Transport across cell membrane

What is simple diffusion?

Movement of molecules from high to low concentration directly through the membrane; no ATP needed.

Why is simple diffusion passive transport?

It moves substances down their concentration gradient, so the cell does not need to supply energy.

What is facilitated diffusion?

Diffusion of specific hydrophilic/polar molecules through membrane proteins, from high to low concentration.

Why does facilitated diffusion need a protein?

Polar or hydrophilic molecules cannot easily cross the membrane’s hydrophobic interior, so proteins help them through.

What do protein channels do in facilitated diffusion?

They provide specific hydrophilic pathways that allow particular molecules to cross the membrane quickly.

What is active transport?

Movement of solutes against their concentration gradient, usually low to high, requiring cellular energy.

What is primary active transport?

ATP directly powers a protein pump that moves a specific solute against its concentration gradient.

What happens to ATP in primary active transport?

ATP loses a phosphate and becomes ADP; the phosphate transfers energy to the pump.

What is a conformational change?

A change in a protein’s shape. In pumps, ATP-driven shape changes move solutes across the membrane.

Why is primary active transport called “primary”?

The transport protein receives energy directly from ATP to perform the transport work.

What does active transport move against?

It moves solutes against their concentration gradient, from lower concentration toward higher concentration.

What are antiport and symport?

They describe how two substances move: antiport = opposite directions; symport = same direction.

What is antiport?

Two substances are transported across a membrane in opposite directions.

What is symport?

Two substances are transported across a membrane in the same direction.

What is secondary active transport?

It uses energy stored in an electrochemical gradient to move another solute against its gradient.

What is an electrochemical gradient?

The combined effect of a concentration gradient and an electrical charge gradient on an ion.

How does secondary active transport get its energy?

A solute moves down its electrochemical gradient, and that downhill movement powers another solute’s transport.

Why doesn't secondary active transport use ATP directly?

ATP was used earlier to create the gradient; the secondary transporter uses the gradient’s stored energy.

How does the H⁺/sucrose pump demonstrate secondary active transport?

ATP first creates an H⁺ gradient. H⁺ then moves down its gradient and powers sucrose transport.

Is secondary active transport passive or active?

Active. Although one solute moves down its gradient, the system uses that energy to move another solute against its gradient.

What is osmosis?

The diffusion of water across a selectively permeable membrane from higher to lower water potential.

What determines the direction of osmosis?

Water moves toward the side with lower water potential, which generally has the higher solute concentration.

What does hypertonic mean?

A solution has more solute and lower water potential than the cell; water tends to leave the cell.

What does hypotonic mean?

A solution has less solute and higher water potential than the cell; water tends to enter the cell.

What does isotonic mean?

The cell and solution have equal water potential, so there is no net movement of water.

What happens to an animal cell in fresh water?

Fresh water is hypotonic, so water enters; the cell swells and may lyse, meaning it bursts.

What happens to a plant cell in fresh water?

Water enters and the cell becomes turgid—firm and full. Its cell wall prevents it from bursting.

What happens to an animal cell in salt water?

Salt water is hypertonic, so water leaves and the animal cell shrivels and may die.

What happens to a plant cell in salt water?

Water leaves, causing plasmolysis: the membrane pulls away from the cell wall and the plant wilts.

What does flaccid mean for a plant cell?

Limp or not firm because the cell lacks enough internal water pressure; typical of an isotonic solution.

What does turgid mean for a plant cell?

Firm and full of water due to water entering the cell; this is a healthy state in fresh water.

What does plasmolyzed mean?

Water loss causes the plant cell membrane to pull away from the cell wall.

What does lysed mean?

The cell takes in excessive water, swells, and tears apart or bursts.

How are fresh water and salt water related to tonicity?

Fresh water has relatively little solute and is hypotonic; salt water has more solute and is hypertonic.

How does a plant cell differ from an animal cell in hypotonic water?

Both gain water, but the plant becomes turgid because its cell wall resists bursting; an animal cell may lyse.

How does a plant cell differ from an animal cell in hypertonic water?

Both lose water; an animal cell shrivels, while a plant cell becomes plasmolyzed and may wilt.

How does water potential relate to osmosis?

Water moves from higher water potential toward lower water potential across a selectively permeable membrane.

How does endocytosis move large molecules?

The membrane surrounds material and pinches inward, forming a vesicle that brings the material into the cell.

What is phagocytosis?

“Cellular eating”: endocytosis in which the cell engulfs large particles or solid material.

What is pinocytosis?

“Cellular drinking”: endocytosis in which the cell takes in extracellular fluid and dissolved substances.