Plant Transport

Question 1

What do plants need to transport?

Answer 1

• water and mineral ions : from soil via roots to all cells using the xylem
• co2: during daylight from air or water
• o2: for respiration
• assimilates : not all plants photosynthesis to produce glucose so they need the phloem to transport sucrose and amino acids
• plant hormones

Question 2

Vascular tissue distribution in young roots

Answer 2

• Xylem : transport of inorganic ions and water
• Phloem : Transport of assimilates
• Endodermis : Sheath of cells surrounding the vascular bundle - has a key role in getting water into xylem vessels
• Pericycle : Layer of meristem cells - undifferentiated cells able to divide for new growth of tissues

Question 3

Vascular tissue distribution in young stems

Answer 3

• Xylem : transport of inorganic ions and water
• Phloem : Transport of assimilates
• Cambium : Layer of meristem cells - undifferentiated cells unable to divide for new growth of tissues = xylem + phloem
• Parenchyma : Packing and support tissue capable of cell division
• Collenchyma - Cells divide structural support in growing shoots and leaves

Question 4

Vascular tissue distribution in leaves

Answer 4

• The vasculkar bundles form from the midrib ( main vein) and side veins of the leaf
• The branching net work of veins spreading throughtout of the leaf veins support the leaf as well as as transporting substances to and from the leaf cells
• Within each vein the xylem is above the phloem
• The palisade and mesophyll cells are adapted for photosythesis and air spaces for diffusion

Question 5

Structure of xylem tissue

Answer 5

• Xylem vessels : columns of fused hollow ( dead) cells which transport the water and mineral ions up the plant
• Fibres : long dead cells with thick cell walls which provide support
• Xylem parenchyma : living cells forms packing tissue and stores food . May contain bitter tasting tannin to protect against insect attacks

Question 6

Structure of the xylem tissue

Answer 6

• Starts at column of live cells which lay down, waterproof lignin inside the cell walls
• Cell contents die and cell wall breaks down
• Without the plates long vessel elements are formed lignin increases as cell wall ages. It is arranged in spirals annular or reticular patterns.
• These are gaps in the lignin where there is only cellulose these are called pits

Question 7

Adaptions of xylem vessels to functions

Answer 7

• Continuos column
• No contents or end walls to impede walls
• Lignin prevents walls from collapsing inwards due to tension
• Diameter most effective for flow - small enough to maintain water column for capillary action
• Lignin allows for adhesion for water molecules
• Pits allow sideways movement ins and out of vessels
• Arrangement og lignin allows plant to grow bend and branch

Question 8

Why do plants need a transport system?

Answer 8

• Plants are multicelullar so not all cells will be in direct contact with their environment
• Substances need to be transported to all cells to allow metabolic reactions to occur and waste products need to be removed
• Plants can continue growing throughtout their lives which means that they can be very large . This means that substances such as water and minerals may be transported a long distance from the soil to their leaves

Question 9

Structure of phloem tissue

Answer 9

• Sieve tube elements : Columns of cells that transport the assimlates
• Companion cells : linked to sieve tubes , contain dense cytoplasm and carry out the metabloic reactions required transport
• Parenchyma : Packing cells
• Fibres : Thick lignin walls, dead hollow for support , narrow and long
• Sclerieids : Ligninfied paranchyma cells th at provides rigidity and support

Question 10

Sieve elements

Answer 10

• Liquid within phloem sap
• Thin layer of cytoplam contain mitochondria and endoplasmic reticulum
• No nucleus or ribosomes
• End cells walls are pertorated formning sieve plates
• 10-15 um in width
• usually 5-6 sided cross section

Question 11

Companion cells

Answer 11

• linked to sieve elements by plasmodesmata
• Have dense cytoplasm with large molecules
• More mitochandria and ribosomes than normal

Question 12

Water transport rin multicellular plants

Answer 12

1. Water vapour diffuses out of the stomata
2. Water evaporates from mesophyll cell walls
3. Tension pulls water from the veins into the apoplast of the mesophyll cells air gaps
4. Tension pulls the water column upwards and outward in the xylem veins in the leaves
5. Tension pulls the water column upwatds in the the xylem of the root and stem
6. Water molecules form a cohesive column from the roots to the leaves
7. Water moves into the stele by osmosis

Question 13

Water potential

Answer 13

• The tendency of water to move in or out of a solution
• The more solute in a solution the lower the water potential
• Water moves from a higher water potential to a lower water potential

Question 14

Cells in high and low water potential

Answer 14

High water potential
* When the cell is placed in water solution where the water potential is higher outisde the cell then inside the cell. Water moves via osmosis through a partially permeable membrane . This causes the cell to swell , grow in size and increase the volume of the cytoplasm. This makes the cell become turgid and increase turgor pressure.
Low water potential
* When the cell is placed in water potential is highter inside the cell then outside the cell. Water moves out the cell by osmosis through a partially permeable membrane this causes the cytoplasm to pull away from the cell wall and become incipant plasmadoysed then plasmadoysed

Question 15

Water and mineral ions uptake from the soil

Answer 15

• The epidermis has root hair cells which increase the roots the SA and absorb water and mineral ions
• These minerals are absorbed by active transport
• These minerals together with sugars and amino acids lower the water potential of the cytoplasm and vacuolar sap
• Water moves in by osmosis

Question 16

Movement of water into the root

Answer 16

• Soil water has a low conentration of dissolved minerals so a very high water potential
• The cytoplasm and the vacuolar sapof the root hair cell contains many solutes so the water is lower
• Water moves into the root hair cell by osmosis down the water potential gradient
• The cell becomes turgid
• The adjacent cell will have a lower water potential than the root hair cell
• Water will continue moving from cell to cell by osmosis

Question 17

Pathways - Movement of water across the root

Apoplast pathway

Answer 17

• The cellulose cell wall is fully permeable
• Water moves through gaps in the cellulose cell walls (adhesion)
• Dissolved mineral ions are carried with the water
• It does not pas through the cell membrane ( not osmosis )
• Thw water moving up the xylem together with the cohesion between water molecules means there is a continous flow
• The open structure of cellulose offers little resistance to the flow
• Hwever the apoplast pathway stops at the endodermis

Question 18

Pathways - Movement of water across the root

The symplast pathway

Answer 18

• Water travels through the cytoplasm of cellsy
• The cytoplasm’s of the cells in the root are connected by plasmodesmata through holes in the cell walls
• The plasmodesmata are composed of a thin strands of cytoplasm

Question 19

Pathways - Movement of water across the root

The vacuolar pathway

Answer 19

• Very similar to the symplast pathway
• The pathway which water takes is not confined the cytoplasm
• Water can travel through the vacuoles

Question 20

Pathways - Movement of water across the root

The casparian strip

Answer 20

• Cells in the endodermis contain a band of waterproof meterial called the casparian strip
• This is made from suberin
• The strip blocks the apoplast pathway and forces water into cells via the symplast pathway
• Transport proteins in the cell surface membrane which is slectively permeable
• Transport proteins in the cell surface carry out active transport to move mineral ions into the cytoplasm
• Lowering the water potential mwans water will enter the xylem by osmosis.
• Once water has enterned the xlem it cannot pass back into the cortex as the apoplast pathway is blocked

Question 21

Root pressure

Answer 21

• Mineral ions are actively transported into the xylem vessels at the roots
• This lowers the water potential , causing water to enter by osmosis and forces water up the stem
• Root pressure will be affected by metabolic poisons, temperature and oxygen concentration
• This actions only moves water a small distance

Question 22

Transpiratioin pull

Answer 22

• Water molecules are attacted to each other by cohesive forces
• This forms a long column of water in the xylem
• As water is lost at the top by transpiration , the column is pulled through the xylem
• The pull of water creates a tension in the column.The lignin prevents the xylem from collapsing inwards with this tension - evidence for this is that the diameter of the tree trunck decreases in the day
• If a column is broken , water can flow into adjacent xylem vessels via pits
• This called the cohesion theory

Question 23

Capillary action

Answer 23

• Adhesive forces between water molecules and lignin in the narrow walls of the xylem vessels help pull the water up the xylem vessels
• Water can rise up a tube against the force of gravity
• A narrower container means there is a greater proportion of water in contact with the walls = greater adhesive forces

Question 24

Mass flow

Answer 24

This is an overall mass flow of water and mineral ions from a high hydrostatic pressure at the roots ( root pressure) to a lower pressure at the leaves due to transpiration

Question 25

Transpiration

Answer 25

• The loss of water vapour from the upper plant especially the leaves
• Leaves have a very large SA: V ration but evaporation is limited by the waxy cuticle. Most evaporation occurs through the stoma which have to be open to allow for gas exchange for photosynthesis

Question 26

Three main processes of transpiration

Answer 26

1. Water enters the leaves travelling in the xylem (pits). It then passes into the mesophyll cells by osmosis. Water may move from cell to cell by the symplast and apoplast pathway
2. The water evaporates from the surface of the mesophyll cells to form water vapour. Water vapourb collects in the air spaces between meosphyll cells raising the water vapour potential
3. Once the water potential is higher inside the leaf than outside water molecules will diffuse out of the leaf through the stoma
   Water vapour is carried away from the leaf by air movements
   **The evaporation of water from cells lowers the water potential so maintaining the water potential gradient **

Question 27

Guard cells

Answer 27

Turgid
1. Water enters by osmosis
2. Guard cell becomes turgid opening stoma
**Flaccid **
1. water leaves by osmosis
2. Guard cell becomes flaccid closing stoma
3. Water enters by osmosis along the water potential gradient

• Only epidermal cell with chloroplast
• Daylight when the stomata opens so co2 can enter the leaf
• Lowers water potential: Chloroplast make sugars and guard cells actively pump k+ ions
• Hoops of cellulose microfibrils prevent cells getting wider when they swell

Question 28

How can you estimate the rate of transpiration?

Answer 28

It is very difficult to measure the water vapour that evaporates from a plant (and separate it from water lost in respiration).
However water uptake is easier to measure and since 99% of this is lost in transpiration it gives us a good estimation of water loses

Question 29

How would the volume of water take up be calculated ?

Answer 29

Pie r squared x L

Question 30

Method for potometer

Answer 30

1. Fill the apparatus with water , ensure there in no air bubbles
2. Take a fresh healthy soot and cut it under water : removed 2-3 cm off then end at a slant
3. Keeping everything under water and insert the shoot into apparatus
4. Remove the apparatus from the water and use vaseline to seal joints
5. Dry off leaves and allow time to acclimatise
6. Use the screw clip adjust the water in the capillary tube to the start of the scale
7. Keep conditions constant and measure how far the water moves in a set period of time

Question 31

Factors affecting transpiration rates

Answer 31

• Number of leaves : More leaves means a bigger surface area and more stomata for gaseous exchange.
  This will increase the transpiration meaning a higher water loss

Question 32

Factors affecting transpiraition rates

Answer 32

• Number , size and postion of stomata : More stomata means more pores for transpiration so increased rate of transpiration .
  Size : Bigger / larger leaf means a larger SA so will transpire faster then a leaf with smaller SA

Question 33

Factors affecting transpiration rates

Answer 33

• Presence of cuticle : A waxy cuticle is releatively imperable to water and water vapour and reduces evaporation from the from the plant surface except from the stomata. A cuticle reduces solar heating and temperature rise of the leaf reducing the rate of tramspiration

Question 34

Factors affecting transpiration rates

Answer 34

• Light: Light is required for photosynthesis and gas exchange
• Increasing light intensity means an increase number of open stomata , increased rate of water vapour diffusing out
• Increases the evaporation from the surface of the leaf therefore an increase in transpiration

Question 35

Factors affecting transpiration rates

Answer 35

Temperature: An increase in temp means and a increase in kinetic energy of the water molecules which increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
Increase in temp increases the concentration of water vapour that the external air can hold before it becomes saturated - decreases humidity water potential

Question 36

Factors affecting transpiration rates

Answer 36

• Relative humidity : A very high relative humidity will lower the rate of transpiration because:
  Reduced water vapour potential gradient between the inside of the leaf and outside the leaf
  Very thin air has the opposite effect and increases the rate of transpiration

Question 37

Factors affecting transpiration rates

Answer 37

• Air movement / wind: Water vapour that diffuses out of the leaf accumulates underneath the leaf so the potential (water) increases (stomata) in turn reducing the diffusion gradient
• Air movement or wind will increase the rate of transpiration and a long period of still will reduce transpiration

Question 38

Factors affecting transpiration rates

Answer 38

• Water availability: The amount of water available in the soil can affect transpiration rate if it very dry the plant will be under water stress and the rate of transpiration will be reduced

Question 39

Xerophytes - adapted to dry conditions

Answer 39

• Leaves reduced to spines : reduces the SA from which transpiration can take place and protects the plant from being eaten by animals and reducing loss of heat by transpiration
• Succulents :Store water in specialised parenchyma tissue in their stems and roots
• Hairs :Help to keep the humid air trapped inside a rolled leaf , when unrolled the hairs begin to help trap a layer of moist air close to the leaf surface , reducing air movement and increasing the thickness of the layer . The thicker the layer , the more slowly water is lost by transpiration
• Dense spongy mesophyll :Reduces the cell surface area that is exposed to the air inside the leaves meaning less water will evaporate into the leaf air spaces and reduces water loss
• Rolled leaves : Rolling the leaves is done so the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated . This reduces or eliminates the water vapour potential gradient.
• Sunken stomata : Have their stomata located in pits to reduce air movement producing a microclimate of still humid (moist) air that reduces the water vapour potential gradient and so reduces transpiration
• Thick waxy cuticle : To help minimise water loss , helps them survive hot and dry summers and impermeable
• Reduced number of stomata :Reduces their water loss by transpiration and reduces their gas exchange capabilities
• Leaf loss : Loss of their leaves when water is unavailable and minimises water loss by transpiration
• Root adaption : Long tap roots growing deep to reach water down below the surface , a mass of wide spread shallow roots with a large SA is able to absorb any available water before rain water evaporates

Question 40

Precautions of potometer

Answer 40

• No air bubbles in the system - air bubbles would block the continuous column of water (cohesion)
• All joints water tight with vaseline - stops water leaks , giving valid results
• Shoot is fresh and healthy - ensures water will flow and transpiration rate is normal
• Cut stem at angle - Increases SA for water uptake
• Cut shoot under water and dont expose to air : less chance of air entering and blocking the continuous column of water
• Dry the leaves - maintain water potential gradient for transpiration
• Allow shoot acclimate to each condition before taking results - plant can adjust to new conditions

Question 41

Hydrophytes ( submerged in water)

Answer 41

• **Wide flat leaves **: Spread across the surface of the water to capture as much light as possible
• Aerenchyma /air spaces: makes leaves and stem more buoyant, forms a low resistance internal pathway for the movement of oxygen to tissues below the water which helps the water cope in low oxygen conditions ( anoxic)
• Stomata on upper surface: no risk of plant losing turgor due to the abundance of water available so the stomata are open all the time for gas exchange and the guard cells are inactive and a constant concentration gradient
• Very thin / no waxy cuticle: dont need to conserve water as plenty eater is available so water loss by transpiration is not a problem
• **Reduced structure to the plant ** : The water support the leaves and flowers so There is no need for strong supporting structures
• ** Small roots** : Water can diffuse directly into stem and leaf tissue so need for uptake by roots
  -Air sacs: Enable the leaves and or flowers to float on the surface of the water
  -Large SA for stems and roots underwater: Maximises area for photosynthesis

Question 42

Translocation

Answer 42

• The movement of assimilates ( products of photosynthesis) up and down the plant in the phloem
• Glucose is converted to sucrose for transporte
• Movement from source to sink

Question 43

Sources

Answer 43

• Assimiilates loading into the phloem
• Photosynthesising green leaves and stems
• Storage organs at the start of the growth season
• Seeds when they germinate

Question 44

Sinks

Answer 44

– assimilates removed from the phloem
- Roots – when growing
-Buds / any actively dividing meristems
- Storage organs – laying down stores
- Flowers / Fruit – developing

Question 45

Mass flow at source

Answer 45

• Sucrose is loaded into sieve tube elements involving an active process.
• This reduces the water potential of the sap.
• Therefore water follows the sucrose into the sieve element moving down a water potential gradient by osmosis.
• This increases hydrostatic pressure (turgor)

Question 46

Mass flow at sinks

Answer 46

• Sucrose diffuses out of sieve tube elements (as there is a low concentration in surrounding cells as they are using it up).
• This increases the water potential of the sap.
• Therefore water follows the sucrose out of the sieve element moving down a water potential gradient by osmosis.
• This decreases hydrostatic pressure (turgor) in the phloem at the sink

Question 47

Mass Flow

Answer 47

• Water moving in at the source and out at the sink creates a hydrostatic pressure difference in the phloem sieve tube.
• This forces the sap to move from source to sink by mass flow.

Question 48

Loading sucrose into the phloem

Answer 48

• Symplast route : In some plants sucrose moves from photosynthesising mesophyll cells to companion cells through plasmodesmata.
• Apoplast. Route : Some plants have few, if any plasmodesmata connecting to the companion cells and sucrose passes across the cell walls. This involves the movement of hydrogen ions.

Question 49

Hydrogen ion pumps

Answer 49

-Hydrogen ions are pumped out of the companion cell using ATP (active transport).
- This leads to a higher conc. of H+ ions outside the cell than inside.
-The H+ ions can move back into the companion cell down the concentration gradient through a special co-transporter protein (facilitated diffusion).
-The H+ ions carry with them sucrose molecules – moving them against their concentration gradient.

Question 50

Loading of sucrose into the phloem

Answer 50

• The sucrose molecules then diffuse from the companion cell into the sieve tube events, though the plasmodesmata

Question 51

Unloading sucroses from the phloem

Answer 51

• Unloading occurs in any tissues that need sucrose
• It is likely that the sucrose moves into the tissues by facilitated diffusion
• Once in the tissues the sucrose is converted into something else by enzymes – this maintains a concentration gradient from the phloem to the tissue
  e.g. invertase converts sucrose to glucose and fructose

Question 52

How do we know that phloem is used?

Answer 52

1. Radioactively labelled carbon -14CO2 found to be incorporated into soluble carbohydrates. Trackable as we are able to track the movement of carbon in the phloem
2. Tree ringing - Bark swells above the region - solutes accumulate, causing water to enter by osmosis, increasing in storage cells
3. Aphid feeding - aphids can be used to collect sap by cutting off the stylet neat the aphids head

Question 53

How do we know that it needs metabolic energy ATP ?

Answer 53

1. Companion cells have many mitochondria
2. Use of metabolic poison stops translocation
3. High rate of flow of sugars (10,000 times more quickly than by diffusion alone)

Question 54

How do we know it uses this mechanism?

Answer 54

1. Phloem sap has a relatively high pH (around 8) – this is what you would expect if hydrogen ions are being actively transported out of the neighbouring companion cell
2. Higher conc. of sucrose in the source than the sink
3. There is a difference in electrical potential across the plasma membrane of companion cells – more negative inside than outside - caused by the higher conc. of positively charged H+ ions outside.

Question 55

What evidence is there against this mechanism?

Answer 55

1. Not all the solutes in the sap flow at the same rate
2. Sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration
3. The role of sieve plates is unclear