L7 membrane trafficking 1 Flashcards
membrane composition?
Membranes consist primarily of lipids. Since the cytosol is aqueous, hydrophobic and hydrophilic interactions drive membrane formation. Lipids, including fats and oils, are hydrophobic, meaning they do not mix well with water due to the absence of stabilizing interactions with polar molecules.
amphipathic molecules and triglycerides?
Membranes consist of amphipathic molecules, which have both hydrophilic (water-loving) and hydrophobic (water-repellent) parts. Detergents (e.g., SDS) are an example of amphipathic molecules, having a long hydrophobic acyl chain and a polar hydrophilic head group. Fatty acids are composed of long-chain hydrocarbons attached to a carboxyl group (e.g., palmitic acid (16:0), oleic acid (18:1), stearic acid (18:0), arachidonic acid (20:4).
These fatty acids serve as building blocks for membrane lipids. They are synthesized via the fatty acid synthase pathway.
Triglycerides (fats in the body/food) are formed by esterification of fatty acids to glycerol. These are stored in lipid droplets (spherical organelles form from budding from er membranes and form a single monolayer surrounding bundled triglycerides. all dietary and biosynthetic fats are stored here) within fat cells for energy storage when mobalised. l
formation of membrane phospholipids?
Triglycerides, while excellent for energy storage, are not suitable for membrane formation because they are hydrophobic and lack a hydrophilic head group. To form membranes, the fatty acids can be modified to phospholipids by removing one acyl chain and adding a hydrophilic head group, making them amphipathic.
Common Phospholipid Head Groups:
Phosphatidic acid (PtdOH) – contains a phosphate group.
Phosphatidyl choline (PtdCho) – contains a choline group.
Phosphatidyl serine (PtdSer) – contains a serine group.
Phosphatidylinositol (PtdIns) – contains an inositol group.
These hydrophilic heads project outward, stabilizing the molecule in an aqueous environment, while the hydrophobic tails cluster together in the interior of the bilayer.
lipid self-assembly?
Phospholipids in an aqueous environment will self-assemble into structures like micelles (spherical) and liposomes (bilayer spherical vesicles), driven by the need to shield their hydrophobic tails from water.
The shape of the lipid molecule (affected by the size of the head group and the presence of double bonds) influences the bilayer structure. Type 1 lipids (larger head groups) favor micelle formation, while Type 2 lipids (larger acyl chains) prefer forming bilayers.
membrane curvature and vesicle formation?
The curvature of the bilayer is essential for organelle formation, vesicle budding, and the transport of molecules. Phospholipid composition and the degree of acyl chain saturation affect the packing density of lipids in the bilayer:
Saturated fatty acids allow tight packing, forming a solid-ordered phase.
Unsaturated fatty acids (with double bonds) prevent tight packing, creating a liquid-disordered phase.
role of cholesterol in membrane fluidity?
Cholesterol is a crucial lipid in the plasma membrane, making up to 30% of the plasma membrane (less in intracellular membranes). Its small polar head fits beneath phospholipid heads, while its rigid ring structure stabilizes the bilayer.
Cholesterol helps to form a liquid-ordered phase by allowing looser packing, enhancing membrane fluidity, and adding plasticity to the membrane system. It also stabilizes lipid rafts, which are cholesterol-rich microdomains that concentrate specific proteins. Enables looser packing, high lateral mobility, thicker and allows plasticity.
lipid rafts and membrane microdomains?
Lipid rafts are specialized regions of the membrane enriched in cholesterol and specific lipids, believed to concentrate certain membrane proteins. While evidence for lipid rafts in live cells is limited, artificial membranes have shown the existence of segregated domains. In artificial membranes you can make bilayers showing liquid ordered and liquid disorders immiscibility. Lipids segregate based on affinity for each phase so different domains for different types of lipids and lets cells incorporate proteins into these different phases that conc or deconcentrate proteins in these specific areas.
membrane proteins and their interactions?
Membrane proteins are categorized by their interaction with the lipid bilayer:
Integral membrane proteins: Span the membrane with hydrophobic residues facing the lipid bilayer.
Peripheral membrane proteins: Associate with the membrane indirectly, often by interacting with other membrane proteins.
Proteins can present either hydrophobic or hydrophilic residues on their surface:
Hydrophobic residues (e.g., leucine, valine) are embedded in the bilayer.
Hydrophilic residues (e.g., arginine, lysine) interact with the aqueous cytosol. They are stabilised by polar water molecules in the aqueous cytosol.
types of membrane proteins?
Single-pass transmembrane proteins: Cross the membrane once.
Multi-pass transmembrane proteins: Have multiple membrane-spanning regions.
Post-translational modifications: Examples include acylation and GPI-anchoring.
Peripheral membrane proteins: Associate with the membrane through protein-protein interactions.
extracellular peripheral membrane proteinss
transport across membranes and transport proteins?
Membranes are hydrophobic, making it difficult for most molecules, especially charged ions, to cross. Transport proteins facilitate movement across membranes via various mechanisms:
Passive transport: Moves molecules down a concentration gradient (e.g., channel-mediated and transporter-mediated).
Active transport: Moves molecules against a concentration gradient using energy (e.g., from ATP hydrolysis or light). e.g: coupled transporter: Na+/Ca++ antiporter, atp=driven pump: Na+/K+ atpase, vacuolar H+ atpase, light-driven pump: bacterorhodpsin
Functional Types of Transmembrane Transport Proteins:
Ion pumps: Use energy to pump ions against their concentration gradients. Example: the Na+/K+ ATPase pump.
In neurons, this pump maintains ionic gradients crucial for action potentials.
Ion channels: Allow ions to flow down their concentration gradient, often regulated by gating mechanisms (e.g., voltage-gated, ligand-gated, or mechanosensitive). excess sodium and chloride on the outside and excess intracellular potassium. for each molecule of atp hydrolised, 3 sodium ions are pumped our and 2 potassium ions are pumped in. enables neuron polarisation and so proppogate an action potential. It is important for maintaining physiological ion conc and keeping cytoplasm ionically bufered.
Aquaporins: Water channels that facilitate the rapid movement of water across the membrane, important in cells like those in the kidney and gut lining.
biosynthetic and endocytic pathways?
These pathways involve membrane-bound compartments that either synthesize secretory products or mediate endocytosis, allowing cells to regulate the internal environment and communicate with the extracellular space.
esterification?
Glycerol (a three-carbon molecule) forms the backbone.
Two of the hydroxyl groups on the glycerol are esterified to fatty acids, forming diacylglycerols.
The third hydroxyl group is esterified to a phosphate group, which can then be attached to a variety of polar head groups (like choline, serine, or inositol)
This gives the molecule both hydrophobic fatty acid tails and a hydrophilic phosphate-containing head group, making phospholipids a key component of biological membranes, allowing them to form bilayers that separate intracellular and extracellular environments.
lipid shape and determining structure?
If put double bond in acyl chain, causes chain to kink so takes up more space than head group. Type 2 have larger acyl chains than head group , type 0 lipid will be roughly equal and type 1 will have a larger head group and strsighter smaller acyl chains. So some of these changes in lipid structure when placed in inner or outer leaflet of bilayer changes shape or curvature of that bilayer. Essential to help cell make organelles, vesicles and transport intermediates of different sizes and shapes.
Type 1 lipids often prefer to form micelles
