Plus One Botany NotesChapter 7 Transport in Plants is part of Plus One Botany Notes. Here we have given Kerala Plus One Botany Notes Chapter 7 Transport in Plants.
|Text Book||NCERT Based|
|Chapter Name||Transport in Plants|
|Category||Plus One Kerala|
Kerala Plus One Botany Notes Chapter 7 Transport in Plants
The water taken up by the roots has to reach all the parts of the plant, up to very tip of the growing stem. This upward transport take place through the xylem of vascular bundles. The prepared food material from the leaves to the other parts of the plant through the phloem of vascular bundle.
In a flowering plant the substances that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators. Over small distances substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over longer distances proceeds through the vascular system (the xylem and the phloem) and is called translocation.
Means of transport
It is a physical process in which passive trans-port of solvent molecules or solute ions occur without the expenditure of energy. It is a slow process and is independent of living system. During the process, the molecules or ions flow in a random fashion from the region of higher concentration to region of lower concentration be it a gas, liquid or solids.
Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, temperature and pressure.
ii. Faciliated diffusion
Substances that have a hydrophilic moiety, find it difficult to pass through the membrane, their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient, a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion.
- In facilitated diffusion special proteins help move substances across membranes without expenditure of ATP energy.
- It is very specific, it allows cell to select substances for uptake. It is sensitive to inhibitors which react with protein side chains.
- The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
- An extracellular molecule bound to the transport protein, the transport protein then rotates and releases the molecule inside the cell, e.g., water channels made up of eight different types of aquaporins.
Passive symports and antiports
Some carrier or transport proteins allow diffusion only if two types of molecules move together. In a symport, both molecules cross the membrane in the same direction; in an antiport, they move in opposite directions. When a molecule moves across a membrane independent of other molecules, the process is called uniport.
iii. Active transport
Active transport uses energy to pump molecules against a concentration gradient. Active transport is carried out by membrane proteins. Pumps are proteins that use energy to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration
Comparison of different transport processes
|Requires special membrane proteins||No||Yes||Yes|
|Requires ATP- energy||No||No||Yes|
Plant – water relation
It provides the medium in which most substances are dissolved. The protoplasm of the cells is nothing but water in which different molecules are dissolved and suspended. Distribution of water within a plant varies woody parts have relatively very little water, while soft parts mostly contain water. A seed may appear dry but it still has water, otherwise it would not be alive and respiring. Terrestrial plants take up huge amount water daily but most of it is lost to the air through evaporation from the leaves, is known as transpiration.
Water potential (φw) is a concept fundamental to understanding water movement. This is denoted by the Greek symbol – Psi (φ) and is expressed in pressure units such as Pascal (Pa). Solute potential (φs) and pressure potential (φp) are the two components that determine water potential. Water potential of a cell is affected by solute potential and pressure potential.
φw = φs +φp
Osmosis is the term used to refer specifically to the diffusion of water across a differentially or semi-permeable membrane. Osmosis occurs spontaneously in response to a driving force. The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient.
Below given the two chambers, A and B, containing solutions are separated by a semi- permeable membrane.
Solution of chamber A has higher water potential and lower solute potential. So osmosis take place from A to B. When two chambers will have same water potential it attain equilibrium.
The funnel experiment
where a solution of sucrose in water taken in a funnel is separated from pure water in a beaker through a semi-permeable membrane, membrane intact. Water will move into the funnel,. resulting in rise in the level of the solution in the funnel. This will continue till the equilibrium is reached.
If the external solution balances the osmotic pressure of the cytoplasm, it is said to be isotonic. If the external solution is more dilute than the cytoplasm, it is hypotonic and if the external solution is more concentrated, it is hypertonic. Plasmolysis occurs when water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall.
- If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic. When water flows into the cell and out of the cell and are in equilibrium, the cells are said to be flaccid.
- When the cells are placed in a hypotonic solution (higher water potential or dilute solution as compared to the cytoplasm), water diffuses
into the cell causing the cytoplasm to build up a pressure against the wall, that ¡s called turgor pressure.
- The pressure exerted by the protoplasts due to entry of water against the rigid walls is called pressure potential.
- The rigidity of the cell wall, the cell does not rupture. This turgor pressure is ultimately responsible for enlargement and extension growth
of cells. This process is called deplasmolysis.
Imbibition is a special type of diffusion when water Is absorbed by solids, colloeds, causing them to enormously increase in volume. 1mbi. bitlon s also diffusion since water movement.Is along a concentration gradient, the seeds and other such materials have almost no water
hence they absorb water easily, Water potential gradient between the absorbent and the liquid imbibed is essential for imbibition.In addition, for any substance to imbibe any liquid. affinity between the adsorbant and the liquid is also a pie-requisite.
Long distance transport of water
- The bulk movement of substances through the conducting or vascular tissues of plants called translocation.
- The higher plants have highly specialised vascular tissues, xylem and phloem.
- Xylem is associated with translocatiori of mainly waler, mineral salts, some organic nitrogen and hormones, from roots to the aerial
parts of the plants.
- The phloem translocates a variety of organic and inorganic solutes. mainly from the leaves to other parts of the plants.
How do plants absorb water?
- Water flows across the cell membranes of root hairs or epidermal cells into the root cortex by diffusion,
- Root hairs are thin-walled slender extensions of root epidemial cells that greatly increase the surface area for absorption.
- The movement of water from the root hairs to the xylem occurs by two pathways.
i. Apoplast pathway
The apoplast is the system of adjacent cell walls that is continuous throughout the plant,except at the caspariari strips of the endodermis in the roots. The apoplectic movement of water occurs exclusively through the intercellular spaces and the wails of the cells. Movement through the apoplast does not involve crossing the cell membrane and energy.
ii. Symplastic pathway
The symplastic system is the system of inter connected protoplasts Neighbouring cells are connected through cytoplasrnic strands that
extend through plasmodesmata.
- Symplastic movement may be aided bycytoplasmic streaming. You may have observed cytoplasmic streaming in cells of the Hydrilla leaf; the movement of chloroplast due to streaming is easily visible.
- Most of the water flow in the roots occurs via the apoplast.
- The inner boundary of the cortex, the endodermis, is impervious to water because of a band of suberised matrix called the casparian strip.
- Water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes. The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem,
- In young roots, water enters directly into the xylem vessels and/or tracheids. These are non-living conduits and so are parts of the apoplast
It is a symbiotic association of a fungus with a root system. The fungal filaments form a net-work around the young root or they penetrate the root cells.
The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae. Some plants have an obligate association with the mycorrhizae. For example, Pinus seeds cannot germinate and establish without the
Water movement up a plant
The upward movement of water through stem is called ascent sap. It occurs’ mainly through xylem. The upward movement of water can be explained by two ways,
i. Root pressure
The positive hydrolic pressure developed in the root cells due to accumulation of water absorbed by the root. Which pumps the sap up in the xylem elements.
ii. Transpiration pull
Root pressure does not account for the majority of water transport, most plants meet their need by transpiratory pull. The pull or tension developed due to transpiration is called transpiration puli. Water is pulled up as a continous coloumn due to the cohesive force among the water molecules and adhesive force between the water molecules and the lignoceliulosic cell wall of xylem vessels.
The loss of water in the form of water vapour from the plant to the atmosphere, is known as transpiration. It occurs mainly through the stomata in the leaves.
- Stomata are minute pores found on the epidermis of leaves and young stems. ‘
- The loss of water vapour in transpiration, exchange of oxygen and carbon dioxide in the leaf also occurs through pores called stomata.
- Normally stomata are open in the day time ; and close during the night. The immediate
cause of the opening or closing of the stomata is a change in the turgidity of the guard cells.
- The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic.
- The epidermal cells or accessory cells are epidermal cells surrounding the guard cell.
- The guard cells differ from the other epidermal cells in presence of chloroplast.
- The lower surface of a dorsiventral (often dicotyledonous) leaf has a greater number of stomata while in an isobilateral
(often mono- cotyledonous) leaf they are about equal on both surfaces.
- Transpiration is affected by several external factors, temperature, light, humidity, wind speed, number and distribution of stomata, number of stomata open per cent, water status of the plant, canopy structure etc.
- The transpiration driven ascent of xylem sap depends mainly on cohesion, adhesion, and surface tension.
Transpiration and Photosynthesis – a Compromise
- Transpiration creates transpiration pull for absorption and transport of plants.
- It supplies water for photosynthesis.
- It transports minerals from the soil to all parts of the plant.
- It cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling.
- It maintains the shape and structure of the plants by keeping cells turgid.
- Photosynthesis is limited by available water which can be swiftly depleted by transpiration.
- The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil.
Uptake of mineral ions
Most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP. The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively.
Translocation of mineral ions
After the ions have reached xylem through active or passive uptake, or a combination of the two, their further transport up the stem to all parts of the plant is through the transpiration stream.
Phloem trasport: Flow from source to sink
- Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a sink.
- Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, act as sink, they need energy for growth and development of the photosynthetic apparatus.
- The direction of movement in the phloem can be upwards or downwards (bi-directional). This contrasts with that of the xylem where the movement is always unidirectional (upwards).
- Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem.
The Pressure flow or mass flow hypothesis
The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.
According to mass flow hypothesis, the movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the mass movement in the phloem. The mass flow of ions through roots occur as result of transpiration pull in the absence of energy.
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