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