Topic 11 - Transport in Plants (Unit 3)

Question 1

What is osmosis?

Answer 1

Osmosis is the net diffusion of water molecules across a partially permeable membrane from a solution with high water potential to a solution of low water potential.

Question 2

How does osmosis work?

Answer 2

Although water molecules can pass through the membrane in either direction, the water molecules in the more concentrated solution are attracted to the other molecules in the solution, causing them to slow down and be less free to move - they have less kinetic energy. As a result of this, more water molecules will diffuse from the dilute solution to the concentrated solution in oppose to the opposite direction.

Question 3

What is Visking tubing?

Answer 3

Visking tubing is an artificial partially permeable membrane, used in kidney dialysis machines.

Question 4

How does Visking tubing work?

Answer 4

Visking tubing has microscopic holes in it, which let small molecules, like water, pass through it but is not permeable to some larger molecules, such as the sugar sucrose. This is why it is called a ‘partially’ permeable membrane.

Question 5

What is water potential?

Answer 5

Water potential is a measure of the ability of water molecules to move in a solution - pure water, for example, has the highest water potential.

Question 6

How does osmosis work in plant cells?

Answer 6

Around the plant cell is the tough cellulose cell wall. This outer structure keeps the shape of the cell and can resist changes in pressure inside the cell.

Question 7

How does osmosis work in plant cells?

Answer 7

The cell surface membrane of plant cells (and animals cells) is a partially permeable membrane, like Visking tubing, and so is the inner membrane around the plant cell’s vacuole.

Question 8

Why is a tough cellulose cell wall important?

Answer 8

A tough cellulose cell wall is important because it keeps the shape of the cell and can resist changes in pressure inside the cell. This is very important and critical in explaining the way that plants are supported.

Question 9

How does a plant cell become turgid?

Answer 9

The cell contents, including the sap vacuole, contain many dissolved solutes, such as sugars and ions. If a plant is put into pure water or a dilute solution, the contents of the cell have a lower water potential than the external solution, so the cell will absorb water by osmosis. The cell then swells up and the cytoplasm pushes against the cell wall - the plant is now turgid.

Question 10

How does a plant cell become flaccid?

Answer 10

If the cell is placed in a concentrated solution that has a lower water potential than the cell contents, it will lose water by osmosis. The cell decreases in volume and the cytoplasm no longer pushes against the cell wall.

Question 11

How does a plant cell become plasmolysed?

Answer 11

Eventually, the cell contents and gaps appear between the wall and the membrane, causing it to become plasmolysed.

Question 12

Why is turgor important to plants?

Answer 12

The pressure inside the cells pushes neighbouring cells against each other, like a box full of inflated balloons. This supports the non-woody parts of the plant, such as young stems and leaves, and holds stems upright, so the leaves can carry out photosynthesis properly. Turgor is also important in the functioning of stoma. If a plant loses too much water from its cells so that they become flaccid, this makes the plant wilt.

Question 13

How does water move from cell to cell in a plant?

Answer 13

Water moves from cell to cell in a plant by osmosis. If a cell has a higher water potential than the cell next to it, water will move from the first cell to the second, with the process repeating across a plant tissue, down a gradient of water potential.

Question 14

How do you investigate the effects of osmosis in onion epidermis cells?

Answer 14

Add a drop of concentrated sucrose solution on a microscope slide and a drop of distilled water on another. Two small squares of inner epidermis are removed from an onion, placing one on each solution. A drop of the correct solution is added to the top of each specimen, followed by a cover slip. Each slide is examined: the specimen in water will show turgid cells, whilst the cells in sucrose solution will gradually plasmolyse.

Question 15

How do you investigate the effects of osmosis on potato tuber tissue?

Answer 15

A boiling tube is half filled with distilled water and a second with concentrated sucrose solution. A third tube is left empty. A potato is cut into chips, without any skin. Each chip is gently botted to remove excess moisture and the mass of each chip is found by weighing on a balance. One chip is placed in each boiling tube. After 30 minutes, the chips are removed and reweighed to find that the potato in water has become turgid, the one in sucrose has lost water so will become flaccid (and, eventually, plasmolyse) and the one in air lost a small volume of water due to the fact that there is a higher concentration of water in the potato than in the air.

Question 16

What are the tips of the roots of a plant covered in?

Answer 16

The regions just behind the growing tips of the roots of a plant are covered in thousands of tiny root hairs. These areas are the main sites of water absorption by the roots, where the hairs greatly increase the surface area of the root epidermis.

Question 17

How do roots uptake water?

Answer 17

Each root hair is actually a single, specialised cell of the root epidermis, which penetrates between the soil particles, reaching the soil water. The water in the soil has some solutes in it, such as mineral ions, but their concentrations are much lower than the concentrations of solutes inside the root hair cell. The soil water has a higher water potential than the inside of the cell, allowing water to enter the root hair cell by osmosis; this water movement dilutes the contents of the cell, increasing its water potential.

Question 18

How is osmosis involved in the movement of water through leaves?

Answer 18

The epidermis of leaves is covered by a waxy cuticle, which is impermeable to water. Most water passes out of the leaves as water vapour through pores called stomata. Water leaves the cells of the leaf mesophyll and evaporates into the air spaces between the spongy mesophyll cells. The water vapour then diffuses out through the stomatal pores. Loss of water from the mesophyll cells sets up a water potential gradient which draws water by osmosis from surrounding mesophyll cells. Xylem vessels supply the leaf mesophyll tissues with water.

Question 19

What is transpiration?

Answer 19

Transpiration is the loss of water vapour from the leaves of plants.

Question 20

What is the transpiration stream?

Answer 20

The transpiration stream is the passage of water and minerals through the roots, stem and leaves of a plant.

Question 21

What are the functions of the transpiration stream?

Answer 21

The transpiration stream supplies water for the leaf cells to carry out photosynthesis, carries mineral ions dissolved in the water, provides water to keep the plant cells turgid and allows evaporation from the leaf surface, which cools the leaf, in a similar way to sweat cooling the human skin.

Question 22

What does the xylem transport?

Answer 22

Xylem transports water and mineral ions throughout the plant.

Question 23

What does xylem contain?

Answer 23

Xylem contains dead cells arranged end-to-end, forming continuous vessels. When they are mature, the vessels contain no cytoplasm and, instead have a hollow centre space or lumen through which the water passes. The walls of the xylem vessels contain a woody material called lignin, which provides strength and makes them impermeable to water.

Question 24

How do xylem vessels begin life?

Answer 24

Xylem vessels begin life as living cells with normal cytoplasm and cellulose cell walls. As they develop, they become elongated and, gradually, their original cell walls become impregnated with lignin, made by the cytoplasm. As this happens, the cells die, forming hollow tubes.

Question 25

What does lignification do to xylem vessels?

Answer 25

Lignification makes them very strong and enables them to carry water up tall plants without collapsing. Lignin is also impermeable to water.

Question 26

What do phloem vessels consist of?

Answer 26

Phloem vessels consist of living cells at all stages of its developments.

Question 27

How are phloem tubes formed?

Answer 27

Tubes in the phloem are formed by cells arranged end-to-end, but they have cell walls made of cellulose, and retain their cytoplasm. The end of each cell is formed by a cross-wall of cellulose with holes, called a sieve plate. The living cytoplasm extends through the holes in the sieve plate, linking each cell with the next, forming a long sieve tube.

Question 28

What is the sieve plate?

Answer 28

The sieve plate is the specialised end wall of the sieve tube, with holes allowing connections between one cell and the next.

Question 29

What is the sieve tube?

Answer 29

The sieve tube is a cell found in phloem, consisting of tube transporting the products of photosynthesis.

Question 30

What does the phloem transport?

Answer 30

The phloem transports the products of photosynthesis form the leaves to other parts of the plant. Sugars for energy or amino acids for building proteins are carried to young leaves and other growing points in the plant.

Question 31

How are sieve tubes controlled?

Answer 31

Despite being living cells, the phloem sieve tubes have no nucleus; they seem to be controlled by other cells that lie alongside the sieve tubes, called companion cells.

Question 32

What are companion cells?

Answer 32

Companion cells are specialised cells living next to a sieve tube in the phloem and controlling its activities.

Question 33

What are vascular bundles?

Answer 33

Vascular bundles are collections of xylem and phloem in a stem or root.

Question 34

How is vascular tissue arranged in roots?

Answer 34

In the root, vascular tissue is in the central core. Xylem vessels are tough, so the vascular bundles are here to resist force.

Question 35

How is vascular tissue arranged in stems?

Answer 35

Vascular bundles are arranged near the edge of stems, with the phloem on the outside and the xylem on the inside, in order to resist compression.

Question 36

How is vascular tissue arranged in older stems?

Answer 36

In older stems, the vascular tissue grows to form complete rings around the stem. The inner xylem forms the woody central core of a stem, with the living layer of phloem outside this.

Question 37

Why are there more stomata on the lower surface of leaves than on the upper surface?

Answer 37

If they were mainly on the upper leaf surface, the leaf would lose too much water because the stomata would be exposed to direct sunlight, which would produce a high rate of evaporation from them. There is also less air movement on the underside of leaves.

Question 38

How are the stomata opened in the light?

Answer 38

In the light, water enters the guard cells by osmosis from the surrounding epidermis cells. This causes the guard cells to become turgid and, as they swell up, their shape changes. They bend outwards, opening up the stoma.

Question 39

How are stomata closed in the dark?

Answer 39

In the dark, the guard cells lose water again, becoming flaccid, which causes the stoma to close.

Question 40

Why do stomata close in the dark?

Answer 40

Stomata are adapted to close in the dark because, without the sun, there is no need for loss of water vapour from the stomata to cool the leaves. In addition, leaves cannot photosynthesise in the dark, so they do not need water for this purpose.

Question 41

Which four main factors affect the rate of transpiration?

Answer 41

Light intensity, temperature, humidity and wind speed all affect the rate of transpiration.

Question 42

How does light intensity affect the rate of transpiration?

Answer 42

The rate of transpiration increases in the light because of the opening of the stomata in the leaves, so that the leaf can photosynthesise.

Question 43

How does temperature affect the rate of transpiration?

Answer 43

High temperatures increase the rate of transpiration, by increasing the rate of evaporation of water from the mesophyll cells.

Question 44

How does humidity affect the rate of transpiration?

Answer 44

When the air around the plant is humid, this reduces the diffusion gradient between the air spaces in the leaf and the external air. The rate of transpiration, therefore, decreases in humid air and speeds up in dry air.

Question 45

How does wind speed affect the rate of transpiration?

Answer 45

The rate of transpiration increases with faster air movements across the surface of the leaf. The moving air removes any water vapour which might retain near the stomata. This moist air would otherwise reduce the diffusion gradient and slow down diffusion.

Question 46

What is a potometer?

Answer 46

A potometer is a simple piece of apparatus which measures the rate of transpiration or the rate of uptake of water by a plant.

Question 47

What are the two types of potometers?

Answer 47

‘Weight’ (or mass) potometers and volume (or bubble) potometers are the two types.

Question 48

What does a ‘weight’ (or mass) potometer measure?

Answer 48

A ‘weight’ potometer measures the rate of loss of mass from a potted plant or leafy shoot over an extended period of time, usually several hours.

Question 49

Why is there a polythene bag around the pot in a ‘weight’ (or mass) potometer?

Answer 49

The polythene bag around the pot prevents loss of moisture by evaporation from the soil. Although, most of the mass lost by the plant will be due to water evaporating from the leaves during transpiration, there will be small changes in mass due to respiration and photosynthesis.

Question 50

What is a volume (or bubble) potometer used for?

Answer 50

A volume (or bubble) potometer is used to find the rate of uptake of water by a leafy shoot, by ‘magnifying’ this uptake in a capillary tube.

Question 51

How do you investigate the effects of environmental factors on the rate of transpiration using a simple potometer?

Answer 51

The capillary and attached rubber tubing is placed in a sink of water and any air is removed by squeezing. A shoot is taken from a plant and the end is cut at an angle (to stop air from entering the shoot). The angled cut makes it easier to push the stem into the rubber tubing. The assembled apparatus is allowed to dry and any joints sealed with petroleum jelly. The potometer is clamped in a vertical position and left to adjust. The distance moved by the column of water in the capillary tube is measured over a certain time and the rate of movement is calculated. The plant is then exposed to different environmental conditions.