Definition: Passive transport is the movement of molecules across a cell membrane without the use of energy. It occurs along the concentration gradient, from areas of greater concentration to areas of lesser concentration.
Role: Channel proteins facilitate the transport of polar, ionic, and large molecules through the cell membrane.
Example: Aquaporins are specialized channel proteins that specifically allow water to pass through the membrane.
Hormonal Control: The hypothalamus releases Antidiuretic Hormone (ADH), which signals the collecting ducts in the kidneys to insert more aquaporins into their membranes.
Purpose: Increased aquaporins enhance water reabsorption by allowing more water to diffuse back into the bloodstream through osmosis.
Definition: Facilitated diffusion is a type of passive transport that relies on transport proteins to help move specific molecules across the membrane.
Characteristics: It also does not require energy, but the transport protein typically carries one specific type of molecule at a time.
Example: Glucose transport occurs via both channel proteins and facilitated diffusion proteins, allowing glucose to move from higher to lower concentrations without energy expenditure.
Definition: Active transport is the movement of molecules against their concentration gradient, requiring energy input from the cell, usually in the form of ATP.
Concept: Active transport essentially fights the natural process of diffusion and entropy, moving substances from areas of lower concentration to higher concentration.
Structure of ATP: ATP (Adenosine Triphosphate) consists of an adenosine molecule and three phosphate groups, which are all negatively charged and repel each other, creating high-energy bonds.
Energy Release: The last phosphate group in ATP can transfer to another molecule, releasing energy that changes the shape of the target molecule, allowing for active transport processes.
Process: This transfer is known as phosphorylation, and the result is often the conversion of ATP to ADP (Adenosine Diphosphate).
Mechanism: The sodium-potassium pump is an example of an active transport protein that moves sodium and potassium ions against their gradients.
Sodium ions are pumped out of the cell (3 ions out for every 2 potassium ions that are pumped in).
This results in a higher concentration of sodium outside and a higher concentration of potassium inside.
Purpose: Establishes a voltage gradient (resting potential) across the membrane, essential for nerve signal transmission.
Rapid Communication: Nerve cells, particularly axons, utilize the sodium-potassium pump to maintain a positive charge outside and a negative charge inside the cell, enabling quick signal transmission across long distances.
Example: In response to pain, the rapid changes in voltage enable quick reflexes, demonstrating the significance of ion gradients and active transport in survival.