Lesson 2 of 5 / Membranes and transport
Boundaries create useful conditions
Why are membranes needed inside a cell too?
In this lesson: Outline the functions of cell surface and internal membranes.
About 4 min
The key ideaMembranes control exchange, separate reactions into compartments and provide surfaces on which organised processes occur.
Explore the idea
Compare surface and internal boundaries
The surface boundary regulates exchange with the surroundings and carries recognition and signalling components.
Explanation
The cell surface membrane separates the cell from its surroundings. Selective transport helps regulate internal composition, while receptors enable responses to signals and surface molecules help cells recognise or attach to one another.
Internal membranes divide a eukaryotic cell into compartments. Different enzyme systems can then operate under different conditions. A lysosomal membrane helps maintain an acidic interior and separates hydrolytic enzymes from much of the cytosol.
Membranes also provide organised reaction surfaces. Mitochondrial inner membranes and chloroplast thylakoids hold components involved in energy transfer. Vesicles move material between compartments while keeping their contents enclosed. Separation is controlled communication, not total isolation.
Step by step
- 1
Define the boundary
Identify the two environments separated by the membrane.
- 2
State the useful difference
Different contents, pH or gradients can support a particular function.
- 3
Include regulated exchange
Membranes contain routes for appropriate substances to enter and leave.
Worked example
An acidic digestive compartment
Why is enclosing hydrolytic enzymes in a lysosome useful?
One way to explain it
The membrane helps maintain conditions suited to those enzymes and separates them from many cytosolic components. Controlled delivery of material allows digestion without exposing the whole cell to the same compartmental conditions.
Why this answer works
- Link separation to a maintained environment.
- Avoid claiming the membrane is impermeable to everything.
Is this true? "Internal membranes only take up space."
They create compartments and reaction surfaces, allowing different processes and gradients to coexist.