Transport in plants Flashcards
Xylem FUNCTION
Carries water and mineral ions from the roots of the plant up the stem to the leaves.
Phloem FUNCTION
- Transports ORGANIC MOLECULES
- e.g sugar produced by photosynthesis in the leaves
- These molecules can move in the phloem DOWN the plant to the roots or UP the plant to the flowers.
Vascular bundles
- consist of cells specialised for TRANSPORTING fluids by mass flow.
- Xylem vessels and phloem vessels are grouped together in vascular bundles.
- The arrangement is different in the:
- roots
- stem
- leaves
Vascular bundle in the ROOTS
- Root hair cells frow from a layer of external tissue called the EPIDERMIS
- CORTEX ~ thick layer of cells that contains parenchyma cells
VASCULAR BUNDLE ~ centre of root:
- Surrounded by a layer of cells called the EPIDERMIS
- XYLEM VESSELS:
- mechanically strong
- group together in the centre of the root
- helps prevent the root from being pulled out of the soil e.g by strong winds
Vascular bundle in the STEM
MEDULLA ~ centre of the plant stem which consists of parenchyma cells
- PHLOEM vessels located around the EDGE and XYLEM vessels found closer to the CENTRE.
- VASCULAR BUNDLE ~ found around the edge of the stem to help withstand bending due to wind.
- CAMBIUM ~ a layer of meristem cells that divide to produce new xylem and phloem.
Vascular bundle in LEAVES
Vascular bundle
- found in the CENTRE
- known as the MIDRIB
- Allows transport and gives support
- Leaf is also supported by smaller vascular bundles connected to the main one.
- XYLEM found in upper part of vascular bundle and PHLOEM found on lower part.
- PHOTOSYNTHESIS mainly takes place in the PALISADE MESOPHYLL which is the UPPER part of the leaf.
SUBSTANCES transported in plants
MINERAL IONS :
- cells in root tissue absorb mineral ions by active transport
- e.g NITRATE IONS used by plants to make amino acids
HORMONES:
- Transported from where they are synthesised to their target tissues.
XYLEM TISSUE
- Used to TRANSPORT water and mineral ions from the roots up to the leaves and other parts of the plant.
- The interior contents of the cell DIES.
Consists of:
- VESSELS to carry water and dissolved mineral ions
- FIBRES provide mechanical strength
- PARENCHYMA CELLS for the storage of starch and contain tannins which deter herbivores.
STRUCTURE related to FUNCTION of XYLEM VESSELS
LIGNIN :
- strengthens
- waterproof
- prevents collapsion
- Spiral shaped prevents vessel from becoming too rigid and allows some flexibility of the stem or branch.
NARROW:
- prevents water column from breaking easily allowing capillary action to be effective
BORDERED PITS:
- occur where lignification is incomplete
- allow water to move sideways from one vessel to another
HOLLOW:
- No cell contents, nucleus or cytoplasm
NO CROSS WALLS:
- One continuous tube so movement of substances is not disrupted.
How are ROOT HAIR CELLS adapted to allow the rapid uptake of water by osmosis?
DENSLEY PACKED:
- massively increases the SA:V of the root
THIN:
- Outer surface consists only of the cell wall and cell membrane
- short osmosis pathway
LOW WATER POTENTIAL:
- contains dissolved mineral ion and sugars
- the cell has a lower water potential than the soil
- water moves into the cell by osmosis down the water potential gradient
Symplast Pathway
- water moves from the CYTOPLASM of one cell to the cytoplasm of the adjacent cell.
- To do this , water moves through the PLASMODESMATA :
a microscopic channel through the cell wall connecting the cytoplasm of cells. - Relatively SLOW ~ obstructed by organelles in the cytoplasm
Driven by the WATER POTENTIAL gradient between the root hair cells & xylem:
- water continually moves into root hair
- the WP of the root hair rises above the WP in the cortex cells
- WP in xylem is relatively low, causing water to move across the cortex
Apoplast Pathway
- Water moves within the CELL WALLS and the SPACES BETWEEN cells
- Cellulose cell walls have a relatively OPEN STRUCTURE, allowing water to move easily between the cellulose fibres
- As water is carried away through xylem, more water moves along the apoplast pathway due to the COHESION of the water molecules.
- Offers much LESS RESISTANCE to water flow
The ENDODERMIS
- Before water enters xylem, it passes through a LAYER OF CELLS called endodermis
SUBERIN ~ a band of waterproof material which runs around the cell walls of these cells
CASPARIAN STRIP ~ formed from this band and prevents water moving through the apoplast pathway
- Instead the water passes through the cell membrane and into the CYTOPLASM , becoming part of the symplast pathway.
Passage of water into the xylem from the cytoplasm
- By forcing all the water through the cytoplasm, the cell membrane can CONTROL what substances can enter the xylem
- Cells in the endodermis use ACTIVE TRANSPORT to pump mineral ions into the xylem
- This LOWERS the WP of the xylem
- This TRIGGERS water to move IN to the xylem vessels by OSMOSIS
ROOT PRESSUE:
- Cause by water moving into the xylem vessels by osmosis
- The resulting force which helps to move water up the plant
- ACTIVE PROCESS ~ requires energy from respiration
INHIBIT RESPIRATION = ROOT PRESSURE STOPS:
- Using METABOLIC POISONS e.g cyanide
- Excluding OXYGEN
Movement of water through plants
Pathway water has to take:
- soil
- root hair cell
- root cortex
- xylem
THREE PATHWAYS:
- Symplast pathway ~ cytoplasm
- Apoplast pathway ~ cell walls
- Vacuolar pathway ~ similar to symplast , water can move through vacuole as well as cytoplasm
