Chapter 8: Biological Membranes Flashcards
Lipid rafts
Phospholipid bilayer also includes proteins and distinct signaling areas within lipid rafts; collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules; often serve important roles in signaling; both lipid rafts and protein also travel within the plane of the membrane, but more slowly
Glycoprotein coat
Carbohydrates associated with membrane-bound proteins; cell walls of plants, bacteria, and fungi contain higher levels of carbohydrates
Flippases
Assist in the transition or “flip” between layers
Most plentiful to least plentiful membrane components
Lipids > proteins > carbohydrates > nucleic acids
What are two essential fatty acids for humans?
Α-linolenic acid and linoleic acid
What can phospholipids spontaneously form?
Micelles (small monolayer vesicles) or liposomes (bilayered vesicles)
What are glycerophospholipids used for?
Membrane synthesis and can produce a hydrophilic surface layer on lipoproteins such as VLDL (lipid transporter); primary component of cell membranes; serve as secondary messengers in signal transduction; the phosphate group provides an attachment point for water-soluble groups such as choline or inositol
Sphingolipids
Do not contain glycerol but contain 2 fatty acid-derived hydrophobic tails and a hydrophilic region
What are the classes of sphingolipids?
Ceramide; sphingomyelins; cerebrosides; gangliosides
By mass, what percentage of the cell membrane is composed of cholesterol? By mole fraction?
20%; 1/2
Waxes
Class of lipids that are extremely hydrophobic and are rarely found in the cell membranes of animals, but are sometimes found in the cell membranes of plants; composed of a long chain fatty acid and a long-chain alcohol which contribute to the high melting point of these substances; role in waterproofing, protection and stability and rigidity (within the nonpolar tail region only)
Transmembrane proteins
Pass COMPLETELY through the lipid bilayer; usually transporters, channels, and receptors
Embedded proteins
Associated with only the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane
Integral proteins
Transmembrane and embedded proteins; associated with the interior of the plasma membrane, which is usually assisted by one or more membrane-associated domains that are partially hydrophobic
Peripheral (membrane-associated) proteins
May be bound through electrostatic interactions with the lipid bilayer, especially at lipid rats or to other transmembrane or embedded proteins (like G proteins)
Carbohydrate role in cell membrane
Signaling and recognition molecules
Membrane receptors
Tend to be transmembrane proteins; mostly protein, can be carbohydrates or lipids too
Cell adhesion molecules
Cell-cell junctions; proteins that allow cells to recognize each other and contribute to proper cell differentiation and development
Gap junctions
Allow for direct cell-cell communication and are often found in small bunches together; these connexons are formed by the alignment and interaction of pores composed of six molecules of connexin; permit movement of water and some solutes directly between cells (not proteins though)
Tight junctions
Prevent solutes from leaking into the space between cells via a paracellular route; found in epithelial cells and function as a physical link between the cells as they form a single layer of tissue; can limit permeability enough to create a transepithelial voltage difference; must form a continuous band around the cell
Desmosomes
Bind adjacent cells by anchoring to their cytoskeletons; formed by interactions between transmembrane proteins associated with intermediate filaments inside adjacent cells; primarily found at the interface between two layers of epithelial tissue
Hemidesmosomes
Similar function to desmosomes but their main function is to attach epithelial cells to underlying (basement) membrane
How does cholesterol provide membrane fluidity? Membrane stability?
Interfering with the crystal structure of the cell membrane and occupying space between phospholipid molecules; cross-linking adjacent phospholipids through interactions of the polar head group and the hydrophobic interactions at the nearby fatty acid tails
Passive transport
Do not require energy (negative delta G); includes diffusion, facilitated diffusion, and osmosis; generally increase in rate as temperature increases; primary thermodynamic motivator in passive transport is an increase in entropy
Active transport
Nonspontaneous and require energy (positive delta G); may or may not be affected by temperature depending on the enthalpy
Osmotic pressure
Colligative property; physical property of solution that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles; water will move towards the compartment with the highest osmotic pressure; opposed by hydrostatic pressure
Van’t Hoff factor
The number of particles obtained from the molecule when in solution
Carriers
Only open to one side of the cell membrane at any given point; for a brief time, the carrier is in the occluded state in which the carrier is not open to either side of the phospholipid bilayer
Channels
May have an open or closed conformation; open conformation - channels are exposed to both sides of the cell membrane like a tunnel for particles to diffuse through
Primary active transport
Uses ATP or another energy molecule to directly transport molecules across a membrane
Secondary active transport
Coupled transport; also uses energy to transport molecules across the membrane but without direct coupling to ATP hydrolysis; uses energy released from one particle going down its electrochemical gradient
What molecules are transported via simple diffusion?
Small, non-polar (O2, CO2)
What molecules are transported via facilitated diffusion?
Polar molecules (glucose) or ions (Na+, Cl-)
What molecules are transported via active transport?
Polar molecules or ions (Na+, Cl-, K+)
Endocytosis
Occurs when the cell membrane invaginates and engulfs material to bring it into the cells
Pinocytosis
Endocytosis of fluids and dissolved particles
Phagocytosis
Ingestion of large solids such as bacteria
What initiates endocytosis?
Substrate binding to specific receptors embedded within the plasma membrane
Exocytosis
Occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment; important in the NS and intercellular signaling
Membrane potential
The difference in electrical potential across cell membranes; the impermeability of the cell membrane to ions and the selectivity of ion channels both lead to an electrochemical gradient between the exterior and interior of the cell
What is the resting potential for most cells?
-40 to -80 mV, although the membrane potential can rise as high as +35 mV during depolarization
Faraday constant
96 485 C/mol e-
Leak channels
Ions passively diffuse through the cell membrane over time; cell membranes are more permeable to K+ than Na+ at rest because there are more K+ leak channels
Sodium-potassium pump
Maintains the membrane potential; pumps 3 Na+ out for every 2 K+ ions in
Nernst equation
E = RT/zf*ln [ion out]/[ion inside] E = 61.5/z log [ion out]/ [ion inside]
Goldman-Hodgkin-Katz voltage equation
Vm = 61.5 log [P ion * [ion] outside/ Pion * [ion] inside]
Outer mitochondrial membrane
Highly permeable due to many large pores that allow for the passage of ions and small proteins; completely surrounds the inner mitochondrial membrane with the presence of a small intermembrane space in between the two layers
Inner mitochondrial membrane
Much more restricted permeability compared to the outer mitochondrial membrane; contains numerous infoldings known as cristae which increase the surface area available for the integral proteins associated with the membrane; encloses the mitochondrial matrix where the CAC occurs; contains a very high level of cardiolipin and does not contain cholesterol
