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OCR Biology Paper 1 > 3.3 - Transport in plants > Flashcards

3.3 - Transport in plants Flashcards

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1
Q

Why do plants need transport systems?

A
  • 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
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2
Q

Define dicotyledonous plant

A
  • Seeds contain two cotyledons
  • Cotyledons are organs that store food for developing plant
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3
Q

Define vascular system

A
  • Transport vessels
  • that run through roots, stem and leaves
  • i.e. xylem and phloem
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4
Q

Define vascular bundle

A

Arrangement of xylem and phloem tissue

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5
Q

Describe the arrangement of the vascular bundle in the stem

A
  • Around edge of plant
  • Gives strength and support
  • Phloem closest to outside edge
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6
Q

Describe the arrangement of the vascular bundle in the root

A
  • Middle of root
  • Protects bundle from tugging strains in the wind
  • Xylem forms an ‘X’ shape
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7
Q

Describe the arrangement of the vascular bundle in the leaf

A
  • 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
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8
Q

Describe and explain the structure of leaves

A
  • 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
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9
Q

Describe how plants carry out gas exchange in the leaves

A
  • 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
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10
Q

Explain how water is absorbed by the roots

A
  • By root hair cells
  • Osmosis
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11
Q

What is the role of the xylem?

A
  • Transport of water and mineral ions
  • From roots upwards
  • Provides mechanical support
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12
Q

Describe the structure of the xylem

A
  • Continuous column
  • Made from dead cells
  • Lignin to strengthen - either spiral or annular (rings)
  • Pores in outer cellulose cell wall - allow water to leave xylem into adjacent leaf cells or xylem vessels
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13
Q

Explain why lignin is essential in the wall of a xylem vessel

A
  • Provides support to prevent collapse of xylem
  • Necessary because transpiration produces tension
  • Waterproofs cell
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14
Q

Why is water important for plants?

A
  • Provides turgor (hydrostatic) pressure
  • Gives support to stems
  • Provides force for roots to push through ground
  • Mineral ions and sugars are transported in aqueous solutions
  • Required for photosynthesis
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15
Q

How are mineral ions absorbed by roots?

A
  • By root hair cells
  • Against the concentration gradient
  • Requires active transport - protein pumps and ATP
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16
Q

List mineral ions absorbed by roots

A
  • Potassium
  • Phosphates
  • Nitrates
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17
Q

How is the surface area for absorption of mineral ions increased?

A
  • Branching of roots
  • Root hair cells
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18
Q

How does water enter root hair cells?

A

Osmosis
- High water potential in soil
- Lower water potential in root hair cell
- Due to dissolved mineral ions, sugars, amino acids

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19
Q

Define plasmodesmata

A

Continuous cytoplasm channels
that link plant cells

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20
Q

What are the two pathways that water uses to move through the root?

A
  • Symplast pathway
  • Apoplast pathway
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21
Q

Describe the symplast pathway

A
  • Water moves through cytoplasm
  • By osmosis
  • Plasmodesmata link adjacent cells
  • Water potential gradient maintained by water leaving roots and entering xylem
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22
Q

Describe the apoplast pathway

A
  • Water moves through the cell walls and intercellular spaces
  • Cohesive forces pull water molecules along
  • Creates tension, so continuous flow of water forms
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23
Q

What is the Casparian strip?

A
  • Band of suberin (a waxy material)
  • Lines cells in endodermis
  • Provides waterproof layer around xylem
24
Q

What is the role of the casparian strip?

A
  • Prevents water moving through the apoplast pathway
  • Forces water into symplast pathway
25
Q

How does water enter the xylem?

A
  • 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’
26
Q

Define transpiration

A
  • Evaporation of water vapour from the surface of leaves
  • Water vapour lost through stomata
27
Q

What is a potometer used to measure?

A

Rate of water uptake

28
Q

Define transpiration stream

A
  • Movement of water up xylem vessels
  • From roots to leaves
29
Q

What is the role of a transpiration stream?

A
  • Cools plants
  • Delivers water and mineral ions to leaves
  • Provides support
30
Q

What properties of water enable it to form a transpiration stream?

A
  • Due to hydrogen bonding and polarity of water molecules
  • Water molecules are cohesive with other water molecules
  • Adhesion between water and xylem
31
Q

Describe how a transpiration stream is brought about

A
  • 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
32
Q

Explain the evidence for the cohesion-tension theory

A
  • 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
33
Q

How does water travel from leaves to the stomata?

A

Through apoplast and symplast pathways

34
Q

What controls the opening and closing of stomata?

A

Guard cells

35
Q

How can plants control water loss?

A
  • 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
36
Q

How are guard cells adapted for their role?

A
  • 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
37
Q

Why is transpiration unavoidable during the day?

A
  • 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
38
Q

What factors affect the rate of transpiration?

A
  • Light intensity
  • Temperature
  • Humidity
  • Wind speed
39
Q

Explain how abiotic factors affect the rate of transpiration in terrestrial plants

A

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

40
Q

What are xerophytes?

A

Plants adapted to live in very dry conditions e.g. deserts
- e.g. cacti, marram grass

41
Q

Describe and explain how xerophytes are adapted for life in deserts

A
  • 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
42
Q

What are hydrophytes?

A

Plants that are adapted to live in water e.g. lakes
- e.g. waterlilies

43
Q

Describe and explain how hydrophytes are adapted for life in water

A
  • 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
44
Q

What is the role of the phloem?

A
  • Transport of sugars e.g. sucrose
  • From leaves (source) to storage regions (sink)
45
Q

Describe the structure of phloem

A
  • Sieve tube elements
  • Companion cells
  • Plasmodesmata between sieve elements and companion cells
  • Connects cytoplasm of the cells
  • Allows exchange of metabolites
46
Q

Describe the structure of sieve tube elements

A
  • 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
47
Q

Describe the structure of companion cells

A
  • 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
48
Q

Define translocation (mass flow)

A

Movement of organic compounds (e.g. sucrose, amino acids) from sources to sinks

49
Q

Define source and give examples

A

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

50
Q

Define sink and give examples

A
  • Where assimilates are unloaded for use or for storage
  • e.g. roots, fruits and seeds
51
Q

Explain how sucrose is loaded into the phloem sieve tube elements

A
  • 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
52
Q

Explain how water moves in the phloem

A
  • 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
53
Q

Describe in detail the transport of organic compounds in vascular plants

A
  • 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
54
Q

Outline how glucose produced in photosynthesis is transported and stored in plants

A
  • 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
55
Q

How can you measure the phloem transport rates?

A
  • 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
56
Q

What is the other evidence to support the theory of mass flow?

A
  • 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
57
Q

What evidence is there to suggest mass flow is not the correct model?

A
  • 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

OCR Biology Paper 1 (20 decks)

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