chapter 8 transport in plants

Question 1

function of xylem

Answer 1

1. Provides mechanical support for the plant
2. Conducting water and dissolved mineral salts (DMS) from roots to the stems and leaves -> does not require energy

Question 2

adaptations of xylem

Answer 2

1. Walls are lignified (covered with hard woody substance): provide mechanical support = prevent collapse of vessel
2. Continuous empty lumen without any protoplasm or cross-walls: reduce resistance = water and DMS can flow and be transported easily from roots to other parts of the plant

Question 3

shape of xylem

Answer 3

looks like a star (transverse cut), is hollow and transparent (black-white photo), found closer to the centre of stem

Question 4

function of phloem

Answer 4

transports manufactured food substances such as amino acids and sucrose from the leaves to other parts of the plant

Question 5

structure of phloem

Answer 5

→structure: made up of sieve tubes and companion cells

1. Sieve tube consist of: sieve tube elements (made up of sieve tube cells - has protoplasm without nuclei) -> joined end to end with sieve plates
2. Each sieve tube is connected to a companion cell beside it: provides nutrients and helps STC to transport manufactured food

Question 6

adaptations of phloem

Answer 6

1. Sieve tube elements have little protoplasm: reduced resistance = increased flow of manufactured food substances
2. Sieve plate has pores: rapid flow of manufactured food substances
3. Companion cell has numerous mitochondria: release more energy for phloem to use in active transport (loading sugars from mesophyll cells to sieve tubes)

Question 7

function of root hair cells

Answer 7

Absorption of water by osmosis
Uptake of dissolved mineral salts by diffusion and active transport

Question 8

how is water taken in by plant

Answer 8

1. cell sap of root hair cells has a lower water potential compared to the soil solution surrounding RHCs <- cell sap of RHC is concentrated solution of sugars and salts
2. Creates water potential gradient = Water molecules moves into cell sap of root hair cell from soil solution through a partially permeable membrane by osmosis down the water potential gradient
3. Water potential of RHC’s cell sap is higher than inner cells’ cell sap = creates water potential gradient
4. Water molecules moves from cell sap of root hair cells to cell sap of inner cells by osmosis through a partially permeable membrane down a water potential gradient
5. continues until water reaches the xylem = xylem will transport water to other parts of the plant

Question 9

how is mineral salts taken in by plant by diffusion

Answer 9

1. concentration of mineral salts in soil solution is more concentrated than cell sap of RHC = creates concentration gradient
2. DMS will move from soil solution to cell sap RHC to other cells via diffusion down a concentration gradient
3. Cell sap of RHC is more concentrated than cell sap of other cells = creates concentration gradient
4. DMS will move from cell sap of RHC to other cells via diffusion down a concentration gradient
5. continues until DMS reaches xylem = DMS and water taken up xylem by transpiration pull (+ capillary action and root pressure)

Question 10

how is food transported in phloem

Answer 10

1. through translocation: transport of manufactured food substances such as sucrose and amino acids that occurs through the phloem from the leaves to other parts of the plant
2. translocation can occur the other direction: from other parts of the plant to the leaves = active transport -> require energy

Question 11

how are mineral salts taken in by active transport

Answer 11

1. concentration of mineral salts is lower in soil solution than in cell sap
2. DMS will move from soil solution to cell sap (higher conc) against concentration gradient by active transport = difference in concentration of mineral salts between cell sap and soil solution is greater
3. water potential of soil solution is higher than cell sap = steeper water potential gradient
4. water is taken in by osmosis from soil solution to cell sap of RHC at a higher rate

Question 12

when will mineral salts be taken in by active transport

Answer 12

when concentration of mineral salts in cell sap and soil solution are similar eg: cell sap of RHC has 5 mol of DMS, soil solution has 4mol of DMS

water potentials of cell sap and soil solution do not have big difference = water potential gradient to take in water molecules from soil solution to RHC is not very steep = rate of osmosis is low

Question 13

advantages of taking in mineral salts by active transport

Answer 13

results in faster water intake
1. Photosynthesis: faster water intake = higher rate of p/s
2. Wilting: faster water intake = plant will not continue wilting
3. Allows steep ion concentration gradients to be maintained for active transport = high rate of water intake

Question 14

adaptations of root hair cell

Answer 14

1. Has a long and narrow protrusion = increase surface area to volume ratio = increases the rate of absorption of water and dissolved mineral salts
2. has partially permeable cell surface membrane = prevents cell sap from leaking out = maintains the lower water potential than that of the solution in the soil = creating steep water potential gradient = high rate of water absorption by osmosis
3. contains mitochondria = carry out aerobic respiration to release energy = used in active transport of ions into the cells

Question 15

what forces transport water in the plant

Answer 15

1. Capillary action: tendency of water molecules to move up inside very narrow tubes -> liquid moves up to occupy empty spaces = moves up water in vessels of small plants
2. Root pressure: pressure resulting from constant entry of water molecules into roots -> process is after water entry
   ^1 and 2 only account for in small plants
3. transpiration pull - for trees

Question 16

what is transpiration (definition + process)

Answer 16

definition: loss of water in the form of water vapour mainly through the stomata, or aerial parts of the plant
→ process:
1. water continuously moves out of the mesophyll cells = forms a thin film of moisture over the surfaces of spongy mesophyll cells
2. water evaporates into water vapour from the thin film of moisture = water vapour accumulates in the intercellular air spaces in the spongy mesophyll layer
3. higher concentration of water vapour in the leaf than that outside the leaf = steep water vapour concentration gradient created
3. water vapour diffuses out the leaf through the stomata (water loss)

Question 17

what is transpiration pull

Answer 17

1. Water continuously moves out of mesophyll cells = water potential of cell sap in mesophyll cells decreases
2. water potential of cell sap of cells deeper in the leaf are higher = creates a steep water potential gradient
3. mesophyll cells absorbs water by osmosis from the cells deeper in the leaf = process continues until cells remove water from veins (xylem vessels)
4. Creates suction force = pulls whole column of water up xylem vessels -> transpiration pull

Question 18

how to measure rate of transpiration

Answer 18

potometer
- assume rate of transpiration is proportional to rate of water absorption (to replace water loss by plant as it transpires)
- volume (cm3) or mass (g) of decrease in water/time taken (h)

Question 19

why is transpiration important

Answer 19

1. Transpiration pull: draws water (and dissolved mineral salts) from the roots to the leaves
2. transpiration = evaporation of water from cells in the leaves -> removes latent heat of vaporisation = cools plant
3. Transpiration pull transports water to leaves = keeps leaf cells turgid -> keep leaves spread out widely = trap sunlight for photosynthesis

Question 20

factors affecting transpiration (4)

Answer 20

1. humidity of air (inverse r/s)
2. temperature of air (positive r/s)
3. light intensity (positive r/s)
4. presence of wind

Question 21

how does humidity affect rate of transpiration

Answer 21

High humidity of air: high concentration of water vapour in surrounding air = concentration gradient between leaf and surrounding air becomes less steep = rate of diffusion is lower = rate of t/p decrease

Question 22

how does temp affect rate of t/p?

Answer 22

High temp of air: rate of evaporation of water from thin film of moisture on SMC increases = increase water vapour concentration in ICAS = steepen water vapour concentration gradient between surrounding air and leaf = rate of diffusion is higher = rate of t/p increases

Question 23

how does light intensity affect rate of t/p

Answer 23

Higher light intensity: causes stomata to open and become wider = rate of diffusion from leaf to surrounding air through opened stomata increases (more water vapour able to diffuse out at one time) = rate of t/p increases

Question 24

how does presence of wind affect rate of t/p

Answer 24

presence of wind/air movement: water vapour that accumulates outside stomata is blown away = concentration of water vapour in surrounding air decreases = water vapour concentration gradient steepens = rate of diffusion is higher = rate of t/p increases

Question 25

how does a plant wilt

Answer 25

on a hot day
rate of transpiration exceeds rate of water absorption - water loss exceeds water intake = cells lose turgor = plant wilts

Question 26

advantage of plant wilting (1)

Answer 26

prevent excessive water loss:
wilted leaves fold up = surface area exposed to light decrease -> absence of light = guard cells become flaccid = stomata close = rate of transpiration decreases = prevent excessive water loss

Question 27

the disadvantage of plant wilting (1 disadvantage + 3 reasons)

Answer 27

rate of photosynthesis decreases
1. water does not become a LF (does not affect rate of p/s when water increases) = rate of p/s decreases
2. Stomata closed = amt of carbon dioxide taken in by leaf decreases = carbon dioxide becomes LF = rate of p/s decreases
3. Leaf is folded = reduced surface area exposed to light = rate of p/s decreases