Physiology 1A Notes Flashcards
IMPORTANT!!! What is Fick’s Law?
A formula used to calculate flux (movement of substances per time and area) - uses the basis of calculating diffusion
J = DA (dC/dx)
Where:
J = flux, the movement of substance per unit time and area
D = the diffusion coefficient
A = area of interface between two solutions
dC/dx = solute concentration difference across an interface
The two schematic graphs below indicate the relationship between flux and concentration gradients for diffusion of two different substances (A and B) across the cell membrane. In the space provided state which of these graphs is more likely to represent:
diffusion of carbon dioxide (CO2) across a capillary wall, or
diffusion of glucose into a red blood cell following a large meal.
Give a brief reason why you selected these in the space provided.
Substance A: diffusion of carbon dioxide
Substance B: diffusion of glucose
- CO2 → simple diffusion (no energy, protein carriers or channels required)
- Moves across the membrane from high to low concentration without impediment
- Therefore, the linear relationship between the flux of CO2 and the concentration gradient
- Glucose → facilitated diffusion because of high molecular weight
- Only possible with the aid of protein carrier molecules
- Takes longer than simple diffusion because, at one point, all carrier sites will be occupied
- Therefore, the flux/concentration gradient is not linear
What is the key difference between primary and secondary active transport? To which category of transporter does the membrane protein labelled X in the figure below belong?
- Primary active transport directly uses an energy source to move substances against a gradient
- Secondary active transporters indirectly use an energy source (i.e. electrochemical/ion gradients established by primary transporters) to move substances against a gradient
Name two specific membrane transport proteins important in establishing the negative resting membrane potential in pancreatic beta cells and in many other cells. Describe the category of membrane transport process that each protein belongs to, and its role in establishing the resting membrane potential.
Transport proteins:
- The Na+/K+ pump
- K+ leak channels.
- Na+/K+ pump → primary active transporter
3 Na+ out, 2K+ ions in against concentration gradient using ATP - High concentration of Na+ outside cell
- High concentration of K+ inside the cell
- K+ leak channel → passive transport (facilitated diffusion)
- Allows K+ to diffuse out the cell down its concentration gradient
Four categories of cells?
- neurons
- communicate via electrical/chemical signalling (e.g. action potentials) - muscle
- generate force and movement
- three types –> skeletal, cardiac (heart) and smooth (e.g. blood vessels) - epithelial cells
- move substances across barriers (secretory/absorptive)
- found in glands (kidney, pancreas) and linings (lungs, stomach) - connective tissue
- provides structural support, storage and protection (e.g. fibroblasts, immune cells)
Parts of a neuron?
- dendrites: receive/integrate signals
- cell body/soma: generate electrical output
- axon: propagate electrical signal
- terminal: chemical communication
(electrical signals into chemical signals)
What does membrane potential entail? What is the difference in electrical charge across the membrane?
That all cells are polarised
Difference in electrical charge across the membrane = ~100mV)
What is action potential?
- Transient change in neuron membrane potential (-70mV to peak +30mV) for signalling
- Propagated along the axon
What is passive transport? What types are there?
It is passive diffusion, which is the movement of molecules from high to low concentration (no energy required)
Passive diffusion
- can be simple diffusion or facilitated diffusion, and includes permeability
IMPORTANT!! What is simple diffusion? What is its rate?
- When small, non-polar substances (e.g. gases) cross the bilayer
- Rate is directly proportional to the substance’s lipid solubility (J=KDA (dC/dx))
What is facilitated diffusion?
When hydrophilic or larger substances (e.g. ions, glucose) use protein channels or carriers
What is permeability dependent on?
Transporter number and speed (of transporters moving substances across)
Simple diffusion vs. facilitated diffusion
- specificity (lock and key)
- competition
- saturation
What is active transport? What types are there?
Transport that moves against a gradient, thereby requiring energy.
Types include:
Primary
- uses ATP directly
Secondary
- uses electrochemical/ion gradients from primary transporters
Types of secondary transport?
Co-transport (symports):
- moves substances in the same direction as each other (e.g. Na+ glucose co-transporter)
Counter-transports (antiports)
- moves substances in opposite directions (e.g. Na+/H+ exchangers)
What does the Na+/K+ pump do? What is its stoichiometry?
Function:
- maintains Na+ and K+ gradients critical for ion balances inside/outside the cell and cell volume
- 3 Na+ out, 2 K+ in per ATP
Two types of vesicular transport, and their sub-classes?
Exocytosis (EXIT = RELEASE):
- when vesicles fuse with the membrane to release contents (e.g. neurotransmitter release at synapses triggered by Ca2+ influx, enabling nerve cell communication)
Endocytosis (EN = ENGULF):
- vesicles form the membrane to engulf substances
Sub-classes of endocytosis
- phagocytosis: “cellular eating”
- pinocytosis: “cellular drinking”
- receptor-mediated: specific uptake
IMPORTANT! What are the body fluid compartments?
Total body water (TBW): ~42L
Intercellular fluid (ICF): ~28L (or 2/3 of TBW)
- high K+
- low Na+
- low Ca2+
Extracellular fluid (ECF): ~14L (or 1/3 of TWB)
- low K+
- high Na+
- higher Ca+
In extracellular fluid:
- interstitial fluid (fluid between cells): ~11L
- plasma (fluid in the blood): ~3L
Physiological saline: 0.9% NaCl, ~150mM, matching ECF Na+ concentration
What is homeostasis? What are some regulated variables?
The maintenance of stable internal conditions, despite external changes.
Regulated variables include:
- temperature
- glucose
- pH
- blood O2 or CO2 levels
- ion levels
- blood pressure, volume, osmolality
Outline the homeostatic reflex
- Sensors detects change
- Control centre receives sensor input, and determines response
- Effectors mediate the response and restore balance
- Negative feedback loop restores variable back to set range
IMPORTANT?? Outline facilitated diffision and secondary active diffusion in glucose transport.
Facilitated Diffusion (GLUT): Passive, downhill transport (e.g., GLUT1 in most cells).
Secondary Active (SGLT): Na⁺-dependent, uphill transport (e.g., SGLT1 in intestines, SGLT2 in kidneys for glucose reabsorption).
What are ion channels? What is selectivity and gating?
Ion channels: Facilitate PASSIVE ion diffusion through a selective pore, across a concentration gradient
Selectivity: Specific to ions (e.g., K⁺, Na⁺)
Gating: Open/close to allow ions in/out. They function fast (millions of ions/sec)
How does gating of ion channels work? What are some of its main modes?
- occurs between non-conducting to conducting conformations
Main modes:
- binding of extracellular ligands
- phosphorylation/binding of intracellular ligands
- membrane potential (open/closes channels)
- stretch of membrane
- leak channels (always open)
What is osmosis?
What is osmosis?
- passive diffusion of water down a concentration gradient
- simple diffusion, therefore low permeability
IMPORTANT! What is the flux of water like in solutes? What about in aquaporins?
What is the concentration gradient like for water and for solutes?
- high to low water concentration: down conc. gradient
- low to high solute concentration and osmolarity (measure of solute concentration)
Aquaporins (water channels)
- water channels increase permeability (i.e. Peter Agre)
- high permeability
What is the cell membrane?
- phospholipid bilayer with embedded proteins (receptors, transporters, adhesion molecules) and cholesterol
- separates intracellular/extracellular fluids and controls substance transport (e.g. ions, glucose)
What are the 3Rs in animal ethics? Define them.
Replacement
- technology or methods that can replace/avoid animal research
Reduction
- minimise number of animals used in experiment
- involves sharing data and animal resources between researchers
Refinement
- methods that minimise pain, suffering and distress in research animals
Define permeability, permeant and permeable
permeability - how easily a molecule/substance can cross the membrane
permeant - the substances/molecules that can cross the membrane
permeable - a membrane that let’s that substance/molecule through
