Cell Transport (3b)

What are the two major active membrane transport processes, and why are they needed?

Active transport and vesicular transport. Both require ATP and are used when a solute is too large for channels, is not lipid soluble, or cannot move down its concentration gradient.

What is active transport?

Movement of solutes against their concentration gradient (low → high) using energy and carrier proteins called solute pumps.

What are symporters and antiporters?

Symporters move two different substances in the SAME direction. Antiporters move two different substances in OPPOSITE directions.

What are the two types of active transport?

Primary active transport uses energy DIRECTLY from ATP hydrolysis. Secondary active transport uses energy INDIRECTLY from ionic gradients created by primary active transport.

How does primary active transport work?

ATP hydrolysis changes the shape of a transport protein, causing bound solutes to be pumped across the membrane against their gradient.

What are examples of primary active transport pumps?

Calcium pumps, hydrogen/proton pumps, and the Na⁺-K⁺ pump.

What is the Na⁺-K⁺ pump and what does it move?

It is Na⁺-K⁺ ATPase, an antiporter that uses ATP to pump 3 Na⁺ OUT of the cell and 2 K⁺ INTO the cell against their concentration gradients.

Where is the Na⁺-K⁺ pump especially important?

It is found in all plasma membranes but is especially active in excitable cells such as nerve and muscle cells.

What happens through Na⁺ and K⁺ leakage channels?

Na⁺ leaks INTO the cell and K⁺ leaks OUT of the cell, both moving down their concentration gradients.

Why is the Na⁺-K⁺ pump important?

It maintains electrochemical gradients, which involve both ion concentration and electrical charge and are essential for nerve and muscle function.

What is secondary active transport and what drives it?

It uses energy stored in an ion gradient created by primary active transport rather than using ATP directly.

How does the Na⁺ gradient drive secondary active transport?

The Na⁺-K⁺ pump keeps Na⁺ low inside the cell. Na⁺ then strongly wants to move back into the cell and can bring another substance with it through a carrier, usually a symporter.

How does Na⁺-glucose secondary active transport work?

Na⁺ moves DOWN its gradient into the cell and provides the energy to move glucose into the cell, even when glucose must move UP its gradient.

What substances commonly enter cells by secondary active transport?

Some sugars, amino acids, and ions.

What is the main difference between active transport and facilitated diffusion?

Active transport requires energy and can move substances against their concentration gradient; facilitated diffusion does not require ATP and moves substances down their gradient.

What is vesicular transport?

ATP-requiring transport of large particles, macromolecules, and fluids across the membrane using membranous sacs called vesicles.

What are the four major types of vesicular transport?

Endocytosis = into the cell; exocytosis = out of the cell; transcytosis = into, across, then out of the cell; vesicular trafficking = movement from one area/organelle of the cell to another.

What are the three types of endocytosis?

Phagocytosis, pinocytosis, and receptor-mediated endocytosis.

How does endocytosis generally work?

Protein-coated vesicles form to bring substances into the cell. It often uses receptors and can therefore be highly selective. Once inside, a vesicle may fuse with a lysosome or undergo transcytosis.

What is phagocytosis?

"Cell eating." Pseudopods surround and engulf a solid particle, forming a vesicle called a phagosome.

What are pseudopods and a phagosome?

Pseudopods are membrane projections that surround a solid particle during phagocytosis. The resulting vesicle containing the particle is called a phagosome.

Which cells commonly perform phagocytosis?

Macrophages and certain other white blood cells.

What is amoeboid motion?

Movement in which cytoplasm flows into temporary extensions, allowing a cell to creep.

What is pinocytosis?

"Cell drinking." The plasma membrane infolds to bring extracellular fluid and dissolved solutes into the cell in vesicles.

Is pinocytosis specific, and what is it used for?

It is nonspecific and can be used to sample the environment. It is also a main way nutrient absorption occurs in the small intestine.

What is receptor-mediated endocytosis?

A selective form of endocytosis in which specific extracellular substances bind to specific receptors and are brought into the cell.

What are clathrin-coated pits and caveolae?

Clathrin-coated pits contain receptors that capture specific molecules for receptor-mediated endocytosis. Caveolae are smaller pits with a different protein coat that also capture specific molecules and can use transcytosis.

What substances can enter through receptor-mediated endocytosis?

Examples include enzymes, LDL, iron, and insulin. Some viruses and toxins can also enter cells this way.

What is exocytosis?

The process of ejecting material from the cell. A secretory vesicle fuses with the plasma membrane and releases its contents outside.

What can trigger exocytosis and what substances are released this way?

Cell-surface signals or changes in membrane voltage can trigger it. Examples include hormones, neurotransmitters, mucus, and cellular wastes.

What are v-SNAREs and t-SNAREs in exocytosis?

v-SNARE proteins on the vesicle bind to target t-SNARE proteins on the plasma membrane. This docking helps trigger exocytosis.

What is resting membrane potential (RMP)?

Electrical potential energy produced by the separation of oppositely charged particles across the plasma membrane.

What do voltage and polarized mean?

Voltage is the difference in electrical charge between two points. A cell is polarized when there is a difference in charge across its plasma membrane.

Where does membrane voltage occur and what does a negative RMP mean?

Voltage occurs only at the membrane surface; the rest of the cell and ECF are neutral overall. A negative RMP means the inside of the cell is more negative than the outside.

What is the typical membrane voltage range?

About -50 to -100 mV in different cells. Many cells have an RMP around -90 mV.

Which ion is the key player in establishing RMP?

K⁺ (potassium).

How does K⁺ establish the resting membrane potential?

K⁺ diffuses OUT through K⁺ leakage channels down its concentration gradient. Negatively charged proteins cannot leave, so the inside becomes more negative. This negative interior then attracts K⁺ back toward the cell.

When is resting membrane potential established?

When K⁺'s concentration gradient pushing it OUT is balanced by its electrical gradient pulling it IN.

Why does K⁺ have a greater influence on RMP than Na⁺?

The membrane is more permeable to K⁺ than to Na⁺, so K⁺ is the primary influence on RMP.

How can Na⁺ affect resting membrane potential?

Na⁺ is attracted to the negatively charged cell interior. If Na⁺ enters, it can bring the RMP up to about -70 mV.

Why does Cl⁻ not influence RMP according to the slides?

Its concentration gradient and electrical gradient are exactly balanced.

How does the Na⁺-K⁺ pump maintain RMP?

It continuously pumps 3 Na⁺ OUT and 2 K⁺ IN, maintaining the electrochemical gradients needed for RMP.

What is the steady state for Na⁺ at resting membrane potential?

The rate of active pumping of Na⁺ out of the cell equals the rate of Na⁺ diffusion into the cell.

How do neurons and muscle cells disrupt their resting membrane potential?

They intentionally open gated Na⁺ and K⁺ channels.