Module 3 Section 3: Plant Transport Flashcards
Why do plants need transport systems
They are multicellular:
Increasing transport distances
Small surface area : volume ratio
Relatively high metabolic rate
Direct diffusion would be too slow to meet metabolic demands
What do transport systems do in plants
Need transport systems to move substances to and from individual cells quickly
Two types of plants
Herbaceous dicots (non woody stem)
Woody dicots (woody stem)
What are vascular system of a plant
Transport vessels that run through the root, stem, leaves arranged in vascular bundles
Herbaceous dicots have vascular systems made up of two transport vessels: phloem and xylem
Why does size of the plant cause the need for a transport system
The plant is made up of lots of cells and each one requires water, glucose and mineral ions
The roots take up water and mineral ions while the leaves produce glucose by photosynthesis
These molecules need to be transported to the other ends of the plant and this large distance means that simple diffusion cannot be used as it wouldn’t be fast enough to meet the metabolic demands
Why does the surface area to volume ratio of a plant cause the need for a transport system
They have less surface area available for substances to diffuse through, so the rate of diffusion may not be fast enough to meet its cells requirements
Large plants cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products.
How are plants adapted to increase SA:V
Branching body shape
Leaves are flat and thin
Roots have root hairs
Why does having a high metabolic rate cause the need for a transport system in plants
Have more cells so there is a high demand for oxygen and nutrients and more waste is produced
What are the different plant transport systems
Transpiration System
The movement of water molecules and dissolved minerals ions
Xylem vessels
Passive process
Translocation system
The movement of sugars (Sucrose) & amino acids
Phloem vessel – sieve & companion cells
Active process
How are xylem and phloem arranged
Xylem and Phloem are arranged in vascular bundles in the roots, stems and leaves
There is a layer of cambium between these vessels which contain meristem cells
How is xylem and phloem arranged in the root
Xylem in the centre (cross shape) with phloem in four separate structure to provide a drill-like structure and support for the root as it pushes into the soil
How is xylem and phloem arranged in the stems
Xylem and phloem are near the outside to provide a scaffolding that reduces bending
Xylem on the inside phloem on the outside
How is xylem and phloem arranged in the leaf
Xylem and phloem make up a network of veins which supports the thin leaves
Structure of xylem vessels
Lignified cell walls with spiralised lignin
No end plates (mature)
No protoplasm (no nucleus or cytoplasm)
Pits in wall (non-lignified)
Vessels have small diameter
Function of lignified cell walls in xylem vessels
Adds strength to withstand hydrostatic pressure so vessels don’t collapse, impermeable to water
Function of having no end plates in xylem vessels
Allows the mass flow of water and dissolved solutes to be cohesive (between water molecules) and adhesive (between water and the walls)
These forces would be disrupted with end plates
Function of having no protoplasm in the xylem vessel
Doesn’t impede the mass flow of water and dissolved solutes (transpiration stream)
Function of having pits in wall for xylem vessels
Lateral movement of water
Allows flow of water even if air bubbles form in vessels
Function of xylem vessels having a small diameter
Helps prevent the water column from breaking and assists with capillary action
Structures of phloem tissue
Made up of sieve tubes and companion cells
Structure of sieve tubes
Living cells forming a tube for transportation
Joined end to end to form sieve tubes
Sieve section has holes in to allow solutes to pass through
Sieve tube elements have no nucleus, very thin cytoplasm and a few organelles
Cytoplasm of adjacent cells is connected through holes in sieve plates
Function of companion cells
Cells accompany sieve tube elements and carry out living functions for both of them
e.g. they provide energy for active transport of solutes
What is the need for water in plants
Mineral ions and sugars are transported in aqueous solution
Water is a raw materials of photosynthesis
Cooling effect (by transpiration)
Turgor pressure - hydrostatic skeleton
Adaptations of root hair cells
Very thin cellulose walls to provide a short pathway
Microscopic in size
Large SA : V ratio
Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
What are the water movement pathways
Symplastic pathway (through cytoplasm)
Vacuolar pathway (through vacuoles)
Apoplast pathway (through cell wall)
Process of water through the apoplast pathway
Water enters and moves through the cell wall
Moves by diffusion (not crossing a partially permeable membrane
Water may move from cell wall to cell wall, across the intercellular spaces or it may move directly from cell wall to cell wall
This is faster than the symplastic pathway
Process of water through the symplastic pathway
Water enters the cytoplasm across the partially permeable plasma membrane
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls
Process of water through the vacuolar pathway
Water enters the cytoplasm across the partially permeable plasma membrane
Water can move into the sap in the vacuole, through the Tonoplast
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls
Similar to symplast pathway
Slowest route
What is the casparian strip and where is it found
Found in the endodermis
The caspian strip is an impermeable layer of suburin - a waxy material
It forces all the water in the apoplast pathway into the symplastic pathways
What is the endodermis
This is a continuous cylinder of endodermal cells which surrounds the central vascular tissue (xylem and phloem)
Process when water reaches the casparian strip
When water reaches the endodermis of the root, it’s path is blocked.
The endodermis has a waterproof, impenetrable layer called the Casparian strip in its walls.
This is because of the waxy layer of suberin in the walls of endodermal cells.
To cross the endodermis, water in the apoplast pathway moves into the symplast pathway
In this way the selectively permeable plasma membrane of the cells can control what enters the xylem tissue.
This is important as the cell surface membrane can remove any toxic solutes from the soil, and only allow necessary water molecules and mineral ions to enter.
Function of the casparian strip
Helps to control which substances reach the xylem vessels
Plays a part in increasing root pressure
How does active transport allow water to enter the endodermis
Active transport reduces the water potential of endodermal cells
Water moves by osmosis, down a water potential gradient
How does water leave the leaf once it has been transported through the xylem
Xylem vessels transport the water all round the plant
At the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway
Water evaporates from the cell walls into the spaces between cells in the leaf
When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf (down the water potential gradient) into the surrounding air
The loss of water from a plant’s surface is called transpiration
How is water transported by cohesion
Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow (due to hydrogen bonds)
This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
Water enters the stem through the root cortex cells
How is water transported by adhesion
Water molecules are attracted to the walls of the xylem vessels as they are polar
This helps water to rise up through the xylem vessels
This is done through capillary action