8: Plant Transport

Question 1

Define Mass flow

Answer 1

The movement of fluids through tubes so that all of the fluid moves in the same direction within each tube.

Question 2

What are the two main transport systems found in plants?

Answer 2

Xylem and Phloem are plant tissues composed of cells that are specialised for transport.

Question 3

What substances do xylem vessels transport, how are these substances transported and what direction are they being moved at.

Answer 3

Xylem vessels transports water and mineral ions. The root absorb the water and mineral ions. These enter the xylem in the root and travel upwards in the stem to the leaves, flowers, and fruits. Xylem vessels can only transport substances in one direction

Question 4

What substances are being transported using the phloem, and what direction do they flow in.

Answer 4

Phloem vessels transport sucrose, amino acids and hormones throughout the plant.

Substances transported in the phloem move in two directions; downwards and upwards to supply all parts of the plant.

Question 5

What are the functions of a root tip and the root cap.

Answer 5

Root tip is the end of the root where it grows as the cells divide.

Root cap protects the root tip as it grows through the soil.

Question 6

What are the functions of the Cambium, Vascular bundle, Epidermis found in the plant stem

Answer 6

Cambium cells make new xylem and phloem as the plant grows.

Vascular bundle is the bundle made up of phloem, xylem, and cambium.

The epidermis is a single layer of cells on the outside of the stem which protects the stem and reduces water loss.

Question 7

Explain the placement of xylem and phloem vessels in the stem of the root.

Answer 7

The the vascular bundle, xylem vessels are situated nearer to the inside of the stem whereas phloem vessels are placed right behind the xylem vessels nearer to the outside of the stem.

Question 8

What are the main functions or the root.

Answer 8

Roots help anchor the plant in the soil and to take up water and mineral ions.

Question 9

How is the root adapted to maximise efficient uptake of water and minerals?

Answer 9

Root hairs have thin permeable cell walls and provide a large surface area to absorb more water. The cell sap within the root hair cells is more concentrated than the soil water which allows water to diffuse into the cell via osmosis. This can be achieved as the cell membrane is partially permeable allowing water to diffuse into the plant.

Question 10

Explain the movement of water from the soil into the root into the xylem vessel in terms of water potential.

Answer 10

Water passes down a water potential gradient.
From a high water potential in the solution in the soil,
To a lower water potential in the root hair cell.

Water then again passes from the root hair cell to a lower water potential in the cells of the cortex and eventually into a lower water potential in the xylem vessel.

Question 11

How is water transported up the plant?

Answer 11

Transpiration pull is when water is ‘pulled’ up the xylem in the stem from the roots to the leaves.

Question 12

What are the uses of water in a plant?

Answer 12

Water is used for photosynthesis and to stop the plant from wilting.

Question 13

Define cohesion,

Answer 13

When water molecules tend to attract each other stick together.

Question 14

Define adhesion.

Answer 14

Water molecules sticking to the inside of the xylem vessel.

Question 15

What is a transpiration stream?

Answer 15

As water is used up or lost from the leaves, more is ducked up from the soil by the roots into the xylem vessel. This creates a continuous flow of water from the roots to the leaves.

Question 16

What is transpiration?

Answer 16

Transpiration is the loss of water vapour from plant leaves by the evaporation of water at the surfaces of the mesophyll cells, followed by the diffusion of water vapour through the stomata.

Question 17

How does water diffuse out the spongy mesophyll into the atmosphere?

Answer 17

Water transported into the spongy mesophyll cells is evaporated, this makes the air spaces saturated with water vapour.

There is more water vapour in the mesophyll than outside the plant, this causes water to side use through the stomata into the air

Question 18

What conditions cause the stomata to close?

Answer 18

The stomata close at night to reduce the volume of water lost by transpiration.

The stomata also closes in hot dry conditions during the day as water lost in transpiration is not being replaced by water from the soil.

Question 19

What are the causes of wilting? And explain them.

Answer 19

Plants wilt when it does not get enough water. Water loss>Water uptake.

It’s cells have lost so much water that they are no longer turgid. Turgid cells are required to give the plant support. Once these cells become flaccid, the plant becomes soft and the stem is no longer upright, the leaves droop.

Question 20

What are the factors that affect transpiration?

Answer 20

Light intensity
Humidity
Temperature

Question 21

Explain how light intensity affects the rate of transpiration?

Answer 21

Light causes the stomata to open.
As the light intensity increases, the stomata will open wider.

As the light intensity decreases, the stomata will close so that less water is lost by transpiration.

Question 22

Explain how Humidity affects transpiration.

Answer 22

In humid conditions the rate of transpiration is low.

In humid conditions, there may be nearly as much water vapour in the atmosphere as in the air inside the leaves. This means that there is no concentration gradient for water vapour so the rate of transpiration is low.

In dry conditions, the concentration gradient is very steep so water vapour diffuses out of the leaves through the stomata.

Question 23

Explain how temperature affects the rate of transpiration.

Answer 23

As temperature increases, the rate of transpiration increases.

As temperature increases, water molecules on the cell surfaces have more kinetic energy and enter the air inside the leaf as water vapour which is hen diffused out the leaf via the stomata.

Question 24

Define the term Translocation.

Answer 24

The movement of sucrose and amino acids from the leaves to regions of respiration, growth and storage.

Question 25

How is sucrose produced and what is it used for? 4p

Answer 25

The simple sugars produces by photosynthesis are converted in the leaves into sucrose.

• Sucrose is broken down by an enzyme to give simple sugars that are used in respiration.
• Sucrose is changed to starch for storage in the root cortex and in seeds.
• Sucrose is used to make cellulose for new cell walls at the growing root tip and shoot tip.
• Sucrose is stored in some fruits to make them sweet enough to attract animals.

Question 26

Define the terms ‘source’ and ‘sink’

Answer 26

Source is the part of the plant where a substance begins its journey.

Sink is the part of the plant where plant transport ends.

Question 27

Identify the sources and sinks involved in the transport of water and mineral ions.

Answer 27

The roots is the source.

The leaves, flowers, and fruits are the sinks.

Question 28

Identify the sources and sinks involved in the transport of sucrose and amino acids.

Answer 28

The source is the leaf.

The sinks are the respiring plant tissues, regions of growth (root and shoot tips) and regions of storage such as the root cortex and seeds.

Question 29

Explain why the movement of substances via the xylem are passive.

Answer 29

Xylem vessels are columns of dead empty cells.

The movement of water in xylem vessels are passive since it relies upon the evaporation of water vapour from the leaves producing a tension in the xylem.

Question 30

Explain why the movement of substances via the phloem are active.

Answer 30

Phloem tubes are living cells that contain some cytoplasm.

Movement in the phloem tube requires active transport of sucrose at the source. Water enters the phloem tubes to build a head of pressure that forces the phloem sap to the sinks.