BI203-- membrane physiology: passive transport

what part of the cell is crucial for maintaining homeostasis

cell plasma membrane

what is cell plasma membrane composed of?

lipids and proteins

what is the basic structure of a lipid bilayer?

• 75% phospholipids

• 20% cholesterol

• 5% glycolipids

phospholipid structure

• polar head (electrically charged)

• two nonpolar fatty acid tails (electrically neutral)

• bilayer arrangement as lipids are amphipathic (polar head hydrophilic and non polar tails hydrophobic)

• hydrophobic regions cluster together exposing polar end to either extracellular or cytosol fluid

glycoproteins

polar on carbohydrate head and non polar on their tail

only found on extracellular membrane (asymmetric)— important cellular functions

cholesterol

mildly amphipathic and found interspersed between other lipids

two kinds of membrane proteins +functions

integral: extend through lipid bilayer and not easily removed

peripheral: associated with polar heads of lipids or integral proteins and can be removed easily

functions: transport, receptors, self recognition

integral membrane proteins

• some attach to lipids in bilayer

• polypeptide chain traverses lipid bilayer

• the portion within bilayer consists primarily of hydrophobic amino acids

• portion of polypeptide that project out from bilayer tend to have high percentage of hydrophilic amino acids

• those that project into aqueous surroundings of vell are usually glycoproteins— with many hydrophilic sugar residues attached to part of polypeptide exposed at surface of cell

• some transmembrane proteins that span the bilayer several times form hydrophilic channel through which certain ions and molecules can enter or leave cell

peripheral membrane proteins

do not penetrate hydrophobic core of the membrane

often found in association with integral membrane proteins

can be removed from membranes by means that do not require the disruption of the membrane structure

plasma membrane — fluid mosaic model (image)

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membrane transport

• small non charged (non poalr) lipid soluble materials can cross the lipid bilayer easily

• macromolecules, ions and charged (polar) molecules need specialised proteins

• membrane slectively permeable

• passive transport (no cellular energy)

• active transport (cellular energy)

passive transport (diffusion)

• diffusion: random mixing of particles that occur in a solution because of the particles

• in solute/solvent mixture both substances and liquid undergo diffusion

• solute diffuses from area of high concentration to area of lower concentration e.g. crystal dye diffusion

• in a cells case ( a membrane is present)— if the concentration of O₂ in extracellular fluid greater than the cytosol, O₂ will diffuse into cell— similarly CO₂ will diffuse out

• diffusion across plasma membrane will only occur for small, non charged (non polar) lipid soluble molecules (e.g. O₂, CO₂, oestrogen)

factors influencing diffusion rate across membranes

1. concentration gradient: greater the diff in concentration gradient between two sides of membrane→ higher rate of diffusion

2. temperature: higher rates of temperature lead to higher rates of diffusion

3. molecular mass: larger the mass of diffusing particle the slower the rate of diffusion

4. surface area: larger membrane surface area leads to faster diffusion rates (e.g. Lungs)

5. diffusion distance: greater the distance the slower the rate if diffusion (e.g. Pneumonia)

facilitated diffusion

• molecules (e.g. glucose) transported across membrane by special carrier proteins

• facilitated diffusion: net movement is down a concentration gradient and is passive

• molecule to be transported attaches on binding site on protein carrier

• carreir protein changes conformation, exposing boung molecule to other side of membrane (lower concentration side)

• bound molecule detaches from carrier

• carrier returns to its original conformation (binding site on higher concentration side)

diffsuion of ions across membranes— facilitated diffusion

• Na⁺, K⁺,Cl^-,and Ca²⁺ diffuse through ion channels

• when ion channels are open they allow specific ions to move across plasma membrane down their electrochemical gradient

• ions move from where they are more concentrated to where they are less concentrated

• positively charged cations move toward negatively charged area and negatively charged anions move towards positively charged areas

• as ions move the constitute a flow of electrical current that can alter membrane potential

• presence of ion channels in plasma membrane of neurons and muscle fibres gives these cells the property of electrical excitability

• ion channels open/close due to ion gates

4 types of ion channels

1. leakage channels: randomly open/close— more K⁺ than Na⁺— K⁺ membrane permeability higher

2. voltage gated channels: open in respones to change in membrane potential

3. ligand gated channels: open in response to ligand/chemical stimulus e.g. binding of neruotransmitter acetycholine at certain synapses opens channels that admit Na⁺ and Ca⁺ and initate a nerve impulse or muscle contraction

4. mechanical gated channels: open or close in response to mechanical stimulation e.g. auditory receptors and touch receptors

osmosis

• net diffusion of water through a membrane

• occurs when water is mroe concentrated on one side of a membrane— moves down its concentration gradient

• more dilute solution— higher concentration of water molecules and lower concentration of solute

• aquaporins— protein molecules in phospholipid bilayer— transport water across membrane— increases rate of tranpsort of water

osmosis— solution cencentration types

• osmolality: total solute concentration of a cell (osm/L)— typical osmolality of animal cell ~300 mOsm (milliosmoles)

• hypertonic solution: >300 mOsm: cell loses water and shrinks

• hypotonic solution: <300 mOsm: cell gains water and expands

• isotonic solution: ~300 mOsm: no net movement of water

tonicity: measure of solutions ability to change volume of cells by altering their water content

osmosis in organisms

• osmoconformers: organisms isotonic with environment (marine invertebrates)

• osmoregulators: organisms that need to maintain constant osmolaity different from that of surroundings

Salmon(salt water):

• to offset dehydrating effects they drink loadsss of water

• kidneys urine production rates dramatically drop and urine is as concentrated as kidneys can make it

Salmon (fresh water)

• salmon doesnt drink at all

• kidneys produce large volumes of dilute urine ( to cope with all the water thats diffusing into salmon)