How Cells Move Substances Across Membranes
Cells must constantly exchange materials with their environment, and membrane transport is the system that makes this possible. Two major categories—passive and active transport—allow substances to cross membranes in different ways. Understanding the differences between these transport mechanisms is essential for IB Biology students studying cell structure and function.
Passive transport moves substances down a concentration gradient, meaning from an area of high concentration to low concentration. This process does not require energy because it relies on the natural kinetic motion of particles. There are three main types of passive transport:
- Simple diffusion, where small nonpolar molecules move directly through the phospholipid bilayer.
- Facilitated diffusion, where specific membrane proteins help polar or charged molecules cross.
- Osmosis, the diffusion of water through a selectively permeable membrane via aquaporins or directly through the bilayer.
Passive transport is essential for exchanging gases, balancing water levels, and moving small molecules efficiently.
In contrast, active transport moves substances against a concentration gradient, from low concentration to high concentration. Because this movement goes against the natural direction of diffusion, it requires energy in the form of ATP. Active transport relies on specialized proteins called pumps, such as the sodium–potassium pump, which maintains essential ion gradients in animal cells.
There are two types of active transport:
- Primary active transport, where ATP is used directly to power movement.
- Secondary active transport, where the energy from one gradient fuels the movement of another molecule.
These mechanisms allow cells to accumulate essential nutrients, expel wastes, and maintain stable internal conditions regardless of external changes.
Another key difference lies in specificity. While passive transport depends on the permeability of the membrane or the availability of channels, active transport uses pumps that bind specific molecules. This means active transport can precisely regulate internal concentrations.
Passive and active transport also differ in speed and regulation. Facilitated diffusion can be rapid when concentration gradients are steep, but it slows as equilibrium approaches. Active transport continues to move molecules in the desired direction regardless of gradients, as long as ATP is available.
Together, passive and active transport systems allow cells to maintain homeostasis, control chemical composition, and support essential processes such as nerve conduction, muscle contraction, and nutrient absorption.
FAQs
Why does active transport require energy?
Active transport pushes substances against their concentration gradients, which is energetically unfavorable. ATP provides the energy needed for pumps to move molecules in this direction.
Is facilitated diffusion passive or active?
Facilitated diffusion is a type of passive transport. It uses membrane proteins to move molecules down a gradient but does not require energy.
How does osmosis fit into membrane transport?
Osmosis is the passive movement of water across a membrane. It follows concentration gradients of solutes and is essential for maintaining cell turgor and hydration.
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