Transport in plants

Question 1

Why do plants need transport systems (including size, metabolic demands and SA:V ratio)? (6)

Answer 1

• Size: in larger plants, the distance between the roots and leaves (1) means the plant must move water and minerals over challenging distances. (1)
• Metabolic demands: lots of cells (that do not photosynthesise) require oxygen and glucose to be transported; hormones must be transported to target areas; mineral ions required for protein synthesis and cell structure; waste products (of cell metabolism) must be removed. (2)
• SA:V ratio: small plants may be able to rely on diffusion alone, but taking roots, trunks and stems into account, a lot of plants have small SA:V ratios. (1) Diffusion alone cannot supply everything each cell needs. (1)

Question 2

Fill in the blanks about vascular bundle structures:

Stem - Xylem are nearer to the ___, furthest from the outside of the stem to reduce ___ loss by transpiration. The vascular bundles around the edge of the stem provide ___ support.

Leaves - Xylem is closer to the top of the leaf, furthest from the ___ and closest to the ___ cuticle. The structure allows for leaves to be long and ___, maximising their absorption of ___ and SA:V ratio.

Roots - Xylem are often in an ‘___’ shape in the middle of the root. Being in the middle, the vascular bundle resists tugging and keeps the plant ___ in the ___. (10)

Answer 2

• Middle (1)
• Water (1)
• Structural (1)
• Stomata (1)
• Waxy (1)
• Thin (1)
• Light (1)
• X/star (1)
• Upright (1)
• Wind (1)

Question 3

Describe the function and structure of the xylem. (4)

Answer 3

• Transports water and mineral ions. (1)
• Supports the plant. (1)
• Resources travel from the roots to the leaves; upwards. (1)
• Xylem vessels are columns of (mostly) dead cells fused together; long hollow tubes. (1)
• Xylem parenchyma is packed around xylem vessels and stores food; tannin deposits. (1)
• Spirals of lignin are around the xylem lumen providing mechanical strength; preventing the collapse of xylem vessels from the transpiration pull. (1)

Question 4

Describe the structure of sieve tubes in the phloem and their purpose. (4)

Answer 4

• Elongated cells joined end to end; long hollow structure. (1)
• Lacks nucleus and other organelles due to perforated cell wall. (1)
• Allows them to be filled with phloem sap. (1)
• Perforated cell walls at the end of each cell allow materials to pass through. (1)

Question 5

How do companion cells support sieve tube cells? (2)

Answer 5

• Connected by plasmodesmata. (1)
• Acts as a life support system for sieve tube cells that have lost their normal cell functions. (1)

Question 6

Explain two ways root hair cells are adapted for exchange. (4)

Answer 6

• Large SA:V ratio - More area units per volume units for exchange to occur over. (2)
• Cytoplasm with lower water potential - Maintains a concentration gradient between the cell and the soil. (2)
• Many mitochondria - Helps root hair cells actively transport mineral ions into the cell (also lowers water potential). (2)
• They are microscopic - They can push between particles of soil, helping them gather resources. (2)
• Thin surface layers - Minimises the distance water and mineral ions need to diffuse across. (2)

Question 7

What are the names of the two pathways water can travel through the roots to the xylem? (2)

Answer 7

Apoplast and symplast

Question 8

Describe the symplast pathway. (5)

Answer 8

• The symplast is the continuous cytoplasm of plant cells connected by plasmodesmata. (1)
• Water moves by osmosis. (1)
• The root hair cell will have a higher water potential than the next cell along (1), due to the active transport of mineral ions. (1) This repeats until water reaches the xylem. (1)

Question 9

Describe the apoplast pathway.

Answer 9

• The apoplast in the cell walls (loose, open network) and spaces between cells. (1)
• A pull is created by water being pulled into the xylem and cohesive forces between water molecules. (1)
• Maintains continuous flow of water. (1)

Question 10

Fill in the blanks about water moving into the xylem:

Once water in both the ___ and ___ pathway reach the endodermis (cells that surround ___ tissue in the roots), water in the apoplast pathway cannot go further. This is due to the ___ strip, a ___ layer, which runs around each of the endodermal cells. Water in the symplast route can travel through and water from the apoplast pathway joins the symplast pathway.
This forces the water to pass through the ___-___ membrane by ___ diffusion. This prevents toxic solutes entering the ___. (8)

Answer 10

• Apoplast/symplast (1)
• Apoplast/symplast (1)
• Vascular (1)
• Casparian (1)
• Waterproof (1)
• Partially-permable (1)
• Facilitated (1)
• Xylem (1)

Question 11

Explain root pressure and give an example of evidence found for its effect. (5)

Answer 11

• Endodermal cells are usually more dilute than xylem cells. (1)
• This is because they actively transport mineral ions into xylem cells. (1)
• This causes water from endodermal cells to move down a concentration gradient into the xylem cells. (1)
• If a poison enters the root cells and prevents mitochondria producing ATP (1), mineral ions cannot be actively transported causing root pressure to disappear. (1)
• Root pressure increases in increased temperature/falls with decreased temperature (1), suggesting the involvement of chemical reactions. (1)
• If levels of oxygen and glucose fall (1), root pressure falls (cannot as much produce ATP). (1)

Question 12

What is transpiration? (1)

Answer 12

Water being lost as vapour from the leaves or stem of a plant. (1)

Question 13

Plants need to maintain balance between keeping their stomata open and shut. Why is this? (1)

Answer 13

If they are open too long, excessive amounts of water will be lost by transpiration; if they are shut too long, sufficient gas exchange will not occur. (1)

Question 13

Fill in the blanks about stomata:

When guard cells are ___ (swollen), they bend ___ to open the stomata. This is caused by the ___ ___ of solutes into the guard cells, causing water to ___ via osmosis.
When guard cells are ___ (lost turgor), they flop ___ to close the stomata. This is caused by solutes being pumped out of the guard cells, causing water to ___ via osmosis. (7)

Answer 13

• Turgid (1)
• Outwards (1)
• Active transport (1)
• Enter (1)
• Flaccid (1)
• Inwards (1)
• Leave (1)

Question 14

Describe the transpiration stream and cohesion-tension theory. (5)

Answer 14

• Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the leaf into the air (diffusion). (1)
• The evaporation of the water causes water to move into the mesophyll cells (due to the lower concentration gradient). (1)
• This repeats across the leaf to the xylem, causing water to enter the leaf cells from the xylem. (1)
• Water adheres to the carbohydrates in the walls of xylem vessels and coheres to other water molecules. (1)
• Due to this, as well as the need to replace water lost by evaporation (move down a concentration gradient), water is pulled up the xylem. (1)
  (This creates tension in the xylem, hence cohesion-tension theory).

Question 15

How do the factors of light intensity and temperature affect transpiration? (4)

Answer 15

• Higher light intensities will increase the rate of transpiration. (1)
• This is because light is required for photosynthesis, so more stomata will be open (therefore, increasing evaporation from the surfaces of leaves). (1)
• Higher temperatures will increase the rate of transpiration. (1)
• Kinetic energy of water molecules increases with temperature; decreases relative humidity of surrounding air and its water potential (therefore, increasing evaporation from the surfaces of leaves). (1)

Question 16

What is relative humidity. (1)

Answer 16

The amount of water vapour in the air compared to the amount of water vapour the air can hold. (1)

Question 17

Explain how lower air movement will affect the rate of transpiration compared to higher air movement. (3)

Answer 17

• Water vapour will collect in the still layer of air around the leaf; low air movement may not move this layer. (1)
• Concentration gradient between water vapour potential inside and outside of the leaf will be less steep. (1)
• Rate of transpiration will be lower. (1)

Question 18

Describe three xerophyte adaptations. (6)

Answer 18

• Thicker waxy cuticles (1) minimise water loss. (1)
• Sunken stomata (1) reduce air movement; keeps moist humid air near the stomata, decreasing the concentration gradient. (1)
• Less stomata (1) reduces water loss from transpiration. (1)
• Less leaves (1) reduces SA:V ratio, minimising water loss from transpiration. (1)
• Hairy leaves (1) traps a layer of moist humid hair, decreasing the concentration gradient. (1)
• Long tap roots (1) are able to reach water a way below the surface so the plant can get as much water as possible. (1)
• Curled leaves (1) confine the plant’s stomata in a microclimate of still, humid air. (1
• Succulents (like cacti) have specialised parenchyma tissues (1) to store water. The water can be stored when water is plentiful and used in drought. (1)

Question 19

Describe two hydrophyte adaptations. (4)

Answer 19

• No/thin waxy cuticle (1) as water being lost is not a problem. (1)
• They have many/always open stomata (1), there is no risk of losing turgor, so gaseous exchange is maximised. (1)
• Small roots (1). These plants get most of their water from it directly diffusing into the stem and leaf tissue. (1)
• Wide/flat leaves (1) spread across the surface of water to maximise the light they receive. (1)
• Aerenchyma (specialised parenchyma) (1) has many large spaces which make the plant more buoyant and forms a low resistance passage for the movement of oxygen to tissues that may be below the water. (1)
• Less structural features (1) as the water supports the leaves and flowers. (1)

Question 20

What is translocation? (1)

Answer 20

Organic compounds being transported from sources to sinks. (1)

Question 21

Give two examples of a source and two examples of a sink. (4)

Answer 21

• Green leaves.
• Green stems.
• Food stores in seeds when they germinate.