Cells
In the 1830s, two German scientists, Mathias Schleiden and Theodor Schwann, developed one of biology's most significant and unifying concepts: the cell hypothesis. It basically asserts that the cell is the basic organising unit of life. Despite the fact that previous scientists have observed cells, Schleiden and Schwann were the first to propose that all living things are made up of cells. The human body is made up of 100 trillion of them. Cell research has piqued the curiosity of various scientists for over 300 years. But, thus far, these microscopic structures have not revealed all of their mysteries, not even to modern researchers' probing instruments.CELL STRUCTURE AND FUNCTION
The notion of structure and function complementarity was presented in Chapter 1 and is obvious in the correlations that exist between cell size, shape, and function. The majority of human cells are tiny in size. Their sizes range from 7.5 micrometres (m) (red blood cells) to 300 m (white blood cells) (female sex cell or ovum). The period at the end of this phrase is around 100 um in size, which is nearly 13 times the size of our tiniest cells and one-third the size of a human ovum. Cells, like other anatomic structures, have a certain size or shape because they are designed to fulfil a specific function. A single nerve cell, for example, can develop threadlike extensions that are almost a metre long! This type of cell is good for transmitting nerve impulses from one part of the body to another. Muscle cells are trained to contract and shorten, whereas other types of cells may serve or secrete (Table 3-1).
The Typical Cell
Cells have numerous commonalities despite their varied anatomic traits and specific activities. There is no one cell that represents or includes all of the specialised components found in the many types of bodily cells. As a result, students are frequently introduced to cell structure and function by studying a typical or composite cell, which possesses the most significant traits of many diverse cell types. Figure 3-1 depicts such a generalised cell. Remember that no such "typical" cell exists in the body; it is a composite entity produced for research reasons. Refer to Figure 3-1 and Table 3-2 frequently as you learn about the main cell structures described in the following paragraphs.
FIGURE 3-1 Cells might be typical or composite. A, An artist's rendition of cell structure. A color-enhanced electron micrograph of a cell is shown in B. Both depict the many mitochondria, also known as the "power plants of the cell." Take note of the many dots that surround the endoplasmic reticulum. Ribosomes are the cell's "protein producers."
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Table 3-2 Some major cell structure and their
functions
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Cell
Structure
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Functions
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Membranous
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Plasma
membrane
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Serves as a boundary of the cell,
maintaining its integrity; protein molecules on outer surface of plasma
membrane perform various functions; for example, they serve as markers that
identify cells of each individual, as receptor molecules for certain hormones
and other molecules, and as transport mechanisms.
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Endoplasmic
reticulum
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Ribosomes attached to rough ER synthesize
proteins that leave cells via the Golgi complex; smooth ER synthesize lipids
incorporated in cell membranes, steroid hormones, and certain carbohydrates
used to form glycoproteins.
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Golgi
Apparatus
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Synthesizes carbohydrates, combines
it with protein, and packages the product as globules of glycoproteins.
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Lysosomes
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A cell’s “digestive system”.
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Peroxisomes
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Contain enzymes that detoxify
harmful substances.
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Mitochondria
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Catabolism; ATP synthesis; a cell’s “power
plants”.
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Nucleus
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Dictates protein synthesis; thereby
playing essential role in other cell activities, namely, cell transport,
metabolism, and growth.
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Non-Membranous
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Ribosomes
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Synthesize proteins; a cell’s “protein
factories”.
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Cytoskeleton
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Acts as a framework to support the
cell and its organelles; functions in cell movement; forms cell extensions
(microvilli, cilia, flagella)
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Cilia
and Flagella
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Hairlike cell extensions that serve
to move substances over the cell surface (cilia) or propel sperm cells
(flagella).
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Nucleolus
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Plays an essential role in the
formation of ribosomes.
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Cell Structures
Cell structure theories have evolved significantly throughout time. Cells were formerly viewed by biologists as simple, fluid-filled bubbles. Biologists now understand that cells are significantly more intricate than this. Each cell is surrounded by a plasma membrane that isolates it from its surroundings. The interior of the cell is mostly made up of a thick fluid known as cytoplasm (literally, "cell sub stance").
Various or ganelles, containing a central nucleus, are suspended in the cytoplasm. Each distinct or ganelle is architecturally adapted to perform a certain job inside the cell, just as each of your organs is physically suited to perform a specific role within your body. The primary cell structures are
(1) the plasma membrane,
(2) the cytoplasm, and
(3) the organelles (Figure 3-1).
QUICK CHECK
1. What important concept in biology was proposed by Schleiden and Schwann?
2. Give an example of how cell structure relates to its function.
3. List the three main structural components of a typical cell.
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