Transport in plants Flashcards
Why do plants need transport systems?
- To move sucrose and oxygen to roots for respiration
- To move ions from roots to leaves
- Plants can be very large
- Stems, trunks and roots have small SA : V ratio - diffusion too slow
Define dicotyledonous plant
- Seeds contain two cotyledons
- Cotyledons are organs that store food for developing plant
Define vascular system
- Transport vessels that run through roots, stem and leaves
- i.e. xylem and phloem
Define vascular bundle
Arrangement of xylem and phloem tissue
Describe the arrangement of the vascular bundle in the stem
- Around edge of plant
- Gives strength and support
- Phloem closest to outside edge
Describe the arrangement of the vascular bundle in the root
- Middle of root
- Protects bundle from tugging strains in the wind
- Xylem forms an ‘X’ shape
Describe the arrangement of the vascular bundle in the leaf
- Midrib is main vein carrying vascular tissue - gives support to structure of leaf
- Xylem closest to upper side of leaf
- Smaller branching veins also carry vascular bundles
Describe and explain the structure of leaves
- Waxy cuticle - prevents transpiration
- Upper epidermis - transparent to allow light to enter the leaf
- Palisade mesophyll - contains lots of chloroplasts to absorb light
- Spongy mesophyll - air spaces allow gases to diffuse
- Guard cells - control opening and closing of stomata
- Stomata - allow gas exchange - carbon dioxide enters, oxygen (and water) leaves
Describe how plants carry out gas exchange in the leaves
- O2 and CO2 enter and exit the leaf through the stomata by diffusion
- Photosynthesis maintains gas concentration gradients in the leaf
- Guard cells open the stomata during the day and close the stomata at night
- O2 and CO2 move through air spaces in the spongy mesophyll
- CO2 dissolves in moisture in mesophyll cell walls
Explain how water is absorbed by the roots
- By root hair cells
- Osmosis
What is the role of the xylem?
- Transport of water and mineral ions
- From roots upwards
- Provides mechanical support
Describe the structure of the xylem
- Continuous column
- Made from dead cells
- Lignin to strengthen - either spiral or annular (rings)
- Pores/pits in outer cellulose cell wall allow water to leave xylem into adjacent leaf cells or
xylem vessels
Explain why lignin is essential in the wall of a xylem vessel
- Provides support to prevent collapse of xylem
- Necessary because transpiration produces tension
- Waterproofs cell
- Cell dies and creates continuous hollow tube
- Enables adhesion between water molecules and wall
Why is water important for plants?
- Provides turgor (hydrostatic) pressure
- Gives support to stems
- Provides force for roots to push through ground
- Loss of water helps keep plants cool
- Mineral ions and sugars are transported in aqueous solutions
- Required for photosynthesis
How are mineral ions absorbed by roots?
- By root hair cells
- Against the concentration gradient
- Requires active transport - protein pumps and ATP
List mineral ions absorbed by roots
- Potassium
- Phosphates
- Nitrates
How is the surface area for absorption of mineral ions increased?
- Branching of roots
- Root hair cells
How does water enter root hair cells?
Osmosis
- High water potential in soil
- Lower water potential in root hair cell
- Due to dissolved mineral ions, sugars, amino acids
Define plasmodesmata
Continuous cytoplasm channels that link plant cells
What are the two pathways that water uses to move through the root?
- Symplast pathway
- Apoplast pathway
Describe the symplast pathway
- Water moves through cytoplasm
- By osmosis
- Plasmodesmata link adjacent cells
- Water potential gradient maintained by water leaving roots and entering xylem
Describe the apoplast pathway
- Water moves through the cell walls and intercellular spaces
- Gaps between cellulose fibres filled with water
- Cohesive forces pull water molecules along
- Creates tension, so continuous flow of water forms
What is the Casparian strip?
- Band of suberin (a waxy material)
- Lines cells in endodermis
- Provides waterproof layer around xylem
What is the role of the casparian strip?
- Prevents water moving through the apoplast pathway
- Forces water into symplast pathway
How does water enter the xylem?
- Water in apoplast pathway forced into symplast pathway by Casparian strip
- Water must pass through cell membranes to enter cytoplasm
- Cell membranes are selectively permeable - prevents toxic solutes entering cells
- Mineral ions actively transported into xylem
- Water follows by osmosis down the water potential gradient
- Known as ‘root pressure’
Define transpiration
- Evaporation of water vapour from the surface of leaves
- Water vapour lost through stomata
What is a potometer used to measure?
Rate of water uptake
Define transpiration stream
- Movement of water up xylem vessels
- From roots to leaves
What is the role of a transpiration stream?
- Cools plants
- Delivers water and mineral ions to leaves
- Provides support
What properties of water enable it to form a transpiration stream?
- Due to hydrogen bonding and polarity of water molecules
- Water molecules are cohesive with other water molecules
- Adhesion between water and xylem
Describe how a transpiration stream is brought about
- Hydrogen bonding in water
- Give adhesive properties - water sticks to cellulose cell walls
- As water vapour is lost through evaporation more is drawn up from below
- Due to the cohesive properties of water
- Tension created when water evaporates out of stomata
- Due to lower pressure at top of xylem
Explain the evidence for the cohesion-tension theory
- Tree diameter is smaller during the day
- Rate of transpiration at its highest
- Tension in xylem vessels at its highest
- Pulls stem/trunk inwards
- When xylem vessel breaks, air is drawn in
- Plant can no longer move water up stem
- Continuous stream of water is broken
How does water travel from leaves to the stomata?
Through apoplast and symplast pathways
What controls the opening and closing of stomata?
Guard cells
How can plants control water loss?
- Opening and closing of stomata
- When environmental conditions are favourable, solutes actively pumped into guard cells
- Water follows by osmosis
- Increase in turgor makes stomata open, as guard cells becomes bean shaped
How are guard cells adapted for their role?
- Unevenly thickened cell wall
- Wall beside pore is thicker
- Allows guard cell to bend
- Transport proteins present in plasma membrane
- Chloroplasts and mitochondria to provide ATP
Why is transpiration unavoidable during the day?
- Stomata are open to allow gas exchange
- Required for photosynthesis
- Water vapour leaves leaf down water potential gradient
- Higher temperatures during the day cause greater evaporation
What factors affect the rate of transpiration?
- Light intensity
- Temperature
- Humidity
- Wind speed
Explain how abiotic factors affect the rate of transpiration in terrestrial plants
Less transpiration as humidity rises
- Air spaces inside leaf are nearly saturated with water vapour
- Smaller concentration gradient with higher atmospheric humidity
More transpiration as temperature rises
- More kinetic energy of water molecules
- Faster evaporation rate
More transpiration as wind speed increases
- Water vapour blown away from the leaf
- Increasing the concentration gradient of water vapour
More transpiration in the light
- Light causes stomata to open (stomata closed in darkness)
- Low CO2 concentration inside leaf in bright light so stomata open wider
What are xerophytes?
Plants adapted to live in very dry conditions e.g. deserts
- e.g. cacti, marram grass
Describe and explain how xerophytes are adapted for life in deserts
- Thick, waxy cuticle - reduces evaporation through epidermis
- Spines instead of leaves - reduces transpiration as fewer stomata
- Stomata only open at night - reduces rate of transpiration
- Hairs on leaves - reduces air flow near leaf to trap water vapour
- Reduces water potential gradient
- Stomata in pits to increase humidity - reduces water potential gradient
- Long, shallow root systems - absorb as much water as possible
- Succulents - store water in specialised tissue in stems and roots
What are hydrophytes?
Plants that are adapted to live in water e.g. lakes
- e.g. waterlilies
Describe and explain how hydrophytes are adapted for life in water
- Very thin or no waxy cuticle - do not need to prevent water loss
- Many always open stomata on upper surface - maximises gas exchange
- Narrow stems - no need for strong support structure in water
- Wide, flat leaves - capture as much light as possible
- Small roots - water can diffuse directly into stem and leaves
- Air sacs - enable leaves to float
- Aerenchyma - specialised tissue with large air spaces
- Make leaves buoyant and provides pathway for substances such as oxygen
What is the role of the phloem?
- Transport of sugars e.g. sucrose
- From leaves (source) to storage regions (sink)
Describe the structure of phloem
- Sieve tube elements
- Companion cells
- Plasmodesmata between sieve elements and companion cells
- Connects cytoplasm of the cells
- Allows exchange of metabolites
Describe the structure of sieve tube elements
- Long and narrow cells that are connected together to form the sieve tube
- Connected by porous sieve plates at end of sieve elements
- No nuclei and reduced numbers of organelles to maximise space for the translocation of
materials - Little cytoplasm
- Thick and rigid cell walls
Describe the structure of companion cells
- Infolding plasma membrane which increases SA:Vol ratio to allow for more material
exchange - Many mitochondria to provide ATP for active transport of materials between sieve tube and
source/sink - Transport proteins within plasma membrane move materials into or out of the sieve tube
- Plasmodesmata between companion cells and sieve tube elements
Define translocation (mass flow)
Movement of organic compounds (e.g. sucrose, amino acids) from sources to sinks
Define source and give examples
Site where loading of sugars and amino acids into sieve tubes of phloem occurs
- Occurs where organic compounds are synthesised
- i.e. where photosynthesis occurs - the leaves
Define sink and give examples
- Where assimilates are unloaded for use or for storage
- e.g. roots, fruits and seeds
Explain how sucrose is loaded into the phloem sieve tube elements
- Protons (H+) pumped out of companion cells into surrounding tissue by active transport
- Creates proton gradient
- Protons diffuse back into companion cells down the concentration gradient attached to
sucrose molecules - Protons and sucrose flow through co-transporter proteins in membrane
- Move by facilitated diffusion
- Sucrose then diffuses into sieve tube elements via plasmodesmata
- Amino acids can also be loaded in this way
Explain how water moves in the phloem
- High concentration of solutes in phloem sieve tubes at source
- Leads to water uptake from xylem by osmosis
- Hydrostatic pressure increases
- Low concentration of solutes in phloem sieve tubes at sink
- Water moves back into xylem by osmosis
- Hydrostatic pressure decreases
- Water moves down the hydrostatic pressure gradient due to its incompressibility
Describe in detail the transport of organic compounds in vascular plants
- Phloem transports organic compounds
- From sources to sinks
- Through sieve tubes
- H+ ions actively transported out of companion cells
- Sucrose and H+ ions diffuse back into phloem (loading)
- High solute concentration causes water to enter by osmosis at source
- High hydrostatic pressure causes flow from source to sink
- Solutes diffuse out of phloem into sink
- Water moves back out of sieve tube elements by osmosis
- This is known as translocation/mass flow
Outline how glucose produced in photosynthesis is transported and stored in plants
- Glucose transformed to sucrose
- Translocation of sucrose by phloem
- Active process
- Sucrose moves from source to sink
- Source is photosynthetic tissue (leaves)
- Sink is fruits/seeds/roots/storage organs
- Sucrose converted to starch
- Stored in storage organs/roots/tubers
How can you measure the phloem transport rates?
- Using aphids
- Aphids have long piercing mouth parts (stylets)
- Stylet inserted so that it pierces sieve tube
- High pressure inside sieve tube pushes phloem sap into aphid
- To sample phloem, aphid is cut from its stylet
- Radioactive isotopes of CO2 can be used to measure flow rates
What is the other evidence to support the theory of mass flow?
- If mitochondria of companion cells are poisoned, translocation stops
- Suggests ATP required for active transport
- Flow of sugars much faster than by diffusion alone
- When ring of phloem is removed from a tree, the trunk swells above the cut
- Sugars cannot pass the cut
- Leads to water moving into cells and increased cell division to store sugars
What evidence is there to suggest mass flow is not the correct model?
- Not all solutes in phloem move at same rate
- Sucrose moves at same rate regardless of concentration at sink
- Sieve plates should slow rate of movement down
