You're someone who just wants to know a little bit more about the living world around you or your green neighborhood, plants. Good news . . . this book is your oyster! Read it at your leisure, starting with whatever topic fascinates you most. I incl...
Every living thing has cells. The smallest creatures have only one, yet they're as alive as you are. What exactly is a cell?
In plain and simple terms, it's the smallest living piece of an organism - including you. Without cells, you'd be a disorganized blob of chemicals that'd just ooze out into the environment. And that is why the cell is the fundamental unit of life.
You get to explore the purpose and structure of cells in this chapter. And because cells rely on chemical reactions to make things happen, you also find out all about enzymes, which are proteins that help speed up the pace of chemical reactions.
Cells are sacs of fluid that are reinforced by proteins and surrounded by membranes. Inside the fluid float chemicals and organelles, structures inside cells that are used during metabolic processes. (Yes, an organism contains parts that are smaller than a cell, but these structures can't perform all the functions of life on their own, so they aren't considered to be alive.) A cell is the smallest part of an organism that retains characteristics of the entire organism.
For example, a cell can take in fuel, convert it to energy, and eliminate wastes, just like the organism as a whole can. Because cells can perform all the functions of life (as shown in Diagram), the cell is the smallest unit of life.
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Cells can be categorized in different ways, according to structure or function, or in terms of their evolutionary relationships. In terms of structure, scientists categorize cells based on their internal organization:
✓ Prokaryotes don't have a "true" nucleus in their cells. Nor do they have organelles. Bacteria and archaea are all prokaryotes.
✓ Eukaryotes have a nucleus in their cells that houses their genetic material. They also have organelles. Plants, animals, algae, and fungi are all eukaryotes.
Viruses versus bacteria
Viruses (like those that cause the flu, a cold, or AIDS) aren't made of cells.
Although viruses make you sick, just like bacteria do, viruses and bacteria are actually quite different. Viruses are simpler than bacteria and are made of just a few molecules. In fact, viruses have so few parts that they can't even grow and divide by themselves (so they're not considered to be alive). Viruses can only reproduce if they take over a host cell and steal its energy and materials (viruses attack cells).
