The Eukaryotic Cell

I. The eukaryotic cell differs from the procaryotic cell in several ways:

-- most obvious is the difference in size; the eukaryotic cell is huge compared to the procaryotic cell;

-- though some eukaryotic cells have a cell wall (but not animal cells), the cell wall does not contain peptidoglycan. Plant cell walls are made of cellulose which is a polymer of glucose while fungi have either cellulose or a substance known as chitin;

-- Though many organisms exists which are single eukaryotic cells, the eukaryotic cell can also combine to form multicellular plants and animals;

-- Present in the cytoplasm of the eukaryotic cell are internal structures known as organelles and a structure known as the nucleus. Several different organelles have been characterized and will be discussed shortly. Generally, an organelle is enclosed by a phospholipid bilayer. Organelles can be thought of as compartments where specific biochemical reactions occur. In this lecture you will be shown the organelle.

A. Members of the kingdom Protista and yeasts (members of the Kingdom Myceteae) are single celled organisms that have eukaryotic cells. When discussing disease processes, several members of this kingdom are important. Most members of this group are free living and do not cause disease.

B. For the most part, our discussion of members of the Kingdom Animalia will be limited to references to those cells involved in the immune response and to those multicellular organisms that are parasitic on other animal hosts including humans.

II. Most protists and many cells within our body have extensions from the cytoplasm that allow for movement (either of the cell or of substances on the surface of the cell). These extensions include flagella and cilia.

A. The eukaryotic flagellum is radically different from that of a procaryotic cell. In the eukaryotic cell the flagellum does not spin but waves back and forth. This is accomplished by the interaction of protein structures known as microtubules. In most cases the eukaryotic cell has but one flagellum or at most two.

B. Cilia also contain microtubules but are much shorter than flagella. They are usually found in large numbers on the surface of a ciliated eukaryotic cell. Movement of the cilia can allow protists to move through the environment. In the multicellular organism, cilia serve to move substance across their surface.

III. Like the bacterial cell, eukaryotic cells will often be surrounded by a glycocalyx. As its name suggests it is a structure that is very rich in carbohydrates (polysaccharides). In fungi and plants you will find a rigid cell wall beneath the glycocalyx. As in the procaryotic cell this structure helps to protect the cell from alterations in osmotic pressure.

A. Fungal cells (yeasts and molds) have a chitin-containing cell wall.

B. Plants and algae have a cell wall that is rich in a glucose polymer known as cellulose. Animals lack the ability to digest this form of glucose. Those the are unable to utilize cellulose as a food source.
 

IV. The cytoplasmic membrane of the eukaryotic cell is very similar to that found in procaryotic cells. Most eukaryotic cytoplasmic membranes contain the molecule cholesterol which keeps the membrane extremely fluid. Movement of substances across the cell membrane is accomplished by several routes including phagocytosis, receptor mediated endocytosis, simple diffusion, facilitated diffusion and active transport. The last three processes have already been discussed when we talked about the procaryotic cell. They occur in generally the same way as in the procaryotic cell and will not be discussed here. The lack of a cell wall allows the animal cell to perform several types of movement across the membrane that are not possible for the procaryotic cell.

A. Large particles such as bacteria or large pieces of dead cells are engulfed by the cytoplasmic membrane in a process known as phagocytosis. This involves extending the cytoplasmic membrane out and around the substance to be engulfed. This process requires considerable energy. Phagocytosis is essential to the proper functioning of the immune system.

B. Similarly to phagocytosis, receptor mediated endocytosis specifically internalizes molecules bound to a transmembrane protein whose extracellular portion acts as a receptor. The membrane invaginates pulling the attached substance with it. This process requires the activity of the cytoplasmic protein clathrin and the invaginations that form are referred to as clathrin coated pits. When we discuss viral lifecycles, this process is very important in allowing the virus to gain entry to the eukaryotic cell.

C. In both cases, the membrane is "pinched-off" forming an internal vacuole. Other vacuoles known as lysosomes carry digestive enzymes and proteins capable of acidifying the internalized vacuole fuse with the vacuole. Whatever was internalized by the vacuole will be digested within the vacuole and the resulting monomers and simple molecules will be used by the cell for growth. Once again this process is utilized by viruses to gain entry into the cytoplasm.
 

V. The nucleus is the most prominent feature of the eukaryotic cell. In the vast majority of cases cells have one nucleus. (Membrane bound multi-nucleated masses are referred to as a syncytium. Example: the placenta.)

A. The nucleus is surrounded by double membrane known as the nuclear envelope. Gaps in the envelope allow mRNA to leave and proteins to enter. Receptors for a certain class of hormones exist here. Binding of the hormone to these receptors allows the hormone to exert its effect on the DNA.

B. Nucleolus is a dark area of the nucleus that is the site of  rRNA and tRNA synthesis. The rRNA combines with proteins and form the subunits of the ribosomes.

C. DNA is not free in the nucleus but wound around proteins known as histones. The histone-DNA complex is referred to a chromosome.  Usually it is dispersed and invisible, but at certain times during the cell cycle the chromosome condenses and becomes visible under light microscopy.  The DNA of most organisms is divided among at least several chromosomes.  Humans have 46 chromosomes.
 

VI. DNA exists mainly in the nucleus. RNA is synthesized in the nucleus but then travels into the cytoplasm where it functions in the synthesis of proteins. If the protein is destined to stay in the cytoplasm, this synthesis will occur on a ribosome-mRNA translational complex that is free in the cytoplasm. But many proteins are destined to be exported from the cell or to exist embedded in the phospholipid bilayer membranes of the cell. These proteins will be synthesized by ribosomes associated with an organelle referred to as the rough endoplasmic reticulum (rough ER).  Once synthesized, these proteins will be  moved to the golgi apparatus and eventually exported from the cell by a vacuole.

A. Endoplasmic reticulum Within the cytoplasm are networks of membrane bound channels that connect with the nucleus and cytoplasmic membrane. These channels are referred to as endoplasmic reticulum.

1. The cytoplasmic surface of the rough endoplasmic reticulum is covered with ribosomes actively making proteins. The proteins synthesized by the ribosomes on the surface of the ER will be eventually exported from the cell, will be found embedded in the membranes of the cell or will be transported to organelles such as the mitochondria. Within the rough endoplasmic reticulum the proces of adding polysaccharides (glycosylation) is begun.
2. Smooth endoplasmic reticulum lacks the ribosomes and is not involved in processing of proteins for export. Smooth ER is believed to play a role in destroying certain toxins, synthesizing phospholipids for membranes and making steroids, among other functions.

B. Golgi apparatus   The proteins within the rough endoplasmic reticulum will move, by way of membrane-bound vesicles, to the Golgi apparatus (sometimes referred to as the Golgi body). Within the Golgi apparatus the proteins will be further glycosylated. The proteins will then be transported, by way of vacuoles again, to the cytoplasmic membrane where the vacuole will fuse with this membrane and the proteins will be extruded from the cell.

C. Lysosomes   These membrane bound structures are formed by the golgi apparatus and contain enzymes that are capable of breaking down complex macromolecules into simple monomers which the cell can use for nutrition. The lysosome will join with an internalized vacuole which contains complex substances. The fusion of the membranes allows the enzymes of the lysosome to mix with the contents of the vacuole. Immediately, the contents begin to be degraded. Eventually, only non-digestible contents are left in the vacuole.

D. Vacuoles Membrane bound sacs within the cytoplasm are referred to as vacuoles. These can contain substances which are being imported from the extracellular environment, or they can be substances being moved around within the cell. Some vacuoles serve as storage sites for certain nutrients.
 

VII. Eukaryotic cells must produce ATP to be used in most energy-requiring cellular activity. The initial breakdown of glucose (glycolysis) occurs in the cytoplasm and results in the production of two molecules of pyruvic acid. The pyruvic acid moves into the central region of an organelle known as the mitochondria. Within the mitochondria all of the aerobic reactions needed to make large amounts of ATP occur.  The mitochondria has an outer membrane, an inner membrane, a matrix (cavity within the inner membrane) and a intermembrane space. The enzymes of the Kreb's cycle are found in the matrix; the cytochromes of the electron transport chain are within the inner membrane. The inner membrane is folded extensively.  THe folds are referred to as cristae. 

Some good links!!!

MIT Biology hypertextbook    Cell Biology     http://web.mit.edu/esgbio/www/cb/cbdir.html

Wonderful resource.  This will tell you everything you need to know (at this level) about the cell. 

The Grapes of Staph   Eukaryotic cell  http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/index.html

Basic description of those things you need to know about the cell.