WEEK 2 (1.5 and 1.6) Flashcards
What is a membrane? What does the fluid mosaic model refer to?
Membranes physically separate cells from external environment and define spaces within the cells
Fluid mosaic model: lipid bilayer is a fluid structure within which molecules move laterally and is a mosaic (mix) of 2 molecules (proteins and lipids)
What is the main lipid that composes a membrane?
Phospholipids (glycerol backbone attached to a phosphate group and 2 fatty acids)
Phosphate head: hydrophilic, because its polar → able to form H bonds with water
Fatty acids: hydrophobic, because they are nonpolar → aren’t able to form H bonds with water
→ The presence of both a hydrophilic and hydrophobic regions in a single molecule (amphipathic)
What are the functions of membrane proteins?
Transporters: moving ions or other molecules across the membrane
Receptors: allow the cell to receive a signal from environment
Enzymes: catalyze chemical reactions
Anchors: attach to other proteins and maintain cell structure and shape
What are the e main types of membrane proteins?
- Integral membrane proteins: permanently associated with cell membranes and cannot be taken out without disturbing the membrane
-Transmembrane proteins: span the entire lipid bilayer; composed of 3 regions (one protruding from each face of the membrane) and a connecting hydrophobic that spans the membrane - Peripheral membrane proteins: temporarily associated or integral proteins (noncovalent interactions); these could be removed without disturbing the membrane (could be either on the external or internal side of the membrane)
How do phospholipids behave in an aqueous environment?
Arrange themselves in a way where the polar head is on the outside and interacts with water, while the nonpolar tail groups come together on the inside away from the water
Shape is determined by the bulkiness of the head group relative to the hydrophobic tails
- Bulky head and a single hydrophobic tail → wedge-shaped and pack into spherical structures “micelles”
- Less bulky head and 2 hydrophobic tails → rectangular and form a bilayer
Bilayer and liposomes: in water; form depends on amount of phospholipids available
Phospholipid monolayers: between polar and non-polar solvent
What is a phospholipid bilayer?
Bilayer: structure formed of 2 layers of lipids in which the hydrophilic head are the outside surfaces of the bilayer and the hydrophobic tails are sandwiched in between → that way the polar head is in contact with the aqueous environment, while the nonpolar is isolated from it
- Bilayers form closed structures with an inner space, because free edges would expose the hydrophobic chains to aqueous environment
Why is the pH important in preserving the properties of a phospholipid?
pH important since it ensures the head groups are in their ionized form and therefore are hydrophilic
What is the interaction between the fatty acids of the phospholipids in a cell membrane?
Van Der Walls forces between fatty acid tails → which are easily broken and re-formed and thus are able to move in their horizontal and vertical plane
What alters the fluidity of a membrane?
- Length of fatty acid chain: the longer the chain, the more surface available to participate in the VDW interactions
- Presence of double bonds between neighboring C: tighter packing tends to reduce lipid mobility
Saturated fatty acid tails have no double bonds = straight and tightly packed = reducing mobility
Unsaturated fats have double bonds = kinks in the tails, reducing tightness of packing = enhancing lipid mobility
- Alters fluidity depending temperature
Room temperature: decreases fluidity = interaction rigid ring structure of cholesterol with the fatty acids
Low temperature: increases fluidity = prevents phospholipids from packing tightly and preserve a consistent state of fluidity by preventing drastic transition from fluid to solid
How does the plasma membrane maintain homeostasis?
Homeostasis: maintenance of a constant environment within cells
Selective permeability: property of the membrane that allows some molecules in and out freely, while it lets others in and out only under certain conditions and it prevents other molecules from passing through at all → this results from a combination of lipids and embedded proteins
- The hydrophobic interior of the bilayer prevents charged polar molecules from moving across it + proteins and polysaccharides are too large to cross the plasma membrane on their own
- Gasses, small uncharged molecules (water), and nonpolar molecules (lipids) can move across the bilayer
- Protein channels and transporters can facilitate the movement of molecules that can’t cross the bilayer on their own
What is passive transport and what are the 2 main types?
→ Works by diffusion (random movement of molecules; given that molecules are always moving in their environment due to their intrinsic kinetic energy)
Diffusion: leads to a net movement of the substance from one region to another, from a region of high to low concentration (there is a concentration gradient) When there is no longer a concentration gradient, net movement stops but movement of the molecules in both directions continues The molecule moves through the lipid bilayer directly - Molecule diffuses directly across the membrane by itself Facilitated diffusion: when a molecule moves by diffusion through the membrane through a membrane protein - Relies on concentration gradient as well and doesn't require energy
What are the 2 types (and its subtypes) of membrane proteins involved in passive transport
Channel: provides an opening between the inside and outside of the cell that allows some molecules to pass through (depending on shape and charge)
Carrier: binds to and then transports specific molecules → binding of a molecule to the carrier induces a conformational change → allows the molecule to be transported across the bilayer
2 conformations
- Open to one side of the cell (extracellular)
- Open to the other side of the cell (intracellular)
What is osmosis?
Osmosis: net movement of a solvent across a selectively permeable membrane
-Water also moves from high to low concentration
-Water concentration decreases as solute concentration increases
Water moves from regions of lower solute concentration to higher solute concentration
Osmosis continues until a concentration gradient no longer exists or is opposed by another force or the cell wall
Osmosis could be prevented by applying a force to the compartment with higher solute concentration → “osmotic pressure”
What is primary active tansport?
→ If an ion/molecule cannot diffuse directly across the membrane and requires energy → ATP (move from low to high concentration)
→ If it moves against the concentration gradient (could be high to low) and uses ATP in order to do so, then its active transport
→ Cells move substances through transport proteins embedded in the membrane
How does active transport impact size and composition of cells?
→ Hypertonic solution (higher extracellular solute concentration outside than the inside) = water leaves the cell → cell shrinks
→ Hypotonic solution (lower extracellular solute concentration outside than the inside) = water moves into the cell → cell lyses
→ Solution: keeping intracellular fluid isotonic (same solute concentration with extracellular fluid)
What is the hydrophobic effect?
Water pushing the non-polar molecules together in order to maximize its own entropy, allowing water to make stronger interactions and therefore become more stable
- Even though lipid tails are pushed together and have less freedom (decreased entropy), this is overpowered by the increase in entropy of water)
How does free energy behave in the bilayer formation?
Initial state - less stable and free energy is >0 (energy is required)
Final state - more stable and free energy is <0 (energy is being released - since entropy increased)
