The cell membrane, is the most external layer of any living cells, it is composed of two layers of lipid molecules (the lipid bilayer). The lipid molecules each have a hydrophilic (water-loving, or polar) end and a hydrophobic (water-hating, or nonpolar) end. The cell membrane is surrounded by an aqueous environment, lipid molecules of the cell membrane arrange themselves so as to expose their hydrophilic ends and protect their hydrophobic ends. Suspended randomly among the lipid molecules are proteins, some of which extend from the exterior surface of the cell membrane to the interior surface.
Chemicals outside the membrane tend to dissolve more readily in a solvent of similar polarity. Nonpolar chemicals are considered lipophilic (lipid-loving), and polar chemicals are hydrophilic (water-loving). Lipid-soluble, nonpolar molecules pass readily through the membrane because they dissolve in the hydrophobic, nonpolar portion of the lipid bilayer. Although permeable to water (a polar molecule), the nonpolar lipid bilayer of cell membranes is impermeable to many other polar molecules, such as charged ions or those that contain many polar side chains. Polar molecules pass through lipid membranes via specific transport systems.
The characteristics of lipids are that they dissolve readily in organic solvents--but in addition they have a region that is attracted to and soluble in water. This "amphiphilic" property is basic to the role of lipids as building blocks of cellular membranes.
Phospholipids molecules have a head (often of glycerol) to which are attached two long fatty acid chains that look much like tails. These tails are repelled by water and dissolve readily in organic solvents, giving the molecule its lipid character. To another part of the head is attached a phosphoryl group with a negative electrical charge; to this group in turn is attached another group with a positive or neutral charge. This portion of the phospholipid dissolves in water, thereby completing the molecule's amphiphilic character.
A bilayer is composed of two sheets of phospholipid molecules with all of the molecules of each sheet aligned in the same direction. In a water medium, the phospholipid of the two sheets align so that their water-repellent, lipid-soluble tails are turned and loosely bonded to the tails of the molecules on the other sheet. The water-soluble heads turn outward into the water, to which they are chemically attracted. In this way, the two sheets form a fluid, sandwich like structure, with the fatty acid chains in the middle mingling in an organic medium while sealing out the water medium. This type of lipid bilayer, formed by the self-assembly of lipid molecules, is the basic structure of the plasma membrane of the cell. It is the most stable thermodynamic structure that a phospholipid-water mixture can take up: the fatty acid portion of each molecule dissolved in the organic phase formed by the identical regions of the other molecules and the water-attractive regions surrounded by water and facing away from the fatty acid regions. The chemical affinity of each region of the amphiphilic molecule is thus satisfied in the bilayer structure.
The cell membrane allows some dissolved substances, or solutes, to pass while blocking others. Lipid-soluble molecules and some small molecules can permeate the membrane, but the lipid bilayer effectively repels the many large, water-soluble molecules and electrically charged ions that the cell must import or export in order to live. Diffusion is the process, resulting from random motion of molecules by which there is a net flow of matter from a region of high concentration to a region of low concentration Transport of these substances is carried out by the membrane proteins, particularly the intrinsic proteins. These molecules form a variety of transport systems: some are open channels, which allow ions to diffuse directly into the cell; others are "facilitators," which, through a little-understood chemical transformation, help solutes diffuse past the lipid screen; yet others are "pumps," which force solutes through the membrane when they are not concentrated enough to diffuse spontaneously. Particles too large to be diffused or pumped are often swallowed or disgorged whole by an opening and closing of the membrane. Behind this movement of solutes across the plasma membrane is the principle of diffusion. According to this principle, a dissolved substance diffuses down a concentration gradient; that is, given no energy from an outside source, it moves from a place where its concentration is high to a place where its concentration is low. Diffusion continues down this gradually decreasing gradient until a state of equilibrium is reached, at which point there is an equal concentration in both places and an equal, random diffusion in both directions. A solute at high concentration is at high free energy; that is, it is capable of doing more "work" (the work being that of diffusion) than a solute at low concentration. In performing the work of diffusion, the solute loses free energy, so that when it reaches equilibrium at a lower concentration, it is unable to return spontaneously (under its own energy) to its former high concentration.
However, by the addition of energy from an outside source (through the work of an ion pump, for example), the solute may be returned to its former concentration and state of high free energy. This "coupling" of work processes is, in effect, a transferral of free energy from the pump to the solute, which is then able to repeat the work of diffusion. For most substances of biologic interest, the concentrations inside and outside the cell are different, creating concentration gradients down which the solutes spontaneously diffuse, provided they can permeate the lipid bilayer. Membrane channels and diffusion facilitators bring them through the membrane by passive transport; that is, the changes that the proteins undergo in order to facilitate diffusion are powered by the diffusing solutes themselves. For the healthy functioning of the cell, some solutes must remain at different concentrations on each side of the membrane; if through diffusion they approach equilibrium, they must be pumped back up their gradients by the process of active transport. Those membrane proteins serving as pumps accomplish this by coupling the energy required for transport to the energy produced by cell metabolism or by the diffusion of other solutes.
Temperature:
I predict that increasing the temperature will increase the rate at which the red beetroot pigment diffuses out of the beetroot. This is because the molecules of the solution surrounding will gain energy and start to move around at a higher rate The energy gained by the solution molecules is known as kinetic energy. Increasing the temperature will increase the kinetic energy gained by the solution molecules and therefore increase the amount of successful collision between the beetroot membrane and the solution. In many cases increasing the temperature by 10 0c doubles the rate of reaction hence increasing the temperature 10 0c will double the amount and the rate at which the red beetroot pigment diffuses out of the beetroot membrane.
However increasing too high temperature can also denature the solution molecules (detergent). Therefore make the detergent molecules ineffective on affecting the beetroot membrane. If the temperature is raised too high the bonds present in the solution (detergent) will break apart.
Therefore resulting in the detergent losing its shape and enabling it to complete its function, which is to break down or dissolve the lipid layer in the membrane.
I predict that graph for the factor temperature will look something like this.
The diagram above shows at A there is slow rate of diffusion of the red beetroot pigment. This is because the molecule of solution has a very little kinetic energy therefore less movement of molecules therefore result few collisions between the membrane and the solution.
At B the solution tends to have greater kinetic energy therefore resulting in more movement of molecule and more rate of successful collision between the beetroot and the solution. The more movement of solution (detergent) the more damage done to the membrane resulting in more free movement of particle to move freely across the concentration gradient. At this point the intensity of beetroot pigment would be the deepest and the darkest.
At C the solution molecule have become denatured and lost their shape and therefore will became ineffective in damaging the membrane which would in turn would lead to less pigment diffusing out the membrane as it may be controlled by the protein channels. However the diffusion of the pigment will not completely stop merely slow down. The concentration gradient is always present therefore will take place in order establish equilibrium.
Source of the information: Revise AS Biology
Author: R.Fosbery, J.Gregary and L.Stevens
Publisher: Heinemann
Page: 22
