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Lipid bilayer structure and amphipathic organization
Lipids are amphipathic: hydrophilic heads face water; hydrophobic tails face inward.
When organized, tails from opposite leaflets align, heads on the exterior surfaces.
This arrangement drives the formation of a lipid bilayer that hides fatty acid tails from water on both the inside and outside of the cell, effectively creating a hydrophobic core.
Lipids can assemble into spheres (liposomes) under certain conditions, forming closed bilayer structures.
Polysaccharide formation: hydrolysis vs. condensation
In the formation of a polysaccharide, a water molecule is either released or consumed depending on the reaction type.
Condensation (dehydration synthesis): water is released.
Hydrolysis: water is consumed/used to break bonds.
Question example: H2O is released through a condensation reaction when forming a polysaccharide.
Membrane transport: gradients, channels, and pumps
Ions and solutes have different concentrations inside and outside a cell (e.g., higher outside for Na+, Cl−, Ca2+; higher inside for K+).
If ions cannot pass through the lipid bilayer, they rely on membrane transport proteins (channels and transporters).
Transport proteins can be either passive (no energy input) or active (energy input required).
An ion channel that is passive allows movement down its electrochemical gradient; pumps actively move ions against their gradient.
The movement of solutes is driven by concentration gradients and other factors like membrane potential.
Concept of electrochemical gradient combines both factors and serves as the net driving force for solute movement.
Electrochemical gradient and membrane potential
Electrochemical gradient is the combination of the chemical (concentration) gradient and the electrical (membrane potential) gradient.
Net driving force for a given ion is described by the electrochemical potential difference:
\Delta \mux = RT \ln\left(\frac{[x]{in}}{[x]{out}}\right) + zx F \Delta \psi
where:$R$ is the gas constant, $T$ is temperature, $[x]{in/out}$ are intracellular/extracellular concentrations, $zx$ is the charge of the ion, $F$ is Faraday's constant, and $\Delta \psi$ is the membrane potential (inside minus outside).
The resting membrane potential is typically negative inside relative to the outside (e.g., around $-60$ to $-70$ mV in neurons).
When an ion has a positive outside and a more negative interior, there is an inward pull of positive ions unless otherwise regulated by channels/pumps.
The term electrochemical gradient is especially important for predicting movement of ions and for understanding action potentials.
Active vs passive transport
Passive transport: movement down the gradient; no direct energy input required.
Active transport: movement against the gradient; energy input (usually ATP) required.
Energy coupling can enable transport that would otherwise be energetically unfavorable (e.g., many systems couple the uphill transport of one solute with the downhill transport of another).
Sodium-potassium ATPase (Na⁺/K⁺-ATPase) pump
An essential active transporter that uses ATP to move ions against their gradients.
Mechanism: for each cycle, it pumps out 3 Na⁺ ions from the cytosol to the extracellular space and pumps in 2 K⁺ ions from the extracellular space to the cytosol.
ATP hydrolysis provides the energy to drive conformational changes in the pump:
\text{ATP} + \mathrm{H2O} \rightarrow \mathrm{ADP} + Pi + \text{Energy}This activity contributes to the negative resting membrane potential by reducing intracellular Na⁺ and increasing intracellular K⁺.
The pump helps maintain ion gradients and membrane potential, enabling various other transport processes.
Transporters: uniport, symport, antiport
Uniport: one molecule moves in one direction (facilitated diffusion).
Symport (cotransport): two different solutes move in the same direction.
Antiport (countertransport): two different solutes move in opposite directions.
Transporters can be passive (facilitated diffusion) or active (requiring energy).
Glucose uptake in gut epithelia: a coupled transport example
In gut epithelial cells, glucose uptake is coupled to sodium transport (SGLT-like mechanism).
Key features described:
Glucose concentration is highest inside the epithelial cell relative to the gut lumen, so glucose tends to move out via a passive transporter; however, uptake from the lumen into the cell is driven by the Na⁺ gradient.
A Na⁺/K⁺ ATPase on the basolateral membrane maintains a high Na⁺ gradient: high Na⁺ outside the cell (in lumen) and low Na⁺ inside.
This Na⁺ gradient enables active uptake of glucose from the gut lumen into the cell via a sodium-glucose cotransporter (symport).
Glucose exits the cell to the extracellular fluid via a separate passive transporter on the basolateral side.
Conceptual takeaway: coupling an uphill transport (glucose uptake against its gradient) with a downhill ion gradient (Na⁺ moving down its gradient) makes the overall process energetically favorable.
Hypertonic, hypotonic, and isotonic solutions
Hypertonic: the solute concentration is higher on the outside; water tends to move out of the cell.
Hypotonic: the solute concentration is lower on the outside; water tends to move into the cell.
Isotonic: solute concentrations are equal on both sides of the membrane; no net water movement.
The terms relate to the relative solute concentrations across a membrane and the resulting water movement.
Aquaporins: water channels
Aquaporins are transmembrane proteins that form channels allowing water to move across the membrane.
Water moves down its own concentration gradient (from regions of higher water activity to lower water activity; equivalently from lower solute concentration to higher solute concentration).
They enable rapid water movement to equilibrate solute concentrations across membranes.
Transporters: passive transporters and conformational change
Passive transporters facilitate diffusion across the membrane without energy input.
Transporters often undergo a conformational change as they move a solute from one side to the other.
Directionality is determined by gradients; net movement is the difference between directions.
Some transporters couple binding events (e.g., glucose binding) with conformational changes to shuttle substrates across membranes.
Ion channels and membrane potential
Ion channels can be gated, meaning they are closed until activated by a signal (gated channels).
Gates provide faster transport compared to carriers/transporters due to the rapid opening/closing of channels.
Selectivity filters determine which ions can pass through a given channel.
Types of gates:
Voltage-gated channels open in response to changes in membrane potential (voltage change across the membrane).
Ligand-gated channels open in response to a ligand binding to a receptor (could be inside or outside the cell).
Resting condition: membrane is polarized with a more positive outside and more negative inside.
Neuron structure and signaling basics
Neuron anatomy: cell body (nucleus), dendrites (input), and axon (output) that can be very long (from a few millimeters to over a meter in humans).
Signals propagate along the axon as an action potential, then release neurotransmitters at the synapse to communicate with the next cell.
Resting membrane potential in neurons is typically negative inside relative to the outside, due to asymmetric ion distributions.
Action potentials: generation and propagation
An action potential is a rapid, transient change in membrane potential that travels along the axon.
Key phases:
1) Resting state: voltage-gated Na⁺ and K⁺ channels are closed; inside is negative relative to outside.
2) Depolarization: upon stimulation, voltage-gated Na⁺ channels open; Na⁺ rushes into the cell following its electrochemical gradient, causing the inside to become positive relative to the outside.
3) Propagation: depolarization opens Na⁺ channels in adjacent regions; the depolarized zone moves along the axon.
4) Inactivation: Na⁺ channels quickly enter an inactivated state, preventing backward conduction in the depolarized region.
5) Repolarization: voltage-gated K⁺ channels open; K⁺ exits the cell, bringing the membrane potential back toward the resting value.
6) Refractory period: briefly, the Na⁺ channels cannot reopen, ensuring unidirectional propagation.The sodium-potassium ATPase (Na⁺/K⁺-ATPase) helps restore and maintain the membrane potential after an action potential by pumping Na⁺ out and K⁺ in.
Typical dynamic: the pump moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed, contributing to the negative interior and overall ion balance.
Synapses: neurotransmitter release and postsynaptic reception
Synapse structure: presynaptic terminal, synaptic cleft, and postsynaptic membrane with receptors.
Action potential arrival triggers opening of voltage-gated Ca²⁺ channels at the presynaptic terminal.
Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
Neurotransmitters bind to ligand-gated receptors on the postsynaptic cell, opening ion channels and initiating a postsynaptic response (e.g., Na⁺ influx to trigger a new action potential).
Practical notes and study guidance from the instructor (as discussed in the transcript)
Study guidance: spend time with slides, review captions under figures, and download slides for captions and notes when using PowerPoint.
Practice questions from the textbook or with study partners to test understanding.
The study guide for Unit 1 is released on the course portal (D2L) under supplementary materials for Unit 1; aim for early review (e.g., by Monday or the weekend).
Exam format and scope mentioned: 36 points total; 32 multiple-choice questions and a multi-part question; each MCQ worth one point; some written responses worth more than one point.
Reading strategy: focus on sections the course covers; skip material that is not discussed in class; if something in the text sounds unfamiliar, verify if it was actually covered in lectures.
Practical tip: even if you’re nervous, allocate time this weekend to start studying and retesting yourself; repetition helps retention.
Quick recap of key ideas (core takeaways)
Lipid bilayer forms a hydrophobic core, with hydrophilic heads facing water on both sides.
Polysaccharide formation involves either condensation (release of water) or hydrolysis (consumption of water).
Ions move across membranes via channels and transporters; gradients and membrane potential determine net movement.
Electrochemical gradient combines concentration gradient and membrane potential and can be described by the thermodynamic expression \Delta \mux = RT \ln\left(\frac{[x]{in}}{[x]{out}}\right) + zx F \Delta \psi.
Na⁺/K⁺-ATPase maintains ion gradients by pumping 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed, contributing to a negative interior.
Passive transporters and ion channels enable rapid movement; gated channels (voltage- and ligand-gated) regulate flow in response to signals.
In epithelia (gut), Na⁺ gradients drive uptake of glucose via cotransport, with glucose exiting on the basolateral side via facilitated transport.
Neurons rely on resting membrane potential, action potentials, and synaptic transmission to communicate; Ca²⁺-triggered vesicle fusion and ligand-gated receptors propagate signals to the next cell.