3.1.3 - Transport in Plants Flashcards
Why do multicellular plants need transport systems?
Metabolic demands, size, and SA:V
What are dicotyledonous plants?
Plants that make seeds that contain two cotyledons
What is a cotyledon?
An organ that acts as a food store for the developing embryo plant
What is the vascular system?
A series of transport vessels running through the stem, roots, and leaves
What are the two main types of transport vessels?
Xylem and phloem
How are the transport tissues arranged?
In vascular bundles
Describe the structure of the xylem
Non-living tissue. Xylem vessels are the main structures, and they are long, hollow structures made by several columns of cells fusing together end to end. The walls of the xylem vessels are lignified in spirals or non-joined circles. There are also parenchyma packing around the xylem vessels, storing food. The unlignified parts of the xylem vessel are called bordered pits, and it is through these that the water leaves the xylem
What does the xylem transport?
Water and mineral ions
What else does the xylem tissue provide?
Support to the plant
What is the phloem?
Living tissue that transports food in the form of organic solutes around the plant
Describe the structure of the phloem
The main transporting vessels are sieve tube elements. Phloem sieve tubes are made of many cells joined end to end to form a long, hollow structure. In the areas between the cells, walls are perforated to form sieve plates. Companion cells are closely linked to sieve tube elements by plasmodesmata. Companion cells act as a life support system for the sieve tube elements.
How does water enter the root?
Root hair cells on the root actively transport mineral ions from the soil into the root, decreasing the water potential inside the root. This causes water to enter the root via osmosis
How are root hairs adapted to their function?
- They are small so they can penetrate between soil particles
- Large SA:V
- Lots of mitochondria
What are the two ways water can move across the root?
The symplast and the apoplast pathways
Describe the symplast pathway
Water moves through the continuous cytoplasm of living cells that is connected through the plasmodesmata.
Describe the apoplast pathway
Water moves through the cell walls and the intercellular spaces. Water is pulled along by cohesive forces.
What happens when the water reaches the endodermis?
Water from the apoplast pathway joins the symplast pathway, as the Casparian strip prevents water in the apoplast pathway from continuing through.
Why is the diversion of water to the cytoplasm important?
In order to reach the cytoplasm, the water must pass through the plasma membrane, which prevents any potentially toxic solutes in the water from reaching living tissue
What happens once the water is inside the vascular bundle?
Water returns to the apoplast pathway to enter the xylem itself and move up the plant.
What is root pressure?
The active pumping of minerals into the xylem to produce movement of water by osmosis
What is transpiration?
The loss of water vapour from leaves and stems of plants
What is the transpiration stream?
The movement of water through the plant and eventually out
How does the transpiration stream work?
- Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and exit via diffusion
- Loss of water lowers the water potential of the mesophyll cell, so water moves into the cell from an adjacent one via osmosis
- This repeats until the xylem is reached. Water moves out of the xylem by osmosis
- Water molecules form H bonds with the xylem vessels. This is adhesion. They also form H bonds with each other. This is cohesion. The combined effects result in water exhibiting capillary action
What is capillary action?
The process by which water can rise up a narrow tube against gravity. Water is drawn up the xylem in a coninuous stream to replace the water lost. This is the transpiration pull
What does the transpiration pull do?
Creates a tension in the xylem, results in water moving across the roots from the soil.
What is the model of water moving up the plant in a continuous stream called?
The cohesion tension theory
What factors affect transpiration?
Light, humidity, temperature, wind
What is translocation?
The movement of organic compounds in the phloem from source to sink
What is a source?
An area of the plant that produces more assimilates than it uses
What is a sink?
An area of the plant that uses more assimilates than it produces
What are assimilates?
The products of photosynthesis
Describe the active loading of the phloem via the apoplast pathway
In the companion cells, sucrose is moved into the cytoplasm across the cell membrane in an active process. H+ ions are actively pumped out of the companion cell into the surrounding tissue using ATP. H+ ions return to the companion cell with sucrose via cotransport. This increases the sucrose concentration of the companion cells and so sucrose can then enter the sieve tube elements via the plasmodesmata.
What does the increase in sucrose concentration in the sieve tube cause?
Water moves in by osmosis. This increases hydrostatic pressure. The sucrose then moves down the pressure gradient to the sink, in a process called mass flow
Describe the unloading of the phloem
Sucrose is unloaded from the phloem at any point the cells need it. The sucrose diffuses from the phloem into the surrounding cells
What happens after the sucrose is unloaded?
The water potential in the sieve tube increases, so water re-enters the xylem via osmosis.
What are xerophytes?
Plants that are adapted to live and reproduce where water availability is very low.
Describe ways xerophytes conserve water
Thick waxy cuticle, sunken stomata, reduced stomata, reduced leaves, hairy leaves, curled leaves, succulents, leaf loss, widespread shallow roots, deep roots
What are hydrophytes?
Plants that are adapted to living in water
Describe ways hydrophytes are adapted to their environment
Very thin or no waxy cuticle, many always-open stomata on the upper surfaces, reduced structure, wide and flat leaves, small roots, air sacs, aerenchyma
What are aerenchyma and what do they do?
Specialised parenchyma tissue. They have many large air spaces, which makes the leaves and stems more buoyant, as well as form a low-resistance internal pathway for the movement of substances like oxygen to tissues below the water.
