Cell Transport & Homeostasis

Cell Transport & Homeostasis

Big Picture

The cell membrane regulates what enters and exits the cell by allowing only certain substances to pass through. Substances can cross the membrane by either passive or active transport. Passive transport includes diffusion and osmosis, where molecules move from areas of high concentration to areas of low concentration to reach equilibrium. Active transport requires energy to move substances against their concentration gradient. An example is the sodium-potassium pump, which is vital for moving sodium and potassium ions in and out of the cell.

Key Terms

• Homeostasis: The process of maintaining a stable internal environment inside a cell or an entire organism.

• Concentration Gradient: A measurement of how much the concentration of a substance changes across a distance.

• Passive Transport: Substances cross the cell membrane without the cell providing energy.

• Diffusion: The movement of a substance through a membrane from an area of high concentration to an area of low concentration.

• Osmosis: The diffusion of water molecules across a membrane.

• Facilitated Diffusion: Diffusion with the help of transport proteins.

• Transport Protein: Special proteins in the membrane that help substances cross.

• Active Transport: The movement of a substance against the concentration gradient (requires energy from the cell).

• Sodium-Potassium Pump: A protein pump that moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.

• Vesicle Transport: The type of transport in which large molecules cross the cell membrane in vesicles.

• Endocytosis: A type of vesicle transport that moves a substance into the cell by engulfing it in a vesicle.

• Exocytosis: A type of vesicle transport that moves a substance out of the cell by fusing a vesicle with the cell membrane.

Transport Across Membranes

Homeostasis is a central concept in biology, requiring cells to regulate the movement of substances to maintain internal stability. The cell membrane is selectively permeable, allowing some molecules to pass while blocking others.

• Hydrophobic (water-repelling) molecules like oxygen can pass easily through the membrane.

• Hydrophilic (water-attracting) molecules and ions, such as sodium and potassium, require transport proteins to cross the membrane.

• Large molecules like sugars and proteins are too big to go through without assistance.

There are three main ways for substances to move across a cell membrane:

• Passive Transport: Includes diffusion, osmosis, and facilitated diffusion. No energy is required.

• Active Transport: Requires energy (usually from ATP) to move substances against their concentration gradient.

• Vesicle Transport: Used for moving large molecules or particles via endocytosis and exocytosis.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction (relative to gradient)	Examples
Passive Transport	No	High to Low	Diffusion, Osmosis, Facilitated Diffusion
Active Transport	Yes	Low to High	Sodium-Potassium Pump
Vesicle Transport	Yes	Either	Endocytosis, Exocytosis

Key Equations

• Osmosis: The net movement of water can be described by the equation: $J = -P (C_2 - C_1)$ where $J$ is the flux, $P$ is the permeability coefficient, and $C_2 - C_1$ is the concentration difference across the membrane.

• Sodium-Potassium Pump: For every 3 Na+ ions pumped out, 2 K+ ions are pumped in, using 1 ATP molecule.

Examples & Applications

• Example of Diffusion: Oxygen entering a cell from the bloodstream by moving down its concentration gradient.

• Example of Active Transport: The sodium-potassium pump maintaining nerve cell function.

• Example of Vesicle Transport: White blood cells engulfing bacteria by endocytosis.

Additional info: Transport proteins or vesicle transports are needed to move hydrophilic and large molecules across the membrane.