3 Transport in plants

Question 1

What is osmosis?

Answer 1

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

Question 2

What is a partially permeable membrane?

Answer 2

A membrane that is permeable to some molecules but not to others.

Question 3

What is an example of an artificially partially permeable membrane?

Answer 3

Visking tubing.

Question 4

What is visking tubing?

Answer 4

It 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.

Question 5

How can you show the effect of osmosis with visking tubing?

Answer 5

You can show the effects of osmosis by filling a visking tubing sausage with concentrated sucrose solution, attaching it to a capillary tube and placing the Visking tubing in a beaker of water.

Question 6

What is an image showcasing the apparatus used during the experiment that shows the effect of osmosis with visking tubing?

Question 7

What happens to the water levels?

Answer 7

The level in the capillary tube rises as water moves from the beaker to the inside of the visking tubing.

Question 8

What happens in relation to the molecules and how they pass through the visking tubing?

Answer 8

The sucrose molecules are too big to pass through the holes in the partially permeable membrane. The water molecules can pass through the membrane in either direction, but those on the right are attracted to the sugar molecules.

Question 9

What does this do to their speed?

Answer 9

It slows them down and means that they are less free to move - they have less kinetic energy.

Question 10

What happens as a result of this?

Answer 10

More water molecules diffuse from left to right than from right to left. In other words, there is a greater diffusion of water molecules from the more dilute solution (in this case pure water) to the more concentrated solution.

Question 11

What is an image that represents how more water molecules diffuse from left to right than from right to left?

Question 12

What is the word that represents how ‘free’ the water molecules are able to move?

Answer 12

Water potential.

Question 13

What is water potential?

Answer 13

It is the measure of the ability of water molecules to move in a solution. Pure water has the highest water potential.

Question 14

Which molecules can move the most ‘free’?

Answer 14

The molecules in pure water can move most freely, so pure water has the highest water potential.

Question 15

What happens as a result of concentration in relation to water potential?

Answer 15

The more concentrated a solution is, the lower its water potential.

Question 16

What are some other examples of partially permeable membranes?

Answer 16

The cell surface membranes of both animal and plant cells are partially permeable, and so is the inner membrane around the plant cell’s sap vacuole.

Question 17

What is an image that shows which specific structures in the cell are partially permeable?

Question 18

What are the functions of the specific structures in the cell?

Answer 18

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. This is very important, and critical in explaining the way that plants are supported. The cell contents, including the sap vacuole, contain many dissolved solutes, such as sugars and ions.

Question 19

What happens if a plant cell is put into pure water or a dilute solution?

Answer 19

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. A plant cell that has developed an internal pressure like this is called turgid.

Question 20

What does turgid mean?

Answer 20

It is the description of a plant cell with a high internal pressure, so that the cytoplasm pushes against the cell wall.

Question 21

What image shows the effects of osmosis on plant cells?

Question 22

However, what happens if the cell is placed in a concentrated sucrose solution that has a lower water potential than the cell contents?

Answer 22

It will lose water by osmosis. The cell decreases in volume and the cytoplasm no longer pushes against the cell wall.

Question 23

What is the cell called in this state?

Answer 23

Flaccid.

Question 24

What is flaccid?

Answer 24

The condition in a plant cell which has lost internal pressure, so that the cytoplasm no longer pushes against the cell wall.

Question 25

What happens as the cell shrinks so much?

Answer 25

The membrane and cytoplasm split away from the cell wall and gaps appear between the wall and the membrane.

Question 26

What is the cell called in this state?

Answer 26

Plasmolysed.

Question 27

What does it mean if a cell is plasmolysed?

Answer 27

The condition of a plant cell that has lost water by osmosis, resulting in the cell contents shrinking and the cell membrane and cytoplasm pulling away from the cell wall.

Question 28

What does turgid mean?

Answer 28

It is the description of a plant cell with a high internal pressure, so that the cytoplasm pushes against the cell wall.

Question 29

Why is it important for a plants cells to be turgid?

Answer 29

When they are turgid, it supports the non-woody parts of the plant, such as young stem and leaves, and holds stems upright, so the leaves can carry out photosynthesis properly.

Question 30

Why is turgor important for the function of the stomata?

Answer 30

If a plant loses too much water from its cells so that they become flaccid, this makes the plant wilt. You can see wilting in a pot plant which has been left for too long without water. The leaves droop and collapse. In fact this is a protective action. It cuts down water loss by reducing the exposed surface area of the leaves and closing the stomata.

Question 31

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

Answer 31

Through osmosis.

Question 32

So, what happens if a cell has a higher water potential than the cell next to it?

Answer 32

Water will move from the first cell to the second. In turn, this will dilute the contents of the second cell, so that it has a higher water potential than the next cell. In this way, water can move across a plant tissue, down a gradient of water potential.

Question 33

What is an image that shows how water moves from cell to cell down a water potential gradient?

Question 34

What is the first step when investigating the effects of osmosis in onion epidermis cells?

Answer 34

A drop of concentrated sucrose solution is placed on one microscopic slide, and a drop of distilled water on a second slide. Two small squares of inner epidermis are removed from the outer fleshy layers of an onion. One square is transferred to the sucrose solution, and the second to the water.

Question 35

What do we have to ensure is done during this step?

Answer 35

To make sure that this is done as quickly as possible, so that the cells do not dry out.

Question 36

What is an image that shows how we can investigate osmosis in onion epidermis cells?

Question 37

What is the second step when investigating the effects of osmosis in onion epidermis cells?

Answer 37

A drop of the correct solution is added to the top of each specimen, followed by a cover slip. Any excess liquid is blotted (cleaned) up with filter paper.

Question 38

What is the third step when investigating the effects of osmosis in onion epidermis cells?

Answer 38

Each slide is examined through the microscope for several minutes. The specimen in water will show turgid cells.

Question 39

How could we further investigate the effect on plasmolysed cells?

Answer 39

We could replace the sugar solution on the first slide with distilled water. This can be done quite easily without removing the cover slip. Some water is placed on one side of the cover slip and drawn across the slide using filter paper. After a while all the sucrose solution is replaced by water. If the cells are now observed for a few minutes, they will gradually recover from their plasmolysed condition until they are fully turgid again.

Question 40

What is the first step when investigating the effects of osmosis on potato tuber tissue?

Answer 40

A boiling tube is half filled with distilled water and a second with concentrated sucrose solution. A third tube is left empty.

Question 41

What is the second step when investigating the effects of osmosis on potato tuber tissue?

Answer 41

A potato is cut into chips 5cm x 1 cm x 1 cm, making these measurements as accurate as possible so that the three chips are the same size. No skin is left on the potato tissue.

Question 42

What is the third step when investigating the effects of osmosis on potato tuber tissue?

Answer 42

Each chip is gently blotted to remove excess moisture, and the mass of each is found by weighing on a balance. One chip is places in each of the three boiling tubes.

Question 43

What is an image that shows investigating osmosis in potato tissue?

Question 44

What is the fourth step when investigating the effects of osmosis on potato tuber tissue?

Answer 44

After 30 minutes, the chips are removed from the tubes and gently blotted to remove excess liquid, then re-weighed. Each is felt in turn, to compare how flexible or stiff they are.

Question 45

What is the percentage change equation?

Answer 45

Change in mass
———————– x100
Starting mass

Question 46

What is the xylem?

Answer 46

It transports tissue carrying water and minerals up through a plant from the roots.

Question 47

What does the xylem contain?

Answer 47

The xylem contains dead cells arranged end-to-end, forming continuous vessels. When they are mature, the vessels contain no cytoplasm. Instead, they have a hollow central space or lumen through which the water passes. The walls of the xylem vessels contain a woody material called lignin.

Question 48

What is the lumen?

Answer 48

A space in the middle of the tube such as an artery or a xylem vessel.

Question 49

What is the lignin?

Answer 49

It is the woody material present in the cell walls of some plant cells, such as xylem vessels. Provides strength and makes the walls impermeable to water.

Question 50

What is the life cycle of the xylem?

Answer 50

They begin life as living cells with normal cytoplasm and cellulose cell walls. As they develop they become elongated, and gradually their original cellulose cell walls become impregnated with lignin, made by the cytoplasm. As this happens, the cells die, forming hollow tubes. Lignification makes them very strong, and enables them to carry water up tall plants without collapsing. Lignin is also impermeable to water.

Question 51

What is a diagram of the xylem?

Question 52

Where is the xylem found?

Question 53

What are the tubes in the phloem formed by?

Answer 53

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.

Question 54

What is a sieve plate?

Answer 54

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

Question 55

What extends through the sieve plate?

Answer 55

The living cytoplasm extends through the holes in the sieve plates, linking each cell with the next, forming a long sieve tube.

Question 56

What is a sieve tube?

Answer 56

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

Question 57

What does the sieve tube do?

Answer 57

It transports the products of photosynthesis from 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 58

Why may sugar be taken to the roots?

Answer 58

They may also be taken to the roots and converted into starch for storage.

Question 59

What is interesting about the phloem sieve tubes?

Answer 59

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 60

What is a companion cell?

Answer 60

It is a specialised cell lying next to a sieve tube in the phloem and controlling its activities.

Question 61

What is a diagram of the phloem?

Question 62

Where is the phloem found?

Question 63

In a young stem, how are things grouped together?

Answer 63

The xylem and phloem are grouped together in areas called vascular bundles.

Question 64

What is a vascular bundle?

Answer 64

Xylem and phloem grouped together in a stem or root.

Question 65

What is unique about vascular bundles in the stem?

Answer 65

Unlike in the root, where the vascular tissue is in the central core, the vascular bundles are arranged in a circle around the outer part of the stem.

Question 66

What happens in term of the vascular tissue in older stems?

Answer 66

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 67

What are the metabolic reactions that take place in plants?

Answer 67

Aerobic respiration.

Question 67

What is aerobic respiration?

Answer 67

It is the Breakdown of Glucose to release Energy in Cells in the Presence of Oxygen to Produce Carbon Dioxide and Water (and Energy)

Question 68

How do plants excrete waste (Excess carbon dioxide and oxygen)?

Answer 68

Through the stomata in the leaves.

Question 69

How are the waste products formed?

Answer 69

Carbon dioxide is a waste product of aerobic respiration in plant cells. Oxygen is a waste product of photosynthesis.

Question 70

What is metabolism?

Answer 70

It is the chemical reactions taking place inside cells.

Question 71

Why aren’t the stomata on the upper leaf surface?

Answer 71

The leaf would lose too much water as the stomata would be exposed to too much sunlight, which would produce a high rate of evaporation from them. There is also less air movement on the underside of leaves. This is an adaptation so water loss is kept at a minimum.