Transport through cell membranes Flashcards
Channels Vs Carriers
Channels -Watery spaces in the molecule -Free movement of ions/molecules Carriers -Binding of molecules -Conformational change -Transport *Both are selective for molecules they allow to pass
Diffusion
Random movement of each molecule of a substance either through inter-molecular spaces or with the help of carriers Energy: normal kinetic motion of matter
Active transport
Movement across membranes in combination with carriers, against the energy gradient (low to high conc.) Energy: Additional energy required
Simple diffusion and its rate determining factors
-Movement across membranes without interacting with carrier proteins Rate determining factors -Amount of substance -Velocity of motion -Number and size of openings -Solubility of substance
Facilitated diffusion
Requires interaction of a carrier protein which aids transport by chemically binding and shuttling them across -Depends on no. of proteins and speed of translocation
Types of simple diffusion
- Lipid-soluble substances -Dissolve directly and diffuse across membrane -Rate of diffusion directly proportional to lipid solubility 2. Lipid-insoluble substances -transport through channels in protein molecules that penetrate through the membrane -Aquaporins: for rapid transport of water -Penetration is inversely proportional to size
Selective permeability
Results from characteristics like diameter and shape of the channel, nature of electrical charges, chemical bonds inside
Potassium channels
-open when the inside of cell membrane becomes positively charged -tetrameric structure with four identical subunits around a pore -pore loops forming narrow selectivity filter -filter lined by carbonyl oxygens which dehydrate the potassium ions. (These carbonyl oxygens are too far apart to be able to interact with sodium ions and hence exclude them)
Sodium channels
-0.3-0.5 nm diameter -inner surface lined with strongly negative amino acids which pull the dehydrated sodium ions -once in the channel the ions diffuse in either direction
Voltage gating
-conformation of gate responds to electrical potential across membrane -action potentials in nerves
Chemical gating
-ligand binding causes conformational changes which causes it to open/close -acetylcholine channel (negative pore; nerve cell-nerve cell signal transmission; nerve cell-muscle cell signal transmission for contraction)
Factors affecting net rate of diffusion (Conc. difference)
-rate of substances diffusing inside is proportional to conc. of molecules outside -net diffusion directly proportional to conc. outside minus conc. inside
Factors affecting net rate of diffusion (Electrical potential)
- conc. and charges balancing each other
- Nernst equation (balance of conc. difference of univalent ions at normal body temperature)
EMF= +-61log C1/C2
Factors affecting net rate of diffusion (Pressure)
- Pressure referred to here is the sum of all the forces striking a unit surface at a given instant
- increased amount of energy is available to cause net movement of molecules from high to low pressure
Effect of the conc. of a substance on the rate of diffusion
- Simple Diffusion: rate of diffusion through an open channel increases proportionately with conc. of diffusing substance
- Facilitated diffusion: approaches Vmax. as the conc. increases
Osmosis and osmotic pressure
Osmosis: movement of water caused by its conc. difference
Osmotic pressure: Amount of pressure required to stop osmosis
Osmolality, its relationship with pressure, and osmolarity
Osmolality
-expresses conc. of a solution in terms of numbers of particles using osmole instead of grams
1 osmole= 1g mol. wt. of osmotically active solute
Relationship with pressure
- normal body temp., unit osmolarity causes 19,300 mm hg osmotic pressure
- 1 mil.os.= 19.3 mm hg
- total calculated OP of body fluids = 5500 mm hg
Osmolarity
-osmolar conc. expressed as osmoles per liter of solution
Primary and Secondary active transport
Primary
- energy directly derived from breakdown of ATP or other high energy phosphate bonds
- sodium, potassium, calcium, hydrogen, chloride etc. transported by this mechanism
- Energy required is determined by how much substance is concentrated
- Energy (calories per osmole) = 1400log C1/C2
Secondary
-energy derived is from the energy stored in the form of ionic conc. differences of secondary ionic substances bw two sides of a membrane (originally created by primary transport)
Sodium-potassium pump (functions, structure)
- maintains sodium and potassium conc. differences
- maintains negative electrical voltage inside the cell
- carrier protein has two subunits- alpha and beta (smaller)
- larger subunit contains: three binding sites for Na, two for K and has ATPase activity inside
- When Na and K bind, ATPase function is activated which leads to cleavage of ATP. liberated energy causes conformational change
- Reverse is also possible
- important for controlling cell conc.
- electrogenic in nature
Primary active transport of Hydrogen ions
Gastric glands
- by deep lying Parietal cells
- basis for secreting HCl in digestive secretions
- H+ conc. is increased to a million fold at the secretory end and is release into the stomach with chloride ions to form HCl
Renal Tubules
- by intercalated cells in DCT and collecting ducts
- ions are transported from blood to urine for elimination
Co-transport
- under appropriate conditions, diffusion energy of sodium can pull other substances with it
- a coupling mechanism is required, achieved by a carrier protein in the cell membrane
- e.g. glucose and sodium
Counter-transport
- simultaneous transport of two substances across cell membrane in opposite directions either by the same carrier or by different carriers biochemically linked to each other
- e.g.hydrogen and calcium
