Abstract
To qualify as living, units of life called cells must be identifiable, distinct, and demonstrate most or all the qualities of life. Cells tremendously vary in size from about 0.5-500 micrometers. The smallest known single cells are those of bacteria while most higher organisms have multiple cells differentiated and functioning together as a single system. Communication in cells involves cell signaling, reception, transduction, and response. Signals received at the surface of the cell from other cells, or from blood or tissue fluid must be transferred to various parts of the cell and a cell response initiated. Cells actively take in raw materials which they use to function and perform maintenance activities. Collectively these activities are called cellular metabolism catalysed by enzymes. To avoid chaos in the body, cells maintain control of what reactions are needed all the time, needed only certain times or needed very rarely. This chapter explores the cellular basis of life.
TopChapter Outline
3.1 Cell Theory and Types of Cells
3.2 Structure of Cells
3.2.1 Animal and Plant Cells
3.2.2 Unique to Plant Cells
3.3 Cellular Communication and Transportation
3.4 Humans: Complex Cellular Systems
3.5 Metabolic Functions in Cells
Chapter Summary
TopLearning Outcomes
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Understand the cell theory and the basic types of cells
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Demonstrate the various cellular structures
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Differentiate between plant and animal cells
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Explain how communication both outside and within cells
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Outline the various organ systems in complex organisms like humans
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Explain what metabolism means and the role of enzymes and cofactors.
3.1 CELL THEORY AND TYPES OF CELLS
Although life is based on chemistry and the various complex chemical reactions that characterize it, none these are components are holistic enough to be considered living by biologists. To qualify as living units of life must be identifiable, distinct and demonstrate most or all of the qualities of life discussed in Chapter 1 (section 1.2). The Cell Theory outlines three broad elements that characterize all units of life.
- 1.
The smallest units of life occur at the cell level of organization
- 2.
All living organisms are made up of one or more cells
- 3.
New cells arise only from existing cells
Cells tremendously vary in size from about 0.5-500 micrometers. The smallest known single cells of bacteria vary in length from 0.5-2 micrometers. For comparison, the smallest frog eggs visible to the human eye as clusters clinging to vegetation in freshwater are about 1 millimeter in diameter. The largest cells are ostrich eggs which are about 5-6 inches in diameter (Figure 1). As noted in section 1.3, bacteria (prokaryotes) have existed on Earth as single cells for millions of years. Most higher organisms (eukaryotes) have multiple cells differentiated and functioning together as a single system.
Figure 1. The size range of cells. Most cells are between 1 and 100 µm in diameter and are therefore visible only under the light microscope.
At their most basic level using a compound light microscope, cells are seen as having a fluid-filled interior called cytosol which is bounded by an outside plasma membrane that delineates the cell boundary. The cytosol of the cell is about 65-70% water and contains dissolved substances and other products of the chemical reactions occurring both within and outside the cell. Towards the middle part of the cell is the control center or nucleus which carries the genetic information important in cellular operations (Figure 2).
Figure 2. Generalized structure of a cell under a light microscope - the plasma membrane, nucleus and cytosol are visible
If you take a cotton swab or toothpick and lightly scrape the inside of your cheeks, then put the contents onto a slide, add a stain like methylene blue which is absorbed by the cells and increases contrast, cheek cell basic structure is clearly visible (Figure 3). However, using an electron microscope, cells have a very complex structure with many “small organs” called organelles distributed into the cytosol (see below). The cytosol including the organelles is called the cytoplasm. Organelles are made up of membranes and perform specific functions in the cell—much like what organs like the heart do in the organism.
Figure 3. Human cheek cells. Note the stained nucleus.
Key Terms in this Chapter
Organelles: Specialized structures found in the cytoplasm of a living cell that carry out specific functions.
Epithelial Tissue: One of four basic tissue types in the body; covering or lining tissue that serves to protect, support, secrete, and absorb (i.e., skin).
Connective Tissue: One of four basic tissue types in the body; supportive tissue such as bone, cartilage, and fat tissue that serve protection and support functions.
Enzymes: Super selective catalysts used to lower the activation energy of a chemical reaction.
Nervous Tissue: One of four basic tissue types in the body; composed of specialized cells that function to receive stimuli and conduct impulses throughout the body (i.e., nerve cells, neurons).
Active Site: The specific site of an enzyme, in which binding of a certain substrate catalyzes a particular reaction.
Cell Theory: All living things are made up of one or more cells.
Muscle Tissue: One of four basic tissue types in the body; the three type include skeletal, cardiac, and smooth.
Cell: The basic unit of structure and function in all living organisms.
Cytosol: The aqueous component of a cell that make up the cytoplasm, proteins, and other cellular structures.