This cheat sheet covers how substances move into and out of cells by diffusion, osmosis, facilitated diffusion, and active transport. Students need these ideas to understand cell survival, homeostasis, nutrient uptake, and waste removal. It also helps connect cell membranes to real biological examples such as plant wilting, red blood cell swelling, and mineral uptake by roots.
The most important idea is that passive transport moves substances down a concentration gradient without using cellular energy. Osmosis is the diffusion of water across a selectively permeable membrane, while facilitated diffusion uses membrane proteins to move substances down the gradient. Active transport moves substances against the gradient and requires energy, usually from ATP.
Key Facts
- Diffusion moves particles from high concentration to low concentration until equilibrium is reached.
- Osmosis is the movement of water across a selectively permeable membrane from higher water concentration to lower water concentration.
- Passive transport does not require ATP because substances move down their concentration gradient.
- Facilitated diffusion moves molecules through channel or carrier proteins from high concentration to low concentration.
- Active transport requires energy because it moves substances from low concentration to high concentration.
- A hypertonic solution has more solute outside the cell, so water tends to leave the cell.
- A hypotonic solution has less solute outside the cell, so water tends to enter the cell.
- An isotonic solution has equal solute concentration inside and outside the cell, so there is no net water movement.
Vocabulary
- Concentration Gradient
- A difference in the amount of a substance between two areas.
- Diffusion
- The movement of particles from an area of higher concentration to an area of lower concentration.
- Osmosis
- The diffusion of water across a selectively permeable membrane.
- Facilitated Diffusion
- Passive transport that uses membrane proteins to help molecules cross the cell membrane.
- Active Transport
- The movement of substances across a membrane against their concentration gradient using energy.
- Tonicity
- A comparison of solute concentration outside a cell to solute concentration inside a cell.
Common Mistakes to Avoid
- Thinking all movement across a membrane uses energy, which is wrong because diffusion, osmosis, and facilitated diffusion are passive and do not require ATP.
- Confusing solute movement with water movement, which is wrong because osmosis specifically describes water moving across a selectively permeable membrane.
- Saying water moves toward lower solute concentration, which is wrong because water moves toward higher solute concentration when the membrane allows water through.
- Mixing up hypertonic and hypotonic solutions, which is wrong because hypertonic solutions draw water out of cells while hypotonic solutions push water into cells.
- Forgetting that facilitated diffusion still moves down the gradient, which is wrong because using a protein channel does not automatically make transport active.
Practice Questions
- 1 A cell has 5% solute inside and is placed in a solution with 10% solute. Will water move into the cell, out of the cell, or show no net movement?
- 2 A membrane separates side A with 30 oxygen molecules and side B with 80 oxygen molecules. In which direction will oxygen diffuse?
- 3 A plant root cell uses ATP to move mineral ions from low concentration in the soil into higher concentration inside the cell. What type of transport is this?
- 4 Explain why a freshwater organism may have problems if placed in saltwater, using osmosis and tonicity in your answer.
Understanding Osmosis, Diffusion & Active Transport
The cell membrane is not a simple wall. Its lipid bilayer has a water repelling middle, so only certain substances can pass through it easily. Small uncharged molecules such as oxygen or carbon dioxide can slip between lipids.
Charged particles, including sodium, potassium, or calcium ions, face a much greater barrier. Large polar molecules such as glucose need help from proteins in the membrane. Channel proteins provide tiny pores.
Carrier proteins bind to a specific substance, then change shape to move it across. This selectivity helps a cell control its internal conditions instead of becoming identical to its surroundings.
Particles move randomly all the time. A net movement in one direction happens because there are more particles on one side at first. At equilibrium, particles still move, but equal numbers cross in each direction.
The speed of transport depends on several conditions. A steeper concentration difference produces faster net movement. Higher temperature makes particles move faster.
A larger membrane surface gives more space for crossing. A shorter travel distance speeds movement too.
These ideas explain why many cells have folded membranes. Root hair cells and cells lining the small intestine have shapes that increase surface area for exchange.
Tonicity depends mainly on solutes that cannot cross the membrane. These trapped solutes affect where water moves and how much a cell changes shape. Animal cells have no rigid cell wall, so a large water gain can make them burst.
A large water loss makes them shrink and disrupts normal cell processes. Plant cells respond differently because a cell wall resists expansion. Water entering a plant cell creates internal pressure against the wall.
This pressure helps stems and leaves stay firm. When plants lose too much water, that pressure falls, which contributes to wilting. A cucumber placed in salty water loses water for the same basic reason.
Active transport does more than move one substance across a membrane. It builds gradients that cells can use later. For example, the sodium potassium pump uses ATP to maintain different ion levels on opposite sides of an animal cell membrane.
This creates an electrochemical gradient, which includes differences in particle amount and electrical charge. Some cells use the energy stored in this gradient to bring in glucose or other nutrients through linked transport proteins. This is called secondary active transport because the protein does not use ATP directly at that moment.
When reading transport diagrams, check the arrow direction, the type of molecule, the membrane protein shown, and whether energy is supplied. Those clues reveal which transport process is taking place.