The plasma membrane is an integral component of all cells, effectively acting as a barrier between the intracellular and extracellular environments within an organism. It is composed from two layers of phospholipid molecules stacked on top of one another, forming a phospholipid bilayer.
Phospholipid molecules consist of one hydrophilic polar phosphate heads and two hydrophobic nonpolar fatty acid tails. For VCE Biology, it is acceptable to describe hydrophilic and polar substances as being those which are attracted towards water. Conversely, hydrophobic nonpolar substances are therefore those which are repelled by water. The tails face inwards, opposite one another, while the heads face outwards towards the watery extracellular and intracellular environments. This structure can be explained by the chemical composition of phospholipid molecules. Because the heads are hydrophilic, therefore they are attracted towards water, while the hydrophobic tails are repelled by water and instead are attracted towards one another and face inwards. Remember, water and fats do not mix together.
The Plasma Membrane
The plasma membrane serves many different functions, most of these however will not be entirely apparent to you until you progress through the course and learn the background information.
Commonly, the plasma membrane is represented by the fluid mosaic model. Hence, as the name suggests, the plasma membrane is not a static structure, but it is very flexible and fluid. This is facilitated by the phospholipid molecules, since they are not actually bonded to each other. The mosaic appearance is given by the many other components: cholesterol, glycoproteins, glycolipids, protein channels and protein carriers, which are embedded within the plasma membrane. It is also important to note that proteins which span the entire length of the plasma membrane can be referred to as being transmembrane proteins i.e. they connect the intracellular and extracellular environments of a cell.
The primary function of the plasma membrane is to act as an active boundary between the intracellular and extracellular environments. Because the plasma membrane is merely semi-permeable, that is, only certain substances can pass through, there are three conditions which we need to consider when determining whether a substance can readily cross the plasma membrane – the size, polarity and charge of the molecule.
The larger the molecule, the more difficult it is for it to cross the plasma membrane. This is directly attributed to the fact that it will struggle to pass through the tiny gaps between the phospholipid molecules. If the substance is polar, then it will be repelled by the nonpolar tails. Conversely, if the substance is nonpolar, then it can dissolve within the tails, allowing it to cross. Moreover, the more charged a substance is, then the more polar it becomes, and would therefore be repelled. Therefore, very small molecules and nonpolar (hydrophobic) molecules can directly cross the plasma membrane. For example, even though water is a polar substance, it is a very small molecule and thus, it can cross the plasma membrane through the gaps between phospholipid molecules. Other substances which can cross include O2, CO2, urea and steroids. Of course, however, cells are still able to transport larger molecules e.g. glucose into the cell, however these occur through other methods which are outlined below.
Passive transport is the movement of molecules across the plasma membrane without any additional energy, moving down their concentration gradient. This is further categorised into simple diffusion, osmosis and facilitated diffusion.
Simple diffusion is the passive net movement of molecules from a region of high concentration to a region of low concentration, allowing for very small and nonpolar molecules to diffuse directly across the plasma membrane
Osmosis is the passive net movement of free water molecules from a region of low solute concentration to a region of high solute concentration across a semi-permeable membrane. Solutions can be described using the terms hypertonic, hypotonic and isotonic, which essentially compare the relative concentration of solutes dissolved in a cell and the solution surrounding it. A solution is hypertonic when its solute concentration is higher than that of within the cell. Therefore, water will move out of the cell. Conversely, if the solution is hypotonic, it will have a lower solute concentration than that of within the cell. Therefore, water will move into the cell. On the other hand, a solution is isotonic when the concentration of solutes in the cell and the outside solution are equal, resulting in no net movement of water molecules. However, water still readily moves back and forth, but there is no net change as the amount of water leaving the cell is equal to the amount of water entering the cell.
Facilitated diffusion is the passive net movement of molecules across the plasma membrane via protein channels or protein carriers from a region of high concentration to a region of low concentration. Channel proteins are transmembrane proteins which help create hydrophilic passages for polar, charged and hydrophilic substances e.g. glucose, potassium and sodium ions.
Active transport is the energy requiring movement of molecules across the plasma membrane generally from a region of low concentration to a region of high concentration via protein pumps. This process enables cells to dispose of unwanted substances and to take up nutrients even when the concentration outside the cell is very low.
Active Transport - Protein Pump
Exocytosis is the energy dependent bulk transport of large molecules out of a cell through the formation of a vesicle within the cell, which fuses with the plasma membrane before the contents are secreted.
Endocytosis is the energy dependent bulk transport of large molecules into a cell through the formation of a vesicle from the plasma membrane. Phagocytosis is the term used for solids, while pinocytosis is used for fluids.