Membrane Structure and Function Chapter 8

Fluid Mosiac Model

Membrane is a fluid mosaic 1. Of lipids, carbohydrates and proteins 2. Held together by hydrophobic interactions 3. Molecules can drift laterally A. phospholipids move quickly B. proteins slowly C. some proteins attached to cytoskeleton and can not move much Membrane movie

D. Unsaturated hydrocarbon tails enhance fluidity 4. Membranes solidify at critical low temps. A. organisms in colder temps > concentration of unsaturated phospholipids B. cholesterol modulates fluidity 1.Less fluid at warmer temps 2. More fluid at lower temps

Membranes: STRUCTURE and FUNCTION 1.Integral proteins Penetrate the hydrophobic interior of membrane Unilateral – only partway across membrane Transmembrane – across membrane

2. Peripheral – attached to surface May be attached to integral proteins, fibers in ECM (extracellular matrix), or cytoskeleton Gives animal cells stronger framework

Membrane Proteins

Function of proteins 1. Transport (tunnels) 2. Enzymes 3. Signal transduction 4. Cell-cell recognition. 5.

Membranes are bifacial 1. Proteins have distinct orientation 2. Carbohydrates restricted to membranes exterior (oligosaccharide) 1. Glycolipid – attached to phospholipid 2. Glycoprotein – attached to protein 3. Glcocalyx – all extended carbohydrates

Bifacial arrangement

Membrane Carbohydrates 1. Function: Cell to cell recognition 1. Sorting: animal embryo cells into tissue and organs 2. Rejection of alien cells by immune system 3. Vary from species

Membrane permeability 1. Terms: semi-permeable, selectively permeable, differentially permeable 2. Permeability depends upon: 1. phospholipid layer 2. Specific integral proteins

Permeability of lipid bilayer 1. Nonpolar molecules: 1. Cross with ease 2. Smaller molecules cross faster 2. Polar molecules: 1. Small, polar uncharged molecules pass easily ( H 2 O, CO 2 ) 2. Larger polar uncharged will not (C 6 H 12 O 6 ) 3. All ions will not pass easily

Passive transport 1. Does not require energy 2. Direction of movement: 1. away from concentration center 2. Down the concentration gradient 3. Results from random molecular movement in ALL directions (kinetic energy of molecules) 4. Concentration gradient – change in concentration over a distance in a particular direction

Types of Passive transport 1. Diffusion 2. Osmosis 3. Facilitated diffusion

Passive transport 1. Down concentration gradient 2. Spontaneous process 3. Decreases free energy 4. Increases entropy 5. DOES NOT require energy 6. Rate is regulated by permeability of membrane

Diffusion 1. NET movement of a substance down a concentration gradient 2. What direction is this? 3. Continues until dynamic equilibrium is reached (Does movement of molecules stop?) osmosis

Osmosis 1. Diffusion of water across a selectively permeable membrane 2. What direction does water move? 3. Osmotic concentration – total solute concentration of a solution 4. Osmotic pressure – measure of a tendency for a solution to take up water when separated by a membrane

Solutions in relation to membranes 1. Hypertonic solution 1. A solution with a greater solute concentration when compared to another solution (i.e. inside cell) 2. Hypotonic solution 1. Solution with less solute when compared to another solution 3. Isotonic solution 1. Solution with equal solute concentration when compared to another solution

Direction of movement of H 2 O 1. Down its concentration gradient 2. Determined by difference in TOTAL solute concentration (all types solutes) 3. From hypotonic (hypoosmotic) to hypertonic (hyperosmotic) 4. Hypoosmotic – lower osmotic conc. 5. Hyperosmotic – higher osmotic conc.

Osmotic pressure 1. Pure water = zero 2. Proportional to its osmotic concentration 1. The greater the solute concentration 2. The greater the osmotic pressure

Water balance in animal cells 1. In isotonic environments 2. No net movement of water 3. In hypertonic environments 1. Cell will lose water 2. Crenation (shrivel) 4. In hypotonic environments 1. Gain water 2. lyse

Osmoregulation 1. Contractile vacuoles (protists) 1. Pump water out in hypotonic env. 2. Pump out salts 1. Conserve water in hypertonic env.

Water balance in cells with cell walls 1. In hypertonic environments 1. Plasmolyze-plasma membrane pulls away from cell wall 2. In hypotonic environments 1. Pressure against cell wall equals the osmotic pressure of cytoplasm 2. Dynamic equilibrium established 3. turgid

Turgidity 1. Tension found in wall cells 2. In hypoosmotic environment 3. Ideal state for most plants 4. Provides mechanical support for plants 5. Requires cells to be hyperosmotic to environment

3.In isotonic environments 1. No net movement of water 1. Is water moving? 2. Flaccid 1. Loss of structural support

Water Potential (Ψ) and Osmosis Define osmosis Water potential (Ψ) 1. The free energy of water 2. A consequence of solute concentration and pressure 3. Physical property predicting the direction of water flow 4. Measured in units of pressure megapascals (MPa)

Movement of water 1. From solution with higher water potential 2. To solution with lower water potential 3. Pure water = 0 MPa 4. Ψ = 0

Change in water potential 1. Addition of solutes lowers water potential (into negative) 2. Increased pressure raises water potential (into positive range 3. Bulk flow– movement of water due to pressure differences 4. Faster than movement due to concentration differences

Effects of pressure and solute concentration 1. Ψ = Ψ P + Ψ S 2. Example: M solution = Ψ S is – In an open container Ψ P is 0 5. What is the Ψ? 6. Ψ = 0 + (-.23) 7. Ψ = -0.23

Movement of water 1. Water would enter solution due to osmotic pressure only 2. Addition of pressure 1. Counter affects of osmotic pressure 2. Stopping net water movement 3. Forcing water from solution into pure water

Facilitated diffusion 1. Diffusion with help of transport proteins 2. Aids in diffusion of polar molecules and ions

Transport proteins share some properties of enzymes 1. Specific 2. Can be saturated 3. Can be inhibited by molecules resembling solute 4. Do not catalyze reactions

Action of transport proteins 1. Remain in place 2. Alternate between two conformations 3. One conformation binds to solute 4. Another conformation deposits solute 5. Binding and release of solute may trigger conformation

Some are selective channels 1. Permeable to specific solutes 2. Solutes pass through channels 3. Selective channels 4. May open in response to electrical or chemical stimuli 1. Na + and K + ions 2. neurotransmitters

Active Transport 1. Energy (ATP) requiring process 2. Transport protein pumps a molecule against concentration gradient 3. Energetically uphill (+delta G) 4. Requires cell to expend energy 5. Maintain steep ionic gradients across membrane 1. High affinity for K + with binding sites towards ECM

Examples 1. Sodium-potassium pump 2. Transport protein oscillates between 2 conformations 1. High affinity for Na + with binding sites towards cytoplasm

Action 1. ATP phosphorylates transport protein 2. Powers the conformational change 3. From Na + receptive to K + receptive 4. Conformation change translocates bound solutes 5. Na + K + -pump translocates 3 Na + out of cell for every 2 K + into cell

Na + K + Pump

Membrane potential 1. Voltage across membranes 2. Range: -50 to –200 mv 3. Cytoplasm side of cell is negative 4. Affects movement of charged substances across membrane 5. Favors diffusion of cations

Two forces drive passive transport of ions 1. Concentration gradient 2. Effect of membrane potential

Electrochemical gradient 1. Combined effects of membrane potential and concentration gradient 2. Ions: 1. May not always diffuse down their conc. gradient 2. Always diffuse down their electrochemical gradient 3. Distribution of ions may be different from expected

Cotransport 1. Membrane protein couples the transport of one solute to another 2. Single ATP actively transports one solute 3. indirectly drives the transport of other solutes 4. Against concentration gradients

Involves 2 transport proteins 1. ATP powered pump actively transports one solute (protein) 2. Another transport protein allows solute’s downhill diffusion 1. Solute leaks back across membrane 2. As a second solute’s uphill transport across membrane

Endocytosis 1. Importing macromolecules 2. Form vesicles derived from plasma membrane 3. Vesicle forms form a localized region 4. Sinks inward 5. Pinches off into cytoplasm

Types of endocytosis 1. Phagocytosis 1. Solid particles 2. Cell engulfs particle with pseudopodia 3. Vacuole fuses with lysosome 2. Pinocytosis 1. Fluid droplets 2. Droplets taken into small vesicles 3. Not a discriminating process

What is this?

Exocytosis 1. Exporting macromolecules 2. Fusion of vesicles with plasma membrane 3. Vesicle budded from ER to Golgi migrates to plasma membrane 4. Used by secretory cells to export products 1. insulin in pancreas 2. neurotansmitters

Path of export vesicle

Endocytosis and exocytosis