Midterm 1: Lec 4 Slides Flashcards
Importance of membranes (3)
- Permeability barrier that can transport certain molecules
- Important in cell response to external stimuli
- Involved in energy transduction
Biological membrane components (and their functions)
- Lipids: provide physical integrity
- Proteins: carry out specific functions
- Carbohydrates: recognition sites on cell surface
What influences fluidity of lipid bilayer?
Composition and temperature
- Lower temperature means more rigidity (goes through phase transition from fluid to gel)
- Phase transition temp. is lower if hydrocarbon chains are short or have double bonds
How does cholesterol orient itself in the bilayers of animal cells?
Polar head group close to polar head group of phospholipid; it intercollates
How do water-soluble molecules pass through the bilayer?
Presence of specific protein molecules
What does the fluid mosaic model explain?
How proteins are incorporated into lipid bilayer; carbs are attached to proteins or phospholipids on outer surface of plasma membrane
Integral proteins - what do they do?
Penetrate into lipid bilayer; most are transmembrane proteins
What are transmembrane proteins (type of integral protein)?
Contain alpha-helix of nonpolar amino acids extending across membrane’s hydrophobic interior
Where are peripheral proteins found?
Outside lipid bilayer on cytoplasmic surface, attached by noncovalent bonds to polar head groups of lipid bilayer or integral membrane proteins
Where are lipid-anchored proteins found?
Covalently attached to phospholipid or fatty acid embedded in the bilayer
3 classes of membrane proteins
Integral, peripheral, lipid-anchored
Describe cytoskeletal protein network in red blood cells
SPECTRIN forms mesh beneath cytoplasmic surface of cell; spectrin attached to membrane junctional complexes by ACTIN and to peripheral protein ANKYRIN, which is attached to anion channel (integral protein)
Roles of membrane proteins (3)
- Transport
- Receptors for molecular messengers from other cells
- Form junctions between cells
Where are membrane carbohydrates found?
Projecting from the exterior surface of the plasma membrane when attached to proteins or phospholipids within membrane
Most carbohydrates in membranes are bound to ____ forming ______
proteins, glycoproteins
What do glycolipids do?
Form cell identity markers (e.g., blood group markers)
Types of specific cell junctions in animal cells
Tight junctions, desmosomes, gap junctions
Homotypic vs. heterotypic binding
Same membrane proteins vs. different membrane types
What do tight junctions prevent?
Permeability (fluid cannot get through)
What do desmosomes do?
They are structural features joining cells together and don’t prevent molecules slipping between cells
What is the structure of gap junctions and what do they facilitate? Give an example of where they’re important.
Form an aqueous pore between two adjacent cells, allowing small molecules like ions to move between them (important in heart, so action potentials can move between cells and cause heart contraction)
What do integrins do?
Mediate the noncovalent attachment of animal cells to extracellular matrix - integrin attaches to fibronectin
What feature do plants have that is similar to gap junctions in animal cells?
Plasmodesmata (aqueous pores)
What are the four basic mechanisms of movement across membranes?
- Simple diffusion
- Simple diffusion through an aqueous channel (pore)
- Facilitated diffusion involving a membrane protein carrier
- Active transport involving a membrane protein carrier and requiring energy
In which forms of transmembrane transport is the movement down the concentration gradient?
Everything except active transport (diffusion, diffusion w/ aqueous pore, and facilitated diffusion)
What molecules can diffuse across the lipid bilayer?
Small nonpolar molecules and gases (O2 and CO2)
Which molecules diffuse more rapidly across the lipid bilayer?
Those with higher lipid solubility
What is osmosis and why is it important?
Diffusion of water molecules across a selectively permeable membrane - important for maintaining water balance between cells and surroundings
Osmolarity - what is it, and what does it meant to be hypo- or hyperosmotic? And what’s blood plasmas’a osmolarity?
Sum of ions and solutes in solution
- Hypoosmotic: 300 mosmol/L
- Blood plasma has ~300 mosmol/L
What does it mean if a cell is in hypertonic solution?
Water will diffuse out and it will shrink (outside solution has higher concentration of solute than cell and therefore less water, so water moves out of cell)
What does it mean if a cell is in hypotonic solution?
Water will diffuse into cell and it will inflate and either burst (animal cell) or be turgid (plant cell) (outside solution has lower solute concentration than cell and therefore more water, so water moves into cell)
How do ions like Na+ move across membranes?
They pass through water-filled pores created by intrinsic membrane proteins (channels)
What are gated channel proteins?
They don’t stay open for extended periods of time; stimulus molecule binds to them to open them
What does aquaporin do to cells?
Increases membrane permeability to water
How does facilitated diffusion work? Is energy needed?
Molecules (e.g., glucose) bind to carrier proteins in membrane to be transported across - this is passive, as net movement is direction of lowest concentration
What is specific facilitated diffusion?
A particular carrier protein transports only certain molecules
What does it mean if the facilitated diffusion carriers are saturated?
All protein carriers are in use, as there are only a fixed amount of carriers in the present cells
What is active transport?
Transport across a membrane by a carrier-mediated process against the concentration gradient (going uphill); requires chemical energy
What is primary active transport?
Requires direct participation of ATP
What is secondary active transport?
Doesn’t directly use At is TP; instead uses energy in ion gradient established by primary active transport
What is a uniporter protein?
Moves a single substance in one direction
What is a symporter protein?
Moves two substances in the same direction
What is an antiporter protein?
Moves two substances in opposite directions (one into cell/organlle, and one out)
What is a coupled transporter?
Moves two substances at once (symporter and antiporter)
What does endocytosis do and how does it work?
Transports macromolecules, large particles and small cells into eukaryotic cells - plasma membrane envelops materials and forms vesicle inside cell
What are the 3 types of endocytosis?
Phagocytosis, pinocytosis, and receptor-mediated
What is phagocytosis?
Large particles/cells are engulfed
What is pinocytosis?
Small dissolved solutes or fluids enter cell
What is receptor-mediated endocytosis?
Specific membrane receptor binds to particular macromolecules at sites called coated pits (which contain clathrin)
What is exocytosis?
Materials vesicles are secreted from the cell when vesicles fuse with the plasma membrane
