Transport In Plants

Question 1

Why do plants need a transport system?

Answer 1

1. High metabolic demands as they make their own glucose and oxygen which needs to be transported to other parts of the plant for respiration
2. Waste products from metabolism also needs to be removed
3. Hormones made in one part of the plant need to be transported to where they have an effect
4. Multicellular plants have a low SA:V ratio so they cannot rely on diffusion as an effective means of transport

Question 2

State the positioning of the transport tissue in the herbaceous dicot root and explain how this positioning is related to their function (2 marks)

Answer 2

They are located at the centre(1) to help give strength against rigging forces when plants are blown by winds(1)

Question 3

State the positioning of the transport tissue in the herbaceous dicot stem and explain how this positioning is related to their function (2 marks)

Answer 3

The transport tissues are located around the edge (1) which helps give strength and support to structure (1)

Question 4

State the positioning of the transport tissue in the herbaceous dicot leaf and explain how this positioning is related to their function (2 marks)

Answer 4

A large central vein containing vascular tissue (1) gives support to the broad structure of leaf (1)

Question 5

What is the function of the xylem?

Answer 5

• It transports water and mineral ions from the root to the leaves
• It also provides support

Question 6

What is the structure of the xylem?

Answer 6

1. It is made up of dead cells joined end to end with no end walls between the cells
2. They are long, non-living, hollow vessels
3. Parenchyma cells pack around the xylem and store food and tannin. Tannin is a foul tasting chemical that prevents the plant from being attacked by herbivorous
4. The walls of the xylem are reinforced with lignin which helps to support the vessels and stops them collapsing inwards
5. The walls also have bordered pits where water leaves the xylem and moves into other cells of the plant.
6. Movement of water is unidirectional- water can only move from the roots up the xylem

Question 7

What is the function of the phloem?

Answer 7

It transports solutes from the leaves to other parts of the plant

Question 8

Structure of the phloem

Answer 8

1. It contains sieve tube elements and companion cells
2. The sieve tube elements are living cells( that have lost their nucleus, some organelles and cytoplasm) that are fused end to end with perforated sieve plates separating each element, forming a long hollow structure
3. The sieve plates allow phloem sap to move between the cells
4. Companion cells are linked to sieve tube elements by plasmodesmata and they provide the sieve tube element with essential molecules
5. Movement of solutes is bi-directional. This means the assimilates can move both up and down and down to up.

Question 9

What is the plasmodesmata?

Answer 9

They are microscopic channels that link the cytoplasm of two adjacent cells

Question 10

State three differences between transport systems in multicellular plants and multicellular animals (3 marks)

Answer 10

In plants, one type of vessel is made up of dead cells while animal vessels are all made up of living tissues (1). Plants have different transport systems carrying different materials but animals have different vessels but the same transport medium (1). Plants have no heart to act as a central pump whereas multicellular animals have a heart (1)

Question 11

Adaptations of the root hair cells

Answer 11

1. They have a thin surface layer so diffusion and osmosis can take place quickly
2. They are very small in size so they can penetrate soil particles
3. They have a high SA:V ratio for water absorption
4. The concentration of solutes in the cytoplasm of the root hair cells maintains a water potential gradient between the soil water and the cell

Question 12

Process of movement of water into the roots

Answer 12

Water is drawn into the roots via osmosis. Soil water has a high water potential as it has a low concentration of dissolved minerals. The cytoplasm and vacuolar sap of the root hair cells has a high concentration of dissolved minerals which lowers the water potential, so water moves into the root hair by osmosis

Question 13

Process of the movement of water into the xylem (through the root cortex) - the symplast pathway

Answer 13

1. In this pathway, water moves from the cytoplasm of one cell to another via the plasmodesmata.
2. The pathway is driven by the water potential gradient between the root hair cells and the xylem. Water is continually being drawn into the root hair cells by osmosis from the soil. This makes the water potential in the root hair cells greater than the water potential in the cortex cells
3. In the xylem the water potential is low because the root hair cells are actively transporting mineral ions to the xylem which lowers the water potential causing water to move into the xylem by osmosis

Question 14

The apoplast pathway

Answer 14

1. In this pathway, water moves through the cell wall and the intercellular spaces between the cells
2. As the water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive force between the water molecules.
3. Water continues to move through the root cortex until it reaches the endodermis. The wall of the endodermis is surrounded by a band of waxy material called suberin which blocks the path of the apoplast pathway forcing the water into the cytoplasm to join the symplast pathway

Question 15

Why is the casparian strip important?

Answer 15

It is important as it forces all the water and it solutes through the selectively permeable cell surface membrane removing any potentially toxic chemical from reaching living tissues

Question 16

What is the root pressure?

Answer 16

It is the positive pressure exerted by root cells that helps push water and dissolved minerals upwards into the xylem vessels

Question 17

How is the root pressure created?

Answer 17

It is created when the root cells actively transport mineral ions into the xylem, lowering the water potential within the xylem and drawing water in by osmosis which then builds up and creates pressure

Question 18

What is Transpiration?

Answer 18

It is the loss of water through the stomata in the leaves by evaporation

Question 19

Mechanisms to move water from the roots up the xylem

Answer 19

1. Cohesion: This is when water molecules stick together allowing them to easily be pulled up as a column up the xylem vessel. This is called the transpiration pull
2. Tension: This is caused by a water potential gradient between the xylem into the spongy mesophyll cell by osmosis
3. Adhesion: This is when the water molecules are sticking to the cellulose on the cell wall of the xylem.

Question 20

How is transpiration controlled in the leaves?

Answer 20

It is controlled by the stomata which opens and closes to let CO2 in and O2 and water out.

Question 21

Process of opening and closing the stomata.

Answer 21

The inner wall of a guard cell is thicker than the outer wall. When the guard cell fills with water it becomes turgid and the outer wall balloons outwards, drawing the inner wall with it and causing the stomata to open.

Question 22

Factors affecting transpiration: Temperature

Answer 22

As temperature increases, the rate of transpiration increases. This is because the water molecules have more kinetic energy and so they evaporate from the cells inside the leaf faster, increasing the rate of diffusion of the water molecules outside the leaf

Question 23

How does light intensity affect transpiration rate?

Answer 23

As light intensity increases, the rate of transpiration increases. This is because there are more stomata open which increases the rate of diffusion of the water molecules from inside the airspace to the outside air

Question 24

How does humidity affect transpiration rate?

Answer 24

The drier the air around the plant is, the faster the rate of transpiration. This is because there is an increased water vapour concentration gradient between the leaf and the outside air so more water evaporates from the surface of the leaf

Question 25

How does wind affect transpiration rate?

Answer 25

The windier it is, the faster the rate of transpiration. This is because lots of air movement decreases the humidity which then increases the water vapour concentration gradient so more water evaporates from the surface of the leaves.

Question 26

What instrument is used to estimate transpiration rate?

Answer 26

A Potometer

Question 27

Procedure of experiment.

Answer 27

1. Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant angle to increase the SA available for water uptake 2. Assemble the potometer in water and insert the shoot underwater so no air can enter 3. Remove the apparatus from the water but keep the end of the capillary tube submerged in water and check that the apparatus is watertight and airtight 4. Dry the leaves and allow time for the shoot to acclimatise and then shut the tap 5. Remove the end of the capillary tube from the water until one air bubble has formed, then submerge it back into the water 6. Record the starting position of the air bubble 7. Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour

Question 28

What are some of the conditions that are kept constant?

Answer 28

1. Use the same cutting to have the same SA

Question 29

Explain why the rate of water uptake is usually higher than the actual rate of water lost via transpiration

Answer 29

1. Some of the water taken up is used in photosynthesis 2. Some are used in hydrolysis reactions

Question 30

How could air bubbles affect the uptake of water from roots?

Answer 30

The air bubbles could prevent the water molecules from sticking together , breaking the column of water and so less water is pulled up the xylem.

Question 31

What is Translocation?

Answer 31

It is the movement of dissolved substance/assimilates to where they are needed

Question 32

What are assimilates?

Answer 32

They are products of photosynthesis

Question 33

How are the assimilates moved?

Answer 33

They are moved from sources to sink.

Question 34

What is the source?

Answer 34

The source is where the assimilates are produced. e.g. in the leaves

Question 35

What is the sink?

Answer 35

The sink is where the assimilates are needed. e.g. in the roots

Question 36

Process of translocation: Active loading of sucrose from the source into the phloem.

Answer 36

1. In the companion cells, ATP is used to actively transport hydrogen ions out of the companion cells into the surrounding tissue cells 2. This sets up a concentration gradient as there are more H+ outside the companion cells than inside 3. An H+ binds to a cotransport protein in the companion cell membrane and re-enters the cell (down its concentration gradient) 4. A sucrose molecule also binds to the cotransport protein and moves into the companion cell, against its concentration gradient. The sucrose molecules are then transported out of the companion cells into the sieve tube element by diffusion.

Question 37

More on active loading

Answer 37

Once inside the sieve tube element, the water potential of the sieve tube element decreases causing water to move into the cell by osmosis from the xylem creating high pressure at the source end of the phloem

Question 38

What is mass flow?

Answer 38

It is the movement of assimilates from the source end of the phloem at high pressure to the sink end with a lower pressure down the pressure gradient.

Question 39

Process of translocation: Phloem unloading at the sink end

Answer 39

1. The sucrose in the phloem(sieve tube element) diffuses into the sink (e.g. the roots) 2. In the roots, the sucrose can either be moved to other cells or be converted to other forms e.g. glucose to be used for respiration. The purpose of this is to maintain a sucrose concentration gradient between the phloem and the cells. 3. Because the sucrose diffuses out of the phloem, the water potential inside the sieve tube element increases, so water leaves the tubes by osmosis into the cells. This lowers the pressure inside the sieve tubes

Question 40

What are xerophytes?

Answer 40

They are plants that are adapted to live in conditions with little or no water availability.

Question 41

Adaptations of Xerophytes

Answer 41

1. A thick waxy cuticle on the epidermis which reduces water loss by evaporation. Both marram grass and cacti have these 2. Sunken stomata: many xerophytes have sunken stomata in pits which reduces air movement thus reducing water vapour concentration gradient and so transpiration reduces 3. Cacti have spines instead of leaves to reduce the SA:V ratio minimising the amount of water loss by evaporation

Question 42

What are hydrophytes?

Answer 42

They are plants that live in aquatic habitats and have adaptations to help them cope with a low oxygen level

Question 43

Adaptations of Hydrophytes

Answer 43

1. Very thin or no waxy cuticle as they do not need to conserve water as there is always water available 2. Many always open stomata on the upper surfaces maximising gaseous exchange 3. Wide flat leaves- some hydrophytes, including water lilies have wide, flat leaves that spread across the surface of water to capture as much light as possible.