The Cell Membrane

In this lesson, students focus on the structure and function of the cell membrane, which surrounds and protects the cell. Students conduct an experiment to investigate the phenomena of how the cell membrane helps cells regulate their internal environment through osmosis and how this process can affect a cell’s mass.

• The cell membrane maintains the structure of the cell and controls what enters and leaves the cell.
• The cell membrane is semi-permeable, which means it allows some molecules through but blocks out others.
• Osmosis is the diffusion of water across a cell membrane. Osmosis only happens when there are more solutes on one side of the cell membrane than on the other side, and when the membrane is permeable to water but impermeable to the solute.
• Osmosis helps individual cells balance the concentration of water and solutes that enter an organism. A plant or animal’s survival depends on the ability of its cells to balance the concentration of water and solutes.
• Osmosis can cause one of three things to happen when a cell is placed in a solution containing water.
• Tonicity is the measure of water lost or gained from osmosis.

• Why is it important for the cell membrane to be semi-permeable?
• What causes osmosis to happen?
• How would you describe the direction that water travels during osmosis?
• Why do cells rely on osmosis to function?
• How would a cell react to an isotonic environment?
• Why do animal cells need to be in isotonic solutions?

Misconception: Human cells are completely different from other animal cells, plant cells, and prokaryotic cells.

Fact: All cells, whether human, plant, or prokaryote, share certain similarities. All cells must perform essential life functions, and all cells share certain parts, including a cell membrane and cytoplasm.

Misconception:  Individual cells do not need water to function.

Fact: Because cells are living things, cells need water to survive.

Cell : the smallest unit of life; makes up single-celled and multi-celled organisms; surrounded by a cell membrane and filled with cytoplasm

Cell membrane : a protective covering that surrounds a cell and selects which molecules can enter and exit the cell

Organism : a complete living thing that has the ability to carry out all functions for life

Osmosis : the diffusion of water across a semi-permeable

The Bull Shark

Most sharks can only live in the ocean. This is because they cannot survive in fresh water. They depend on salt water for survival.

The bull shark is different. It can survive in both fresh water and salt water, and it has been found 2,000 miles from the ocean in the Amazon River. In the United States, bull sharks have been found as far up the Mississippi River as Illinois.

The bull shark’s ability to survive in both salt water and fresh water has to do with how its body regulates the amount of water it has relative to the amount of salts and other molecules in its body. This happens at the cellular level with the cell membrane and at the organism level with its skin.

Let’s begin with the cell membrane. In all organisms, the cell membrane is semi-permeable (pronounced sem-ee-puhr-mee-uh-buhl) because it allows some molecules through but blocks out others. Molecules that are large or unknown are usually blocked by the cell membrane to protect it, but there are different ways that molecules can pass through the cell membrane. For example, nutrients must pass through the cell membrane to nourish the cell. And water flows in and out of cells easily because the cell membrane has pores large enough for water to travel through.

The ability of water to flow into and out of the cell is essential to the cell’s survival. All cells need to maintain a balanced concentration of water and dissolved particles, called solutes. If the concentration of water to solutes is unequal, water will move from the area of its highest concentration to the area of lowest concentration until the concentrations are equal.

The Flow of Water

This balancing of water and solutes happens through the process of osmosis. Osmosis is the diffusion of water across a semi-permeable membrane. Osmosis only happens when there are more solutes on one side of the cell membrane than on the other side, and when the membrane is permeable to water but impermeable to the solute. Impermeable means it blocks the solutes.

Bull Sharks in Fresh and Salt Water

Osmosis happens at the level of the organism as well. The ability of an organism to keep a constant concentration of water in its body in different kinds of environments is called osmoregulation. In other words, the organism’s body regulates osmosis so that it has a balanced amount of water in its body.

However, most organisms are designed to carry out osmosis in a particular environment. For example, drinking salt water actually speeds up dehydration in humans, causing our cells to lose water molecules and making us thirstier.

Marine animals have the opposite problem. Marine animals have to prevent dehydration because they are surrounded by high concentrations of salt. They do this by keeping a high concentration of solutes in their bloodstream. This causes water to flow from the environment into their bodies. The extra salt that they continually absorb from the ocean water is released from their bodies in their urine.

For most sharks, they cannot change these processes. As a result, if most sharks are put into fresh water, they will absorb too much water and lose too much salt to be able to survive. But bull sharks are different. They can adapt their osmoregulation when they move into a freshwater environment. When bull sharks swim from the ocean to a river, their bodies change their osmoregulation so that less salt is released in their urine. This allows them to survive in both saltwater and freshwater environments.

In this lesson, investigating phenomena caused by the cell membrane, specifically how it helps cells regulate their internal environment through osmosis and how this process can affect a cell’s mass. Students develop experiments to collect and analyze data on the mass of the freshwater cell before and after it is placed in a freshwater and a saltwater environment, looking for patterns that might indicate a relationship between the presence of salt in a watery environment and the structure of a cell in that environment.