Membrane structure and function

• Physical barriar- separates intracellular fluid from extracellular fluid
• Controls entry and exit of ions and nutrients
• Contains proteins which enable the cell to recognise and respond to molecules or changes in the external environment

Contains proteins which support the cell:

• Hold the cytoskeleton in place
• Create junctions between each cell and between the cell and the extracellular matrix

• The fluid mosaic model refers to the fluidity of the membrane structure
• Phospholipids create the phospholipid bilayer
• There are two phospholipid layers arranged tail to tail

Proteins within the bilayer:

Integeral proteins-

• stick out at either end of the bilayer
• Transport large molecules such as glucose across cell membrane

Peripheral proteins-

• Only found on one side of the membrane
• Help with transport and communication

Some proteins can move freely/bob about within the membrane causing a changing pattern or "mosaic"

Cholesteral makes the membrane stiff and more difficult for molecules to pass

• A type of passive transport
• Movement of water molecules from high to low concentration
• Movement of solvent (the liquid in which the molecule is dissolved in)
• Water molecules are polar which means they need channel proteins to move down their concentration gradient
• Movement occurs across a selectively permiable membrane

Different types of solutions:

Hypertonic
Water flows out of the cell leaving the cell to shrink

Hypotonic
The cell swells from a lot of water flowing in

Isotonic
The cell remains the same as there is the same amount of water flowing in and out of the ce

• Diffusion occurs directly over the phospholipid bilayer
• Does not need the help of membrane proteins
• Transport of smaller non polar molecules

• No ATP is needed
• Substances move along the concentration gradient
• Molecules move from high to low concentration

Types

• Osmosis
• Simple
• Facilitated

• Needs a transport or channel protein
• Concentration gradient across membrane and amount of carrier protein available both affect facilitated diffusion
• Transport of polar molecles- glucose and amino acids
• Transport of larger ions- sodium and chloride
• Allow polar and charged molecules to avoi hydrophobic core of plasma membrane

• Create hydrophillic tunnels across the plasma membrane
• Selectively permiable
• Aquaporins are a type of channel protein which allow water to cross the membrane quickly
• Gated channels- the channel can open and close in response to a signal
• Ion channels- highly selective

Different types of ion channels:

Ligand gated- Permiability is increased when binded to a specific ligand

Volt gated- Open and close in response to changes in transmembrane voltage
Play a key roll in electrical signalling by excitable cells such as neurons

Stretch activated- 

• Can change their shape to move a molecule from one side of the membrane to another
• Selective
• Uniporters
• Antiporters
• Cotransporters

Active transport

• ATP is needed
• Substances move against concentration gradient
• Substance moves from an area of low concentration to an area of high concentration
• Requires assistance from proteins
• For the movement of ions, glucose and amino acids

Secondary active transport

• One substrate moves down its concentration gradient whilst the other substrate moves against its concentration gradient

Types of active transport

• Endocytosis: the three types of endocytosis are: phagocytosis, pinocytosis and receptor mediated endocytosis
• Exocytosis
• Ion pumps

Phagocytosis

An example of phagocytosis would be when a white blood cell comes into contact with a bacterium. Phagocytosis is where the bacterium is engulfed by the cell membrane, this vesicle is pinched off and disposed of by the digestive enzymes in the lysosomes. Occurs in the following steps:

1. White blood cell identifies bacterium
2. The white blood cell engulfs the bacterium to create a large vesicle called a phagosome
3. The phagosome pinches off from the cell membrane and moves into the cytoplasm
4. The phagosome binds with the lysosome
5. The lysosomes digestive enzymes will destroy the bacterium

Pinocytosis

• Only deals with single celled molecules unlike phagocytosis
• Cell membrane indents rather than pushes out

Receptor mediated endocytosis

• Takes place in membrane regions called pits
• Receptor protein- Clathrin
• 9 step process

• Used to get rid of waste products from lysosomes and to export large lipophobic molecules
• Two groups of proteins involved:
  RABS- Helps vesicles dock onto the membrane
  SNARES- Facilitate membrane diffusion
• A large surge of calcium begins as a signal to start exocytosis

Resting membrane potential

• Electrical gradient between the extracellular fluid and the intracellular fluid
• Membrane potential is a non changing steady state
• Potential- the electrical gradient created by active transport of ions across the cell membrane is a form of stored/potential energy 
• Sodium (NA) is positively charged and is found in higher concentrations outside the cell than inside
• Potassium is positively charged and is found in higher concentrations inside the cell than outside
• More permiable to potassium than sodium
• Determined by uneven distribution of charged particles inside and outside of the cell