BIOL 2024 Exam 2 USU

Why is cholesterol important to the cell membrane?

Cholesterol stabilizes the membrane's fluidity and mechanical integrity: it prevents phospholipids from packing too closely (limiting rigidity) at low temperatures and restricts their movement (limiting excessive fluidity) at high temperatures. It also significantly reduces permeability to small, water-soluble molecules and enhances the membrane's resistance to deformation.

How thick is the plasma membrane? Can it vary in thickness?

The plasma membrane is typically about 5–10 nanometers thick. Its thickness can vary based on its lipid composition; for instance, a higher concentration of cholesterol makes the membrane thicker and more rigid, while a greater proportion of unsaturated fatty acids tends to make it thinner and more fluid due to their bent tails.

What are the four main phospholipids in the membrane?

The four primary phospholipids in eukaryotic cell membranes are: Phosphatidylcholine (PC), Phosphatidylethanolamine (PE), Phosphatidylserine (PS), and Sphingomyelin (SM). PC and PE are zwitterionic (neutral net charge), while PS carries a net negative charge, and SM is a sphingolipid, differing structurally from the other three glycerophospholipids.

How are phospholipids distributed between the monolayers?

Phospholipids are asymmetrically distributed: Phosphatidylcholine (PC) and Sphingomyelin (SM) are predominantly found in the outer (extracellular) monolayer, while Phosphatidylethanolamine (PE) and Phosphatidylserine (PS) are primarily located in the inner (cytosolic) monolayer. Notably, PS carries a negative charge, and its presence on the inner leaflet is crucial for cell signaling and apoptosis.

Why is the plasma membrane called a “fluid mosaic”?

The plasma membrane is termed a 'fluid mosaic' because of its dynamic and heterogeneous nature. It is 'fluid' due to the rapid lateral diffusion of its lipid and protein components within the plane of the membrane, giving it a viscous, oil-like consistency. It's a 'mosaic' because it's composed of a diverse mixture of phospholipids, cholesterol, proteins (integral and peripheral), and carbohydrates, creating a complex, patterned structure.

Why does the plasma membrane need to be fluid?

Membrane fluidity is essential for vital cellular processes: it enables membrane proteins to diffuse and interact, facilitating cell signaling, transport, and enzymatic reactions. Fluidity also allows for membrane fusion events (like endocytosis and exocytosis), cell growth, cell division (ensuring even distribution of cellular components), and adaptation to environmental changes.

What are lipid rafts?

Lipid rafts are dynamic, specialized microdomains within the plasma membrane, characterized by higher concentrations of cholesterol and sphingolipids (like sphingomyelin) which create a thicker, more ordered phase. These rafts serve as platforms for organizing and concentrating specific lipids and proteins, playing crucial roles in cell signaling, membrane protein trafficking, and receptor sorting.

What determines how easily a molecule passes through the membrane?

The ease with which a molecule crosses the lipid bilayer depends critically on its physical and chemical properties: its size, its electrical charge, and its polarity (hydrophobicity). Small, nonpolar, lipid-soluble molecules (like O2 or CO2) can diffuse directly through the lipid bilayer. In contrast, larger, polar, or charged molecules (like ions, glucose, or proteins) are repelled by the hydrophobic core and require specific transmembrane transport proteins to cross the membrane.

Hydrophobic vs. hydrophilic molecules — what’s the difference?

Hydrophobic (water-fearing) molecules are typically nonpolar and lipid-soluble, allowing them to readily diffuse directly through the lipid bilayer. In contrast, hydrophilic (water-loving) molecules are polar or charged, meaning they interact favorably with water but are repelled by the hydrophobic interior of the membrane, thus requiring specific protein channels or carrier proteins for membrane transit.

Why can’t ions easily cross the lipid bilayer?

Ions cannot easily cross the lipid bilayer due to two main reasons: Firstly, they are typically surrounded by a hydration shell of water molecules, which makes them effectively larger and highly polar. Secondly, the hydrophobic interior of the lipid bilayer strongly repels these charged, hydrated particles. Dehydrating the ion and moving it through the nonpolar core would require a substantial input of energy.

What are the main types of membrane transport proteins?

The two main classes of membrane transport proteins are channel proteins and carrier proteins. Channel proteins form hydrophilic pores through the membrane, selectively allowing specific small, water-soluble molecules or ions to pass rapidly down their electrochemical gradient (passive transport). Carrier proteins bind specifically to the molecule they transport, undergo a conformational change, and then release the molecule on the other side of the membrane, facilitating both passive (facilitated diffusion) and active transport.

What are ion channels used for?

Ion channels are integral membrane proteins that form narrow, water-filled pores, allowing specific ions (e.g., Na^+, K^+, Ca^{2+}, $$Cl^-$) to rapidly flow across the membrane down their electrochemical gradient. They are fundamental for generating and propagating electrical signals in excitable cells (like neurons and muscle cells) and for regulating cell volume and maintaining ionic homeostasis.

Can ion channels be gated and selective?

Yes, ion channels exhibit both gating and selectivity. Selectivity means each channel is highly specific, allowing only certain types of ions to pass based on their size and charge. Gating refers to their ability to open and close in response to specific stimuli, such as changes in membrane potential (voltage-gated), binding of a signaling molecule (ligand-gated), or mechanical force (mechanically-gated).

How fast are ion channels compared to other transport proteins?

Ion channels are remarkably fast; they can transport millions of ions per second through their open pores, making them orders of magnitude quicker than carrier proteins. Their speed is critical for rapid cellular responses, especially in neuronal signaling where quick changes in membrane potential are required.

What happens during passive transport?

During passive transport, molecules move spontaneously across the membrane without the direct expenditure of metabolic energy (ATP). This movement occurs down their concentration gradient (from an area of higher concentration to an area of lower concentration) or, in the case of ions, down their electrochemical gradient (combining concentration and electrical potential differences).

What is an electrochemical gradient?

An electrochemical gradient is a crucial driving force for ion movement across a membrane, representing the combined influence of two forces: the chemical concentration gradient (the tendency for ions to move from an area of high concentration to low concentration) and the electrical potential difference (the tendency for ions to move towards an area of opposite charge). Both components dictate the net direction of an ion's movement.

What is active transport?

Active transport is the process by which cells move molecules across the membrane directly against their concentration or electrochemical gradient—meaning from an area of lower concentration to an area of higher concentration. This 'uphill' movement is non-spontaneous and requires a direct input of metabolic energy, typically in the form of ATP hydrolysis.

Primary vs. secondary active transport

In primary active transport, the transporter protein directly utilizes energy from ATP hydrolysis to pump molecules across the membrane against their gradient (e.g., the Na^+/K^+ pump). In secondary active transport, the transport of one molecule against its gradient is indirectly powered by the electrochemical gradient of another ion (often Na^+), which moves down its gradient. The energy released from the 'downhill' movement of the second ion is used to drive the 'uphill' movement of the first molecule (e.g., the Na^+/glucose symporter).

What is coupled transport?

Coupled transport is a specific mechanism of secondary active transport where two different solutes are transported simultaneously across the membrane by the same carrier protein. One solute moves 'downhill' (down its electrochemical gradient), releasing energy that is then used to drive the 'uphill' movement of the second solute (against its electrochemical gradient). This occurs in either a symport (both in the same direction) or antiport (opposite directions) fashion.

What type of transport protein is GLUT?

GLUT (GLUcose Transporter) is a type of facilitated diffusion carrier protein. It specifically binds glucose molecules and transports them across the cell membrane down their concentration gradient, from an area of higher glucose concentration to lower, without directly consuming ATP. This ensures glucose uptake into cells where it is metabolized.

What type of transport protein is the Na+/K+ pump?

The Na^+/K^+ pump (also known as Na^+/K^+ ATPase) is a crucial primary active transporter found in the plasma membrane of most animal cells. It directly uses the energy from ATP hydrolysis to actively pump 3 sodium ions (Na^+) out of the cell and 2 potassium ions (K^+) into the cell, against their respective electrochemical gradients. This action is vital for maintaining cell volume, establishing the resting membrane potential, and powering secondary active transport systems.

What type of transport protein is the Na+/glucose symporter?

The Na^+/glucose symporter is a secondary active transport protein that moves both sodium ions (Na^+) and glucose into the cell simultaneously (symport). It harnesses the energy stored in the strong electrochemical gradient of Na^+ (established by the Na^+/K^+ pump, driving Na^+ into the cell) to transport glucose against its own concentration gradient. This mechanism is critical for glucose absorption in the gut and reabsorption in the kidneys.

Why are Na+/K+ pumps important?

The Na^+/K^+ pumps are indispensable for several cellular functions: 1. They establish and maintain the steep electrochemical gradients for Na^+ (high outside, low inside) and K^+$+ (high inside, low outside), which are essential for nerve impulse transmission and muscle contraction. 2. They play a critical role in regulating cell volume by controlling intracellular osmolarity. 3. They are a primary contributor to establishing the negative resting membrane potential. 4. The Na^+$$ gradient they create provides the energy for numerous secondary active transport systems.

What is digoxin and what does it do?

Digoxin is a cardiac glycoside medication primarily used to treat heart failure and irregular heartbeats. Its mechanism of action involves specifically inhibiting the Na^+/K^+ pumps in cardiac muscle cells. This inhibition leads to an increase in intracellular Na^+, which in turn reduces the efficiency of the Na^+/Ca^{2+} exchanger, causing an accumulation of intracellular Ca^{2+}. The increased Ca^{2+} enhances the contractility of the heart muscle, leading to a stronger heartbeat and increased excitability.

Who was Alessandro Volta and what did he invent?

Alessandro Volta was an Italian physicist and chemist who, in 1800, invented the voltaic pile, which is considered the first true electric battery capable of providing a continuous current. His invention revolutionized the study of electricity by demonstrating that electricity could be generated chemically, laying the fundamental groundwork for the field of electrochemistry and the understanding of electrical potential.

What is a cell’s resting membrane potential (RMP)?

A cell's resting membrane potential (RMP) is the stable, baseline electrical voltage difference that exists across the plasma membrane of an excitable cell (like a neuron or muscle cell) in its inactive or 'resting' state. At rest, the inside of the cell is typically slightly negative compared to the outside, usually ranging from -40 mV to -90 mV, primarily due to the differential distribution of ions and selective membrane permeability.

Why does an RMP exist?

A resting membrane potential exists primarily due to three factors: 1. Unequal distribution of ions: High extracellular Na^+ and Cl^-, and high intracellular K^+ and negatively charged proteins/anions. 2. Selective permeability of the membrane: The membrane is far more permeable to K^+ ions than to Na^+ ions at rest, largely due to always-open K^+ leak channels. 3. The action of the Na^+/K^+ pump: This pump actively transports 3 Na^+ ions out for every 2 K^+ ions pumped in, contributing a small direct electrogenic component to the negative potential and crucially maintaining the ion gradients over time.

What are fixed intracellular anions?

Fixed intracellular anions are large, negatively charged molecules (like proteins, amino acids, phosphates, and nucleic acids) that are synthesized and metabolized within the cell and are too large to easily cross the plasma membrane. Because they are trapped inside, they contribute significantly to the net negative charge of the cytosol and passively attract positively charged ions, especially K^+, further influencing the resting membrane potential.

Why does membrane thickness matter?

Membrane thickness is a significant factor in electrical signaling because it dictates the distance between the separated charges across the bilayer. A thinner membrane means that the positive and negative charges on opposite sides of the membrane are closer together. This proximity leads to a stronger electrical field and a sharper, more effective potential difference across the membrane, impacting how easily ions can cross and how electrical signals propagate.

How much of a cell’s K+ is needed to establish RMP?

Surprisingly, only a very small fraction—less than 1%—of a cell's total potassium (K^+) ions needs to move across the membrane to establish a stable resting membrane potential. The bulk ion concentrations remain nearly constant: intracellular K^+ is about 140 mM while extracellular is 4–5 mM; conversely, intracellular Na^+ is about 10–15 mM while extracellular is about 140 mM. These gradients are critical for the RMP.

What can the Nernst equation calculate?

The Nernst equation is a mathematical formula used to calculate the equilibrium potential (also known as the Nernst potential) for a specific ion across a selectively permeable membrane at a given temperature. This potential represents the membrane voltage at which the net movement of that particular ion across the membrane due to its concentration gradient is exactly balanced by the electrical force, meaning there is no net flux of that ion. It helps predict the direction of ion movement if the membrane potential is different from the equilibrium potential.

What are the main excitable cells in the body?

The main excitable cells in the human body are neurons (nerve cells) and muscle cells (skeletal, cardiac, and smooth muscle). These cells possess specialized voltage-gated ion channels in their membranes that allow them to rapidly change their membrane potential in response to stimuli, thereby generating and propagating electrical signals (action potentials) crucial for communication and contraction.

What is the soma and where is the axon hillock?

The soma refers to the neuron's cell body, which contains the nucleus and is the metabolic center where proteins are synthesized. The axon hillock is a specialized region found at the junction between the soma and the axon. It is characterized by a high density of voltage-gated Na^+ channels and serves as the trigger zone where graded potentials are integrated, and if the threshold is reached, action potentials are initiated before propagating down the axon.

Why are action potentials all-or-nothing?

Action potentials are considered 'all-or-nothing' events because they either occur maximally and propagate fully or they do not occur at all. Once the membrane potential reaches a critical threshold (typically around -55 mV), a positive feedback loop quickly opens a sufficient number of voltage-gated Na^+ channels, guaranteeing a full-amplitude depolarization. If the stimulus is subthreshold, an action potential will not be generated.

Why are action potentials self-propagating and unidirectional?

Action potentials are self-propagating because the influx of Na^+ ions during depolarization at one segment of the axon creates local currents that depolarize adjacent resting membrane segments to threshold, triggering a new action potential there. They are unidirectional (propagate in only one direction, usually from the axon hillock towards the axon terminal) primarily because of the refractory periods: immediately behind the depolarizing wave, Na^+ channels are temporarily inactivated, preventing backward propagation of the signal.

What are the three states of voltage-gated Na+ channels?

Voltage-gated Na^+ channels exist in three distinct conformational states: 1. Closed (Resting) state: The channel is closed but capable of opening, typically at the resting membrane potential. 2. Open (Activated) state: Upon depolarization reaching threshold, the activation gate opens rapidly, allowing a massive influx of Na^+ ions into the cell, which drives the rising phase of the action potential. 3. Inactivated state: Shortly after opening, a separate inactivation gate swings to block the pore, preventing further Na^+ entry. This state is critical for the refractory period and for ensuring unidirectional action potential propagation, and the channel cannot reopen until it returns to the closed state.

What does the inactivation gate do during an action potential?

During an action potential, the inactivation gate of voltage-gated Na^+ channels promptly swings shut and blocks the channel pore a few milliseconds after the channel has opened. This temporary blockade is crucial: it not only rapidly terminates the Na^+ influx (contributing to repolarization) but also ensures that the channel enters an inactivated (refractory) period. This inactivation prevents the action potential from propagating backward along the axon and helps limit the duration of each action potential, allowing the membrane to repolarize and prepare for the next signal.