TRANSPORT IN PLANTS

Question 1

What does the vascular system consist of?

Answer 1

Xylem tissue and phloem tissue

Question 2

Describe the xylem vessels and their function.

Answer 2

Elongated hollow tubes that are made of dead xylem cells linked end to end.

1. Transport water and mineral salts from the roots to the leaves
2. Provide mechanical support to the plant

Question 3

How are the xylem cells adapted for their function?

Answer 3

1. Absence of protoplasm and cross-walls -> prevent water flow through the lumen from being impeded
2. Deposition of lignin on the cell walls strengthens vessel walls, providing support

Question 4

Describe the phloem tissue and its function.

Answer 4

Consists of sieve tube elements and companion cells.

Conducts sugars and amino acids from the leaves to other parts of the plant.

Question 5

Describe sieve tube elements.

Answer 5

Elongated, thin-walled living cells that degenerate protoplasm, and hence lack most organelles (e.g. nucleus, vacuole). They are arranged end to end, with porous walls called sieve plates between them. These pores allow food to move through the sieve tubes rapidly.

Question 6

Describe companion cells.

Answer 6

Companion cells contain nuclei, cytoplasm and numerous mitochondria. One such cell is closely associated with each sieve tube element.

Companion cells perform the metabolic functions of the sieve tube elements.

Question 7

Describe the arrangement of vascular bundles in dicotyledonous stems.

Answer 7

1. Vascular bundles are arranged in a ring around a central pith.
2. Between the ring of vascular tissue and the epidermis is the cortex.
3. Within the vascular bundles, the phloem tissue is found on the side facing the cortex and the xylem on the side facing the pith.
4. Between the xylem and phloem is the cambium

Question 8

Describe the epidermis and its adaptation.

Answer 8

One-cell thick layer covered by a waterproof cuticle that minimises water loss in the stem.

Question 9

Describe the cortex and its function.

Answer 9

Consists of a few layers of thick-walled, large and rounded cells with intercellular spaces.

Stores food.

Question 10

Describe the pith and its function.

Answer 10

Found inside the ring of vascular bundles. Made up of thin-walled cells which serve as storage tissue for food.

Question 11

What is the function of the cambium?

Answer 11

Cambium cells differentiate into new xylem and phloem tissues that thicken the stem.

Question 12

Describe the arrangement of vascular tissue in dicotyledonous roots.

Answer 12

1. The outermost layer of the root is the epidermis. The outer walls of these cells protrude to form unicellular root hairs. NO cuticle is present
2. Layer below the epidermis is the cortex (same as stem)
3. The central region of the root contains the xylem and phloem tissues. Xylem tissues radiate from the centre, with phloem tissues alternating between them

Question 13

How are root hair cells adapted for their function?

Answer 13

1. Long and narrow -> large surface area to volume ratio for rapid absorption of water and minerals
2. Higher concentration of cell sap -> water enters the roots by osmosis
3. Able to carry out active transport -> provide energy for active transport of mineral salts and ions

Question 14

Describe the process by which water and minerals are taken up by root hair cells.

Answer 14

1. Cell sap in the root hair cells contains sugar and ions, causing WP to be lower than that of the soil solution
2. Water moves across the partially permeable cell membrane from the soil solution into the cell sap by osmosis
3. Cell sap now has a higher WP than the cell sap in the adjacent cell
4. Water moves across the cell membranes into the adjacent cell by osmosis
5. This continues until the water enters the xylem vessels and up the plant

Question 15

What mechanisms enable water to move up the stem through xylem vessels?

Answer 15

1. Root pressure
2. Transpiration
3. Capillary action

Question 16

Explain the root pressure mechanism.

Answer 16

1. Root cells pump mineral salts into xylem vessels by active transport -> WP of xylem vessels is lower than WP of root cortex cells
2. Water moves up into the xylem vessels by osmosis, causing cells to become turgid. This creates a root pressure that forces water to move upwards

Question 17

What is the limitation of root pressure?

Answer 17

Only forces water to travel short distances

Question 18

Explain the capillary action mechanism

Answer 18

Water tends to travel up very thin tubes due to the attractive forces between the water molecules being greater than the forces trying to separate them.

(smaller plants)

Question 19

Define transpiration

Answer 19

Loss of water vapour from the stomata of the leaves through diffusion

Question 20

Explain the transpiration mechanism

Answer 20

1. During photosynthesis, stomata are open to take in carbon dioxide
2. Water vapour from the intercellular air spaces in the leaves diffuses out into the air outside (concentration gradient)
3. As water evaporates from the mesophyll cells, the water potential of the sap decreases.
4. Mesophyll cells absorb water from adjacent cells closer to the vascular bundles by osmosis. These cells, in turn, absorb water from the xylem vessels
5. This pressure suctions water up the xylem from the roots, resulting in transpiration pull

Question 21

Why is transpiration important?

Answer 21

1. Transport water and mineral salts
2. Cooling of leaf

Question 22

What factors affect the rate of transpiration?

Answer 22

1. Humidity of surroundings -> higher humidity = higher concentration of water vapour in surrounding air = less steep concentration gradient = less transpiration
2. Air movement -> wind removes water vapour that accumulates outside the stomata (transpiration) = maintains steep concentration gradient = more transpiration
3. Temperature -> heat increases rate of evaporation = higher rate of movement of water vapour = more transpiration
4. Light intensity -> more light = induced stomatal opening = more transpiration

Question 23

Explain wilting

Answer 23

When a leaf loses water, its turgor pressure will fall -> water can enter the vacuole to restore turgor pressure

1. Rate of transpiration exceeds rate of absorption of water by roots
2. Cells may get water by osmosis from adjacent cells more readily than from xylem vessels
3. Plant cells become flaccid -> WILT

Question 24

What control mechanisms do plants have to control transpiration?

Answer 24

1. Stomata (movement of opening)
2. Leaf fall (in wintertime)
3. Leaf shape and cuticle

Question 25

Define translocation

Answer 25

Transport of sugars from the leaves to other parts of the plant by the phloem tissues

Question 26

Describe the movement of sugars in translocation

Answer 26

1. Sugars are supplied by the leaves
2. Sugars are transported to the companion cells of the phloem by active transport (requires energy)
3. Sugars are transported along the phloem and unloaded at sink (recipient) cells by diffusion or active transport