BDS1.CP.04 Membrane Transport

CP 1.1. Describe structure and function of cell membrane. CP 1.2. Point out the factors which determine the value of membrane potential.

What determines the membrane potential?

Membrane potential is primarily determined by ion concentration gradients and the membrane's permeability to those ions. 

Cell membrane structure. Membrane is selectively permeable. Different ions are maintained at different concentrations inside vs outside. Ions experience chemical + electrical forces. Some ions can cross the membrane more easily than others . This creates a voltage across the membrane = membrane potential (Vm) 

What is the function of a cell membrane? (what helpful analogy can you recall) 

Think Customs & Border Agency analogy is actually very useful = The membrane controls what happens at the boundary of the cell. 

• (1) Mechanical/structural:  keeps the cell intact, interacts with cytoskeleton and ECM, maintains cell shape.  

• (2) Selective permeability  

• (3) Markers & Signalling (Receptors) - ligand → receptor → intracellular response 

• (4) Metabolic Activities, e.g. phosphorylation of receptors.

Define the fluid mosaic model of a cell membrane:  

• Fluid = membrane is not rigid. Proteins/lipids can move laterally within the membrane.  

• Mosaic = contains many different components – including (49%) proteins, (43%) phospholipids, glycolipids, glycoproteins , cholesterol

What structure do the phospholipids within the cell membrane create?  

Phospholipids have dual properties (Hydrophobic fatty acid tails and Hydrophilic phosphate heads). This drives the formation of favourable bilayer structure = creating a self-sealing aqueous environment. This is called amphipathic (contains hydrophobic and hydrophilic elements).  

What relevance does the phospholipid bilayer structure play in membrane potential? 

Hydrophobic tail section makes the membrane impermeable to charged ions. So, Na⁺, K⁺, Ca²⁺ and Cl⁻ cannot simply diffuse through the lipid bilayer. They need membrane proteins. 

Define a membrane protein and describe its 4x roles.  

• Membrane protein = proteins that are temporarily/permanently attached to the cell membrane and organelles.  

• Membrane proteins key functions: Transport, Receptors, Enzymes, Cell-recognition, mechanical- structural role.  

How do membrane proteins play a mechanical/structural role? 

• The membrane proteins interact with structures on the intracellular side: 

  • Interact with cytoskeleton – to maintain shape/anchor proteins/organise membrane/transmit signals

  • AND interacts with extracellular matrix such as collage/elastin/other proteins 

What are the types of cell membrane proteins that give the membrane its selective permeability?

The below proteins give the membrane its selective permeability by allowing the membrane to control what comes in and out.  

(1) Ion channels – allow SPECIFIC ions to cross 

(2) Pumps – use ATP to move ions against their electrochemical gradient. This is active transport. E.g. Sodium-potassium pump (Na+/K+ ATPase = transports Na+ out and K+ in).  

(3) Transporter - Binds a substance and changes conformation to move it across the membrane. 

Describe passive transport.

• Process that relies on electrochemical gradient (substances move down their gradients). No ATP required.  

• Can be facilltated. -

• E.g. Ion channels (and some transporters).  

Describe active transport.

• Substances are moved against their electrochemical gradients therefore require ATP.  Two types:

  • Primary: Substances are moved against their electrochemical gradients therefore require ATP. E.g. Na⁺/K⁺ ATPase (normally more Na+ outside cell and more K+ inside) the transporter moves: 3 Na⁺ → OUT and 2 K⁺ → IN per ATP. This helps maintain the concentration gradients required for membrane potential.

  • Secondary active transport uses free energy of the electrochemical gradient for one component to transport another compartment (not direct hydrolysis of ATP)

• E.g. Na⁺/K⁺ ATPase (normally more Na+ outside cell and more K+ inside) the transporter moves: 3 Na⁺ → OUT and 2 K⁺ → IN per ATP. This helps maintain the concentration gradients required for membrane potential. 

Describe bulk transport.

• Movement of larger quantities of material using vesicles. It is also an active process. 

• Endocytosis → into the cell (enveloping the substance in a portion of its cell membrane and then pinching off that portion of membrane to form an intracellular vesicle). 

  • Phagocytosis: endocytosis of large particles  

  • Pinocytosis: endocytosis/cell drinking of fluid with dissolved substances  

• Receptor mediated endocytosis – selective uptake: Specific molecules bind to receptors on the cell surface. The membrane forms a coated pit, often involving the protein clathrin, and buds off to form a vesicle. Example: uptake of LDL particles carrying cholesterol.

• Exocytosis → out of the cell 

  • Constitutive exocytosis: occurs continuously to deliver membrane components and release products. For example, fibroblasts secrete collagen, and goblet cells release mucus.

  • Regulated exocytosis: occurs in response to a specific signal. For example, a rise in intracellular Ca²⁺ triggers neurotransmitter release at a synapse.

What is membrane potential and what is it measured in?

Potential difference across membrane causes by different concentrations and electrical charges on either side of the membrane.  

Millivolts (mV), cell range varies from –80 to –20 mV.  

e.g. Neurone membrane potential is usually –70mV. But what does that mean?

The inside of the neurone cell is approximately 70 mV more negative than the outside. 

What is voltage of the extracellular side?

0mV 

What does biological electrical activity rely on?  

Not electrons. But the movement of charged ions.  

What are the main ions? 

Cations and anions.  

• Cations: K+, H+, Ca2+. Cations is an ion with fewer electrons than protons.  

• Anions: Cl- (+organic proteins and phosphate compounds). Anion is an ion with more electrons than protons.  

What are normal/typical ion distributions in an excitable cell?  (K+, Na+, Cl-, Ca2+) And movement according to chemical gradient?

• K+: In > Out; thus, main chemical tendency is to go OUT  

• Na+ In < Out; thus, main chemical tendency is to go IN 

• Ca2+: in < Out; thus, main chemical tendency is to go IN 

• Cl-:  In < Out; thus, main chemical tendency is to go IN  

Why is the inside of the cell negatively charged (–ve mV)?

Charged large intracellular organic anions (negatively charged proteins and phosphate-containing compounds) cannot cross the cell membrane.  

How is the cell membrane both a capacitor and a resistor? 

• Capacitor: A capacitor stores separated electrical charge. The membrane does this by separating the positive and negative charges on either side of the lipid membrane. This is because the lipid bilayer is an insulator/dielectric so stops the charges from freely crossing.  

• Resistor: but because ions can cross through ion channels that can be opened or closed the ion movement and therefore the resistance can be controlled: 

  • Open channels: ions move. Lower resistance.  

  • Closed channels: ions don't move. Higher resistance.  

• Capacitator (storing) as well as resistor.

Chemical gradient + electrical gradient = electrochemical gradient. This determines the overall tendency of an ion to move across the membrane.

Predict what would happen with K+. 

• More K+ inside cell. Based off chemical gradient would expect it to move OUT of the cell. 

• Based off the –ve mV charge within the cell would expect K+ to remain in/ be pulled INTO the cell.  In addition, the more K+ that moves out of a cell the more negative it becomes – also pulling K+ INTO the cell.  

• These forces oppose each other. At a particular membrane voltage, they become equal = Nernst equation. 

What question is the Nernst equation answering? 

• "For a particular ion, what membrane potential would balance its chemical gradient?" = At what voltage would the electrical force exactly oppose the chemical force? 

• This is called the: Equilibrium potential (Eion) 

• However, it only considers one ion at a time. It does not consider all ions entering/leaving the cell, so it is not an actual Vm value. IF you wanted to consider multiple ions you would use the GHK equation.  

Why is the Na⁺/K⁺ ATPase described as being electrogenic?  

Na⁺/K⁺ ATPase transporter moves: 3 Na⁺ → OUT and 2 K⁺ → IN per ATP.  Thus producing a small direct contribution to the negative membrane potential. 

According to electrochemical gradient predict what would happen to Na+ ion? Opening Na+ ion channels do what to the charge of the cell?  

• There is more Na+ outside if a cell than inside normally. The chemical gradient would predict that Na+ would move into the cell down the concentration gradient (IN). And because the inside of the cell is –ve mV it would also pull +ve charged Na+ INto the cell.  

• Both forces (electrochemical) favour Na⁺ entering the cell which is why opening Na⁺ channels tends to make the membrane potential more positive. 

Plays an important role in depolarisation.

According to electrochemical gradient predict what would happen to Ca2+ ion?

AND opening Ca2+ ion channels do what to the charge of the cell? What role does Ca2+ play? 

• There is MUCH more calcium OUTSIDE the cell than within (Out>In).  

• Chemical gradient Ca2+ would move in RAPIDLY. Electrical gradient Ca2+ would also move in. 

• Opening the ion channels Ca2+ would move in rapidly increasing the positive charge of the cell. Ca2+ plays a vital role in muscle contraction, neurotransmitter release and intracellular signalling.  

According to electrochemical gradient predict what would happen to Cl- ion?  

• There are more chloride ions OUTSIDE the cell. Cl- OUT> In.  

• Chemical gradient Cl- moves in. Electrical gradient Cl- is repelled so moves OUT.  

• Opposing forces. These forces oppose each other. At a particular membrane voltage, they become equal. = Nernst equation. 

If an ion has a huge electrochemical gradient but does not have a strong membrane potential – why is that? Contrast K+ and Ca2+ 

• If the Ion is unable to move across the cell membrane.  

• Ca2+ has an enormous driving electrochemical force into the cell however is unable to diffuse across the cell membrane so if the ion channel is closed then it plays no role on the membrane potential. 

• K+ in contrast has a substantial gradient and the resting membrane is highly permeable to K⁺. Meaning K⁺ has a major influence on resting Vm. 

Why does K+ ion dominate at rest? 

• High resting permeability. So, the EK of K+ = equilibrium potential for K⁺ 

• Because K⁺ has a high resting permeability, the resting Vm is pulled strongly towards EK. 

• Thus: equilibrium potential of K+ EK ≈ −90 mV and the resting potential of the cell is Vm ≈ −70 mV 

What question is the Nernst equation answering? 

• "For a particular ion, what membrane potential would balance its chemical gradient?" = At what voltage would the electrical force exactly oppose the chemical force? 

• This is called the: Equilibrium potential (Eion) 

• However, it only considers one ion at a time. It does not consider all ions entering/leaving the cell, so it is not an actual Vm value. IF you wanted to consider multiple ions you would use the GHK equation.  

What is the GHK equation? 

• Nernst considers 1x ion.  

• The Goldman-Hodgkin-Katz (GHK) equation considers multiple ions. Vm depends on the concentration gradients of several ions AND how permeable the membrane is to each ion. 

• The GHK equation is better thought of as a permeability-weighted combination 

What are the 5 factors that determine the membrane potential? 

• (1) Ion concentration gradients  

• (2) Electrical gradients 

• (3) Membrane permeability  

• (4) Temperature  

• (5) Ion pumps/transporters  

What are the two types of passive transport?

• Simple – concentration gradient dependent. No transmembrane carriers.

• OR facilitated helped by transmembrane carriers (change shape to move the bound molecule across the membrane) or ion channels.

Neither require ATP.

What are the two types of active transport?

1. Primary: Substances are moved against their electrochemical gradients therefore require ATP. E.g. Na⁺/K⁺ ATPase (normally more Na+ outside cell and more K+ inside) the transporter moves: 3 Na⁺ → OUT and 2 K⁺ → IN per ATP. This helps maintain the concentration gradients required for membrane potential.

2. Secondary active transport uses free energy of the electrochemical gradient for one component to transport another compartment (not direct hydrolysis of ATP)

What are the two types of exocytosis bulk transport?

1. Constitutive exocytosis: occurs continuously to deliver membrane components and release products. For example, fibroblasts secrete collagen, and goblet cells release mucus.

2. Regulated exocytosis: occurs in response to a specific signal. For example, a rise in intracellular Ca²⁺ triggers neurotransmitter release at a synapse.

Contrast uniport vs symport vs antiport

• Uniport: One type of molecule or ion moves across the membrane in one direction. Example: GLUT-mediated glucose transport.

• Symport Two or more different substances move across the membrane in the same direction. Example: Na⁺/K⁺/2Cl⁻ cotransporter (NKCC), which moves all three types of ions in the same direction.

• Antiport: Two or more different substances move in opposite directions.

What is the structure and function of voltage gated ions?

• Transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential. AS the cell is depolarised the channels open up. 

e.g Voltage-gated ion channels are usually specific to Na+, K+, CA2+ and Cl-.

What is the structure and function of Ligand-gated ion channels?

They are receptors (membrane proteins) that open by binding of neurotransmitter, hormone or drug

What is a carrier protein?

• A carrier protein binds a particular substance and changes shape to move it across the membrane. Unlike a channel, a carrier does not normally provide a pore that is open to both sides simultaneously. It alternates between conformations that expose its binding site to one side and then the other.

• Slower than ion channels – seconds-minutes (rather than ms).

• e.g. GLUT proteins transport glucose by facilitated diffusion

What is Stoichiometry of the transmembrane transporters?

• The stoichiometry of a transmembrane transporter refers to the exact numerical ratio of ions and substrate molecules moved across a biological membrane during each transport cycle.

• Na-K-ATPase: 3Na+ out and 2K+ in. Overall 1+ out.

• Na/Ca exchanger: 3Na+ in and 1Ca2+ out. (overall 1+ in)

• Ca2+ ATPase 1 Ca2+ out. (overall 2+ out).