U1 AOS2 PLASMA MEMBRANE & TRANSPORT

Describe the function and features of the plasma membrane

separates the internal components of a cell from the external environment

• Semipermeable: some material cannot cross the membrane without assistance

• Selective: cell can regulate the passage of these materials, allowing only specific molecules/ions/substances to pass through

Outline the 3 principle components of the plasma membrane

1. Phospholipid bilayer 

2. Proteins 

3. Cholesterol

What is the fluid-mosaic model

Representation of the organisation of the plasma membrane.

• Fluid - the phospholipid bilayer is viscous (due to weak hydrophobic associations) = membrane components can move position

• Mosaic - it is embedded with proteins = mosaic of components 

Outline the structure of the phospholipid bilayer

Phospholipid = amphipathic (both hydrophilic and phobic)

Polar phosphate head and two non-polar fatty acid tails

• Head = hydrophilic

• tail = hydrophobic

Outline how the phospholipid bilayer is arranged in membranes

Spontaneously arrange into a bilayer → hydrophilic heads facing out toward (aq) solution + hydrophobic tails inwards to avoid exposure to polar fluids  Bilayer is held together by weak hydrophobic associations between the tails

Outline the properties of the phospholipid bilayer

Hydrophilic / hydrophobic layers restrict the passage of many substances Individual phospholipids are fluid = provide membrane with flexibility Membrane can break and reform → allows vesicular transport (cytosis - active transport mechanism where cells move large quantities of particles across cell membrane using membrane-bound vesicles)

State the 3 main functions of integral proteins

• Transport - facilitates passive or active movement of molecules that cannot freely cross the bilayer 

• Metabolic activity - enzymes may be bound to membrane to localise activity and receptors can detect signalling molecules

• Connections: membrane proteins can join cells together or act as attachment point for intra/extra cellular components

Integral proteins

permanently attached to the membrane and are transmembrane (span across the bilayer)

Peripheral proteins

temporarily attached by non-covalent interactions and associate with one surface of the membrane 

Describe the function of cholesterol in a phospholipid bilayer

• Amphipathic molecule (like phospholipids)

• Acts as a barrier and fits in the spaces between phospholipids → prevents water soluble substances from diffusing across the membrane

• interacts with the fatty acid tails of phospholipids → acts as a modulator of membrane fluidity

Cholesterol functions as a modulator

• It works to immobilize the outer surface of the membrane to reduce fluidity

• Makes the membrane less permeable to very small polar molecules which would otherwise freely cross

• Helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

• At low temp = disrupts the close-packing/crystallisation of phospholipids & increases membrane flexibility 

Effect of temperature on membrane fluidity

• High temp = more fluid

• Low temp = solidifies or becomes static because of the restriction in movement of molecules that compose the membrane

Describe what movement of material depends on

Size and solubility

• Small, non-polar molecules can freely pass across phospholipid bilayer - O2, CO2, H2O, steroids (cholesterol)

• Larger molecules (glucose) or polar molecules (ions) require membrane proteins in order to cross → nonpolar tails repel polar/charged substances

*all hydrophobic molecules, no matter their size, can get through

Outline the 3 types of transport

1. Passive transport: movement of material down a concentration gradient (high → low)

• Does not require expenditure of energy (ATP hydrolysis)

• Simple diffusion, facilitated diffusion, osmosis 

2. Active transport: movement against concentration gradient 

• Requires expenditure of energy

3. Bulk transport: (cytosis) involves materials entering or leaving cell via vesicles (membrane breaks and reforms)

• Materials do not cross membrane directly and can move along/against grad.

• Active process (requires ATP) but is NOT active transport 

Simple Diffusion

The net movement of molecules from a region of high concentration to low concentration

• Directional movement along a gradient is passive and will continue until molecules reach an equilibrium

• Small and non-polar (lipophilic) molecules can freely diffuse across - CO₂, O₂, glycerol
  

Factors affecting rate:

• Temperature (affects K.E.)

• Molecular size 

• Size of concentration gradient

Define facilitated diffusion

Passive movement of molecules across the cell membrane via the aid of a membrane protein

• Utilised by molecules unable to freely cross phospholipid bilayer → large, polar molecules and ions

Outline the proteins that mediate transport

• Channel proteins: hydrophilic pore and may be gated to regulate the passage of ions in response to certain stimuli 

• Carrier proteins: undergo conformational change to translocate solutes and have a comparably slower rate of transport 

Define osmosis

The net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to high solute concentration (until equilibrium is reached)

↳ water = universal solvent → associate with and dissolve polar/charged solutes

↳ solutes cannot cross a membrane unaided so water will move to equalise the two solutions

↳ high solute conc. = less free water molecules because water is associated w/ solute

∴ osmosis = diffusion of free water molecules and hence occurs from regions of low solute concentration

Describe the effects of different tonicities on osmotic pressure

Impacts protoplast

1. Hypertonic: higher solute concentration (will lose water) → turgor pressure (cw), crenated (cells w/o cw)

2. Hypotonic: lower solute concentration (will gain water) → plasmolysis (cw), burst (cells w/o cw)

3. Isotonic: same solute concentration (no net water flow)

Describe the mechanism of Bulk (Vesicular) transport

The fluidity of membranes allow materials to be taken in or released by cells without crossing the phospholipid bilayer 

• Membrane held together by weak hydrophobic associations between fatty acid tails which can be broken and reformed via ATP hydrolysis. 

• Membrane segments can be excised to form internal vesicles, while new segments are added when membrane + vesicles fuse

Define ENDOCYTOSIS

Large substances (or bulk of small substances) entering the cell without crossing the membrane

• The membrane forms a flask-like invagination which engulfs extracellular material 

• The invagination is then sealed off to form an intracellular vesicle containing the material

Define the two types of endocytosis

1. Phagocytosis: process where solid substances are ingested (usually tb transported to lysosome)

2. Pinocytosis: process where liquid/dissolved substances are ingested (allows faster entry via protein channels)

Define EXOCYTOSIS

Substance exiting the cell without crossing the membrane (must be packaged in vesicles)

Describe the mechanism of exocytosis

• Polypeptides synthesized within the rER will be packaged into a vesicle → golgi apparatus 

• Golgi complex will potentially modify the protein then export it out of the cell via a vesicle 

• Proteins can either be secreted by golgi immediately (constitutive secretion) or stored for delayed release (regulatory secretion)

• Proteins synthesized via this pathway can also be shuttled to other organelles (mitochondria, lysosome)