M3, C9 Transport in Plants

Question 1

Why do plants need a transport system?

Answer 1

Metabolic demands –

• Transport glucose and oxygen
• Transport waste products
• Transport of hormones and mineral ions

Size – some plants are small, but as they grow year on year some are huge

SA:Vol ratio – Leaves have a large SA:Vol ratio for gas exchange, but when the stem, trunks and roots are included they have a small SA:Vol ratio

Question 2

what are the two tissues in plants and what do they transport?

Answer 2

Xylem – water and mineral ions upwards
Phloem – assimilates 9mainly sucrose and amino acids) around the plant (up and down)
Found together in the vascular bundles in the leaves, stem and roots

Question 3

where are the vascular bundles in the stem of plants and why are they there?

Answer 3

around the edge to gives strength and support

Phloem on the outside and xylem on the inside

Question 4

where are the vascular bundles in roots and why are they there?

Answer 4

In the middle to help plant withstand the tugging strains that result as the stem and leaves are blown in the wind

Xylem is the starshaped tube in the middle- phloem between in the pits

Question 5

where are the vascular bundles in leaf cells and why are they there

Answer 5

The midrib is the main vein carrying the vascular tissue through the organ.
It also helps to support the structure of the leaf.
Many small branching veins spread through the leaf functioning both in transport and support

Question 6

what is the structure of xylem and how are they adapted for the function of xylem

Answer 6

thick cell wall containing lignin - waterproof and makes vessels extremely strong and prevents them from collapsing

wide lumen and linked end to end to create a long hollow tube - means transport of large volumes of water and ions

bordered pits (unlignified) - allows lateral movement of water

Question 7

what do the thick walled parenchyma and xylem fibre tissues do in the xylem?

Answer 7

Thick walled parenchyma – packs around the xylem, storing food and contains tannin deposits

Xylem fibre – long cells with a lignified secondary wall – provides strength, doesn’t transport water

Question 8

what is the structure of phloem and how are they adapted to their function?

Answer 8

sieve tube and sieve plates - cells connected end to end like xylem but contains living cells, often lacks a nucleus to allow maximum transport

companion cells -contains the organelles which the sieve tube doesn’t have. plasmodesmata links them. controls movement of substances

parenchyma and fibres - provides support

Question 9

what is water potential?

Answer 9

The pressure exerted by water molecules as they collide with a membrane or container
Measured in pascals (Pa) or kilopascals (kPa)
Symbol is Ѱ (Greek letter psi)

Question 10

what is the water potential of pure water?

what is the water potential of solutes?

Answer 10

pure water = 0kPa
Presence of solutes lowers Ѱ, below zero
All solutions have a negative water potential

water can move but NOT solutes

Question 11

what is osmosis?

Answer 11

Net movement of water from a high water potential to a low water potential
This continues until an equilibrium is reached

Question 12

what happens if you put a plant cell in pure water?

Answer 12

the water potential is higher outside the cell than in the cell so this causes osmosis into the cell.
Water inside exerts hydrostatic pressure on cell membrane pushing it against cell wall, called the turgor pressure – as turgor pressure increases it resists entry of further water
Cell is said to be TURGID

Question 13

what happens if you put a plant cell in concentrated salt solution

Answer 13

Water moves out by osmosis because the water potential of the cell is higher (less negative) than the water potential of the solution.

Cell loses its turgidity
Volume of cytoplasm reduced
Cell membrane pulls away from the cell wall = PLASMOLYSIS
Cell is said to be PLASMOLYSED

Question 14

define hydrostatic pressure

define oncotic pressure

Answer 14

Hydrostatic pressure: pressure exerted by a liquid

Oncotic pressure: tendency of water to move into blood by osmosis due to plasma proteins

Question 15

what are the features of a root hair cell?

Answer 15

Microscopic – penetrate between soil particles
Large SA:Vol ratio
1000s on each root tip
Hair has thin surface layer
Concentration of solutes in cytoplasm maintains water potential gradient between soil, water and cell

Question 16

how does water move from the soil to the root hair cell

Answer 16

Mineral ion absorbed by active transport (into root hair cells)
This reduces water potential of cells
Water potential is lower in cell than soil
Water moves from soil to RHC by osmosis

Question 17

how does water move across the root the symplast way?

Answer 17

• Cytoplasm connected by plasmodesmata
• RHC has higher Ѱ than next cell along
• Water moves across by osmosis
• Process continues until xylem reached

Question 18

how does water move across the root the apoplast way?

Answer 18

• Water fills spaces between fibres of cell wall
• Water moves into xylem by cohesive forces between water molecules and as a result of transpiration pull up the xylem, water is pulled through

Question 19

define endodermis

define casparian strip

Answer 19

Endodermis
Layer of cells surrounding the vascular tissue

Casparian Strip
Band of waxy material (suberin) that runs around each endodermal cell

Question 20

how does water pass through the endodermis into the xylem

Answer 20

The waterproof layer in the cell walls of the endodermis, means water can no longer pass through via the apoplast route

Instead, it enters the cytoplasm and passes through the symplast route.

This way, it is forced to pass through the cell surface membrane before entering the xylem.

Question 21

what is the point of the casparian strip

Answer 21

to regulate the water and to stop any potential toxic solutes in the soil water entering the living tissues

Question 22

why does the water move from the endodermal cells into the xylem

Answer 22

Higher water potential in endodermal cell than xylem

Plus endodermal cells actively transport mineral ions out of themselves and into the xylem (thus furthering increasing the water potential in the cell)

So water moves into xylem by osmosis.

Question 23

what happens when the water is at the base of the xylem

Answer 23

Root pressure - the entry of water into the base of the xylem, physically pushes any existing water above it upwards.

Question 24

how did scientist find out active transport was involved in water transport in xylem

Answer 24

• some poisons like cyanide affect the mitochondria and prevent production of ATP. You can see this because there is no energy supply to the roots meaning the pressure disappears
• root pressure increases with an increase in temperature and falls with a fall in temperature suggesting chemical reactions are involved
• if levels of oxygen or respiratory substrates fall, root pressure falls
• guttation - xylem sap may discharge from the cut end of stems at certain times. In the natural world, xylem sap is forced out of special pores at the end of leaves in some conditions

Question 25

what is transpiration

Answer 25

Transpiration is the evaporation of water vapour from the surface of leaves of a plant. Most of this occurs from the underside of a leaf, where there are many stomata. Guard cells open and close the stomata to control water loss. Diffusion of CO2 and O2 also occurs through the stomata.

Question 26

what is a transpiration stream

Answer 26

Flow of water from soil into the root hair cells, through the cortex by osmosis, into the xylem and up the stem by cohesion of water molecules, across the leaf by osmosis and out of the leaf by evaporation and diffusion.

Question 27

what are the 3 ways that cause the water to move up the xylem

Answer 27

transpiration pull adhesion cohesion

Question 28

what is the cohesion-tension theory

Answer 28

Adhesion - Water forms hydrogen bonds with the carbohydrates in the xylem, so attracted to wall Water molecules attracted to each other by cohesion – hold molecules together in a column due to slight electrical charge Adhesion and cohesion results in capillary action, so water can rise against the pull of gravity. Transpiration pull - Water is drawn up by a continuous stream to replace water lost This creates tension so xylem contains lignin to prevent it from collapsing and pulls water in from root.

Question 29

how do stomata control the rate of transpiration

Answer 29

When turgor is low (no pressure pushing back, so less water in guard cells), guard cell closes pore In favourable environments, guard cells pump solutes in increasing turgor as water follows by osmosis. Cellulose hoops prevent swelling in width, so extend lengthways, becoming bean shaped opening pore

Question 30

what is the evidence for the cohesion-tension theory

Answer 30

- changes diameter of trees - tension increases as rate of transpiration increases hence shrinking the diameter - when a xylem vessel is broken air is drawn into the xylem rather than water leaking out. - cohesive forces break when the stem breaks so water no longer moves up the stem

Question 31

how do you measure the rate of transpiration

Answer 31

Potometer - Measures the rate of water uptake - As this is proportional to the rate of transpiration, it can be taken as a measure of transpiration rate if we assume that the rate of water loss equals the rate of water uptake. - The connection must be airtight otherwise water will not be drawn up into the capillary tube and no air bubble movement will be possible. - The shoot must be cut at an angle to increase surface area and remain under water to prevent air bubbles entering the xylem in the shoot

Question 32

whats the equation for the rate of water uptake and what are the units

Answer 32

Rate of water uptake (cms^-1) = distance moved by air bubble / time taken for air bubble to move that distance

Question 33

how does light affect the rate of transpiration

Answer 33

more light opens more stomata which enables more gas exchange. this means more water vapour diffuses out therefore increasing rate of evaporation from the surfaces of the leaf, hence increasing the rate of transpiration

Question 34

how does humidity affect the rate of transpiration

Answer 34

a very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and the outside air. so very dry conditions increases the rate of transpiration

Question 35

how does temperature effect the rate of transpiration

Answer 35

1) increasing temp increases the kinetic energy of water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf 2) increasing temp increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases the humidity hence increasing rate of transpiration)

Question 36

how does air movement effect the rate of transpiration

Answer 36

each leaf has a layer of still air around it trapped by the shape of the leaf and features like hairs on the surface of the leaf decrease air movement close to the leaf. the water vapour that diffuses out of the leaf accumulates here so the water vapour potential around the stomata increases which reduces the diffusion gradient. anything that increases the diffusion gradient increases the rate of transpiration so wind increases the rate of transpiration

Question 37

how does soil-water availability effect the rate of transpiration

Answer 37

the amount of water available in the soil can affect transpiration rate. If it is very dry the plant will be under water stress and the rate of transpiration will be reduced.

Question 38

what is glucose converted to when travelling around the plant

Answer 38

sucrose

Question 39

what is an assimilate

Answer 39

Any product of photosynthesis that is carried around the phloem is called an assimilate. Sucrose is the main assimilate in plants. They move from source to sink

Question 40

define source

Answer 40

Source: a region of the plant that has excess carbohydrate

Question 41

define sink

Answer 41

Sink: a region of the plant that needs carbohydrate

Question 42

define translocation

Answer 42

The movement of assimilates in the phloem from source to sink

Question 43

what happens in the symplast route of the transport of glucose

Answer 43

Sucrose moves from source through cytoplasm and into sieve tubes by diffusion through plasmodesmata Water follows by osmosis No energy required

Question 44

what happens in the apoplast route of the transport of glucose

Answer 44

- H ions are actively (uses ATP) pumped out of companion cell into surrounding tissue - H ions returns to companion cell down concentration gradient via co-transport protein with sucrose - Lots of sucrose in companion cell, low sucrose in sieve tube - Therefore sucrose moves from high to low concentration via diffusion through plasmodesmata - Water moves into phloem from xylem as lower water potential

Question 45

what are the main sinks in a plant

Answer 45

growing roots meristems that are actively dividing developing seeds, fruits or storage organs

Question 46

what are the main sources in a plant

Answer 46

green leaves or green stems storage organs food stores in seeds when they germinate

Question 47

define mass flow

Answer 47

the transport of substances in a fluid from a high to low pressure

Question 48

define turgor pressure

Answer 48

the pressure exerted by the cell-surface membrane against the cell wall

Question 49

define co-transport

Answer 49

process in which 2 substances are simultaneously transported across a membrane by one protein

Question 50

how does sucrose unload at the sink end

Answer 50

1. sucrose diffuses out of the sieve tube into surrounding cells from high to low concentration of sucrose 2. the loss of sucrose from the sieve tube increases the water potential in the tube 3. Water moves from the sieve tube into the xylem by osmosis from a high to low water potential

Question 51

what is the evidence for translocation

Answer 51

Companion cells have many mitochondria The flow of sugars in the phloem is 10 000 times faster than it would be by diffusion alone Let an aphid feed on the phloem. Remove the aphid body and leave the mouthpart in the phloem. Sap flows out the mouthpart faster nearer the leaves than further down the stem - flow rate in phloem is lower closer to the sink than it is near the source Add a metabolic inhibitor and translocation stops - shows chemical reactions are involved because if it was natural then the inhibitor wouldn't stop anything

Question 52

define xerophytes | define hydrophytes

Answer 52

xerophytes - Plants in dry habitats have evolved to enable them to live and reproduce in places where water is very low hydrophytes - Live in water and so need adaptations to cope with growing in water or in permanently saturated soil

Question 53

how are xerophytes adapted for survival and explain how

Answer 53

``` thick waxy cuticle sunken stomata reduced numbers of stomata reduced leaves hairy leaves curled leaves succulents (parenchyma stores water) leaf loss root adaptations avoiding the problems ```

Question 54

how are hydrophytes adapted for survival and explain how

Answer 54

``` very thin or no waxy cuticle many, always open stomata on the upper surface reduced structure wide, flat leaves small roots large surface area of stem and roots under the water air sacs aerenchyma ```

Question 55

what is aerenchyma (hydrophytes)

Answer 55

in leaves, stems and roots large air spaces makes leaves and stems more buoyant forms a low resistance internal pathway for movement of substances like oxygen to tissues below the water. helps plant cope with extreme low oxygen conditions

Question 56

Give examples of xerophytes

Answer 56

Conifers | Marram grass

Question 57

Give examples of hydrophytes

Answer 57

Water lilies Water cress Duck weeds Yellow iris

Question 58

what's the difference between the term transpiration and the transpiration stream

Answer 58

transpiration loss of water vapour / evaporation of water ; from, aerial parts of plant / leaves / stomata transpiration stream movement of water (up xylem vessels) ; from roots to, leaves / air surrounding leaves