9.2 Transport

Question 1

Outline why plants need to maximise the surface area of their roots

Answer 1

Plants take up water and essential minerals via their roots and therefore need to maximise surface area in order to optimise absorption

Question 2

Describe root systems in monocots

Answer 2

A fibrous, highly branching structure which increases surface area for maximal absorption

Question 3

Describe root systems in dicots

Answer 3

A main ‘tap root’ which can penetrate deeply into soil in order to access deeper reservoirs of water and minerals, as well as lateral branches to maximise surface area

Question 4

Outline the function of roots hairs in root systems

Answer 4

The root epidermis may have extensions called ‘root hairs’ which further increase surface area for mineral and water absorption. These root hairs have carrier proteins and ion pumps in their plasma membranes, and many mitochondria within the cytoplasm to aid active transport.

Question 5

List three ways in which mineral ions in the soil move into the root

Answer 5

Diffusion, fungal hyphae, mass flow

Question 6

Outline how mineral ions in the soil move into the root via diffusion

Answer 6

Movement of minerals along a concentration gradient

Question 7

Outline how mineral ions in the soil move into the root via fungal hyphae

Answer 7

Minerals absorbed and exchanged with sugars from the plant. This is a form of mutualism.

Question 8

Outline how mineral ions in the soil move into the root via mass flow

Answer 8

Uptake of mineral ions by means of a hydrostatic pressure gradient. Water being taken into roots via osmosis creates a negative hydrostatic pressure in the soil. Minerals form hydrogen bonds with water molecules and are dragged to the root, concentrating them for absorption.

Question 9

List the two processes by which plants can uptake mineral ions from the soil into their roots

Answer 9

Indirect active transport and direct active transport

Question 10

Outline how plants can uptake mineral ions from the soil into their roots via indirect active transport

Answer 10

• Fertile soil usually contains negatively charged clay particles which are covered with positively charged minerals
  
  • Roots cells contain protein pumps that actively pump H+ ions into the surrounding soil, which displaces the positively charged minerals, allowing for them to be absorbed
  • This method uses energy (protein pumps) to establish an electrochemical gradient by which mineral ions may be absorbed via diffusion

Question 11

Outline how plants can uptake mineral ions from the soil into their roots via direct active transport

Answer 11

• Protein pumps are used to actively translocate ions against their concentration gradient
  
  • This process occurs against a concentration gradient and requires metabolic energy (ATP) for transport
  • This is a selective process in which specific ions can be absorbed based on the needs of the plant
  • It uses protein pumps which select specific ions to be transported to the other side of the membrane
  • If a certain pump is not present, the substance will not be transported

Question 12

List three ways by which terrestrial plants may support themselves

Answer 12

Thickened cellulose, lignified xylem, cell tugor

Question 13

Outline how terrestrial plants may support themselves via thickened cellulose

Answer 13

Thickening of the cell wall provides extra structural support

Question 14

Outline how terrestrial plants may support themselves via lignified xylem

Answer 14

Xylem vessels run the length of the stem and branches; lignification of these vessels provides extra support

Question 15

Outline how terrestrial plants may support themselves via cell tugor

Answer 15

Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells turgid

Question 16

Define lignification

Answer 16

To turn into wood or become woody

Question 17

Define transpiration

Answer 17

Transpiration is the loss of water vapour from the leaves and stems of plants

Question 18

Explain how guard cells can regulate transpiration

Answer 18

• Stomata comprised of two elongated ‘guard cells’
  
  • The transpiration pull is generated by the negative hydrostatic pressure created by the evaporation of water vapour from the leaf
  • Stomata are comprised of two elongated ‘guard cells’ which regulated transpiration by controlling how much vapour can exit the leaf
  • When stomata are open the rate of transpiration will be higher than when they are closed
  • Stomata will open if turgor pressure is too high, and close if plant is wilting due to lack of water

Question 19

Outline the function of plant hormone abscisic acid (ABA)

Answer 19

• When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid
  
  • Abscisic acid trigger the secretion of potassium from guard cells, decreasing the water pressure within these cells and making them flaccid
  • This causes the stomatal pores to close

Question 20

List four factors which affect the rate of transpiration in a typical terrestrial plant

Answer 20

Light, temperature, wind, humidity

Question 21

Outline how light affects the rate of transpiration in a typical terrestrial plant

Answer 21

• Increasing the intensity of light increases the rate of transpiration
  
  • Light stimulates the stomata to open (gas exchange required for photosynthesis to occur)
  • Some of the light energy absorbed by leaves is converted into heat, which increases rate of water evaporation

Question 22

Outline how temperature affects the rate of transpiration in a typical terrestrial plant

Answer 22

• Increasing the temperature increases the rate of transpiration
  
  • Higher temperatures cause an increase in water vaporisation in the spongy mesophyll and an increase in evaporation from the surface of the leaf

This causes an increase in the diffusion of water vapour out of the leaf (via the stomata) which increases the rate of transpiration

Question 23

Outline how wind affects the rate of transpiration in a typical terrestrial plant

Answer 23

• Greater air flow around the surface of the leaf increases the rate of transpiration
  
  • Wind removes water vapour, increasing the concentration gradient between the inside and outside of the leaf

Question 24

Outline how humidity affects the rate of transpiration in a typical terrestrial plant

Answer 24

• Increasing the humidity decreases the rate of transpiration
  
  • Humidity is water vapour in the air, this a higher humidity means there is a higher concentration of water vapour in the air
  • This reduces the rate of diffusion of water vapour from the inside of the leaf as the concentration gradient is smaller resulting in less net flow

Question 25

List 5 adaptations of xerophytes that help to reduce transpiration

Answer 25

Reduced leaves, Rolled leaves, Thickened waxy cuticle, Low growth, Fewer stomata

Question 26

Outline how reduced leaves help to reduce transpiration

Answer 26

Reducing the surface area of the leaf will reduce the area for water loss and thus reduce transpiration

Question 27

Outline how rolled leaves help to reduce transpiration

Answer 27

Rolling up leaves reduces exposure of stomata to air and thus reduces transpiration

Question 28

Outline how thickened waxy cuticle helps to reduce transpiration

Answer 28

Prevents water loss from the surface of the leaves and thus reduces transpiration

Question 29

Outline how low growth helps to reduce transpiration

Answer 29

Plants located near the ground are less exposed to wind and are more likely to be shaded, reducing the rate of transpiration

Question 30

Outline how fewer stomata help to reduce transpiration

Answer 30

Less opportunity for water to escape

Question 31

Explain how water is carried by the transpiration stream

Answer 31

• Transpiration is water loss from the plant by evaporation
  
  • Flow of water through xylem from roots to leaves is the transpiration stream
  • Water lost by evaporation from spongy mesophyll cells or stomata is replaced with water from the xylem via osmosis
  • Water pulled out of xylem creates suction, resulting in a transpiration pull
  • The fact that the xylem vessels are thin allows water molecules stick together due to hydrogen bonding, creating a continuous column of water