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Term Paper on Cell Organization

Term Paper Contents:

  1. Term Paper on the Introduction to Cell Organization
  2. Term Paper on the Cell Size
  3. Term Paper on the Cell Boundaries
  4. Term Paper on the Cell Wall
  5. Term Paper on How Cells Move
  6. Term Paper on the Cell Organelles

1. Term Paper on the Introduction to Cell Organization:

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In the remainder of this article, we shall glimpse the structure and some of the functions of that amazing entity-the living cell. We use the word “glimpse” because, although much is still unknown about the life of the cell, what is known about cell structure and physiology would fill several large volumes. Our narrative is greatly abbreviated and therefore incomplete.

Although we can look at only one structure or process at a time, remember that most activities of a cell go on simultaneously and influence one another. Chlamydomonas, for instance, is swimming, photosynthesizing, absorbing nutrients from the water, building its cell wall, making proteins, converting sugar to starch (or vice versa), and oxidizing food molecules for energy, all at the same time. It is also likely to be orienting itself in the sunlight, it is probably preparing to divide, it is possibly “looking” for a mate, and it is undoubtedly carrying out at least a dozen or more other important activities.

2. Term Paper on the Cell Size:

Most of the cells that make up a plant or animal body are between 10 and 30 micrometers in diameter. A principal restriction on cell size seems to be the relationship between volume and surface area. As volume increases, surface area decreases rapidly in proportion to volume. Materials-such as oxygen, carbon dioxide, ions, food molecules, and waste products-entering and leaving the cell must move through its membrane-bound surface.

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The more active the metabolism of a cell is, the more rapidly these materials must be exchanged with the environment if the cell is to continue to function. In small cells, the proportion of surface area to volume is greater than in large cells; thus, materials can move faster into, out of, and through small cells.

A second limitation on cell size appears to involve the capacity of the nucleus, the cell’s control center, to regulate the cellular activities of a large, metabolically active cell. The exceptions seem to “prove” the rule. In certain large, complex one-celled organisms – the ciliates, of which Paramecium is an example-each cell has two or more nuclei, the additional ones apparently copies of the original.

It is not surprising, therefore, that the most metabolically active cells are usually small. The relationship between cell size and metabolic activity is nicely illustrated by egg cells. Many egg cells are very large. A frog’s egg, for instance, is 1,500 micrometers in diameter. Some egg cells are several centimeters across-for example, the egg cell, or yolk, of a chicken’s egg.

Most of this mass consists of stored nutrients for the developing embryo. When the egg cell is fertilized and begins to be active metabolically, it first divides many times before there is any actual increase in volume. Thus the cellular units are cut down to an efficient metabolic size.

Electron Microscopic Structure of a Plant Cell

3. Term Paper on the Cell Boundaries:

A cell can exist as a separate entity be­cause of the cell mem­brane, which regulates the passage of mat­erials into and out of the cell. The cell mem­brane is only about 9 nanometers thick and cannot be resolved in the light microscope. Now, with the electron microscope, it can be visualized as a continuous, thin double line.

The basic structure of all cell membranes is formed by two layers of phospholipid molecules, arranged with their hydrophobic tails pointing inward. It is essentially the same in all living cells, whether prokaryotic or eukaryotic. However, differences in the proteins and carbohydrates associated with the phospholipid molecules give the membranes of different types of cells unique properties. We shall examine the molecular structure of cell membranes and the ways in which they perform their essential tasks.

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4. Term Paper on the Cell Wall:

A principal distinction between plant and animal cells is that the former are surrounded by a cell wall. The wall is but side the membrane and is constructed by the cell. As a plant cell divides, a thin layer of gluey material forms between the two new cells; this becomes the middle lamella. Composed of pectins (the compounds that make jellies gel) and other polysaccharides, it holds adjacent cells together. Next, under the middle lamella, the plant cell constructs its primary cell wall.

This wall is composed, to a large extent, of cellulose molecules wound together like wires in a cable and laid down in a matrix of gluey polymers. Successive layers of micro-fibrils are oriented at right angles to one another in the completed cell wall. (Those of you familiar with building materials will note that the cellulose cell wall thus combines the structural features of both fiber glass and plywood.)

Structure of Cell Wall

In plants, growth takes place largely by cell elongation. Studies have shown that the cell adds new materials to its walls throughout this elongation process. The cell, however, does not simply expand in all directions; its final shape is determined by the structure of its cell wall.

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As the cell matures, a secondary wall may be constructed. This wall is not capable of expansion, as is the primary wall. It often contains other molecules that have stiffening properties. In such cells, the living material of the cell often dies, leaving only the outer wall, a monument to the cell’s architectural abilities. Cellulose cell walls are also found in many algae; fungi and prokaryotes also have cell walls, but they are not made of cellulose.

5. Term Paper on How Cells Move:

All cells exhibit some form of movement. Even plant cells, encased in a rigid cell wall, exhibit active movement of the cytoplasm within the cell as well as chromosomal movements and changes in shape during cell division. Embryonic cells migrate in the course of development. Amoebas pursue and engulf their prey. Even little Chlamydomonas cells dart toward a light source.

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Two different mechanisms of cellular movement have been identified. The first consists of assemblies of fibrous proteins, usually referred to as muscle protein because they were first identified and studied in muscle tissue. However, it has now been found that these proteins are present in a great variety of cells and appear to be associated with movement within the cells. The second mechanism involves long, thin structures-cilia and flagella-extending from the surface of many types of eukaryotic cells.

6. Term Paper on the Cell Organelles:

Nucleus:

In eukaryotic cells, the nucleus is a large, often spherical body, usually the most prominent structure within the cell. It is surrounded by two membranes, which together make up the nuclear envelope. These two membranes are fused together at frequent intervals to create small pores that appear to form channels between the nucleus and the cytoplasm.

The chromosomes are found within the nucleus. When the cell is not dividing, they are visible only as a tangle of fine threads, called chromatin. The most conspicuous body within the nucleus is the nucleolus, the site at which the ribosomes are assembled.

The Functions of the Nucleus:

The nucleus carries the hereditary information for the cell, the instructions that determine whether a particular cell will be an amoeba, part of a leaf, or part of a human liver. Each time a cell divides, this information is passed on to the two new cells. The nucleus exerts its influence by directing the ongoing activities of the cell, ensuring that the various complex molecules the cell requires are synthesized in the numbers needed.

Not long ago, the cell was visualized as a bag of fluid containing enzymes and other dissolved molecules along with the nucleus and a few organelles. With the development of electron microscopy, however, an increasing number of structures have been identified within the cytoplasm, which is now known to be highly organized and crowded with organelles.

Ribosomes and Endoplasmic Reticulum:

Ribosomes, the most numerous of the cell’s many organelles, are the sites at which amino acids are assembled into proteins. The more protein a cell is making, the more ribosomes it has. The way in which the ribosomes are distributed in the cell seems to be related to the way the proteins are utilized. In cells, such as embryonic cells, that are making proteins for their own use, ribosomes tend to be distributed in the cytoplasm.

In cells that are making digestive enzymes or other proteins for export, the ribosomes are found attached to a complex system of internal membranes called the endoplasmic reticulum. Endoplasmic reticulum with ribosomes attached to it is known as rough endoplasmic reticulum. It is continuous with the outer layer of the nuclear envelope. There is evidence that rough endoplasmic reticulum is involved both in producing proteins and in preparing them for shipment out of the cell.

Cells also contain smooth endoplasmic reticulum, that is, endoplasmic reticulum with no ribosomes on it. It is found largely in the form of tubules and plays a role in transporting substances from the interior of the cell to the surface and in synthesizing lipids.

Golgi Bodies:

A Golgi body consists of a group of flattened sacs composed of membranes stacked loosely on one another and surrounded by tubules and vesicles (very small membrane-enclosed sacs). Golgi bodies serve as packaging and distribution centers, especially for substances formed on the endoplasmic reticulum.

Also, they are the sites of assembly of some complex molecules for instance, combinations of sugars and proteins (glycoproteins) that are found on the surfaces of cell membranes. In plant cells, they bring together the various components of plant cell walls. Golgi bodies are found in almost all eukaryotic cells. Animal cells usually contain 10 to 20 Golgi bodies, and plant cells may have several hundred.

Lysosomes:

One type of vesicle formed in the Golgi body is a lysosome. Lysosomes are essentially membranous bags that enclose destructive enzymes, thereby separating the enzymes from the rest of the cell If the lysosomes break open, the cell itself is destroyed since the enzymes they carry (for example, digestive enzymes) are capable of breaking down all the major compounds found in a living cell.

An example of the function of lysosomes is given by white blood cells, which engulf bacteria in the human body. As the bacteria are taken up by the cell, they are wrapped in a membrane-enclosed sac, a vacuole. (Vacuoles are similar to vesicles but are larger.) When this occurs, the lysosomes within the cell fuse with the vacuoles containing the bacteria, releasing their destructive enzymes into the vacuole. These enzymes then digest the contents of the vacuoles. Why the enzymes do not destroy the membranes of the lysosomes that carry them is a pertinent question yet to be answered.

Chloroplasts and Mitochondria:

The activities of a cell require energy. Some cells (autotrophs) manufacture their own energy-rich organic compounds from inorganic molecules. Other cells (heterotrophs) must obtain organic molecules from outside sources.

Photosynthetic autotrophs capture radiant energy from the sun and transform it to chemical energy stored in organic molecules. This process, photosynthesis, requires special pigments, of which chlorophyll is the most common. Photosynthesis takes place, however, only when the chlorophyll molecules are embedded in a membrane.

In all photosynthetic eukaryotes, the chlorophyll-bearing membranes are organized within a membrane bound organelle, the chloroplast. (In photosynthetic prokaryotes, such as the blue-green alga, the chlorophyll is also contained in membranes, but these membranes are not separated from the rest of the cytoplasm by an outer membrane.)

Virtually all eukaryotic cells (including photosynthetic ones) have mitochondria (singular, mitochondrion), which are also membrane-bound organelles. In the process of cellular respiration, which occurs in the mitochondria, energy-rich molecules are broken down. The process uses oxygen and releases the energy needed for cellular activities. (When we breathe, we are working for our mitochondria, supplying them with the required oxygen.)

Chloroplasts and mitochondria are the essential power generators of eukaryotic cells. Without the energy these organelles make available, most other cellular functions could not be carried out. We shall examine the structure of these organelles when we consider the processes of photosynthesis and respiration.

Cilia and Flagella:

Cilia (from the Latin word for “eyelash”) and flagella (singular, flagellum) are two names for essentially the same structure in eukaryotic cells. (The names were given before the basic similarity was realized.) When they are shorter and occur in larger numbers, the structures are more likely to be called cilia; when they are longer and fewer, they are usually called flagella. Thus we say that a Paramecium is covered with cilia, and Chlamydomonas has two flagella.

Many one-celled eukaryotes and also some very small multicellular ones, such as flatworms, are propelled by cilia. Similarly, the motile power of the human sperm cell comes from its single powerful flagellum, or “tail.”

Many of the cells that form the tissues of our bodies are also ciliated. These cilia do not move the cells, but rather serve to sweep substances across the cell surface. For example, cilia on the surface of cells of the respiratory tract beat upward, propelling bits of soot, dust, pollen, tobacco tar-whatever foreign substances we have inhaled either accidentally or on purpose-to the backs of our throats, where they can be removed by swallowing.

Only a few large groups of eukaryotic organisms-most notably the flowering plants-have no cilia or flagella in any cells. Some bacteria move by means of flagella, but these prokaryotic flagella are so different in construction from those of eukaryotes that it would be useful if they had a different name.

All eukaryotic cilia and flagella have a similar structure. The basic unit of this structure is the microtubule. In each cilium or flagellum, nine pairs of fused microtubules form a ring that surrounds two additional, solitary microtubules in the center. The movement of cilia and flagella comes from within the structures themselves; if cilia are removed from cells and placed in a medium containing energy-rich chemicals, they twitch.

The movement, according to one hypothesis, is caused by one outer pair moving tractor-fashion over its nearest neighbour. The “arms” that you can see on one of each pair of outer tubules have been shown to be enzymes involved in energy-releasing chemical reactions.

Comparison of Prokaryotic, Animal and Plant Cells

Cilia and flagella arise from basal bodies, which are also made up of microtubules. Their number and arrangement are somewhat different. The basal bodies are believed to keep the cilia or flagella supplied with fuel molecules and perhaps other substances as well. Eukaryotic cells with cilia and flagella also contain structures identical to basal bodies that are known as centrioles.

The distribution of centrioles within the cell is different from that of basal bodies, and, until recently, it appeared that their function was also different. Thus they were given a different name, long before electron microscopy revealed their identical structure. Centrioles appear to have an important role in the movements of the chromosomes during cell division.

The discovery of the complex internal structure of cilia and flagella, repeated over and over again throughout the living world, was one of the spectacular revelations of electron microscopy. For biologists, it is another glimpse down the long corridor of evolution, providing overwhelming evidence, once again, of the basic unity of earth’s living things.