TEAS Ready🧬 Science · Biology · Lesson 2 of 7 · 5 min
Levels of organization and cell transport
💧 Why does a red blood cell burst in pure water but shrivel in salty water? The answer is osmosis, and it shows up in IV fluids every day on a nursing unit.
💡 Key idea
Passive transport goes down the gradient with no ATP; active transport goes up the gradient and spends ATP; water follows solute.
🧠 The idea
Living things are organized from small to large: cell, TISSUE (similar cells working together), organ, organ system, organism. Cells move materials across the membrane in two ways. PASSIVE TRANSPORT needs no energy and moves substances from high to low concentration: simple diffusion (oxygen, carbon dioxide), facilitated diffusion through channel or carrier proteins (glucose), and OSMOSIS, the diffusion of water across a selectively permeable membrane. Water moves toward the side with more dissolved solute. In a hypotonic solution water enters the cell and it swells; in a hypertonic solution water leaves and it shrinks; in an isotonic solution there is no net movement. ACTIVE TRANSPORT uses ATP to move substances against their gradient, from low to high concentration; the sodium-potassium pump moves 3 Na+ out and 2 K+ in. Endocytosis and exocytosis move large particles in vesicles and also use energy.
✏️ Worked example
A red blood cell is placed in a solution that is 3% salt, while the cell's interior is about 0.9%. Step 1: compare solute: outside is higher, so the solution is hypertonic. Step 2: water moves toward more solute, so water leaves the cell. Step 3: the cell shrinks (crenates). In distilled water, the reverse happens and the cell can swell and burst (hemolysis).
📌 Takeaways
- Order: cell, tissue, organ, organ system, organism.
- Hypotonic outside = cell swells; hypertonic outside = cell shrinks; isotonic = no net change.
- Active transport (like the Na+/K+ pump) moves solutes from low to high and uses ATP.
📖 Terms in this lesson
Osmosis: The movement of water across a selectively permeable membrane toward the side with more dissolved solute; for example, a red blood cell swells in pure water.
Diffusion: The movement of particles from an area of higher concentration to lower concentration without using energy; for example, oxygen moving from the lungs' air sacs into the blood.
✅ Check yourself
Question 1 of 3
Which list shows the levels of organization from smallest to largest?
- Cell, organ, tissue, organ system
- Tissue, cell, organ, organ system
- Organ, tissue, cell, organ system
- Cell, tissue, organ, organ system
Show the answer and rationale
Answer: D
Cells group into tissues, tissues form organs, and organs work together as organ systems. The other orders place tissue and organ or cell and tissue in the wrong sequence.
Question 2 of 3
A cell is placed in a solution with a lower solute concentration than its cytoplasm. What happens?
- Solute is pumped out using ATP
- Nothing, because the solution is isotonic
- Water enters and the cell swells
- Water leaves and the cell shrinks
Show the answer and rationale
Answer: C
Lower solute outside means the solution is hypotonic, so water moves into the cell, toward the higher solute concentration, and the cell swells. Shrinking happens in a hypertonic solution, and an isotonic solution would have equal concentration.
Question 3 of 3
Which process requires ATP?
- The sodium-potassium pump
- Facilitated diffusion of glucose
- Diffusion of oxygen into the blood
- Osmosis of water into a cell
Show the answer and rationale
Answer: A
The sodium-potassium pump is active transport, moving ions against their gradients and spending ATP. Osmosis, simple diffusion and facilitated diffusion all move substances down their gradients and need no ATP.
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