How do carrier proteins function in facilitated diffusion?

They bind to a specific solute, which triggers a conformational change in the protein that moves the solute to the other side of the membrane without using energy.

What is the primary difference between simple diffusion and facilitated diffusion?

Simple diffusion occurs directly through the phospholipid bilayer without assistance, while facilitated diffusion requires specific transmembrane proteins (channels or carriers) to move substances down their gradient.

How does active transport differ from facilitated diffusion regarding energy and direction?

Active transport moves substances against their concentration gradient and requires metabolic energy ($ATP$), whereas facilitated diffusion moves substances down their gradient and is energy-passive.

What is the difference between endocytosis and exocytosis?

Endocytosis is the process of bringing materials into the cell by invaginating the plasma membrane to form a vesicle, while exocytosis is the fusion of internal vesicles with the membrane to release contents outside.

What is a common misconception regarding the energy source for secondary active transport?

Students often think it uses $ATP$ directly; however, it actually uses the energy from an existing electrochemical gradient (often $Na^+$) established by a primary active transport pump.

Why is it incorrect to say water moves toward a hypotonic solution during osmosis?

Water moves from a hypotonic solution (lower solute/higher water potential) toward a hypertonic solution (higher solute/lower water potential) to reach equilibrium.

What error occurs when assuming all small molecules can cross the membrane via simple diffusion?

While small, ions (like $H^+$ or $Na^+$) cannot cross the hydrophobic core of the bilayer due to their charge; they require facilitated diffusion or active transport.

Define 'Selective Permeability'.

It is the property of a membrane that allows some substances to pass through while blocking others, effectively regulating the cell's internal composition.

Aquaporins are specialized channel proteins that facilitate the rapid transport of water molecules across the plasma membrane, significantly increasing the rate of osmosis.

Tonicity is the ability of an extracellular solution to make water move into or out of a cell by osmosis, determined by the relative solute concentrations.

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Biology

Unit 2: Cell Structure & Function

Membrane Transport

Summary

Membrane transport encompasses the diverse mechanisms by which cells regulate the movement of ions, small molecules, and macromolecules across the phospholipid bilayer. These processes are fundamental for maintaining internal homeostasis, establishing electrochemical gradients, and facilitating cellular communication, categorized primarily by their energy requirements and the specific proteins involved.

1. Foundations of Membrane Permeability

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Selective permeability allows the cell to maintain a distinct internal environment; small, nonpolar molecules (like $O_2$ and $CO_2$ ) pass easily, while large or charged substances require specialized transport mechanisms.
The hydrophobic core of the bilayer acts as the primary barrier to the passage of hydrophilic substances, including ions and polar molecules like glucose.

Diagram showing simple diffusion through the bilayer, facilitated diffusion through a channel protein, and active transport against a gradient using a carrier protein.

2. Passive Transport: Diffusion and Osmosis

3. Active Transport and Electrochemical Gradients

4. Bulk Transport: Endocytosis and Exocytosis

Bulk transport is used for moving large particles or large quantities of substances across the membrane via the formation and fusion of vesicles.
Exocytosis involves internal vesicles fusing with the plasma membrane to release contents (like hormones or neurotransmitters) into the extracellular space.
Endocytosis is the process of taking in substances by folding the membrane inward to form a vesicle; types include phagocytosis (cell eating) and pinocytosis (cell drinking).
Both processes require significant energy and involve the dynamic rearrangement of the cell's cytoskeleton and membrane structure.

5. Key Distinctions in Transport Mechanisms

6. Exam Strategy & Common Pitfalls