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Osmosis and Cell Function — Reading Comprehension

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This informational reading passage for grades 6-8 provides a comprehensive look at osmosis and its critical role in cell function. Aligned with NGSS standard MS-LS1-2, the content explains how water moves across selectively permeable membranes, defines hypertonic, hypotonic, and isotonic solutions, and details their effects on animal and plant cells. The passage illustrates scientific mechanisms, presents real-world scenarios (like why slugs die from salt or why plants wilt), and connects to broader scientific principles of systems and interactions. Glossary terms are highlighted for vocabulary development. The resource includes a Spanish translation, a differentiated version for struggling readers, quizzes, writing activities, and graphic organizers. Audio integration supports diverse learners, making it ideal for classroom or independent study.

CONTENT PREVIEW

Osmosis is a critical process in biology that controls how water moves in and out of cells. In living organisms, the balance of water inside and outside each cell affects its shape, health, and ability to function. When this balance is disturbed, cells can swell, shrink, or even burst. Understanding osmosis explains why plants wilt, why drinking salt water is dangerous, and why salt kills slugs.

Mechanism of Osmosis: Moving Water Across Membranes

Osmosis occurs when water molecules move across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. The membrane allows water to pass, but not most dissolved substances. Solutions outside the cell can be hypertonic, hypotonic, or isotonic. In a hypertonic solution (higher solute concentration outside), water leaves the cell, causing it to shrink. In a hypotonic solution (lower solute outside), water enters the cell, causing it to swell. In an isotonic solution, water moves equally in both directions, and the cell stays the same size.

For example, if an animal cell is placed in pure water (a hypotonic solution), water rushes in, and the cell may burst. In salty water (hypertonic), the cell loses water and shrivels. Plant cells behave differently: in a hypotonic solution, water entering the cell increases turgor pressure, making the plant firm. In hypertonic solutions, plant cells lose water, and the cell membrane pulls away from the wall—a process called plasmolysis.

Real-World Effects and Applications

Osmosis has important consequences for living things. When gardeners sprinkle salt on slugs, water leaves their bodies by osmosis, causing them to dehydrate and die. Plants wilt when the soil becomes too salty, drawing water out of their roots. Drinking salt water can be dangerous for humans because it makes blood hypertonic, pulling water out of cells and causing dehydration. In medicine, saline solutions are used in IV fluids to match the body's isotonic conditions and avoid harming cells.

Scientists study osmosis using experiments with plant cells and artificial membranes. Data shows that water can cross membranes quickly—sometimes at rates of thousands of water molecules per second per channel protein. The principles of osmosis also guide food preservation (like salting meat or pickling) and environmental management.

Interactions and Broader Implications

Osmosis demonstrates how cells interact with their environments and maintain homeostasis. The balance of water is part of a larger system that includes nutrient transport, waste removal, and cell communication. Disrupting osmosis can lead to disease, stress, or death in cells. Understanding osmosis helps scientists develop better treatments, improve agriculture, and protect ecosystems.

Osmosis is a key concept in understanding how life maintains stability in changing environments. It connects to larger scientific ideas about systems, energy, and equilibrium in living things.

Interesting Fact: Some desert plants have adapted cell membranes that control osmosis so well they can survive in extremely salty soils!

What is osmosis?

The movement of water across a selectively permeable membraneThe movement of oxygen into cellsThe movement of solutes through a cell membraneThe process of photosynthesis

What happens to an animal cell placed in a hypertonic solution?

It swells and burstsIt shrivels as water leavesIt stays the same sizeIt becomes more flexible

Why do plant cells become firm in a hypotonic solution?

Water leaves the cellTurgor pressure increases as water entersThe cell wall dissolvesPlasmolysis occurs

What is the main function of a selectively permeable membrane?

It allows all substances to pass freelyIt blocks all moleculesIt lets some substances, like water, pass while blocking othersIt produces energy for the cell

What does isotonic mean in relation to cell solutions?

More water outside the cellLess water outside the cellEqual water and solute concentration inside and outside the cellNo water present

What happens to plant cells in a hypertonic solution?

They gain water and become turgidThey lose water and plasmolysis occursThey burst openThey begin photosynthesis

Which real-world example is caused by osmosis?

Slugs dying when salt is sprinkled on themBirds migrating in winterFrogs croaking at nightLeaves changing color in autumn

Why is drinking salt water dangerous for humans?

It provides no nutrientsIt makes blood hypertonic, pulling water out of cellsIt causes plants to wiltIt increases turgor pressure

T/F: In an isotonic solution, water moves only into the cell.

TrueFalse

T/F: Plasmolysis happens when a plant cell loses water in a hypertonic environment.

TrueFalse