The Cell –
The basic unit of structure and function in almost all organisms. The human body is composed of billions of cells.
Cytology – the study of cells.
1. In 1665, the English scientist Robert Hooke observed a very thinly sliced piece of cork under his primitive microscope. He observed thousands of tiny empty chambers which he called cells.
2. Between 1673 and 12716, Anton von Leeuwanhoek manufactured a microscope using multiple lenses. This was the first compound (more than one lens) microscope. He was able to see living microbes, e.g., bacteria and protozoa.
3. In 1831, the science of cytology began in earnest with the studies of Robert Brown. He discovered the nucleus of the cell.
4. By 1839 the tissues of many animals and plants had been studied with the microscope and found to be constructed from cells. In that year, Schleiden and Schwann proposed the “Cell Theory” which states that all plants and animals are composed of cells.
5. In 1863, Rudolf Virchow expanded on the Cell Theory by stating the principle that “all cells arise from previously existing cells”. This statement was a refutation of the theory of spontaneous generation. Proponents of spontaneous generation believed that living things could germinate form non-living matter.
6. In the latter decades of the nineteenth century improvements in the microscope and knowledge of cells was fueled by the discoveries of Pasteur, Koch and others that microbes could cause human disease.
7. At the present time, we can update the Cell Theory based on discoveries made during the 20th century.
Size – Cells have an enormous range in size. A typical RBC measures 7.5 um. A skeletal muscle cell may be several inches long. Some nerve cells may be over one meter long.
Shape – There art some 200 different types of cells showing a great diversity in shape and overall appearance. For example,
Adipose (Fat) Cell Spherical
Squamous Flat, plate-like
Cuboidal Box-like
Skeletal Muscle Cell Long Cylinder
The parts of the cell are referred to as organelles. Each organelle performs specific functions in the cell.
Cell organelles can be divided into two categories:
I. Membranous Structures – These are organelles surrounded or associated with a lipid bilayer membrane. Included in this category are the following organelles.
A. Cell Membrane – a lipid bilayer containing phospholipids, glycolipids, cholesterol and proteins. The overriding role played by the cell membrane is to regulate the passage of ions, molecules and larger particles between the intracellular and extracellular environments.
1.
Membrane
Lipids
a. The phospholipids in the membrane form a bilayer in which the hydrophilic phosphate ends of the molecules face the aqueous environments outside and inside the cell. The hydrophobic fatty acid ends face each other within the membrane.
b. Cholesterol and other lipids are closely associated with the hydrophobic “tails” of the phospholipids.
c. The basic membrane structure closely resembles the micelle described before. Ions and other water soluble materials do not pass across the hydrophobic barrier of the membrane’s interior. This helps to isolate the cytoplasm from the surrounding fluid environment.
2.
Membrane
Proteins
a. They account for about 55% of the weight of the cell membrane.
b. Proteins are associated with the membrane in two basic ways:
1. Integrated proteins are part of the membrane’s structure. The degree of penetration and integration of proteins depends on the relative proportions of hydrophilic and hydrophobic regions on the protein.
2. Peripheral proteins are more loosely bound to the inner and outer surfaces of the membrane. Most membrane proteins are integral.
c. Functions of membrane proteins include:
1. Anchoring – membrane protein may attach the membrane to the cytoskeleton inside the cell. Outside the cell proteins can bind the membrane to extracellular fibers of a connective tissue or to another cell.
2. Recognition Proteins – immune cells recognize the cells of the body by the presence of special recognition proteins. One important group of these molecules are the Major Histocompatability proteins.
3. Enzymes – will catalyze reactions in the extracellular or intracellular fluid depending on the position of the enzyme molecule within the membrane. For example, dipeptides are broken down into amino acids by enzymes on the surfaces of intestinal cells. The enzyme adenyl cyclase faces the intracellular environment and catalyzes the formation of the important 2nd messenger cyclic AMP.
4. Receptor proteins – will bind specifically to extracellular molecules called ligands. This binding will trigger changes in the activity of the cell. For example, the ligand insulin will bind to its receptor in the membranes of body cells. This binding leads to the uptake of glucose by the cell. The degree of activation of the cell will depend on the concentration of ligand or the number of receptor molecules on the target cell.
5. Carrier Proteins – will bind solutes and transport them across the cell membrane. This binding involves a shape change in the carrier protein. After the solute molecule has made the transit across the membrane, the carrier returns to its original shape.
This transport may or may not involve an energy cost on the part of the cell.
Glucose is transported free of charge into body cells by carrier molecules. On the
other hand, sodium and calcium movement out of the cell through carrier molecules
requires the expenditure of energy from the breakdown of the high energy compound
adenosine triphosphate.
6. Channel Forming Proteins – many integral proteins contain a central pore or channel. There are two principal types of channels:
a. Leak channels – allow a passive movement of water, ions and other small molecules into and out of the cell.
b. Gated channels – open and close to regulate the movement of small molecules and ions into and out of the cell.
3. Membrane Carbohydrates
a. Constitute about 3% of the membrane’s weight.
b. Are components of larger molecules. For example, proteoglycans, glycoproteins and glycolipids are large integral molecules. The carbohydrate portions of these molecules extend out from the cell’s surface forming a layer called the glycocalyx.
c. The glycocalyx functions to
1. lubricate and protect the cell’s surface
2. helps to anchor the cell in place
3. serve as receptor molecules for ligands
4. be used as recognition molecules by cells of the immune system. This property of the molecules of the glycocalyx is determined genetically. For example, blood type is based on the presence or absence of specific membrane glycolipids on the surfaces of the red blood cells.
B. Microvilli are closely bunched, tiny projections from the free surface of cells lining the intestinal
cavity (striate border) and kidney tubules (brush border). In these tissues their chief function is
absorption. In the stomach and uterine epithelium they function primarily for secretion.
C. Sensory hairs are membrane covered processes found on certain sensory cells. They are concerned with taste (taste buds), smell (olfactory epithelium), hearing (Organ of Corti) and equilibrium (vestibular apparatus).
D. Stereocilia are nonmotile, cilia-like extensions from the free surfaces of epithelial cells lining the
male sex duct. They do not show the 9:2 arrangement of microtubules internally. They have a
secretory function.
E. Cilia are short extentions of the cytoplasm and cell membrane at the free surface of some body
cells. As revealed by electron microscopy, a cross section of a cilium shows a central pair of tiny,
hollow microtubules surrounded by nine peripheral pairs of microtubules. Cilia are anchored just
below the cell membrane to a basal body. Cilia beat rhythmically on the surface of cells lining the
respiratory tract and oviduct tubes in mammals.
F. Flagella (singular flagellum) closely resembles a cilium internally in microtubule arrangement and anchorage on a basal body. They are much longer than cilia and usually found one to a cell.
Flagella move sperm cells from where they are deposited in the female’s vagina to the oviduct
where they fuse with the egg cell.
G. Intercellular Junctions – are found at the places where the cell membrane of one cell makes contact with the membrane of another. They serve a number of important functions.
1. Desmosomes – are regions where the membranes of two cells are held together very strongly. In the epidermis, a tough fiber called keratin joins the cells at desmosomes.
2. Zona occludens or “tight junctions” consist of an apparent fusion of the cell membranes of adjacent cells. In the lining of the small intestine these junctions seem to prevent materials from the intestinal lumen from leaking into the intercellular spaces.
3. Gap Junctions appear to be a series of very small (20 A) pores in the membranes between two cells. They may be sites of cell to cell communication.
A. Cytoskeleton – is a microscaffolding within the cell produced by a number of structural elements. Besides giving the cell shape and structure, these filamentous elements prodkuce movements of structures within the cell and movements of the cell itself. There are four primary typ3es of filaments:
1.
Microtubules
a. Composed of molecules of tubulin arranged to form hollow tubes about 25 nm in diameter and possibly many micrometers (um) long.
b. Produce movement of certain cellular structures by assembly and disassembly of the microtubules attached to those structures, e.g., chromosomal movement during cell division.
c. Produce changes in cell shape associated with cell motility and phagocytosis.
d. Microtubules can serve as an anchorage or “footpath” for proteins that behave as molecular motors. These specialized proteins, kinesin and dynein produce the intracellular movements of small structures like vesicles.
e. Microtubules within cilia and flagella produce the whip-like motions of these organelles.
2.
Microfilaments
a. Very slender protein strands less than 6nm in diameter.
b. The most abundant microfilaments are composed of actin, a contractile protein.
c. Actin filaments are most common in the periphery of the cell and rare in the region around the nucleus. They attach the cell membrane and the nuclear envelope to the cytoplasm.
d. Actin interracts with the thicker (18 nm) myosin microfilaments to produce cell contractions. This arrangement is abundant in skeletal muscle cells.
3. Intermediate filaments are intermediate in thickness between the actin (thin) and myosin (thick) microfilaments. They are insoluble and the most durable of the cytoskeletal components.
a. Keratins are the tough, waterproof proteins found in the skin, hair and nails.
b. Neurofilaments provide the sturctural support for the very long cell processes called axons in nerve cells.
c. Desmin filaments are the anchorage for the intercellular junctions (desmosomes) which hold cells together in epithelial tissues.
1. A cylindrical body composed of a circular arrangement of 9 sets of microtubule triplets. It is
referred to as a 9 + 0 array since there are no central microtubules.
2. During cell division of animal cells the centrioles (one at each pole of the cell) form the spindle of microtubules which attach to and move the chromosomes during mitosis.
3. There are no centrioles in mature erythrocytes, skeletal muscle cells, cardiac muscle cells or
neurons. These cells are not capable of dividing.
1. Located in the cytoplasm as unattached, free ribosomes or as fixed ribosomes attached to flattened cytoplasmic sacs, the endoplasmic reticulum. Mitochondria also have their own ribosomes.
2. Each ribosome is made of two subunits, a small and a large ribosomal subunit. These subunits
will come together around a strand of messenger RNA to begin the process of protein synthesis.
3. Proteins prepared on fixed ribosomes enter the endoplasmic reticulum for further processing.
1. The largest organelle (5um in diameter) in the cell. It is usually centrally located.
2. The nucleus is surrounded by a series of flattened membranous sacs making up the nuclear envelope. The envelope separates the contents of the nucleus from the cytoplasm. The nuclear envelope is perforated with nuclear pores (9nm). These pores allow for chemical communication between the nucleoplasm and cytoplasm.
3. The nucleoplasm contains the nucleic acids Ribonucleic acid and Deoxyribonucleic acid, as well as, their building blocks, the nucleotides. Basic dyes like methylene blue stain this material intensely. For this reason, the content of the nucleus is referred to as chromatin.
4. DNA stores the vital, chemically coded information needed by the cell to carry out its life activities. The DNA molecule is a double helix or a “twisted ladder”. The sides of the ladder consist of alternating molecules of deoxyribose sugar and phosphate. The rungs of the ladder are formed by complementary base pairing of two kinds of nitrogen bases, the purines and the pyrimidines.
Purine
Pyrimidine
Adenine combines with Thymine
Guanine combines with Cytosine
5. During most of the cell’s activities, the DNA molecules are dispersed and loosely coiled
chromatin. When the cell is dividing, the DNA coils very tightly around certain proteins, the
histones. At this time, the DNA becomes clearly seen as discrete structures called
chromosomes.
6. The two principal activities of DNA are:
a. Replication – each DNA molecule copies itself precisely before the cell, as a whole, divides. In this way each daughter cell can receive the same number and same kind of DNA found in the mother cell.
b. Transcription – DNA transcribes its coded message into a complementary molecule of RNA. This form of RNA is called the messenger since it is in this form that the DNA “information” is carried into the cytoplasm. In the cytoplasm, the mRNA binds to ribosomes to carry out the process of translation – the synthesis of a protein strand.
7. The chief differences between DNA and RNA can be summarized as follows:
Contains Deoxyribose sugar Contains Ribose sugar
Usually double-stranded Usually
single-stranded
Contains Thymine Contains
Uracil
8. The nucleus contains the nucleolus.
a. This is an organelle in which ribosomal RNA is synthesized. It is located in the nucleoplasm near the DNA that has the code for producing ribosomal RNA.
b. Nucleoli are prominent in cells that carry out a great deal of protein synthesis such as liver, muscle cells and neurons.
B. Mitochondrion ( plural ‘dria’) – The powerhouse of the cell.
1. Very small organelles about the size of bacteria (1.5 um x 2 to 8 um).
2. Surrounded by an unusual double membrane. The inner membrane contains many folds called cristae.
3. The surface of the inner membrane facing the interior or matrix of the mitochondrion is covered with respiratory enzymes and other molecules which are responsible for the reactions which release the stored energy in glucose. Some of that energy is stored in molecules of adenosine triphosphate, a form the cell can readily use for life activities.
4. The matrix of the mitochondrion contains mitochondrial ribosomes and a unique mitochondrial DNA which has been associated with a number of human diseases.
5. When the cell is preparing to carry out cell division, all of the mitochondria of the cell divide so that each of the daughter cells will have a full complement of mitochondria.
C.
Endoplasmic Reticulum (ER)
1. The ER consists of a network of intracellular membranes forming hollow tubes, flattened sacs and rounded chambers called cisternae.
2. This network is connected to or continuous with the nuclear envelope surrounding the nucleus.
3. The ER carries out a number of intracellular functions:
a. Protein synthesis:
1. Straight protein chains are produced on the fixed ribosomes attached to the sacs of the ER.
2. The secondary, tertiary and quartenary structures of the protein molecules are developed within the spaces of the ER.
3.
Protein synthesis is primarily carried out in that
portion of ER covered with ribosomes.
Due to its appearance under the electron microscope, this portion of the
ER is called Rough ER or RER.
b. Lipid and Carbohydrate synthesis is accomplished in that portion of the ER lacking ribosomes. From its appearance it is called Smooth ER or SER. Examples of products synthesized in the SER include:
1. Synthesis of phospholipids for growth and repair of cell membrane and organelle membrane.
2. Synthesis of sex steroids in the SER of reproductive cells.
3. Synthesis and storage of triglycerides in liver and fat cells.
4. Synthesis and storage of glycogen in skeletal muscle and liver cells.
c. Transport of materials from one ER sac to an adjacent one by the formation and absorption
of membranous vesicles.
1. Consists of a stack of five or six flattened saccules. A single cell may have several Golgi Bodies.
2. Generally, the Golgi Body is located near the nucleus.
3. Receives vesicles from the RER containing newly synthesized protein. Within the saccules, the proteins are modified. For example, phosphate, fatty acid or sugars may be attached to the protein.
4. These proteins move from saccule to saccule by small transfer vesicles. The proteins leave the last saccule in vesicles. The fate of these vesicles depends on the product they contain.
a. Secretory products move to the cell membrane for release within secretory vesicles.
b. Lysozymes are packaged in specialized vesicles called lysosomes. Lysozymes are powerful enzymes used to degrade intracellular food particles, released by certain white blood cells to destroy microbes or released within the cell to carry out autolysis following trauma (suicide sacs).
c. Peroxisomes are vesicles generally smaller than lysosomes. They remove and neutralize toxins within the cell such as free radicals. They are most abundant in liver cells where they neutralize and degrade toxins absorbed by the digestive tract.