Concentrated sugar solution Sugar molecules (Water molecules not shown) 100ml 100ml Hypertonic [S] g [H2 Hypotonic [H O] 2 O] [H 2 O] g Semipermeable Dilute sugar solution (100ml) Time 125ml Osmosis 75ml Movement of Membrane water A membrane is impermeable when nothing can pass through, semipermeable when only water can pass through, and selectively permeable when water and only certain small solute particles can pass through. The membrane above is impermeable to sugar and permeable to water. The solute particles are not the only particles in solution? Water molecules are particles! The diagram shows the concentration difference between solutions as the number of sugar molecules on each side of the membrane. The more solute particles, the fewer free water molecules because water molecules interacting with solute particles are not free to diffuse. Because the membrane is not solute permeable, only water will diffuse. Tonicity is the osmotic power of a solution to draw water to itself. Hypertonic (), Hypotonic (), Isotonic (=[S]). Osmosis is the diffusion of water through membranes! 15
Osmosis Pressure Pressure [S] g [H 2 O] [H 2 O] Hypotonic [H 2 O] Hypertonic g The significant difference between this example and the previous is a lid with pressure gauges. Osmotic pressure is the pressure produced in a closed system by the diffusion of water through a membrane (osmosis). In a closed system (lid), the pressure on the hypotonic side of the membrane will decrease and on the hypertonic side will increase. Either condition can be catastrophic for a living cell!!! Osmolarity must be precisely regulated in living systems (osmoregulation to maintain homeostasis). Osmotic pressure is dangerous! 16
The animal cells in this example are RBCs. Osmosis Plant cells have a cell wall and central vacuole. Animal cells do not. The solution surrounding the cells is hypertonic when [H 2 O]. For both cell types, water will diffuse along it s concentration gradient OUT of the cell causing the RBCs to crenate (shrivel) and the plant cell to plasmolyze (plasma membrane shrinks away from the cell wall). The solution surrounding the cell is hypotonic when [H 2 O]. For both cell types, water will diffuse along it s concentration gradient INTO the cell Crenation Normal Cytolysis causing the RBCs to cytolyze (swell & burst) and the plant cell to become turgid Plasmolysis Flaccid Turgid (increased osmotic turgor pressure). (wilted) (Normal) Plant cells prefer a hypotonic environment to maintain turgor pressure so they do not become flaccid (wilt). Animal cells prefer an isotonic environment. Interstitial fluid is the aqueous solution that baths all of our (animal) cells. Osmoregulation is the homeostatic system to maintain isotonic interstitial fluid. In animals, this is performed primarily by the excretory system (kidneys). If your hospital IV solution (saline) is hypertonic, your RBCs will crenate. If it is hypotonic, they will cytolyze. Either is fatal and requires medical technicians to mix their concentrations VERY PRECISELY! Osmoregulation is crucial! 17
Extracellular Cell membrane Proteins Cell Membrane Constituents Carbohydrate chains Intracellular (cytoplasm) Protein channel Phospholipid bilayer Passive Transport The fluid mosaic model of the plasma membrane describes the phospholipid bilayer as very dynamic (fluid) with a wide variety of protein molecules embedded in it (mosaic). Attached to some proteins are carbohydrate polymers forming glycoproteins that function in cell to cell recognition (Ex. Self vs. Non-self immunity). Membrane proteins can also transport particles across the membrane, be enzymes catalyzing metabolic reactions, and be receptors for signal molecules that bind causing a change in the cell. The plasma membrane phospholipid bilayer alone is a selectively permeable barrier to large, charged particles. Certain proteins embedded in the bilayer increase membrane permeability and selectivity. There are also passive facilitated carrier proteins, passive cotransport proteins, and active transport pumps, all very specific. Membrane function depends on proteins! 18
Membrane permeable solute particles are: 1. Small 2. Uncharged 3. Non-polar Passive transport is diffusion of particles across a membrane along their concentration gradient, without requiring cellular energy (ATP). Channel proteins create a pore of the proper size and properties to facilitate the passive transport of specific particles. Carrier proteins bind specific solute particles and through conformational change facilitate their passive translocation across the membrane. Facilitated diffusion requires a concentration gradient. Facilitated diffusion is a passive form of channel or carrier transport. A gated channel can alter permeability by being opened or closed. Gated channels can be either chemically gated and open/close when a signal molecule binds to them, or electrically gated and open/close in response to a change in charge across the membrane. Facilitated Diffusion Glucose molecules Protein channel or carrier Facilitated diffusion is a passive form of channel/carrier transport! 19
Active transport is the pumping of particles across the membrane AGAINST their concentration gradient using cellular energy (ATP). Active transport does not require a concentration gradient. Active transport proteins are also known as pumps. The energy for active transport is supplied by ATP, the energy currency of all living cells. Unlike passive transport, active transport enables cells to highly concentrate specific particles in the cytoplasm or other cellular compartments (organelles). Active Transport Molecule or ion to be carried Low Solute Concentration PUMP High Solute Concentration Active transport uses ATP to pump particles against their [ ] g! 20
The direction of the [Na + ] g is extracellular to intracellular. The [K + ] g is intracellular to extracellular. If membrane permeable, each ion would diffuse along their concentration gradient. The phospholipid bilayer is impermeable to these ions because they are charged. Both these ions are being pumped against their [ ] g indicating that this is active transport. The other indication of active transport is ATP! This active transport protein pumps 3 sodium ions out and 2 potassium ions in. Pumping more positive charge out than in results in a slight intracellular negative charge, [Na + ] g Active Transport [K + ] g making this an electrogenic pump. The sodium-potassium pump is a ubiquitous electrogenic pump! 21