MCAT Biochemistry - Biological Membranes

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cell (plasma) membrane

semipermeable phospholipid bilayer; permits fat-soluble compounds to cross easily, while larger and water-soluble compounds must seek alternative entry

fluid mosaic model

the theory that underlies the structure and function of the cell membrane

glycoprotein coatt

Carbohydrates associated with membrane-bound proteins

cell wall

rigid structure outside the cell membrane; plants, bacteria, and fungi, contain high levels of carbohydrates

Phospholipid bilayer

hydrophobic tail and hydrophilic head; two layers makes up membrane; move via simple diffusion

Lipid raft

collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules; toften serve roles in signaling

flippases

assist in the transition or “flip” between layers as the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane

Fatty acids

carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group

Triacylglycerols / triglycerides

storage lipids involved in human metabolic processes; contain three fatty acid chains esterified to a glycerol molecule; saturated or unsaturated

Unsaturated fatty acids

regarded as “healthier” fats, tend to have one or more double bonds and exist in liquid form at room temperature; impart fluidity to the membrane

chylomicrons

transport dietary lipids, such as fats and cholesterol, from the intestines to other locations in the body, within the water-based solution of the bloodstream

α-linolenic acid & linoleic acid

important essential fatty acids

Saturated fatty acids

main components of animal fats; tend to exist as solids at room temperature; found in processed foods and are considered less healthy; decrease the overall membrane fluidity

glycerophospholipid / phospholipid

replace a fatty acid chain of triacylglycerol with a phosphate group; used for membrane synthesis and can produce a hydrophilic surface layer on lipoproteins

micelles

small monolayer vesicles

liposomes

bilayered vesicles of phospholipids

low-density lipoprotein (VLDL)

a lipid transporter

Sphingolipids

do not contain glycerol but contain a hydrophilic region and two fatty acid–derived hydrophobic tails

Cholesterol

regulates membrane fluidity; contains both a hydrophilic and hydrophobic region; prevents the formation of crystal structures in the membrane, increasing fluidity at lower temperatures; limiting movement of phospholipids, decreases fluidity and helps hold the membrane intact at high temperatures; 20% mass, 50% mole fraction of cell membrane

steroids

hormones derived from cholesterol

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; long-chain fatty acid and a long-chain alcohol, which contribute to the high melting point; stability and rigidity within the nonpolar tail region; serve an extracellular function in protection or waterproofing

Transmembrane proteins

pass completely through the lipid bilayer; Transporters, channels, and receptors

Embedded proteins

are associated with only the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane

integral proteins

association with the interior of the plasma membrane, which is usually assisted by one or more membrane-associated domains that are partially hydrophobic; transmembrane and embedded

Membrane-associated (peripheral) proteins

bound through electrostatic interactions with the lipid bilayer, especially at lipid rafts, or to other transmembrane or embedded proteins

ex. G proteins found in G protein-coupled receptors

ABO antigens

red blood cells; sphingolipids that differ only in their carbohydrate sequence; allows for identification and targetting

biofilm

carbohydrates are generally hydrophilic, interactions between glycoproteins and water can form a coat around the cell

membrane receptors,

transporters for facilitated diffusion and active transport can be activated or deactivated; tend to be transmembrane proteins

ex. ligand-gated ion channels, G protein-coupled receptors

cell adhesion molecules (CAM)

proteins that allow cells to recognize each other and contribute to proper cell differentiation and development

intercellular junctions

provide direct pathways of communication between neighboring cells or between cells and the extracellular matrix; generally comprised of cell adhesion molecules (CAM)

Gap junctions / connexons

direct cell–cell communication and are often found in small bunches together; formed by the alignment and interaction of pores composed of six molecules of connexin

connexin

molecules that create pores in the cell membrane

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; limit permeability enough to create a transepithelial voltage difference based on differing concentrations of ions on either side of the epithelium; 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 structures, especially the basement membrane

passive transport

Spontaneous transport processes that do not require energy (negative ΔG); affected by temperature positively

active transport.

nonspontaneous transport that requires energy (positive ΔG); temperature effect depends on enthalpy

simple diffusion

substrates move down their concentration gradient directly across the membrane; only particles that are freely permeable to the membrane

Osmosis

specific kind of simple diffusion that concerns water; water will move from a region of lower solute concentration to one of higher solute concentration

hypotonic solution

concentration of solutes inside the cell is higher than the surrounding solution; cause a cell to swell as water rushes in, sometimes to the point of bursting (lysing)

hypertonic solution

solution that is more concentrated than the cell; water will move out of the cell

isotonic solution

solutions inside and outside are equimolar; equal movement in and out

Osmotic pressure

driving force behind osmosis; water level will only rise to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane

∏ = iMRT

where M is the molarity of the solution, R is the ideal gas constant, T is the absolute temperature (in kelvins), i is the van’t Hoff factor

colligative property

a physical property of solutions that is dependent on the concentration of dissolved particles but not on the chemical identity of those dissolved particles

van’t Hoff factor

the number of particles obtained from the molecule when in solution

ex. glucose = 1, NaCl = 2

Facilitated diffusion

simple diffusion for molecules that are impermeable to the membrane (large, polar, or charged); requires integral membrane proteins to serve as transporters or channels for these substrates

Carrier proteins

only open to one side of the cell membrane at any given point

occluded state

carrier is not open to either side of the phospholipid bilayer

channel proteins

open or closed conformation, open are exposed to both sides of the cell membrane and act like a tunnel for the particles to diffuse through, thereby permitting much more rapid transport kinetics

Active transport

net movement of a solute against its concentration gradient

Primary active transport

uses ATP or another energy molecule to directly power the transport of molecules across a membrane involving the use of a transmembrane ATPase

Secondary active (coupled) transport

no direct coupling to ATP hydrolysis; harnesses the energy released by one particle going down its electrochemical gradient to drive a different particle up its gradient

symport

both particles flow the same direction across the membrane

antiport

particles flow in opposite directions

Endocytosis

when the cell membrane invaginates and engulfs material to bring it into the cell; encased in a vesicle

Pinocytosis

endocytosis of fluids and dissolved particles

phagocytosis

ingestion of large solids such as bacteria

vesicle-coating proteins

Invagination will then be initiated and carried out

ex. clathrin

Exocytosis

when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment

membrane potential, Vm

difference in electrical potential across cell membranes

resting potential

between −40 and −80 mV

leak channels

ions may passively diffuse through the cell membrane over time, changing membrane potential

sodium–potassium pump (Na+/K+ ATPase)

regulates the concentration of intracellular and extracellular sodium and potassium ions; maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping three sodium ions out for every two potassium ions pumped in

Nernst equation

used to determine the membrane potential from the intra- and extracellular concentrations of the various ions

where R is the ideal gas constant, T is the temperature in kelvins, z is the charge of the ion, and F is the Faraday constant (96, 485 C/mol e− )

simplification assumes body temperature

Goldman–Hodgkin–Katz voltage equation

flows from the Nernst equation, taking into account the relative contribution of each major ion to the membrane potential

Mitochondria

the “powerhouse” of the cell because of their ability to produce ATP by oxidative respiration

outer mitochondrial membrane

highly permeable due to many large pores that allow the passage of ions and small proteins; completely surrounds the inner mitochondrial membrane

intermembrane space

space between inner and outer mitochondrial membranes

inner mitochondrial membrane

involved in the electron transport chain and ATP synthesis; very high level of cardiolipin and does not contain cholesterol

cristae

inner mitochondrial membrane infoldings; numerous; increase the available surface area for the integral proteins associated with the membrane

mitochondrial matrix

where the citric acid cycle produces high-energy electron carriers used in the electron transport chain; inside inner mitochondrial membrane