Chapter 7: Transport In Plants

Question 1

How do plants transport carbon dioxide and oxygen?

Answer 1

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

Question 2

What are the two transport systems in a plant?

Answer 2

Xylem and phloem

Question 3

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

Answer 3

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.

Question 4

What does the phloem carry and where? Direction?

Answer 4

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.

Question 5

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

Answer 5

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.

Question 6

What are tissues?

Answer 6

They are collections of cells specialised for a particular function.

Question 7

What is the epidermis and what is its function?

Answer 7

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

Question 8

What are some of the functions of a waxy cuticle?

Answer 8

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

Question 9

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

Answer 9

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.

Question 10

What is parenchyma and where is it found?

Answer 10

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.

Question 11

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

Answer 11

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.

Question 12

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

Answer 12

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.

Question 13

Where is the endodermis found?

Answer 13

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

Question 14

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

Answer 14

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.

Question 15

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

Answer 15

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.

Question 16

What are the tubes of the xylem called?

Answer 16

Vessels

Question 17

What are xylem vessels made of?

Answer 17

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

Question 18

What are some functions of xylem?

Answer 18

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

Question 19

What are the tubes of the phloem called?

Answer 19

Sieve tubes

Question 20

What are the sieve tubes made of?

Answer 20

They are made from living cells called sieve tube elements

Question 21

What is a function of the phloem?

Answer 21

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

Question 22

Give a brief overview of transpiration.

Answer 22

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.

Question 23

What is transpiration? Proper

Answer 23

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.

Question 24

Describe the leaf to atmosphere part of transpiration.

Answer 24

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.

Question 25

Are the cells in the mesophyll layer wet?

Answer 25

Yes

Question 26

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

Answer 26

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

Question 27

How does humidity affect transpiration?

Answer 27

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.

Question 28

How do wind speed and temperature affect transpiration?

Answer 28

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

Question 29

How does light intensity affect transpiration?

Answer 29

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.

Question 30

How do very dry conditions affect transpiration?

Answer 30

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.

Question 31

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

Answer 31

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.

Question 32

How can transpiration lead to wilting of plants?

Answer 32

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.

Question 33

What are xerophytes?

Answer 33

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.

Question 34

What are some adaptations of xerophytes?

Answer 34

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

Question 35

How do rolled leaves help xerophytes?

Answer 35

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

Question 36

How do thick waxy cuticle help xerophytes?

Answer 36

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

Question 37

How do hair help xerophytes?

Answer 37

Trap moisture to reduce water potential gradient.

Question 38

How do sunken stomata/ stomata in pits help xerophytes?

Answer 38

Moist air trapped in pits reduces water potential gradient

Question 39

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

Answer 39

Not exposed to sunlight, reducing evaporation rate

Question 40

How do small leaves, reduced to spines help xerophytes?

Answer 40

It reduces surface area for transpiration.

Question 41

Explain the transport from xylem across the leaf in transpiration.

Answer 41

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.

Question 42

What is the symplastic pathway?

Answer 42

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

Question 43

What is the apoplastic pathway?

Answer 43

It is where water moves through the cell walls.

Question 44

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

Answer 44

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.

Question 45

What are pits?

Answer 45

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.

Question 46

What is hydrostatic pressure?

Answer 46

It is the pressure exerted by a liquid.

Question 47

What gradients are present in the xylem tube?

Answer 47

Hydrostatic pressure gradient and water potential gradient.

Question 48

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

Answer 48

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.

Question 49

What is mass flow?

Answer 49

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

Question 50

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

Answer 50

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.

Question 51

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

Answer 51

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.

Question 52

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

Answer 52

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.

Question 53

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

Answer 53

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.

Question 54

How does water move from root hair to the xylem?

Answer 54

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.

Question 55

What happens when endodermal cells get older?

Answer 55

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.

Question 56

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

Answer 56

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.

Question 57

How do mineral ions enter the plant?

Answer 57

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.

Question 58

Describe the structure of a phloem sieve tube element.

Answer 58

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.

Question 59

Describe the structure of a companion cell.

Answer 59

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.

Question 60

How does clotting occur in a plant?

Answer 60

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.

Question 61

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

Answer 61

Phloem uses active transport and xylem uses passive transport.

Question 62

How is sucrose loaded into the phloem?

Answer 62

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).

Question 63

How does translocation occur?

Answer 63

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.