Cell Transport and Signalling Flashcards
♣ Also called plasma membrane
♣ 8 to 10 nm thick
♣ a phospholipid bilayer
♣ barrier against movement of water molecules and water-soluble substances
Cell Membrane
♣ Cell membrane is composed of phospholipid bilayer
♣ It is fluid-like
Fluid Mosaic Model
♣ Lipid-soluble substances diffuse easily
♣ Water and water-soluble substances passes through transport proteins
Membrane Transport
allow free movement of water and selected ions
Channel Proteins
conformational change to transport molecules
Carrier Proteins
- random movement of substance through intermolecular spaces or in combination with a carrier
- Normal kinetic motion of matter
- “downhill”
Passive Transport
- Movement across the membrane in combination with a carrier protein
- against the concentration gradient
- Requires additional energy
- “Uphill”
Active Transport
♣ simple movement though the membrane
♣ caused by the random motion or kinetic movement of the molecules
♣ cell membrane pores
♣ lipid matrix of the membrane (for lipid soluble substances)
Diffusion
♣ Occurs downhill from an electrochemical gradient
♣ Via membrane opening or intermolecular spaces
♣ No interaction with carrier proteins
♣ Governed by Fick’s Law of Diffusion
Simple Diffusion
The rate of diffusion is determined by
- the amount of substance
- velocity of kinetic motion
- number and sizes of openings
Predicts the rate of diffusion of molecules across a biological membrane
Fick’s Law of Diffusion
According to Fick’s Law, diffusion is_________ at higher concentration gradients
Fast
According to Fick’s Law, diffusion is _______ at higher permeability
Fast
According to Fick’s Law,diffusion is _______ at higher areas for diffusion
Fast
According to Fick’s Law, diffusion is ______ when diffusing membrane is thicker
Slow
2 important characteristics of Simple Diffusion
- selectively permeable
2. Voltage or ligand -gated channels
♣ molecular conformation of the gate or of its chemical bonds responds to the electrical potential across the cell membrane; Eg Na and K Pump
Voltage-Gated Channels
♣ Also called chemical gating
♣ Channels are opened by a chemical substance with the protein
♣ Causes conformational/structural change in the channel
♣ Example: Acetylcholine Channel
Ligand-Gated Channels
♣ Also called uniport
♣ Occurs downhill from an electrochemical gradient
♣ Does not require metabolic energy (passive)
♣ More rapid than simple diffusion
♣ carrier-mediated process
♣ NOT governed by Fick’s law of diffusion
Facilitated Diffusion
♣ Net movement of water through a semipermeable membrane caused by a concentration
Osmosis
homogeneous mixture composed of two or more substances
Solution
undergoes diffusion from an area of high solute concentration to an area of low solute concentration.
Solute
undergoes osmosis from an area of low solute concentration to an area of high solute concentration.
Solvent
- concentration of all osmotically active
particles (osmoles) per liter of solution (osmol/L) - colligative property that can be measured by freezing point depression
Osmolarity
- concentration of all osmotically active particles (osmoles) per kg of solvent (osmol/kg)
- determines osmotic pressure between solutions
Osmolality
Two solutions that have the same osmolarity
Isosmotic
Solution with the higher osmolarity
Hyperosmotic
Solution with the lower osmolarity
Hyposmotic
- The exact amount of pressure required to stop osmosis
- pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane
- calculated using Van’t Hoff’s Law
Osmotic Pressure
Van’t Hoff’s Law physiologic implications
♣ osmotic pressure is HIGHER with higher osmolality
♣ osmotic pressure is HIGHER with higher temperature
♣ the higher the osmotic pressure of a solution, the greater the tendency for water to flow into the solution
- measure of the osmotic pressure of two solutions separated by a semipermeable membrane
- influenced only by solutes that cannot cross the membrane
Tonicity
OSMOLARITY vs TONICITY
OSMOLARITY accounts for all solutes.
TONICITY accounts for only non-permeating solutes
♣ Transport of glucose, amino acids, and other polar molecules through the cell membrane
♣ Mediated by carrier proteins in cell membrane
Carrier-Mediated Transport
Characteristics of Carrier-Mediated Transport
♣ Stereospecificity
♣ Saturation
♣ Competition
Each carrier protein is specialized to transport a specific substance
Stereospecificity
Transport rate increases as solute concentration increases until all carriers are saturated (transport maxima, Tmax)
Saturation
Structurally related solutes compete for transport sites on carrier molecules
Competition
♣ Occurs uphill against an electrochemical gradient
♣ Requires direct input of metabolic energy (active)
♣ Carrier-mediated transport that exhibits stereospecificity, saturation and competition
Primary Active Transport
♣ Transport Na+ from ICF to ECF and K+ from ECF to ICF -against electrochemical gradients
♣ Energy provided from terminal bonds of ATP
♣ Usual stoichiometry is 3NA+/2K+
Na+/K+-ATPase
Functions of Na+/K+-ATPase
♣ CONTROL OF CELL VOLUME - large numbers of proteins and other osmotically active particles are present inside the cell
♣ ELECTROGENIC NATURE - net of one positive charge is moved from the interior of the cell to the exterior for each cycle of the pump; creates an electrical potential across the cell membrane
♣ Transport of two or more solutes is coupled
♣ One of the solutes (usually Na+) is transported downhill and provides energy for the uphill transport of other solute(s)
♣ Metabolic energy is provided indirectly from the Na+ gradient
♣ inhibition of Na+/K+-ATPase inhibits secondary active transport
Secondary Active Transport
Types of Secondary Active Transport
- Co-Transport
2. Countertransport
- also called symport
- occurs if the solutes move in the same direction across the cell membrane
Co-Transport
- called exchange transport or antiport
- occurs if the solutes move in opposite directions across the cell membrane
Countertransport
Transport of hormones along the blood stream to a distant target organ
Endocrine Signaling
♣ Also called synaptic transmission
♣ Transport of neurotransmitter from a presynaptic cell to a postsynaptic cell
Neurocrine Signaling
♣ Release and diffusion of local hormones with regulatory action on neighboring target cells
Paracrine Signaling
♣ A cell secretes hormones or chemical messengers that binds to the same cell
Autocrine Signaling
♣ Also called contact-dependent signaling
♣ transmitted via oligosaccharide, lipid or protein components of a cell membrane
♣ occurs between adjacent cells linked by gap junctions
Juxtacrine Signaling
2 Basic Classes of Receptor
Plasma Membrane receptors
Nuclear receptors
♣ Ion-channel linked
♣ G-protein coupled receptors
♣ Catalytic receptors
♣ Transmembrane receptors
Plasma Membrane receptors
♣ hormone-receptor complex binds to DNA and regulates the transcription of specific genes
♣ Early primary response –>gene activation to stimulate other genes –> biological effect
Nuclear receptors
♣ Process by which an extracellular signal activates a membrane receptor
♣ second messengers
♣ Involves small molecules in complicated networks within the cell
♣ Results to single amplification
♣ Alteration of intracellular molecules creating a response
Signal Transduction
♣ Family of integral transmembrane proteins that possess seven transmembrane domains
♣ Heterotimeric complexes - α, β, and γ subunits
♣ Linked with more than 1000 different receptors
G Protein-Coupled Signal Transduction Pathways
♣ Mediate direct and rapid synaptic signaling between electrically excitable cells
♣ neurotransmitters bind to the receptors and either open or close the ion channel
♣ Chemical electrical signal response
♣ Examples: Voltage gated-channels in NMJ, ryanodine receptor, Arachidonic acid, caffeine
Ion Channel Linked Signal Transduction Pathway
♣ Kinase is an enzyme that modifies other proteins by phosphorylation
♣ Phosphorylation usually results in a functional change of the target protein
Protein Kinases
♣ calcium (Ca2+) binding causes conformational alterations in calmodulin; Eg. Muscle cells
Calmodulin-dependent Protein Kinases
♣ Adenylyl cyclase facilitates the conversion of ATP to cAMP
♣ Increased cAMP activates protein kinase A
cAMP-dependent Protein Kinases
♣ Binding of ANP causes dimerization and activation of guanylyl cyclase, which metabolizes GTP to cGMP
♣ cGMP activates cGMP-dependent protein kinases; phosphorylates proteins on specific serine and threonine residues
cGMP-dependent Kinases
