Plasma membrane components

• Integral/intrinsic protein


  A protein that is permanently embedded in the membrane, as they are hydrophobic on at least one area of their surface. They are mostly transmembrane proteins- meaning that they extend across the membrane. They allow molecular transport through the membrane, and if they contain a channel, they are known as channel proteins. Some of these proteins may also be enzymes with active sites exposed to the extracellular fluid. Those enzymes are organised to carry out reactions as part of metabolic activity.
• Channel protein


  Intrinsic proteins with a channel within them. The channel is a pore that connects the cytoplasm to the aqueous solution outside of the cell. Both the diameter of the pore and its chemical properties determine that only one specific type of particle can pass through - for example, a specific ion. They are used in facilitated diffusion. Some channels can also be opened and closed. They are orientated specifically within the membrane for optimum function. The more active a membrane, the higher its protein content.
• Peripheral/extrinsic protein


  Do not cross the membrane, and are involved with transportation of materials to various locations within the cell, signalling, moving molecules through the bilayer and maintaining cell shape. They are also hydrophilic, and most are attached to the surface of integral proteins. They are orientated specifically within the membrane for optimum function. The more active a membrane, the higher its protein content.
• Pump proteins/globular protiens - They use energy (ATP from aerobic cell respiration) to carry out active transport. They only move particles across the membrane in one direction, against the concentration gradient. The molecule or ion enters the globular protein and binds in its central chamber. Then the hydrolysis process of ATP releases energy that changes the shape of the globular protein, (conformational change- from one state (stable) to the other (less stable) – the reverse change does not require energy) meaning the molecule or ion is able to pass through the membrane.
• Fatty acid/lipid tail

  They are the lipid tails of the phospholipids that make up the bilayer. They are also hydrocarbon chains. As they are lipids, they are hydrophobic, meaning that they protect the cell by having low permeability to hydrophilic particles, such as ions.
• Phosphate head


  They are the heads of the phospholipid bilayer. Each head has two lipid tails, and two sets of these tails face each other in the core of the amphipathic bilayer. The phosphate heads are polar, and therefore hydrophilic, meaning that they attract water.
• Carbohydrate branch /tail

  a branch of carbohydrates that are involved in cell-to-cell recognition and cell protection. They appear on the outside of the cell. They are usually 3-10 sugar units
• Glycoprotein

  Consists of proteins and carbohydrate tails. they are involved in cell-to-cell interactions, such as cell adhesions and recognition. They appear on the outside of a cell. With glycolipids, they form a carbohydrate-rich layer on the outside of the membrane, known as the glycocalyx. The glycocalyx of adjacent cells can fuse, binding the cells together and preventing tissue from falling apart.
• Glycolipid

  Consists of a lipid tail and a carbohydrate tail. They maintain the stability of the plasma membrane and aids cellular recognition. They appear on the outside of a cell. With glycoproteins, they form a carbohydrate-rich layer on the outside of the membrane, known as the glycocalyx. the glycocalyx of adjacent cells can fuse, binding the cells together and preventing tissue from falling apart.
• Cholesterol


  Are within the lipid bilayer, and are there for added strength.
• Phospholipid bilayer

  The membrane may be called a semi-permeable phospholipid bilayer with proteins. The phospholipid bilayer is amphipathic - both hydrophilic and hydrophobic. it is arranged as two sheets with molecules of each sheets aligned in the same direction. Sheet-like Phospholipid bilayers form spontaneously in water due to their hydrophilic and hydrophobic regions
• Functions of the phospholipid bilayer

  Form continuous sheet-like barriers in water

  They may be used for borders between a cell and its environment or to divide the cell into compartments.

  It allows cells to have different internal conditions to their environment, either side of it to have an aqueous solution, and its core has low permeability to large and hydrophilic particles. Hydrophilic molecules may penetrate the phospholipid heads but then are more attracted to the aqueous solution than the hydrophobic tails so they move back out of the membrane.
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  Phospholipids

  Made of two fatty acids, a phosphate group, and a glycerol molecule. Fatty acids are hydrocarbon chains, while phosphate groups are phosphorus molecules with four oxygen molecules attached. These two components are connected via a third molecule, glycerol.

  Contain polar, charged, phosphate heads

  The fatty acids tails are attactched to glycerols, forming tri-glyceride if they have three tails- they are non-polar
• Factors affecting the fluidity of the phospholipid bilayer

  If they are saturated or unsaturated affects the fluidity of the membrane:

  Saturated hydrocarbon tails are packed together, which increases membrane viscosity.

  Unsaturated hydrocarbon tails are kinked, which prevents packing and enhances membrane fluidity.

  At low temperatures, cholesterol hinders solidification by disrupting the regular packing of the phospholipids. At moderate temperatures, cholesterol hinders membrane fluidity by reducing phospholipid movement.