The Eukaryotic Cell

THE EUKARYOTIC CELL

Eukaryotes come from the domain Eukarya and are represented by plants, animals, protozoa, fungi, and algae. Many of those we will study in this course are unicellular but some are multicellular. The defining feature of these cells is the presence of membrane bound organelles. These are relatively fixed in place by a cell cytoskeleton, which helps to maintain the shape of the cell. The genetic material is usually not circular but is arranged linearly in one or more chromosomes. Figure 13 describes a eukaryotic cell:

Figure 13.

Eukaryotic cells tend to be larger than prokaryotes. They divide not by binary fission but through meiosis or mitosis, which will be covered later. Some have cell walls that can be made from chitin, cellulose, or silica, which is a component of some algae cell walls. Most, however, do not have a cell wall. Motility happens with flagella or cilia that are made from microtubules. The shapes of eukaryotes can vary widely from species to species.

The nucleus is membrane bound and contains the genetic material of the cell. The DNA inside the nucleus is tightly packed and is accompanied by histone proteins that help to wrap and organize the genetic material. A few microbes will have two nuclei that perform reproductive and metabolic functions. There are cells that have nuclei that divide without expansion or division of the cytoplasm. These are called coenocytes.

The nuclear membrane is called a nuclear envelope. It has two separate lipid bilayers to form an inner and outer membrane. Each bilayer is unique. There are pores in the nuclear envelope that allow molecules like RNA to pass. The nuclear lamina consists of intermediate filaments that form a mesh inside the membranes in order to give the nucleus its shape. There are other intermediate filaments on the outside of the envelope that keep the nucleus in one place within the cell.

The nucleolus is located inside the nucleus. It is the site for ribosomal RNA or rRNA. As this is the major component of ribosomes, it is also the site for the start of the assembly of ribosomes. The partial ribosomes are transported outside to the cytoplasm, where assembly is finished.

Ribosomes in eukaryotic cells are larger than that found in prokaryotic cells. There is both a large and small subunit making up the total structure. There are free ribosomes and ribosomes associated with a membrane. The free ribosomes will synthesize watersoluble proteins, while membrane-bound ribosomes, bound to the rough endoplasmic reticulum, make proteins for export or for insertion into membranes. Some antibiotic drugs specifically make use of the fact that there are differences in prokaryotic and eukaryotic ribosomes.

The endomembrane system involves a number of sacs, flattened disks, and membranous tubules. The endomembrane system is what makes up many organelles, such as the endoplasmic reticulum, Golgi apparatus, vesicles, and lysosomes.

The endoplasmic reticulum is made from cisternae, which are flattened sacs, and tubules that are connected to one another and located just outside the nucleus. The rough endoplasmic reticulum is called rough because they are studded with ribosomes, while the smooth endoplasmic reticulum does not contain ribosomes. The rough endoplasmic reticulum is linked to protein synthesis and helps to make vesicles to

transport these proteins. The smooth endoplasmic reticulum makes lipids, detoxifies toxic molecules, and participates in carbohydrate metabolism. Figure 14 shows the endoplasmic reticulum:

Figure 14.

The Golgi apparatus consists of multiple stacked discs that contain enzymes. These enzymes will modify proteins and lipids to make proteoglycans, glycoproteins, and glycolipids that then can be transported out into the cytoplasm or outside the cell. There are two sides to the Golgi apparatus. The cis face is the receiving end of the apparatus, while the trans face is the exiting end of the apparatus.

The Golgi apparatus, basically the post office system of the cell, contains cisternae. It makes secretory vesicles that leave the cell in the process of exocytosis, in which the

vesicles fuse with the plasma membrane in order to extrude the contents to the outside of the cell. Figure 15 shows what the Golgi apparatus looks like:

Figure 15.

Lysosomes are small membrane-bound structure in the cell that are responsible for digestion of unwanted larger particles. They contain multiple digestive enzymes that break down debris, nutrients, immune complexes, and even small microorganisms. These need to be compartmentalized in order to protect the cell from digesting itself.

Peroxisomes are also membrane-bound organelles that contain hydrogen peroxide, necessary for the breakdown of amino acids, uric acid, and fatty acids. They also contain catalase, which has the capacity to degrade the hydrogen peroxide. In some circumstances, peroxisomes also participate in lipid synthesis. The hydrogen peroxide needs to be compartmentalized because hydrogen peroxide will degrade the cell.

The cytoskeleton of the eukaryotic cell is important in the internal cell structure. There are three types of fibers within the cytoskeleton, which are microtubules, intermediate filaments, and microfilaments. They are also important in the movement of structures within the cell. Things like mitosis, meiosis, and exocytosis all depend on the cytoskeleton.

Microfilaments are made from two strands of actin wound together. These will work with motor proteins, such as myosin, in human muscle contraction as well as in the movement of the pseudopodia of amoeba. Microfilaments can build up and break down rapidly so as to allow for cellular movement.

Intermediate filaments are the cables of the cell that form the nuclear lamina inside the nuclear envelope. They anchor adjacent cells in multicellular animals. Some will contain desmin, which is a protein important for the making of desmosomes that hold adjacent cells together. Others contain keratin found in skin, hair, and nails.

Microtubules are made from two different types of the tubulin protein. The core of the microtubule is hollow. They form part of the cytoskeleton and work with two motor proteins called dynein and kinesin, which act to move organelles within the cell structure. Both the flagella and cilia of eukaryotic cells are made from microtubules that can rapidly break down and reassemble themselves. Microtubules form the mitotic spindle that can separate chromosomes during cell division. Centrosomes make microtubules that separate the chromosomes in mitosis.

Mitochondria are considered the powerhouses of the cell because they participate in cellular respiration that ultimately makes ATP energy for the cell. They contain DNA and have a double membrane system. The inner membrane is highly convoluted and are where the metabolic enzymes are located. These invaginations are referred to as cristae. Figure 16 shows the internal structure of the mitochondria:

Figure 16.

Plants and algae contain chloroplasts, which are organelles that participate in photosynthesis. There are three membrane systems associated with them. There is an outer membrane, an inner membrane, and what’s called the thylakoid membrane system. The stroma is between the inner and outer membrane. The thylakoids are folded sacs containing chlorophyll pigment, where the photosynthesis takes place. The stacks of thylakoids contain grana in plant organisms. Figure 17 describes chloroplasts:

Figure 17.

The plasma membrane of eukaryotes is similar to that of prokaryotes. One major difference is the presence of cholesterol and other sterols in eukaryotes but not prokaryotes. Sphingolipids are specialized lipids in some eukaryotes that participate in cell to cell communication. Eukaryotes are different from prokaryotes because they can participate in endocytosis, which further specializes into phagocytosis or “cell eating” that applies mainly to immune cells. Pinocytosis is called “cell drinking” and involves taking in solutes and water by the cell. Exocytosis involves the release of vesicles from the inside of the cell to the outside of the cell.

Those cells that do not have a cell wall will make an extracellular matrix, which is a mass of proteins and carbohydrates that exists between the adjacent cells. The basement membrane beneath epithelial cells in animals is made from the extracellular matrix. Collagen is one of the proteins made that can provide tissue strength. This matrix is responsible for protecting the cell from external stressors.

As mentioned, eukaryotes contain flagella and cilia for movement. The flagella in eukaryotes are different from those found in prokaryotes. The eukaryotic flagella are

flexible rather than rigid and are made from microtubules. Dynein proteins provide the movement of the flagella.

Cilia are similar to flagella but are shorter and cover the entire cell surface. They are also made from microtubules and have a basal body on the base of the cilium as well as the flagellum. Cilia have rapid, wave-like movements that can sweep particles past the cell or can sweep nutrients into the mouths of some cells, such as protozoa. Cilia are found in the respiratory tract of humans and other mammals.