Cell Junctions, Membrane Transport, and Diffusion in Biology

BIO212 - PART 2 (EXAM 1)

What is the function of tight junctions?

Seal adjacent cells together and prevent substances from passing between cells.

Which proteins are found in tight junctions?

Claudins and occludins.

Where are tight junctions commonly found?

Intestinal epithelium, kidney tubules, and the blood-brain barrier.

What is the function of desmosomes?

Provide mechanical strength and prevent cells from pulling apart.

Which proteins are associated with desmosomes?

Cadherins.

Where are desmosomes commonly found?

Skin and cardiac muscle.

What is the function of gap junctions?

Allow communication between neighboring cells.

Which proteins form gap junctions?

Connexons.

Which cell junction is known as the communicating junction?

Gap junctions.

Where are gap junctions commonly found?

Cardiac muscle and smooth muscle.

What does semipermeable or selectively permeable mean?

The membrane allows some substances to pass while restricting others.

Define concentration gradient.

A difference in concentration between two areas.

Define net movement.

The overall direction molecules move after accounting for movement in both directions.

Define equilibrium.

A state where molecules continue moving but there is no net movement.

What is simple diffusion?

Movement of molecules from high concentration to low concentration without assistance.

Does simple diffusion require ATP?

No.

What direction does diffusion occur relative to the concentration gradient?

Down the concentration gradient.

What are the variables in Fick's Law?

Permeability (P), surface area (A), concentration gradient (ΔC), and membrane thickness (d).

How does increasing surface area affect diffusion?

It increases the rate of diffusion.

How does increasing the concentration gradient affect diffusion?

It increases the rate of diffusion.

How does increasing membrane thickness affect diffusion?

It decreases the rate of diffusion.

What is membrane permeability?

A measure of how easily a substance can cross a membrane.

What factors affect membrane permeability?

Molecular size, electrical charge, and lipid solubility.

What is a hydration shell?

A layer of water molecules surrounding an ion.

Why do Na+ and K+ cross membranes poorly?

Their hydration shells make them effectively larger and less permeable.

What is the partition coefficient?

A measure of a substance's lipid solubility.

How does a high partition coefficient affect membrane transport?

It allows substances to cross membranes more easily.

Give examples of substances with high membrane permeability.

O2, CO2, and steroid hormones.

What are the four major types of membrane transport?

Simple diffusion, channel-mediated diffusion, facilitated diffusion, and active transport.

Which transport processes are passive?

Simple diffusion, channel-mediated diffusion, and facilitated diffusion.

Which transport process requires ATP?

Active transport.

Which transport processes are carrier-mediated?

Facilitated diffusion and active transport.

Does simple diffusion require a transport protein?

No.

Does facilitated diffusion require a transport protein?

Yes, a carrier protein.

What is an example of active transport?

The sodium-potassium pump.

Which substances commonly use channel proteins?

Ions and water.

Which substances can diffuse directly through the phospholipid bilayer?

O2, CO2, and steroid hormones.

Why is the list of substances that diffuse directly through the membrane short?

The membrane interior is hydrophobic and excludes large or charged molecules.

Are channel proteins specific?

Yes.

Do channel proteins typically become saturated?

No, they generally do not.

What is a leak channel?

A channel that is always open.

What is a gated channel?

A channel that can open or close in response to stimuli.

What are the three types of gated channels?

Voltage-gated, ligand-gated, and mechanically gated.

What controls voltage-gated channels?

Changes in membrane potential.

What controls ligand-gated channels?

Binding of a chemical ligand such as a neurotransmitter.

What controls mechanically gated channels?

Stretching, pressure, or deformation of the membrane.

What is mediated transport?

Transport that requires a transport protein.

How do carrier proteins transport substances?

They bind the substance, change shape, and release it on the other side.

How are carrier proteins different from channel proteins?

Carrier proteins change shape; channel proteins form a pore.

Are carrier proteins specific?

Yes.

Do carrier proteins become saturated?

Yes.

What is transport maximum (Tm)?

The maximum transport rate reached when all carriers are occupied.

How can a cell increase transport through a specific carrier protein?

Make more carriers or insert more carriers into the membrane.

What is GLUT?

A glucose transporter used for facilitated diffusion of glucose.

What type of transport does GLUT perform?

Facilitated diffusion.

Does GLUT require ATP?

No.

What is the primary function of GLUT-1?

Basal glucose uptake.

Where is GLUT-1 found?

Most tissues, red blood cells, and the blood-brain barrier.

What is the primary function of GLUT-2?

High-capacity glucose transport and glucose sensing.

Where is GLUT-2 found?

Liver, pancreatic beta cells, intestines, and kidneys.

What is the primary function of GLUT-3?

Ensuring glucose uptake by neurons.

Where is GLUT-3 found?

Neurons.

What is the primary function of GLUT-4?

Insulin-responsive glucose transport.

Where is GLUT-4 found?

Skeletal muscle, cardiac muscle, and adipose tissue.

What is GLUT-4 translocation?

The movement of GLUT-4 transporters from intracellular vesicles to the cell membrane.

What triggers GLUT-4 translocation?

Insulin and exercise.

What happens to glucose uptake after GLUT-4 translocation?

Glucose uptake increases.

How do adipose and resting skeletal muscle cells increase glucose transport when insulin arrives?

By inserting GLUT-4 transporters into the membrane.

How does exercise increase glucose transport?

By stimulating GLUT-4 translocation.

What is hypoglycemia?

Abnormally low blood glucose.

Common symptoms of hypoglycemia?

Sweating, trembling, weakness, confusion, and dizziness.

What is insulin shock?

Severe hypoglycemia caused by excess insulin.

Why can insulin shock be dangerous?

The brain becomes deprived of glucose, potentially causing seizures or loss of consciousness.

What is diabetes mellitus?

A disorder characterized by chronically elevated blood glucose levels.

What causes Type 1 diabetes mellitus?

Autoimmune destruction of pancreatic beta cells leading to little or no insulin production.

What causes Type 2 diabetes mellitus?

Insulin resistance.

How does exercise help people with diabetes?

It increases GLUT-4 translocation, glucose uptake, and insulin sensitivity.

Which GLUT transporter is insulin-sensitive?

GLUT-4.

Which GLUT transporter is associated with liver and pancreatic beta cells?

GLUT-2.

Which GLUT transporter is associated with neurons?

GLUT-3.

Which GLUT transporter is associated with red blood cells and the blood-brain barrier?

GLUT-1.

Which junction contains connexons?

Gap junctions.

Which junction contains cadherins?

Desmosomes.

Which junction contains claudins and occludins?

Tight junctions.

At equilibrium, do molecules stop moving?

No, molecules continue moving but there is no net movement.

Are Na+ and K+ more likely to cross by simple diffusion or through channels?

Through channels.

Which transport process moves substances against their concentration gradient?

Active transport.

Which transport process uses ATP and carrier proteins?

Active transport.

Which transport process uses carrier proteins but does not require ATP?

Facilitated diffusion.