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
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)
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
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
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
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
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
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
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
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
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
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!
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
14
Q
Root Pressure (RP)
A
- evidence to prove active transport is involved in moving water up the xylem👇🏽
- Poisons (e.g. cyanide) affect the mitochondria. Therefore, the production of ATP - root pressure disappears if present
- RP has a direct relationship with changes in temperature - suggesting chemical reactions are involved
- RP falls if oxygen falls
- Guttation - plants secrete excess liquid/ sap from its leaves when transpiration is low
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
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
- 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 - A high concentration of H+ ions build up in the mesophyll cells
- H+ ions move back in comp. cells with sucrose molecules by facilitated diffusion
^ using a co-transporter protein - Sucrose builds inside comp. cells until there’s a higher concentration inside
- Sucrose diffuses into sieve tube elements through the plasmodesmata
- The water pot. in the sieve tube lowers
- Water from surrounding cells (xylem) move into sieve tube elements by osmosis
- Turgor pressure increases
- Water containing assimilates (phloem sap) go from source to sink by mass flow
26
Q
Unloading at the Sink
A
- Sucrose diffuses into companion cells, then into sink
- Cells convert sucrose into glucose for respiration & starch for storage
- Conversion creates a concentration gradient between sucrose in sieve tube elements and companion cells
- More sucrose diffuses out of sieve tube so its water potential increases
- Water in sieve tube moves into surrounding cells by osmosis
^ some moves back into xylem to join the transpiration stream - 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
