A&P 1.6 Membrane Transport

Why are most cells relatively small?

Small cells have a higher surface area-to-volume ratio, allowing more efficient exchange across the cell membrane

What is selective permeability?

The ability of a membrane to allow some substances to cross more easily than others

What are the two major categories of membrane transport?

Molecular transport and vesicular transport

What is diffusion?

Net movement of a substance from high concentration to low concentration

What drives diffusion?

A concentration gradient

What types of molecules can cross the membrane by simple diffusion?

Small nonpolar or lipid-soluble molecules

Give examples of substances that cross by simple diffusion.

Oxygen, carbon dioxide, alcohol, fatty acids, and steroid derivatives

What is facilitated diffusion?

Passive movement of a substance down its concentration gradient through a membrane protein

What types of substances commonly use facilitated diffusion?

Large, polar, or charged substances such as glucose, ions, and amino acids

What factors affect the rate of diffusion?

Temperature, molecular weight, concentration gradient, surface area, membrane permeability, and electrical forces

How does a larger concentration gradient affect diffusion?

It generally increases the rate of diffusion

How does higher temperature affect diffusion?

It generally increases diffusion because molecules move faster

How does molecular weight affect diffusion?

Smaller molecules generally diffuse faster than larger molecules

How does surface area affect diffusion?

Greater surface area increases the rate of diffusion

What is osmosis?

Net movement of water across a selectively permeable membrane

In which direction does water move during osmosis?

Toward the side with the higher solute concentration

Does osmosis require ATP?

No

What are aquaporins?

Protein channels that allow water to cross the membrane more efficiently

What is osmolarity?

The total concentration of osmotically active solutes in a solution

What is normal blood osmolarity?

Approximately 300 mOsm/L

What is tonicity?

The ability of a solution to affect cell or fluid volume by causing water movement

What is the difference between hypotonic, hypertonic, and isotonic solutions?

Hypotonic has lower solute concentration, hypertonic has higher solute concentration, and isotonic has equal effective solute concentration

What IV solution is approximately isotonic with blood?

0.9% NaCl

What is hydrostatic pressure?

Pressure exerted by fluid against the walls of a container or blood vessel

What is osmotic pressure?

Pressure associated with the movement of water across a membrane due to differences in solute concentration

What are carrier proteins?

Integral membrane proteins that bind specific substances and transport them across the membrane

What is transport maximum (TM)?

The maximum transport rate reached when all available carrier proteins are occupied

Why does carrier-mediated transport eventually reach a maximum rate?

All available carrier proteins become saturated

What is a uniport?

A carrier that transports one type of solute in one direction

What is a symport?

A carrier that transports two types of solutes in the same direction

What is an antiport?

A carrier that transports two types of solutes in opposite directions

What does uniport, symport, and antiport describe?

The number and direction of substances transported by a carrier

What is primary active transport?

Movement of substances against their concentration gradient using ATP

What is an example of primary active transport?

The sodium-potassium pump

What is the sodium-potassium pump?

A membrane protein that uses ATP to move 3 Na+ out of the cell and 2 K+ into the cell

How many Na+ ions does the sodium-potassium pump move out of the cell?

3 Na+

How many K+ ions does the sodium-potassium pump move into the cell?

2 K+

What type of transport is the sodium-potassium pump?

Primary active transport

What are major functions of the sodium-potassium pump?

Creates ion gradients, supports secondary active transport, regulates cell volume, helps establish membrane potential, and contributes to heat production

Why is the sodium-potassium pump important in excitable tissues?

It helps maintain ion gradients needed for membrane potential and electrical signaling

What is secondary active transport?

Transport that uses energy stored in an ion gradient to move another substance against its gradient

Does secondary active transport directly use ATP at the transporter?

No

What is an example of secondary active transport?

SGLT, the sodium-glucose cotransporter

How does SGLT transport glucose?

It uses the downhill movement of Na+ to drive glucose uphill against its concentration gradient

What is vesicular transport?

ATP-dependent bulk movement of materials using membrane-bound vesicles

What is endocytosis?

Movement of substances into a cell by forming a vesicle from the cell membrane

What is phagocytosis?

Endocytosis in which a cell engulfs large particles or cells

What is an example of phagocytosis?

Macrophages engulfing pathogens or cellular debris

What is pinocytosis?

Nonspecific uptake of extracellular fluid and dissolved substances

What does "pinocytosis" literally mean?

Cell drinking

What is receptor-mediated endocytosis?

Specific uptake of substances after they bind to specific membrane receptors

Why is receptor-mediated endocytosis selective?

Only substances that bind to the appropriate receptors are taken into the cell

What is exocytosis?

Movement of materials out of a cell when a vesicle fuses with the cell membrane

Give examples of exocytosis.

Release of hormones, neurotransmitters, enzymes, and sweat

What is the difference between endocytosis and exocytosis?

Endocytosis moves materials into the cell, exocytosis moves materials out

What is the difference between phagocytosis, pinocytosis, and receptor-mediated endocytosis?

Phagocytosis takes in large particles, pinocytosis takes in fluid nonspecifically, and receptor-mediated endocytosis takes in specific ligands