Plasma membrane: Composition & Functions Flashcards
What is the role of plasma membrane?
Separates the living cell from its surroundings - essential for all cells
A barrier for contents of the cell from escaping and mixing with other molecules from surrounding environment
In eukaryotic cells only…..
compartmentalisation of individual organelles - specific composition for specific reactions
lipid bilayer = explain structure
Lipids are arranged in two closely apposed sheets - forming lipid bilayer
proteins are embedded and some carbohydrates
Phospholipids
most abundant lipids in plasma membrane - arranged in two-layer sheets
(phosphatidylcholine is MOST abundant)
phosphate group are:
hydrophilic- water loving
Heads are arranged outward - exposed to aqueous solutions
what are hydrophilic - water repellent in the bilayer?
fatty acid tails - arranged inward, shield water and form a sealed comportment
weak hydrophobic interaction between fatty acid tails to form a bilayer
What is the fluid mosaic model?
dynamic and complex structure of plasma membrane
Mosaic (of components) - besides phospholipids, glycolipids, sterols (cholestreol in mammalian cells) and various proteins and glycoproteins
Fluid - dynamic and flexible = viscosity of the lipid bilayer
lipid molecules able yo 2- dimensionally move along
the plane of the bi-layer
Phospholipid movements??
can lead to
- Rapid lateral diffusion of lipid molecules within the plane of each monolayer
- Spin in place (rotation - up to 500 revolutions per s)
Flexion; contraction movement
Flip-flop; from a side to the other
Do proteins usually move?
no - anchored to cytoskeleton
ONLY some proteins can slightly and slowly move driven by the motor proteins
Amphiphatic
compounds that are polar and non-polar
hydrophobic regions (non-polar a.a) - often coiled into a helics - lie in the interior of the bilayer
Hydrophilic regions of a.a are exposed to external aqueous environment
Which other molecules are also amphiphatic?
cholesterol
glycolipids
Cholesterol - 20% of membrane lipids = modulates the membrane fluidity and improving stability
stiffen the membrane ( less flexible and permeable)
Reducing molecules passage
Glycolipids :
lipids containing sugars representing their hydrophilic head
Which way do sugar groups face
cell exterior (bilayer is asymmetrical)
How are membrane protein synthesised?
rough ER
other proteins embedded in membrane are exclusively on the _______
cytosolic
how are peripheral membrane proteins attached to membrane lipids or other integral membrane proteins
covalently
what are the main functions of membrane proteins?
1) transport of molecules
2) enzymatic activity
3) signal transduction/ cell communication
4) structural support / attachment to the extracellular matrix (ECM)
most membrane proteins are glycoproteisn -
short chains of sugars linked to a.a (facing extracellular space)
= protect the cell surface from mechanical damage and involved in cell-to-cell communication
- protein glycosylation = post - translational modoficsagtion in rough ER and Golgi appartus
most membrane proteins are glycoproteisn -
short chains of sugars linked to a.a (facing extracellular space)
= protect the cell surface from mechanical damage and involved in cell-to-cell communication
- protein glycosylation = post - translational modification in rough ER and Golgi apparatus
Trasport Across a cell membrane are
selectively permeable -0 regulating traffic molecules and ions in adn out of cells
Two main types of transport
1) passive
2) Active transport
Passive transport ( 3 types) - movement of molecules w/o expenditure energy
movement DOWN gradient - reach equilibrium
1) diffusion
2) osmosis
3) facilitated diffusion
Diffusion, which molecules?
FREE movement of molecules across the lipid bilayer
NO energy (ATP) cost
DOWN gradient concentrations (until equilibrium)
Gases - O2 + CO2
small uncharged polar molecules - urea, ethanol
(many are too large molecules)
Osmosis
SPONTANEOUS movement (passive transport) of water/ solvent across a semipermeable membrane
DOWN conc
Water potential of a solution is inversely porportional to the solutes concentration
when SOLUTES cannot go across the membrane - osmosis occurs
What is osmotic pressure?
hydrostatic
pressure required to stop the net flow of water across a membrane separating solutions of different water potentials
how can water flow in/out of cells?
through aquaporins (facilitated diffusion)
Types of osmosis;
- isotonic solution
- hypertonic solution
- hypotonic solution
- isotonic solution
a solution with the same concentration of solutes both inside and outside the cell
equal
(NORMAL no net water movement)
- hypertonic solution
a solution that has a higher solute concentration outside the cell than inside
(SHRIVELED - cell loses water)
- hypotonic solution
a solution that has a higher solute concentration inside the cell than outside
(LYSED/ BURSED - cell gains water)
Effect of Osmotic pressure on blood cells
The movement of water across the plasma membrane determines the volume of an individual cell, which must be regulated to avoid damage to the cell.
changes in osmotic conditions cause cells to swell or shrink rapidly
Facilitated diffusion – Passive transport
DOWN conc via transport proteins
Highly specific
Reversible
No energy (ATP) cost
Transport protein;
- protein channel
- protein carrier/ transporters (small molecules or ions)
Protein channels
form a hydrophilic “tube” across the membrane through which specific molecules/ions diffuse (down a concentration gradient)
Do not interact with the molecules, although specific
Move at a very rapid rate
Passive transport (no energy cost)
Protein carrier/transporters – passive transport
(facilitated diff)
Shift/Transport a single type of molecule (ions or small molecules) across the membrane (not as a channel.
It involves the Interaction with the specific molecule (shape complementarity) that induces a conformational change of the protein to mediate the transport of the molecule
Work at a much slower rate than channels
Three transporter types
uniporters, symporters* and antiporters
Uniporter is the only transporter based in passive transport.
Symporters and antiporters (
based on cotransport of more than 1 molecule are active transport
against the concentration/electric gradient
Symporters and antiporters (
based on cotransport of more than 1 molecule are active transport
against the concentration/electric gradient
Active transport
Moves substances against their concentration or electrochemical gradients
Requires energy, usually in the form of ATP
Performed by specific proteins embedded in the membranes
Allows for stockpiling
Common features of transport proteins and enzymes
Specific binding sites for the solute
Can be saturated
Can be inhibited by molecules that resemble the normal “substrate”
Catalyse a physical process
Protein carrier/transporters – Active transport
based on binding with the molecule - shape complimentary
Protein carrier/transporters – Active transport
based on binding with the molecule - shape complimentary
Symporters moving _ different molecules across the cell membrane ( _____ direction)
2
same
Antiporter moving _ distinct molecules across the membrane ( ______ directions) E.g. Na+-K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in.
2
opposite
Vesicle Transport - Transport of large macromolecules
Bulk transport requires vesicle transport (endomembranous system)
two main types of transport - endo/exocytosis
Endocytosis
> Uptake (from outside to inside) of macromolecules and large particles
- Invagination of the cell surface to form an intracellular membrane-bounded vesicle containing extracellular fluid
Exocytosis
> Secretion or excretion (from inside to outside) e.g. insulin (protein hormone) by pancreatic cells
- Transport vesicles (from Golgi) migrate to the plasma membrane, fuse with it, and release their contents
Types of endocytosis
1) Phagocytosis (“cellular eating”)
2) Pinocytosis (“cellular drinking”)
3) Receptor-mediated endocytosis
