Ch 7: Membrane Structure and Function

Fluid Mosaic Model Cell membrane  Selectively permeable – allows some substances to cross more easily than others  8 nm thick  Fluid structure of phospholipids w/ a mosaic of various proteins embedded in it  Cholesterol maintains fluidity – acts as temperature buffer Less fluid as temp increases by restricting movement More fluid as temp decreases by maintaining space

Structure of Cell Membrane

Membrane Proteins Different types of cells contain different types of membrane proteins Integral proteins – embedded into the hydrophobic core of lipid bilayer, can go all the way through membrane Peripheral proteins – on surface of membrane

Types of Membrane Proteins Transport  Channel/ Carrier proteins (hydrophilic tunnel) allow polar molecules and ions to pass through hydrophobic layer  Channel and Carrier proteins are specific for the substance they transport Enzymatic Signal Transduction (chemical messages)  Receptor proteins transmit information from outside of the cell to inside of the cell Cell Recognition  ID tags – glycolipids or glycoproteins Intercellular joining  Join together adjacent cells Cell Support  Attach to cytoskeleton or extra-cellular matrix for stability

Passive Transport Passive Transport - Movement across the cell membrane that does not require energy Diffusion – movement of particles from an area of high concentration to an area of low concentration until an equilibrium is reached (go with/ down its concentration gradient) Concentration gradient – the difference in the concentration of a substance across a space Equilibrium – conc. of a substance is equal throughout a space (doesn’t stop moving)

Osmosis Osmosis – diffusion of free water across a selectively permeable membrane Water diffuses across the cell membrane from the region of low solute conc. to that of a higher solute conc. until it reaches equilibrium

Osmoregulation in Cells w/o cell walls Osmoregulation – control of water balance Tonicity – ability of a solution to cause a cell to gain /lose water  Isotonic solution – no net movement of water across the cell membrane  Hypertonic solution – more free water inside the cell :. cell will lose water to its environment, and shrivel  Hypotonic solution – less free water inside the cell :. water will enter cell, it will swell and lyse Some cells have a contractile vacuole to pump water out of cell

Osmoregulation of Cells w/ walls Plant cells are healthiest in a hypotonic solution, osmotic pressure keeps cell wall turgid (very firm) Plant cells are flaccid (limp) in an isotonic solution In a hypertonic solution, the cell membrane will shrink and pull away from the cell wall…called plasmolysis (wilt)

Facilitated Diffusion: Passive Transport aided by proteins Channel Proteins – hydrophilic passageways  Some are always open for diffusion  Rate of movement is determined by conc. Gradient  (+) charged ions more likely to diffuse INTO cell  (-) charged ions morel likely to diffuse out of the cell  Some ion channels have gates and can be opened by: Stretching of cell membrane Change in electrical charge Binding of specific molecule

Voltage Gated Channels Nerve Cells transmit electrical signals by opening a series of Na + gated channels 1.Channel is closed 2.Area changes voltage 3.Channel opens briefly 4.Na + flood into cell – voltage changes 5.Channel closes and electrical signal passes on to the next voltage channel

Chemically Gated Channels Nerve cells send out a neurotransmitter called acetylcholine (ACh) to nearby muscle cells to signal muscle to contract 1.ACh binds to ACh receptor protein 2.Receptor gate opens for a microsecond to allow Na + in 3.Na + sets off muscle contraction

Facilitated Diffusion: Passive Transport aided by proteins Carrier Proteins 1.Specific substance binds to carrier protein 2.Protein changes shape and transports substance across cell membrane 3.Molecules is released into the cell, and carrier protein returns to its original shape

Active Transport Active Transport - uses energy to move solutes AGAINST conc. gradient Carrier proteins act as “pumps” powered by ATP Examples  Sodium Potassium pump  Proton Pump  Cotransport

Sodium Potassium pump Pumps 3 Na+ out of the cell and 2 K+ into the cell Actively transports both ions against their conc. Gradient, powered by ATP Prevents Na+ from accumulating in the cell 1.3 Na+ and a P (from ATP) bind to inside protein pump 2.Pump changes shape transporting 3 Na+ across membrane and out 3.2 K+ bind to pump and are transported across membrane 4.2 K+ and P are released inside of cell

Proton Pump Actively transports protons (H + ) through the internal membranes of mitochondria and chloroplasts

Cotransport (Coupled Transport) Cotransport – an ATP powered pump that transports a specific solute, can indirectly drive the active transport of several other solutes  Ex. As proton pump pumps H+ out, H+ diffuse back into the cell pulling sucrose molecules into the cell with it!

Bulk Transport: Substances that are too large to be transported by carrier proteins Exocytosis (export)  Secretion of macromolecules by fusion of vesicle with membrane, releasing the contents outside of cell

Bulk Transport (cont’d) Endocytosis  Cell membrane engulfs particles and pinches off to form vesicle inside the cell. Vesicle may fuse w/ lysosomes or other organelles  3 Types: Phagocytosis Pinocytosis Receptor-mediated