Exchange between Organisms (Mass Transport) - Transport in Plants

Question 1

Compare diffusion and mass flow.

Answer 1

Diffusion

• individual molecules/ions
• random
• down concentration gradient

Mass Flow

• volume of liquid/gas
• directional
• down pressure gradient (heart generates pressure)

Question 2

What direction does water flow in a plant?

Answer 2

up - from a higher water potential (roots) to a lower water potential (leaves)

Question 3

What is transpiration?

Answer 3

the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves

Question 4

How does water move out through stomata?

Answer 4

• humidity of atmosphere is less than that of the air spaces next to stomata
• so there’s a water potential gradient from the air spaces through the stomata in the air
• if stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
• water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells

Question 5

How can plants control their rate of transpiration?

Answer 5

by changing the size of the stomatal pores

Question 6

How does water move across the cells of a leaf?

Answer 6

• water is lost from the mesophyll cells by evaporation from their cell walls to the air spaces of the leaf
• this is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm

Question 7

Why does water movement occur in the case of the cytoplasmic route?

Answer 7

• 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 of water from these neighbouring cells lowers their water potential
• they, in turn, take in water from their neighbours by osmosis

Question 8

What is the overall movement of water in a plant?

Answer 8

a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere

Question 9

How does water move up the stem in the xylem? What is the cohesion-tension theory?

Answer 9

• water evaporates from mesophyll cells due to heat from the sun leading to transpiration
• water molecules form hydrogen bonds between one another and hence tend to stick together (cohesion and capillary action)
• water forms a continuous, unbroken column across the mesophyll cells and down the xylem
• as water evaporates from the mesophyll cells in the lead into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion
• a column of water is therefore pulled up the xylem as a result of transpiration (transpiration pull)
• transpiration pull puts the xylem under tension so there is a negative pressure within the xylem (more negative at top (pull) and more positive at bottom (push)), hence the name cohesion-tension theory

Question 10

What are the two ways water moves along a water potential gradient in a plant?

Answer 10

• cell wall pathway

- cytoplasmic pathway

Question 11

What evidence is there that support the cohesion-tension theory?

Answer 11

• Change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter. At night, when transpiration is at its 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 it, 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 12

How is energy for transpiration supplied?

Answer 12

• transpiration pull is a passive process and therefore does not require metabolic energy to take place
• the xylem vessels through which the water passes are dead and so cannot actively move the water
• xylem vessels have no end walls which means that xylem forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory of water flow up the stem
• energy is nevertheless needed to drive the process of transpiration
• this energy is in the form of heat that evaporates water from the leaves and it ultimately comes from the sun

Question 13

How does water evaporate in plants?

Answer 13

Water evaporates from the surface of the cell wall to the air spaces.

Question 14

How does water diffuse in plants?

Answer 14

Water vapour diffuses out of the leaf through the stomata, from the air spaces to the air.

Question 15

Why does water move in a plant by mass flow?

Answer 15

• volume of liquid is moving
• directional
• down a pressure gradient

Question 16

How is breathing in similar to transpiration?

Answer 16

breathing in creates a negative pressure, which would make the trachea collapse if they didn’t have rings of cartilage (this is the same with xylem tubes and lignin)

Question 17

What happens when xylem are matured and differentiated?

Answer 17

• only have dead cell walls

- have holes to free up space, minimising negative pressure and minimising movement restriction

Question 18

How can you have a positive water potential?

Answer 18

if you compress pure water (hydrostatic pressure)

Question 19

What are xylem tubes?

Answer 19

• carry water and mineral ions from the roots to the stem and leaves
• made of dead cells and have no cytoplasm so form an empty tube for water to flow through (flow is not slowed)
• thick side walls and rings of lignin form rigid tubes that will not burst or collapse, and that provide support (strengthen xylem tubes)
• tiny pores allow water and mineral ions to enter and leave the xylem vessels
• contain pits which enable water to move sideways between the vessels
• directional

Question 20

What are phloem tubes?

Answer 20

• transport food substances (mainly sucrose) made in the leaves to the rest of the plant for immediate use (e.g. in growing regions) or for storage (use energy)
• made of elongated living cells (sieve tubes) that have small holes in the cell wall where the cells meet each other to allow substances to flow through
• sieve cells have small amounts of cytoplasm and no nucleus so there’s more room for the central channel
• contains companion cells with many mitochondria to produce energy needed for active transport, and load sucrose into sieve tubes
• bidirectional
• the cytoplasm of the sieve tube elements and companion cells is linked through structures known as plasmodesmata which are gaps between cell walls which allow communication and flow of substances such as minerals between the cells

Question 21

What is root pressure?

Answer 21

• only used in young plants or deciduous trees when leaves are developing and transpiration rate is low
• plants use energy to bring in mineral ions to roots via active transport, and this takes in water by diffusion (from high water and low salt concentration, to low water and high salt concentration)

Question 22

What is the transpiration stream?

Answer 22

movement of water and dissolved mineral ions up the plant

Question 23

What does transpiration provide?

Answer 23

• water for leaves/photosynthesis
• growth and elongation
• movement of mineral ions from roots to leaves
• evaporation has cooling effect on leaves preventing enzymes denaturing

Question 24

What are the factors affecting the rate of transpiration?

Answer 24

Any factor that will increase the water potential gradient between the air spaces in the leaf and surrounding air will have an effect.

• temperature
• humidity
• wind speed
• light

Question 25

How does temperature affect the rate of transpiration?

Answer 25

• increase in temperature increases kinetic energy of water molecules
• more molecules evaporate into leaf air spaces
• water potential gradient increases, more water diffuses out of the leaf

Question 26

How does humidity affect the rate of transpiration?

Answer 26

• decrease in humidity increases water potential gradient between leaf air spaces and surrounding air
• therefore increase in diffusion of water

Question 27

How does wind speed affect the rate of transpiration?

Answer 27

• increase in air movement around leaf will move water molecules away from leaf
• this maintains the water potential gradient

Question 28

How does light affect the rate of transpiration?

Answer 28

• light stimulates stomata to open

- the higher the light intensity, the more stomata are open

Question 29

What adaptations do xerophytic plants have to minimise water loss?

Answer 29

• smaller leaves to reduce surface area for water loss
• thicker waxy cuticle on both sides of plant
• succulent leaves store water for dry periods
• respond to low water availability by closing stomata
• contain hairs and pits which serve as a means of trapping moist air, reducing water vapour potential gradient
• roll their leaves in order to reduce the exposure of the lower epidermis to the atmosphere, trapping air that is moist

Question 30

What adaptations do cacti have to minimise water loss?

Answer 30

• no leaves, only succulent stems reduces sa:v ratio
• water storage in stems
• spines and hairs trap moist air next to stomata
• stomata open at night
• deep roots access water deeper below the surface
• shallow roots absorb rainwater before it evaporates

Question 31

What is translocation?

Answer 31

the process by which organic molecules and some mineral ions are transported from one part of a plant to another

Question 32

Why is translocation bidirectional?

Answer 32

• plants transport sugars produced during photosynthesis from the sites of production (sources) to places where they’ll be used directly or stored for future use (sinks)
• sinks can be anywhere in a plant, sometimes above or below the source
• therefore the translocation of molecules in phloem can be in either direction

Question 33

What does the phloem transport?

Answer 33

• organic molecules that include sucrose and amino acids and other assimilates
• inorganic ions such as potassium, chloride, phosphate and magnesium ions

Question 34

How are substances transported in the phloem?

Answer 34

• unsure about precise mechanism as the rate of movement is too fast to be explained by diffusion
• current thinking favours the mass flow theory

Question 35

How is sucrose transferred into sieve elements from photosynthesising tissue?

Answer 35

• Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts.
• The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells.
• Hydrogen ions are actively transported (active loading) from companion cells into the spaces within cell walls using ATP.
• These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
• Sucrose molecules are transported along with the hydrogen ions in a process known as co-transport. The protein carriers are therefore also known as co-transport proteins.

Question 36

How does sucrose move through the sieve tube elements?

Answer 36

Mass flow is the bulk movement of a substance through a given channel or area in a specified time.

• the sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes
• this causes the sieve tubes to have a lower water potential
• as the xylem has a much higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them (because the volume increases)
• at the respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage
• these cells therefore have a low sucrose content and so sucrose is actively transported into them from the sieve tubes lowering their water potential
• due to this lowered water potential, water also moves into these respiring cells, from the sieve tubes, by osmosis
• the hydrostatic pressure of the sieve tubes in this region is therefore lowered
• as a result of water entering the sieve tube elements at the source and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink
• there is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes

Question 37

Is translocation active or passive?

Answer 37

• while mass flow is a passive process, it occurs as a result of the active transport of sugars
• therefore the process as a whole is active which is why it is affected by temperature and metabolic poisons etc.

Question 38

How is sucrose transferred from sieve tube elements into storage or other sink cells?

Answer 38

the sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells

Question 39

What evidence supports the mass flow hypothesis?

Answer 39

• there is a pressure within sieve tubes, as shown by sap being released when they are cut
• the concentration of sucrose is higher in leaves (source) than in roots (sink)
• downward flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night
• increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later
• metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem
• companion cells possess many mitochondria and readily produce ATP

Question 40

What evidence questions the mass flow hypothesis?

Answer 40

• the function of the sieve plates is unclear, as they would seem to hinder mass flow (it has been suggested that they may have a structural function, helping to prevent the tubes from bursting under pressure)
• not all solutes move at the same speed, they should do so if movement is by mass flow
• sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to ones with the lowest sucrose concentration, which the mass flow theory would suggest

Question 41

Which direction is sucrose transported in the phloem?

Answer 41

from a source to a sink

1. source: photosynthesising leaf -> sink: roots for storage
2. source: photosynthesising-leaf -> sink: flower, developing seed
3. source: root storage tuber -> sink: growing leaf bud

Question 42

What is the overall process of mass flow in a plant?

Answer 42

The mass flow hypothesis:

1. Sucrose is loaded into the sieve element by the companion cell at the source.
2. Sucrose is unloaded from the sieve element by the companion cell at the sink.
3. A positive hydrostatic pressure pushes the phloem sap from source to sink.

Question 43

What is the vascular bundle in the roots?

Answer 43

• xylem and phloem are components of the vascular bundle, which enable the transport of substances as well as provide structural support
• the xylem vessels are arranged in an x shape in the centre of the vascular bundle
• this enables the plant to withstand various mechanical forces
• the x shape arrange of xylem vessels is surrounded by endodermis, which is an outer layer of cells which supply xylem vessels with water
• an inner layer of meristem cells known as the pericycle

Question 44

What is the vascular bundle in the stem?

Answer 44

• the xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem
• phloem is found on the outside of the vascular bundle
• there is a layer of cambium in between the xylem and phloem, which are meristem cells involved in the production of new xylem and phloem tissue

Question 45

What is the vascular bundle in the leaf?

Answer 45

• the vascular bundles forms the midrib and veins of a leaf
• dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards which are involved in transport and support

Question 46

What is a potometer?

Answer 46

The rate of transpiration can be investigated with the help of a potometer where water lost by the leaf is replaced by water in a capillary tube. Therefore, measuring the movement of the meniscus or a bubble can be used to determine the rate of transpiration.

Factors which affect the rate of transpiration include:

• number of leaves
• number/size or position of stomata
• presence of waxy cuticle
• the amount of light present
• the temperature
• humidity of the air
• air movement
• water availability

Question 47

What are the two ways water is taken up by root hair cells and moves across the cortex of the root into the xylem?

Answer 47

• It can either occur via the symplast pathway where water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata, the channels which connect the cytoplasm of one cell to the next.
• The other pathway is the apoplast pathway where the water moves through the water filled intercellular spaces between cellulose molecules in the cells walls. In this pathway, water doesn’t pass through any plasma membranes therefore it can carry dissolved mineral ions and salts.
• When the water reaches a part of the root called the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water.
• Therefore, in order for the water to cross the endodermis, the water that has been moving through the cell walls must now enter the symplast pathway.
• Once it has moved across the endodermis, the water continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel which is the entry point of water

Question 48

What are ringing experiments?

Answer 48

In order to investigate if the phloem is responsible for mass flow a ringing experiment can be used. In this the bark and phloem of a tree are removed leaving just the xylem in the centre. Overtime the tissues above the missing ring swell with sucrose solution and the tissue below dies. This shows that sucrose is transported in the phloem

Question 49

What are tracer experiments?

Answer 49

Tracer experiments can also be used to investigate the transport of sucrose in plants. Plants are grown in a environment that contains radioactivity labelled carbon dioxide (14CO2). The presence of this means that they are incorporated into the sugar produced in photosynthesis.

The movement of these sugars can now be traced through the plant using autoradiography. Those areas that have been exposed to the radiation produced by the 14C in the sugars will appear black. It follows that these regions correspond to the area where the phloem is and therefore suggest that this is where the sugars are transported.

Question 50

What do observations in the ringing experiment suggest?

Answer 50

These observations suggest that removing the phloem around the stem has led to:

• the sugars of the phloem accumulating above the ring, leading to swelling in this region
• the interruption of flow of sugars to the region below the ring and the death of tissues in this region

Question 51

What evidence is there that translocation of organic molecules occurs in the phloem?

Answer 51

• When phloem is cut, a solution of organic molecules (sap) flow out.
• Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time.
• Aphids are a type of insect that feed on plants. They have needle-like mouthparts which penetrate the phloem. They can therefore be used to extract the contents of the sieve tubes. These contents show daily variations in the sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content of the phloem.
• The removal of a ring of phloem from around the whole circumference of a stem loads to the accumulation of sugars above the ring and their disappearance from below it.

Question 52

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 52

• sugars are actively transported into the phloem from the source via companion cells
• this lowers the water potential of the sieve tube elements so water enters by osmosis, causing an increased hydrostatic pressure
• this causes mass movement of sugars towards the sink for respiration for storage