Fluid-Mosaic Model
- Fluid because phospholipids are in continuous lateral movement
- Mosaic because membrane is a mosaic of different proteins embedded and distributed randomly in dynamic phospholipid bilayer
How do phospholipids affect membrane fluidity?
- Structure of Phospholipids (Chapter 2)
- This allows phospholipids to be held together by weak hydrophilic interactions between phosphate heads and weak hydrophobic interactions between non-polar hydrophobic hydrocarbon chains within hydrophobic core, thus can drift laterally
- This contributes to membrane fluidity and transient pore formation
- When temperature increases, the kinetic energy of phospholipids increase, causing increase in fluidity. Conversely, when temperature decreases, KE decreases and phospholipids settle in a closely packed arrangement
- Ratio of saturated to unsaturated hydrocarbon chains. With increased saturation, phospholipids packed more closely, decreasing fluidity. On the converse, increased unsaturation allows for kinks in C=C bonds to prevent close packing at cold temps and prevent overly fluid at higher temperatures
How does Cholestrol affect membrane fluidity?
- Structure of Cholestrol (Ch 2)
- Prevents membrane from being overly fluid at warmer temperatures by restricting lateral movement
- Prevents membrane from being overly firm at colder temperatures by preventing close packing
- Increased fluidity also increases permeability by increasing formation of transient pores
Proteins (write for everything)
- AAs with non-polar hydrophobic R groups forms hydrophobic interactions with hydrophobic core of plasma membrane
- AAs with polar/charged hydrophilic R groups forms hydrophilic interactions with hydrophilic phosphate heads + aqueous environment
- Hydrophilic pore interior lined with AAs with hydrophilic R groups that allow passage of polar molecules/ions while exterior lined w AAs w hydrophobic R groups to stay attached in hydrophobic core
- Hydrophobic core would be impermeable to charged/polar molecules
Channel vs Carrier Proteins
- Channel proteins are transmembrane proteins that provide hydrophilic pore for a specific molecule —> eg aquaporins for H2O to flow thru much quicker
- Carrier proteins exist in 2 alternate conformations. Each side contains a solute-specific binding site which undergoes a conformation change upon solute binding, moving solute to the other site and releasing it.
- Binding site returns to original conformation and direction of flow is bidirectional depending on relative concentrations of solute (eg Glucose)
Role of Proteins
- Allow facilitated diffusion of polar/charged molecules via transmembrane channel/carrier proteins
- Assist in active transport of polar/charged molecules via protein pumps against concentration gradient using ATP
- Membrane proteins interact with the extracellular matrix on the exterior side and cytoskeleton on the cytoplasmic side to maintain shape of the cell and fix the location of some membrane proteins
- Function as receptor proteins for signal transduction and cell-cell recognition
Role of Glycolipids and Glycoproteins
Always projecting outward into ECM
1. Cell-Cell Recognition
- Diversity of glycoproteins differentiate between cell types and can be used to recognise self/other
2. Cell Receptors
- Receptors for hormones in cell signalling
- Receptors for viruses, bacteria and toxins as point of attachment
- Receptors for WBC recognition
Functions of Membranes
- Regulate Movement of substances by being selectively permeable
- Regulates concentrations of ions, polar and large molecules
- Serve as barrier between cytoplasm and external enviro to retain cell contents - Compartmentalisation
- Formation of unique environments with optimum enviro for specialised reactions eg acidic enviro for hydrolytic enzymes of lysosomes
- Accumulation of ions and formation of chemical/proton gradients due to impermeability of hydrophobic core eg proton gradient for ATP synthesis via chemiosmosis formed by buildup of H+ in intermembrane space/thylakoid space
- Storage of food sources eg starch in amyloplasts - Localisation of proteins of a related metabolic function along membrane
- Facilitates sequential biochemical processes eg photosystems II and I and electron carriers of ETC
- But also links spatially separated biochemical processes eg endomembrane system with nucleus RER and Golgi all having specific but related function - Increase SA for chemical reactions eg cristae that hold many ETCs and ATP synthase or thylakoid membrane that have many photosynthetic pigments
- Embed proteins within phospholipid bilayer in correct orientation for proper function eg ATP synthase where active site faces stroma
- Cell-cell recognition and adhesion
- Glycoproteins act as surface antigens for self/other recognition
- Membrane proteins can hook together to form tight/gap junctions - Signal transduction act as cell surface receptors by recognising ligand, triggering proteins to change shape and trigger cell response
Simple Diffusion
Small, non-polar molecules pass through hydrophobic core down their concentration gradients without ATP — bidirectional movement until dynamic equilibrium
Factors affecting rate of simple diffusion
1. Smaller Molecular Size
2. Solubility in lipid bilayer
3. Steeper concentration gradient
4. Greater KE via greater temp
5. Larger SA of cell membrane
6. Shorter Distance to diffuse
Facilitated Diffusion
Polar/charged molecules move down conc gradient from higher to lower without ATP with help of channel/carrier proteins
(If necessary, talk abt structure and function of channel/carrier proteins)
- Demonstrate saturation kinetics in which rate increases then plateaus at saturation point when maximum facilitation occurs and all carrier proteins in use
Osmosis
Water moves from region of higher to lower water potential through a selectively permeable membrane
1. Simple diffusion of water through transient pores
2. Facilitated diffusion through aquaporin channels
Active Transport
Specialised solute-specific pumps moving polar/charged molecules against their concentration gradients requiring ATP
Eg Na+/K+ pump
Bulk Transport (Endocytosis)
- Phagocytosis cell eating of large solid insoluble macromolecules
- Filaments rearranged to form pseudopodia using ATP which are outward extensions of membrane that engulf particle
- Ends of pseudopodia fuse and vesicle is pinched off, moving into cytoplasm as endosome - Pinocytosis taken up in liquid (soluble form)
- Small area of plasma membrane invaginates to form pinocytic vesicles - Receptor-mediated endocytosis — specific transport
- Protein receptors embedded on exterior surface are complementary in shape and charge to ligand, are often found in clathrin-coated pits
- Plasma membrane invaginates when specific ligand binds, forming an endocytic vesicle with the whole ligand-receptor complexes
- Helps cell to acquire large quantities of specific substances
Bulk Transport (Exocytosis)
- Export of macromolecules out of cell such as waste materials, manufactured proteins or hormones
- Secretory vesicle moved to cell surface membrane and fuses to release vesicle contents to extracellular environment
- Large molecules like peptide hormones often released by exocytosis and received at receptors instead of entering the cell directly
