Membranes have different functions in cellular metabolism. Most of the significant activities of cells are related with membranes. For instance, proteins intended for secretion or for insertion in cell membranes are composed by ribosomes which are joined to membrane system known as endoplasmic reticulum. Phospholipids have dual solubility. The long hydrocarbon tails are non polar and hydrophobic (water-fearing).
Structure of Membranes:
Membranes have electron-transparent inner region sandwiched between 2 electron-dense outer layers. Other layers were supposed to be composed of proteins and phospholipids heads, and inner region was thought to be composed of hydrocarbons in phospholipids tails. Mitochondria membrane is thinner than plasma membranes and has higher proportion of protein. These newer version suggested in 1972 is known as fluid mosaic model. It holds that proteins happen as mosaic in fluid bilayer of phospholipids. Visual evidence for fluid mosaic model appears from scanning electron microscopy of freeze-fractured membranes. Interior of membrane is then observed as dotted plain. Plain is sea of lipids and dots are proteins inserted in them. Primary structure of thytakoid protein 5 regions of protein happens in membrane, remaining regions give or into interior of thytakoid. The membrane protein comprise of 352 each designated by one-letter.
Another significant feature of membrane structure is its asymmetry, which is, on side of membrane is different from other. This difference comes generally from carbohydrates which are joined to proteins on outside surface of membrane proteins with carbohydrates joined to them are known as glyco-proteins, and they don't generally occur on inner surfaces of membranes.
Functions of Membranes:
Few significant activities are explained below:
1. Movement of water and solutes: Plasma membrane usually permits unrestricted passage of water and certain dissolved substances in or out of cell.
2. Differential permeability: Membranes manage or block passage of some types of molecules such membranes are termed to as differentially permeable membranes.
3. Ion pumps: certain ions, like K+ and H+, are pumped by membranes. Ion pumps in plasma membranes are energy from ATP to move Ions from cell while Ion pumps in mitochondria and chloroplast membranes are significant for making ATP.
4. Enzyme activity: Enzymes which cooperate in multistep procedures like ATP synthesis or absorption of light energy frequently occur together in particular spot on membrane.
5. Cellular communication: Plasma membrane has proteins which bind molecules released from other cells. Once bound to external molecule, such proteins activate other proteins in membrane which cause metabolic changes in cell.
Movement of Water and other Molecules through Membranes:
The most common type of molecule in cells is water, Ions and other polar molecules are dissolved in water, i.e. they are solutes and water is solvent which dissolves them. Solutes comprise protons (H+), mineral ions potassium (K+) and magnesium (Mg++). And organic compounds like sugars and amino acids. Passage of the substances through membranes is determined both by phospholipids bilayer and by proteins embedded in it. Non-polar molecules, like hydrocarbons and oxygen pass simply by membranes lipid core. Small and uncharged polar molecular, like water and carbon dioxide, also pass by membrane lipid bilayer. This inhibits substances from leaking freely in or out of cell. Some terms that relates to water and solute movements are given below:
Movement of Solutes:
Every molecules show random thermal motion, or kinetic energy, that is solute molecule has tendency to move around in solution. One result of random movement is that molecules diffuse outward from regions of high concentration to regions of lower concentration. Diffusion by random movement continues until distribution of molecules becomes homogeneous throughout solution.
As diffusion is based on random movements, every molecule has equal change of moving toward or away from region of high solute concentration. However, there are more high solute molecules near crystal that means there is greater possibility of net movement away from source. Molecular movement goes on after homogeneity is reached, although there is no longer net change in concentration from one region of solution to another. This signifies that, after balance is achieved, for each molecule which vacates the spot, another molecule takes its place simply by chance. Diffusion is net movement of solutes from area of greater concentration to area of lesser concentration, which is down concentration gradient.
Similar to solutes, water also contain potential energy to flow to where it is less concentrated, potential energy of water has special name. Water inclines to move down water potential gradient, which is from region of high water potential to region of low water potential. Also, similar to solutes, water needs energy to move up water-potential gradient. Through general agreement water potential of pure water is zero. This signifies that water potential of solution has negative value as water is less concentrated than pure water. Water potential is stated in units of pressure instead of units of energy, as pressure is easy to measure. Therefore, water potential may be stated in bars or in megapascals (Mpa).
Several substances comprising water move through biological membranes as freely as they move by aqueous solution. Such unrestricted movement of substance by biological membrane is known as passive transport. Energy for passive transport is kinetic energy which is inherent in all molecules. Passive transport doesn't need energy from cellular metabolism. Passive transport of water influences several activities of cell, comprising cell growth, structural firmness and photosynthesis. Due to its roles in several processes, diffusion of water through selectively permeable membrane has special name; Osmosis. Osmosis is affected by different water potentials on either side of membrane. Consider the membrane which is permeable to water but impermeable to glucose. When such membrane separates two halves of container each having different concentration of glucose, water diffuses by osmosis in side having higher glucose concentration (i.e. lower water potential). Net movement of water will stop either when both sides of container have same concentration of glucose, or when force of gravity equals force of water movement, whichever happens first. Side that started with higher concentration of glucose increases in volume.
Most plant cells are enclosed by hypotonic environment. Consequently, cells absorb as much water as they can sustain. Outward pressure of plasma membrane against cell walls is known as turgor pressure as it keeps cell turgid. Turgor pressure is very important to plants in many ways. During development, cell expansion is occurred by turgor pressure on cell walls which have turn out to be relaxed. Turgor pressure also keeps herbaceous (non-woody) plants upright and supports fleshy stalks and leaves of trees and shrubs, and keeps vegetables crisp when they are sprayed with water. Changes in turgor also cause movements in plants like opening and closing of stomata and curling of grass leaves.
Few movements caused by changes in turgor are dramatic; example is leaf movement in sensitive plant. Cells lose turgor when they are placed in hypertonic solution. Continued loss of turgor causes cytoplasm to shrink away from cell wall.
Inducing Osmosis-the Control of Turgor:
Though water moves freely across biological membranes, plants can manage osmosis by regulating concentrations of solutes in the cells. Loss of turgor causes uptake of potassium ions (K+), that are osmotically active as they change cell's osmotic potential. As concentration of K+ increases, osmotic potential increases, and move water enters cell. On the contrary, cells decrease osmotic potential by secreting K+, this causes water to leave cell. Uptake of K+ occurs against concentration gradient, i.e., K+ moves from region of low concentration outside cell to region of high concentration inside cell. This procedure that is known as active transport needs metabolic energy.
Plants which live in high-salt environments like salt flats, close to ocean bays or inland where oceans once were, collect large amounts of osmotically active solutes like amino acid praline and carbohydrate mannitol. Such organic solutes help plant absorb water (via osmosis) from dry, salty soil.
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