Membrane Transport
A cell is not a sealed box. Its membrane constantly lets some things in and keeps others out, and how that traffic works decides whether a cell thrives or dies. Membrane transport is a compact topic with a few clear rules, and once you know them, the questions almost answer themselves.
This lesson explains how substances cross the cell membrane, with and without energy.
The cell membrane is a selectively permeable barrier that controls what enters and leaves a cell. Passive transport, including diffusion and osmosis, moves substances from high to low concentration without energy. Active transport moves substances from low to high concentration and requires energy from the cell.
What is the cell membrane’s job?
The cell membrane is selectively permeable, meaning it lets some substances through while blocking others. This control keeps helpful materials in, keeps harmful ones out, and lets the cell manage its internal environment. Think of it as a gatekeeper rather than a wall.
What is passive transport?
Passive transport needs no energy from the cell because substances move naturally from where they are crowded to where they are sparse, from high concentration to low. The simplest form is diffusion, the spreading of particles until they are evenly distributed, like a scent filling a room. A special case is osmosis, the diffusion of water across a membrane. Water moves toward the side with more dissolved material until balance is reached.
| Type | Direction | Energy? |
|---|---|---|
| Diffusion | High to low concentration | No |
| Osmosis (water) | High to low water concentration | No |
| Active transport | Low to high concentration | Yes |
What is active transport?
Sometimes a cell needs to move a substance the “wrong” way, from low concentration to high. That is uphill work, so it requires active transport, which uses energy, usually in the form of a molecule called ATP. This is how cells pull in nutrients even when they already have plenty inside, or push out substances against the natural flow. The key contrast: passive transport goes downhill and is free; active transport goes uphill and costs energy.
Watch: A Short Video Lesson
RegisteredNurseRN walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for transport questions
- Find the direction: high-to-low concentration or low-to-high?
- High to low means passive transport, no energy needed.
- Low to high means active transport, energy required.
- If the substance moving is water, the process is osmosis.
- Remember the membrane is selectively permeable, so it chooses what crosses.
Practice questions
- What does “selectively permeable” mean?
- Does diffusion require energy?
- What is the special name for the diffusion of water?
- Which type of transport moves substances from low to high concentration?
- Where does the energy for active transport usually come from?
- True or false: osmosis is a form of passive transport.
Answers:
- The membrane lets some substances through while blocking others.
- No. Diffusion is passive.
- Osmosis.
- Active transport.
- ATP, the cell’s energy molecule.
- True. It moves water from high to low water concentration without energy.
Where this fits
Membrane transport builds on the structure of the cell and on cell theory and cell types. The energy that active transport spends is made by the processes you study next, photosynthesis and respiration. Explore all topics on the ASVAB General Science Learning Hub.
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