mod 2.3 - transport Flashcards
What does xylem transport?
Xylem: transports water and inorganic nutrients (mineral ions) absorbed by the roots from the soil to the aerial (above ground) parts of the plant.
What does phloem transport?
Phloem: transports organic nutrients (dissolved sugars) produced in the leaves by photosynthesis throughout the plant. Other organic substances, such as amino acids, are also transported in the phloem.
How do water and mineral ions get absorbed?
Plants absorb water and mineral ions through root hair cells.
Why are mineral ions absorbed?
Potassium is needed to regulate the opening and closing of the stomata; calcium is needed to build cell walls; magnesium is important in the production of chlorophyll; and nitrogen is necessary for making proteins and aa’s.
Why is water absorbed?
Water is essential for dissolving and transporting mineral ions through the plant.
What is xylem in general?
Xylem is the vascular tissue that transports water and mineral ions obtained from the soil throughout the plant. It is mainly composed of xylem vessels and elongated cells called tracheids.
How are xylem vessels formed?
Mature xylem vessels (or vessel elements) are long, water-filled tubes consisting of elongated cells joined end to end.
As the cells mature, the cell wall is strengthened with lignin (a polymer related to cellulose), making them stronger and more rigid.
The cytoplasm and nucleus in the xylem vessel cells then disintegrate and the cells die, creating hollow lignin tubes.
What do mature xylem vessels have?
Cylindrical skeletons of dead cells joined end to end to form continuous tubes.
Perforated or complete openings at each end, like a straw, so that fluid can flow directly through them.
Pits (unthickened areas) and perforations in the side walls that allow sideways movement of substances between neighbouring vessels in the vascular bundle.
No nucleus or cytoplasm.
How are tracheids formed?
Single, large, tapering water-filled cells that form part of the xylem tissue in all vascular plants.
When mature, tracheids lose their nucleus and cytoplasm, leading to cell death, but also creating an open structure for water to flow through.
What do mature tracheids have?
Cylindrical skeletons of death cells joined to form continuous tubes, like xylem vessels.
Pits and perforations in their lignified cell walls.
No nucleus or cytoplasm.
What is the difference between xylem vessels and tracheids?
Unlike xylem vessels, tracheids are not connected end to end; their ends overlap and water is transferred horizontally through the adjoining pits.
How to roots optimise the absorption?
Roots have a branched structure that increases both SA and capacity to absorb water and mineral ions.
What are the two possible pathways for movement of water and mineral ions absorbed from the soil via the roots?
The extracellular pathway and the cytoplasmic pathway.
What is the extracellular pathway?
Most water and some mineral ions pass in or between cell walls.
What is the cytoplasmic pathway?
Most mineral ions and some water pass through the cytoplasm of living root cells.
Involves substances entering a root hair cell by crossing the cell’s membrane, then passing from cell to cell through plasmodesmata, which are strands of cytoplasm that connect one cell with the next.
What are the three types of transport that move substances across cell membranes and along the cytoplasmic pathway?
Active transport: most dissolved mineral ions are selectively taken (by proteins) into roots by active transport.
Osmosis: the high concentration of ions in the vascular tissues of terrestrial plants creates a very large osmotic concentration gradient.
Diffusion: some mineral ions (potassium and phosphate) enter the roots by diffusion; the uptake of these nutrients depends on the rate of water uptake.
What is root pressure?
In some plants, the osmotic gradient draws in so much water from the roots that it can travel up to 10 m up the stem; known as root pressure. In some trees like BIRCH MINECRAFT, root pressure causes the rising of sap (water and mineral ions) in spring when the soil is warm and the rainfall is high.
What is guttation?
In some small plants, root pressure can result in the process of guttation. This is the loss of liquid water, and sometimes other substances from leaves (different to transpiration).
In guttation, water is lost through specialised pores at the ends of leaf veins.
What is the Casperian strip?
The Casperian strip between the roots and xylem is a waterproof layer of cells that form a barrier.
At this barrier, water travelling through the extracellular pathway is forced into the cytoplasm, therefore regulating the substances entering the xylem.
What is translocation?
The transport of organic solutes from the leaves to other tissues in the plant is known as translocation. –> phloem
Leaves produce carbohydrates in the form of sugars during photosynthesis and are transported to other parts of the plant where it is needed.
What is phloem?
Phloem transports organic solutes, such as sugars and aa’s, from the leaves to the stems and roots; where it is used or stored in its cells to produce energy for growth and reproduction.
Plants can store sugar in their cells as starch, which can be used for structural support, or as an energy source when the plant cannot photosynthesise.
Phloem tissue is composed of sieve tubes, companion cells, parenchyma cells and sclerenchyma cells.
What are sieve tubes?
Mature sieve tubes are living cells with no nucleus and no lignin in the cell walls.
They form linear rows of elongated cells, and their cell walls are thin and perforated at each end by holes or pores → forming sieve plates.
Plasmodesmata pass through the perforations in sieve tubes, acting like straws through which sugars and other materials can move.
Sieve tube cells are connected with one or more companion cells, connected by plasmodesmata; and are able to function without a nucleus with companion cells.
What are companion cells?
A type of parenchyma cell that provides metabolic support and helps load and unload materials throughout the plant. Like sieve tube cells, companion cells have thin cell walls.
They retain their nuclei and carry out all the metabolic processes required by the sieve tube cells, sharing metabolic products through plasmodesmata; keeping sieve tube cells alive.
What are parenchyma cells?
Make up the soft tissue of a plant and have many important functions.
In leaves, they contain the chloroplasts and make up the mesophyll. Parenchyma cells that contain chloroplasts are called chlorenchyma cells.
In roots and tubers, they have large vacuoles that store starch, fats, proteins and water. They also provide buoyancy in aquatic plants and play a role in wound repair.
Their structure varies from elongated to spherical.
What are sclerenchyma cells?
Provide strength and structural support for the plant.
Mature sclerenchyma cells are dead and have very thick cell walls made of cellulose and lignin.
There are two types: fibres and sclereids.
Fibres are found in stem roots and the vascular tissue of leaves.
Sclereids are found in the outer layer of seeds and the shell of nuts.
The fibres of some plants, such as flax and hemp, have important uses as textiles.
What are sources?
Sources: the sites where sugars are produced during photosynthesis. (leaves)
What are sinks?
Sinks: the sites where sugars are translocated to. (roots, bulbs, stems, flowers and fruits)
Where is glucose produced?
Glucose (monosaccharide) is produced in the chloroplasts of the chlorenchyma cells and converted into sucrose in the cytosol of cells; this sucrose is then pumped into the companion cells and from there, flows into the sieve tube cells.
Is translocation an active transport?
Translocation is an active process; involving the flow of cytoplasm in sieve tubes driven by a pressure gradient. This pressure gradient begins in the leaves, where sucrose is actively pumped into phloem sieve tube cells.
Why do sieve tube cells have thick and rigid cell walls?
Sieve tube cells have thick and rigid cell walls to withstand hydrostatic pressure, which assists the flow of solutes. Transport in individual sieve tube cells is in one direction only, but bundles of sieve tube cells transport sap in both directions.
What happens when water enters sieve cells?
As water enters, it increases the fluid pressure (turgor) in sieve cells, which pushes fluid into the adjacent sieve cells.
While this is happening in the leaves, sucrose is being actively removed from sieve cells in roots, and used for growing shoots and developing fruit; causing an osmotic gradient that draws water out of sieve cells and lowers their turgor pressure.
Fluid pressure is therefore high in sieve tube cells in leaves and low in thos in roots and growing shoots.
Most phloem sap in sieve tubes flows along this fluid pressure gradient from sources to sinks, allowing the phloem to translocate solutes away from the source and towards the sink.
Translocation stops if the cells in the stem die.
What is transpiration?
The passive movement of water though the xylem of vascular plants, from the roots to the leaves.
Why is transpiration a vital process?
Transpiration is a vital process that enables plants to: absorb the water necessary for photosynthesis, transport mineral salts to leaf cells and fruits, cool down and not become overheated.
Who proposed the transpiration-cohesion-tension theory?
Proposed by John Joly and Henry Horatio Dixon in 1894, and is now the most accepted theory that explains the upward movement of water through the xylem of plants.
What is the transpiration-cohesion-tension theory?
Theory explains the primary mechanisms of water in plants; cohesion between water molecules, adhesion between water molecules and plant cell walls, and the tension (differential pressure) created when water evaporates from the leaves.
Are water molecules cohesive?
Water molecules are very cohesive; they have a strong tendency to stick together.
Cohesion also causes the water molecules evaporating from the surface of a leaf to pull adjacent water molecules with them.
Water in nearby xylem vessels is then drawn up to the leaves to replace the water lost via evaporation; in this way, thousands of lead cells, each drawing water from the xylem, create tension that pulls water up the xylem from the roots.
Are water molecules adhesive?
They are also adhesive, meaning they have a tendency to stick to other molecules.
How does adhesion and cohesion work together for transpiration
The adhesiveness allows it to stick to the hydrophilic cell walls in the xylem of the plant, and along with cohesion, moves against the force of gravity.
What is the transpiration stream?
This continuous one-way flow of water from roots to leaves is called the transpiration stream.
What factors affect transpiration rates?
Humidity, temperature and wind.
Also:
The SA across which transpiration takes place is related to the degree of opening of all stomata; this is by far the most important factor affecting the rate of transpiration. The greater the number of stomata and the more open they are, the more SA there is from which water can be lost.
The rate of transpiration is higher during the day than at night because stomata open during the day to exchange gases during photosynthesis, and close at night to minimise water loss.
How does humidity affect transpiration rates?
Humidity: rates decrease when there is a lot of water vapour in the air (high humidity). Humidity reduces the water concentration gradient between leaf spaces and air, so fewer molecules evaporate into the air. (potassium ions)
How does temperature affect transpiration rates?
Temperature: rates increase as temperature increases, because heat energy increases the rate of evaporation of water.
How does wind affect transpiration rates?
Wind: air currents increase the rate, by moving water vapour away from the leaf and increasing the rate of evaporation of water.
What are the two transport systems of mammals?
Mammals have two transport systems: the cardiovascular system (circulatory system) and the lymphatic system.
What is the cardiovascular system of mammals?
The cardiovascular system is a closed circulatory system. It Highly branched network means no cells are more than 1mm from a capillary, ensures efficient nourishment and waste removal. Specialised circulatory systems with networks of pipes and chambers have evolved to transport vital nutrients to all cells in complex multicellular organisms.
The cardiovascular system delivers oxygen from the lungs to the brain in less than 4 seconds.
What is the circulatory fluid of cardiovascular system in mammals?
Blood.
What are the two circulation pathways?
Pulmonary circulation and systemic circulation.
What is pulmonary circulation?
Pulmonary circulation: transports blood to and from the lungs. Deoxygenated blood is pumped from the heart to the lungs, where it is oxygenated before returning to the heart.
