Cell Biology: Osmosis, Homeostasis, and Specialization

Osmosis

Osmosis is the diffusion of water through a selectively permeable membrane. Water molecules move from an area of higher concentration to an area of lower concentration.

Isotonic Solutions

Isotonic means "same strength" and refers to the solute concentration. In isotonic solutions, water moves equally in both directions. For example:

• Animal cells: Water goes in and out.
• Plant cells: Water goes in and out.

Hypertonic Solutions

Hypertonic means "stronger." In hypertonic solutions, there is a net movement of water molecules out of the cell, causing it to shrink. For example:

• Animal cells: Water goes out.
• Plant cells: Water goes out.

In a hypertonic environment, the outside is more concentrated than the inside of the cell. The cell shrinks because it loses so much water that it starts to look crenated or shriveled.

Hypotonic Solutions

Hypotonic means "weak." In hypotonic solutions, there is a net movement of water into the cell, causing it to swell. For example:

• Animal cells: Water goes in.
• Plant cells: Water goes in.

In a hypotonic environment, the outside of the cell is less concentrated than the inside. Water pushes into the cell wall but won't burst because the cell wall protects it.

In a hypertonic solution, water exits the cell at a higher rate than it enters, resulting in a net loss of water. The outside becomes less concentrated.

In a hypotonic solution, more water enters the cell, resulting in a net gain of water. The cell will fill up to the point it bursts, which is called cell lysis. This only happens in animal cells. Plant cells have a cell wall that provides extra protection. When a plant cell is in a hypertonic solution, it will undergo plasmolysis, causing it to shrivel and look stretched.

Homeostasis

Homeostasis is the stable condition of a cell and its internal environment. To maintain homeostasis, unicellular organisms grow, respond to the environment, transform energy, and reproduce. Any changes on the outside would force the cell to modify itself.

Cell Specialization

Cell specialization means that different cell types play different roles. Some cells are specialized to move, others to react to the environment, and some produce substances that other organisms need. The change from a stem cell to a specialized cell allows the cell to perform specific jobs.

Lysosomes are small, sac-like structures surrounded by a single membrane and containing strong digestive enzymes. When released, these enzymes can break down worn-out organelles or food. The lysosome is also known as a "suicide sac."

Mitochondria are round, tube-like organelles surrounded by a double membrane, with the inner membrane being highly folded. Mitochondria are often referred to as the "powerhouse" of the cell. They release energy from food molecules to be used by the cell through a process called respiration. Some cells, like muscle cells, require more energy than others and therefore have many more mitochondria.

Differences Between Plant and Animal Cells

Structurally, plant and animal cells are very similar because they are both eukaryotic cells. They both contain membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes. Both also contain similar membranes. The functions of these organelles are extremely similar. However, the few differences between plant and animal cells are very significant and reflect a difference in the functions of each cell. Beyond size, the main structural differences between plant and animal cells lie in a few additional structures found in plant cells. These structures include chloroplasts, the cell wall, and vacuoles.

Endocytosis vs. Exocytosis

Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane and bringing it into the cell. Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell.

Endocytosis: The membrane forms a pocket around a particle. The pocket then breaks loose from the outer portion of the cell membrane and forms a vesicle within the cytoplasm.

Exocytosis: The membrane of a vesicle surrounds the material and then fuses with the cell membrane. The contents are forced out of the cell.