AS-LEVEL OCR BIOLOGY NOTES 
TOPIC 3: transport in plants

< BACK TO TOPIC INDEX

1. the need for transport systems in multicellular plants

In large, multicultural plants, only epithelial cells receive enough oxygen and nutrients from diffusion. Cells deeper within the plant would not receive enough nutrients to survive were it not for transport systems:

• Roots can access water but not sugars
  
• Leaves can access sugar, but not water
  

System of transport are therefore necessary and require highly specialised tissue

2. the vascular system in the roots, stems and leave of herbaceous dicotyledonous plants

 ​​​​The vascular system in plants consists of the vascular bundle

• Xylem and Phloem found together are found in vascular bundle
  
• Xylem moves water and minerals upwards
  
• Phloem moves sugars and products of photosynthesis downwards
  

Young Root

• Vascular bundle at centre
  
• Large central core of xylem, often in an x shape
  
• Phloem found in between x-shaped arms
  
• Provides strength for roots
  
• Bundle surrounded by endodermis and pericircle of meristem cells
  

Stem

• Bundles found near outer edge of stem
  
• Discrete in non-woody plants
  
• Xylem found towards inside
  
• Phloem on outside
  
• Layer of cambium in-between
  
• Layer of meristem cells
  

Structure and Function of xylem and phloem

Xylem

• Xylem vessel elements
  
• Long cells
  
• Thick walls, impregnated by lignin
  
• Cells die and contents decay
  
• Leaves a long, hollow tube
  
• Lignin strengthens and waterproofs walls
  
• Lignin form patterns, allowing for flexibility
  
• Bordered pit allow for water to pass in and out of xylem vessels
  

Phloem

• Sieve Tubes
  
  • Lined up end to end
    
  • Form a tube
    
  • Sugars dissolved into water, forming sap
    
  • Contains cross-walls, allowing sap to flow
    
  • Cross-walls are called sieve plates
    
  • 5/6 sided
    
  • Thin walls
    
• Companion Cells
  
  • Lie between sieve tubes
    
  • Large nucleus, dense cytoplasm
    
  • Numerous mitochondria
    
  • Carry out metabolic processes needed by sieve tube elements
    
  • Cells linked by plasmodesmata
    
  • These gaps allow for the flow of minerals between cells

3. transpiration and transpiration rate

​Transpiration is the Loss of water by evaporation from aerial parts of the plant. Water enters the leaves through the xylem, passes through mesophyll cells by osmosis, and diffuses through air spaces in spongy layer. As water vapour collects, water vapour potential rises. When water vapour potential is higher inside the leaf, it diffuses out.

Transpiration is an inevitable Consequence of Gaseous Exchange. Loss of water through transpiration is unavoidable. Plants exchange gases via stomata. During the day plants must take up carbon dioxide and release oxygen. The stomata must be open to allow this. This provides an easy route for water loss.

Factors that Affect Transpiration

• Number of leaves
  
  • More leaves means greater surface area
    
  • Greater area over which water can be lost
    
• Number and size of stomata
  
  • Large stomata means water is lost more rapidly
    
  • If stomata are on the lower surface of the leaf, water loss is slower
    
• Presence of cuticle
  
  • Waxy cuticle reduces evaporation
    
• Light
  
  • Light causes stomata to open
    
  • Allows gaseous exchange for photosynthesis
    
• Temperature
  
  • This will increase the rate of water loss in three ways:
    
    • Increases rate of evaporation from surface
      
    • Increase rate of diffusion through the stomata
      
    • Decreases relative water vapour potential in the air
      
• Relative Humidity
  
  • Higher humidity will decrease water loss
    
  • This will reduce the water vapour potential gradient between the air in the leaf and outside
    
• Air movement and wind
  
  • Moving air will carry water vapour away
    
  • This maintains a high water vapour potential gradient
    
• Water availability
  
  • If there is low water availability, the plant cannot replace the lost water
    
  • Transpiration rates are therefore reduced
    

Potometer

• What is it?
  
  • Can be used to estimate the rate of water loss
    
  • Only an estimate as it only measures the rate of water uptake
    
  • Gives an accurate estimate
    
• Set Up
  
  • Leafy shoot is placed in a glass tube full of water
    
  • Make sure there are no bubbles in the apparatus
    
  • The tube is raised out of the beaker
    
  • Movement of the meniscus and the end of the water column can then be measured
    
  • The distance travelled by the meniscus over a certain time period will give the rate of water uptake/loss
    

EXAM TIP Use the information on this page to describe how you might set up an experiment to collect data which tells you how different factors (e.g. light intensity, temperature) affect the rate of transpiration. When describing the effect of factors on transpiration, remember to specify what is causing the change in rate - is it an increase or decrease in factor?​

4. the movement of water & plants

• Water Potential
  
  • Water potential is a measure of the tendency of water molecules to diffuse from area to another
    
  • Cytoplasm of plant cells contains salts and sugars, reducing water potential
    
    • Water potential in plants is always negative
      
• Moving Between Cells
  
  • When cells are touching, water can diffuse from one to another
    
  • Water will move from the cell with the higher water potential to the cells with the lower water potential
    
• Routes
  
  • There are three pathways that water can take:
    
    • Apoplast Pathway
      
      • Cellulose walls have water-filled spaces between the cellulose molecules
        
      • Water can move through these spaces and between cells
        
      • Water does not pass through plasma membrane
        
      • Allows dissolved minerals and ions to be carried in the water
        
    • Symplast Pathway
      
      • Water enters cytoplasm through plasma membrane
        
      • Then passes through plasmodesmata from cell to the next
        
      • Once in the cytoplasm, water can move from cell to cell
        
      • Cytoplasm between cells is continuous and connected by one thin strand of cytoplasm through plasmodesmata
        
    • Vacuolar Pathway
      
      • Similar to symplast
        
      • Water is not just confined to the cytoplasm
        
      • It can pass through the vacuoles as well

• Movement into Roots
  
  • Plant roots are surrounded by soil particles
    
  • Root hair cells use ATP to transport minerals from soil into the root
    
  • This reduces water potential of cell cytoplasm
    
  • Creates concentration gradient
    
  • Water is taken up across plasma membrane by osmosis
    

• Movement into Xylem
  
  • Movement of water across the root is driven by active process in the endodermis
    
  • Endodermis is layer of cells surrounding xylem
    

• Contains granules of starch
  

• Endodermis has waterproof Casparian strip in its walls
  
• This blocks the apoplast pathway
  
• Endodermis cells move minerals from the cortex into the xylem, decreasing water potential
  
• Water moves through the endodermis cells into xylem by osmosis
  
• This reduces water potential in cells just outside endodermis
  
• Creates a water potential gradient
  
• Water moved along the symplast pathway from root hair cells, across cortex and into xylem
  
• Water can move by the apoplast pathway, but must switch to the symplast at last minute to pass into xylem
  

• Movement up the Stem
  
  • Root Pressure
    
    • Action of moving nitrate ions into xylem drives water into xylem by osmosis
      
    • Pushes water up xylem
      
    • Can push water a few metres, but not to the top of a tree

• Transpiration Pull
  
  • Water is lost through transpiration at leaves
    
  • Water must be replaced
    
  • Water molecules are attracted to each other through cohesion
    
  • This forms a water column
    
  • As water molecules are lost through the stomata in leaves, the water chain is pulled up
    
  • This creates a transpiration stream
    
  • The pull of the water creates tension within the column
    
  • This tension is why xylem needs to be structurally strong and lignin is required
    
• Capillary Action
  
  • Water molecule also attracted to the sides of the xylem vessels
    
  • This is adhesion
    
  • Because xylem are narrow, these forces can pull water upwards
    

• Role of the Casparian Strip
  
  • Blocks apoplast pathway between cortex and xylem
    
  • Ensures water and dissolved ions have to pass through the cell cytoplasm and membrane
    
  • Transporter proteins help to transport nitrate ions from cortex into xylem
    
  • This lowers water potential in xylem, creating a water potential gradient
    
  • Prevents water from flowing back into endodermal cells

EXAM TIP This topic overlaps a little with the water topic in module 2, in the sense that transpiration is made possible thanks to the unique properties of water, especially these two: Cohesion = the attraction between water molecules, caused by hydrogen bonds, that allows it to form one continuous column Adhesion = the attraction of water molecules to the walls of the xylem, allowing the column of water to ‘stick’ to the xylem and draw itself up

5. adaptations of plants & the availability

Xerophytes (eg. cacti and marram grass) are plants that are specially adapted to survive higher temperatures by controlling transpiration. At higher temperature water molecule have greater kinetic energy: they move with more speed, increasing the rate of the diffusion through the stomata. This Increases the rate of evaporation from the spongy mesophyll to the air spaces. If the water loss is greater than the water uptake, the plant may suffer water stress, and the cells may lose turgidity.
Adaptations:

• Small Leaves
  
  • Particularly needle shaped leaves
    
  • Reduces total surface area
    
  • Less water lost through transpiration
    
• Dense Spongy Mesophyll
  
  • Reduces cell surface area exposed to air inside leaves
    
  • Less water able to evaporate into leaf air spaces
    
• Thicker Cuticle
  
  • Reduces evaporation
    
• Surface Hairs
  
  • Trap layer of air close to surface
    
  • Air can become saturated with water and reduce the diffusion of water vapour
    
  • Water vapour potential is reduced
    
• Pits
  
  • Pits contain stomata at the base
    
  • Trap air
    
  • Air can become saturated with water vapour
    
• Rolled leaves
  
  • Lower epidermis not exposed to atmosphere
    
  • Air trapped
    
  • Air can become saturated with moisture
    
  • Eliminates water vapour potential gradient
    
• Low Water Potential Inside Leaves
  
  • This is achieve by maintaining a high salt concentration in the cells
    
  • Low water potential reduces the evaporation of water from the cell surface
    
  • Reduces water potential gradient
    
  
  

Hydrophytes (eg. water lilies) are plants that are specially adapted to survive while submerged entirely or partially in water.
Adaptations:

• Large leaves
  
  • Contributes to plant’s ability to float
    
  • Increases total surface area
    
  • More water lost through transpiration
    
• Air savs in Spongy Mesophyll
  
  • Reduces density of leaves, promoting flotation
    
• Thinner Cuticle
  
  • Increases evaporation
    
• Lots of stomata
  
  • Increase transpiration and amount of water lost by evaporation per unit time
    
• Stomata open most of the time
  
  • Release water through transpiration evaporation
    

6. the mechanism of translocation

• Translocation is the transport of assimilates (sugars and chemicals made by plant cells) throughout the plant
  
  • Sugars are transported in the form of sucrose
    

Translocation occurs in the phloem tissue. Sucrose is released into the phloem at sources and taken out of the phloem at sinks.

• How does sucrose enter the Phloem?
  
  • Loaded in by active process
    
  • Companion cells use ATP to transport hydrogen out of cells into cytoplasm
    
  • This establishes a diffusion gradient
    
  • Hydrogen ions diffuse back in through special cotransporter proteins
    
  • These allow hydrogen to bring sucrose molecule back in with them
    
  • Sucrose molecules build up in the companion cell
    
  • Diffuse into sieve tube elements through plasmodesmata
    

• Source
  
  • Sucrose enters sieve tube element
    
  • Reduces water potential
    
  • Water molecules move in by osmosis
    
  • Increase hydrostatic pressure at the source
    

• Along the Phloem
  
  • Water enters at source
    
  • Moves down hydrostatic pressure gradient towards sink
    
  • Produces water flow
    
  • This carries sucrose and other assimilates along the phloem
    
  • This is called mass flow
    
  • Can occur both up and down a plant
    

• Sink
  

• Sucrose used in cells surrounding phloem
  
  • May be converted to starch for storage
    
  • May be used in metabolic reactions
    
• This reduces sucrose concentration
  
• Sucrose molecules move out of the sieve tube elements
  
• Water potential in sieve tube elements increases, so water moves out by osmosis
  
• This reduces hydrostatic pressure at the sink

​EXAM TIPS You should be familiar with the evidence supporting and contradicting the theory of translocation: Use of Phloem If radioactive carbon dioxide is supplied, it soon appears in the phloem Ringing a tree results in sugars collecting above the ring Use of ATP Companion cells have many mitochondria Translocation can be stopped by inhibiting ATP Rate of flow of sugars is very high and some energy must be used to drive the flow Mechanism pH of companion cells is higher than surrounding cells Concentration of sucrose is higher in the source than in the sink Evidence Against Not all solutes in phloem move at the same rate Sucrose is moved to all areas at the same rate, rather than quickly going to areas with lower concentrations Role of sieve plates is unclear