An intact membrane is essential to a cell. If the plasma membrane is disrupted, the cell loses its content and dies. The membrane is very important; two vital functions are:
The regulation of the entrance and exit of molecules: The interior and exterior of the cell is mainly fluid. The membrane functions to keep the intracellular fluid constant despite molecules such as nutrients and waste constantly moving in and out.
Communication: The components of a membrane signal other cells as to what type of cell it is. It may also serve as receptors for various signal molecules that affect the cell’s metabolism.
Membrane Models:
At the beginning of the last century, scientist noted that lipid-soluble molecules entered the cells more rapidly than water-soluble molecules. This caused them to think that lipids were a component of the plasma membrane.
Later it was discovered that it consists of phospholipids and proteins. Phospholipids are lipids in which one of the fatty acid groups is replaced by H3PO4. The phosphoric acid is hydrophillic, the rest of the molecule is hydrophobic.
The ‘Fluid’ Membrane
A membrane is held together by weak hydrophobic interactions. Most membrane lipids are able to drift laterally within the membrane and occasionally flip vertically, known as ‘flip-flopping’. Phospholipids move quickly along the membrane plane, where as the proteins move relatively slowly.
Unsaturated hydrocarbon tails enhance membrane fluidity because the kinks at the carbon-carbon double bonds hinder close packing of the phospholipids. Membranes solidify at the critical temperature. This is lower in a membrane with a higher concentration of C=C bonds.
Cholesterol found in the plasma membranes of eukaryotes modulates membrane fluidity by keeping the membrane fluid in cold environments and solid in hot temperatures. Cells may also the concentration of unsaturated fats to better suit their environment.
Integral proteins, which are inserted into the membrane have hydrophobic regions, surrounded by the hydrophobic areas of the phhospholipids. Their hydrophillic ends are exposed at both sides of the membrane.
The proteins in the plasma membrane may provide a variety of major cell functions:
- Transport
- Intercellular joining
- Enzymatic activity
- Cell-cell recognition
- Signal transduction
- Attachment to the cytoskeleton and extracellular fliud.
Substances can be moved through the membrane via: Active Transport; Diffusion; and Osmosis.
Active Transport is the movement of molecules from an area of low solute concentration to an area of high solute concentration, against the concentration gradient, in a process that requires energy.
Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration.
Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a semi-permeable membrane.
Diffusion is influenced by:
- The permeability of the membrane
- The shape and size of the molecule to be transported
- Number of proteins on the cell surface
- Concentration of molecules on either side of the membrane
- Surface area of the membrane
Facilitated diffusion is a method of diffusion that uses proteins to transport substances that find it difficult to pass through a membrane. e.g. polar molecules. The proteins are known as carrier proteins.
Fick’s Law
Rate of diffusion is proportional to the surface area multiplied by the difference in concentration, all divided by the thickness of the membrane.
Rate of diffusion (fish) Surface area x Concentration gradient
thickness of membrane
Osmosis – Thermodynamics
1 molecule of water will move quickly if heat is applied, or if the water concentration in a solution is high.
If water molecules are moving from left to right, then the potential energy is greater on the left than on the right. The potential energy is known, in Biology, as water potential.
Water diffuses from an area of high water potential to a region of low water potential through a semi permeable membrane. Water potential can be regarded as the tendency of water to leave a solution.
If solute molecules are present, they always slow down the movement of the water molecules in a solution. The tendency of the water to leave the solution is reduced because water is always attracted to the solute.
Water Potential Gradient
High Pure water = 0 kPa
Dilute solution = -500 kPa
Low Concentrated solution = -1000 kPa
Water potential is never positive. When the potential is more negative, water will flow into the cell.
Isotonic – two solutions are of the same water concentration, and as such there is no net movement of water.
Hypotonic – The water potential outside of the cell is greater than the intracellular potential. As such, there is a net inflow of water. The inside of the cell is more negative.
Hypertonic – The water potential inside of the cell is greater then the extracellular potential. As such, there is a net outflow of water. There inside of the cell is less negative.
Active Transport
Active transport requires energy in the form of ATP. it trasports molecules and ions in a direction that is not natural to the normal flow. This means that there will be many mitochondria present.
The following use ATP to transport molecules and ions:
1. Membrane pumps
- An active transport mechanism that moves ions in order to obtain polarisation
- For active transport two factors need to be considered: concentration and electrical charge.
- Ions generally diffuse to form an area of high concentration to an area of low concentration and are attracted to regions with an opposite charge. Therefore we take into consideration both the concentration and elecrtochemical gradient.
- Cells maintain a potential difference across the membrane. Many studies have shown that the inside of a cell is -ve and therefore cations are attracted and anions repulsed.
- however, their relative concentrations inside and outside the of the cells helps to decide which way they move.
- Three common ions to be transported are K+, Na+ and Cl-
1. Sodium Potassium pump
- Cell surface membranes have pumps that are intrinsic proteins that span the membrane. The sodium pump removes Na+ from the cell. K+ is taken into the cell and so is coupled with the Na+ pump. It is therefore known as the Na+/P+ pump.
- The pump requires more than one third of the ATP produced by a resting animal. It is very important.
- The pump is essential for:
- controlling cell volume (osmoregulation)
- Maintaining electrical activity in nerve and muscle cells
- Driving active transport of other substances (e.g. sugars and amino acids.)
Active transport in the intestine:
Soon after feeding there is a high concentration of food in the gut. Absorption is mainly due to diffusion but it is very slow and so it is coupled with the active transport and the movement of Na+. As the sodium is actively transported out by the Na+/K+ pump, it will start to diffuse back in. A membrane rquires both Na+ and glucose and so another pump is used that transports glucose at the same time as Na+.
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