Transport in flowering plants
Plants are less active than animals so their cells don’t need materials so quickly. Also, the shape and structure of plants means that oxygen and carbon dioxide can diffuse into and out of cells. Substances such as glucose molecules and nitrate ions move naturally from a region of higher concentration to one of lower concentration by diffusion. Water diffuses through a selectively permeable membrane from a region of higher water potential to one of lower water potential, a process known as osmosis. These vital processes are essential but adequate only for a transport of molecules and ions over short distances within a living system. Diffusion by itself is not sufficient; it is often too slow, possibly in the wrong direction and inadequate over longer distances. The evolution of flowering plants has seen the development of efficient transport systems, which have enabled plants to colonise the land, to increase their size and to develop an established habit. Plants have two transport systems; the xylem carries water and dissolved minerals and the phloem carries the ‘food’ materials that the plant has made. These enable the movement of water, organic molecules and ions over long distances.
Xylem and phloem tubes are found together in a group called a vascular bundle.
Xylem vessels run from the roots, up through the stem and out into each leaf. The xylem is made of hollow dead cells joined end to end. The cells don’t contain any cytoplasm or nuclei. The cell walls are made of cellulose and lignin, which is very strong and helps keep the plant upright. The ends of the cells have disappeared so that a long continuous tube is formed. There are two types of xylem cells involved in the transport of water, tracheids and vessel elements. Both types of cell have cell walls thickened with lignin. Lignin strengthens the wall enabling it to provide support. It also waterproofs the cell so that it can conduct water.
A new xylem is formed each year, and the old xylem is pushed to the centre and becomes compacted. The xylem at this point is almost entirely lignin (this is wood). This is the structure of the xylem:
The xylem vessels also allow sideways flow of water through the vessels. This is done by bordered pits. Xylem vessels can transport water even when a plant is dead. This is because they work passively. Metabolic poisons e.g. potassium cyanide (KCN) do not prevent transport of water in plants.
The phloem is also made up of cells joined end to end. The ends of the cells are not completely broken down and form a kind of sieve (called a sieve plate). Each of the cells is called a sieve tube element. These cells contain strands of cytoplasm, but no nucleus, and there is no lignin in the cell wall but there is cellulose. Each sieve tube element has a companion cell attached to it. The companion cells have a nucleus, organelles and dense cytoplasm, and provide the sieve tube elements with some of their requirements. This is the structure of the phloem:
The plant to make proteins, oils and other organic substances uses carbohydrates made in the leaves during photosynthesis. Organic food material, particularly sugar, is transported in the phloem tubes to where it is needed. This is called translocation. The sap inside phloem tubes contains lots of sugar. Moving it is an active process that uses energy.
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Food, primarily sucrose is transported by the vascular tissue called phloem from a source to a sink.
Unlike transpiration's one-way flow of water sap, food in phloem sap can be transported in any direction needed so long as there is a source of sugar and a sink able to use, store or remove the sugar.
The source and sink may be reversed depending on the season, or the plant's needs. Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, the sink, need energy for growth and development of the photosynthetic apparatus.
This is a transverse section of a dicotyledonous root:
This is a transverse section of a dicotyledonous stem:
This is a vertical section of a dicotyledonous leaf:
In the root of a dicot the xylem and phloem are centred to provide stability to the root, whereas in the stem they are more circulated around he outside. There are three ways that substances can move across the root. These are simple diffusion, active transport and osmosis.
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. Active transport is the uptake of substances against the concentration gradient, and osmosis is the movement of water particles across a partially permeable membrane from a region of low concentration to a region of high concentration.
Water enters the plant through root hairs on the root tip. Each root hair is a long epidermal cell. These cells are short-lived and are replaced frequently. The very end of the root tip is called the root cap. This is a layer of cells that protects the root as it grows. A layer of cells covers the rest of the root, which is the epidermis. The root hairs start above the root cap. Water is absorbed into the root hair by osmosis. The cytoplasm and cell sap in the root hairs has a high concentration of solutes, which means a low concentration of water. The concentration of water (or water potential) is higher outside the root tip than inside it, so the water diffuses into the cell. (Plant cells have a partially permeable membrane across which osmosis occurs.) The water travels by osmosis from cell to cell and into the xylem in the centre of the root. Water is literally sucked up the xylem. This happens because there is a pressure gradient: the pressure at the top of the xylem is lower than at the bottom, so the water moves upwards. (Capillary action aids this to a small degree.) The pressure at the top of the xylem system is kept low because of transpiration. This is the evaporation of water from the plant’s leaves. The cells inside the leaf are each covered in a film of moisture. Some of this moisture evaporates from the cells and diffuses out through the stomata, which are pores on the underside of the leaf. The moisture that evaporates is replaced by water from the xylem by osmosis. This reduces the effective pressure at the top of the plant, so water is drawn upwards. This process is known as the transpiration stream.
Root cells take up ions dissolved in the soil water by active transport. This is because the minerals are in a higher concentration inside the root hair than in the soil, so the minerals have to be taken up against the concentration gradient. Special carrier proteins in the cell membrane of the root hair take up the minerals; the energy is supplied by the mitochondria.
As various ions from the soil are actively secreted into the root's vascular tissue water follows (its potential gradient) and osmotic pressure increases. This osmotic pressure is called root pressure.
Root pressure can only provide a modest push in the overall process of water transport. Its greatest contribution maybe to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
Active transport establishes lower water potential and helps the root hairs take in the necessary minerals dissolved in soil water. Lower water potential allows water to be drawn into the root cells by osmosis.
When a water potential gradient is established between two areas, water will spontaneously diffuse from the high end (soil) to the low end (air). This gradient is necessary for plants to transport water. Water potential may be established by either increasing the concentration of solutes. Pure water has the highest potential while a saturated solution of ions etc. would have the lowest potential. Or on the other hand, converting water to a gas. Water potential is highest when water is a liquid and lowest when water is a gas in air.
Four important forces combine to transport water solutions from the roots, through the xylem elements, and into the leaves. These forces are transpiration, adhesion, cohesion and tension.
Transpiration involves the pulling of water up through the xylem of a plant utilising the energy of evaporation and the tensile strength of water. There are certain conditions that will effect the transpiration occurring in a plant. As temperature increases, there is more energy available for evaporation, so transpiration increases. Also humidity, this is the amount of moisture in the air. As humidity increases, there is more water in the air. This reduces the difference in water concentration between the inside and outside of the leaf, so diffusion is slower. So, the higher the humidity, the slower the transpiration rate. Transpiration also increases in windy weather. Water evaporates quickly on a windy day because the diffused molecules are taken away by the wind. Light intensity increases transpiration; the light provides energy for evaporation. Also, on bright days, plants photosynthesise very quickly. The stomata open to allow carbon dioxide to diffuse in; this also allows water to diffuse out. Lastly water supply, plants close their stomata when water is in short supply. This will decrease the rate of transpiration. Transpiration is useful because it keeps water moving up through the plant. However, it is important that the plant doesn’t lose more water than the root can take up, if this happens the plant wilts.
Adhesion is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.
Cohesion is the attractive force between molecules of the same substance. Water has an unusually high cohesive force again due to the 4 hydrogen bonds each water molecule potentially has with any other water molecule. It is estimated that water's cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter.
Tension can be thought of as a stress placed on an object by a pulling force. This pulling force is created by the surface tension, which develops in the leaf's air spaces.
Transpiration has more that one purpose however, supplies water for photosynthesis, transports minerals from the soil to all parts of the plant, it also cools leaf surfaces some 10 to 15 degrees by evaporative cooling and maintains the plant's shape and structure by keeping cells turgid.
The bulk flow of water to the top of a plant is driven by solar energy since evaporation from leaves is responsible for transpiration pull.
The flow of water and minerals from the soil to the cells of the root is accomplished by transpirational pull, active transport and a special layer of cells called the casparian strip.
In order to regulate the quantity and type of minerals and ions reach the xylem, the root has a waxy layer between the endodermis and pericycle called the casparian strip. Water and mineral normally can travel through the porous cell walls of the root cortex; this is the apoplastic route. But in order for water and minerals to reach the stele (xylem) the highly regulated (cytoplasmic) symplastic route must be taken. The casparian strip blocks the apoplastic route.
The symplastic route involves special openings between adjacent cell walls called plasmodesmata. The vacuolar route however is the route that travels through the vacuoles of the cells.
In the apoplast pathway most water travels from cell to cell via the cell wall, made up of cellulose fibres, between which are water-filled spaces. As the water evaporates into the sub-stomatal air space from the wall of one cell, it creates a tension that pulls in water from the spaces in the walls of surrounding walls.
In the symplast pathway, some water is lost to the sub-stomatal air space from the cytoplasm of cells surrounding it. The water potential of this cytoplasm is thereby made more negative. Between adjacent cells are tiny strands of cytoplasm, known as plasmodesmata, which link the cytoplasm of one cell to that of the next.
Lastly in the vacuolar pathway, a little water passes by osmosis from the vacuole of one cell to the next, through the cell wall, membranes and cytoplasm of adjacent cells. In the same way as the symplast pathway, a water potential gradient between the xylem and the sub-stomatal air space exists. It is along this gradient that the water passes.
Water moves in the direction it does (root to leaf) because of the water potential gradient. The gradient is highest in the water surrounding the roots and lowest in the air space within the spongy parenchyma of the leaf. (Liquids have higher potential than gases and the purer the liquid the higher it’s potential).
The energy of evaporation is needed to pull molecules away from the film of water coating air spaces within the spongy parenchyma.
As more molecules evaporate from the film coating the air spaces the curvature of the meniscus increases which increases the surface tension. Water from surrounding cells and air spaces will then be pulled towards this area to reduce the tension.
Finally these forces are communicated to water molecules within the xylem because each water molecule is bound to the next by hydrogen bonds.
Plants have adaptations that help prevent water loss. (Very dry conditions are called xerophytic conditions.)
Firstly closure of the stomata. When the stomata are short of water, the guard cells become flaccid and the stomata close. When there is plenty of water, the guard cells become turgid (full of water) and the stomata open. The structure of the guard cells allows this to happen: the inner wall is thick and cannot stretch as much as the thinner outer wall, so when the guard cells absorb water, the cells curve, forming the opening. Plants need the stomata to be open so that carbon dioxide can diffuse in for photosynthesis. The stomata only close at night, and on hot, dry days, when the plant can’t photosynthesise. There is a waxy cuticle layer made by the cells in the epidermis and keeps the leaf waterproof. Holly leaves have a thick waxy cuticle. Some plants have hairy leaves to prevent water loss. The hairs trap a layer of moisture next to the leaf. This reduces the concentration gradient between the inside and outside of the leaf, so evaporation is slowed down. Stomata on the underside of the leaf, is another beneficial adaptation. Most plants have more stomata on the underside of the leaf than the upper surface. Less light reaches this surface so it is cooler and transpiration is slower here than on the upper surface. Some stomata may be in pits. Plants that live in very hot conditions often have spines rather than leaves; this decreases the surface area. These have a much smaller surface area than leaves, so there is less area for evaporation. Although this helps to conserve water, it also means that photosynthesis is slow.
An actively photosynthesising plant has a strong need for water. The efficiency of photosynthesis increases as the surface area for CO2, the second reactant, increases. This is the purpose of the spongy mesophyll that is honeycombed with air sacs. The spongy mesophyll has 10 to 30 times more surface area than the corresponding external surface of a leaf.
Photosynthesis requires water. The system of xylem vessels from root to leaf vein can supply the needed water. Several forces combine to overcome the pull of gravity. These combined forces culminate in a process called transpiration. Ultimately water is pulled molecule by molecule into the leaf. The pulling forces and energy needed involves; free energy of the water potential gradient, of evaporation, force of surface tension and also the force of hydrogen bonding between water molecule. Each force can be communicated to the next because water forms a strong continuous chain from root to leaf.
In conclusion flowering plants have many different ways of transporting different substances e.g. xylem and phloem, and also many ways of moving substances within the plant. They can also adapt to different environments and treat different substances in different ways to get the most efficient use out of them. All of these characteristics of the plant make up the transport system which allows them to photosynthesise, transpire and respire at a quick rate in order to carry out all of the needs of the plant.