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Osmosis, Diffusion, and Cell Transport
Osmosis, Diffusion, and Cell Transport
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Cells must constantly move materials in and out to stay alive, grow, and respond to their environment. Osmosis, diffusion, and cell transport explain how water, gases, nutrients, and wastes cross the cell membrane. These processes are essential in digestion, nerve signaling, kidney function, and maintaining stable conditions inside the body. Understanding them helps students connect cell structure to real biological function.
The cell membrane is selectively permeable, which means some substances cross easily while others need help or cannot cross at all. Small nonpolar molecules often move by simple diffusion, while water moves by osmosis and many ions or large molecules require transport proteins. Passive transport moves substances down a concentration gradient without energy, but active transport uses ATP to move them against the gradient. Cells also use vesicles in endocytosis and exocytosis to move large materials across the membrane.
Key Facts
- Diffusion is the net movement of particles from high concentration to low concentration.
- Osmosis is the diffusion of water across a selectively permeable membrane.
- Passive transport requires no cellular energy and moves substances down their concentration gradient.
- Active transport requires energy, often ATP, and moves substances against their concentration gradient.
- Concentration gradient = change in concentration across a space or membrane.
- Surface area to volume ratio affects transport efficiency; smaller cells exchange materials more easily.
Vocabulary
- Selective permeability
- The property of a membrane that allows some substances to pass through more easily than others.
- Concentration gradient
- A difference in the amount of a substance between two regions.
- Channel protein
- A membrane protein that forms a passageway for specific ions or molecules to cross the membrane.
- Carrier protein
- A membrane protein that changes shape to move a specific substance across the membrane.
- ATP
- A molecule that stores and transfers energy for many cellular processes, including active transport.
Common Mistakes to Avoid
- Confusing diffusion with osmosis, because osmosis refers only to the movement of water while diffusion can describe many different particles.
- Thinking all transport across membranes needs energy, which is wrong because passive transport happens without ATP when substances move down their gradient.
- Assuming large or charged particles can pass directly through the lipid bilayer, even though many of them need channel proteins, carrier proteins, or vesicles.
- Mixing up hypertonic, hypotonic, and isotonic solutions, which leads to wrong predictions about whether a cell will gain water, lose water, or stay the same size.
Practice Questions
- 1 A cell is placed in a solution where the solute concentration is 12% outside the cell and 4% inside the cell. Predict the direction of water movement and state whether the cell will shrink or swell.
- 2 Oxygen concentration is 18 units outside a cell and 5 units inside. Describe the net movement of oxygen and identify whether this is passive or active transport.
- 3 Explain why sodium ions usually need membrane proteins or pumps to cross the cell membrane, but small nonpolar molecules like oxygen can often cross directly.