Section 5: Cell Processes Flashcards
Plasma membrane structure
A thin, 8nm flexible and sturdy barrier that surrounds cytoplasm of a cell
2 back-to-back layers of 3 types of lipid molecules; glycolipid and cholesterol, which are scattered among a double row of phospholipid molecules
Fluid mosaic model
Describes membrane structure
‘sea of lipids in which proteins float’
What makes up the membrane? (%)
50% lipid and 50% protein, held together by H bonds
Lipid is barrier to entry/exit of polar substances
Proteins are ‘gatekeepers’ - regulate traffic across lipid bilayer
Why is the plasma membrane critical for cellular function and evolution?
DNA, mitochondria and cytoplasm can’t be freely floating around in primordial suit and must be contained in a membrane so there’s a difference between the inside and outside of the cell
What is the ‘outside’ for a single cell
The outside world, so must have a barrier which enables it to partition itself from the outside world
Phospholipids - lipids
Comprises 75% of lipids
Phospholipid bilayer
2 parallel layers of molecules
Phospholipid - amphipathic
Phospholipids will orient themselves to provide the lowest energy structure
Each molecule has both a polar and non-polar region
Non-polar hydrophobic tails face each other and exclude water so water is outside of lipid bilayer
Water interacts with polar head groups - excluded from hydrophobic core
Membrane fluidity
Membranes are fluid structures and lipids can move around within the plane of the membrane leaflet and allow lateral diffusion of proteins within the lipid bilayer
Lipids rarely flip flop between membrane leaflets –> lipid composition of leaflets can be asymmetric
Fluidity of membrane is determined by…
Lipid tail length: longer tail = less fluid
No. of double bonds: more double bonds = increased fluidity
Amount of cholesterol: more cholesterol embedded = decreased fluidity
What does fluidity determine
Properties of lipid bilayer - how many molecules can get through it
Can maintain differences in lipid composition - diff on one side of membrane facing inside and membrane facing outside
e.g. water diffusion
Membrane fluidity - double bonds
Introduces kinks in the tail, which allows them to pack less tightly to give more fluidity –> membrane is less stable
Types of membrane proteins
Integral proteins
Peripheral proteins
Integral proteins
AKA transmembrane protein
Amphipathic
Extend into / completely across cell membrane - able to sense molecules on outside and inside of cell for movement across membrane
Peripheral proteins
Attached to either inner or outer surface of cell membrane and are easily removed from it (by changes in ionic strength)
Peripheral membrane proteins - cytoskeleton proteins
Linked to membrane proteins embedded in lipid bilayer, which can bend and change shape of membrane or hold membrane proteins in a particular place
Can easily break these interactions by exposing membrane to an ionic solution to break chemical bonds and strip peripheral proteins from membrane
Integral proteins - hydrophobic regions
Have hydrophobic regions that span hydrophobic core of lipid bilayer
Usually consist of non-polar amino acids coiled into helices to form a protein
Integral proteins - hydrophilic ends
Interact with aqueous solution
Removal of an integral protein
Must break interactions between hydrophobic lipids and hydrophobic amino acids
To break the lipid, use detergent to dissolve lipid and stabalise membrane –> isolate integral membrane proteins
Membrane proteins can act as…
Receptor proteins - sense signals, e.g. from blood, and bind those receptors and transfer signals inside the cell
Cell identity markers - can be a sense of ‘self’
Linkers - provide links to other cells, sheets of tissue, or parts of CT e.g. tendons and BM
Enzymes - on surface of membrane, can catalyse enzymatic activity, e.g. break down glucose
Ion channels and transporter proteins - move molecules across cell membrane
Ion channels vs transporters
Transport diff things and use diff forces to do the transferring
Membrane - selective permeability
Membrane allows some substances to cross but excludes others because of the way specific molecules interact with lipid bilayer
What is the lipid bilayer (im)permeable to
Permeable to:
- nonpolar, uncharged molecules (O2, N2, benzene)
- lipid soluble molecules (steroids, fatty acids, some vitamins)
- small uncharged polar molecules (water, urea, glycero, CO2)
Impermeable to:
- large uncharged polar molecules (glucose, amino acids)
- ions (Na+, K+, Cl-)
Why are ions impermeable
Although they are small, they have an electric charge and so will be repelled by non-polar hydrophobic core of lipid bilayer - can only be moved through integral proteins
Diffusion
The random mixing of particles in a solution as a result of the particle’s kinetic energy
More molecules move away from an area of high conc to an area of low conc until conc across the membrane is equal
Factors affecting rate of diffusion
Greater diff in conc between 2 sides of membrane = faster rate
Higher temp = faster rate
Larger size of diffusing substance = slower rate
Increased SA available for diffusion = faster rate
Increased diffusion distance = slower rate
Thicker membrane = slower rate
Diffusion - size limit
Rate of diffusion sets limit on size of cells of about 20μm
What diffuses down concentration gradient
Non-charged molecules diffuse down conc gradients
Electrical gradient
Membrane potential
Electrochemical gradient
Movement of ions will be influenced by the electrochemical gradient
Passive transport
If there is a conc gradient and membrane is permeable, molecules will rapidly move until they reach an equal conc on both sides of membrane
Membrane charge
Across the cell membrane, there is a membrane charge which determines how molecules move across the cell
Movement of ions influenced by…
Sum of electrical and chemical (electrochemical) gradient
Selective permeability - conc gradient
Selective permeability of membrane enables a difference in conc (gradient) across the membrane to be established
Cells can maintain a difference in charged ions between the inside and outside of membrane, establishing gradient / membrane potential
Membranes - capacitors
Membranes mimic capacitors and can separate and store charge
Cytoplasm: -vely charged
ECM: +vely charged
Why is it important that the lipid bilayer is not permeable to ions
If membrane had holes in it that allows ions to diffuse down, you could never have a conc gradient
Crucial for establishing conc differences across the membrane
What do ion gradients represent
Stored charge and energy
Extracellular ion concentrations
High Na+: 150 millimoles
Low K+: 5 millimoles
High Cl-: 150 millimoles
Cytoplasmic ion concentrations
Low Na+
High K+
Low Cl-
How much resting energy do cells use to maintain conc and electrical gradients
~30% of resting energy
Accumulation of ions on one side of a membrane creates a…
Concentration difference
Electrochemical gradient - Na+
Product of conc gradient
Directed into the cell where there is a -ve membrane potential so electrical and conc gradient of Na+ will always be directed inside the cell
Na+ always want to move into cell down its electrochemical gradient
Osmosis
Diffusion of water across membranes
Net movement of water through a selectively permeable membrane from an area of high water conc to an area of lower water conc
When does osmosis occur
Only occurs if membrane is permeable to water but not to certain solutes, e.g. biological membranes
If an osmotic gradient exists…
Water will want to move to eliminate it
Electrochemical gradient - K+
K+ moves out of cell and down conc gradient until electrical gradient puts a brake on and slows it down –> reaches electrochemical equilibrium
Electrochemical gradient - Cl-
One vector with high Cl- conc wanting to pull it into the cell, but inside the cell is a -ve membrane potential - not attracted –> 2 opposing vectors; conc wanting to push in and electrical gradient wanting to push out
If membrane potential becomes less -ve than norm (-80mV), it depolarises and Cl- comes into cell. if it hyperpolarises, Cl- will leave the cell
What is used to establish gradients
Energy of metabolism, Na/K ATPase used to establish gradients and put in chemical work to create gradients that can then create other forms of energy to do numerous cellular processes
Membrane permeability to water (Pw)
Pw = Pd + Pf Where Pd = through lipid bilayer Pf = through water channel Pf > Pd Pf mediated by aquaporins (9 isoforms) Cells have diff Pw because they express diff aquaporin isoforms
Membrane permeability to water - properties
Pd:
Small
Mercury insensitive
Temp dependent (lipid fluidity)
Pf:
Large
Mercury sensitive
Temp independent
Osmotic pressure
The pressure applied by a solution to prevent inward flow of water across a semi-permeable membrane
Hypersmotic solution –> hyperosmotic solution
Types of transport across plasma membrane
Non-mediated transport
Mediated transport:
Passive transport
Active transport
Vesicular transport
Why are there a variety of processes for transport across plasma membrane
Lipid bilayer has certain permeability to diff molecules so there will be diff ways to get molecules across lipid bilayer
What type of transport do ions undergo
Non-mediated transport, as they don’t involve a transport protein
Why don’t non-mediated transport need integral membrane proteins
They’re permeable across hydrophobic core / bilayer
Non-mediated transport
Does not directly use a transport protein
Always passive diffusion
Important for absorption of nutrients and excretion of waste
Soluble, non-polar, hydrophobic molecules, e.g. O2, CO2, fatty acids
Diffusion through ion channels - speed
Ions don’t bind to channel pore, therefore transport is very fast (passive diffusion)
Diffusion through ion channels - process
Within the ion channel there are many charged (hydrophilic) amino acids, creating a pathway for ions to get through hydrophobic core
Channel forms a water-filled pore that shields ions from hydrophobic core of lipid bilayer
Water in ions flow through channel across bilayer down electrochemical gradient
Ionic selectivity
Large diversity of ion channels specific for a particular ion
Specific amino acids lining pore determine selectivity of channel to ions
By being selective to a particular ion, the channel can harness energy stored in diff ion gradients
Ionic selectivity - factors
Individual amino acids in protein backbone with a -ve charge effectively repel -ve ions going through the channel Shape of selectivity filter can discriminate between diff ions based on size of ions and amount of water they have around them --> specific filter that allows only one class of ions to go through it
If there was no lipid bilayer with a hydrophobic core…
Ions would be allowed to go through it and there wouldn’t be an ion gradient
Channels - gating
Channels contain gates that control opening and closing of the pore
Diff stimuli can control gate channel opening and closing
Channels - gating - stimuli
Voltage - a change in membrane potential can open an ion channel and cause generation of an action
Ligand binding - a molecule from blood binds to channel, causing it to open
Cell volume - can be sensed by cytoskeleton, causing it to stretch and open channel
pH - can change through differences in metabolism; if O2 deficit, can go into anaerobic metabolism –> can open ion channel
Phosphorylation - phosphorylate ion channels and open them to change properties of cells
Channels - open gate
Allows ions to flow down electrochemical gradient
Patch clamp technique
Used to measure ion channel function
Isolates a small patch of membrane that contains one of the channels - can see current flowing through channel
Patch clamp technique - current
Diffusion of > 1 million ions / sec through a channel generates a measurable current
Flow of ions is a pA (10^-12 amp) current
Current fluctuations represent conformational changes in channel structure associated with channel gating
Patch clamp technique - binding pocket
When molecule (e.g. acetyl choline) binds to closed channel, it causes it to open --> current starts to flow When molecule is removed, channel closes
Carrier mediated transport
Substrate to be transported directly interacts with transporter protein
For a molecule to be transported from one side to the other, must first bind to binding pocket, which induces a change in structure of that protein (e.g. binding of ion to protein)
Carrier changes its conformation and allows molecule to go across the membrane
Carrier mediated transport - rate
Since transporter undergoes a conformational change, transport rates are slower than those obtained for channels
Carrier mediated transport - properties
Similar to those of enzymes
Specificity - fits into binding pocket - specific for shape of a specific molecule
Inhibition - if inhibit/change the binding pocket of transporter, can block transport across - can be competitive or non-competitive
Competition - if 2 diff molecules can fit in binding pocket, it slows down rate of transport as they will compete for binding pocket
Saturation (transport max) - limited no of binding pockets; after a while if you keep increasing conc gradient, no effect
Do transport proteins catalyse chemical reactions
No, they mediate transport across cell membrane at a faster than normal rate
Mediated transport can be…
Passive (facilitated) or active
Glucose transport - saturation
Occurs until all binding sites are saturated
Facilitated diffusion of glucose - steps
- Glucose binds to transport protein (GLUT) - NOT a glucose channel or electrochemical gradient
- Transport protein changes shape. Glucose moves across cell membrane (down conc gradient)
- Kinase enzyme reduces glucose conc inside cell by transforming glucose into glucose-6-phosphate - conversion maintains conc gradient for glucose entry
Active transport
Uses energy to move molecules and ions against their concentration or electrochemical gradients
Forms of active transport
Primary:
- energy directly derived from hydrolysis of ATP
- typical cell uses 30% of energy (ATP) on primary active transport
- establishes ion gradients
Secondary:
- energy stored in an ionic conc gradient is used to drive active transport of a molecule against its gradient
Work together to do active transport
Primary active transporters: Na/K ATPase - overall mechanism
3 Na+ ions removed from cell as 2 K+ brought into cell
Pump generates a net current and is electrogenic
Primary active transporters: Na/K ATPase - steps
- Na+ binds to binding pocket (carrier). Binding converts ATP –> ADP, leaving a phosphate on the ion channel, so ATPase part of carrier protein attaches a phosphate group to it
- Phosphate has a -ve charge, so changes conformation of protein so sodium binding sites are opened up to outside of cell –> Na+ pushed out
- K+ binds, causing phosphate molecule to fall off –> changes conformation back to resting state where binding sites are now inside membrane –> K+ pushed in
Primary active transporters: H/K
Pumps H out to create acidic environment, e.g. low pH in stomach
Primary active transporters: Na pump function
Maintains low conc of Na+ and high conc of K+ in cytosol
Why is diff in conc of Na+ and K+ important
Maintains 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
Pump-leak hypothesis
Since Na and K are continually leaking back into cell down their respective gradients, the pump works continuously to compensate
Secondary active transport
Uses energy stored in ion gradients created by primary active transporters to move other substances against their own conc gradient
Transporters indirectly use energy obtained by hydrolysis of ATP
Cells - secondary active transport
Cells have many secondary active transporters powered by Na+ gradient initially established by Na pump
Types of secondary active transporters
Na+ antiporter or exchangers:
- Na+ ions rush inward
- Ca2+ or H+ pushed out
- movement of Na+ is passive and occurs when Ca2+ binds and goes against its electrochemical gradient
- uses energy of Na+ gradient to actively remove H+ from cells
Na+ symporters or co-transporters:
Glucose or amino acids rush inward together with Na+ ions
Epithelial tissues consist of…
Cells arranged in continuous sheets in either single or multiple layers
Epithelial tissue - physical breakdown
Subject to physical breakdown and injury, so undergo constant and rapid renewal process
How are epithelial cells separated
Separated from their neighbours by lateral intercellular/paracellular space
How are epithelial cells held together
Held together at their luminal edges (apical membrane) by tight junctions
Tight junction structure
Composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells
More ridges = more tightly packed cells tgt
In ECM it appears the membranes are fused together
In freeze fracture, it appears as an interlocking network of ridges in the plasma membrane
Tight junction function
Barrier - restrict movement of substances through intercellular space between cells
Fence - prevent membrane proteins from diffusing in lane of lipid bilayer
Hence, they separate epithelial cells into 2 distinct membrane domains; apical and basolateral
Epithelial cells - apical and basolateral membrane
Apical/luminal/mucosal membrane - faces lumen of organ or body cavity
Basolateral: adheres to adjacent BM (made up of collagen) and interfaces with blood
Epithelial transport properties
Distinct membrane domains - diff transport proteins can be inserted into either the apical or basolateral layer
Transport can occur via paracellular, transcellular pathway or both
Paracellular transport is governed by…
Laws of diffusion and tightness of junctions
Paracellular transport - electrical resistance
Electrical resistance to ion flow (current) through tight junctions can be measured
Higher electrical resistance to ion flow = greater no of tight junction strands holding cell tgt
Paracellular transport
Allows some molecules to cross them but not others
Gradients set up by transcellular transport for paracellular transport
Tight junction proteins
Many diff tight junction proteins have distinct permeabilities to diff ions and diff proteins
Functional classification of epithelial tissues
Leaky epithelium - paracellular transport dominants
Tight epithelium - transcellular transport dominates
Leaky epithelium
Provides a low resistance pathway for ion movement via the paracellular pathway
Tight epithelium
No transport via paracellular pathway because junctions are very tight and electrical resistance is very high
Changes in tight junction resistance
Tight junction resistance changes in a proximal to distal direction in the GI tract and kidney
Changes in tight junction resistance - proximal
Leaky epithelium Low electrical resistance Low no of strands Bulk transport (paracellular) e.g. duodenum, proximal tubule
Changes in tight junction resistance - distal
Tight epithelium High electrical resistance High no of strands Hormonally controlled (transcellular) e.g. colon, collect duct
Transcellular transport
Epithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels to produce transport across epithelial tissues
Diff ion channels and carrier mediated proteins in basolateral and apical membrane produce transport across tissue
Types of transcellular transport
Absorption: transport from lumen to blood
Secretion: transport from blood to lumen
Transepithelial transport - rules
Entry and exit steps: entry for absorption is apical but for secretion is basolateral membrane - diff transport proteins in diff membranes depending on which direction it is going in
Electrochemical gradient: is the entry or exit step passive or active
Electroneutrality: movement of a positive or 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
Membrane permeability to water
Membrane very permeable to water = lots of aquaporins
Membrane not very permeable to water = no aquaporins
Aquaporin
If there is an osmotic gradient across the membrane, water will flow through aquaporin, but if there’s no aquaporins, there’ll be no water flow
Can dissociate water flow from ion flow by presence of aquaporin
Transepithelial transport - cells can select from…
Repertoire of primary active transporter, entry step and exit step
Which surface are tight junctions found
Only on lumen surface not blood surface
Transepithelial transport - secretion
Must have energy to carry out secretion - requires primary active transporter (Na/K ATPase sets up Na+ electrochemical gradient which can be used to drive secretion)
Entry step in basolateral membrane
Exit step in apical membrane
Transepithelial transport - absorption
Primary active transporter set up ion gradients, which are utilised to drive absorption
Entry step in apical membrane
Exit step in basolateral membrane
If we move a positive ion across the cell…
A negative ion will want to come via the paracellular pathway
If create a conc / osmotic difference…
Water will want to flow, but only if the tight junctions allow those molecules to flow through them, i.e. must have both the gradient and junctions that allow it to happen
Leaky and tight epithelium - movement
Leaky junctions give rapid movement via paracellular pathway in response to absorption driven by transcellular pathway
Tight epithelium where resistance is low, will be little movement via this pathway because junctions are very tight
Glucose absorption in small intestine / kidneys - net effect
Absorbed glucose, NaCl, and water
Absorbed water so there’s no change in volume
Isotonic fluid absorption
Oral rehydration therapy
The ability of glucose to enhance absorption of Na+ and hence Cl-, and water is exploited in oral rehydration therapy
A simple sugar solution when given to dehydrated babies suffering from diarrhea saves millions of lives per year
Glucose-galactose malabsorption syndrome
A mutation to the glucose symporter (SGLT) in the small intestine means glucose is retained in the intestine lumen
Increased conc of glucose –> increased osmolarity of lumen of intestine –> creates osmotic gradient that requires water to move to make conc on either side of small intestine equal
The associated increase in lumen osmolarity induces a water efflux - not absorbing glucose –> lose water –> water moves from blood into small intestine to make conc on either side of epithelial wall the same –> diarrhoea
What is galactose
The sugar in milk
Treatment for glucose-galactose malabsorption
Remove glucose and galactose from diet
Use fructose as as source of carbohydrate, which can be moved across basolateral membrane
Utilises a factilitative transporter (GLUT5) that is specific for fructose
Glucose reabsorption in kidneys
In kidneys, glucose in plasma is filtered and needs to be reabsorbed or it will appear in urine
Not a primary absorption - only re-absorption
Glucose re-absorption in kidney - amount absorbed
100% uptake of all glucose that is filtered
60-80% water re-absorbed in proximal tube (small part of kidney)
Glucosuria
Most common cause is diabetes mellitis because insulin activity is deficient and blood sugar is too high (> 200mg/mL)
In diabetes, the glucose symporter can’t absorb glucose fast enough and glucose appears in urine
Glucose in urine - transporter kinetics
If glucose absorption is impaired or transporter is saturated, glucose will appear in urine
All filtered glucose is reabsorbed until renal threshold is reached
Once renal threshold is reached and glucose has saturated all binding pockets, there’s no further increase of transport and glucose appears in urine
Glucose in urine - transporter kinetics - renal threshold
Reflects transport maximum of SGLT
Once this maximum is exceeded, there is no more uptake of glucose and instead starts to appear in urine
Chloride secretion - net result
Accumulates Na+, Cl- and water
Isotonic solutions and blood
Have same osmolarity, so by moving Na+ and Cl- in same conc as blood across lumen and moving water with it, it moves the same type of solution as the blood to the other side of the tissue
Chloride secretion - rate limiting step
Cl- can’t leave the cell unless the Cl- channel is open; if channel is shut, no Cl- moves across apical membrane –> no isotonic fluid secretion
Opening of Cl- channel is strictly regulated (gated), so is the rate limiting step of Cl- secretion
Cystic Fibrosis Transmembrane conductance Regulator (CFTR)
Cl- channel identified at molecular level as CFTR
Regulated by protein kinase A dependent phosphorylation of R domain and binding of ATP to NBD (nucleotide binding domain)
Contains 2 NBDs
CFTR over-stimulation has been implicated in secretory diarrhoea and its dysfunction causes cystic fibrosis; everyone has CFTR - only a defect/mutation in Cl- channel causes cystic fibrosis
What is secretory diarrhoea caused by
Excessive stimulation of secretory cells in crypts of small intestine and colon, which could be due to abnormally high conc of endogenous secretagogues produced by tumours or inflammation
More commonly due to secretion of enterotoxins from bacteria, e.g. vibrio cholerae (contaminated water –> die of dehydration)
Secretory diarrhoea - what do enterotoxins do
Irreversibly activate adenylate cyclase, causing maximal stimulation of CFTR –> secretion that overwhelms absorptive capacity of colon
Secretory diarrhoea - over-stimulation
Overly stimulated secretory cells –> pump out lots of Cl-, Na+ and H2O which exceeds capacity to absorb fluid –> ends up with lots of fluid in gut –> secretory diarrhoea
Molecular mechanism of cholera - normal
Bind to GPCR which releases a G-protein which binds to adenylate cyclase
ATP converted into cAMP –> acts on protein kinase A –> phosphorylates CFTR –> allows it to open –> chloride secretion
Stop by removing GPCR –> turns off adenylate cyclase –> remove phosphorylation, channel shuts –> Cl- secretion stops
Molecular mechanism of cholera - affected by cholera toxin
Cholera toxin irreversibly binds to adenylate cyclase and causes activation of CFTR, i.e. bypasses GPCR
Produces lots of cAMP –> phosphorylates CFTR –> channel permanently open
Effectively gotten rid of rate limiting step and all ion gradients are accumulating Cl- into cell and Cl- immediately leaves via CFTR
Secretory diarrhoea: Secretion vs absorption - normal and over-stimulation
In normal circumstances, secretion and absorption are matched
If overstimulation of secretion, overwhelms ability for absorption –> secretory diarrhoea
Crypt cells vs villus cells
Crypt cells - epithelial cells involved in Cl- secretion
Villus cells - cells involved in Na+ absorption
Crypt cells migrate and change properties and become absorption cells - ~5 day life cycle of crypt –> villus cell
Treatment after survival of initial insult of cholera toxin (secretory diarrhoea)
Maintain hydration - use oral rehydration therapy
Stimulates water influx and tries to offset some effects of overstimulation
What is cystic fibrosis
A complex inherited disorder than affects children and young adults
Mortality usually due to respiratory failure
Exhibit defects in Na+ absorption and Cl- secretion in the lung
Cystic fibrosis (CF) - genes
Inherited in an autosomal recessive fashion
Heterozygotes have no symptoms but are carriers
Children of 2 carriers have a 1/4 chance of getting CF
Cystic fibrosis - disease frequency
Less common in other ethnic groups
Sickle cell anaemea provides protection against…
Malaria, so its been maintained in the population
CF - symptoms
Diverse range
Common theme is involvement of epithelial tissues
CF - most cases of mortality are due to…
Respiratory failure
Organs affected by cystic fibrosis - airways
Clogging and infection of bronchial passages impede breathing
Infections progressively destroy lungs
Lung disease accounts for most deaths from cystic fibrosis
Organs affected by cystic fibrosis - liver
Plugging of small bile ducts impedes digestion and disrupts liver function
Organs affected by cystic fibrosis - pancreas
Occlusion of ducts prevents pancreas from delivering critical digestive enzymes to bowel
Diabetes can result
Organs affected by cystic fibrosis - small intestine
Obstruction of gut by thick stool necessitates surgery; particularly newborns
Organs affected by cystic fibrosis - reproductive tract
Absence of fine ducts renders most males infertile
Occasionally women are made infertile by a dense plug of mucous that blocks sperm from entering uterus
Organs affected by cystic fibrosis - skin/sweat gland
Malfunctioning of sweat glands causes perspiration to contain excessive salt (NaCl)
Clinical management of cystic fibrosis
Chest percussion to improve clearance of infected secretions; clear mucous –> less infections
Antibiotics to treat infections of bacteria in lungs
Pancreatic enzyme replacement; eating meals with pills that contain enzymes required to break down food that are no longer being produced by pancreas
Attention to nutritional status
CF - median survival
38 years of age
Cystic fibrosis: Defect in Cl- secretion - normal lung epithelial cells
Balance between secretion and absorption keeps lung surface moist but prevents excessive fluid build up
Enough water so there’s a fluid surface for gas exchange, but not full of secretions
Cystic fibrosis: Defect in Cl- secretion - lung epithelial cells in CF
Defective/absent Cl- channel prevents isotonic fluid secretion and enhances Na+ absorption through open Na+ ion channel –> not secreting, reabsorbs more –> dry lung surface
Blocking Cl- secretion: Lung pathology - pathway
CFTR gene defect --> Defective ion transport --> Airway surface liquid depletion --> Defective mucocillary clearance --> Cycle of: Mucous obstruction --> infection --> inflammation
Lung pathology: normal lung
Moist surface through Cl- secretion and Na+ reabsorption
Layer of mucous that floats above cells, which have cilia which beat to move mucous
Protects from particles of bacteria that are inhaled into lung surface as it sticks to mucous and is moved out towards back of throat and coughed out
Lung pathology: CFTR lung
Dry dehydrated lung surface
Mucous sticks to cells and becomes a rich environment allowing bacteria to proliferate
Infection and immune system starts to attack bacteria
Overtime, results in damaged tissue which are no longer available for gas exchange - decreased SA eventually becomes fatal
Lung pathology: CFTR lung - therapies
Remove mucous and target infection by having specific antibodies to try break the cycle
Eventually, you can’t overcome that so intervene with gene therapy, where rather than treating symptoms, treat the cause by trying to replace gene with a functional copy to have good ion transport
Or, increase ion transport by bypassing CFTR gene to stop symptoms occurring
CF - sweat formation
People with CF have a very salty sweat
Formation of sweat - processes
2 stage process:
- a primary isotonic secretion of fluid by acinar cells
- a secondary reabsorption of NaCl but NOT water –> hypotonic solution
What causes salty sweat in CF patients
Failure of epithelial cells in ducts of sweat glands to reabsorb NaCl
Purpose of sweat production
Remove heat to surface where it can evaporate
Wet surfaces remove heat better
Sweat production - Hypotonic solutions
Don’t want to put ions (Na+ and Cl-) that a lot of time is spent absorbing in diet out with sweat, so want to have more hypotonic solutions which don’t have same osmolarity as body fluids
At surface where Na+ and Cl- was re-absorbed has less salt and more water –> hypotonic
Sweat formation - acinar channels
2 diff channels that can mediate Cl- release:
CFDR - stimulation causing cyclic AMP, which stimulates protein kinase A to phosphorylate channel to open it
CLCA - activated by elevated intracellular Ca2+ to open and cause Cl- to be secreted
In both, the final pathway in Cl- secretion occurs because Cl- is elevated above electrochemical equilibrium by secondary active transporter
Sweat formation - duct cells
Only have CFTR and Na+ channel
Cl- comes into cell through CFTR down electrochemical gradient where it can be removed from lumen of duct
Water doesn’t move even though there’s an osmotic diff because cells aren’t permeable to water (no aquaporin)
Sweat production - no cystic fibrosis genes
No Cl- secretion at CFTR or Cl- reabsorption occurring, but instead will have another mechanism producing isotonic fluid secretion
CF and sweat formation - normal sweat duct
-ve membrane potential is depolarised and Cl- wants to enter cell down its electrochemical gradient and Na+ moves with it –> removes Na and Cl, and water doesn’t flow –> water retained in lumen –> hypotonic sweat
CF and sweat formation - CF patients
In CF patients, CFTR channel is defective and Cl- accumulates –> affects movement of Na –> Na and Cl retained in duct lumen –> salty sweat
What is osmolarity measured in
Osmoles
Osmolarity - when comparing a solution to the reference solution…
If solution has same osmolarity –> isosmotic
If solution has lower osmolarity –> hyposmotic
If solution has higher osmolarity –> hyperosmotic
Osmolarity - body fluids
~300 mOsmol
Osmolarity of intracellular and extracellular fluids must be equal (isosmotic) so no net water flow (osmosis) occurs
If osmosis occurs, change in cell volume occurs
Tonicity
The effect a solution has on cell volume
Depends on membrane permeability of solute, so osmolarity doesn’t always indicate effect if will have on cell volume
Osmolarity vs tonicity
Not always the same thing - can have same osmolarity but diff tonicity
Effects of tonicity on RBCs
Isotonic solution: no change in cell volume - no net water movement
Hypotonic solution: cause cell swelling and eventually cell lysis (hemolysis) - net gain of water
Hypertonic solution: cause cell shrinkage (crenation) - net water loss
Effects of membrane permeable osmolytes - NaCl
Na pump maintains steady-state cell volume by effectively making Na+ completely impermeable because it always removes it when it comes into the cell
Not balancing Na conc, instead balancing osmolarities inside and outside the cell - no net water flow, and at steady-state, there’s no change in volume
Effects of membrane permeable osmolytes - urea
Lipid permeable, so can cross hydrophobic core of lipid bilayer –> can diffuse down conc gradient into the cell, which can change the osmolarity inside the cell –> osmolarity increases –> water flows into cell –> swell and potentially burst
In some cases, urea will keep moving into the cell until it reaches an equilibrium –> won’t be any change in net volume
Cl- changes its direction of movement based on…
Changes in membrane potential which changes electrical gradient and chemical equilibrium –> drives Cl- movement either into or out of the cell, or maintaining it at electrochemical equilibrium
Glucose in kidney vs gut
In kidney, always have glucose being filtered
In the gut, only have glucose to be re-absorbed when you’re eating
If fasting, no reabsorption occurs because no glucose to absorb across the membrane - conc gradient for glucose favours uptake of glucose from the blood by facilitated diffusion
Is the lipid bilayer formed by cholesterol
No
How does Ca2+ move across a membrane
Against its chemical gradient by its electrochemical gradient
Cl- when MP = -80mV
Electrochemical gradient for Cl- drives no net diffusion of Cl-
Passive diffusion of Cl- increases if MP is depolarised
Flow of an ion through an ion channel is often determined by…
Ion selectivity filter
Isotonic fluid secretion is stimulated by…
Oral rehydration therapy
In Cl- secretion, changing gating of CFTR from closed to open drives…
Isotonic fluid secertion
In sweat glands, acinar cells produce a primary ______ secretion and secondary ____ secretion
Primary isotonic secretion
Secondary hypotonic secretion
Epithelial transport in tight epithelium is often under…
Hormonal control
Cl- secretion - secretagogues
Binds to receptors in basolateral membrane to activate signalling pathways that activate CFTR in apical membrane
Hyperosmotic solution of 0.15 NaCl and 0.05M urea will cause…
An initial cell swelling before returning to the original volume
Urea goes through membrane and water follows, so cell swells. Since hyperosmotic, goes back down since water moves out of cell
Placing RBCs into an isosmotic solution of a membrane permeable solute causes…
Swelling and rupture
Sodium dependent amino acid transporters expressed in apical membrane in small intestine
Used to accumulate amino acids above their conc gradient
Glucose uptake in small intestine drives…
Isotonic fluid reabsorption
Tight junctions - transmission electron microscopy
Appear as membrane fusions
Diffusion of water - water channels
Diffusion of water through a cell membrane is increased by presence of water channels
When does swelling of cells occur when placed in an isosmotic solution
When solution contains a membrane permeable solute
CFTR - ATP binding
Only occurs if R domain is phosphorylated
Uptake of amino acids from gut lumen is mediated by….
Secondary active transport
Apical membrane of duct cells - permeability to water
Low permeability to water
