Transport In Plants Flashcards

1
Q

Plants

A
  • work at very high pressures
  • have high metabolic demands
  • parts of plants that don’t photosynthesise still need oxygen & glucose (+ waste products removed)
  • hormones & mineral ions (from roots) need to be transported
  • have a small SA:vol ratio
  • ^ meaning they cannot rely on diffusion alone for substance transport
  • ^ leaves are adapted to try increase ratio
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2
Q

Dicotyledonous Plants
(aka. ‘Dicots’)

A
  • contain 2 cotyledons in their seeds
  • ‘cotyledon’ = first embryonic leaf that appears in plants containing seeds
  • ‘woody dicots’ have hard, lignified tissues & long-life cycles (e.g. trees)
  • ‘herbaceous dicots’ have soft tissues & short-life cycles (e.g. flowers)
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3
Q

Vascular Transport System

A
  • transport vessels run throughout the stem, roots and leaves
  • the xylem & phloem are the transport vessels of herbaceous dicots
  • ^ they are arranged together in a ‘vascular bundle’ (VB)
  • on the edge of stems, VBs provide strength & support
  • in the middle of roots, VBs help plants withstand “tugging strains” from wind
  • in the midrib of a leaf, VBs support its transport & structure
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4
Q

Xylem Vessels

A
  • non-living tissue transporting water & mineral ions from roots to leaves
  • has a long, hollow structure made by columns of cells fused together
  • ‘spirals of ‘lignin’ form within the cell walls to support its structure
  • ^ it also prevents the xylem from collapsing due to the ‘transpiration pull’
  • ^ lignin is impermeable
  • pits = lignin-free regions of the cell wall that allow water & dissolved substances to pass through
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5
Q

Xylem Fibres

A
  • do not transport water or mineral ions
  • provide mechanical support for the plant
  • ‘parenchyma cells’ store food (starch) & contain ‘tannin’ deposits
  • ^ ‘tannin’ are bitter tasting compounds protecting plants against herbivores
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6
Q

Phloem

A
  • living tissue transporting sugars, amino acids & assimilates from leaves to roots
  • flow can go up & down the plant
  • sieve tube elements = lignin-free cells joined end to end
  • sieve plates let phloem sap through due to their holes between cells
  • ^ the tonoplast and nucleus in cells breakdown when they’re formed
  • fibres & sclereids have thickened cell walls with lignin to support its shape
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7
Q

Companion Cells

A
  • still have their nucleus
  • very active due to being the “life-support system” for sieve tube cells
  • ^ hence why they have many mitochondria
  • linked to sieve tube elements by plasmodesmata
  • plasmodesmata = microscopic channels through the cellulose cell wall that link the cytoplasm of adjacent cells
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8
Q

Importance of Water in Plants

A
  • it’s essential for photosynthesis
  • turgor pressure (as a result of osmosis) provides a ‘hydrostatic skeleton’ to support the stem & leaves
  • ^ it also drives cell expansion - allowing roots to force their way through ground
  • allows mineral ions to be transported
  • loss of water by evaporation helps to keep plants cool
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9
Q

Root Hair Cells

A
  • the exchange surface for water to be taken up from soil into the plant body
  • each microscopic hair has a large SA:vol ratio & thin surface layer for transport
  • concentration of solutes in its cytoplasm maintains a water potential gradient between soil water and cell
  • soil water has a low concentration of dissolved minerals
  • ^ therefore a low water potential so water moves into cell by osmosis
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10
Q

Symplast Pathway

A
  • root hair cells have a higher water potential than the next cell along
  • ^ due to water diffusing into the cytoplasm from the soil
  • water moves from one cell into the next cell through plasmodesmata by osmosis
  • ^ this process continues until the xylem is reached
  • the constant rise & fall of the water potential in the cytoplasm maintains as steep water potential gradient
  • ^ this ensures as much water as possible moves into cells from the soil
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11
Q

Apoplast Pathway

A
  • water fills spaces between the loose, open network of fibres in the cell wall
  • water molecules move into the xylem and more are pulled through
  • ^ due to cohesive forces
  • tension is created so there’s a continuous flow of water through the open structure of the cellulose wall
  • ^ tension doesn’t come with resistance
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12
Q

Water Movement in the Xylem

A
  • water moves across the roots in the apoplast (A) & symplast (S) pathways until it reaches the endodermis
  • ^ the endodermis is noticeable in roots due to the effect of the ‘Casparian strip’
  • Casparian strip = waxy material bands running around each endodermal cell to form a waterproof layer
  • water in the A. pathway can go no further – it’s forced into the cytoplasm of the endo. cell to join the S. pathway
  • ^ it must pass through the semi-permeable cell surface membrane first!
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13
Q

Water Movement in the Xylem 2

A
  • the cytoplasm of cells have a higher solute concentration that those in endodermal cells
  • endodermal cells move mineral ions in by active transport
  • ^ causing the water pot. of xylem cells to become lower than endodermal cells
  • ^ the rate of water moving in by osmosis now increases (root pressure)
  • once inside the vascular bundle, water returns to the A. pathway to enter
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14
Q

Root Pressure (RP)

A
  • evidence to prove active transport is involved in moving water up the xylem👇🏽
    1. Poisons (e.g. cyanide) affect the mitochondria. Therefore, the production of ATP - root pressure disappears if present
    1. RP has a direct relationship with changes in temperature - suggesting chemical reactions are involved
    1. RP falls if oxygen falls
    1. Guttation - plants secrete excess liquid/ sap from its leaves when transpiration is low
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15
Q

Transpiration

A
  • majority of photosynthesis occurs in the palisade mesophyll cells
  • a waxy cuticle covers leaves to prevent rapid water loss by evaporation
  • water vapour diffuses out of leaf through stomata during gas exchange
  • ^ it’s an inevitable process
  • the evaporated water comes from the mesophyll cells’ surface
  • ^ this lowers their water potential
  • ^ water then moves back into these cells from adjacent cells (that border the xylem) along both pathways
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16
Q

Cohesion-Tension Theory

A
  • water molecules form H bonds with carbs in xylem walls (aka. ‘adhesion’)
  • water molecules form H bonds with each other & stick together (aka. ‘cohesion’)
  • ^ this results in the ‘capillary action’ of water up the xylem
  • capillary action = (water) moving up a narrow tube against the force of gravity
  • water is drawn up the xylem from the roots in a continuous stream
  • ^ this ‘transpiration pull’ is used to replace the water lost by evaporation
17
Q

Measuring Transpiration

A
  • very hard to make direct measurements of transpiration due to practical difficulties
  • ^ e.g. separating water vapour from transpiration vs water vapour (waste product) from respiration
  • 99% of water taken up by plant is lost during transpiration
  • ^ so it’s better to measure water uptake than water vapour lost
18
Q

Measuring Transpiration
(Potometer)

A
  • air bubble travels along a capillary tube connected to a water reservoir syringe
  • potometer joints are sealed with petroleum jelly to minimise water loss
  • stem must be cut under water to prevent air bubbles AND leaves must not come into contact with any water!
  • ^ plant needs a couple minutes to adapt to its surroundings
  • rate of water uptake = distance moved by air bubble/time taken
19
Q

Cohesion-Tension Theory Evidence

A
  • the diameter of trees change throughout the day due to changes in the rate of transpiration & tension in the xylem
  • air is drawn into the xylem if a vessel is broken, instead of water leaking out
  • ^ this air breaks the cohesion so continuous stream of water is broken
20
Q

Controlling the Rate of Transpiration
(Stomata)

A
  • a ‘turgor-driven’ process
  • low turgor = asymmetric shape of the guard cell walls keep stomata closed
  • high turgor = guard cells swell & extend lengthwise to open stomata
  • ^ cellulose rings in the cell wall ensure cells extend this way
  • turgor increases when environmental conditions are favourable (daytime)
  • ^ i.e. guard cells fill with solutes so water is drawn into them
  • inner walls of guard cells are thicker & less flexible so they become bean-shaped to open the pore
  • hormonal signals from roots trigger turgor loss when water is scarce (drought)
  • ^ pores close to conserve water
21
Q

Factors Affecting Transpiration Rate
(Light & Humidity)

A
  • light intensity affects the number of open stomata
  • ^ therefore, the rate of water vapour loss & evaporation (ultimately transpiration)
  • relative humidity affects the water vapour potential gradient
  • ^ therefore, high humidity in surrounding air means less transpiration
22
Q

Factors Affecting Transpiration Rate
(Temperature)

A
  • temperature affects Ek of water molecules
  • ^ therefore at higher temperatures, more water molecules are likely to be evaporated in the spongy mesophyll later
  • temperature also affects the relative humidity of surrounding air
  • ^ higher temp = less humid air = more transpiration
23
Q

Factors Affecting Transpiration Rate
(Air & Soil)

A
  • each leaf has a layer of still air around it
  • ^ the air is trapped by the leaf’s shape
  • water vapour that diffuses out of accumulates in this air
  • ^ this reduces the diffusion gradient
  • wind reduces the amount of still air around leaf so transpiration will increase
  • the more water available in soil means more transpiration
24
Q

Translocation

A
  • an active process that transports substances up & down a plant
  • glucose produced by leaves are converted to sucrose for easier transport
  • ^ it is less likely to be metabolised
  • sucrose converts back to glucose when it reaches respiring cells
  • sucrose also converts to starch for storage or it’s used to make amino acids
  • assimilates = products of photosynthesis that travel up & down phloem
  • sources: green leaves, green stems, storage organs & food stores in seeds
  • sinks: roots needing mineral ions, dividing meristems & developing seeds
25
Q

Loading at the Source

A
  1. H+ ions are pumped out of companion cells into surrounding mesophyll cells
    ^ the ions move by active transport
    ^ comp. cells have many mitochondria that provide ATP needed for active transport
  2. A high concentration of H+ ions build up in the mesophyll cells
  3. H+ ions move back in comp. cells with sucrose molecules by facilitated diffusion
    ^ using a co-transporter protein
  4. Sucrose builds inside comp. cells until there’s a higher concentration inside
  5. Sucrose diffuses into sieve tube elements through the plasmodesmata
  6. The water pot. in the sieve tube lowers
  7. Water from surrounding cells (xylem) move into sieve tube elements by osmosis
  8. Turgor pressure increases
  9. Water containing assimilates (phloem sap) go from source to sink by mass flow
26
Q

Unloading at the Sink

A
  1. Sucrose diffuses into companion cells, then into sink
  2. Cells convert sucrose into glucose for respiration & starch for storage
  3. Conversion creates a concentration gradient between sucrose in sieve tube elements and companion cells
  4. More sucrose diffuses out of sieve tube so its water potential increases
  5. Water in sieve tube moves into surrounding cells by osmosis
    ^ some moves back into xylem to join the transpiration stream
  6. Turgor pressure in sieve tube reduces
27
Q

Translocation Evidence

A
  • microscopy allow us to see companion cell adaptations for active transport
  • translocation stops if mitochondria in companion cells are poisoned
  • the flow of assimilates is 10,000 X faster than it would be by diffusion alone
  • ^ suggesting that it’s an active process
28
Q

Xerophyte Adaptions

A
  • plants that survive in dry/icy habitats
  • succulents = xerophytes that store & release water when needed
  • ^ ‘cacti’ are an example
  • thicker waxy cuticle layer
  • a reduced number of leaves & stomata
  • usually have sunken stomata that create a microclimate of still, humid air
  • ^ this reduces transpiration
  • ^ curled & hairy leaves are the same
  • some completely get rid of their leaves
  • long roots that penetrate deep into soil
  • some just survive as storage organs e.g. bulbs (onions) & tubers (potatoes)
29
Q

Hydrophyte Adaptations

A
  • plants that live in water or saturated soil
  • water lilies are an example
  • little to no waxy cuticle
  • always-open stomata on the upper surface to prevent water logging
  • wide, flat leaves for light to be captured
  • small roots as water can directly diffuse
  • aerenchyma = specialised parenchyma that have large air spaces
  • ^ creates buoyancy & a pathway for oxygen due to anaerobic conditions