Anatomy PhysiologyHow Cells Move Substances Across the Membrane
Atlas, a calm guide in a lab coat, crouches beside a glowing wall-sized cross-section of a cell membrane, tracing with one finger as tiny water and oxygen particles drift through protein channels while beakers of salt solution sit ready on the bench.
- Identify the four primary tissue types and one function of each.
- Distinguish between passive transport and active transport based on energy use and gradient direction.
- Compare diffusion and osmosis and explain what each substance moves.
- Predict the direction of water movement when a cell is placed in a solution of known concentration.
Key terms
- Selectively permeable membrane
- A cell boundary that allows some substances to cross while restricting others, controlling internal composition.
- Diffusion
- The passive net movement of particles from high to low concentration, requiring no cellular energy.
- Osmosis
- The diffusion of water across a selectively permeable membrane toward the region of lower water concentration.
- Active transport
- The energy-requiring movement of a substance against its concentration gradient using ATP-powered pumps.
- Connective tissue
- A tissue type that supports and binds, including bone, tendon, and blood suspended in a liquid matrix.
Gradient Direction Decides the Mechanism
The single most useful question for any transport problem is whether the substance moves with its concentration gradient or against it. Movement from high to low concentration is downhill and passive — it happens spontaneously and costs no ATP, as in diffusion of oxygen into blood or osmosis of water into a cell. Movement from low to high concentration is uphill and impossible without an energy source, so it requires active transport powered by ATP, as in the sodium-potassium pump. Identifying gradient direction therefore tells you immediately whether energy must be spent.
Four Tissues, Four Jobs
Multicellular bodies organize cells into four primary tissue types, each defined by a job. Epithelial tissue covers surfaces and lines cavities, forming protective and absorptive barriers like skin and gut lining. Connective tissue supports and binds — including bone, tendon, and blood, which counts because its cells float in a liquid matrix. Muscle tissue contracts to generate force and movement. Nervous tissue conducts electrical signals for rapid communication. Matching an unfamiliar example to one of these four functions — cover, connect, move, or signal — reliably classifies the tissue.
Worked examples
Classify how oxygen enters a cell where oxygen is scarce inside.
- Compare concentrations: oxygen is higher outside the cell than inside.
- Determine the direction of net movement: from high (outside) to low (inside) — downhill.
- Downhill movement is passive and needs no ATP, and since the molecule is not water, it is diffusion.
Answer: Oxygen enters by diffusion, a passive process requiring no energy.
Predict water movement for a cell placed in pure water.
- Identify water concentrations: pure water outside is higher than the solute-containing interior.
- Apply osmosis: water moves toward the lower water concentration, which is inside the cell.
- Conclude water enters the cell; the process is passive, so no ATP is used.
Answer: Water moves into the cell by osmosis, and the cell may swell.
Activity
Sort each example into the transport process that best describes it: diffusion, osmosis, or active transport.
Practice
Predict the direction of water movement and the cell's fate when a red blood cell is placed in concentrated salt water.
Classify whether a cell pumping sodium out against its gradient is using diffusion, osmosis, or active transport, and justify it.
Common mistakes to avoid
- All particle movement across membranes uses energy.Only movement against the gradient (active transport) needs ATP; diffusion and osmosis are passive and free.
- Osmosis can move any dissolved substance.Osmosis specifically names the movement of water across a membrane, not the movement of solutes.
Check your understanding
Which tissue type is mainly responsible for contracting to produce movement?
Osmosis is best described as the movement of which substance across a membrane?
A student says, 'Active transport, like diffusion, never needs energy because both just move particles.' Why is this wrong?
A cell is placed in a solution with more water outside than inside. By osmosis, water will mostly move:
Recap
Cells form four primary tissues defined by their jobs: cover, connect, move, or signal. Across the selectively permeable membrane, downhill movement (diffusion, osmosis) is passive and free, while uphill movement against the gradient (active transport) requires ATP — so gradient direction decides the mechanism.
Reflect
Why is it advantageous for cells to spend ATP on active transport instead of relying only on diffusion?