Lec2 Transport of Substances Through the Cell Membrane and Action Potentials

Comprehensive flashcards covering membrane transport, fluid compartments, the fluid-mosaic model, diffusion laws, active transport pumps, and the physiological mechanics of resting and action potentials.

What are the two main transport mechanisms of the cell membrane?

• Passive (no energy from ATP, in favor of gradient)

• Active (energy from ATP, against concentration gradient).

What are common examples of passive transport?

• Diffusion

• Facilitated diffusion

• Filtration

• Osmosis

What are the three forms of bulk transport categorized under active transport?

Exocytosis, endocytosis (which includes phagocytosis and pinocytosis).

How is the cell membrane represented in the fluid-mosaic model?

It is fluid because the phospholipid bilayer is viscous and individual phospholipids can move position, and mosaic because it is embedded with proteins.

What is the structure of the phospholipids in the plasma membrane?

They form a bilayer with:

• phosphate heads facing outwards

• fatty acid tails facing inwards.

What is the function of cholesterol in animal cell membranes?

It improves stability and reduces fluidity.

What are the two types of proteins found in the plasma membrane?

Integral (transmembrane) and peripheral proteins.

Which type of membrane protein can be removed without disrupting the integrity of the membrane?

Peripheral proteins.

What defines the extracellular fluid (ECF) compartment?

It includes all fluid outside the cells, consisting of interstitial fluid (75 %) and plasma (25 %)

Which compartment contains 2/3 of the total body water volume?

The intracellular fluid (ICF).

What is the composition of the interstitial fluid (IF)?

It lies b/w the circulatory system and cells :: 75% of the ECF volume.

What must substances cross to move b/w the plasma and interstitial fluid?

The leaky exchange epithelium of the capillary wall.

Upon what is simple diffusion based?

The kinetic energy of molecules which causes their movement in favor of the concentration gradient.

In Fick's law of diffusion, what does the variable V represent?

The net rate of diffusion.

In the equation $V = \frac{C \times A \times AS \times T}{R}$, what does $C$ stand for?

The diffusion coefficient.

In Fick's law, what does the symbol AS represent?

The concentration gradient ($s1 - s2$).

What does the variable R represent in Fick's law?

The molecule radius.

How is osmosis defined?

The movement of solvent.

How is filtration defined?

The movement of solvent and solute due to a hydrostatic pressure gradient.

What is bulk flow?

The movement of water + solutes together due to a pressure gradient.

Which transport process uses the energy of molecular motion and does not require ATP?

Passive transport (e.g., Simple diffusion, Facilitated diffusion).

What are the two main types of transporters?

Channel proteins and carrier proteins.

What is the function of the sodium-potassium pump (Na+/K+ ATPase)?

It moves 3 sodium ions out of the cell and 2 potassium ions into the cell using ATP.

In which cells is the activity of Na+/K+ pumps so great that it accounts for majority of their ATP usage?

Nerve cells.

What is the characteristic of an active transport carrier?

Typically a lipoprotein that binds to the substrate and transports it to the other side.

What is 'Transport Maximum' ($T_m$) in the context of active transport?

The limited transport capacity of carriers.

What are the three types of gated channels mentioned?

• Chemically gated channels

• Voltage-gated channels

• Mechanically gated channels

How does the sodium-glucose symporter move glucose into the cell?

It uses the energy from the inward diffusion of sodium ions to pull glucose molecules against their concentration gradient.

What is the purpose of the sodium-hydrogen ion antiporter?

It uses the outward movement of hydrogen ions ($H^+$) to maintain the pH of the cell's interior.

What are the relative concentrations of Na+ and K+ in the intracellular fluid (ICF)?

Low $[Na^+]$ and High $[K^+]$.

What are the relative concentrations of Na+ and K+ in the extracellular fluid (ECF)?

High $[Na^+]$ and Low $[K^+]$.

What is the definition of endocytosis?

The process by which cells ingest materials by folding the cellular membrane around them.

What is the specific name for endocytosis of liquid particles?

Pinocytosis.

What is the specific name for endocytosis of solid particles?

Phagocytosis.

What is the role of lysosomes in endocytosis?

Enzymes from lysosomes are used to digest the molecules absorbed during the process.

Where does pinocytosis commonly occur in humans?

In the small intestine, where cells engulf fat droplets.

What is exocytosis?

The process by which cells excrete waste and other large molecules from the protoplasm.

What are the three main stages of an action potential?

Depolarization, repolarization, and hyperpolarization.

What causes depolarization during an action potential?

Positively charged sodium ions rushing into a neuron through open voltage-gated sodium channels.

What causes repolarization during an action potential?

The closing of sodium ion channels and the opening of potassium ion channels.

What causes hyperpolarization?

An excess of open potassium channels and potassium efflux from the cell.

What factors primarily determine resting membrane potential?

K+ concentration gradient and the cell's resting permeability to $K^+$, $Na^+$, and $Cl^-$.

What is the typical range for resting cell membrane potential?

$-70\,mV$ to $-90\,mV$.

Why is potassium ($K^+$) considered the most important electrolyte for resting membrane potential?

Because the resting potential is basically determined by levels of $K^+$ leakage.

How do Na+ channels behave at rest compared to K+ channels?

Na+ channels are perfectly closed, while K+ channels never close completely.

What prevents the equalization of charges despite the permeability of K+ channels at rest?

Intracellular negativity attracts some $K^+$ back into the cell.

What is a resting potential?

The membrane potential of a cell when it remains significantly unchanged for a long period, seen in non-excitable and quiet excitable cells.

What is one role of the membrane potential in all cells?

It allows the cell to function as a battery to power molecular machines.

How is membrane potential used in neurons and muscle cells?

For transmitting signals b/w different parts of a cell.

What level must a membrane depolarize to in order to initiate an action potential?

The threshold potential.

What characterizes the rising phase of an action potential?

A sudden rush of $Na^+$ ions into the neuron.

What characterizes the falling phase of an action potential?

Decrease in $Na^+$ permeability and the rush of $K^+$ ions out of the cell.

What happens when voltage-gated K+ channels remain open after repolarization?

Additional $K^+$ leaves the cell, hyperpolarizing it.

Define the absolute refractory period.

The time during an action potential when another stimulus cannot initiate a new action potential.

What are excitable cells?

Cells such as neurons, muscle cells, and some secretory gland cells that can produce action potentials.

How do graded potentials differ from action potentials regarding size?

Graded potentials vary in size (amplitude), while action potentials are all-or-none.

Where do graded potentials usually occur in a neuron?

Dendrites and the cell body.

What initiates a graded potential?

The entry of ions through gated channels (mechanically, chemically, or voltage-gated).

What is the 'all-or-none' phenomenon?

The principle that action potentials either occur fully if threshold is reached, or not at all.

Where is the trigger zone for an action potential?

The axon.

What happens to the strength of a graded potential signal as it moves?

It decreases over time and space.

How is the intensity of a stimulus encoded if action potentials are all-or-none?

By the frequency of the action potentials.

What is an electrical gradient in cellular transport?

A difference in electrical charge across a membrane that causes ions to move regardless of concentration.

In the electrical gradient, what attracts negative ions?

A positive charge.

How does the concentration gradient of K+ affect its movement?

Since intracellular $[K^+]$ ($\sim 130\,mmol/L$) is much higher than extracellular ($\sim 4\,mmol/L$)

• $K^+$ tends to exit the cell.

How does the electrical gradient affect K+ movement?

The relative negativity of the intracellular space attracts positively charged $K^+$ ions back inside.

What occurs at the equilibrium potential?

The rate of ions leaving by concentration gradient = the rate of ions entering via the electrochemical gradient.

What equation is used to calculate the equilibrium potential for a specific ion?

The Nernst Equation.

What is the overall impact of sodium entering the cell on the resting potential?

It makes the resting potential less negative (more positive).

What impact do organic anions have on the resting potential?

They cannot cross the membrane and make the interior more negative.

What value would diffusion potentials alone (mostly K+) give for the membrane potential?

$\sim -86\,mV$.

How much does the Na+/K+ pump contribute specifically to the membrane potential?

An additional $-4\,mV$.

What is the net membrane potential resulting from both diffusion and the pump?

$-90\,mV$.

What is a common function shared by nerve and muscle tissue?

They both allow $Ca^{2+}$ ions to move back and forth across membranes and allow $Na^+$ to enter and $K^+$ to exit.

Which ions are found in higher concentrations inside most cells?

$K^+$ and $Mg^{2+}$.

Which ions are found in higher concentrations in the surrounding tissue fluids?

$Na^+$ and $Ca^{2+}$.

What are nerve signals?

Action potentials that spread rapidly along the nerve fiber membrane.

What are the dendrites of a neuron?

Narrow, branchlike parts that receive incoming neurotransmitters.

What is the axon?

The long, narrow part of a neuron connecting the cell body to the terminals.

What is the synapse?

The gap or empty space surrounding the neuron.

What are the terminal ends of a neuron?

Places where neurotransmitters wait to cross the synapse.

What happens when a cell membrane reaches the threshold potential?

It initiates an action potential.

Name the textbook reference used for these lecture notes.

Guyton and Hall Textbook of Medical Physiology by John E Hall PhD, 14th Edition (2020).

In a graded potential, what happens if two signals come close together in time?

They will sum.

Why can't two action potential signals sum if they are too close together?

Due to the refractory period.