Lesson 2 of 5 / Membranes and transport
Boundaries create useful conditions
Why are membranes needed inside a cell too?
I can outline the functions of cell surface and internal membranes.
I can outline surface and internal membrane functions.Syllabus
Syllabus 8876, Core 1(i). Outline surface and internal membrane functions.
About 4 min
Make a guess. You are not marked.
A lysosome is full of digestive enzymes. Why does it not digest the cell around it?
Show the answer
Its membrane keeps the enzymes inside.
Compartments let conditions inside differ from the cytosol and keep harmful reactions contained.
The key idea
Membranes 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.
Watch out for this
A student says: "Internal membranes only take up space." What is wrong with this?
Show the answer
They create compartments and reaction surfaces, allowing different processes and gradients to coexist.
Check your understanding
Which is a benefit of an internal membrane?
Show the answer
Maintaining a compartment with conditions different from the cytosol.
Selective boundaries support different local reaction conditions.
Quick recall
Why are membranes needed inside a cell too?