Week 6 plasma membrane: composition & functions Flashcards
Cell membranes main function
-Boundary that separates the cell from its surroundings -> found in all cells
-Provides structural integrity to the cell and protects its contents
-Exhibits selective permeability - allowing only specific molecules to pass through
-It plays a key role in interacting with other cells (glycoproteins) & cell signalling
Internal membranes of organelles allow compartmentalisation (for a specific composition -> specific
reactions/functions)
Lipid bilayer
Phospholipid Bilayer - the basic structural framework of the membrane
Lipids are arranged in two closely apposed sheets, forming a lipid bilayer, in which proteins are embedded and some carbohydrates
This creates a semi-permeable barrier, isolating the intracellular content
Fluid mosaic model
The fluid mosaic model - describes the dynamic and complex structure of the bilayer
MOSAIC:
-Phospholipids
-Glycolipids
-Sterols (cholesterol in mammalian cells)
-Various proteins
-Glycoproteins
FLUID:
Dynamic, fluid and flexible
->Phospholipids and proteins move laterally within the layer
Phospholipids
Phospholipids (75% lipids of the membrane) and are arranged in two-layer sheets
Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions
-Hydrophilic heads
-Hydrophobic tails
Hydrophilic heads
Outwards
A phosphate group and a glycerol
->Polar, allowing interacting with water
Hydrophobic tails
Nonpolar, repelling water and
interacting with other lipid tails
->Self-Assembly into the bilayer to shield water - no energy needed
Membrane fluidity
Phospholipid fluidity can lead to:
-Rapid lateral diffusion within the plane (107 times/sec)
-Spin in place (rotations – up to 500/sec.)
-Flexion (contraction movement)
-Very rarely, Flip-flop (side-to-side movement)
Membrane proteins :
-Usually, no movements -> are anchored to the cytoskeleton
-Only some proteins can slightly and slowly move driven by the motor proteins
Cholesterol
-20% of membrane lipids
Stabilise the membrane affecting its fluidity and stiffness
->Improve stability
->Reduce permeability - down molecule passage
Glycolipids
-5%
->Lipids containing sugars
Sugar groups (hydrophilic) face outwards -> asymmetry
Membrane proteins
-Integral proteins
-Peripheral proteins
Integral proteins (embedded)
Trans-membrane proteins spanning the membrane
->Integral proteins facing only one side
-Amphipathic nature:
->Hydrophobic amino acids lie in the bilayer
->Hydrophilic regions facing the aqueous environment
Peripheral proteins
-Temporary attached by interacting with membrane lipids or proteins
Functions of membrane proteins
Major functions of membrane proteins;
-Transport of molecules
-Enzymatic activity
-As receptors in cell signalling
-Structural support (linked to cytoskeleton)
Glycoproteins
With sugar groups facing outwards (Glycosylation occurs in RE or Golgi)
-Protection from mechanical damage
-Cell-to-cell communication
Transport across a cell membrane
Membranes are selectively permeable to control molecules / ions passage
Two types: Passive and Active Transport
Passive transport
-Movement of molecules/ions across membranes without energy expenditure
-Movement DOWN the concentration (or electrical) gradient;
->From areas with high concentration to area with a low concentration of the molecule, until the equilibrium is reached
Three categories: diffusion, osmosis and facilitated diffusion
Simple diffusion
Movement of small, nonpolar
molecules* that can diffuse
directly through the lipid bilayer
->No energy cost
->Down their concentration gradient
Small and non-polar -> O2, CO2, steroid hormones
Small uncharged polar molecules too, such as urea and ethanol
Each solute diffuses down its concentration gradient (independent to others)
Many molecules are too large or polar to directly diffuse the membrane
Osmosis
Movement of water across a cell
membrane from an area with higher water potential (lower solute conc.) to an area with lower water potential (higher solute conc.)
Water potential is inversely proportional to the solute’s concentration
->As solute concentration increases, water potential decreases
->When the solutes are unable to across, osmosis balances solute concentrations, to achieve equilibrium
Osmotic tonicity
Tonicity -> the surrounding solution can affect the volume / pressure of a cell by altering their water content, through osmosis
Isotonic solution
Solute concentration is equal inside and outside the cell
-No net water movement
Hypertonic solution
Solute concentration is higher outside than inside the cell
-Water moves out of the cell -> cell shrinkage
Hypotonic solution
Solute concentration is lower outside than inside the cell
-Water moves into the cell -> cell swelling
Effect of osmotic pressure on cells
Abnormal tonicity can damage cells
Shrivelled -> cell loses water (hypertonic)
Normal -> No net water movement (isotonic)
Lysed -> cell gains water (hypotonic)
Intravenous fluid
Intravenous fluids must match the bodies osmotic balance
To prevent cell shrinkage or
swelling -> major adverse effects
Isotonic only
Facilitated diffusion
The passive movement of molecules down a
concentration gradient (or electric potential) via a
transport protein (transmembrane proteins):
-Protein channel
-Protein carrier/transporters
They are;
->Highly specific (glucose vs. fructose)
->Reversible
->No energy (ATP) cost
Which molecules;
-Specific ions (e.g. Cl-)
-Hydrophilic small molecules (e.g. glucose)
-Water facilitated diffusion (via Aquaporins)
Facilitated diffusion - protein ion channels
Form a hydrophilic “tube / corridor” across the membrane through which specific molecules / ions diffuse
-Specific
-No molecule-channel interaction
-Rapid rate
-Can be gated -> open or close in response to signals
(e.g. voltage changes in neurons)
Protein carriers / transporters
-Uniporter -> one molecule/ion interaction at a time
1) The solute interacts with the protein carrier
2) Triggering a change in shape that translocate the solute across the membrane
-Specificity
-Slower rate than protein channels
Other protein carrier transports (symporters and antiporters) are active transport
Active transport
-Moves substances against their concentration or electrochemical gradients
-Requires energy, usually in the form of ATP
-Performed by specific membrane proteins or vesicles
-To maintain concentration gradients
Protein transporter – active transport
-Carriers that shift more than 1 molecule type at a time, against their concentration gradient
-Specific and based on binding with the molecule
Symporters -> moving 2 distinct molecules in the same direction across the cell membrane
->Antiporter - moving 2 molecules in opposite directions across the membrane
Na+-K+ pump
Na+-K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in
Vesicle transport
Bulk transport requires active vesicle transport (endomembranous system)
Two main types according the direction:
-Exocytosis
-Endocytosis
Exocytosis
From inside to outside
->Transport vesicles (from Golgi) fuse with the plasma membrane, and release their contents
Endocytosis
Uptake of macromolecules and
large particles (from outside to inside)
->Substances are brought into the cell by engulfing them in a membrane-bound vesicle
Types of endocytosis
3 types of endocytosis:
->Phagocytosis (“cellular eating”)
->Pinocytosis (“cellular drinking”)
->Receptor-mediated endocytosis
Summary
