* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Human Physiology Lecture Reading Notes
Survey
Document related concepts
Organ-on-a-chip wikipedia , lookup
Cell nucleus wikipedia , lookup
Magnesium transporter wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Cytokinesis wikipedia , lookup
Mechanosensitive channels wikipedia , lookup
Extracellular matrix wikipedia , lookup
Theories of general anaesthetic action wikipedia , lookup
SNARE (protein) wikipedia , lookup
Ethanol-induced non-lamellar phases in phospholipids wikipedia , lookup
Lipid bilayer wikipedia , lookup
Membrane potential wikipedia , lookup
Model lipid bilayer wikipedia , lookup
Signal transduction wikipedia , lookup
Cell membrane wikipedia , lookup
Transcript
Human Physiology Lecture Reading Notes Lecture 2: Homeostasis Membranes • • • • • General function of cell membrane: o Physical isolation: separates intra- and extra-cellular fluid o Regulation of exchange: controls entry of ions and nutrients into cell, and release of waste + products o Communication: contains proteins that recognise and respond to molecules/changes in external environment o Structural support: proteins hold cytoskeleton to maintain cell shape, also specialised junctions All membranes consist of lipid, protein and SMALL amount of carbohydrate o More metabolically active = more protein o Lipids mostly phospholipids (as a bilayer, tails inwards) o Proteins studded within bilayer o Extracellular surface of glycoproteins + glycolipids 3 types of lipids in membrane: o Phospholipids o Sphingolipids: fatty acid tail, head either phospholipid or glycolipid, longer than phospholipid o Cholesterol: hydrophobic, make membrane impermeable to water-soluble molecules, increase flexibility in range of temps Membrane proteins: o Integral proteins: tightly bound to the membrane, removed by disrupting the membrane structure with detergents to destroy membrane’s integrity. Eg. transmembrane proteins o Peripheral proteins: attached to other membrane proteins by non-covalent interactions and can be separated from the membrane by chemical methods that do not disrupt the integrity of the membrane. Eg. enzymes o Transmembrane proteins: called membrane-spanning proteins b/c the protein’s chains extend all the way across the cell membrane. When a protein crosses the membrane more than once, loops of the amino acid chain protrude into the cytoplasm + extracellular loops + phosphate groups may attach to the intracellular loops Membrane carbohydrates – mostly sugars attached either to membrane proteins or lipids. o Found on external surface of cell where they form a protective layer known as the glycocalyx o Glycoproteins on cell surface play a key role in the body’s immune response Homeostasis • • • • • • • 2 distinct fluid compartments: cells + fluid surrounding cells Extracellular fluid = buffer b/n cells and environments outside the body Water moves freely b/n cells and extracellular fluid Osmotic equilibrium: because of free movement of water, extra- and intracellular compartments reach this Chemical disequilibrium: some solutes are more concentrated in one body compartment than the other o Extracellular fluid: sodium, chloride, calcium more concentrated outside than inside o Intracellular: potassium more concentrated inside Proteins concentrated in plasma but cannot cross blood vessels Smaller molecules and ions pass freely b/n endothelial cells thus have same concentration in plasma and interstitial fluid • • • If solutes leak across cell membranes, energy is required to return them to the compartment they left o K+ leaks out of cell, Na+ leaks into cell… returned by Na+K+ATPase Distribution of electrical charge between intra- and extra-cellular comparments… o Body electrically neutral o Few extra negative ions found in intracellular fluid, positive ions in extracellular o Inside of cells slightly –ive related to outside o Electrical disequilibrium – ionic imbalance o Changes in disequilibrium create electrical signals Homeostasis is not the same as equilibrium Diffusion • • • • • • • • • • • • • • • • Movement of molecules from an area of higher concentration to lower concentration Passive process – does not require input of energy from some outside source Uses only kinetic energy possessed by all molecules Diffuse down the chemical gradient Rate of diffusion depends on magnitude of concentration gradient Net movement of molecules occurs until the concentration is equal everywhere Diffusion is rapid over short distances but much slower over long distances Diffusion is directly related to temperature Diffusion rate is inversely related to molecular weight and size – smaller molecules require less energy thus diffuse faster Diffusion can take place in an open system or across a partition that separates two compartments Ions do not move by diffusion – influenced by electrical gradients because of the attraction of opposite charges and repulsion of like charges Diffusion = passive movement of uncharged molecules down concentration gradient due to random molecular movement Diffusion across membranes… o Lipid-soluble molecules can pass thru phospholipid bilayer o Lipophilic substances that can pass thru lipid centre of a membrane move by diffusion Diffusion directly across phospholipid bilayer = simple diffusion o Rate of diffusion depends on the ability of the diffusing molecule to dissolve in the lipid layer of the membrane o Diffusion rate depends on how permeable the membrane is to the diffusion molecules o Rate of diffusion across a membrane is directly proportional to the surface area of the membrane § Larger the surface area, the more molecules can diffuse across per unit time Flick’s law of diffusion has several factors: o Size of diffusing molecule § As molecule size increases, membrane permeability decreases) o Lipid solubility of the molecule § As solubility increases, membrane permeability increases o Composition of the lipid bilayer across which it is diffusion – alterations in lipid composition of the membrane change how easily diffusing molecules can slip between the individual phospholipids Flux of a molecule across a membrane depends on the concentration gradient and the membrane’s permeability to the molecule Protein-Mediated Transport • • Vast majority of solutes cross membranes with the help of membrane proteins Mediated transport = passive and moves molecules down their concentration gradient • • Facilitated diffusion = net transport stops when concentrations equal on both sides of memb. Active transport = protein-mediated transport requires energy from ATP or another outside source and moves a substance against its concentration gradient Carried out by proteins known as transporters • 4 broad categories of membrane proteins… o Structural: connect membrane to cytoskeleton to maintain shape of cell, create cell junctions that hold tissues together, attach cells to extracellular matrix by linking cytoskeleton fibers to extracellular collagen and other protein fibers o Enzymes: catalyse chemical reactions that take place on cell’s external surface or just inside the cell o Receptors: part of body’s chemical signally system, binding of receptor with its ligand usually triggers another event o Transporters: moves molecules across membranes – channel proteins (water-filled passageways that directly link the intracellular and extracellular compartments) and carrier proteins (bind to the substances that they carry but never form a direct connection between the intracellular and extracellular fluid) Channel proteins – have pore + gap junctions, movement through these smaller channels is mostly restricted to water and ions o May be specific for one ion or may allow ions of similar size and charge to pass o Selectivity of a channel determined by diameter of its central pore + electrical charge of amino acids that line the channel o If channel is positively charged, positive ions repelled, negative ions pass thru o Channel gates take several forms – § Open channels: gate is mostly open, allowing ions to move back and forth across membrane without regulation – ie. leak channels § Gated channels: gate mostly closed, allows channels to regulate movement of ions through them… controlled by chemicals or voltage or physical forces Carrier proteins – bind with specific substrates and carry them across the membrane by changing conformation, small organic molecules that are too large to pass through use carriers o Uniport carriers – use one kind of molecule o Contransporter – move more than one kind of molecule at one time o Symport carriers – molecules being transported are moving in same direction o Antiport carriers – molecules are being carrier in opposite directions • • • • • • • • • • • • • Channel proteins move many more molecules per second than carrier proteins Carriers never create a continuous passage between inside and outside of cell Carries transport molecules across a membrane in both directions – can restrict a protein to 1D One side of carrier protein creates a barrier that prevents free exchange across membrane Molecule being transported binds to carrier on one side of membrane – changes conformation of the carrier protein so that the opening closes Facilitated diffusion uses carrier proteins Primary active transport – known as ATPases – sometimes caused pumps. Eg. sodium potassium pump Secondary active transport – uses kinetic energy of one molecule moving down its concentration gradient to push other molecules against their concentration gradient Specificity – ability of a transporter to move only one molecule or only one group of closely related molecules Competition – transporter may move several members of a related group of substances, but those substrates compete with one another for binding sites on the transporter. Saturation – rate of substrate transport depends on substrate concentration and the number of carrier molecules.