2015-10-13
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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 per-form all the functions of life, the cell is the smallest unit of life.
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.
Prokaryotes include cells you’ve probably heard of, such as the bacteria E. Coli and Streptococcus (which causes strep throat), the blue-green algae that occasionally cause lake closures, and the live cultures of bacteria in yogurt, as well as some cells you may never have heard of, called archaeans.
Whether you’ve heard of a specific prokaryote or not, you’re likely well aware that bacteria have a pretty bad rap. They seem to make the papers only when they’re causing problems, such as disease. Behind the scenes, though, bacteria are quietly performing many beneficial tasks for people and the rest of life on planet Earth.
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Ex. 27. Translate the following text and fill the gaps in Figure 4 with the words from the list below.
Eukaryotes have the following characteristics:
✓ A nucleus that stores their genetic information.
✓ A plasma membrane that encloses the cell and separates it from its environment.
✓ Internal membranes, such as the endoplasmic reticulum and the Golgi apparatus, that create specialized compartments inside the cells.
✓ A cytoskeleton made of proteins that reinforces the cells and controls cellular movements.
✓ Organelles called mitochondria that combine oxygen and food to transfer the energy from food to a form that cells can use.
✓ Organelles called chloroplasts, which use energy from sunlight plus water and carbon dioxide to make food. (Chloroplasts are found only in the cells of plants and algae.)
