Mass transport in plants

Question 1

define mass Transport

Answer 1

the bulk movement of substances from one area of an organism to another at a similar rate

Question 2

Water uptake from the soil

Answer 2

water enters through the root hair cell within the epidermis by osmosis
root hair cells actively transport ions using energy from the hydrolysis of ATP, from soil across their membranes into their cytoplasm via carrier proteins
this lowers the water potential of the cytoplasm, water enters the root hair cells by osmosis down a water potential gradient
water then passes through the cortex, across endodermis and into the xylem vessels.

Question 3

what are the adaptations of the root hair cell

Answer 3

hair like extensions that increase their surface area to increase the uptake of water and increase the area for channel and carrier proteins.

thinner cellulose cell walls to enable a shorter diffusion pathway

Question 4

how are mineral ions pumped in

Answer 4

by active transport- uses carrier proteins that change shape using energy from ATP hydrolysis. Higher conc in the root hair cell so lower water potential, enters by osmosis through aquaporins

Question 5

what is the problem with waterlogged soil

Answer 5

less O2 available, less respiration, less ATP for active transport

Question 6

xylem tissue structure

Answer 6

Dead cells that form hollow tubes with no cytoplasm: faster water flow as less resistance

End walls break down forming a continuous tube with no end walls: Continuous column of water forms due to cohesion

Cell walls strengthened with lignin: xylem becomes waterproof and rigid, provides support and withstands tension

Xylem pits: Allow water to move laterally between xylem vessels

Question 7

Cohesion-tension theory

Answer 7

water evaporates/ is transpired from leaves
water lost, lowers water potential in leaf cells
water replaced from xylem
water potential gradient creates tension
hydrogen bonds form continuous column of water

Question 8

describe the long process of transpiration

Answer 8

stomata open causing water to diffuse from the air spaces at higher water potential inside the leaf. the loss of water is called transpiration
loss of water causes water to move down a water potential gradient from the mesophyll cells to air spaces
this lowers water potential of mesophyll so water moves to adjacent mesophyll
this creates a water potential gradient across the leaf to the xylem cells
water from xylem enters leaf and causes water to be pulled up under tension through xylem from the root
water forms a continuous column
water molecules from weak hydrogen bonds which create cohesion
they are also attracted to the walls of the xylem- adhesion

Question 9

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.

Answer 9

(In the root) 1. Casparian strip blocks apoplast pathway / only allows symplast pathway;
2. Active transport by endodermis;
3. (Of) ions/salts into xylem;
4. Lower water potential in xylem / water enters xylem by osmosis /down a water potential gradient; (Xylem to leaf)
5. Evaporation / transpiration (from leaves);
6. (Creates) cohesion / tension / H-bonding between water molecules / negative pressure;
7. Adhesion / water molecules bind to xylem;
8. (Creates continuous) column of water

Question 10

Root pressure moves water through the xylem. Describe what causes root pressure.

Answer 10

1. Active transport by endodermis;
2. ions/salts into xylem;
3. Lowers water potential (in xylem);
4. (Water enters) by osmosis;

Question 11

Describe how a high pressure is produced in the leave

Answer 11

1. Water potential becomes lower/becomes more negative (as sugar enters phloem);
2. Water enters phloem by osmosis;
3. Increased volume (of water) causes increased pressure;

Question 12

Affect of light intensity on transpiration

Answer 12

high light intensity= more stomata open
Stomata open in the light and close in the dark due to guard cells decreasing the Ψ of their cytoplasm and water enters by osmosis. The guard cells increase in volume which causes the stoma to open.

Question 13

Affect of temp on rate of transpiration

Answer 13

increased temp= increased rate of transpiration
↑ Temperature = ↑ KE = ↑ rate of Diffusion and Evaporation of water If the soil is dry, then roots will release abscisic acid into the xylem. This triggers the closure of stomata to preserve water loss.

Question 14

affect of humidity on rate of transpiration

Answer 14

increased humidity=decreased rate of transpiration
The air spaces in the leaf are saturated with water vapour. The air outside the leaf contains much less water vapour. The greater the difference in humidity between the air spaces and the air outside the leaf, the greater the rate of diffusion of water vapour out of the leaf, so the greater the rate of transpiration. Water leaves the leaf down a water potential gradient.

Question 15

Wind speed and rate of transpiration

Answer 15

Increased wind speed= increased rate of transpiration
Air movement over a leaf moves the water vapour away from the stomatal pores. This increases the water potential gradient between the inside and the outside of the leaf. Remember xerophytes have sunken stomata. This keeps a higher humidity outside the stomata, so reduces transpiration.

Question 16

stomatal density and rate of transpiration

Answer 16

higher stomatal density= higher rate of transpiration
Stomatal density refers to the total number of stomata in a given area.

Question 17

measuring the rate of transpiration

Answer 17

A leafy shoot is diagonally cut under water, preventing damage to the xylem ensuring a continuous column of water

Don’t get water on the leaves as could prevent the loss of water vapor through the stomata

potomoter filled with water but no air bubbles as ensures a continuous column of water

use a rubber tube and fit it under the water

introduce an air bubble into the capillary tube, as transpiration occurs the bubble moves

to determine volume of water entering the shoot : 𝐿𝜋𝑟2

distance moved over a period of time is recorded and a mean is calculated

then calculate rate of uptake- Using: 𝐿𝜋𝑟2/time

Question 18

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot

Answer 18

1. Seal joints / ensure airtight / ensure watertight;
2. Cut shoot under water;
3. Cut shoot at a slant;
4. Dry off leaves;
5. Insert into apparatus under water;
6. Ensure no air bubbles are present;
7. Shut tap;
8. Note where bubble is at start / move bubble to the start position;

Question 19

Xerophyte adaptations

Answer 19

1. Reduced number of stomata
2. Stomata in pits
3. Hairs to trap water
4. Rolled leaves
5. Leaves reduced to spines
6. Thick waxy cuticles

Question 20

Phloem tissue structure

Answer 20

· Solutes are dissolved substances.

· Phloem tissue transports organic solutes (mainly sucrose) around plants.

· Phloem tissue is formed from cells arranged in long tubes.

· Sieve tube elements and companion cells are important cell types in phloem tissue.

· Sieve tube elements are living cells that form the tube for transporting solutes. They have no nucleus and few organelles.

· Each sieve tube element has it’s own companion cell, that carries out living functions for sieve cells.

· Companion cells contain many mitochondria to synthesise ATP through aerobic respiration for the active transport of solutes.

· Sieve tubes are connected to each other through sieve plates.

Question 21

Translocation

Answer 21

· Translocation is the movement of solutes to where they are needed in a plant. Solutes are sometimes called assimilates

· Translocation is an energy requiring process, that happens in the phloem.

· It moves solutes from SOURCES (where assimilates are produced in higher concentrations) to SINKS where assimilates are used, (so lower concentration).

· Enzymes maintain a concentration gradient from the source to the sink by converting the solutes at the sink to other storage substances (e.g., Starch).

· This makes sure there is always a lower concentration at the sink than the source.

· E.g. the source for sucrose is usually the leaves where it is made and the sinks are the storage organs and the meristems (areas of growth) in the roots stems and leaves.

· In potatoes, sucrose is converted to starch, in the sink areas so there is always a low concentration of sucrose at the sink than inside the phloem.

Question 22

mass flow hypothesis

Answer 22

Source

· Companion cells actively transport (using hydrolysis of ATP) sucrose into the sieve tubes of the phloem at the source

· This lowers water potential in sieve tubes

· Water enters the phloem sieve tubes by osmosis from xylem

· This creates a high hydrostatic pressure inside the sieve tubes at the source end of the phloem

2. Sink

· Sucrose is actively transported from the phloem sieve tubes into sink cells and converted to starch or repired.

· This increases the water potential inside the sieve tubes so water exits the tubes by osmosis

· This lowers the hydrostatic pressure inside the sieve tubes

3. Mass Flow

· This forms is a pressure gradient from the source end to the sink end.

· This gradient pushes solutes towards the sink (roots & growinging regions).

· When they reach the sink the solutes will be used (e.g. in respiration) or stored (e.g. as starch)

· The higher concentration of sucrose at the source the higher rate of translocation

Question 23

Describe the mass flow hypothesis for the mechanism of translocation in plants

Answer 23

1. In source/leaf sucrose actively transported into phloem;
2. By companion cells;
3. Lowers water potential of sieve cell/tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink/root); 5. Sucrose (sugars) used/converted in root for respiration for storage;