Plant Transport

Question 1

How do plants transport water and mineral ions?

Answer 1

Photosynthetic cells in the leaf need water and mineral ions which are available only in the soil
The transport of water and mineral irons from the root to the leaves is essential. These molecules are transported upwards in a hollow tube called xylem.

Question 2

How do plants transport the products of photosynthesis?

Answer 2

The glucose produced during photosynthesis is used to make sucrose and amino acids

These molecules must be moved from the leaves to the other organs of the plant
The photosynthates are transported upwards and downwards in phloem tissue which is called bidirectional movement

Question 3

How are mineral ions taken up by plants?

Answer 3

Mineral ions are actively transported from the soil into the root hair cells. This lowers the water protection inside the root hair cell so water enters by osmosis.

Question 4

What is transport by the Apoplast pathway?

Answer 4

Water is taken up by the root hair cell and moves across the cortex by cohesion via cell walls

Question 5

What is the symplast pathway transport system?

Answer 5

Water moves from the cytoplasm of one cell to the next by osmosis via the plasmodesmata

Question 6

Summarise the uptake of water and mineral ions into the xylem

Answer 6

The cell walls are all endodermal cells contain a Casperian strip made over a waxy substance called Suberin that is in permeable to water molecules and mineral irons so the apoplast pathway is blocked at this point

Water mineral irons are forced across the cell membrane into the symplast pathway

Active transport is needed to move ions into the cytoplasm of the endodermal cell mineral ions are then diffused into the pericycle and then into the xylem

Therefore, the only way that water and mineral irons can pass through the endodermis to the pericycle and into the xylem is by the simplest pathway water follows by osmosis down water potential gradient

This gives the plant to greater control over which ions enter the xylem and are transported to the rest of the plant

Question 7

What is root pressure?

Answer 7

When water moves from the endodermal cells of the root and into the xylem by osmosis this generates hydrostatic pressure and forces of water is small distance up the xylem

Question 8

How would cyanide result in the reduction in root pressure?

Answer 8

Casperian strip stops apoplast pathway because it is waterproof

The movement of ions into the endodermis cytoplasm across membrane requires active transport through carrier proteins

Cyanide is a respiratory inhibitor so it prevents cells from respiring and stops ATP synthesis so active transport of ions cannot take place

Smaller water potential gradient reduces the movement of water into the endodermis cytoplasm by osmosis from cell wall

So less water moves by osmosis into the xylem which lowers the hydrostatic pressure

Question 9

Why does the plan need to control entry of mineral ions into the xylem

Answer 9

Some minerals are toxic if they are absorbed and accumulate inside plant tissue

Question 10

How does the plant ensure toxic ions cannot enter the cell

Answer 10

Fewer to no carrier protein specific to these ions can be made on the membrane so these ions cannot enter by active transport

Question 11

Summarise the transpiration stream

Answer 11

Water is absorbed by the root hair cells

Water moves through the root tissue into the xylem and transported up the xylem in the plant stem to the leaf

Water is transported by osmosis from the xylem and the leaf to the cells of the spongy mesophyl evaporate from the surface of the cells into the air space

Water vapour, then diffusers from the air spaces out of the leaf through the stomata down and water potential gradient

Question 12

What pressure is created as the water enters the xylem by osmosis?

Answer 12

Small hydrostatic pressure

Question 13

Why is it important that the xylem is made from lignin ?

Answer 13

Stops the xylem from collapsing

Question 14

The the volume of water going up the xylem isn’t the same as the water diffused out why?

Answer 14

Water is used to make plant cells turgid

Water is a reactant in photosynthesis

Water is a reactant in hydrolysis

Question 15

What kind of pressure is created when the water is being pulled from the xylem ?

Answer 15

Negative pressure

Question 16

What is cohesion and adhesion?

Answer 16

Cohesion is where water molecules are attracted to each other by hydrogen bonds

Adhesion is where water molecules are attracted to the hydrophilic lining of the lignified xylem vessels

Question 17

Summarise the cohesion tension theory

Answer 17

As water vapour diffuses out of the stomata of the leaf by transpiration water molecules are drawn up from behind to replace those that are lost

Water molecules are drawn across the leaf and up the xylem. This is possible because of cohesion between water molecules due to hydrogen bonds and adhesion between water molecules and xylem vessel walls.

This upward movement of water creates tension on the xylem vessels known as cohesion tension

Question 18

What is capillarity?

Answer 18

The forces of adhesion and cohesion allow water molecules to rise up narrow tubes for short distances which is useful in small plants

Question 19

What happens to transpiration if temperature increases?

Answer 19

There is a higher kinetic energy of water molecules and so the rate of transpiration increases because it increases the rate of evaporation and diffusion of water vapour into the atmosphere

There is a lower water potential of the atmosphere in higher temperatures which increases the water potential gradient

Question 20

What happens to transpiration if there is faster wind/air speed

Answer 20

Faster air speed results in faster rate of transpiration

Still at results in a layer of water vapour around the stomata of the leaf known as diffusion shell

This reduces the water potential gradient between the inside and outside of the leaf

Air movement blows away the diffusion shell and increases the rate of transpiration from the leaf

Question 21

What happens to rate of transpiration if the air is more humid ?

Answer 21

The more humid air the slower rate of transpiration

The water potential gradient between the inside and outside of the leaf decreases where there is more water vapour in the atmosphere

However, there is usually a steep water potential gradient between the inside of the leaf and the atmosphere as the leaf is saturated with water vapour

Question 22

What happens to rate of transpiration if light intensity increases

Answer 22

More light intensity is faster rate a transpiration this is because light causes more stomata to open to allow gas exchange for photosynthesis

Question 23

How would you set up a potometer

Answer 23

Cut a leaf shoot underwater to prevent the entry of air bubbles into the xylem vessel the bubbles would break hydrogen bonds affecting cohesion

Completely fill the apparatus with water to avoid introducing air bubbles into the glassware

Fit leafy shoot and seal all joints with Vaseline so apparatus is airtight

Pat the lead to dry otherwise the water potential radiant will be reduced and this could affect the results

Introduce one air into the capillary tube

Measure the distance the air moves along the scale in a specific time

Question 24

How do you calculate volume of water taken up from capillary tube per minute?

Answer 24

pi radius squared x height

Question 25

What does a low standard deviation mean?

Answer 25

It indicates that the values are close to the mean so therefore more consistent and more reliable

Question 26

What does a high standard deviation mean?

Answer 26

Indicates that the data values are further from the mean so are more spread out meaning they are less consistent and less reliable

Question 27

How do you calculate standard deviation?

Answer 27

Calculate the mean from the data set Subtract the mean from each value Square each number Add up all the squares Divide this by the total number of pieces of data minus one Find the square root of that value

Question 28

What are the four parts of the xylem tissue?

Answer 28

Vessels - dead cells- main cells that conduct water Tracheids- dead cells- also conduct water but less efficient than vessels - more elongated with tapered ends Fibres- living cells- no role in transport but provide structural support Xylem parenchyma- living tissue- act as a packing tissue

Question 29

Why is it important that vessel walls are impermeable to water ?

Answer 29

Water keeps moving up the leaves in one unbroken stream

Question 30

The polysaccharide component of plant cell walls are highly hydrophilic. Why is this important for xylem function?

Answer 30

The inside of the xylem is charged water molecules are attracted to the lignified walls of the xylem vessel

Question 31

What is the structure of the phloem ?

Answer 31

Sieve tubes- transport sucrose and amino acids up and down the plant stem Companion cells- connect to sieve tubes via plasmodesmata Phloem fibres- for support Phloem parenchyma- living tissue

Question 32

Summarise how each component helps the phloem to transport sucrose up and down

Answer 32

Phloem tissue contains sieve tubes to transport organic compounds (photosynthates) e.g. sucrose and amino acids. The sieve tubes are formed from cells called sieve elements placed end to end. The thin cellulose wall at the ends of these cells is perforated to form sieve plates that allow the cytoplasm from one cell to run into an adjacent cell. Phloem protein (filaments in cytoplasm) runs through the sieve plates. Smaller strands of cytoplasm run through the side walls of sieve tube cells into adjacent companion cells through the plasmodesmata. The companion cells do not transport organic materials but they have a nucleus and they provide ATP for the active transport of sugars into/out of the sieve tubes. This is important since sieve tube cells lose their nucleus and other organelles (e.g. mitochondria) as they mature.

Question 33

Give ways that sieve tube elements are adapted to their function

Answer 33

Few organelles - more space for transport of sugars Elongated shape - sieve tube elements are joined end to end to form long tubes Have companion cells- atp is synthesised by mitochondria which is then used by sieve tubes Sieve plates have pores to allow the passage of dissolved solutes between elements Protein filaments- run through sieve plates and help transport sucrose up and down

Question 34

Summarise translocation

Answer 34

Translocation is the transport of soluble organic materials produced by photosynthesis e.g. sucrose and amino acids, in the phloem. Movement of these molecules is bi-directional, i.e. may be downwards e.g. from the leaf to the root or upwards e.g. from the leaves to fruits and shoot tips. The liquid inside the phloem tubes is called sap. The source is the region where the products of photosynthesis (photosynthates) Ce produced and exported. The sink is the region where the products of photosynthesis (photosynthates) are stored or used for growth.

Question 35

How is the ringing experiment evidence for translocation

Answer 35

Ringing experiments: Removal of a ring of outer bark tissue from a woody stem removes the phloem. Analysis of the phloem contents just above and below the ring shows that organic compounds (e.g. sucrose and amino acids) cannot be transported past the region where the bark has been removed. Bulge is seen due to accumulated phloem sap that cannot move down any further Results show phloem is removed when bark is removed Phloem transports sucrose from source to sink Transport is prevented down from leaves to roots if phloem is removed

Question 36

Summarise the aphid experiment

Answer 36

Small insects can be used to collect the contents of individual phloem sieve tube cells. Aphids, such as greenfly, have specialised mouthparts called stylets which they use to penetrate sieve tubes in order to feed on the sugary sap inside. If the aphids are anaesthetised with carbon dioxide, the stylet can be cut off and left in the stem. This means pure phloem sap can be collected through the stylet for analysis. This technique is more accurate than a human with a syringe/needle as the aphid's enzymes ensure that the stylet doesn't get blocked.

Question 37

Summarise radioisotope labelling

Answer 37

Radioisotope labelling: The image on the right shows the result of an experiment using a plant supplied with radioactive 14CO2. The radioactive 24CO2, is supplied to one leaf (marked with an arrow). Q1. What will the plant do with this 14COг? red respirce After 20 minutes the whole plant is placed on photographic film. Dark areas on the film negative (autoradiograph) show areas containing radioactivity. • Пуся heir oun • sove Sid (ear Can nau The result shows that the radioactive carbon (14C) is fixed into the sugar at the source (leaf marked with an arrow A). It is then translocated to sink parts of the plant (dark regions). This technique shows that the sugar is transported bidirectionally (upwards and downwards) since the radioactivity is seen in the aerial parts of the plant as well as the roots.

Question 38

How does the mass flow hypothesis work?

Answer 38

• Sucrose made in photosynthesis is loaded by active transport into the sieve tubes, using ATP. • Water enters the sieve tubes along a water potential gradient by osmosis. • The pressure in the sieve tubes increases and the sucrose moves down a pressure gradient through the phloem towards the sinks. • Sucrose is unloaded by active transport into the cells at the sinks. • Water moves by osmosis out of the phloem as the sucrose is removed and the pressure in the phloem tissue becomes lower at the sink.

Question 39

Arguments against the mass flow theory

Answer 39

• No explanation of sieve plates which seem to act as barriers to flow. • Sucrose and amino acids have been observed moving at different rates and in different directions. Stops producter of lATe in comperian cels Sieve tubes have a high rate of ATP consumption, and translocation is slowed or stopped if respiratory inhibitors such as cyanide are added. The companion cells are found all the way along the sieve tubes (not just in the sources and sinks) and contain numerous mitochondria for production of ATP (energy). If companion cells purely load and unload photosynthates from the sieve tubes they would not be needed anywhere but sources and sinks.

Question 40

What are mesphytes?

Answer 40

Plants in temperate regions and most crop plants Grow best in well drained soils and moderately dry air

Question 41

What are the mesophytes behaviours that allow them to survive ?

Answer 41

• Deciduous trees shed leaves in autumn to survive unfavourable conditions over winter. New leaves then grow in spring. • Bulbs and corms (storage organs) are produced by non woody plants to survive underground over winter. • Annual plants produce seeds and die in the same year, seeds survive winter frost and germinate the next spring when conditions are more favourable.

Question 42

What are hydrophytes ?

Answer 42

Water plants that live submerged or partially submerged in water

Question 43

What are the features of hydrophytes and what do they do ?

Answer 43

Stomata on upper epidermis- to allow gas exchange with the air above Large air spaces- to provide buoyancy for the leaves and act as a reservoir of oxygen and carbon dioxide Thin or no waxy cuticle - no need to reduce water loss as they live in or on water Poorly developed xylem tissue - no need to transport large quantities of water as plant is aquatic Little lignin - water is a supportive medium and so little lignin is required to support the xylem tissue

Question 44

What are xerophytes?

Answer 44

Adapted to conditions of low water availability. They live in hot dry dessert conditions , cold regions where soil is frozen for much of the year or exposed windy locations

Question 45

What are the features of marram and what do they do?

Answer 45

Sunken stomata - water vapour is trapped in pits which decreases diffusion gradient between inside and outside of the leaf so less water is lost by transpiration Hairs on leaf and surface of leaf - same reason as above Thick cuticle - thick waxy cuticle reduces water loss from epidermis Rolled leaves - stomata are less exposed to the atmosphere water vapour is also trapped and saw the diffusion gradient between the inside and the outside of the leaf decreases. Less water is lost by transpiration. Reduced leaf size /spines- reduces the surface area from which transpiration can occur

Question 46

Why do pine trees have needle like leaves

Answer 46

Reduce the surface area of a leaf available, which means less water loss by transpiration Many species have sunken stomata in pits and thick waxy cuticles to reduce water loss

Question 47

Why do many xerophytes open stomata at night and close them during the day ?

Answer 47

To conserve water