Transport in plants

Question 1

Describe how water is taken up by the root hair cells

Answer 1

Soil water contains dilute solution of mineral salts and has high water potential. The vacuole and cytoplasm of the root hair cell contain concentrated solutions of solute and a lower water potential. Water passes into the root hair cell via osmosis

Question 2

Apoplast pathway

Answer 2

Water moves through cell walls. This is the quickest and most significant

Question 3

Symplast Pathway

Answer 3

Water moves from the cytoplasm of one cell to the next via the plasmodesmata

Question 4

Vacuolar pathway

Answer 4

Water moves from vacuole to vacuole until reaching the endodermis

Question 5

Describe the role of the endodermis in the control of transport of mineral ions

Answer 5

The cell walls of the endodermis contain a waxy substance called suberin. This forms a distinctive band called the Casparian strip. This is waterproof and impermeable
prevents further transport along apoplast, forcing them to travel along the symplast pathway.

Question 6

Why do plants need to absorb nitrates from the soil?

Answer 6

Required for synthesis of amino acids, for nucleotides (nitrogenous bases) etc

Question 7

Why is Mg2+ absorbed by plants?

Answer 7

It is a constituent of chlorophyll

Question 8

Xylem

Answer 8

• Transports water and minerals
• Provide mechanical support and strength

Question 9

Name the 3 types of cell in the xylem

Answer 9

• Xylem vessel = transport minerals and water from the roots to the other parts of a plant
• Tracheids = transport
• Xylem parenchyma = support

Question 10

Adaptations of root hair cells

Answer 10

• short diffusion pathway
• large sa

Question 11

How does water move into the xylem

Answer 11

• water pressure is raised in the endodermis by water driven in by the casparian strip
• Sodium ions are actively transported into the xylem cell.
• This lowers the water potential and water moves into the xylem via osmosis

Question 12

What are the 3 mechanisms water moves from the soil through the plant and into the air ?

Answer 12

Cohesion-tension
Root Pressure
Capilliarity

Question 13

Cohesion tension

Answer 13

• as water is evaporated through the stomata, water is lost and needs to be replaced
• As water molecules leave they pull up other water molecules behind them. They move as one column and as these bonds are created it causes tension.
• Charges of water cause attraction to the hydrophilic lining (lignin). This is adhesion

Question 14

Capillarity

Answer 14

• Movement of water in narrow tubes by capillary action
• Short distances e.g moss

Question 15

Root Pressure

Answer 15

As water moves into the xylem via osmosis it pushes water already there

Question 16

What are the 4 factors that affect transpiration ?

Answer 16

• Humidity
• Temperature
• Air movement
• Light Intensity

Question 17

How does temperature affect the rate of transpiration?

Answer 17

• A rise in temperature provides additional kinetic energy for the movement of water molecules
• Increasing the rate of evaporation
• As temp increases so does the rate of transpiration

Question 18

How does humidity affect the rate of transpiration?

Answer 18

• A lower humidity = higher rate of transpiration
• Humidity causes a lot of water vapour molecules in the air meaning a steep concentration gradient is not maintained.

Question 19

Air Movement affect on rate of transpiration?

Answer 19

• Transpiration in still air allows water vapour to accumulate on the leaf surface
• Wind increases the rate of diffusion as water is swept off the leaf

Question 20

Light Intensity affect on rate of transpiration

Answer 20

• Light intensity controls the degree the stomata opens
• Higher light intensity = greater number of stomatal opening therefore increased rate of transpiration
  (levels off after all stomatas have opened and no more water to lose)

Question 21

What measures the rate of transpiration in an experiment ?

Answer 21

• Potometer

Question 22

What does the potometer actually measure?

Answer 22

• The rate of water absorption
• but if the cells are fully turgid the rate of transpiration and water absorption should be the same

Question 23

Process of setting up the potometer

Answer 23

• Cut root under water to prevent air bubbles in the xylem
• Keep leaf dry
• Ensure that it is air tight (usually a layer of oil on top of the water)

Question 24

Why may the results recorded from the potometer be inaccurate ?

Answer 24

• Water may of been used in photosynthesis instead of travelling up the transpiration stream.

Question 25

Translocation

Answer 25

• products of photosynthesis are
  transported in the phloem, away from the site of synthesis in the leaves (the source) to all other parts of the plant.
• products of photosynthesis are
  used for growth or storage (the sink).
• Translocation can happen up, down or sideways

Question 26

What are the 4 types of cell in the phloem tissue?

Answer 26

• Sieve plates
• Companion cells
• Phloem fibres
• Phloem parenchyma

Question 27

Sieve plates

Answer 27

A sieve tube is made of sieve element or sieve cells. Its function is the transport of organic materials such as sucrose and
amino acids
- allow bidirectional flow from element to element through the plant

Question 28

Experiments to prove translocation

Answer 28

1) Ringing experiments (removal of phloem) show accumulation
of sucrose products on leaf side of the ring but none on root
side. Movement of sucrose was blocked by removal of phloem.
Therefore, phloem is the route of transport.
2) Aphids have hollow,
needle-like mouthparts called a stylet. The stylus
extends into sieve tube elements, laser is used to remove the
stylus from the body, the stylus then becomes a micropipette
and sap drips out. This can be analysed to show that sucrose and amino acids are carried in the phloem, both above and below
leaves.

Question 29

The theory of mass flow

Answer 29

suggests there is a passive mass flow of sugars from the phloem of the source leaf, which has the highest concentration of sugar to
other areas of the plant, such as
growing tissues, which have a lower sugar concentration.
- The theory states that translocation
occurs from source to sink.

Question 30

evidence for the mass flow theory

Answer 30

Sucrose made at source
lowers water potential. Water
enters cells and sucrose is
forced into phloem (loading).
This increases hydrostatic
pressure and therefore mass
flow occurs along the phloem
to the root where sucrose
is stored as starch, water
potential is less negative and
water moves into the xylem.

Question 31

Evidence against mass flow

Answer 31

Sieve plates impede flow.
This theory does not explain
bidirectional flow or different rates of flow of sucrose and amino acids.

Question 32

The transpiration stream

Answer 32

• The continual flow of water in at the roots, up the steam of the leaves and out to the atmosphere is the transpiration stream
  -Plants have to balance uptake with loss