots absorb water and minerals but not photosynthesise.

Question 2

What are the main things plants take up?

Answer 2

They need C, H and O2.
Water and minerals, CO2, transport water up and sugars up and down.

Question 3

What are the 2 types of vascular tissue?

Answer 3

1. Xylem transports water and mineral ions
2. Phloem transports sugars up and down from sources to sinks.

Question 4

How is a vascular bundle arranged?

Answer 4

The middle of a young root - central core of xylem in an X. with phloem between the arms.
Strength to withstand the pull on the roots.

Surrounded by an endodermis to deliver water to the xylem vessels.

Question 5

Where is phloem and xylem found in the stem?

Answer 5

On the outer edge. In young stems of woody plants, the bundles are separate and form a ring in older stems.
A layer of cambium is found between the phloem and xylem.

Question 6

Where are xylem and phloem found in the leaf?

Answer 6

The midrib and veins of a leaf are formed by vascular bundles. The xylem is on top.

Question 7

What drives water transport?

Answer 7

Evaporation of water at the leaves drives transpiration - energy from the sun, so is a passive process.

Question 8

How can water move through plants?

Answer 8

Apoplast pathway - through spaces between cellulose in cell walls
Symplast pathway - through cytoplasm
Vacuolar pathway - similar to symplast but also through vacuoles.

Question 9

How is the movement of water driven by osmosis?

Answer 9

Water moves from areas with a higher water potential to areas with lower.
Plant cell with high water potential = turgid - pressure on cell wall, reducing influx of water.
Plant cell with low water potential = flaccid - lose water, leading to plasmolysis.

Question 10

What is the structure of xylem?

Answer 10

Dead cells joined end to end - continuous. Fibres support the plant. Hollow tubes grouped in vascular bundles. Lined with waterproof coat made of lignin (polysaccharide).

Question 11

What are the adaptations of xylem?

Answer 11

• gaps without lignification = bordered pits so water can move between vessels
• dead cells aligned - continuous column
• narrow enough so water travels in unbroken column
• no end walls so water can move continuously
• narrow tubes reduce chances of air bubbles
• lignin spirals strengthens walls to stop them collapsing under negative pressure
• no cell contents to obstruct flow

Question 12

How does water move in the leaves?

Answer 12

Most transpiration occurs form the stomata.
- Water from xylem to cells of spongy mesophyll (or apoplast pathway).
- Water evaporates within leaf.
- Water vapour moves out through open stomata.

Question 13

What changes does the evaporation of water have to the water potential of the leaves?

Answer 13

• Water evaporates into the air spaces then diffuses down a water vapour potential gradient through stomata
• Loss of water from mesophyll lowers water potential - water diffuses in by osmosis
• A water potential gradient is established

Question 14

What is transpiration?

Answer 14

The loss of water via evaporation from the aerial parts of a plant mainly through stomata.

Question 15

What are the benefits of a constant water supply to the leaves?

Answer 15

Maintain cell turgidity
For metabolic processes eg. photosynthesis
Transport dissolved mineral ions in xylem
Evaporating water cools the leaves

Question 16

What is cohesion?

Answer 16

The attraction between water molecules due to hydrogen bonds.

Question 17

What is the cohesion-tension theory?

Answer 17

Cohesion cause water to be attracted in an unbroken column across mesophyll and the xylem. So water evaporating out the xylem pull more water up.
= transpiration pull, putting xylem under tension.
Water also adheres to hydrophilic polysaccharides in the xylem - pull water up the sides.

Question 18

What is adhesion?

Answer 18

The attraction of water molecules to hydrophilic substances.

Question 19

What factors affect transpiration?

Answer 19

Light intensity - stomata open if light.
Temperature - high temp increases transpiration by increasing evaporation and diffusion rate, and decreasing water vapour potential of air.
Humidity - high, decreases transpiration because lower potential difference.
Air movement - decreases humidity so increases transpiration rate.
Water availability - if not enough, stomata close.

Question 20

How are root hair cells adapted to the uptake of water/mineral ions?

Answer 20

Ions actively transported from soil, reducing water potential of cytoplasm. So water enters by osmosis.

Question 21

What is the root endodermis?

Answer 21

A layer of cells surrounding the medulla and xylem vessels (contain starch). Location for active process driving movement of water.

Question 22

What is the Casparian strip?

Answer 22

In the endodermis - blocks apoplast pathway so water etc. pass into the cytoplasm.
- Creates a checkpoint for plant immune systems before transport.
- Blocks water from going back into the cortex from the xylem.
- Water and ions have to pass into the cytoplasm before the xylem.

Question 23

What is root pressure?

Answer 23

The action of the roots loading water into the xylem –> positive pressure at base of stem.
Ions actively transported from endodermis to xylem and water follows by osmosis. Casparian strip stops water going back to roots.

Generates force that can push water a few metres up xylem.

Question 24

What is transpiration pull?

Answer 24

Loss of water at leaves drags water up xylem.
Water molecules are cohesive - stick.

Question 25

What is capillary action?

Answer 25

Water molecules adhere to xylem walls. Narrow vessels, so water pulls itself up.
Thin xylem vessels packed at high density (vascular bundles) maximises this effect.

Question 26

What is a xerophyte?

Answer 26

Plants adapted to living in dry conditions.

Question 27

What are common features of xerophytes to reduce water loss?

Answer 27

• Thick waxy cuticle forms a waterproof barrier
• Closes stomata when low water availability
• Hairs on leaves slow air movement - high local humidity. Lowers water potential gradient.
• Stomata in pits on epidermis increases local humidity
• Small leaves with low SA:V reduce diffusion rate of water

Question 28

How are cacti adapted to living in deserts?

Answer 28

• Succulent plants, so store water in fleshy stems
• Stem is fluted, can expand when lots water available
• Spines instead of leaves, decrease total SA
• Long tap root to reach water deep underground

Question 29

How is marram grass adapted to living in sand dunes?

Answer 29

• Rolled up leaves trap humid air with higher water potential close to the leaf. Less lost via evaporation
• Thick, waxy cuticle slows water loss
• Spongy mesophyll is very dense with few airspaces
• Stomata in pits in the lower epidermis are folded and covered in hairs - reduce air movement.

Question 30

What is a hydrophyte?

Answer 30

Plants adapted specifically to living in water.

Question 31

What are the main challenges faced by hydrophytes?

Answer 31

Deliver oxygen to submerged tissues, keep afloat to get sun, high humidity above the water reduces transpiration rate.

Question 32

What are the adaptations of water lilies?

Answer 32

• Leaves with large air spaces in spongy mesophyll - float.
• Leaf stems with many air spaces so oxygen can diffuse to the roots.
• Stomata on upper epidermis for gas exchange.
• Using hydathodes.

Question 33

What are hydathodes?

Answer 33

Structures in plants that can release water droplets which may then evaporate from the leaf surface.

Question 34

What is an assimilate?

Answer 34

A general term for an organic molecule that can be used by an organism.
eg. amino acid, carbohydrate

Question 35

What are sieve tube elements?

Answer 35

Elongated tubular cells aligned end to end –> sieve tubes.
No nuclei and very little cytoplasm - max flow of sap.
Living and separated by sieve plates.

Question 36

What are companion cells?

Answer 36

Lie in between sieve tubes, control transport of solutes in the STEs, connected by plasmodesmata pores. Actively load sucrose etc. into STEs.
Large nucleus and dense cytoplasm.

Question 37

What is translocation?

Answer 37

The transport of assimilates through a plant.

Question 38

Why must phloem be bidirectional?

Answer 38

Sinks can be anywhere on the plant.

Question 39

How does the process of active loading take place?

Answer 39

1. H+ actively pumped out companion cells to leaf tissue –> conc gradient of H+.
2. H+ and sucrose diffuse down gradient into CC through cotransporter proteins (secondary active transport as sucrose can be transported against its gradient).
3. Increase in sucrose conc in CC causes sucrose to diffuse through plasmodesmata into STEs.

Question 40

What is mass flow?

Answer 40

The bulk movement of a substance in a given time through a given area.
The mass flow hypothesis suggests this is responsible for solute transport in plants.

Question 41

How does translocation by mass flow work?

Answer 41

• Sucrose actively loaded into STEs from source cells (decrease water pot.)
• Water potential gradient - water moves into ST from xylem.
• Sap moves down sieve tube due to hydrostatic pressure gradient
• Higher pressure at source and lower at sink.
• Removal of sucrose by cells surrounding ST increase water pot.
• Exit of water from ST decreases hydrostatic pressure.

Question 42

What causes the flow which transports solutes?

Answer 42

The difference in hydrostatic pressure. Water enters at the source and leaves at the sink, so sap flows the same way.