1.3 Cell membrane

• The width of the cell membrane does not vary between organisms it is 7-8 nm (as

  measured with an electron microscope).
• The cell surface membrane or plasma membrane is the boundary that separates

  the living cell from its non-living surroundings.
• The cell membrane controls which substances pass into and out of the cell.
• The cell membrane controls the uptake of nutrients.
• The cell membrane is responsible for secreting substances such as enzymes and glycoproteins.
• The cell membrane allows waste products to pass out of the cell.
• The cell membrane is used for cell recognition
• The cell membrane is made up of almost entirely

  phospholipids and proteins (and a small amount of sugar).
• Phospholipids can form bilayers, with one sheet of phospholipid forming

  over another. The phosphate head of the phospholipid is a polar molecule (hydrophilic)

  and is attracted to other polar molecules such as water. The 2 fatty acid tails of the

  phospholipid are non-polar (hydrophobic) and repel water. This phospholipid bilayer

  forms the basis of membrane structure.
• The phospholipid bilayer allows lipid-soluble

  (non-polar) molecules to enter and leave the cell, but prevents water soluble (polar)

  molecules from doing so.
• In the membrane the proteins are arranged randomly in contrast to

  the more regular patterns of phospholipids.
• Extrinsic proteins occur on the surface of the

  bilayer, or are partly embedded in it. They provide structural support. They also form

  recognition sites by identifying cells.
• Intrinsic proteins span (go right through) the

  phospholipid bilayer; some act as channels or carriers to facilitate the diffusion of polar

  (water soluble) molecules, such as ions, across the cell membrane. Other intrinsic proteins

  form pumps and carry out active transport against a concentration gradient.
• Singer and Nicholson proposed the fluid

  mosaic model. The phospholipids are fluid as each molecule can move in relation to the

  others within the membrane. The proteins form a mosaic pattern within the phospholipid

  bilayer.
• The phospholipid layer is capable of movement. Components of the

  membrane are free to move with respect to each other. This is why

  Singer and Nicholson’s model is called the Fluid Mosaic model.
• Cholesterol is found in animal cells. It fits between the phospholipid

  molecules, increasing the rigidity and stability of the membrane.
• Glycolipids are lipids which have combine with polysaccharide; they

  are found in the outer layer of the membrane and are involved in cell

  to cell recognition.
• Glycoproteins (proteins combined with polysaccharide) also stick out

  of some membranes.
• The cell surface membrane is selectively permeable to

  water and some solutes. Lipid soluble (non-polar) substances can move through the

  membrane more easily than water-soluble (polar) substances
• Small uncharged molecules, such as oxygen and carbon dioxide,

  freely pass through the membrane by simple diffusion. Lipid soluble molecules such as

  glycerol can also pass through the membrane, through the phospholipid bilayer.
• The hydrophobic core of the membrane impedes the transport of ions

  and polar molecules. Charged particles, and relatively large

  molecules such as glucose cannot diffuse across the non-polar centre of

  the phospholipid bilayer as they are insoluble in lipid. Intrinsic proteins allow these particles to cross the membrane.
• Channel proteins and carriers allow facilitated diffusion (diffusion helped by an intrinsic protein).

  Pumps carry out active transport.
• tonoplast- the

  membrane which surrounds the vacuole in plant cells
• At temperatures above 40C the cell

  membrane and tonoplast become increasingly unstable.

  Increased heat energy leads to increases kinetic energy. The

  phospholipids vibrate more and more and move further

  apart. This increases the permeability of the membrane.
• Organic solvents such as ethanol dissolve phospholipids. The

  greater the concentration of ethanol the more permeable

  the membranes become.
• As sodium chloride concentration

  increases the permeability will decrease. Sodium ions (Na+) attach to the oxygen atoms on the

  hydrophilic (phosphate) heads of the phospholipid bilayer.

  This reduces mobility of the phospholipid molecules so less

  betalain is released.
• Detergents reduce surface tension of phospholipids and

  disperse the membrane. As the concentration of detergent

  increases the permeability of the membranes increase.
• Molecules or particles in a liquid or gas move randomly, but if they are highly

  concentrated in one area there will be a net movement away from that area until

  equilibrium is reached (uniform distribution). This process is called diffusion; it is a

  passive process.
• Diffusion is the movement of molecules or ions from a region where they are in high

  concentration to a region of lower concentration until they are equally distributed.

  Molecules move down a concentration gradient.
• Diffusion-This is a passive process, which needs

  no ATP from the cell.
• A concentration gradient

  is the difference in concentration

  between two areas, e.g. outside

  and inside a cell. Cells can

  maintain a concentration gradient

  as the substances transported into

  the cell by diffusion are either

  used up or transported

  somewhere else.
• Unless the molecule is used up by the cell equilibrium will be reached, which means the

  concentration of molecules is equal either side of the membrane. At equilibrium

  molecules and particles continue to cross the membrane in both directions, but there is

  no net movement in a particular direction.
• The greater concentration gradient (the difference in

  concentration of ions of molecules in two areas) the

  greater the rate of diffusion.
• The shorter the distance of travel the greater the rate of

  diffusion.
• The larger the surface area of the membrane the greater the rate of

  diffusion.
• The thinner the membrane the greater the rate of

  diffusion (the diffusion path is short).
• An increase in temperature increases molecular kinetic

  energy and therefore increases the rate of diffusion.
• Small particles diffuse faster than larger molecules.
• Simple diffusion: Diffusion of non-polar molecules such as oxygen and carbon dioxide occurs across the

  phospholipid bilayer.
• Polar molecules cannot cross the

  phospholipid bilayer and therefore must use an intrinsic (membrane spanning) protein to

  facilitate transport across the membrane, this is called facilitated diffusion.