transport in plants

Question 1

What does xylem and phloem transport?

Answer 1

Xylem tissue: water and mineral salts (in the form of ions)

Phloem tissue: Manufactured foods (sugar + amino acids)

Question 2

What is the structure and function of the xylem?

Answer 2

Long hollow tubes with dissolved end walls
- absence of organelles (ie dead cells)
- allows for a continuous column of water
Hardened by lignin
- able to resist negative pressure without collapsing on itself

Question 3

The lignification of xylem vessels…

Answer 3

The lignification of xylem vessels

• makes them impermeable to water: to transport water and dissolved mineral salts from the roots to all other parts of the plant
• hardened to provide structural support to plant

Question 4

Incomplete lignification of xylem vessels…

Answer 4

Incomplete lignification of xylem vessels resulting in pits in walls
- for lateral movement of water to surrounding tissue

Question 5

What is the structure of the phloem?

Answer 5

• Living tissue with a high rate of aerobic respiration during transport (ie active process)
• Consists of sieve tubes and companion cells

Question 6

What are sieve tubes? Features?

Answer 6

Sieve tubes are narrow, elongated elements connected end to end to form tubes
The cytoplasm of mature sieve tubes contain no nucleus or many other organelles

Question 7

What is the function of companion cells?

Answer 7

They are connected to and maintains the cytoplasm of each sieve tube element

Question 8

Translocation is…

Answer 8

Translocation is the process by which molecules are synthesised (sucrose, amino acids and hormones) are transported around in the plant through the phloem tissues of the vascular bundles

Question 9

Xylem vessels have a ___-way flow while phloem vessels have a ___-way flow

Answer 9

Xylem vessels have a one-way flow while phloem vessels have a two-way flow

Question 10

What are root hairs? What do they do? (structure determines function!!)

Answer 10

Root hairs are extensions of epidermal cells. Root hairs dramatically increase the surface area available for absorption of water and dissolved ions.

Question 11

What is the path of water? Soil –> ?

Answer 11

Soil –> root –> stem –> leaf –> air

Question 12

Water moves into the roots by ___. Why / how so?

Answer 12

Osmosis. Root tips are in close contact with the soil solution which has a very high water potential, cells of the root cortex have a water potential much lower than that of the soil solution.

Question 13

There is also the ___ ___ / ____ of soluble ions into the roots (depending on the concentration in soil / root cells)

Answer 13

There is also the active uptake / diffusion of soluble ions into the roots (depending on the concentration in soil / root cells)

Question 14

What are the 2 main forces that enable mass flow of water against gravity?

Answer 14

1. Root pressure
   - water potential of root xylem vessel lowered by active transport of ions, water accumulates in the roots by osmosis and flows upwards up to a few meters (insufficient for tall trees)
2. transpiration
   - cohesion-adhesion theory

Question 15

What is transpiration?

Answer 15

Transpiration is a process through which water vapour is lost from aerial parts of plants as a result of the evaporation of water

Question 16

What are the 3 places / ways water is lost?

Answer 16

• at the surface of mesophyll cells into the air spaces within the leaf,
• followed by diffusion of water vapour out of the leaf, mainly through the stomata, down a water vapour concentration gradient
• from the surface of the spongy mesophyll cells via the air spaces in the leaf to the atmosphere

Question 17

How do plants control transpiration?

Answer 17

Stomata helps plants adjust their transpiration rates to external environmental conditions. They are formed by a pair of guard cells which open / close stomata as they change in shape

Question 18

Transpiration is a consequence of…

Answer 18

Transpiration is a consequence of gaseous exchange in plants.

Question 19

Factors increasing rate of transpiration: Humidity? Low / high

Answer 19

Low humidity will increase the water vapour concentration gradient between the leaf and atmosphere, hence increasing water evaporation.

Question 20

Factors increasing rate of transpiration: Temperature? Low / high

Answer 20

High temperatures increase heat energy which in turn increases the evaporation of water from the surface of mesophyll cells.

Question 21

Factors increasing rate of transpiration: Wind? Low / high

Answer 21

The stronger the wind the higher the rate of transpiration, wind blows away saturated air from around the leaves, thereby maintaining a concentration gradient between the leaf interior and the air outside stomata.

Question 22

Factors increasing rate of transpiration: Light? Low / high

Answer 22

In sunlight, the stomata become open and wider.

Question 23

Factors increasing rate of transpiration: Water supply? Low / high

Answer 23

An ample water supply will keep cells turgid and stomata remain open.

Question 24

Why does a plant wilt?

Answer 24

It wilts under strong sunlight when the rate of transpiration exceeds the rate of absorption of water by roots, cells become flaccid and plant wilts

Question 25

The rate of transpiration is reduced during wilting by…

Answer 25

• reduction of exposure of leaf surface area to sunlight

- closure of stomata from excessive loss of water

Question 26

Reduced rate of photosynthesis due to the reduction in…

Answer 26

• water
• carbon dioxide entering the leaf
• exposure of leaf surface area to sunlight