Chapter 7: Transport In Plants Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

How do plants transport carbon dioxide and oxygen?

A

The gases diffuse through air spaces within stems, roots and leaves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are the two transport systems in a plant?

A

Xylem and phloem

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What does xylem carry and where? How ref. direction?

A

Xylem carries mainly water and inorganic ions(mineral salts) from the roots to the parts above ground. The movement in the xylem is in one direction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does the phloem carry and where? Direction?

A

The phloem carries the substances made by photosynthesis from the leaves to other areas of the plant. At any one time, phloem sap can be moving in different directions in different parts of the phloem.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How are monocots and dicots different and similar in terms of transport?

A

The mechanisms of transport through both types of plant are the same, but there are differences in the distribution of xylem and phloem in their roots, stems and leaves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are tissues?

A

They are collections of cells specialised for a particular function.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the epidermis and what is its function?

A

It is the continuous layer on the outside of the plant, one cell thick, that provides protection. In leaves, it has pores called stomata.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are some of the functions of a waxy cuticle?

A

It is waterproof and helps to protect the organ from drying out and from infection.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What special features does the epidermis have in these parts of the plants and what are their functions?

  1. stem and leaves
  2. leaves
  3. roots
A
  1. In stems and leaves, the epidermis is covered with a waxy cuticle which is waterproof and helps to protect the organ from drying out and from infection.
  2. In leaves, it also has pores called stomata which allow entry of carbon dioxide for photosynthesis.
  3. In roots, they may have extensions called root hairs to increase the surface area for absorption of water and mineral salts.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is parenchyma and where is it found?

A

Parenchyma is made up of thin walled cells. It forms the cortex of stems and roots, and the pith(central region) in stems. The parenchyma in leaves contain chloroplasts, where it is modified to form the palisade and spongy mesophyll layer.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are some of the functions of parenchyma? (6)

A
  1. They are used as packing tissue.
  2. They may be used for storage of food like starch.
  3. When they are turgid(inflated with water), they help to support the plant, preventing wilting.
  4. The air spaces between the cells allow gas exchange.
  5. Water is transported through the walls and through the protoplasts of cells.
  6. They can be specialised to form the palisade and spongy mesophyll layer.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is collenchyma and what is its function? Where is it found?

A

Collenchyma is a modified form of parenchyma with extra cellulose deposited at the corners of cells. This provides extra strength. The midrib of leaves contain collenchyma.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Where is the endodermis found?

A

The endodermis is a one cell thick layer that surrounds the vascular tissue in stems and roots.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the two types of mesophyll? What are the differences?

A

Spongy mesophyll is so called because in three dimensions it is spongy in appearance because it has many large air spaces between the cells.
Palisade mesophyll cells are near the upper surface of the leaf where they receive more sunlight. They are column shaped and contain more chloroplasts than spongy mesophyll cells.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the pericycle? Where is it found? Describe this layer with reference to roots and stems.

A

This is a layer of cells, one to several cells thick, just inside the endodermis and next to the vascular tissue.
In roots, it is one cell thick and new roots can form from this layer.
In stems, it is formed from a tissue called sclerenchyma. This has dead lignified cells for extra strength.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are the tubes of the xylem called?

A

Vessels

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What are xylem vessels made of?

A

They are made from dead lignified cells known as xylem vessel elements.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What are some functions of xylem?

A

Xylem allows the long distance transport of water and minerals salts. It also provides mechanical support and strength.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What are the tubes of the phloem called?

A

Sieve tubes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What are the sieve tubes made of?

A

They are made from living cells called sieve tube elements

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is a function of the phloem?

A

It allows for the long distance transport of organic compounds, particularly the sugar sucrose.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Give a brief overview of transpiration.

A

The energy of the Sun causes water the evaporate from the leaves, a process called transpiration. This reduces the water potential in the leaves and sets up a water potential gradient throughout the plant. Water moves down this gradient from the soil into the plant, and then across the root into the xylem tissue in the centre. Once inside the xylem vessels, the water moves upwards through the roots to the stem and from there into the leaves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is transpiration? Proper

A

Transpiration is the loss of water vapour from a plant to its environment, by diffusion down a water potential gradient. Most transpiration takes place through the stomata in the leaves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Describe the leaf to atmosphere part of transpiration.

A

The cells in the mesophyll layer are not tightly packed and have many spaces between them filled with air. The walls of the mesophyll cells are wet and some of this water evaporates into the intercellular air spaces, so that the air inside the leaf is usually saturated with water. The air in the intercellular spaces of the leaf has direct contact with the air outsides the leaf, through small pores called stomata. If there is a water potential gradient between the air outside the leaf(lower water potential) and the air inside the leaf(higher water potential), then water vapour will diffuse out of the leaf down this gradient.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Are the cells in the mesophyll layer wet?

A

Yes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What are the factors that affect transpiration? |(7)

A
  1. Humidity
  2. Wind speed and temperature
  3. Light intensity
  4. Very dry conditions
  5. Number of stomata
  6. Surface area of leaf
  7. Thickness of cuticle
27
Q

How does humidity affect transpiration?

A

If the water potential gradient between the air spaces in the leaf and the air outside becomes steeper, the rate of transpiration will increase. In conditions of low humidity, the gradient is steep, so transpiration will take place more quickly than in high humidity.

28
Q

How do wind speed and temperature affect transpiration?

A

Transpiration may also be increased by an increase in wind speed or rise in temperature.

29
Q

How does light intensity affect transpiration?

A

In most plants, stomata open during the day and close at night. Most transpiration takes place through the stomata, so the rate of transpiration is almost zero at night.

30
Q

How do very dry conditions affect transpiration?

A

In especially dry conditions, when the water potential gradient between the internal air spaces and external air is steep, a plant may have to partially or completely close its stomata to prevent its leaves drying out, even if it means reducing the rate of photosynthesis.

31
Q

How does transpiration play a role in helping plants in hot conditions?

A

Transpiration plays a role in cooling the plants in hot conditions. As water evaporates from the cell walls inside the leaf, it absorbs heat energy from these cells, thus reducing their temperature.

32
Q

How can transpiration lead to wilting of plants?

A

If the rate at which water vapour is lost by transpiration exceeds the rate at which a plant can take up water from the soil, then the amount of water in its cells decreases. The cells become less turgid and the plants wilt. This is due to the plant losing the support provided by the turgid cells.

33
Q

What are xerophytes?

A

Xerophytes are plants that live in places where water is in short supply and have evolved special adaptations of their leaves that keep water loss down to a minimum.

34
Q

What are some adaptations of xerophytes?

A
  1. Rolled leaf
  2. Thick waxy cuticle
  3. Hairs/ trichomes on surface
  4. Sunken stomata/ stomata in pits
  5. No stomata on upper surface
  6. Small leaves, reduced to spines
35
Q

How do rolled leaves help xerophytes?

A

It increases humidity around the stomata, reducing the water potential gradient.

36
Q

How do thick waxy cuticle help xerophytes?

A

Increases distance for diffusion, acting as a barrier for transpiration. Is waterproof. Shiny surface reflects heat, lowering temperature.

37
Q

How do hair help xerophytes?

A

Trap moisture to reduce water potential gradient.

38
Q

How do sunken stomata/ stomata in pits help xerophytes?

A

Moist air trapped in pits reduces water potential gradient

39
Q

How does the absence of stomata on the upper surface of leaves help xerophytes?

A

Not exposed to sunlight, reducing evaporation rate

40
Q

How do small leaves, reduced to spines help xerophytes?

A

It reduces surface area for transpiration.

41
Q

Explain the transport from xylem across the leaf in transpiration.

A

As water evaporated from the mesophyll cells, more water is drawn into the walls to replace it. This water comes from the xylem vessels in the leaf. Water leaves the xylem vessel through a non-lignified area such as a pit. From here, water moves along two possible pathways. In one pathway, known as the symplastic pathway, water moves from cell to cell via the plasmodesmata. In the other pathway, known as the apoplastic pathway, water moves through cell walls.

42
Q

What is the symplastic pathway?

A

It is where water moves from cell to cell via the plasmodesmata and through the cytoplasm or vacuoles via osmosis.

43
Q

What is the apoplastic pathway?

A

It is where water moves through the cell walls.

44
Q

Describe how two types of xylem vessel elements are formed. ref. lignin; plasmodesmata; pits

A

Each xylem vessel element begins life as a normal plant cell in whose wall lignin is laid down. Lignin is a very hard, strong substance, which is impermeable to water. As lignin builds up around the cell, the contents of the cell die, leaving a completely empty space, or lumen, inside.
However, in those parts of the original cell walls where plasmodesmata are found, no lignin is laid down. These non-lignified areas are called pits.

45
Q

What are pits?

A

Pits are those cells in the xylem tube where plasmodesmata are found and where lignin is not laid down. These non-lignified areas are called pits.Pits are not open pores and are crossed by permeable, unthickened cellulose cell wall. The pits in one cell link with those in the neighbouring cells, so water can pass freely from one cell to the next.

46
Q

What is hydrostatic pressure?

A

It is the pressure exerted by a liquid.

47
Q

What gradients are present in the xylem tube?

A

Hydrostatic pressure gradient and water potential gradient.

48
Q

Why do xylem vessels have lignified walls in terms of hydrostatic pressure?

A

The walls may collapse inwards as a result of the pressure differences created. Therefore, xylem vessels have strong, lignified walls to stop them from collapsing in this way.

49
Q

What is mass flow?

A

The movement of water molecules moving together as a body of water up through xylem vessels is called mass flow.

50
Q

Give a brief overview of how water is transported through hydrostatic pressure.

A

The removal of water from the xylem vessels in the leaf reduces the hydrostatic pressure in the xylem vessels. The hydrostatic pressure at the top becomes lower than the pressure at the bottom. This pressure difference causes the mass flow of water.

51
Q

What two factors keep water in a xylem vessel moving in a continuous column and how?

A
  1. Cohesion- Water molecules are attracted to each other by hydrogen bonding.
  2. Adhesion- Water molecules are also attracted to the cellulose and lignin in the walls of the xylem tube.
52
Q

Describe how certain features of xylem vessels help in preventing disruptions due to air lock. Explain what is an air lock.

A

If an air bubble forms in a column, the column of water breaks and the difference in pressure between the water at the top and bottom will be disrupted. We say there is an air lock. The small diameter of xylem vessels help to prevent such breaks from occurring. Also, the pits in the vessel walls allow water to move out into neighbouring vessels and so bypass such an air lock. Air bubbles cannot pass through pits.

53
Q

Describe how roots cause an increase in water pressure in the process of active transport?

A

Transpiration reduces the water potential at the top of a xylem vessel compared with the pressure at the base, so causing the water to flow up the vessels. Plants may also increase the pressure difference between the top and bottom by raising the water pressure at the base of vessels. The pressure is raised by the cells surrounding the xylem using energy to actively transport solutes e.g. mineral salts across their membranes and into the xylem vessels. The presence of solutes decreases the water potential of the solution in the xylem, thus drawing in water from the surrounding root cells. The influx of water increases the water pressure at the base of the xylem vessel.

54
Q

How does water move from root hair to the xylem?

A

Water is taken up by the root hair and moves by means of symplastic or apoplastic pathway to reach the endodermis surrounding the cortex. Once the water has reached the endodermis, the apoplastic pathway is blocked. This is because the cells in the endodermis have a thick, waterproof , waxy band of suberin in their cell walls. This band is called the casparian strip which forms an impenetrable barrier to water in the cells of the endodermis cells. The only way to cross the endodermis is through the cytoplasm of the endodermal cells. Certain cells called passage cells allow water to pass freely. Once across the endodermis, water continues to move down the water potential gradient across the pericycle and towards the xylem vessels. Water moves into the xylem vessels through the pits in their walls.

55
Q

What happens when endodermal cells get older?

A

As the endodermal cells get older, the suberin(waxy waterproof band) gets more extensive, except in passage cells through which water can continue to pass freely.

56
Q

How does water enter the root from the soil with reference to structure?

A

The tips of roots are covered by a tough, protective root cap that is not permeable to water. However, just behind the tip are extensions of the epidermis called root hairs. These reach into spaces in the soil where they absorb water. Water moves down the water potential gradient into the root hair from the soil due to the solutes present in the root cells. The large number of root hairs provides a large surface area, thus increasing the rate at which water can be absorbed.

57
Q

How do mineral ions enter the plant?

A

The mineral ions in solution are absorbed along with the water by the root hairs. They move through the plant past the cortical cells via symplastic or apoplastic pathway, eventually reaching the casparian strip of the endodermis and entering via the symplastic pathway through the pericycle and into the xylem through the pits(unlignified). Mineral ions can move either by active transport or diffusion.

58
Q

Describe the structure of a phloem sieve tube element.

A

Sieve tubes are made up of many elongated sieve elements joined from end to end vertically to form a continuous tube. Each sieve element is a living cell. Each of these cells has a cellulose cell wall, cytoplasm, mitochondria, endoplasmic reticulum, cell surface membrane. There is no nucleus, nor are there any ribosomes. Where the end walls of sieve tube elements meet, a sieve plate is formed where there are large open pores allowing the free flow of liquid.

59
Q

Describe the structure of a companion cell.

A

Each sieve element has at least one companion cell lying close beside it. Companion cells have a cytoplasm, cell membrane, cellulose cell wall, nucleus and a small vacuole. However, the number of mitochondria and ribosomes is rather larger than normal and the cells are metabolically very active. Plasmodesmata pass through the cell walls between the companion cells and sieve tube elements making direct contact between both cytoplasms.

60
Q

How does clotting occur in a plant?

A

When a sieve tube is cut, the release of pressure inside the phloem causes a surge of its contents towards the cut and the sieve plate blocks this surge. This helps to prevent the escape of the contents of the sieve tube. Then, a carbohydrate called callose in a process of clotting seals the sieve plate.

61
Q

Of xylem vessels and phloem tubes, which uses passive transport?

A

Phloem uses active transport and xylem uses passive transport.

62
Q

How is sucrose loaded into the phloem?

A

Mesophyll cells in leaves make sugars during photosynthesis. These sugars are converted to sucrose. Sucrose moves from the mesophyll cell to the phloem tissue either through the symplastic pathway or the apoplastic pathway. The sucrose is loaded into a companion cell or directly into the sieve element by active transport. Hydrogen ions are pumped out of the companion cell into its cell wall using ATP. This creates a large excess of hydrogen ions in the apoplastic pathway outside the companion cell. In order to re-enter cell, the H ions move down their concentration gradient through a carrier protein called a co-transporter molecule which carries both hydrogen ions and sucrose (sucrose is carried against concentration gradient). The sucrose molecules can then move from the companion cell into the sieve tube, through the plasmodesmata which connect them(symplastic).

63
Q

How does translocation occur?

A

To create the pressure differences needed for mass flow in phloem, the plant has to use energy. Phloem transport is therefore an active process. The pressure difference is created by active loading of sucrose into the sieve elements from the source (the place from which sucrose is to be transported). Any area where sucrose is taken out of the phloem is called a sink.
Loading a high concentration of sucrose into a sieve element greatly decreases the water potential in the sap inside it. Therefore, water enters the sieve element by osmosis. This causes a high build up in pressure known as hydrostatic/turgor pressure or pressure potential. A pressure difference is therefore created between the source and the sink. This pressure difference causes a mass flow of water and dissolved solutes through the sieve tubes from the high pressure area to the low pressure area. Within any vascular bundle, phloem sap may be flowing upwards in some sieve tubes and downwards in others.