Compare and contrast passive and active transport.
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/transport.html
http://biology.kenyon.edu/HHMI/Biol113/passive_vs_active.htm
Passive and active transport are two different processes that cells use to move substances across their cell membranes. Let's compare and contrast these two types of transport:
1. Definition:
- Passive transport: Passive transport is the movement of molecules or ions across a cell membrane without the need for energy expenditure by the cell.
- Active transport: Active transport is the movement of molecules or ions across a cell membrane with the expenditure of energy by the cell.
2. Energy Requirement:
- Passive transport: No energy is required for passive transport since it occurs spontaneously, driven by the concentration gradient of the particles.
- Active transport: Active transport requires energy, usually in the form of adenosine triphosphate (ATP), to move substances against their concentration gradient. This is crucial for maintaining concentration differences across the cell membrane.
3. Direction of Movement:
- Passive transport: Passive transport always occurs down the concentration gradient, moving from an area of higher concentration to an area of lower concentration. It aims to equalize the concentration on both sides of the membrane.
- Active transport: Active transport can move substances in either direction, against or with the concentration gradient, depending on the needs of the cell. It can concentrate substances on one side of the membrane, creating concentration gradients.
4. Examples:
- Passive transport: Diffusion (simple and facilitated), osmosis, and filtration are examples of passive transport.
- Active transport: Sodium-potassium pump, proton pump, and endocytosis (phagocytosis and pinocytosis) are examples of active transport.
5. Carrier Proteins:
- Passive transport: In passive transport, carrier proteins may be involved in facilitated diffusion to transport specific molecules across the membrane.
- Active transport: Active transport relies on carrier proteins, also known as pumps, to move molecules against their concentration gradient. These proteins undergo conformational changes fueled by ATP hydrolysis.
In summary, passive transport occurs spontaneously, requires no energy, and moves substances down the concentration gradient, while active transport requires energy, can move substances against the concentration gradient, and is crucial for maintaining concentration gradients in cells.