Lecture 4 - Transport Across the Plasma Membrane

What is the formula for mass balance?

Existing body load + intake or metabolic production – excretion or metabolic removal.

What is clearance?

The rate at which a molecule disappears from the body.

How is mass flow calculated?

Concentration × volume flow.

Homeostasis ≠ equilibrium. What three disequilibria exist?

Chemical, Electrical, Osmotic.

What are the two major fluid compartments of the body?

Intracellular fluid (ICF) and Extracellular fluid (ECF).

How permeable is the plasma membrane?

• Highly permeable: nonpolar molecules

• Moderately permeable: small uncharged polar molecules

• Impermeable: ions and large uncharged polar molecules

What is a concentration gradient?

Difference in chemical concentration across a membrane.

What is an electrical gradient?

Difference in electrical charges across regions.

What is an electrochemical gradient?

Combined influence of concentration and electrical gradients on ion movement.

What law does diffusion follow?

The Second Law of Thermodynamics (entropy).

What are the seven properties of diffusion?

Passive, high → low, net movement until equal, rapid over short distances, directly related to temperature, inversely related to size, occurs in open systems/partitions.

What does Fick’s Law of Diffusion describe?

The rate of diffusion = to surface area × concentration gradient × membrane permeability / membrane thickness

What is simple diffusion?

Solute crosses membrane without transport proteins.

What is facilitated diffusion?

Solute crosses with help of channel or carrier protein.

What is channel-mediated facilitated diffusion?

Ions move through gated channels that open/close.

What is carrier-mediated facilitated diffusion?

A carrier protein binds solute, changes shape, releases solute on other side.

What is GLUT?

A glucose transporter that moves glucose into the cell via conformational change.

What is primary active transport?

Direct use of ATP to move substances (e.g., Na⁺/K⁺-ATPase pump).

What does the sodium-potassium pump do?

Pumps 3 Na⁺ out, 2 K⁺ in (concentration vs electrical gradient).

What is secondary active transport?

Uses energy stored in ion gradients maintained by ATP pumps.

What is the difference between symporters and antiporters?

Symporters = same direction; Antiporters = opposite direction.

How is glucose absorbed in epithelial transport?

• Na⁺-glucose symporter moves glucose in (against gradient).

• GLUT transporter moves glucose to ECF.

• Na⁺/K⁺-ATPase maintains Na⁺ gradient.

What is osmosis?

Diffusion of water across a selectively permeable membrane.

What is osmolarity?

Measure of total solute concentration in osmoles/L.

Define isoosmotic, hyperosmotic, and hypoosmotic.

• Iso = same osmolarity;

• Hyper = higher osmolarity;

• Hypo = lower osmolarity.

What is tonicity?

A solution’s ability to change cell volume via water movement.

What happens in a hypotonic solution?

Cell swells (water in).

What happens in an isotonic solution?

Cell size remains constant.

What happens in a hypertonic solution?

Cell shrinks (water out).

What determines tonicity?

Concentration of nonpenetrating solutes.

When are hypotonic IV solutions used?

Dehydration (rehydrate cells).

When are isotonic IV solutions used?

Blood loss (fluid remains in ECF).

What transporter brings glucose into epithelial cells against its gradient?

Na⁺-glucose symporter (secondary active transport)

What maintains the Na⁺ gradient for glucose transport?

Na⁺/K⁺-ATPase pump.

What is osmosis?

Diffusion of water across a semipermeable membrane from low solute → high solute concentration.

What generates osmotic pressure?

The tendency of water to move due to solute concentration differences.

What are the two types of diffusion that do not require ATP?

Simple diffusion and facilitated diffusion.

What type of transport directly uses ATP?

Primary active transport (e.g., Na⁺/K⁺ pump).

What transport uses gradients established by ATP pumps?

Secondary active transport.

What are the two key properties of carrier proteins?

Saturation (Tmax) and competition.

Which IV solution is isotonic and isosmotic?

0.9% saline (normal saline).

Which IV solution is hyperosmotic but isotonic?

5% dextrose in saline.

Which IV solution is initially isosmotic but becomes hypotonic after metabolism?

5% dextrose in water (D5W).

What does the Na⁺/K⁺ pump do?

Pumps 3 Na⁺ out, 2 K⁺ in, using ATP.

Why does glucose uptake continue in facilitated diffusion?

Glucose is converted to G-6-P, keeping intracellular glucose low.

What are the three types of carrier proteins?

Uniport, symport, antiport.

What are the two types of channel proteins?

Open channels and gated channels.

What does Fick’s Law of Diffusion describe?

Rate of diffusion depends on surface area, concentration gradient, membrane permeability, and thickness.

What factors increase membrane permeability?

High lipid solubility, small molecular size, thin membrane.