Basics of Cells and Life
Every living thing, from the tiniest bacteria to the largest whale, follows the same levels of organization: atoms build organelles, which form cells, tissues, organs, and finally organ systems. To be considered alive, organisms must show eight key characteristics including being cellular, organized, and able to grow, reproduce, and respond to their environment.
The cell theory explains that all living things are made of cells, cells are the basic unit of life, and all cells come from pre-existing cells. Scientists like Redi and Pasteur provided evidence for this theory through experiments that disproved spontaneous generation—the old idea that life could appear from non-living matter.
There are two main types of cells: prokaryotes and eukaryotes. Prokaryotic cells (like bacteria) are simpler with no nucleus or membrane-bound organelles. Eukaryotic cells (like those in plants, animals, and fungi) are more complex with a nucleus and specialized organelles.
Quick Tip: Remember the difference between cell types using "pro" means "before" and "eu" means "true" - prokaryotes evolved before true nuclei existed!
Prokaryotic cells have special parts like a protective capsule, a rigid cell wall for structure, and a cell membrane that controls what enters and exits the cell. They also have pili tinyhair−likestructures to sense their surroundings and flagella for movement.