MCAT Biochemistry Chapter 8: Fluid Mosaic Model

Fluid Mosaic Model

• Describes the cell (plasma) membrane as a semi permeable phospholipid bilayer

  • Semi-permeable: chooses which particles can enter and leave the cell

    • Phospholipid bilayer permits fat-soluble compounds to cross easily, while larger and water-soluble compounds seek alternative entry 

Membrane Dynamics (cell membrane)

• Phospholipid move rapidly in plane of membrane through simple diffusion 

  • Lipid rafts;

  • Flippases: assist in transition or “flip” between layers 

• Concentrations of membrane proteins are mediated by gene regulation, endocytotic activity and protein insertion 

General Membrane Structure and Function

• Mainly functions in protecting interior of cell from external environment, intracellular/intercellular communication and transport 

• Contain proteins within lipid bilayer that act as cellular receptors and signal transduction 

  • Glycoprotein coat: carbohydrates associated with membrane-bound proteins

Lipid Rafts

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

Membrane Components: Lipids

• fatty acids and triglcyerides

• phospholipids (glycerophospholipids)

• shingolipids

• cholesterol and steroids

• waxes

Fatty acid

• carboxylic acids w/ hydrocarbon chain and terminal carboxy

Triglyceridse and Cell Membrane

• storage lipids involved in metabolic processes 

  • Unsaturated: increase membrane fluidity

  • Saturated: decrease membrane fluidity 

Humans synthesize only some of fatty acid others are ingested and then incorporated from small intestine via chylomicrons

Phospholipids (glycerophospholipid)

• formed from substituting one of the heads of triglyceride with a phosphate group 

• Used for membrane synthesis 

• Can produce hydrophilic surface layer on lipoproteins 

• Can act as secondary messengers in signal transduction

• Provide attachment group for water-soluble groups

Sphingolipids (cell membrane)

• Similar in structure to phospholipis

  • contain hydrophilic region and two fatty-acid derived hydrophobic tails 

• Classes differ in their hydrophilic groups 

Choelsterol and Cell Membrane

• choleseterol helps produces steroids and stabilizes adjacent phospholipid sand occupies space between them

  • Prevents formation of crystal structures in the membrane 

    • Increases fluidity at lower temperatures 

  • Limits movement of phospholipids

    • Decreases fluidity at higher temperatures 

Waxes (cell membrane)

• Composed of a long fatty acid chain and a long-chain alcohol

• Provide stability and rigidity within nonpolar tail region of membrane

Integral Proteins

• associate with interior of plasma membrane 

  • Transmembrane proteins: pass completely through lipid bilayer 

  • Embedded proteins: associated with only the interior or exterior surface of the cell membrane 

Membrane associated (peripheral) proteins

• Bound through electrostatic interactions with lipid bilayer 

• Are prominent at lipid rafts; may bound to integral proteins

Carbohydrates (cell membrane)

• Generally attached to protein molecules on the extracellular surface of cells 

• Generally hydrophilic, can act as signaling and recognition molecules

• Can form glycoprotein coats

  EX: differing carbohydrate sequences of ABO antigens (are sphingolipids)

Membrane Receptors

• Usually transmembrane proteins 

• Activate some of the transporters for facilitated diffusion and active transport 

• May participate in biosignaling 

  • Ex.: G-protein coupled receptors 

Cell-cell Junctions

• Generally comprised of cell adhesion molecules (CAM) that allow cells to recognize each other 

• Provide direct pathways of communication, between cells and also to the extracellular matrix

• include….

  • gap junctions

  • tight juncrtions

  • desmosomes

Gap Junctions

• Allow for direct cell-cell communication and are often found in small bunches together 

• Are connexons; formed by alignment and interaction of pores composed of six molecules of connexin 

• Permit water and some solutes directly between cells

Tight junctions

• Prevent solutes from leaking into the space between cells via paracellular routes

• Found in epithelial tissue and renal tubules; act as watertight junctions that prevent solute leakage; 

• Can be tight enough to induce a voltage difference 

Desmosomes

• Bind adjacent cells by anchoring to their cytoskeletons 

• Formed by interactions between transmembrane proteins associated intermediate filaments inside adjacent cells 

• Primarily found in interface between two layers of epithelial tissue 

Passive Transport

Do not require intracellular energy but use concentration gradients to supply energy for particles to move

• simple diffusion

• osmosis

• facilitated diffusion

Simple Diffusion

•  substrate moves down their gradient directly across the membrane 

  • Only freely permeable particles can use this 

  • Potential energy is dissipate as substrate moves down gradient

Osmosis

• specific kind of simple diffusion that concerns water; 

  • Moves from region of lower solute concentrations to one of higher solute concentration 

    • Moves from region of higher water concentration to one of lower water concentration 

  • Most notable when solute is impermeable to the membrane 

Hypotonic

• concentration of solutes inside the cell is higher than the surrounding solution 

  • Cell fills w/ water and lyses

Hypertonic

• concentration of solutes outside the cell is higher than the surrounding solution 

  • Water moves out of cell and cell shrivels

isotonicity

•  equimolar solute concentration exist outside and inside cell

Prevents net movement of water inside and outside cell

Collagative 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 

  • ex: oxmostic pressure

Osmotic Pressure

• describes the point at which water exerts a sufficient pressure to counterbalance the the tendency of water to flow across a membrane to produce equimolar environments 

  • In cells, is maintained against cell membrane rather force of gravity 

Calculating Osmostic Pressure

Equation 8.1

Ⅱ = iMRT

M = molarity of solution 

T = absolute temperature (Kelvins) 

i= van’t Hoff factor (# of particles obtained from molecule in solution) 

i_NaCl = 2 (Na⁺, Cl^-)

I_glucose = 1 (does not dissociate)

Failitated Diffusion

• Simple diffusion for molecules impermeable to membrane 

• Requires integral membrane proteins to serve as transporters

Carriers

•  example of facilitated diffusion where substrate binds to transport protein, remains in the transporter during conformational change; after which it dissociates from transporter 

  • Occluded state: carrier is not open to either side of phospholipid bilayer and is undergoing conformational change

Channels

• exist in open or closed conformations and are exposed to both sides of membrane and permit much more rapid diffusion 

  • viable transporters for faciliated diffusion

Active Transport

• Results in net movement of a solute against its concentration gradient 

• Always requires energy but that source of energy varies

Primary Active Transport

•  uses ATP or another energy molecule to directly power transport of molecules across a membrane 

  • Generally involves use of a transmembrane ATPase

  • Used in nervous system: maintains membrane potential of neurons

Secondary Active Transport

• uses anergy to transport molecules across membrane but is not directly directly coupled to ATP hydrolysis (dissimilar from primary transport) 

  • Harnesses energy released from one particle going down its electrochemical gradient to move another particle up its gradient

    • Symport: particles move in same direction

    • Antiport: particles move in opposite directions 

Summary of Membrane Transport Processes

Endocytosis

• cell membrane invaginates and engulfs material to bring it into the cell via a vesicle

  • Pinocytosis: endocytosis of fluids 

  • Phagocytosis: ingestion of large solids 

Veiscle coating proteins initiate invagination

Exocytosis

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

  • Important to nervous system and extracellular signlaing 

Membrane potential (V_m) 

• Difference in electrical potential across cell membranes 

  • Most cells -40-80mV

  • Ions may passively diffuse through leak channels so sodium-potssium pumps maintain potential

Calculating Membrane Potential

EQUATION 8.2

R = ideal gas constant 

T = temperature (K) 

Z = charge of the ion 

F = Faraday constant 

Goldman Hughes Katz Voltage Equation

EQUATION 8.3

P = permeabliity of relative ion

Sodium Potassium Pump (Na"+/ K+ ATPase)

• Maintain low concentration of sodium ions and high concentration of potassium ions intracellularly 

• Pumps 3 sodium out for every 2 potassium in

• Cells are more permeable to K⁺ than Na⁺ because there are more K⁺  leak channels than Na⁺ channels 

Mitochondrial Membrane Potential

• Outer mitochondrial membrane 

  • Highly permeable; many pores allow passage ion/small proteins

• Inner mitochondrial membrane 

  • Much more restricted permeability 

  • Has integral proteins along cristae that facilitate events of ATP synthesis and the ETC (electron transport chain) 

  • Does not contain cholesterol