Chapter 5: Plasma membranes

5.1 the structure and function of membranes

Membrane structure All the membranes in a cell have the same basic structure. The cell surface membrane which separates the cell from its external environment is known as the plasma membrane. Membranes arc formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane. Cells normally exist in aqueous environments. The inside of cells and organelles arc also usually aqueous environments. Phospholipid bilayers arc perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.

Cell membrane theory Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution. Images taken in the 1950s showed the membrane as two black parallel lines - supporting an earlier theory that membranes were composed of a lipid bilayer. In 1972 American scientists Singer and Nicolson proposed a model, building upon an earlier lipid-bilayer model, in which proteins occupy various positions in the membrane. The model is known as the fluid- mosaic model because the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position (in the same way as the tiles of a mosaic) . This model forms the basis of our understanding of membranes today.

Intrinsic proteins Intrinsic proteins, or integral proteins, arc transmembrane proteins that arc embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core or the membrane, keeping them in place. Channel and carrier proteins arc intrinsic proteins. They are both involved in transport across the membrane. Channel proteins: provide a hydrophilic channel that allows the passive movement or polar molecules and ions down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins. Carrier proteins: have an important role in both passive transport (down a concentration gradient) and active transport (against. a concentration gradient) into cells . This often involves the shape of the protein changing.

5.2 Factors affecting membrane structure

Temperature Phospholipids in a cell membrane are constantly moving. When temperature is increased the phospholipids will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure. If temperature continues to increase the cell will eventually break down completely. This loss of structure increases the permeability of the membrane, making it easier for particles to cross it. Carrier and channel protein s in the membrane will be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.

Solvents Water, a polar solvent, is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipids are orientated away from the water. forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping to keep the bilayer intact. Many organic solvents are less polar than water for example alcohols, or they are non-polar like benzene. Organic solvents will dissolve membranes, disrupting cells. This is why alcohols are used in antiseptic wipes. The alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection. Pure or very strong alcohol solutions are toxic as they destroy cells in the body. Less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but still cause damage. The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane. When the membrane is disrupted it becomes more fluid and more permeable. Some cells need intact cell membranes for specific functions, for example, the transmission of nerve impulses by neurones (nerve cells). When neuronal membranes are disrupted. nerve impulses are no longer transmitted as normal.

5.3 Diffusion

Diffusion Diffusion is the net. or overall, movement of particles (atoms, molecules or ions) from a region of higher concentration to a region of lower concentration. lt is a passive process and it will continue until there is a concentration equilibrium between the two areas. Equilibrium means a balance or no difference in concentrations. Diffusion happens because the particles in a gas or liquid have kinetic energy. This movement is random and an unequal distribution of particles will eventually become an equal distribution. Equilibrium doesn't mean the panicles stop moving, just that the movements are equal in both directions. Particles move at high speeds and are constantly colliding, which slows down their overall movement. This means that over short distances diffusion is fast, bm as diffusion distance increases the rate of diffusion slows down because more collisions have taken place. For this reason cells are generally microscopic- the movement of particles within cells depends on diffusion and a large cell would lead to slow rates of diffusion. Reactions would not get the substrates they need quickly enough or ATP would be supplied too slowly to energy- requiring processes.

Diffusion across membranes Diffusion across membranes involves particles passing through the phospholipid bilayer. It can only happen if the membrane is permeable to the particles. Non-polar molecules such as oxygen diffuse through freely down a concentration gradient. The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions), so they cannot easily pass through. Polar molecules, such as water with partial positive and 2 negative charges can diffuse through membranes, but only at a very slow rate. Small polar molecules pass through more easily than larger ones. Membranes are therefore described as partially permeable. The rate at which molecules or ions diffuse across membranes is affected by: surface area - the larger the area of an exchange surface, the higher the rate of diffusion thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion.

Factors affecting rate of diffusion temperature: the higher the temperature the higher the rate of diffusion. This is because the particles have more kinetic energy and move at higher speeds. concentration difference: the greater the difference in concentration between two regions the faster the rate of diffusion. because the overall movement from the higher concentration to lower concentration will be larger.

5.4 Active transport

Active transport Active transport is the movement o[ molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. The process requires energy and carrier proteins. Energy is needed as the particles are being moved up a concentration gradient. in the opposite direction to diffusion. Metabolic energy is supplied by ATP. Carrier proteins span the membranes and act as 'pumps'. The general process of active transport is described below - in this example transport is from outside to inside a cell

Bulk transport Bulk transport is another form of active transport. Large molecules such as enzymes, hormones, and whole cells like bacteria arc too large to move through channel or carrier proteins. so they arc moved into and out of cell by bulk transport. Endocytosis is the bulk transport of material into cells. There are two types of endocytosis. phagocytosis for solids and pinocytosis for liquids - the process is the same for both. The cell-surface membrane first invaginates (bends inwards) when it comes into contact with the material to be transported. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell. For example, vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes. Exocytosis is the reverse of endocytosis. Vesicles. usually formed by the Golgi apparatus, move towards and fuse with the cell surface membrane. The contents of the vesicle are then released outside of the cell.

5.5 Osmosis

Water potential A solute is a substance dissolved in a solvent {[or example water) forming a solution. The amount of solute in a certain volume of aqueous solution is the concentration. Water potential is the pressure exerted by water molecules as they collide with a membrane or container. It is measured in units of pressure pascals (Pa) or kilopascals (kPa). The symbol for water potential is the Greek letter psi 'l'. Pure water is defined as having a water potential of 0 kPa (at standard temperature and atmospheric pressure - 25 oc and I00 kPa). This is the highest possible value for water potential, as the presence of a solute in water lowers the water potential below zero. All solutions have negative water potentials- the more concentrated the solution the more negative the water potential.

Effects of osmosis on plant and animal cells The diffusion of water into a solution leads to an increase in volume of this solution. If the solution is in a closed system, such as a cell, this results in an increase in pressure. This pressure is called hydrostatic pressure and has the same units as water potentiaL kPa. All the cellular level this pressure is relatively large and potentially damaging. Animal cells: If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell. All cells have thin cell-surface membranes (around 7nm) and no cell walls. The cell-surface membrane cannot stretch much and cannot withstand the increased pressure. It will break and the cell will burst, an event called cytolysis. Plant cells: Like animal cells, plant cells contain a variety of solutes, mainly dissolved in a large vacuole. However. unlike animals, plants are unable to control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water.  Plants cells have strong cellulose walls surrounding the cell-surface membrane. When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls. This pressure against the cell wall is called turgor.