Transport In Plants

Question 1

Explain the need for transport systems in plants in terms of metabolic demands.

Answer 1

• Underground and internal parts of plant can’t photosynthesise (as no chlorophyll or choloroplasts) but still need O2 and glucose and removal of their waste products.
• Hormones need to be transported from where they are made in plant to areas where they have an effect
• mineral ions need transporting from roots to all cells to make proteins required for enzymes and structure of cell.

Question 2

Explain the need for transport systems in plants in terms of size.

Answer 2

• plants continue to grow throughout lives so many perennial (long living and reproducing) plants are large or enormous.
• so need effective transport systems to move substances e.g. Glucose, O2 and waste products both up and down from tip of roots to upmost leaves and stems.

Question 3

Explain the need for transport systems in plants in terms of surface area : volume ratio.

Answer 3

• leaves adapted to have large SA : V ratio for exchange of gases within air, but size and complexity of multicellular plants means when stems, trunks and roots taken into account, still have relatively small SA : V ratio, so cannot rely on diffusion alone to supply cells with nutrients and remove waste

Question 4

Describe the structure of the vascular system in the roots, stems and leaves of herbaceous dicotyledonous plants.

Answer 4

Stem:

• vascular bundles around edge (of circular shape TS of stem) to give strength and support
• phloem on outer side of vascular bundle (in cortex) xylem facing more inside to centre of stem (the parenchyma)
• epidermis surrounds perimeter of TS stem

Roots:

• vascular bundles in centre (phloem as circles, surrounding plus shape xylem) to help plant withstand tugging strains from wind
• endodermis surrounds vascular bundles in centre, then cortex for rest of TS of root, then epidermis, then exodermis lined with root hairs

Leaf:

• main vein in leaf (in the midrib of it running up centre of it) carries vascular tissue through the organ
• also helps support structure of leaf
• vascular bundle is circular, with xylem on top half, phloem on bottom
• main photosynthetic tissue, palisade mesophyll, on surface of leaf

Question 5

Describe dicotyledonous plants, then describe herbaceous dicots.

Answer 5

• plants that make seeds containing two cotyledons
• cotyledons are organs that act as food stores for developing embryo plant and form first leaves when seed germinates
• herbaceous dicots have soft tissues and a short life cycle

Question 6

Describe the function of the vascular systems in the roots, stems and leaves of herbaceous dicotyledonous plants.

Answer 6

Xylem:

• non living tissue that has two main functions in a plant: transport of water and mineral ions and support
• material flows in xylem up from the roots to the shoots and leaves
• made up of several types of mostly dead cells
• long hollow structures made by several columns of cells fusing together end to end

Question 7

Describe the role of the xylem in transporting water and support on plants.

Answer 7

• Two main roles: transporting water and mineral ions, and support
• flow of materials in it is up from roots to shoots and leaves
• made of several cell types, mostly dead when functioning
• they are long hollow structures made of several columns of cells fused together at the ends
• thick walled xylem parenchyma packs around xylem vessels: storing food and containing tannin deposits
• Tannin is a bitter chemical that protects plant tissues from attack by herbivores

Question 8

Describe how xylem vessels are adapted to carry out their two main roles.

Answer 8

• transporting water and mineral ions: no ends to walls, gives an uninterrupted tube allowing water to pass through middle easily
• thick substance called lignin supports walls and prevents collapse, can be in spirals, annular: rings, reticulate, or pitted vessel: a solid tube of lignin w/ small unlignified areas called ‘bordered pits’ (where water can leave xylem and move into other cells of plant, as cell ages, lignin increases for more support

Question 9

Describe the function of the phloem in transport.

Answer 9

• living tissue that transports food as organic solutes around plant from leaves where they are made by photosynthesis.
• supplies cells w sugars and amino acids for cellular respiration and for synthesis of molecules
• flow of materials can go up and down in phloem

Question 10

Describe the structure of the phloem for transport.

Answer 10

• sieve tube elements: main transporting vessels of phloem, living cells that transport sugars through plant, joined from end to end with holes in end of each of their walls ‘sieve plates’, allowing solutes to pass through. Have no nucleus, but a thin cytoplasm and a few organelles.
• companion cells: one for every sieve element, needed as sieve cells lack nucleus and other organelles, so can’t survive alone, linked to sieve cells by many plasmodesmata - microscopic channels through cellulose cell walls linking cytoplasm of adjacent cells, enabling transport and communication between them

Question 11

Describe why the transport of water in plants is vital.

Answer 11

• turgor pressure as a result of osmosis provides a hydrostatic skeleton to support stems and leaves
• turgor also drives cell expansion
• loss of water by evaporation keeps plants cool
• mineral ions and photosynthesis products need to be transported in aqueous solutions

Question 12

Describe the movement of water into the root.

Answer 12

• taken into plant body from soil via root hair cells, root hairs are specialised epidermal cell found near growing tip of root hair cell
• Soil water has v low concentration of dissolved minerals so has a high water potential
• cytoplasm and vacuolar sap of root hair cell have lower water potential, due to many diff solvents
• so water moves into root hair cells by osmosis

Question 13

Describe the adaptations of the root hairs for water uptake from the soil.

Answer 13

• microscopic size: can penetrate easily between soil particles
• large SA:V ratio and are thousands on each root tip
• thin surface layer for quick diffusion and osmosis
• concentration of solutes in their cytoplasm maintains water potential gradient between soil water and cell

Question 14

What two ways can water move across the root to the xylem after being taken into the plant via the roots?

Answer 14

• the symplast pathway

- the apoplast pathway

Question 15

Describe the apoplastic pathway of water across the root to the xylem.

Answer 15

• water moves through cell walls and intercellular spaces
• fills spaces between loose fibres in cellulose cell wall
• as water molecules move into xylem, more water molecules pulled through apoplast behind them due to cohesive forces between molecules
• this creates tension meaning a continuous flow of water through cell wall

Question 16

Describe the symplastic pathway of water across the root to the xylem.

Answer 16

• water moves by osmosis through cytoplasm of plant cells, connected through plasmodesmata
• root hair cell has a higher water potential than next cell along
• so water moves from root hair cell to next door cell by osmosis, and so on until gets to xylem

Question 17

Describe the movement of water into the xylem.

Answer 17

• water moves across roots in pathways until reaches the endodermis (layer of cells surrounding vascular tissue of roots)
• water in apoplastic pathway can then go no further due to casparian strip (band of waxy material called suberin that forms a waterproof layer), so water is forced into cytoplasm of cell (through selectively permeable cell surface membranes that exclude toxic solutes) joining water in symplastic pathway
• endodermal cells solute concentration is solute compared to that of xylem
• endodermal cells move mineral ions into xylem by active transport
• water potential in xylem lower than in endodermal cells
• so rate of water moving into xylem by osmosis down a water potential gradient is increased

Question 18

Describe the movement of water once inside the vascular bundle to the leaves.

Answer 18

• once inside vascular bundle, water returns to apoplastic pathway to enter xylem itself and move up plant
• active pumping of mineral ions into xylem results in root pressure
• root pressure gives water an extra push up the xylem and water travels to leaves

Question 19

Describe transpiration.

Answer 19

The loss of water from the leaves of a plant.

Question 20

Explain the process of transpiration.

Answer 20

• when stomata are open (controlled by guard cells) to allow exchange of CO2 and O2 between air in and out of leaf, water vapour also moves out by diffusion and is lost from plant (transpiration)
• stomata are opened and closed by guard cells to control amount of water lost by plant, but during day plant needs CO2 for photosynthesis and at night needs O2 for cellular respiration, so some stomata must be open at all times, causing transpiration

Question 21

Describe the transpiration stream.

Answer 21

• water evaporates from surface of mesophyll cells
• this lowers water potential of air spaces inside mesophyll cells
• water moves into air spaces by osmosis from adjacent cells along both pathways
• water moves out of xylem by osmosis into cells of leaf
• water molecules form H bonds with other water molecules (cohesion) and form H bonds with carbohydrates in walls of the xylem vessel (adhesion)
• cohesion and adhesion result in capillary action, where water rises up narrow tube against gravity
• so water is drawn up xylem in a continuos stream to replace water lost by evaporation: transpiration pull.

Question 22

Describe the cohesion-tension theory.

Answer 22

• water = polar, so O atom has negative charge but H atoms have positive charge
• so means in xylem, water molecules spontaneously arranged so +ve and -ve poles lie next to eachother
• causes molecules to cohere (stick together) (explaining transpiration pull in transpiration stream)

Question 23

Give evidence for the cohesion-tension theory.

Answer 23

• changes in tree diameter: decreases at high transpiration rates due to tension, and vice versa
• cut flowers: often draw air in rather than leaking water out
• broken xylems: stops drawing water up as air drawn in breaks the transpiration stream

Question 24

Describe stomata controlling water movement.

Answer 24

• turgid: open: water moves into vacuoles by osmosis, outer wall is more flexible than inner wall, so cell bends and opens stoma
• flaccid: closed: water moves out of vacuoles by osmosis, outer wall more flexible than inner wall, so cell bends back and closes stoma

Question 25

Describe the factors affecting the rate of transpiration.

Answer 25

• light: increased intensity will increase no of open stomata, so increases transpiration rate
• humidity: high humidity lowers transpiration
• temp: increased temperature increases transpiration rate
• wind / air movement: increases transpiration rate.

Question 26

Describe a practical experiment to investigate transpiration rate.

Answer 26

Potometer:

• cut shoot underwater to prevent air entering xylem
• cut at a slant to increase SA for water uptake
• assemble potometer and insert shoot underwater so air tight
• remove apparatus from water but keep end of capillary tube submerged
• check apparatus is water tight and air tight
• dry leaves, allow time for shoot to acclimatise then shut tap
• remove end of capillary tube from water until one air bubble forms, then put end back in water
• record starting position of air bubble
• start stopwatch, record distance moved by bubble per unit time, rate of movement = estimate of transpiration rate
• only change one variable at a time, all other conditions must be kept constant

Question 27

Define translocation.

Answer 27

The movement of assimilates and organic compounds in plants from their source, where they are produced, to their sink, where they are used or stored.

Question 28

Describe the first process of translocation (loading at the source - phloem loading and the apoplastic route)

Answer 28

• two main cell types in phloem are sieve tube elements and companion cells
• H+ ions pumped out of companion cells and into neighbouring mesophyll cells, by active transport
• companion cells contain mitochondria to provide ATP needed for active transport of H+
• high concentration of H+ builds in mesophyll cells
• H+ move back into companion cells down a concentration gradient, using co transporter protein; sucrose
• these two move by facilitated diffusion, resulting in a sucrose build up in companion cells
• sucrose moves by diffusion into sieve tube elements, increasing concentration inside it
• so, water potential in sieve tube elements now lowers, so water moves (from high WP to low WP) into sieve tube elements from surrounding cells e.g. Xylem, by osmosis
• increase in water inside sieve tube elements also increases turgor pressure inside them
• causes movement of water and assimilates by mass flow from source to sink

Question 29

Describe the second process of translocation (unloading at the sink - phloem unloading)

Answer 29

• water and assimilates arrive in sieve tube elements at sink
• sucrose diffuses into surrounding cells of sieve tube elements
• surrounding cells concert sucrose into other substances e.g. Starch for storage
• this lowers concentration and creates a concentration gradient, so more sucrose diffuse out of sieve tube element
• reduced sucrose in sieve tube leads to increase in water potential
• so, water moves out of sieve tube into surrounding cells by osmosis, reducing turgor pressure inside sieve tube element
• this maintains mass flow inside the phloem

Question 30

Describe some evidence for translocation.

Answer 30

• microscopy advances allow us too see adaptation soft companion cells for active transport
• if mitochondria in companion cells are poisoned, translocation stops
• flow of sugars in phloem is roughly 10,000 times faster than it would be by diffusion alone, so suggests an active process is driving the mass flow

Question 31

Describe xerophytes.

Answer 31

Plants that live in dry habitats where the availability of water very low.
Examples: conifers, marram grass.

Question 32

Describe the adaptations of xerophytes to conserve water.

Answer 32

• thick waxy cuticle: prevents water loss in hot and cold conditions
• sunken stomata: reduces air movement and makes air humid so lowers water potential gradient and reduces transpiration
• less stomata: reduces water loss by transpiration, but also reduces gas exchange
• less leaves: reduced leaf area reduces water loss greatly
• hairy leaves: traps air from movement and makes air humid so lowers water potential gradient and reduces transpiration
• curled leaves: reduces diffusion of water vapour from stomata as confines them
• succulent: to store more water
• leaf loss: prevent water loss by losing leaves when water is not available
• root adaptations: he’ll get as much water as poss (either long and deep or shallow and wide, so large in SA)
• avoid problems: become dormant if water is too short and reappear again following year

Question 33

Describe hydrophytes.

Answer 33

Plants that live in water, either submerged in it or on the surface or at edges of water bodies.
Examples: water lilies, water cress, duck weeds, bulrushes, yellow iris
Main problem for them: water logging, air spaces in plant need to be full of air, not water, for survival

Question 34

Describe the adaptations of hydrophytes to living in conditions of water.

Answer 34

• thin w no waxy cuticle: no need to conserve water so transpiration not an issue
• many stomata, always open, on upper surfaces: maximises gas exchange, always open as no risk of turgor loss, so guard cells inactive, on upper surface so in contact with air
• reduced structure of plant: water supports them so no need for strong supporting structures
• wide, flat leaves in some: spread across surface of water to capture as much light as poss
• small roots: less need for water uptake in them as water can diffuse
• large SA of underwater stems and roots: maximises area for photosynthesis and oxygen diffusion
• air sacs: enable leaves of surface water plants to float to get light for photosynthesis
• arenchyma forms: (specialised packing tissue) in the leaves, stems and roots to make leaves and stems more buoyant and form a low resistance internal path for substances e.g. O2.