Cell Membrane Transport Quiz

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Questions and Answers

What factor does NOT affect the rate of diffusion across a cell membrane?

Temperature of the solution (correct)

Number and sizes of openings in the membrane

Amount of substance available

Velocity of kinetic motion

Which process allows molecules to move through a membrane without interacting with carrier proteins?

Simple diffusion (correct)

Facilitated diffusion

Bulk transport

Active transport

What is the role of a carrier protein in facilitated diffusion?

It enhances energy production

It forms a barrier against all transport

It actively pumps molecules against the gradient

It binds with molecules to aid their passage (correct)

Which of the following best describes the energy source for diffusion?

Kinetic energy of the particles Signup and view all the answers

Select the correct statement about selective permeability in cell membranes.

Some channels are selective for specific ions. Signup and view all the answers

Which of the following substances primarily utilizes facilitated diffusion?

Glucose Signup and view all the answers

What characterizes the opening and closing of gated channels in the cell membrane?

They are controlled by chemical and electrical signals. Signup and view all the answers

In which scenario would the rate of facilitated diffusion approach its maximum?

When all carrier proteins are saturated Signup and view all the answers

What characterizes the resting stage (polarization stage) of a neuron's membrane potential?

The membrane potential is at -70 mV Signup and view all the answers

What initiates the depolarization stage of an action potential?

Inflow of sodium ions Signup and view all the answers

What process quickly restores the negative resting membrane potential after depolarization?

Diffusion of potassium ions out of the cell Signup and view all the answers

Which phase of an action potential corresponds to the period when sodium channels are open allowing sodium ions to enter the neuron?

Depolarization stage Signup and view all the answers

How does a typical action potential conclude after depolarization has occurred?

By rapidly closing all sodium channels and opening potassium channels Signup and view all the answers

What is the major factor that leads to the overshooting of the membrane potential beyond 0 mV in large nerve fibers?

Sustained sodium ion inflow Signup and view all the answers

What feedback mechanism is involved in the initiation of an action potential?

Positive feedback Signup and view all the answers

Which ion is primarily responsible for causing depolarization during an action potential?

Sodium (Na+) Signup and view all the answers

What role does the sodium-potassium pump play in nerve function?

It establishes a negative electrical voltage inside the cells. Signup and view all the answers

What happens to ATP during the activity of the sodium-potassium pump?

It is cleaved into ADP and phosphate ion. Signup and view all the answers

What is the primary function of the calcium pumps in a cell?

To maintain low calcium ion concentration in the cytosol. Signup and view all the answers

In which parts of the body is the hydrogen pump primarily active?

Gastric glands and kidneys. Signup and view all the answers

What is the typical resting membrane potential of large nerve fibers?

−70 millivolts. Signup and view all the answers

Which of the following is an example of co-transport?

Glucose moving with sodium ions into the cell. Signup and view all the answers

What is the effect of sodium counter-transport in cells?

It facilitates calcium ion removal from the cell. Signup and view all the answers

What is the primary purpose of maintaining ion concentration differences across the cell membrane?

To enable electrical signaling in nerves. Signup and view all the answers

What percentage of the body is composed of skeletal muscle?

40% Signup and view all the answers

What is the role of the sarcoplasmic reticulum in muscle fibers?

Stores calcium ions Signup and view all the answers

What process occurs between the cross-bridges of myosin and actin during muscle contraction?

Cross-bridge cycling Signup and view all the answers

What is the sarcomere?

The area between two Z discs Signup and view all the answers

What initiates the secretion of acetylcholine at the motor nerve endings?

Action potential Signup and view all the answers

Which statement is true about skeletal muscle fibers?

All fibers extend the entire length of the muscle. Signup and view all the answers

What causes muscle contraction to cease after stimulation?

Pumping of calcium ions back into the sarcoplasmic reticulum Signup and view all the answers

Which of the following is not a component of the myofibrils?

Sarcoplasm Signup and view all the answers

What is the threshold level of membrane potential required to initiate an action potential?

-55 millivolts Signup and view all the answers

Which ions must be restored to their original concentration gradients after an action potential?

Sodium and potassium Signup and view all the answers

What primary mechanism allows for the propagation of an action potential along a nerve fiber?

Depolarization stimulating adjacent areas Signup and view all the answers

In heart muscle fibers, what is the duration of the plateau phase during an action potential?

0.2 to 0.3 seconds Signup and view all the answers

Which type of channels are involved in producing the plateau in heart muscle action potentials?

Both fast sodium channels and slow calcium-sodium channels Signup and view all the answers

What mainly causes the explosive development of an action potential?

Greater number of sodium ions entering than potassium ions leaving Signup and view all the answers

What is the state of the membrane potential after reaching the threshold for stimulation?

It rapidly depolarizes and becomes positive Signup and view all the answers

What is the role of the Na+-K+ pump after an action potential has occurred?

To restore the original sodium and potassium gradients Signup and view all the answers

What distinguishes multi-unit smooth muscle from single-unit smooth muscle?

Multi-unit smooth muscle fibers operate independently. Signup and view all the answers

What is the primary difference in the structure of actin and myosin filaments between smooth muscle and skeletal muscle?

Actin filaments are attached to dense bodies in smooth muscle. Signup and view all the answers

Which statement about the contraction of smooth muscle is correct?

Smooth muscle contraction is slower than that of skeletal muscle. Signup and view all the answers

What is the function of gap junctions in single-unit smooth muscle?

They enable electrical signals to pass quickly between muscle fibers. Signup and view all the answers

How does the energy required for sustained contraction in smooth muscle compare to that of skeletal muscle?

Smooth muscle requires significantly less energy than skeletal muscle. Signup and view all the answers

Which muscle type is NOT an example of multi-unit smooth muscle?

Muscles in the walls of the viscera Signup and view all the answers

What best describes the contraction time of smooth muscle compared to skeletal muscle?

Smooth muscle requires more time to contract than skeletal muscle. Signup and view all the answers

Which of the following accurately describes the role of dense bodies in smooth muscle?

They anchor actin filaments and transmit contractile force. Signup and view all the answers

Study Notes

Membrane Physiology

Extracellular Fluid:
- Na+: 142 mEq/L
- K+: 4 mEq/L
- Ca²⁺: 2.4 mEq/L
- Mg²⁺: 1.2 mEq/L
- HCO₃⁻: 103 mEq/L
- Phosphates: 28 mEq/L
- Glucose: 90 mg/dL
- Amino Acids: 30 mg/dL
- Cholesterol: 0.5 g/dL
- Phospholipids: 0.5 g/dL
- Neutral Fat: 2 to 95 g/dL
- PO₂: 46 mm Hg
- PCO₂: 35 mm Hg
- pH: 7.4
- Proteins: 2 g/dL
Intracellular Fluid:
- Na+: 10 mEq/L
- K+: 140 mEq/L
- Ca²⁺: 0.0001mEq/L
- Mg²⁺: 58 mEq/L
- HCO₃⁻: 4mEq/L
- Phosphates: 4 mEq/L
- Glucose: 10 to 20 mg/dL
- Amino Acids: 200 mg.dL
- PO₂: 20 mm Hg?
- PCO₂: 50 mm Hg?
- pH: 7.0
- Proteins: 16 g/dL

Diffusion

All molecules and ions in body fluids are in constant motion.
Motion of particles is called "heat".
Motion never ceases except absolute zero.
Substances move from areas of high concentration to low concentration.

Transport through Cell Membrane

Transport occurs through diffusion and active transport.
Diffusion through the cell membrane involves random movement of substances either through intermolecular spaces within the membrane or through carrier proteins.
Diffusion is caused by the kinetic energy of molecules.

Simple Diffusion

Molecules or ions move through membrane openings or intermolecular spaces without interacting with carrier proteins.
Rate is determined by amount of substance, velocity of kinetic motion, and number and size of openings in the membrane.

Facilitated Diffusion

A carrier protein aids the passage of molecules or ions.
Binding chemically with molecules/ions.
Rate approaches maximum (Vmax) at high concentrations.
Glucose and most amino acids are carried through this process.

Diffusion Through Protein Pores and Channels

Protein channels are selective and have gates.
Voltage Gating: Channels open/close in response to changes in electrical potential (ex. sodium channels respond to membrane losing negativity).
Chemical (Ligand) Gating: Channels open/close upon binding of a ligand (ex. acetylcholine gated sodium channels).

Active Transport

Movement of ions or substances against an energy gradient (uphill).
Requires energy besides kinetic energy.
Carrier protein initiates movement and involves ATP, usually.
Substances actively transported include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, urate, several sugars, and most amino acids.

Primary and Secondary Active Transport

Primary active transport: Energy comes from direct ATP breakdown
Secondary active transport: Energy originates from ion concentration gradients created by primary active transport. Uses ion movement to drive movement of other substances.

Sodium-Potassium Pump

Pumps 3 sodium ions outward and 2 potassium ions inward.
Important for maintaining sodium and potassium concentration differences across the membrane.
Establishes a negative electrical voltage inside the cells.
Controls cell volume.

Calcium Pumps

Two pumps for calcium to the outside and inside of cells / organelles.
Maintains low intracellular Ca²⁺ concentrations.

Hydrogen Pumps

Primary active transport of hydrogen ions in gastric glands of the stomach, and distal tubules and cortical collecting ducts of kidneys.

Co-Transport and Counter-Transport

Co-transport (symport): Coupled movement of two substances in the same direction.
Counter-transport (antiport): Coupled movement of two substances in opposite directions.

Membrane Potentials and Action Potentials

Resting Membrane Potential: About -70 mV in large nerve fibers
Action Potentials: Rapid changes in membrane potential that spread along nerve fibers (nerve impulse).

Stages of Action Potentials in Nerves

Resting Stage: Membrane is polarized (-70mV).
Depolarization: Membrane becomes permeable to sodium ions; rapid inward diffusion of Na⁺ neutralizes negative potential; potential rises rapidly.
Repolarization: Sodium channels close, potassium channels open; rapid outward diffusion of K⁺; potential returns to negative resting level.

Initiation of Action Potential

Any event causes initial rise in membrane potential towards zero.
Opening of voltage-gated sodium channels allows rapid Na⁺ inflow.
Positive feedback cycle leading to action potential.

Threshold for Action Potential

Sudden membrane potential rise of 15-30 mV required in large nerve fibers.
-55mV often considered threshold.

Propagation of the Action Potential

Action potential elicited at any point excites adjacent membrane resulting in propagation along the membrane.
Depolarization process travels along the entire fiber in both directions (nerve/muscle impulse).

Re-establishing Sodium and Potassium Ionic Gradients

After AP completes, sodium ions that diffused inside, and potassium ions that diffused outside, must be returned to original state.
Achieved by action of the Na⁺-K⁺ pump.

Plateau in Some Action Potentials

In some cases, the membrane does not repolarize immediately; instead, the potential remains on a plateau.
Happens in heart muscle fibers where plateau lasts 0.2-0.3 seconds and causes sustained contraction.

Skeletal and Smooth Muscle Contraction

Skeletal Muscle Fiber:
- Contains myofibrils (with actin and myosin filaments).
- Has a sarcolemmal and sarcoplasmic reticulum.
Smooth Muscle:
- Composed of fibers that are both shorter and smaller than skeletal muscle fibers.
- Two types: multi-unit and single-unit.
Multi-unit smooth muscle: Discrete, separate fibers that contract independently.
Single-unit smooth muscle (unitary): Arranged in masses that contract as a single unit.

Sources of energy for muscle contraction

Glycolysis: Breaks down glycogen in absence of oxygen, about 2.5x faster than oxidative source (sustains max contraction ~1 minute).
Phosphocreatine: Combined energy of stored ATP and phosphocreatine can cause max muscle contraction for ~5-8 seconds.
Oxidative metabolism: Combining oxygen with end products of glycolysis/carbohydrates, fats and proteins, creates ATP (sustains long term, hours)

Excitation of Skeletal Muscle (Neuromuscular Junction)

Neuromuscular junction: Junction of myelinated nerve fiber and muscle fiber.
**Acetylcholine (ACh) ** released when a nerve impulse reaches the junction.
ACh Receptors: Acetylcholine binds to receptors on the muscle fiber membrane.
Action potential generation: This initiates an action potential to the muscle fiber stimulating contraction.
Re-excitation prevention: The enzyme acetylcholinesterase breaks down acetylcholine preventing continued muscle re-excitation.

Excitation-Contraction Coupling

Transmission of the action potential from the outside to interior of the muscle fiber.
T-tubules communicate with the extracellular fluid and contain extracellular fluid in their lumens.
Potential changes spreads along the T tubules to the deep interior.
T-tubule action potentials cause release of calcium ions into the muscle fiber initiating contraction.

Smooth Muscle Contraction

Chemical Factors:
- Low oxygen levels/High Carbon Dioxide/High H⁺ levels cause smooth muscle relaxation.
Hormonal Factors:
- Many hormones affect smooth muscle contraction. (ex. norepinephrine, epinephrine, acetylcholine, angiotensin)
Smooth muscles can be stimulated to contract by nervous signals, hormonal, stretch of the muscle, and chemical environment changes.

Membrane Potentials and Action Potentials in Smooth Muscle

Intracellular potential in smooth muscle is usually -50 to -60 mV.
Action potentials of visceral smooth muscle (unitary):
- Spike potentials: Similar to skeletal muscle.
- Action potentials with plateaus: Repolarization is delayed for hundred milliseconds - long contractions (e.g., ureter and uterus).

Calcium Channels and Action Potential in Smooth Muscle

More voltage-gated calcium channels.
Sodium channels participate less in action potential (compared to skeletal muscle).
Calcium ion flow to the interior of the fiber mainly responsible for the action potential (calcium channels open slower than sodium channels, causing plateau in some smooth muscles)

Spontaneous Generation of Action Potentials in Unitary Smooth Muscles

Some smooth muscle cells are self-excitatory and generate their own action potentials (e.g., slow wave rhythm in digestive tract), without external stimuli.
Slow wave rhythm originates from waxing and waning of calcium ions pumping out of the cell. Strong enough slow waves initiate action potentials, but waves alone cannot cause contraction.

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Description

Test your knowledge on the mechanisms of cell membrane transport including diffusion and facilitated diffusion. This quiz covers key concepts such as selective permeability, carrier proteins, and gated channels. Challenge yourself to understand how molecules move in and out of cells and the factors that influence these processes.