36 Resource Acquisition and Transport in Vascular Plants

Question 1

How were primitive plants anchored to the soil?

Answer 1

By the bas of the stem of by threadlike rhizoids

eventually roots developed

Question 2

In what direction does water move in xylem?

Answer 2

In only one direction: root to leaf

Question 3

In what direction do sugars move through the phloem?

Answer 3

Either way - its multidirectional

Question 4

What does phloem carry?

Answer 4

Phloem spa which contains sugars etc.

Question 5

Where does carbon dioxide enter and leaf the plant?

Answer 5

Gas exchange happens in both the leaves and the roots.

Not that in the leads there is a net entry of carbon dioxide and a net loss of oxygen. This is due to photosynthesis.

In the roots only respiration occurs so there is a net intake of oxygen from the soil and a net release of carbon dioxide into the soil

Question 6

What is the principal factor that determines the size of a plant’s leaves?

Answer 6

Water availability: where water is plentiful many plants have large leaves to maximise photosynthesis.

In drier areas small leaves minimise water loss.

Question 7

What is the arrangement of leaves on a stem called?

Answer 7

Phyllotaxy.

Question 8

Why is phyllotaxy important?

Answer 8

As the shoot extends new leaflets form.

If the leaves are in the same place when looks at from above they would shade each other.

If the leaves were all on one side the stem would be unbalanced.

Question 9

As the stem ascends what angle will the next leaf be from the previous? Why is this important?

Answer 9

Approximately 137.5º.

This is important as its prevents leaves shading each other.

If the angle was lower i.e. 30º the leaves wouldn’t shade each other but would the first few to develop will all be on one side and thus the stem will be unbalanced.

Question 10

What indicates the extend of leaf coverage?

Answer 10

The ‘leaf area index’

Question 11

What does the ‘leaf are index’ actually represent and thus how is it calculated?

Answer 11

It is the total leaf area of the plant divided by the total ground area. The total ground area is found by drawing a circle from the stem and increasing its radius so that the circle includes all of the plant.

Therefore the leaf index area represents the ‘efficiency’ of the arrangement of the leaves whilst also predicting how much it shades the ground below.

Question 12

Can a plant have a ‘leaf area index’ greater than 1?

Answer 12

Yes, if there are multiple layers of leaves.

Question 13

What is a the highest ‘leaf area index’ seen in nature?

Answer 13

They usually go as high as ‘7’ - any higher is frivolous.

Question 14

As new leaves are added to the plant they shade the leaves below. What happens to these leaves?

Answer 14

Eventually these leaves will be so shaded that they perform more respiration than photosynthesis and thus are a net loss to the plant.

These leaves or even entire branches are removed by programmed cell death and are shed. This is called ’self-pruning’

Question 15

What factor besides shading and height, can effect the amount of light hitting a leaf?

Answer 15

Its orientation i.e. horizontal or vertical.

Question 16

How does orientation of a leaf effects the sunlight it receives. Therefore where is each orientation seen?

Answer 16

Horizontal leaves i.e. in trees are better at collecting light as they are perpendicular to the rays of sun. However as a consequence they shade the leaves below and the top leaves to be exposed to damagingly intense light.

Vertical leaves i.e. those of grass are parallel to the sun. This means that they do not shade the leaves below as much and thus the entire blade receives light. This prevents the top region from receiving a damagingly high intensity of light.

Question 17

What are two ways a plants growth affects its ability to receive light?

Answer 17

Growing tall may allow it to extend above the canopy and thus receive direct sunlight.

A distinctly branching pattern can help it receive light by extending into gaps where there is little shading. It also maximises the surface area as viewed from above and thus increase the sun light it gets (even if that sunlight has gone through the canopy.)

Question 18

Typically speaking, what types of plant grow taller and why? What is the exception

Answer 18

Eudicots and gymnosperms are generally taller as they have the strong anchoring taproots

Monocots generally not grow so tall as their fibrous roots provide less anchoring, The exception is palm tress which even as monocots, grow relatively tall.

Question 19

What type of root system do gymnosperms have?

Answer 19

Taproots.

Question 20

How is the growth of roots adapted to maximise nutrient acquisition?

Answer 20

They are highly branched to maximise to collect water form a large region.

They are also undergo hydrotropism and growth towards nitrates.

Question 21

How is competition between roots prevented?

Answer 21

When they are near roots of a different plant but the same species they avoid growing in that direction.

Therefore they avoid infraspecific competition for water etc.

Question 22

Besides roots hairs, what improves the efficiency of roots?

Answer 22

Most plants have symbiotic fungi called ‘mycorrhizae’ growing in the roots. These vastly increase the surface area.

Note that the mycorrhizae take in the water, then give it to the plant.

Question 23

What structure of the mycorrhizae increases surface area?

Answer 23

As fungi they have a vast network of branching fibres called ‘mycelium’

It fibre of the mycelium is called a ‘hyphae’. It is the vast number of these thin hyphae that absorbs the water.

Question 24

What are the “compartments” of plant tissue?

Answer 24

The apoplast and the symplast

Question 25

What is the apoplast and what is the symplast?

Answer 25

The apoplast is everything external to the plasma membrane of living cells. This includes cell walls, extracellular spaces and the interior of dead cells. The symplast is the interior of the cells and thus consists of the cytosol of the living cells as well as the plasmodesmata that connect the cells.

Question 26

What are the routes substance can take as they move through tissues?

Answer 26

The ‘apoplastic route’, the ’symplastic route’ and the ’transmembrane route'

Question 27

What is the ‘apoplectic route’?

Answer 27

The solutes and water move exclusively through the apoplast i.e. through the cell walls and extracellular spaces.

Question 28

What is the ’symplastic route’?

Answer 28

The solute/water moves exclusively through the symplastic route. This involves moving through to cytosol and the plasmodesmata.

Question 29

What is the ’transmembrane route’?

Answer 29

The solute moves in and out of the cells and thus travels through both the apoplast and the symplast. This is called the ’transmembrane route’ as it passes from the apoplast to the symplast and vice versa it must cross the plasma membrane.

Question 30

How is membrane potential established in plants and how does this differ from animal cells?

Answer 30

In animal cells membrane potentials are established primarily by pumping Na+ (and potassium) In plant cells such potentials are more commonly initiated through the movement of H+ ions

Question 31

How does co-transport differ between plants and animals?

Answer 31

In plants H+ ions are typically actively transported then allowed to return, causing cotransport. In animals the mechanism is the same but it is typically Na+ ions, not H+ ions, that are pumped.

Question 32

What are some specific examples of cotransport in plants?

Answer 32

By pumping H+ ions against their concentration gradient plants are able to transport Nitrate ion and Sucrose into their cells.

Question 33

What predicts what direction water will flow in?

Answer 33

‘Water potential’ which includes the effects both of solute concentration and physical pressure. Water will move form a region of higher water potential to a region of lower water potential.

Question 34

In what unit is water potential measured?

Answer 34

Megapascals (MPa)

Question 35

What is the equation for water potential?

Answer 35

ѱ = ѱs + ѱp where: ѱ = water potential ѱs = solute potential ѱp = pressure potential

Question 36

What does solute potential relate to?

Answer 36

The potential due to the solute concentration of a solution. As solutes are added the solute potential decreases so that it is always either 0 (pure water) or negative in the case of a solution

Question 37

What is solute potential also known as?

Answer 37

Osmotic potential

Question 38

What does pressure potential refer to?

Answer 38

The physical pressure exerted on the solution. It is positive if a positive pressure is applied to the solution i.e. a "push" It is negative if a negative pressure is applied i.e. suction.

Question 39

A solute is added to a solution on one side of a semipermeable membrane. In what direction will the water flow.

Answer 39

Towards the solution with the solute. This is because its solute potential decreases and thus water moves form high water potential to low.

Question 40

What happens if a plant cell is placed in a hyper osmotic solution?

Answer 40

It will lose water and plasmolyse as the cell wall detaches from the protoplast (cell’s living contents including the plasma membrane)

Question 41

Do animal cells undergo plasmolysis?

Answer 41

No, this is because plasmolysis is technically the separation of the cell wall and the plasma membrane. Since animals don’t have plasma membranes this is not possible.

Question 42

What is the living part of the cell called, including the plasma membrane?

Answer 42

The protoplast.

Question 43

What does ‘protoplast’ refer to?

Answer 43

The living content of the cell including the plasma membrane.

Question 44

What channel facilitates the diffusion of water through the plasma membrane?

Answer 44

Aquaporins.

Question 45

What affects the permeability of aquaporins?

Answer 45

Their permeability is decreased if the cytosolic Ca2+ levels drop. If the cytosolic pH increases so does their permeability.

Question 46

What is the general principle in which long distance transport occurs in a plant?

Answer 46

‘Bulk flow’ which is the movement of a liquid and its contents does to water potential differences. This is opposed to each solute diffusion through the extracellular fluid individually.

Question 47

What adaption of vascular structures allows water to flow through more easily?

Answer 47

Mature tracheids and vessel elements are dead cells and therefore have no cytoplasm, and the cytoplasm of sieve-tube elements is almost devoid of internal organelles.

Question 48

What path does water in the soil take to the before entering the xylem. Why is this important?

Answer 48

The epidermis of the root hair cell is highly permeable to water and thus allow water, and dissolved solutes i.e. ions, to easily enter. Once in the root it takes either a symplastic or apoplectic root. As it does this it passes through various cells of the cortex. This flow enhances the exposure of the cells of the cortex to the soil solution, providing a much greater membrane surface area for absorption than the surface area of the epidermis alone. For example the cortex cells filter the water as it passes through them and therefore important solutes i.e. K+ ions are accumulated.

Question 49

What is the route that water takes between when it enters the cortex and when it is in the xylem?

Answer 49

Either by the symplastic or apoplectic route it reaches the endodermis which separates the cortex from the vascular tissues. Water in the simplistic route pass through into the plasmodesmata of the endodermis and leave into a xylem vessel. Surrounding the apoplast of the endodermis is a thick impermeable strip called the ‘casparian strip’ Therefore water cannot enter the xylem without entering the endodermal cell i.e. joining the symplastic route.

Question 50

Why is the ‘Casparian strip’ important?

Answer 50

It ensure that water heading to the xylem passes through at least one plasma membrane. This is important as it prevents nutrients that have accumulated in the xylem from leaking back into the soil solution.

Question 51

How does water enter the 'water-conducting cells’ of the xylem?

Answer 51

Endodermal cells and also living cells within the vascular cylinder discharge water and minerals into their walls (apoplast).

Question 52

What is the Casparian strip composed of?

Answer 52

’Suberin’, a waxy material that is impervious to water and solutes

Question 53

If water is in a water conducting cell of the xylem is it is the apoplast or symplast?

Answer 53

The apoplast as the water-conducting cells of the xylem are dead.

Question 54

What are the contents tag trove through the xylem called?

Answer 54

The 'xylem sap'.

Question 55

What does ‘xylem sap’ include?

Answer 55

Water and also dissolved minerals

Question 56

What propels the movement of water up the xylem?

Answer 56

‘Root pressure’ and ’transpiration'

Question 57

What is ‘root pressure’ and how does it propel water up the xylem?

Answer 57

When there is little transpiration i.e. at night the root cells actively pump ions into the xylem of the vascular cylinder. The Casparian strip prevents these ions from leaking back. Because of this the water potential of the xylem lowers as the solute potential becomes progressively lower. This causes water to be forced into the xylem through osmosis. This is ‘root pressure.'

Question 58

Is root pressure a ‘push’ or a ‘pull’?

Answer 58

A push

Question 59

Does transpiration pull or push water up the xylem?

Answer 59

It pulls it up.

Question 60

What is evidence for root pressure?

Answer 60

The root pressure sometimes causes more water to enter the leaves than is transpired, resulting in "guttation", the release of water droplets onto the leaf’s surface. (not to be confused with dew)

Question 61

What does ‘guttation’ refer to?

Answer 61

Liquid water that forms on the surface of the leaf when root pressure forces more water into the xylem that is transpired.

Question 62

Water droplets are seen on the leaves of plants in the morning. If these droplets are not caused by dew what do they provide evidence for?

Answer 62

This formation is called ‘guttation’ and is evidence for root pressure. These droplets are seen when more water is forced into the xylem than is transpired. This causes liquid water to form on the leaf’s surface.

Question 63

What type of transport do substances undergo as they pass through the xylem and phloem?

Answer 63

Bulk flow.

Question 64

How does ‘bulk flow’ differ from osmosis?

Answer 64

In ‘bulk flow’ direction is determined exclusively by pressure. Unlike osmosis, relative solute concentration has no influence.

Question 65

What hypothesis describes how transpiration pulls water up the xylem?

Answer 65

The ‘Cohesion-Tension Hypothesis'

Question 66

What does cohesion refer to in terms of water?

Answer 66

The property of water that causes it to send to stick together due to the interaction of hydrogen bonds.

Question 67

What does adhesion refer to in terms of water?

Answer 67

The fact that water will tend to stick to hydrophilic surfaces.

Question 68

When explaining how transpiration pulls water up the xylem what two factors must be understood? (not adhesion or cohesion)

Answer 68

1) How the transpiration pull is generated | 2) How that transpiration pull is conducted to the water in the xylem.

Question 69

How is the transpiration pull generated?

Answer 69

Water from the xylem travels to the cell wall of the spongy mesophyll cells. Here the water evaporates. As this evaporation occurs the water sticks to the microfibrils of the cell wall. Therefore as the water is lost the water film curves backs. This creates surface tension which pulls water from the xylem and thus up the stem.

Question 70

Where does the actual evaporation occur during transpiration?

Answer 70

In the cell walls of the mesophyll cell.

Question 71

How does water get from the xylem to the mesophyll cells?

Answer 71

Through the apoplectic route i.e. across cell walls.

Question 72

What explains the cohesion seen in water?

Answer 72

It is able to form hydrogen bonds with other water molecules.

Question 73

What explain the adhesion seen in water?

Answer 73

Hydrogen bonds forming with the wall of the xylem etc.

Question 74

How is the transpiration pull conducted throughout the xylem?

Answer 74

The water molecules are attracted to each other through ‘cohesion.’ Therefore as water is forced out of the xylem by transpiration this pulls all of the water molecules up the xylem. Due to ‘adhesion’ the water molecules “stick” to the ‘hydrophilic’ xylem wall. This prevents the water molecules from falling back down.

Question 75

Does transpiration affect the vessel elements and tracheids?

Answer 75

It causes tension that causes them to narrow as transpiration increases. This is why these vessels have thick secondary cell walls. This explains why the diameters of a tree decreases as transpiration rate increases.

Question 76

What structure regulates the rate of transpiration?

Answer 76

The stomata.

Question 77

Why is it advantageous that water evaporates from the mesophyll cells?

Answer 77

Since there are many of them they constitute surface are greater than that of the xylem or leaf the leaf’s lamina.

Question 78

What opens and closes the stomata?

Answer 78

The guard cells.

Question 79

What factors control how many stomata there is on a leaf?

Answer 79

Mainly genetics. However high light expose and low CO2 levels during a leaf’s development will causes more stomata to develop

Question 80

The concentration of CO2 in the earths atmosphere is increasing. What impact will this likely have on the concentration of stomata found on leaves?

Answer 80

These lighter CO2 levels will result in less stomata developing as it is easier for the plant to get CO2. Therefore having less stomata is advantageous as it minimises water loss.

Question 81

What is the structure of guard cells and thus how do the guard cells open and close?

Answer 81

Each stomata has two guard cells with one on each side like ||o||. The guard cells have ‘radially orientated cellulose microfibrils’ That appear horizontally in the perspective of ||o|| The guard cells have large vacuoles. As these vacuole take on water this causes the cell to swell. However due to the cellulose microfibrils this swelling causes the guard cells to bend outwards like ((O)) and thus the stomata opens.

Question 82

If the vacuoles of a guard cell are swollen is the stomata open or closed

Answer 82

Open. As water elves the vacuole the guard cell will close the stomata.

Question 83

How does the guard cell regulate when it opens?

Answer 83

The guard cells actively transports H+ ions out of the cells. This establishes a membrane potential that makes the interior of the cell more negative. This membrane potential draws K+ ions into the cell, specifically the vacuole. This raises the solute concentration of the cell and thus lowers the water potential. This causes water to move into the guard cell through osmosis. The uptake of anions, such as malate and chloride ions also contributes to guard cell swelling.

Question 84

Is malate an anion or a cation?

Answer 84

An anion

Question 85

How does the guard cell close?

Answer 85

It releases K+ ions from its vacuole, causing them to leave the cell. This causes water to move out of the guard cell by osmosis and thus the stoma is closed.

Question 86

What are the main factors that regulate the opening of the stomata?

Answer 86

Light, CO2, Water availability, the circadian rhythms and certain other environmental stimuli during the day

Question 87

How does Light affect stomatal opening?

Answer 87

Blue-light receptors in the plasma membrane of the guard cell are activated when the light levels increase. This actives the proton pumps that promote the absorption of K+ ions and thus the opening of the stomata. Therefore bright light stimulates stomata to open. This means transpiration occurs only when the light needed for photosynthesis is present

Question 88

How does carbon dioxide affect stomatal opening?

Answer 88

If there is low levels of CO2 inside the leaf the stomata open. This is to ensure that there is always CO2 available for photosynthesis.

Question 89

How do circadian rhythms affect stomatal opening?

Answer 89

Generally speaking the stomata is open during the day and close during the night, This is to prevent water begin lost during the night when no photosynthesis is occurring.

Question 90

How does water availability affect stomatal opening?

Answer 90

If the leaves or roots are low on water they release the hormones abscisic acid (ABA). This stimulates the closure of the stomata.

Question 91

How does dehydration affect growth of the plant?

Answer 91

Much of the growth in a plant occurs through expansion. Therefore when little water is available the growth rate decreases.

Question 92

What ‘other environmental factors’ i.e. not light, CO2, time and water availability affect stomatal opening?

Answer 92

Drought, high temperature or excessive wind may trigger the stomata to close during the day.

Question 93

What are the general conditions that will lead to the greatest rate of transpiration?

Answer 93

A day that is sunny, warm, dry and windy.

Question 94

Besides providing water to the leaves, what is a major purpose of transpiration?

Answer 94

It provides ‘evaporative cooling’ to the leaves and thus prevents them form overheating which would cause the enzymes to denature.

Question 95

What are plants that are adapted to dry environments called?

Answer 95

Xerophytes

Question 96

What does ‘xerophyte’ refer to?

Answer 96

A plant that is adapted to dry conditions.

Question 97

Why do low water levels reduce photosynthesis?

Answer 97

The primary causes of this is NOT actually the fact that water is a reactant of photosynthesis. Instead it is because the need to conserve water causes the stomata to be closed so frequently that it is actually CO2 that becomes the limiting factor

Question 98

What are some typical adaptations of xerophytes?

Answer 98

Ability to stay dormant then blossom during rain, long roots, fleshy stem to sore water, small leaves (i.e. cactus spines) long roots. Also the use of CAM photosynthesis.

Question 99

What is the movement of sugars through the plant called?

Answer 99

Translocation.

Question 100

What does ’translocation’ refer to?

Answer 100

The movement of the products of photosynthesis i.e sugars through the plant

Question 101

What does phloem carry specifically?

Answer 101

Phloem sap which is an aqueous solution.

Question 102

What can phloem sap contain besides sugar?

Answer 102

Amino acids, hormones and minerals.

Question 103

Why is it important that hormones etc. can be carried by phloem?

Answer 103

Xylem is unidirectional and thus only phloem, which moves both ways, can carry hormones etc. from the leaves to the roots.

Question 104

How can plant structures be classed based on their use of sugars?

Answer 104

'Sugar sinks’ are net users of sugars whereas ’sugar sources’ lead to a net increase of sugars.

Question 105

What are some examples of sugar sinks?

Answer 105

Growing roots, buds, stems and fruits.

Question 106

Are leaves sugar sinks or sugar sources?

Answer 106

A growing leaf is a sugar sink. Conversely a mature illuminated leaf is a sugar source.

Question 107

Besides a leaf, what are some examples of structures that can be both sugar sources and sinks depending on time?

Answer 107

Storage organs such as tubers or bulbs. When stockpiling carbohydrates in summer these structures are sugar sinks. When they release these during winter they are sugar sources.

Question 108

How are sugars loaded into the phloem?

Answer 108

In some species, it moves from mesophyll cells to sieve-tube elements via the symplast, passing through plasmodesmata. In other species, it moves by symplastic and apoplastic pathways. Sucrose diffuses through the symplast from photosynthetic mesophyll cells into small veins. It then moves into the apoplast and is accumulated by nearby sieve-tube elements (directly or by companion cells). In some plants, the walls of the companion cells feature many ingrowths, enhancing solute transfer between apoplast and symplast.

Question 109

There is typically a higher sugar concentration in the phloem than out. How is sugar moved against its concentration gradient into the sieve tube elements?

Answer 109

The 'sieve tube elements' and 'companion cells’ have proton pumps which pump H+ ions out of the cell. As H+ diffuses down its concentration gradient back into the phloem cell this allows sucrose to move into the phloem cell through a ‘H+-sucrose cotransporter'

Question 110

What is phloem transport a form of?

Answer 110

Bulk flow.

Question 111

What principal explains the movement of phloem?

Answer 111

The ‘pressure flow’ hypothesis.

Question 112

How does ‘pressure flow’ work in phloem?

Answer 112

Loading of sugar into the sieve tube at the source reduces water potential inside the sieve-tube elements. This causes the tube to take up water from the xylem by osmosis. This uptake of water generates a positive pressure that forces the sap to flow along the tube. The unloading of sugars at the sink relieves the pressure. This unloading causes water to flow back to the xylem.

Question 113

What happens if a plant is detecting that it is running low on sugars?

Answer 113

It many cut of some flowers, seeds or fruits to save sugar usage. This is called ’self-thinning’ and allows these resources to be used by other sugar sinks.

Question 114

What important feature of the symplast distinguishes it from the apoplast?

Answer 114

The symplast is highly dynamic.

Question 115

In what ways are plasmodesmata dynamic?

Answer 115

They can open and sloes in response to changes in turgor pressure, cytosolic Ca2+ levels or cytosolic pH. Some form during cytokinesis but others form later in the cell’s life in reopen to need. They also interact with viruses.

Question 116

How do plasmodesmata respond to viruses?

Answer 116

Typically viruses are too large to fit through a plasmodesmata. To allow viral infections to spread through plants the viruses have ‘viral movement proteins’ not their surface. These bind with the plasmodesmata causing them to dilate and thus allowing the virus to pass through.

Question 117

What is a large group of cells connected to each other by plasmodesmata called?

Answer 117

A ’symplastic domain’ i.e there is a continuous symplastic route from the phloem cells in the roots to those at the top of the stems.

Question 118

What are some examples of macromolecules transported through phloem?

Answer 118

Proteins and even RNA

Question 119

Besides carrying hormones, how can phloem carry signals?

Answer 119

In some plants it can carry electrical signals/

Question 120

What are some examples of plant which have electrical signalling in the phloem?

Answer 120

Venus fly traps and other plants which have rapid leaf movements.

Question 121

Besides movement, what are some processes that are at least partly mediated by phloem electrical signalling in some plants?

Answer 121

These signals can carry instructions for gene transcriptions, respiration, photosynthesis, phloem unloading or hormone release.

Question 122

Define protoplast?

Answer 122

The living content of a cell i.e. the plasma membrane and the cytoplasm. It does not include the cell wall. The protoplast can also be referred to as the apoplast