Lesson 3 - The Generalized Cell
Student Performance Objectives
1. Draw and label a diagram of a typical eucaryotic cell as found in a human tissue.
2. Identify organelle names with their functions from a matching list.
3. Describe the function of the following major, eucaryotic cellular organelles: cell membrane
(plasma membrane), nucleus, chromosomes, nuclear membrane, mitochondria, Golgi,
lysosomes, endoplasmic reticulum (rough and smooth), ribosomes, centrioles (and
centrosome), microvilli and cilia.
4. Explain the difference in the variety of organelles found in procaryotic and eucaryotic cells.
A. Cell shapes and sizes.
1. Form follows function: Cell shape, or form, is correlated with its function. Examples follow.
a. The red blood cell (rbc), or erythrocyte, is a tiny, biconcave disc - it appears circular in appearance under 430X using a light microscope, and seen on its edge, it appears concave on both surfaces. The biconcave aspect of its shape derives from the extrusion of the spherical rbc nucleus during its maturation in the red bone marrow; the cell cytoplasm collapses around the site formerly occupied by the nucleus. The basic spherical cell shape derives from the minimal amount of energy required to maintain such a shape. It can also be appreciated that rbc's must display flexibility in their 4 months (120 days) of life as they squeeze through tiny capillaries and through the sinusoids of the liver and spleen during normal circulatory patterns. The basic, spherical shape, along with its small size, facilitates shape shifting and ease of movement through the tissues of the body. The tiny erythrocytes comprise 25% of all the cells of a typical human body (25 trillion out of a total of about 100 trillion).
b. The muscle fibers of our skeletal muscles are long and spindle shaped which complements their contribution to the overall length of a muscle. The muscle binds two distant bodily areas and, through contraction, brings them closer together.
c. The neurons of the central nervous system (brain and spinal cord) are reminiscent of snowflakes in that no two have identical shapes and their shapes are complex with free form aspects to them. This pattern results from the need for the system of cell extensions, or dendrites, that emanate from the neuron's cell body, or cyton. Receptors on the dendrites associate with the tips of other neurons, often as many as 25,000 associations (synapses) per cell (sometimes even more).
d. The epithelial cells may be flat and tile-like, and in multiple layers (stratified) when they are lining an area like the mouth or anus that require impermeable and thin boundaries. They may be cuboidal in shape and in only a single layer (simple) when they must form a resilient barrier that possesses many transporting enzyme systems as in the tubules of the kidneys. They are shaped like solid, rectangular columns when they must form a watertight barrier (between cells) that is absorptive through its cells and their receptors and transport systems.
e. The white blood cell (wbc), or leukocyte, is a small, free form cell whose shape complements its function of hunting for invading microorganisms throughout the body's tissues by squeezing, twisting and turning. It is estimated that there are about 25 trillion leukocytes in a human body possessing 100 trillion cells - 25% of the total. So the red and white blood cells of a human body make up 50% of all the cells in the body.
2. The typical cell: biologists speak about "the typical cell" despite the literally infinite variations in the exact shape of cells, along with their varying sizes. (All human cells are microscopic from the point of view of seeing the details of their structure; the human ovum is the largest human cell - about 0.1 mm or 100 micra - and is actually visible as a just-visible point with 20-20 vision, although no detail can be discerned). The typical cell is a descriptive reference structure possessing all the parts seen in many different cells and with its parts displayed in a stylized manner. It is useful for introducing students to the concept of the cell and its parts. It is not meant to be an accurate representation of any individual, specific cell. The basic cellular regions are the cell surface, the cytoplasm, and the nucleus.
a. Cell surface - the cell membrane (plasma membrane) separates the living state within the cell from its acellular, nonliving environment. The membrane's molecules are 98% lipid and 2% protein with some of the lipids and proteins being complexed to carbohydrate. The protein molecules are heavy so that the membrane is 50% protein and 50% lipid by weight. The lipid-protein-carbohydrate barrier is selectively permeable meaning that some molecules can move rapidly through the membrane, some molecules can not pass through at all, and most pass through at rates varying with their ability to bind with surface membrane receptors. For further details see .
b. Cytoplasm - cytoplasm is a general term referring to the semi-solid substance of the cell. It exists between the inner side of the cell membrane and the membrane surrounding the cell's nucleus (nuclear membrane).
(1) Cytoskeleton - The cytoplasm is criss-crossed by extremely thin fibers that make up the cell's cytoskeleton: from thinest to slightly thicker - microfilaments, intermediate filaments and mictrotubules. These fibers give the cell tensile strength, help to maintain cell shape, and serve to anchor the cell's organelle's in place and to permit limited movement of organelles. For further details on the structure and role of the fibers of the cytoskeleton, see .
(2) Organelles - these are small organs located within the cytoplasm that carry out the processes of life. Organelles can not survive independently, outside the cell. Consequently, organelles are not considered to be "alive" when outside a cell. However, their presence and metabolic activities when inside a cell contribute to the "living state" of the cell (the cell being considered the smallest unit of life).
(a) Mitochondria - these organelles resemble rod-shaped bacteria and are found distributed throughout the cytoplasm of human cells. They possess their own DNA in a single circular chromosome and reproduce within the cytoplasm by fission (as do bacterial cells).They can move around in the cytoplasm and can swell and shrink. They have a complex metabolic role in that they can transport into themselves, from the cytoplasm, fats, proteins and carbohydrates derived from foods we eat, and enzymatically oxidize them producing the energy (as ATP) required for most cellular processes. The mitochondria synthesize many specific molecules required by the cell as building materials during these oxidative transformations. A typical hepatocyte (liver cell) or cardiocyte (cardiac muscle fiber) may have 800-1200 mitochondria per cell, which makes sense in that they require lots of energy considering their role in human metabolism. Mature, circulating erythrocytes do not possess any and derive their energy from the anaerobic (meaning without oxygen) breakdown of glucose. Most cells have several hundred mitochondria which can be seen with a light microscope utilizing aappropriate staining techniques and the oil immersion objective (1000x); they will appear as tiny rods. Detailed views of these organelles require the electron microscope and magnifications of 100,000x and higher. Ask you instructor if you can do a report on "the endosymbiont hypothesis" which will help to clarify the relationship between mitochondria and bacteria.
(b) Ribosomes - these are protein-synthesizing organelles present in the thousands in the cytoplasm of cells. The ribosome is the link between the genetic code and cellular action: DNA's code, transcribed onto a messenger molecule in the cell's nucleus, is translated in the cytoplasm into specific proteins that are built on the ribosomes. The DNA molecule, whose structure was first clearly described in 1953 by Francis Crick and James Watson (based on data from many other molecular biologists including Maurice Wilkins and Rosalind Franklin), carries a code built from sequences of molecular "letters" that the ribosome can "read" and from which the ribosome can produce specific proteins. These proteins carry out the messages in DNA through their enzymatic activities and structural properties. Remember, from generation to generation, life has to remember and have the means to build cells that can continue to carry out life's processes. Without DNA there would be no code or blueprint of how to build anything; without ribosomes there would be no means to convert the code into practical structures (enzymes and structural proteins) that can actually accomplish anything. For further details on the processes of replication, transcription and translation of the genetic code, see . While some ribosomes are associated with cytoskeletal elements and are relatively "free" in the cytoplasm, others are attached to the surface of the endoplasmic reticulum (see the next organelle), where they work in association with each other producing proteins at rates of thousands per second.
(c) Endoplasmic reticulum - this intracellular membrane system provides the interior of the cell with increased surface area for the synthesis of proteins from attached ribosomes, and the synthesis of many other important cellular molecules and the breakdown of other molecules. The protein products of the ribosomes can be transported along the endoplasmic reticulum (ER) to other cellular locations like the Golgi for packaging and secretion from the cell, or to the cell surface for placement as receptors or transport proteins. The ER membranes are continuous with the outer of the two nuclear membranes and with the cell boundary (cell membrane) in selected locations. The portion of the ER that holds the ribosomes is called the rough endoplasmic reticulum (RER). Portions of the ER without ribosomes are called smooth endoplasmic reticulum (SER). A particularly important enzyme system located in the SER is one that breaks down drugs and other molecules foreign to the cell. Our need to take medicines, like antibiotics, every few hours, is due to their breakdown and transformation, by liver enzymes located in the SER, into products the body can readily excrete through the intestinal or urinary systems.
(d) Golgi - these are a groupo of cytoplasmic membranes that package proteins transferred to them, from the RER, in preparation for secretion from the cell. Golgi are prominent in pancreatic cells producing and secreting enzymes of digestion into the small intestine, in hepatocytes of liver that secrete proteins that are found in the blood (e.g., albumin, clotting proteins), and in glands producing products for secretion from the cells.
(e) Lysosomes - these digestive sacs contain powerful digestive enzymes that can break apart the chemical constituents of microbes (e.g., bacteria, viruses) that invade areas of the body where they are not permitted. Lysosomes are abundant in leukocytes that phagocytize microbes. After phagocytosis of an invading microorganism, the phagocytic vacuole fuses with a lysosome and the digestive enzymes of the lysosome have access to the microbes and can destroy them. Lysosomes are also important in the developing fetus in that certain fetal regions must be reabsorbed as newer areas are formed; the lysosomes contribute to the cellular death (apoptosis) occurring in the tissue areas being reabsorbed. Some cells of our immune system kill cancer cells by inducing the cells to initiate a death program (apoptosis) involving the release of the cancer cells' own lysosomal enzymes into their cytoplasm which kills them.
(f) Centrioles - these are non-membranous structures composed of microtubules that take part in cell division (mitosis). The details of their action are studied under the topic of mitosis.
c. The nucleus - this region of the cell is bounded by the double-layered nuclear membrane.
1. Chromosomes - these are the carriers of our inheritance and are composed of DNA and protein. Chromosomes can be observed with a light microscope (at about 100x) during mitosis. Their visibility when the cell is in the process of dividing is due to coiling and supercoiling of the long strings of genes and proteins. When a cell is not dividing the chromosomes are uncoiled and become too thin to be seen clearly with the light microscope. Under these conditions, the chromosomes are called chromatin.
2. Nucleolus - this nuclear organelle stores some of the RNA components of the cytoplasmic ribosomes prior to their transport to the cytoplasm.
B. Student Assignment: Using text references and Internet resources, students are to
draw diagrams of a typical eucaryotic human cell and a typical prokaryotic bacterial cell neatly labeling organelles and indicating organelle functions. The website is excellent for this assignment.
C. Organelles to be found: cell membrane, nuclear membrane, nucleus, chromosomes, nucleoli, mitochondria, endoplasmic reticulum (rough and smooth), ribosomes, Golgi, lysosomes, centrioles and the centrosome region, others at your discretion
rough endoplasmic reticulum
smooth endoplasmic reticulum