Module 5 (cell processes) Flashcards
Main constituents of the cell membrane
Phospholipids and proteins
Functions of the cell membrane
Acts as a barrier which isolates the cells from their external environment;
Membrane structure
thin 8nm structure; flexible and sturdy surrounding the cytoplasm; fluid mosaic model; 50% lipid and 50% protein held together by H bonds; proteins are gatekeepers and lipid is the barrier for polar substances
Consequences of the barrier function
Concentration gradients can be maintained; spatial organisation of chemical and physical cellular processes; controlled up-take of nutrients and discharge of waste products and the secretion of molecules; development of a membrane potential
Phospholipids
Comprises 75% of lipids; 2 parallel layers of molecules; each molecule is amphipathic (non-polar and polar regions)
Membrane fluidity
Fluid structures where lipids can move freely (rarely flip-flop between membrane leaflets so lipid composition can be asymmetric); determined by the lipid tail length (longer less fluid), number of double bonds (more, more fluid), amount of cholesterol (more, less fluid)
Types of membrane proteins
Integral and peripheral
Integral proteins
Extend into or completely across the cell membrane (transmembrane); can sense molecules on the inside/outside of the cell or form a pathway; amphipathic, hydrophobic regions span the hydrophobic core of the bilayer (consist of non-polar amino acids coiled into helices); hydrophilic ends interact with the aq. solution
Removing an integral protein
The bonds between the hydrophobic lipids and the hydrophobic amino acids in the protein need to be broken
Peripheral proteins
Attached to either inner or outer surface of the membrane (associated but not embedded); inner: attached to specific membranes/integral membrane proteins embedded in the bilayer or outer: attached to the lipid on the surface due to presence of ion charges
Removing a peripheral protein
Changes in ionic strength (breaking ionic bond); can be broken by increasing ionic strength of the solution and then strip them off
Functions of membrane proteins
Can act as: receptor proteins, cell identity markers, linkers, enzymes, ion channels and/or transporter proteins
membrane selective permeability
Results from the molecular organisation; permeable to: nonpolar uncharged molecules (O2, N2, benzene), lipid-soluble molecules (steroids, fatty acids, some vitamins), small uncharged polar molecules (water, urea, glycerol, CO2); impermeable to: large uncharged polar molecules (glucose, amino acids) and ions (Na+, K+, Cl-, Ca++, H+)
Transport across membranes
Membrane proteins mediate the transport of substances across the membrane that can not permeate the hydrophobic core of the lipid bilayer
Diffusion
More molecules move away from an area of high concentration to an area of low concentration
Diffusion principles
The greater the difference in conc. between the two sides of the membrane, the faster the rate of diffusion
The higher the temperature, the faster the rate of diffusion
The larger the size of the diffusing size, the slower the rate of diffusion
Increase in surface area increases rate of diffusion
Increase in diffusion distance slows rate of diffusion
Physical consequences of diffusion
The rate of diffusion sets a limit on the size of cells of about 20micrometers; increasing diffusion a cell can increase the membrane area available for exchange; the thicker the membrane the slower the RoD - very fast over small distances (circulation)
Concentration gradient
non-charged molecules will diffuse their concentration gradients until equilibrium
Ion movement
Influenced by the electrochemical gradient; dependent on the sum of the electrical and chemical gradients
Gradients across the membrane
Selective permeability enables gradients to occur; cells can maintain a gradient of charged ions and establish an electrical gradient or membrane potential; membranes mimic capacitors and can separate and store charge
Ion gradients and movement
Influenced by the electrochemical gradient; dependent on the sum of the electrical and chemical gradients; cells use ~30% of resting energy to maintain these gradients (which represent stored energy)
Osmosis
The net movement of water through a selectively permeable membrane from an area of high water concentration to an area of low concentration; only occurs if the membrane is permeable to water but not to certain solutes (situation in biological membranes); if osmotic gradient exists, water will move to eliminate it
Membrane permeability to water
Pw = Pd (through lipid bilayer) + Pf (through water channel); cells have different Pw because they express different aquaporin isoforms
Permeability of water through the bilayer
Pd; small, mercury insensitive, temperature-dependent (lipid fluidity); less than it is through water channel;
Permeability of water through a channel
Pf; large, mercury sensitive (which can block the ion channel), temperature-independent; mediated by the aquaporins (9 isoforms)
Osmotic pressure
Pressure applied by a solution to prevent the inward flow of water across a semi-permeable membrane
Types of membrane transport
Non-mediated transport, mediated transport, passive transport, active transport and vesicular transport
Non-mediated transport
Does not directly use a transport protein; molecules (nonpolar, hydrophobic) are permeable across the bilayer; diffusion down a concentration gradient; important for the absorption of nutrients and excretion of wastes; e.g. O2, CO2, N2, fatty acids, steroids, small alcohols, ammonia and fat-soluble vitamins (A, E, D and K)
Diffusion through ion channels
Channel forms a water-filled pore that shields the ions from the hydrophobic core of the lipid bilayer; ions do not bind to a channel pore and transport is very rapid; diversity of ion channels each specific to a particular ion
Protein channels ionic selectivity property
Specific amino acids lining the pore determine the selectivity of the channel to ions; by being selective to a particular ion, the channel can harness the energy stored in the different ion gradients
Protein channels gating property
Channels contain gates that control the opening and closing of the pore; different stimuli can control/gate channel opening and closing (increase the probability of them being open); stimuli include voltage, ligand binding, cell volume (the stretch of cytoskeletal elements), pH, phosphorylation
Measuring ion channel function
Can use the patch-clamp technique where a small part of the membrane containing one channel is isolated; current flowing through the individual channel can be observed and recorded
Ion channels electrical current property
the diffusion of over 1 million ions per sec through a channel generates a measurable current; current fluctuations represent the opening and closing of single ion channels (the conformational changes in channel structure that are associated with channel gating)
Carrier-mediated transport description
The substrate being transported directly interacts with the transporter protein; the transporter undergoes a conformational change (when the substrate binds to a pocket) and so transport rates are slower than those obtained for channels
Carrier-mediated transport properties
Similar to enzymes; exhibit specificity (only one stereoisoform can be transported), inhibition, competition (between 2 molecules), saturation (transport maximum); they do not catalyse chemical reactions but mediate transport across the membrane at a faster-than-normal rate; can be passive or active
Saturation of transport proteins
Display enzyme kinetics; glucose transport occurs until all binding sites are saturated
Facilitated diffusion of glucose
- Glucose binds to transport protein (GLUT)
- Transport protein changes shape, glucose movies across the membrane (down conc. gradient)
- Kinase enzyme reduces glucose conc. inside the cell by transforming glucose into glucose-6-phosphate
Conversion of glucose maintains cons. gradient for glucose entry
Active transport
An energy-requiring process which moves molecules and ions against their conc. or electrochemical gradients; primary and secondary active transports
Primary active transport
Energy is directly derived from the hydrolysis of ATP; a typical cell uses 30% of its energy (ATP) on this; examples: Na/K-ATPase and Na pump
Secondary active transport
Energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient; they indirectly use the energy obtained by hydrolysis of ATP; cells have many of these transporters which are powered by the Na+ gradient initially established by the Na pump
Na/KATPase primary active transport
3 Na+ ions are removed from the cell as 2K+ brought in; the pump generates a net current and is electrogenic; other examples: Ca/K-ATPase (muscle SR), H/K-ATPase (stomach)
Na pump primary active transport
The Na pump maintains a low conc. of Na+ and a high conc. of K+ in the cytosol; because Na and K are continually leaking back into the cell down their respective gradients, the pump works continuously (pump-leak hypothesis)
Importance of ion conc. differences
Maintains the RMP; electrical excitability; contraction of muscle; maintenance of steady-state cell volume; uptake of nutrients via secondary active transporters; maintenance of intracellular pH by secondary active transporters
Examples of secondary active transporters
Na+ anti-porter or exchangers (Na+ ions passively rush forward, Ca+ or H+ pushed out against gradient)
Na+ symporters or transporters (glucose or amino acids rush inwards together with Na+ ions)
Epithelial tissues
Consist of cells arranged in continuous sheets in either single or multiple layers, which sit on a basement membrane; form the boundary between the body’s organs or between the body and the external environment; subject to physical breakdown and injury (undergo constant and rapid renewal process)
Basic types of epithelial tissue
Covering and lining (skin, the lining of blood vessels and ducts) and glandular (thyroid, adrenal and sweat glands)
Epithelia arrangements
Simple, stratified, pseudostratified; squamous, cuboidal, columnar and transitional
Tight junctions
Hold epithelial cells together at their luminal edges; composed of thin bands that encircle the cell and make contact with thin bands of adjacent cells
Tight junctions structure
In electron microscopy, it appears as though the membranes are fused together; in freeze-fracture, they appear as an interlocking network of ridges in the plasma membrane
Tight junctions function
Act as a barrier by restricting the movement of substances through the intercellular space between cells; act as a fine by preventing membrane proteins from diffusing in the pane of the lipid bilayer; they separate the cells into the apical and basolateral domains
Apical membrane
(luminal or mucosal) faces the lumen of the organ or body cavity
Basolateral membrane
Adheres to the adjacent basement membrane and interfaces with the blood
Epithelial transport properties
Distinct membrane domains mean the different transport proteins can be inserted into either the apical or basolateral membrane; transport can occur via the paracellular or transcellular pathway or via both
Paracellular transport
Governed by the laws of diffusion and the tightness of the junctions (barrier); the electrical resistance to ion flow through tight junctions can be measured with ohm’s law where the higher the resistance, the greater the number of tight junction strands holding the cell together
Epithelial tissues classification
Leaky epithelium (paracellular transport dominates due to the ability to move molecules this way); tight epithelium (transcellular transport dominates due to the electrical resistance being so high and no paracellular transport occurs)
Changes in tight junction resistance
Changes in a proximal to distal direction in the GI tract and kidney
Proximal
Leaky epithelium (gut, processing); low electrical resistance; low number of strands; bulk transport (paracellular); e.g. duodenum, proximal tubule of the kidney
Distal
Tight epithelium; high electrical resistance; high number of strands; hormonally controlled (transcellular); e.g. colon, collect duct (more sodium stimulates collecting duct to absorb the Na to change dietary requirement)
Transcellular transport
Epithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels to produce this; have different ion channels and carrier-mediated proteins into the basolateral or apical membranes to produce transport across the tissue
Types of transcellular transport
Absorption which is transport from lumen to blood; or secretion which is transport from the blood to the lumen; both in opposite directions with different proteins mediating them in separate membranes
Rules of transcellular transport
- entry and exit steps: entry for absorption is apical, but secretion is the basolateral membrane (different proteins and membranes depending on which direction and first step)
- electrochemical gradient: entry/exit step passive or active (using energy or not)
- electroneutrality (movement of a positive/negative ion will attract a counter ion)
- osmosis (net movement of ions will establish a difference in osmolarity that will cause water to flow by osmosis)
Aquaporins
Channels which allow only water to flow through them without other ions; by changing which aquaporin channels you put into the membrane you can change its water permeability
Transepithelial transport
Epithelial cells use different collections of transporters and channels to mediate either secretion or absorption; primary active transporter (sets up ion gradients), entry step (often secondary active transport) and exit step (often passive diffusion)
Glucose absorption in the small intestine
Two-step process; glucose and Na+ are co-transported across the apical membrane by the SGLT protein; glucose exits across the basolateral membrane by facilitative diffusion mediated by the GLUT transporter protein; energy for glucose entry is provided by the Na+ gradient maintained by the Na+ pump; the movement of glucose and Na+ across the epithelium creates an osmotic balance and drives the absorption of water and flux of Cl-
Result of glucose absorption in the small intestine
Absorbed glucose, NaCl and H2O
Oral rehydration therapy
The ability of glucose to enhance the absorption of Na+ and hence Cl- and water is exploited; sugar, salt and water
Glucose/galactose malabsorption syndrome
Caused by a mutation to the SGLT (sodium-dependent glucose (co-) transporter protein); results in accumulation of glucose and galactose in the lumen of the small intestine; produces an osmotic imbalance which attracts water and results in severe diarrhea
Treatment of glucose/galactose malabsorption syndrome
Remove glucose and galactose from the diet and to use fructose as a source of carbohydrate; utilises a facilitative transporter (GLUT5) that is specific for fructose
Glucose reabsorption in the kidney
In the kidney, glucose in the plasma is filtered and needs to be reabsorbed or it will appear in the urine
Glucosuria
Glucose in the urine and occurs if the transport maximum of the SGLT is exceeded; the commonest cause is diabetes mellitus because insulin activity is deficient and blood sugar is too high; in diabetes, the glucose symporter cannot absorb glucose fast enough and it appears in the urine
Glucosuria (transporter kinetics)
If glucose absorption is impaired or the transporter is saturated, glucose will appear in the urine; all filtered glucose is reabsorbed until the renal threshold is reached before it will occur in the urine; the renal threshold reflects the transport maximum of SGLT
Chloride secretion
Two-step process; cotransporter located in the basolateral membrane accumulates Cl- above its electrochemical equilibrium which enables Cl- to leave the cell via a Cl- channel in the apical; Na+ moves via the paracellular pathway to preserve electroneutrality and water moves in the same way
Rate-limiting step of chloride secretion
Although Cl- is accumulated above the electrochemical equilibrium, it cannot leave the cell unless the Cl- channel is open; the opening of the Cl- channel is strictly regulated/gated (limiting step); the channel is called the CFTR (Cystic Fibrosis Transmembrane conductance Regulator)
CFTR
Cystic Fibrosis Transmembrane conductance Regulator; Cl- channel involved in chloride secretion; over-stimulation is implicated in secretory diarrhea and its disfunction causes cystic fibrosis; regulated by protein kinase A dependent phosphorylation of the regulatory domain and binding of ATP to the nucleotide binding domain
Secretory diarrhea
Caused by over-stimulation of the secretory cells in the crypts of the small intestine and colon - they pump out a lot of Cl- and therefore Na+ and H2O; could be due to abnormally high concentrations of neurotransmitters/hormones (released by cells to stimulate Cl- secretion) produced by tumours or inflammation; more commonly due to the secretion of enterotoxins from bacteria such as Vibrio cholerae
Enterotoxins
Commonly cause secretory diarrhea; irreversibly activate adenylate cyclase causing a maximal stimulation of CFTR, leading to a secretion that overwhelms the absorptive capacity of the colon
Molecular mechanism of Cl- secretion (normal)
Secretagogue binds to basolateral membrane; NT binds to GPCR, releases G-protein which binds to andenylate cyclase; ATP –> cyclic AMP (intracellular secondary messenger) which acts on enzyme protein kinase A, which phosphorylates CFTR so it opens; Cl- secretion
Molecular mechanism of cholera
Cholera toxin irreversibly activates adenylate cyclase causing activation of CFTR (bypasses its control and produces lots of cyclic AMP which phosphorylates it) and the channel is permanently open; the rate-determining step is removed and all ion gradients are accumulating Cl- in the cell which immediately leaves via CFTR
Treating secretory diarrhoea
Oral rehydration therapy can be used to treat this which is caused by cholera; glucose stimulated water flux to balance out the over stimulation of secretion
Villus cells
Cells involved in the Na+ absorption
Crypt cells
Cells involved in the Cl- secretion
Cystic fibrosis
A complex inherited disorder that affetcs children and young adults; heterozygotes have no symptoms but are carriers; range of symptoms, commonly involving epithelial tissues; most cases of mortality are due to respiratory failure
Organs affected by cystic fibrosis
Airways (clogging and infection); liver (plugging of small bile ducts); pancreas (occlusion of ducts); small intestine (obstruction of the gut by thick stool); reproductive tract (absence of fine ducts, like vas deferens); skin (malfunctioning of sweat glands)
Clinical management of CF
Chest percussion to improve clearance of infected secretions; antibiotics to treat infections; pancreatic enzyme replacement; attention to nutritional status
Normal lung epithelial cells
A balance between secretion and absorption keeps the lung surface moist but prevents excessive build up; Cl movement means there is Na+ and water movement across the tissue (water on the surface of lungs); Na brings Cl and water
Lung epithelial cells in CF
The defective Cl- channel prevents isotonic fluid secretion and enhances Na+ absorption to give a dry lung surface; don’t have enough movement of Cl- and Na+, no fluid flow; blocking Cl CFTR causes Na channel to open more, more ions diffuse in; no secretion, reabsorbing it more (dehydrating lung surface)
Blocking Cl- secretion leading to lung pathology
Normal: wet, thin mucus traps inhaled particles; cilia push mucus to the throat for removal; airways stay clear for breathing
CF: mucus becomes thick and difficult ot move; bacteria proliferate and attract immune cells, which can damage healthy tissue; DNA released from bacteria and lung cells adds to the stickiness; airways become plugged and begin to deteriorate, no longer available for gas exchange
New idea for treating lung pathology due to CF
Gene therapy when the CFTR gene defect is recognised; before the stages escalate
Formation of sweat
Two stage process; a primary isotonic secretion of fluid by acinar cells; a secondary reabsorption of NaCl but not water produces a hypotonic solution, which doesn’t have the same osmolarity as the body fluids; in the duct cells, the membrane potential is depolarised and Cl- wants to enter the cell down its electrochemical gradient
CF and sweat formation
Failure of epithelial cells in the ducts of sweat glands to reabsorb NaCl produces the salty sweat in CF patients; the CFTR is defective and Cl- accumulates in the duct lumen producing salty sweat
