Eukaryotic Cells

Eukaryotic cells are generally larger and more complex than prokaryotic cells. They also contain a variety of cellular bodies called organelles. The organelles function in the activities of the cell and are compartments for localizing metabolic function. Microscopic protozoa, unicellular algae, and fungi have eukaryotic cells.

Nucleus. Eukaryotic cells have a distinctive nucleus, composed primarily of protein and deoxyribonucleic acid, or DNA. The DNA is organized into linear units calledchromosomes, also known as chromatin when the linear units are not obvious. Functional segments of the chromosomes are referred to as genes.

The nuclear proteins belong to a class of proteins called histones. Histones provide a supportive framework for the DNA in chromosomes. The DNA replicates in eukaryotic cells during the process of mitosis.

The nucleus of eukaryotic cells is surrounded by an outer membrane called the nuclear envelope, which is a double-membrane structure consisting of two lipid layers similar to the cell membrane. Pores exist in the nuclear membrane, and the internal nuclear environment can therefore communicate with the cytoplasm of the cell.

Within the nucleus are two or more dense masses referred to as nucleoli (singular,nucleolus). The nucleolus is an RNA-rich area where ribosomes are assembled before passing out of the nucleus into the cytoplasm.

Cellular organelles. Within the cytoplasm (also known as the cytosol) of eukaryotic cells are a number of microscopic bodies called organelles (“little organs”). Various functions of the cell go on within these organelles.

An example of an organelle is the endoplasmic reticulum (ER), a series of membranes that extend throughout the cytoplasm of eukaryotic cells. In some places the ER is studded with submicroscopic bodies called ribosomes. This type of ER is referred to as rough ER. In other places there are no ribosomes, and the ER is calledsmooth ER. The endoplasmic reticulum is the site of protein synthesis in the cell. Eukaryotic ribosomes are 80S bodies where the amino acids are bound together to form proteins. The spaces within the ER membranes are known as cisternae.

Another organelle is the Golgi body (also called the Golgi apparatus). The Golgi body is a series of flattened sacs, usually curled at the edges. The outermost sac often bulges away to form droplike vesicles known as secretory vesicles. It is in the Golgi body that the cell's proteins and lipids are processed and packaged before being sent to their final destination.

Another organelle, the lysosome, is derived from the Golgi body. It is a somewhat circular, droplike sac of enzymes within the cytoplasm. These enzymes are used for digestion in the cell. They break down the particles of food taken into the cell and make the products available to the cell. Enzymes are also contained in a cytoplasmic body called the peroxisome.

The organelle where much energy is released in the eukaryotic cell is themitochondrion (plural, mitochondria). The energy released is used to form adenosine triphosphate (ATP). Because they are involved in energy release and storage, the mitochondria are called the “powerhouses of the cells.”

An organelle found in certain protozoa is a large, fluid-filled, contractile vacuole. The vacuole may occupy over 75 percent of the cell interior and is used for eliminating water. Water pressure building up within the vacuole may cause the cell to swell.

Still another organelle within the cell is the cytoskeleton, an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell. The main components of the cytoskeleton are microtubules, microfilaments, andintermediate filaments. All are assembled from subunits of protein.

Many eukaryotic cells contain flagella and cilia. Eukaryotic flagella, like prokaryotic flagella, are long, hairlike organelles that extend from the cell. Eukaryotic flagella whip about and propel the cell (as in protozoa) and are composed of nine pairs of microfilaments arranged about a central pair. Cilia are shorter and more numerous than flagella. In moving cells, they wave in synchrony and move the cell. Paramecium is a well-known ciliated protozoan.

The cell wall. Many species of eukaryotes, such as fungi, contain a cell wall outside the cell membrane. In fungi, the cell wall contains a complex polysaccharide called chitinas well as some cellulose. Algal cells, by contrast, have no chitin; rather, their cell walls are composed exclusively of the polysaccharide cellulose.

Cell walls provide support for eukaryotic cells and help the cells resist mechanical pressures while giving them a boxlike appearance. The cell walls are not selective devices, as are the cell membranes.

The cell membrane. The eukaryotic cell membrane conforms to the fluid mosaic model found in the prokaryotic membrane. In eukaryotes, the membrane is a dynamic structure governing passage of dissolved molecules and particles into and out from the cytoplasm. However, it neither contains the enzymes found in the prokaryotic cell nor functions in DNA replication.

In order for the cytoplasm of prokaryotic and eukaryotic cells to communicate with the external environment, materials must be able to move through the cell membrane. There are several mechanisms by which movement can occur. One method, calleddiffusion, is the movement of molecules from a region of high concentration to one of low concentration. This movement occurs because the molecules are constantly colliding with one another, and the net movement of the molecules is away from the region of high concentration. Diffusion is a random movement of molecules, and the pathway the molecules take is called the concentration gradient. Molecules are said to move down the concentration gradient in diffusion.

Another method of movement across the membrane is osmosis, the movement of water from a region of high concentration to one of low concentration. Osmosis occurs across a membrane that is semipermeable, meaning that the membrane lets only certain molecules pass through while keeping other molecules out. Osmosis is a type of diffusion involving only water.

A third mechanism for movement across the membrane is facilitated diffusion, a type of diffusion assisted by certain proteins in the membrane. The proteins permit only certain molecules to pass across the membrane and encourage movement from a region of high concentration of molecules to one of low concentration.

A fourth method for passing across the membrane is active transport. When active transport is taking place, a protein moves a certain material across the membrane from a region of low concentration to one of high concentration. Because this movement is happening against the concentration gradient, it requires that energy be expended, energy usually derived from ATP.

The final mechanism for movement across the cell membrane is endocytosis, a process in which a small patch of cell membrane encloses particles or tiny volumes of fluid at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and pinches off from the membrane. When the vesicle contains particulate matter, the process is called phagocytosis; when it contains droplets of fluid, the process is calledpinocytosis.