Transport in Plants

Question 1

Define transpiration.

Answer 1

The evaporation of water vapour, meanly through the stomata in the surface of a leaf.

Question 2

Is transpiration a passive or active process?

Answer 2

Passive process. It uses energy from the sun.

Question 3

How does transpiration work?

Answer 3

It uses energy from the sun, which evaporates water from mesophyll cells.

As water vapour diffuses from the air spaces in the mesophyll and through the stomata, it’s replaced by water from the mesophyll cells. This in turn is replaced by water from the xylem in the veins of the leaf due to cohesiveness which is when some water molecules are pulled into the leaf, and others then follow.

This means that the whole column of water in the xylem moves upwards, against the force of gravity.

Question 4

Water cannot be pulled up further than 10m, so how can tall trees overcome things problem?

Answer 4

Xylem vessels are very narrow, and water molecules tend to stick together due to the hydrogen bonds between them.

As water moves out of the xylem in a leaf, it drags other molecule behind it due to COHESION.

Because the vessels are so narrow, the column of water behind does not breaks and water is pulled up all the way from the roots. The pulling force is so great that the column of water is being stretched - its under tension. This tension in the column of water tends to pull the walls of the vessels inwards slightly.

But the lignin in the walls is strong enough to stop vessels collapsing.

Question 5

Disadvantage of transpiration?

Answer 5

The loss of so much water by transpiration seem wasteful. However it is the unavoidable effect of the need for leaves to take in CO2 for gas exchange.

Question 6

Advantages of transpiration?

Answer 6

• the stream of water transports mineral ions around the plant.
• the evaporation of water from the leaves has a cooling effect.
• when leaves are exposed to bright sunlight, transpiration can reduce the possibility of leaves overheating and the enzymes begin denatured.

Question 7

Where is water water potential greatest?

Answer 7

In the root and bottom of the plant.

Question 8

Where is water potential lowest?

Answer 8

At the top of the plant.

Question 9

Define polar molecule.

Answer 9

A molecule with a slight positive charge in one part of the molecule, and a slight negative charge in another part.

Question 10

How is water a polar molecule?

Answer 10

Because the hydrogen atoms have a slight positive charge.

But the oxygen atom has a slight negative charge.

Question 11

Why do water molecules have a strong cohesion?

Answer 11

Because water is a polar molecule. (Its hydrogen ions have a slight positive charge, but oxygen atom has a slight negative charge).

Opposite charges attract one another so water has a strong cohesion between the molecules.

Question 12

The cooing effect of transpiration that prevents leaves from overheating is due to water’s large…

Why is this?

Answer 12

Latent heat of vaporisation.

This is due to the fact that water molecules are cohesive and so relatively difficult to separate.

Question 13

Define latent heat vaporisation.

Answer 13

The heat required to turn liquid at its boiling temperature into a gas at the same temperature.

Question 14

Define solvent.

Answer 14

A liquid that’s able to dissolve other substances.

Question 15

List 4 featured of xylem vessels that make them specialised for the transport of water.

Answer 15

Water is…

• polar molecules (so strong cohesion)
• latent heat vaporisation (so transpiration has cooling effect the protects leaves from overheating)
• heat capacity (so large volumes of water eg lakes don’t change temperature quickly)
• solvent (wide range of substances soluble in water, which is why solute should can be transported in blood).

Question 16

What’s translocation?

Answer 16

The movement of solutes (eg sugars like sucrose, also amino acids) to where they’re needed in a plant.

Translocation occurs from a source of a sink.

Question 17

How does translocation keep working? Ie how does it keep going from a source to a sink without the sink eventually becoming a source?

Answer 17

Because enzymes maintain the concentration gradient from the source to the sink by changing the solutes at the sink (eg breaking them down).
This ensures there’s always a lower concentration at the sink.

Question 18

Where does translocation occur?

Answer 18

In phloem sieve tubes which are elongated cells joined end to end to form a chain. Where the cells meet, they have holes which us why they’re called sieve plates.

Question 19

Each separate cell in a phloem sieve tube is called a…

Answer 19

Sieve element. It also has another cell alongside it called a companion cell.

Question 20

In phloem tissues, sieve elements have another cell alongside it: companion cells. What do these do?

Answer 20

Companion cells provide metabolic support to the sieve elements. This is important because the cells of sieve tubes lose many of their organelles including their nucleus as they become specialised.

This allows easier flow of phloem sap through the cell, but also means they cannot repair and maintain themselves so well so rely on companion cells for many of these functions.

Question 21

What are xylem tubes made of?

Answer 21

Tubes of dead cells linked end to end.

Question 22

The ‘Mass Flow Hypothesis’ best explains phloem transport.

Describe it.

Answer 22

1. Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source. 🏃🏽‍♀️🏃🏽
2. This lowers the water potential inside the sieve tubes. So water enters the tubes by osmosis from the xylem and companion cells. 👬
3. This creates high pressure inside the sieve tubes at the source end of phloem. ‼️
4. At the sink end, solutes are removed from the phloem to be used.🚰
5. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. 👋🏼
6. This lowers the pressure inside the sieve tubes. ⬇️
7. The result is a pressure gradient from the source to sink end. ↔️
8. The gradient pushes solutes along the sieve tubes towards the sink.
9. When they reach the sink, the solutes will be used (eg respiration) or stored (eg as starch).

Question 23

In translocation, what is the direction of transport?

Answer 23

Where the sinks are in the plant at any point in time.

Question 24

Supporting evidence for Mass Flow Hypothesis.

Answer 24

• a radioactive tracer (eg radioactive carbon ^14C) can be used to track the movement of organic substances in a plant.
• if a metabolic inhibitor (which stops ATP production) is put into the phloem, then translocation stops - evidence that active transport is involved.
• if a phloem tube is punctured the phloem sap oozes out, suggesting it is under pressure. Phloem sap sampled from a source has a higher sucrose conc. than sap sampled from a sink - confirming that different water potentials would cause osmosis into it out of the sieve tubes into 2 locations.

Question 25

Evidence against Mass Flow Hypothesis.

Answer 25

- sugar travels to many different sinks, not just to the one with the highest water potential, as the model suggests. - the sieve plate would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 26

What 4 things affect transpiration rate?

Answer 26

- light - temperature - humidity of the air - wind

Question 27

Explain light as a factor affecting transpiration rate.

Answer 27

The lighter it is, the faster the transpiration rate. This is because the stomata open when it gets light to let in CO2 for photosynthesis. When it's dark the stomata are usually closed, so there's little transpiration.

Question 28

Explota temperature as a factor affecting transpiration rate.

Answer 28

The higher the temperature, the faster the transpiration rate. Warmer molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the conc. gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

Question 29

Explain humidity as a factor affecting transpiration rate.

Answer 29

The lower the humidity, the higher the transpiration area. If the air around a plant is dry, the conc. gradient between the keaf and air is increases, which increases evaporation.

Question 30

Explain wind as a factor affecting transpiration rate.

Answer 30

The windier it is, the faster the transpiration rate. Lots of air movement blown away water molecules from around the stomata. This increases the conc. gradient, which increases the rate of transpiration.

Question 31

What is the energy provided by in transpiration? | What does this mean?

Answer 31

The sun. This means it's passive energy.

Question 32

In what way is there a water potential gradient in transpiration?

Answer 32

The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.

Question 33

How can plants control the rate of transpiration?

Answer 33

By changing the size of the stomata pores

Question 34

Water lost from mesophyll cells from evaporation from their cell walls into the air spaces of the leaf is replaced by water reaching the mesophyll cells from the xylem via the cytoplasm. This water movement occurs because...

Answer 34

- mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun. - these cells now have a lower water potential and so water enters by osmosis from neighbouring cells. - the loss in water by from neighbouring cells lowers their water potential. - they, in turn, take in water of surrounding cells by osmosis. In this way, a water potential gradient is established that pulls up water from the xylem, across the leaf mesophyll and finally out into the atmosphere.

Question 35

What is the main factor responsible for movement of water up the xylem, from the roots to the tips?

Answer 35

Cohesion-tension

Question 36

The movement of water up the stem occurs by...

Answer 36

1. Water evaporates from the mesophyll cells due to heat from the sun leading to transpiration. 2. Water molecules form hydrogen bonds between one another and hence stuck together. This is known as COHESION. 3. Water forms a continuous, unbroken column across the mesophyll cells and down the xylem. 4. As water moves from the mesophyll cells in the leaf into air spaces beneath the stomata, more water molecules are brought up behind it as a result of cohesion. 5. A column of waiter is therefore pulled up the xylem as a result of transpiration. This is called the transpiration pull. 6. Transpiration puts the xylem under tension so there is a negative pressure within the xylem.

Question 37

3 pieces of evidence to support the cohesion-tension theory?

Answer 37

- if a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken so the water molecules can no longer stick together. - when a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure. Instead, air is drawn in, which is consistent with the idea of it being under tension. - during the day, when transpiration is greatest, there is more tension (negative pressure) in the xylem. This pulls water of the xylem vessels inwards and causes the trunk to shrink in diameter. As night when transpiration is at its lowest, there is less transpiration in its xylem, so the diameter of the trunk increases.

Question 38

What are the 4 main factors that affect transpiration rate?

Answer 38

- Light. - Temperature. - Humidity. - Wind.

Question 39

Describe the appearance of xylem vessels.

Answer 39

Long tubes consisting of only cell walls linked end to end. Containing no cytoplasm. They're created only when living cells die, leaving only cell walls.

Question 40

Define transpiration.

Answer 40

The evaporation of water vapour through the stomata on the surface of the leaf. This is a passive process, as the energy is supplied by the sun.

Question 41

In what way does water move out through stomata?

Answer 41

There is a water potential gradient from the air spaces through the stomata to the air (the humidity of the atmosphere is usually less than that of the air spaces neck to the stomata). As long as the stomata are open, the water vapour molecules diffuse out of the air spaces into the surrounding air. This water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.

Question 42

How is water lost by transpiration replaced?

Answer 42

By water evaporating from the cell walls of surrounding mesophyll cells.

Question 43

How do plants control their rate of transpiration?

Answer 43

By changing the size of the stomata pores, plants can control the size of stomatal pores.

Question 44

How is water pulled up in transpiration?

Answer 44

1. Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun. 2. These cells now have a lower water potential and so water enters by osmosis from neighbouring cells. The loss of water from these neighbouring cells lowers their water potential. 3. They, in turn, take in water from their neighbours by osmosis. In this way, a water potential gradient is established that pulls water from the xylem, across leaf mesophyll, and finally into the atmosphere.

Question 45

Outline the cohesion tension theory.

Answer 45

1. Water evaporates from mesophyll cells due to heat from the sun leading to transpiration. 2. Water molecules form hydrogen bonds between one another and hence stick together (cohesion). 3. Water forms a continuous, unbroken column across the mesophyll cells and down the xylem. 4. As the water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules are drawn up behind it as a result of this cohesion. 5. A column of water is therefore pulled up the xylem as a result fo transpiration - transpiration pull. 6. Transpiration pull puts the xylem under tension, so there is a negative pressure within the xylem - hence 'cohesion tension theory'.

Question 46

Evidence to support cohesion tension theory?

Answer 46

- change in the diameter of tree trunks according the the rate of transpiration (in the day, when transpiration is greatest, there is more tension (negative pressure) in the xylem. This pulls the water inwards and causes the tree to shrink in diameter. At night, when transpiration is lowest, there is less tension in the xylem and so the diameter of the trunk increases). - if a xylem vessel is broken, and air enters, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together. - when a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure. Instead, air is drawn in, which is consistent with it being under tension.

Question 47

Transpiration pull is a _________ process. Therefore...

Answer 47

Passive. Therefore, it doesn't require metabolic energy to take place.

Question 48

How do xylem vessels form a series of continuous, unbroken tubes?

Answer 48

New use xylem vessels have no end walls.

Question 49

What energy is needed for transpiration pull?

Answer 49

Energy in the form of heat that evaporates water from the leaves and it ultimately comes from the sun.

Question 50

What equipment do you use to measure transpiration rate?

Answer 50

Using a potometer.

Question 51

Outline how to measure transpiration rate.

Answer 51

1. Cut a shoot underwater (to prevent air entering the xylem) at a slant (to increase SA available for water uptake). 2. Assemble the potometer in water and insert the shoot underwater. 3. Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water. 4. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap. 6. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into water. 7. Record starting position of air bubble. 8. Start a stopwatch and record the distance moved by the bubble per unit of time. The rate of air bubble movement is an estimate of the transpiration rate.

Question 52

How can light affect transpiration rate?

Answer 52

The lighter it is, the faster the transpiration rate. (Positive correlation between light intensity and transpiration rate).

Question 53

How can temperature affect transpiration rate?

Answer 53

The higher the temperature, the faster the transpiration rate.

Question 54

How can humidity affect transpiration rate?

Answer 54

The lower the humidity, the faster the transpiration rate.

Question 55

How can wind affect transpiration rate?

Answer 55

The windier it is, the faster the transpiration rate

Question 56

How does water have a strong cohesion between the molecules in transpiration?

Answer 56

Water is a polar molecule. So a molecule with a slight positive charge in one part of the molecule and a slight negative charge in another part. Because opposite charges attract one another, water has a strong cohesion between the molecules.

Question 57

The height of the tallest tree is limited by...

Answer 57

The cohesiveness of water.

Question 58

What is translocation?

Answer 58

The process by which organic molecules and some mineral ions are transported from one part of a plant to another. This process uses phloem sieve tubes.

Question 59

What are phloem?

Answer 59

Phloem is made up of sieve tube elements, which are long thin structures arranged end to end. Where the cells meet, their end walls have holes. The chains run parallel to one another in bundles in the stem. Associated with companion cells.

Question 60

Each separate cell in a phloem siege tube is called a _______ __________. And has another cell alongside it called a _______________ _______.

Answer 60

Each separate cell in a phloem siege tube is called a SIEVE ELEMENT. And has another cell alongside it called a COMPANION CELL.

Question 61

What are companion cells used for?

Answer 61

Companion cells provide metabolic support to the sieve elements.

Question 62

Why do sieve elements need companion cells?

Answer 62

The cells of sieve tubes lose many of their organelles, including their nucleus as they become specialised (in order to allow easier flow of phloem sap through the cell). However it means that they cannot repair and maintain themselves as well so instead rely on the companion cells for these functions.

Question 63

Translocation occurs from _________ to ________.

Answer 63

Sources to sinks.

Question 64

Outline translocation of sucrose.

Answer 64

Sucrose is made in leaves from the products of photosynthesis. It's then transported to the parts of the plant that are growing, or to places where sucrose can be stored - often to make starch. These areas are sources (as they produce sucrose), whereas buds & developing seeds are sinks.

Question 65

What is sap?

Answer 65

The fluid that's transported in the phloem.

Question 66

In translocation, how is there always a lower concentration at the sink than at the source?

Answer 66

Enzymes maintain the concentration gradient from the source to the sink by changing the solutes at the sink (eg by breaking them down, making sure there's always a lower concentration at the sink than at the source).

Question 67

Outline the 'Mass Flow Hypothesis'.

Answer 67

1. Active transport actively loads the solutes (eg sucrose) from companion cells into the sieve tubes at the source (eg leaves). 2. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells. 3. This creates a higher hydrostatic pressure inside the sieve tubes at the source. 4. At the sink, solutes are removed from the phloem to be used up (or converted to starch). 5. This increases the water potential inside the sieve tube, so water also moves into the sinks by osmosis. 6. This lowers the hydrostatic pressure inside the sieve tubes. 7. As a result of water entering the sieve tube elements at the source and leaving the sink, there is a high hydrostatic pressure at the source and a low one at the sink. 8. The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells.

Question 68

The higher the concentration of _________ at the source, the higher the rate of translocation.

Answer 68

Sucrose.

Question 69

The mass flow hypothesis is a _______ process. It occurs as a result of the ________ ___________ of sugars.

Answer 69

Passive. | Active transport.

Question 70

Why is translocation not affected by temperature or metabolic activities for example.

Answer 70

While mass flow is a passive process, it occurs as a result of the active transport of sugars.

Question 71

Evidence supporting the Mass Flow Hypothesis?

Answer 71

- the conc. of sucrose is high in leaves (source) than in roots (sink). - increases in sucrose levels in the leaf are followed by similar increases in sucrose keels in the phloem a little later. - companion cells possess many mitochondria and readily produce ATP. - there's a pressure within sieve tubes, as shown by sap being released when they're cut.

Question 72

Evidence opposing the Mass Flow Hypothesis.

Answer 72

- sugar travels to many different sinks, not just to the one with the highest water potential, as the model would suggest. - not all solutes move at the same speed; they should do so if movement is by mass flow. - the sieve plates would create a barrier; hindering mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 73

How can you demonstrate translocation?

Answer 73

1. Supply part of a plant (often a leaf) with radioactive carbon dioxide (14CO2). 2. This 14CO2 will then be incorporated into organic substances produced by the leaf (eg sugars produced by the leaf), which will be moved around the plant by osmosis. 3. Track the movement of this substance using autoradiography. To reveal where the radioactive tracer has spread to in a plant, the plant is killed and the plant is place on photographic film - and the radioactive substance is present wherever the film turns black. 4. This demonstrates the translocation of substances from source to sink over time - eg autoradiographs of plants killed at different times show an overall movement of solutes from leaves towards roots.

Question 74

What are the structural differences between xylem and phloem?

Answer 74

Xylem: - made of dead cells - thick cell wall - cell wall made of lignin - impermeable Phloem: - made of living cells - thin cell wall - cell wall made of cellulose - permeable

Question 75

Difference between where the xylem and phloem carries products to?

Answer 75

Xylem carries to leaves. | Phloem carries to growing parts and storage organs.

Question 76

Difference between direction of flow in xylem and phloem?

Answer 76

Xylem - upwards | Phloem - bidirectional