Chapter 13 - Transport in plants

Question 1

Explain the need for a transport system

Answer 1

• Have very large bodies
• All regions need to get all materials
• Diffusion would be too slow

Question 2

Discuss mass flow

Answer 2

• They have specialised transport systems
• These systems transport large volumes of fluid to other parts if organism
• It’s called mass flow

Question 3

Describe the uptake of water by osmosis

Answer 3

• Water moves from the soil to the root hair by osmosis
• Moves from a high potential (soil) to a low potential (root)
• Root hairs provide a large surface area for osmosis

Question 4

Describe water movement through the root

Answer 4

• From root hair through the epidermis to the cortex tissue
• From cortex tissue through endodermis into xylem
• Pathway 1: Apoplast pathway - cell to cell via cell walls. Fastest
• Pathway 2: Symplast pathway - cell to cell via cytoplasm. Slower
• Pathway 3: Vacuolar pathway - osmosis one vacuole to next. Slowest

Question 5

Describe the movement of water up the stem in the xylem

Answer 5

• Water moves up the stem xylem because of different forces
• Water absorbed by roots create a force called root pressure which forces water up the xylem
• Cohesion is the tendency of water molecules to attract one another. Water molecules “pull” other water molecules up the xylem
• Transpiration pull (the evaporation of water during transpiration) causes a sucking force for water up the xylem
• Adhesion (the tendency of water molecules to be attracted to other molecules) between water molecules and the walls of the xylem helps water to move upwards
• Capillarity. Water moves automatically up a tube, and the smaller the diameter the higher it rises. Xylem has a small diameter and therefore has considerable capillarity forces

Question 6

Describe how water moves through a leaf

Answer 6

• Water moves by osmosis from the stem xylem into the leaf xylem
• From there on to the mesophyll cells
• Then evaporates from the mesophyll cell walls into air spaces as water vapour
• Diffuses out of the stomata by transpiration which causes transpiration pull

Question 7

Describe the mechanisms by which water moves from the roots to the air, in terms of water potential gradients

Answer 7

• Soil to root hairs (soil has high potential, root hairs has low potential)
• There is a water potential gradient between the soil and root hair
• Root hair to cortex cells (high and low again)
• Water potential gradient between root hair and cortex cells
• Through cortex cells to endodermis cells (high and low again)
• Water potential gradient between cortex cells and endodermis cells

Question 8

Name where xylem could be found in a plant

Answer 8

Vascular bundles of roots, stems & leaves

Question 9

State two functions of xylem

Answer 9

• Transport of inorganic substances (water and mineral ions) in plants
• Provides support to plants

Question 10

Describe the structure of xylem and state how the structure contributes to the function

Answer 10

• Long hollow tubes connected end to end to form a continuous column
• No end walls
• Cells are dead and contain no cytoplasm nor organelles so there is no obstruction to the flow of water and mineral ions
• Walls are thickened with lignin to strengthen it - to support the plant
• Unlignified areas called pits for lateral movement of water

Question 11

Define transpiration

Answer 11

The process by which water is lost as water vapout from the aerial parts of plants

• Through stomata in leaves
• Through lenticels in stems

Question 12

Name the advantages of transpiration

Answer 12

• Causes transpiration pull
• Has a cooling effect on plants
• Transpiration pull brings mineral ions with the water to the plant

Question 13

Name the disadvantages of transpiration

Answer 13

• Rate of water loss can exceed the uptake of water

- Can lead to wilting, desiccation and death of the plant

Question 14

Explain how temperature can affect the rate of transpiration

Answer 14

• Transpiration rate increases as temperature does
• Higher temperatures causes an increase in kinetic energy of water molecules, increasing the rate of transpiration
• Water molecules move faster when heated
• Air with high temperature holds more water molecules
• High temperature lowers relative humidity of air
• A fall in temperature causes the exact opposite of everything named above

Question 15

Explain how wind speed can affect the rate of transpiration

Answer 15

• Wind increases transpiration rate
• In the absence of wind water vapour accumilates near the leaf surface
• Any wind tends to remove this humid layer, thus increasing the rate of transpiration
• The faster the wind speed, the more rapidly humid air is removed, and the larger the rate of transpiration
• Creates a water potential gradient between the moist air on the leaf and the air outside the leaf

Question 16

Explain how humidity can affect the rate of transpiration

Answer 16

• Low humidity increases transpiration rate
• Creates a water potential gradient between the moist air on the leaf and the air outside the leaf
• When the external air has a high humidity the gradient is reduced and less water is transpired

Question 17

Describe how wilting occurs in plants

Answer 17

• Wilting refers to the loss of rigidity in non-woody parts of plants
• Occurs when the turgor pressure in non-lignified cells falls towards zero
• Cells lose their turgidity and become flaccid
• If the soil around the roots lack water
• or the rate of absorbtion is slower than the rate of water loss
• the cells in the plant will contain less water and fail to support the plant and the leaves
• Stems become soft
• `Wilting often indicates a lack of soil moisture
• Mild wilting does not permanently harm a plant if it is remedied immediately

Question 18

Describe the precautions for setting up a potometer

Answer 18

• Cut the shoot under water (no air goes into the system)
• Ensure that the apparatus is completely filled with water
• Cut the shoot at a slant
• Dry of the leaves
• Ensure air and water tight joints
• Use a healthy, undamaged, unwilted shoot
• Allow the shoot to acclimatise
• Keep environmental conditions constant
• Measure distance travelled by bubble with time
• Reset the air bubble by opening the tap

Question 19

Explain how one would control the different environmental factors when doing an experiment with a potometer

Answer 19

• Fan for wind
• Heater for temperature
• Light bulb for light intensity
• Transparent plastic bag for humidity

Question 20

Define xerophytes

Answer 20

Plants adapted for growing in dry places, able to withstand long periods during which water is unavailable

Question 21

Name and explain the adaptations of xerophytes

Answer 21

• Thick cuticles to reduce water loss
• Reduced number of stomata
• Stomata are situated in pits (sunken)
• The surface has a layer of hair, which traps water vapour
• Leaves roll up to hide stomata and expose waterproof cuticle
• Leaves are smaller (smaller surface area)
• Leaves reduced to spines (thorns)
• Have thick succulent stems for water storage
• Thick succulent leaves for water storage
• Lose leaves during winter

Question 22

Define translocation

Answer 22

The transport of organic molecules from the region of production to the region of utilisation or storage in phloem sieve tubes

Question 23

Describe the translocation of systemic pesticides in phloem

Answer 23

• I shall be doing this when I feel like it

Question 24

Describe the structure of the phloem

Answer 24

• consists of phloem sieve tubes and companion cells.
• phloem sieve tubes responsible for actual transport.
• companion cells plays no direct role.

Question 25

Describe the phloem sieve tubes.

Answer 25

• Living cells, with cytoplasm and organelles.
• phloem sieve tubes connected end to end for easy flow of amino acids and sucrose.
• although have very little cytoplasm and few organelles.
• this enables flow of sucrose and amino acids with little resistance.
• organelles are at the edges, to make more space for flowing.
• end walls with sieve plates to permit continuous flow from cell to cell
• sieve plates prevent bulging of cell under pressure and bursting.
• cytoplasm extends from sieve tube through sieve plate to next cell, to allow easy movement.
• Small plasmodesmata in cell walls allow connection between phloem sieve tubes and companion cells. it allow flow to and from.

Bi- directional flow allow sugar to go both up and down.

Question 26

Describe the companion cells.

Answer 26

• Living cells, with nucleus,cytoplasm, organelles.

- metabolically active cells ( many mitochondria)

Question 27

What is the functions of the companion cells?

Answer 27

• mitochondria in cell release energy for active transport in phloem sieve tubes.
• sucrose are actively loaded into or pumped by companion cells into the phloem sieve tubes.
• maintain and service cytoplasm of phloem sieve tubes.