Cellular Transport: Passive and Active Processes

Factors Influencing Diffusion Rate

The speed at which a substance diffuses is not constant. Several physiological factors can speed up or slow down this process:

• Steepness of the concentration gradient: The greater the difference in concentration between the two sides, the faster the rate of diffusion.
• Temperature: Higher temperatures increase the kinetic energy of molecules, resulting in a quicker rate.
• Surface area: A larger membrane surface area allows more substances to cross at once.
• Diffusion distance: The shorter the distance the substance has to travel, the faster it will move.
• Mass of the substance: Larger, heavier molecules move more slowly than smaller, lighter ones.

Exam Tip: If a question asks why a substance is moving faster in a warm environment, remember that temperature increases kinetic energy, which directly accelerates the rate of diffusion.

Active Processes

When a cell needs to move substances "uphill"—against the concentration gradient—it cannot rely on passive movement. Instead, it must use active processes.

Energy and ATP

Active processes require the expenditure of metabolic energy. The primary "energy currency" used by the cell for this purpose is Adenosine Triphosphate (ATP). ATP is produced within the cell, specifically in the mitochondria, to power these essential movements.

Mechanisms of Active Transport

Active transport is categorized into two main types:

1. Active Transport (Protein-Mediated)

This involves the use of specific membrane proteins to pump molecules across the membrane against their concentration gradient.

2. Vesicular Transport (Endocytosis and Exocytosis)

Sometimes, substances are too large to pass through channels or carriers. In these cases, the cell manipulates its own membrane to create a vesicle—a small, membrane-bound sac.

• Endocytosis: The process of taking in or ingesting molecules into the cell by wrapping the cell membrane around them to form a vesicle.
• Exocytosis: The process of removing waste molecules or secreting useful substances out of the cell via a vesicle.

Because both endocytosis and exocytosis require the cell to physically reshape its membrane and move vesicles, they are both considered forms of active transport and require the addition of energy.

Summary Table: Passive vs. Active Transport