CELL MEMBRANE Chapter 7

Cell Membrane Bilayer of phospholipids Phospholipid –The 2 tails are hydrophobic fatty acids –The head is a hydrophilic phosphate group –The tails and head are connected by glycerol Cell membranes also contain proteins and carbohydrates

Figure 8.1 Artificial membranes (cross sections)

Figure 8.4 The fluidity of membranes

Held together by weak hydrophobic interactions Lipids and proteins can drift laterally within membrane Cholesterol regulates membrane fluidity (makes less fluid when it is warmer, more fluid when it is colder) and is only found in animal cells Unsaturated fatty acids have “kinks” at the double bonds causing them to not pack together closely – makes membrane more fluid

Membrane proteins –Integral proteins - generally transmembrane with hydrophobic regions –Peripheral proteins - generally attached to membrane’s surface Cell-to-cell recognition –“Markers”- glycoproteins, glycolipids, & oligosaccharides (short polysaccharides)

Figure 8.9 Some functions of membrane proteins

Figure 8.7 The structure of a transmembrane protein

Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in cross section

TRAFFIC ACROSS MEMBRANE Selective Permeability - regulates the type and rate of molecular traffic into and out of cell –Nonpolar (hydrophobic) molecules dissolve in membrane cross easily ex. CO 2 and O 2

–Polar (hydrophilic) molecules Small, uncharged pass through easily (ex. water) Larger, uncharged will not easily pass (ex. glucose) All ions, even small ones, have difficulty penetrating hydrophobic region (ex. Na +, H + )

Passive Transport –Requires no energy –Diffusion - the net movement of a substance down a concentration gradient (from high to low concentration) Concentration gradient - regular, graded concentration change over a distance Net movement - the overall movement away from center of concentration

Results from random molecular motion, although net movement may be directional Increases entropy (increases disorder) Decreases free energy (-ΔG) so it is a spontaneous process Rate of diffusion depends on permeability of membrane Water diffuses freely across most membranes Net movement stops at equilibrium

Figure 8.10 The diffusion of solutes across membranes

–Osmosis -diffusion of water Water diffuses down its concentration gradient Direction is determined by total solute concentration, regardless of type or diversity of solutes in solutions

Figure 8.11 Osmosis

Hypertonic solution- solution with a greater solute concentration than inside the cell Hypotonic solution- solution with a lower solute concentration than inside the cell Isotonic solution- solution with the same solute concentration as inside the cell Water moves from hypotonic to hypertonic areas.

Water potential ( Ψ ) is the measure of the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane Water moves from high to low water potential Water potential depends on solute potential and pressure potential (in cells)

–Ψ = Ψ p + Ψ s »Ψ p is pressure potential »Ψ s is solute potenial –Water potential of pure water is zero so Ψ s of any solution will always be negative –Increasing solute, makes Ψ s more negative –Increasing Ψ s decreases water potential.

–Pressure potential is influenced by water movement into and out of plant cells. –Pressure potential is the physical pressure exerted on either side of a membrane. –Increasing Ψ p increases Ψ –A positive pressure potential means a plant cell is turgid and a negative pressure means it is flaccid.

Effects of osmosis –In hypertonic solution: animal cells shrivel, plant cell are plasmolyzed (cell membrane pulls away from cell wall) –In a hypotonic solution: animal cells are lysed (pop), plant cells are turgid (firm) –In isotonic: animal cells normal, plant cells are flaccid (limp)

Figure 8.12 The water balance of living cells

Osmoregulation - controlling water balance –Contractile vacuoles pump out water (ex. Paramecium) –Pumping out salts (ex. Bony fish) –Facilitated Diffusion - diffusion across membrane with the help of transport proteins Passive because solutes move down their concentration gradient and no energy is required Facilitated Diffusion Animation

Figure 8.13 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation

Figure 8.14 Two models for facilitated diffusion

Transport proteins –Specific for the solutes that they transport –Conformational change in protein allows solute to be transferred to other side –Gated proteins - channel opens in response to electrical or chemical signal –Ex. Aquaporins – channel proteins that transport water via facilitated diffusion »Problems with aquaporins associated with glaucoma, cataracts, and kidney diseases

Active Transport - an energy requiring process during which a transport protein pumps a molecule across the membrane, against its concentration gradient (from low to high concentration) –Requires +ΔG –Helps cell maintain steep ionic gradients across cell membrane –Uses ATP (energy) –Sodium Potassium Pump animationSodium Potassium Pump animation

Figure 8.15 The sodium-potassium pump: a specific case of active transport

–Ex. Sodium-potassium pump Transport protein has binding sites for Na + on interior side and sites for K + on exterior side Na + binds to protein and stimulates ATP to phosphorylize the protein thereby changing its shape This changed shape allows Na + to be expelled outside of cell and allows K + to bind on outside of protein

K + binding triggers release of phosphate from protein Loss of phosphate restores proteins original shape and expels K + into cell Na + K + pump translocates 3 Na + ions out of cell for every 2 K + ions pumped into cell.

Ion pumps generate voltage across membrane –Because anions and cations are distributed unequally across cell membranes, all cells have voltages across their membranes (batteries) –Membrane potential - voltage across membrane ranges from -50 to -200 mv (the inside of the cell is more negative than the outside)

–That negative inside favors the passive transport of cations into the cell and anions out of cell. –Electrochemical gradient - diffusion gradient resulting from combined effects of membrane potential and concentration gradient Ions may not always diffuse down their concentration gradient, but always diffuse down their electrochemical gradient

–Electrogenic pump - a transport protein that generates voltage across membrane Ex. Sodium potassium pump (in animals): 3 Na + move out and only 2 K + move in (net charge of +1 on outside of cell) Ex. Proton pump (in bacteria, fungi, and plants): actively transports H + outside of cell

Figure 8.17 An electrogenic pump Ion pump animation

Cotransport - a process where a single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their concentration gradient Cotransport animation

Figure 8.18 Cotransport

–One example in plants: An ATP driven proton pump sends H + outside of the cell Then H + diffuses back into cell via a specific transport protein As H + diffuses, sucrose can ride the proton’s “coattails” and move into the cell (against its own concentration gradient).

–One example in humans: If someone has severe diarrhea or is badly dehydrated from running… –Give person solution high in glucose and salt –Solutes transported to blood causes water to move into blood from colon (rehydration) –Cotransport involves Na + and glucose so both needed

Figure 8.19 The three types of endocytosis in animal cells

Exocytosis - process of exporting macromolecules (ex. proteins and polysaccharides) from a cell by fusion of vesicles with a cell membrane Endocytosis - process of importing macromolecules (ex. proteins and polysaccharides) into a cell by forming vesicles derived from the cell membrane Endo animation More…

–Phagocytosis - endocytosis of solid particles –Pinocytosis - endocytosis of fluid droplets –Receptor-mediated endocytosis - a ligand binds to a receptor site in a coated pit and causes a vesicle to form and ingest material. This is more discriminating than pinocytosis.