The Cell Membrane Structure, Function, and Transport

Structure Composed of lipids (mostly phospholipids) and proteins Phospholipids are amphipathic (contain a hydrophilic “head” and a nonpolar, hydrophobic “tail”) Hydrophobic interactions cause the tails to turn inward so they are not exposed to water Creates a phospholipid bilayer (heads on the outside, tails inside)

Phospholipid Bilayer

Structure Phospholipids are not bonded, making the double layer fluid  Phospholipids may move laterally  Unsaturated hydrocarbon tails enhance fluidity Proteins are embedded throughout the membrane  Peripheral proteins – sit on the surface  Integral proteins – embedded within the membrane  May be hydrophobic or hydrophilic

Structure Several types of proteins in the membrane:  Channel – small openings for molecules to diffuse through  Carrier – binding site on the surface that binds to molecules and pulls them inward  Receptor – receive signals and set off cell responses  Cell recognition – ID for the cell  Enzymatic – carries out metabolic reactions

Structure: Fluid Mosaic Model

Structure Other components:  Cholesterol – makes membrane stronger and less fluid Also lowers temp. required for the membrane to solidify.  Carbohydrates – usually branched oligosaccharides bonded to lipids (glycolipids) or proteins (glycoproteins) Distinguish cells and help cell-to-cell recognition

Transport Cell membranes are selectively permeable, or semi-permeable  Certain substances may pass through, but others cannot  Depends on lipid bilayer and specific transport proteins  Typically, small, uncharged polar molecules and small nonpolar molecules pass freely  Larger molecules and ions and water require channel proteins

Transport The cytoplasm is a solution of many substances in water This creates a concentration gradient – different concentrations inside and out  Concentration = mass solute/volume solvent Remember: Molecules will naturally move from high to low concentration

Diffusion Particles move from high concentration to low concentration Ex: dissolved oxygen diffuses across membrane for cellular respiration Equilibrium – when concentration is the same inside and out; no net movement  Is dynamic  Particles still move at this point, but equally in both directions

Osmosis Some substances are too large to pass through the membrane (impermeable) Water passes quite easily (permeable) through a membrane  Through proteins called aquaporins Osmosis is the diffusion of water through a selectively permeable membrane

Osmosis As the water moves from high to low, it causes the right side to rise Remember: Salt Sucks  Draws water in its direction

Osmosis and Solutions Isotonic – concentrations of water and solute are the same Hypertonic – higher concentration of solute (low water) Hypotonic – lower concentration of solute (high water) (These terms always refer to solute!)

Solutions

Osmosis Water balance is crucial for cells  not a problem if the cell is in isotonic surroundings A cell without rigid cells walls cannot tolerate extreme: they can shrivel or burst (lyse) Osmoregulation – the control of water balance for organisms  May involve special adaptations  Paramecium have a contractile vacuole to pump out excess water

Water Balance with Cell Walls Turgor Pressure – the pressure of water against the cell wall  Turgid – the cell wall exerts pressure back on the cell; firm; plenty of water; no more water can enter the cell  Flaccid – not enough turgor pressure; limp Plasmolysis – the plasma membrane pulls away from the cell wall as the cell shrivels

Passive Transport Does not require energy  Includes diffusion and osmosis Also includes facilitated diffusion – carrier proteins in the membrane help larger molecules pass through  Still moves molecules from high to low concentration

Facilitated Diffusion

Transport proteins are specific to the solute it transports  Can be saturated Carrier protein channels allow water molecules and ions to flow through  Aquaporins – water channel proteins  Gated channels – stimulus causes them to open and close

Active Transport Sometimes cells must move materials in the opposite direction – against the concentration gradient This requires energy Carried out by protein “pumps” in the cell membrane

Active Transport Performed by specific proteins in the membrane Energy supplied by ATP  Phosphate group may be transferred to the transport protein and allows solute to be passed through  Sodium-potassium pump – exchanges sodium ions for potassium ions Moves three Na+ out and two K+ in

Ion pumps generate voltage… All cells have voltages (electrical potential energy) across their membranes – membrane potential  Cytoplasm has a negative charge in comparison to outside the cell Combination of chemical forces (concentration gradient) and electrical force is called the electrochemical gradient  An ion may diffuse down its electrochemical gradient

Active Transport Electrogenic pump – transport protein that generates voltage across a membrane  Sodium-potassium pump  Proton pump – actively transports H+ out of the cell in plants Electrogenic pumps store energy to be used in cellular work

Cotransport A single ATP- powered pump can indirectly drive the active transport of other solutes One solute’s “downhill” diffusion drives the other’s “uphill” transport

Other types of Transport Endocytosis – cell takes in large molecules by creating pockets and engulfing them  Phagocytosis – “cell eating;” extensions of cytoplasm surround food particle, packages it within a food vacuole and cell engulfs it Used by ameobas  Pinocytosis – tiny pockets along the membrane fill with liquid and pinch off; used to take in liquid Exocytosis – opposite of endosytosis; cell removes materials

More transport Receptor-mediated endocytosis – extracellular substances bind proteins with specific receptor sites found in coated pits  Ligands – molecule that binds to receptor When ligand bind to receptor proteins, the coated pits form vesicles which pulls the substance into the cell Very specific Enables cell to acquire bulk quantities of specific substances