9 - Transport in plants

Question 1

The need for transport systems

Size

Answer 1

Large tress around 100m high need effective transport systems to move substances both up and down from the tip of the roots to the topmost leaves and stems.

Question 2

The need for transport systems

Metabolic rate

Answer 2

Many underground plants and internal pants need oxygen and glucose transported to them and the waste products of cell metabolism removed. Hormones need to be transported to other areas.
Mineral ions absorbed by the roots need to be transported to all the cells to make proteins for enzymes and the structure of the plant.

Question 3

The need for transport systems

SA:V

Answer 3

Size and complexity of stems, trunks and roots are taken into account as they have a small SA:V ratio. This means they cannot rely on diffusion alone to supply the cells with everything they need.

Question 4

Structure of Xylem

Answer 4

Made of dead tissue
Large, non-living tissues

Xylem Vessels;
Long, hollow structure - made by fusing columns together.

Question 5

Function of Xylem

Answer 5

Transport of water/minerals

Support the plant

Question 6

Xylem tissues

Tannin/Lignin

Answer 6

Tannin protects plants tissue attacks from herbivorous.

Lignin forms rings to form bordered pits.

Question 7

Structure of Phloem

Answer 7

Living tissues

…..

Question 8

Function of Phloem

Answer 8

Transports organic solutes

Supplies the cells with sugars and amino acids.

Flow of materials can go up or down

Sieve tubes = made from cells joined together

Question 9

Sieve tubes

Answer 9

Areas between the cell walls are perforated to form sieve plates.

As large pores appear in the cell wall, tonoplast, nucleus and other organelles break down

Phloem fills with phloem sap, mature phloem cells have no nucleus.

Companion cells form sieve tubes:

• they maintain their nucleus
• very active cells
• function = support systems for sieve tubes.

Question 10

The dissection of stems, and their examination of the structure of xylem vessels

Answer 10

• Put roots/germinating seeds into water filled with coloured dye for 24 hrs
• Remove the plant from the dye, rinse and look for xylem vessels which should have been stained by the dye.

Question 11

Water in plants

Answer 11

Turgor pressure = result of osmosis

Turgor also drives cell expansion

Lose of water due to cell expansion, to keep plants cool.

Mineral ions and the product of photosynthesis are transported in aqueous solutions

Water is a raw material for photosynthesis

Question 12

Movement of water via Root Hair Cells

Answer 12

Root Hair Cells : 
long
thin
extensible
specialised epidermal cells 
large SA 
thin surface layer

Question 13

The Symplast Pathway

Answer 13

Water moves via the symplast by osmosis.
The root hair cells have a higher water potential than the next cell along.
So the water moves from the root cells to the next door cell via osmosis. This will continue until the xylem is reached.
As water leaves the root hair cells
- water potential of the cytoplasm falls again
- This maintains a steep water p[potential gradient to ensure as much water as possible continue to move

Question 14

The Apoplast Pathway

Cell Wall

Answer 14

Water moves via the apoplast
Water fills the spaces between the loose fibres in the cell wall
The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules create tension .
Meaning there is a continuous flow of water throughout the structure of the cell wall.

Question 15

Process of Transcription

Answer 15

Leaves have a large SA for photosynthesis and are covered with a waxy cuticle that makes them waterproof.
This is important as it prevents the leafs cell from losing water rapidly.
It is also important that gases can move into and out of the air spaces so that photosynthesis is possible.
CO2 moves in to the leaf and O2 out of the leaf by diffusion down concentration gradients through the stomata. The stomata can be opened and closed by guard cells.
Water vapour also moves out of the stomata with oxygen by diffusion and is lost this is called transcription

Question 16

Transpiration Stream

Answer 16

Water enters the roots of the plant by osmosis and is transported up in the xylem until it reaches the leaves.
From the leaves it moves via osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates for the cellulose cell walls of the mesophyll cells in to the air spaces.
The water vapour then moves into the external air via the stomata along a diffusion gradient = Transpiration Stream
The transpiration stream moves water up from the roots of a plant to the highest leaves.

Question 17

Factor Affecting Transpiration

LIGHT

Answer 17

Required for photosynthesis. The higher light intensity more stomata’s open, increasing the rate of water vapour diffusing out and increasing evaporation form the surface of the leaf.

Question 18

Factor Affecting Transpiration

HUMIDITY

Answer 18

A very high humidity will lower the rate of transcription because of the reduced water vapour potential gradient between inside the leaf and the outside air.
Very dry air increases the rate of transpiration.

Question 19

Factor Affecting Transpiration

TEMPERATURE

Answer 19

Increase in temperature increases the KE of the water molecules and therefore increases the rate of evaporation.
Increase in temperature increases the concentration of water vapour that external air can hold before it becomes saturated.

Question 20

Factor Affecting Transpiration

SOIL-WATER AVAILABILITY

Answer 20

The amount of water available in the soil affects the transpiration rate.
If the soil is very dry the plant will be under stress and the transpiration rate will decrease.

Question 21

Practical Investigations to Estimate Transpiration Rates

Answer 21

The rate of water uptake can be measured using a potometer.
A plant is placed in a potometer in a bright light. All the joints have to be sealed with waterproof jelly to make sure any water loss is measured as a result of transpiration.

Rate of water uptake = distance moved / time taken

Question 22

Xerophytes

EXAMPLES

Answer 22

Plants in dry habitats have a wide range of adaptations that enables them to live and reproduce in places where there isn’t much water availability.
Many plants that survive in very cold icy conditions - water that is available is usually frozen

Marram grass
Conifers
Cacti

Question 23

Conserving water: Xerophytes

Answer 23

Thick waxy cuticle 
Sunken stomata 
Reduced n.o of stomata 
Reduced leaves
Hairy leaves
Succulents 
Leaf loss

Question 24

Hydrophytes

EXAMPLES

Answer 24

Plants that live in water - they need special adaptations to cope with growing in water
Surface water plants need their leaves to float so that they can lay on the surface of the water so that they can photosynthesise

Lilies
Duckweed

Question 25

Conserving water: Hydrophytes

Answer 25

Very thin/no waxy cuticle
Many open stomata’s on the upper surface
Reduced structure of the plant 
Wide flat leaves
Small roots
Large SA of stems/roots

Question 26

Translocation

Answer 26

Plant transporting organic compounds form the source to the tissues that need them.
It is an active process and requires energy to transport substances up or down the plant.

Question 27

Assimilates

Answer 27

Products of photosynthesis that are transported

Sucrose is the main assimilate transported around the plant

Question 28

Sucrose in Plants

Answer 28

The sucrose content of most cell sap is only 0.5%

It can be 20%-30% of the phloem sap content

Question 29

Main Sources of Assimilates in Plant

Answer 29

Green leaves and green stems
Storage organs such as tubers and tap roots are unloading their stores
Food stores in seed when they germinate

Question 30

Main Sinks in a Plant

Answer 30

Roots that are growing and absorbing mineral ions
Meristems that are actively dividing
Any parts of the plant they are laying down food stores

Question 31

Phloem Loading

Answer 31

Soluble product of photosynthesis are moved into the phloem from the sources
Sucrose is the main carbohydrate transported

Question 32

Phloem Loading: ACTIVE

THE APOPLAST ROUTE

Answer 32

Sucrose from the source travels through the cell walls and intercellular spaces to the companion cells and sieve elements by diffusion
In companion cells suitcases mood in the cytoplasm across the cell membrane in an active process
Hydrogen ions are ideally pumped out of the companion cells into the surrounding tissues using ATP the hydrogen ions return to the companion cells down a concentration gradient
This increases the sucrose concentration in the companion cells and in the sea elephants through many plasmodesmata between the two linked cells
Companion cells have many infoldings in the cells membrane they give an increased service area for active transport they also have many mitochondria to supply the ATP needed for the transport pumps
The pressure generated in the phloem is 2 MPa which is higher than the human artery which is 0.016 MPa

Question 33

Phloem Unloading

Answer 33

Sucrose is unloaded from the phloem at any point into the cells that need it
Phloem unloading occurs via diffusion
Sucrose rapidly moves into other cells all is converted into another substance
The loss of solutes leads to a rise in the water potential
Water moves out of the cell by osmosis
Some of the water that carried the solute to the sink is drawn into the transpiration stream in xylem