Membranes

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Structure of Membranes

Cell membranes consist of a phospholipid bilayer embedded with proteins and carbohydrates (and cholesterol in animal cells)

This arrangement is called the fluid mosaic model because the bilayer can shift position (fluidity) and is embedded with other molecules (mosaic) 


Structure of the Plasma Membrane


Phospholipid Bilayer

Structure of Phospholipids

  • Consist of a polar head (hydrophilic) made from glycerol and phosphate
  • Consist of two non-polar fatty acid tails (hydrophobic)


Arrangement in Membrane

  • Phospholipids spontaneously arrange in a bilayer
  • Hydrophobic tail regions face inwards and are shielded from the surrounding polar fluid while the two hydrophilic head regions associate with the cytosolic and extracellular environments respectively


Structural Properties of Phospholipid Bilayer

  • Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations)
  • Hydrophilic / hydrophobic layers restrict entry and exit of substances 
  • Phospholipids allow for membrane fluidity / flexibility (important for functionality)
  • Phospholipids with short or unsaturated fatty acids are more fluid
  • Phospholipids can move horizontally or occasionally laterally to increase fluidity
  • Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis)


Functions of Membrane Proteins


Transport:  Protein channels (facilitated) and protein pumps (active)

Receptors:  Peptide-based hormones (insulin, glucagon, etc.)

Anchorage:  Cytoskeleton attachments and extracellular matrix

Cell recognition:  MHC proteins and antigens

Intercellular joinings:  Tight junctions and plasmodesmata

Enzymatic activity:  Metabolic pathways (e.g. electron transport chain)


Properties of Membranes

  • Cell membranes have two key properties:
    • They are semi-permeable - meaning some things can cross the membrane unaided (small, lipophilic) while other things cannot (large, hydrophilic)
    • They are selective - meaning cells can regulate the passage of certain materials across the membrane (via protein channels)


  • There are three ways materials can pass across the membrane:
    • Pores:  Small hydrophilic materials (e.g. water, urea) and uncharged molecules (e.g. oxygen, CO2) may pass through gaps between phospholipids
    • Lipids:  Lipid-soluble substances (e.g. alcohol, chloroform) can dissolve into the bilayer and thus cross the membrane
    • Proteins:  Large, water-soluble substances (e.g. monosaccharides, amino acids) may pass through protein channels


  • All transport across the cell membrane involves one of two processes:
    • Passive Transport:  Involves the movement of material along a concentration gradient (high to low) and does not require the expenditure of ATP
    • Active Transport:  Involves the movement of materials against a concentration gradient (low to high) and requires the expenditure of ATP


Types of Passive Transport

Simple Diffusion:  

  • The net movement of particles from a region of high concentration to a region of low concentration (along the gradient) until equilibrium
  • Simple diffusion occurs for molecules that can freely cross the membrane (small or lipophilic)


Osmosis:  

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

Osmosis


Facilitated diffusion:

  • Larger, polar substances (ions, macromolecules) cannot freely diffuse and require the assistance of transport proteins (carrier proteins and channel proteins) to facilitate their movement (facilitated diffusion)


Simple Diffusion versus Facilitated Diffusion


Types of Active Transport

Active Transport:

  • The passage of materials against a concentration gradient (from low to high) by use of protein pumps (which use the energy from ATP)
  • The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance
  • Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients)
  • An example of an active transport mechanism is the Na+/K+ pump which is involved in the generation of nerve impulses


Facilitated Diffusion versus Active Transport


Bulk Transport:

  • The membrane is principally held together by hydrophobic associations between phospholipids, which are the relatively weak and easy to form
  • This allows for the breaking and remaking of membranes, allowing larger substances access into and out of the cell



Endocytosis:

  • The process by which large substances (or bulk amounts of smaller substances) enter the cell without travelling across the plasma membrane
  • An invagination of the membrane forms a flask-like depression which envelopes the material; the invagination is then sealed off forming a vesicle
  • There are two main types of endocytosis:
    • Phagocytosis:  The process by which solid substances (e.g. food particles, foreign pathogens) are ingested 
    • Pinocytosis:  The process by which liquids / solutions (e.g. dissolved substances) are ingested by the cell (allows quick entry of large amounts) 


Exocytosis

  • The process by which large substances exit the cell without travelling across the plasma membrane
  • Vesicles (usually derived from the golgi) fuse with the plasma membrane expelling their contents into the extracellular environment 


The Process of Exocytosis


Vesicular Transport

  • Polypeptides destined for secretion contain an initial target sequence (a signal recognition peptide) which directs the ribosome to the endoplasmic reticulum
  • The polypeptide continues to be synthesised by the ribosome into the lumen of the ER, where the signal sequence is removed from the nascent chain
  • The polypeptide within the rough ER is transferred to the golgi apparatus via a vesicle, which forms from the budding of the membrane
  • The polypeptide moves via vesicles from the cis face of the golgi to the trans face and may be modified along the way (e.g. glycosylated, truncated, etc.)
  • The polypeptide is finally transferred via a vesicle to the plasma membrane, whereby it is either immediately released (constitutive secretion) or stored for a delayed release in response to some cellular signal (regulatory secretion = for a more concentrated and more sustained effect)


Overview of Vesicular Transport


Surface Area to Volume Relationships

  • The rate of metabolism of a cell (i.e. its energy production and expenditure) is a function of its mass / volume
  • The rate of material exchange in and out of a cell is a function of its surface area
  • As the cell grows, volume increases faster than surface area (leading to a decreased SA:Vol ratio)
  • If the metabolic rate is greater than the rate of exchange of vital materials and wastes, the cell will eventually die 
  • Hence the cell must consequently divide in order to restore a viable SA:Vol ratio and survive
  • Cells and tissues specialised for gas or material exchange (e.g. alveoli) will increase their surface area to optimise the transfer of materials


Microvilli increase surface area allowing for a more efficient exchange of materials / heat