At this time five kingdoms are recognized. The most primitive organisms are in the Kingdom Monera, made up exclusively of prokaryotes. Protista, Fungi, Plants, and Animals, are the other four kingdoms and they are made up exclusively of organisms which are eucalypts. We have already discussed Monera and we will talk more about the latter four kingdoms later.

Eukaryotic cells contain many organelles. The most prominent one is the nucleus which contains the unwound chromosomes. In the unwound state the chromosomes are referred to as chromatin. The nucleus is one of three double-membrane bound organelles found in eukaryotic cells. The other two are the mitochondria (sing. mitochondrion) and chloroplasts. All eukaryotic cells have a nucleus and mitochondria but only photosynthetic cells have chloroplasts.

The outer nuclear membrane is continuous with the endoplasmic reticulum. When a cell divides, the nuclear membrane breaks down and when the cell has completed cell division, the nuclear membrane reforms from the endoplasmic reticulum. The nuclear membrane has protein pores which allow certain molecules into the nucleus and certain molecules out of the nucleus. The outer membrane of the nucleus has ribosomes on it, but the inner one does not. No protein synthesis occurs within the nucleus. All the proteins in the nucleus are made on free ribosomes in the cytoplasm and imported into the nucleus through the pores. Inside the nucleus is a structure (with no membrane) called the nucleolus. It is composed of those portions of the chromosomes that contain the DNA that codes for ribosomal RNA (rRNA). (Ribosomes are composed of rRNA and proteins.)

Outside the nucleus is the cytoplasm which contains a variety of biomolecules, membranes and organelles. There is a system of membrane known as the endoplasmic reticulum. The rough endoplasmic reticulum (RER) is the site of protein synthesis and is prominent in cells which make proteins for export. Examples of these kinds of cells are the cells in your pancreas. Some cells in your pancreas make insulin to send via your blood to all the cells of your body. Other cells in your pancreas make digestive enzymes to send to your intestine to digest your food. Both insulin and digestive enzymes are made in the RER and modified and packaged into vesicles in the Golgi apparatus. These vesicles are released by the cell by a process known as exocytosis. Material can come into the cell by the reverse of this process, called endocytosis. Microscopic portions of the cell membrane are pinched off and taken into the cell often to merge with lysosomes and the contents degraded. Low density lipoprotein (LDL) enters the cell by this process and is degraded to amino acids and fatty acids. Smooth endoplasmic reticulum (SER) is more tubular and is the site of steroid and lipid synthesis. The cells in the testes or ovaries that make the sex hormones have highly developed SER, otherwise cells have only enough to meet their needs for lipid synthesis. The proteins found in the nucleus, mitochondria, chloroplasts, cytoplasm and peroxisomes are all synthesized on free ribosomes in the cytoplasm and not on the RER.

Lysosomes are the garbage disposals of the cell. They are single membrane bound organelles which are released from the Golgi but stay in the cell. Lysosomes are full of a variety of digestive enzymes (amylases, peptidases, nucleases) which break down "old" molecules and organelles. Their function is essential to the health of the cell. If molecules are not recycled, the cell becomes engorged with the useless molecules and as a consequence, the cell dies. There are a variety of human genetic disorders known as lysosomal storage diseases, most of which are lethal. The people that have the disorder are missing one or more of the lysosomal digestive enzymes. Tay-Sachs Disease and the mucopolysaccharidoses are examples of these disorders.

The cytoskeleton of the cell has three components: microtubules, microfilaments and intermediate filaments. Microtubules look something like a straw. The tubulin protein subunits of microtubules associate in a cylindrical arrangement. Microtubules form from centrioles and basal bodies which are short microtubular structures at the base of the spindle and flagella and cilia, respectively. Microtubules form the mitotic and meiotic spindle, flagella, cilia and are intracellular supports for a variety of cell structures including the axons of neurons. Microfilaments look like two pearl necklaces intertwined. They are involved in cell movement and contraction. They also support the microvilli that arise from the surface of many cell types and which increase the cell surface area. The most common microfilament protein is called actin. Actin is found in every organism that has been studied. It is a very old and evolutionarily conserved molecule. The two microfilaments, actin and myosin, are the major proteins in our muscles. There is another cytoskeletal protein component called intermediate filaments. As their name implies they are intermediate in size between the larger microtubules and the smaller microfilaments. They form parts of some tight junctions between cells. Keratin is such a protein and because the keratins are unique to certain cell types, they are sometimes used to identify the origin of cancer in people in whom cancer has metastasized. This is because cancer is clonal, having originated from a single cell that mutated so that it is no longer under normal cell division controls.

Unlike the nuclear membrane, the plasma membrane surrounding the cell has no pores. The intake of materials into and out of the cell is carefully monitored by a variety of proteins in the cell membrane that act as molecular gates. Only lipid soluble molecules can enter easily by merging with the lipid bilayer of the plasma membrane. The eukaryotic cell contains a wide variety of structures and organelles which are composed of membranes or are surrounded by a single or double layered membrane. All cellular membranes are built on a similar basic plan of a lipid (phospholipids, cholesterol, etc.) bilayer with specific proteins embedded in the bilayer. This is called the fluid mosaic model. The term fluid refers to the fact that the lipid and protein molecules in the membrane can move around within the membrane, and the term, mosaic, refers to the variety of lipids and proteins which form the membrane. The types of proteins found in each membrane determine the function(s) of each membrane. In the lecture I discussed how a kidney tubule resembles a sewer pipe with specialized cells lining it. The kidney tubule cells have different proteins in the plasma membrane facing the inner side of the tubule from the proteins in the plasma membrane on the outer surface of the cells. The inner side of each tubule cell selectively withdraws useful molecules from the filtrate of the blood which comes down the tubule and the outer side of the same cell returns those molecules to the blood capillaries surrounding the tubule. Energy in the form of ATP is required when the cell is pumping molecules in or out against the concentration gradient. This is called active transport. A dialysis machine which is used for people with kidney failure cannot differentiate between the useful and toxic molecules in the blood filtrate. All the small and middle-sized organic molecules are dialyzed away, irrespective of their value to the person. Therefore, a person on dialysis must receive supplemental nutrition to replace what is lost. The living kidney cells on the other hand "know" what to leave and what to take back because of the protein carriers in their membranes.

The mitochondria have an outer membrane and an inner membrane which is thrown into folds called cristae. The mitochondria contains the enzymes that carry out the Krebs Cycle which is involved in the oxidation of the food we eat. These enzymes are in the fluid part of the matrix. Many protein complexes containing the cytochromes (which contain iron) are embedded in the inner membranes. They are responsible for the transport of electrons from the food we eat to the oxygen we breathe. The ultimate site of oxygen utilization is, therefore, in our mitochondria. In the process of shuttling electrons from one cytochrome complex to the next, ATP is produced. We need a lot of ATP just to stay alive and we need even more to engage in the myriad of activities we each do every minute of the day. You will find mitochondria in all eukaryotic cells but you may find more in those cells which high energy needs such as those in the central nervous system (CNS), muscles and kidney. The tail of a mammalian sperm is a flagellum made of microtubules around which many mitochondria are wrapped to provide energy for their movement. Mitochondria contain prokaryotic type DNA and prokaryotic type ribosomes. They make some of their own proteins on their own ribosomes from their own DNA and messenger RNA but some of the proteins found in the mitochondria are supplied by the host cell.

Chloroplasts also have double membrane but instead of having the inner one thrown into folds, there are stacks of disk shaped membranes where the light reactions of photosynthesis occur. The light reactions require the movement of electrons that are activated by light and the proteins that shuttle the electrons are once again embedded in membranes for the smooth flow of the electrons and the production of ATP. The fluid matrix contains the enzymes required for the so-called dark reactions of photosynthesis where glucose is synthesized from carbon dioxide, ATP and the electrons from the light reactions. Chloroplasts, too, have prokaryotic DNA and prokaryotic ribosomes. They also make some of their own proteins but the host cell also supplies some of the proteins found in chloroplasts.
 
 
 

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