Eukaryotic Cell Structure

Eukaryotes (eu = true; karyo refers to a nut or nucleus) are so named because they have a true nucleus, in that their DNA is enclosed by a nuclear membrane. Most animal and plant cells are 10 to 30 μm in diameter, about 10 times larger than most prokaryotic cells. Figure 1 illustrates a typical eukaryotic animal cell. This illustration is a composite of most of the structures that might be found in the various types of human body cells.  Figure 2 shows a transmission electron micrograph of an actual yeast cell. A discussion of the functional parts of eukaryotic cells can be better understood by keeping the illustrated structures in mind.
Figure.1 : A typical eukaryotic animal cell. (Redrawn from Cohen BJ. Memmler’s The Human Body in Health and Disease. 11th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009.)
Figure.1 : A typical eukaryotic animal cell. (Redrawn from Cohen BJ. Memmler’s The Human Body in Health and Disease. 11th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009.)
Figure.2 : Cross section through a yeast cell. The cross section shows the nucleus (N) with nuclear pores (P), mitochondrion (M), and vacuole (V). The cytoplasm is surrounded by the cell membrane. The thick outer portion is the cell wall. (From Lechavalier HA, Pramer D. The Microbes. Philadelphia, PA: JB Lippincott; 1970.)
Figure.2 : Cross section through a yeast cell. The cross section shows the nucleus (N) with nuclear pores (P), mitochondrion (M), and vacuole (V). The cytoplasm is surrounded by the cell membrane. The thick outer portion is the cell wall. (From Lechavalier HA, Pramer D. The Microbes. Philadelphia, PA: JB Lippincott; 1970.)

Cell Membrane

The cell is enclosed and held intact by the cell membrane, which is also referred to as the plasma, cytoplasmic, or cellular membrane. Structurally, it is a mosaic composed of large molecules of proteins and phospholipids (certain types of fats). The cell membrane is like a “skin” around the cell, separating the contents of the cell from the outside world. The cell membrane regulates the passage of nutrients, waste products, and secretions into and out of the cell.

Because the cell membrane has the property of selective permeability, only certain substances may enter and leave the cell. The cell membrane is similar in structure and function to all of the other membranes that are found in eukaryotic cells. Cell membranes have selective permeability, meaning that they allow onlycertain substances to pass through them.

Nucleus 

As previously mentioned, the primary difference between prokaryotic and eukaryotic cells is that eukaryotic cells possess a “true nucleus,” whereas prokaryotic cells do not. The nucleus (pl., nuclei) controls the functions of the entire cell and can be thought of as the “command center” of the cell. The nucleus has three components: nucleoplasm, chromosomes, and a nuclear membrane.

Nucleoplasm (a type of protoplasm) is the gelatinous matrix or base material of the nucleus. The chromosomes are embedded or suspended in the nucleoplasm. The membrane that serves as a “skin” around the nucleus is called the nuclear membrane; it contains holes (nuclear pores) through which large molecules can enter and exit the nucleus.

A “true nucleus” consists of nucleoplasm, chromosomes, and a nuclear membrane. Eukaryotic chromosomes consist of linear DNA molecules and proteins (histones and nonhistone proteins). Histones are positively charged, low-molecular-weight proteins found in eukaryotic cell nuclei. They act as spools around which DNA winds. This winding enables the compaction necessary to fit the large genomes of eukaryotes inside cell nuclei.

A compacted DNA molecule is about 40,000 times shorter than the non compacted molecule. Genes are located along the DNA molecules. Although genes are sometimes described as “beads on a string,” each bead (gene) is actually a particular segment of the DNA molecule. Each gene contains the genetic information that enables the cell to produce one or more gene products. Most gene products are proteins, but some genes code for the production of two types of ribonucleic acid (RNA): ribosomal ribonucleic acid (rRNA) and transfer ribonucleic acid (tRNA) molecules. The organism’s complete collection of genes is referred to as that organism’s genotype (or genome).

The number and composition of chromosomes and the number of genes on each chromosome are characteristic of the particular species of organism. Different species have different numbers and sizes of chromosomes. Human diploid cells, for example, have 46 chromosomes (23 pairs), each consisting of thousands of genes. It has been estimated that the human genome consists of between 20,000 and 25,000 genes.

Although the Human Genome Project was completed in 2003, the  exact number of genes encoded by the human genome is still unknown. The reason for the uncertainty is that the various predictions are derived from different computational methods and gene-finding programs.  Defining a gene is problematic for a number of reasons, including (1) small genes can be difficult to detect, (2) one gene can code for several protein products, (3) some genes code only for RNA, and (4) two genes can overlap. Even with improved genome analysis, computation alone is insufficient to generate an accurate gene number. Gene predictions must be verified by labor-intensive work in the laboratory before the scientific community can reach any real consensus.

When observed using a transmission electron microscope, a dark (electron dense) area can be seen in the nucleus. This area is called the nucleolus; it is here that rRNA molecules are manufactured. The rRNA molecules then exit the nucleus and become part of the structure of ribosomes.

Something To Think About 

According to findings of the Human Genome Project, humans possess between 20,000 and 25,000 genes. How does this compare with the genome size of other organisms? It has been reported that the animal with the largest genome is a tiny aquatic crustacean called a water flea (Daphnia pulex), with about 31,000 genes. But what about other organisms? Examples reported by the Human Genome Project and wikipedia include the bacterium Haemophilus influenzae (1,700), Escherichia coli (3,200), Cryptosporidium parasites (~4,000), a red alga (~5,300), a malarial parasite (~5,300), bakers’ yeast (~6,000), other fungi (from ~2,000 to ~11,800), a green alga (~8,000), a mosquito (~13,600), a fruit fly (13,600), a roundworm (19,000), a mouse (~25,000 genes), a puffer fish (from 22,000 to 29,000), a wild mustard plant called Arabidopsis thaliana (25,000), rice (from 32,000 to 50,000), and a cottonwood tree (~45,500). Isn’t it interesting that the genomes of certain plants are larger than the human genome? Equally interesting is the fact that more than 97% of human genetic material is identical to a chimpanzee’s, and, although not a microbiology topic, it might be something that you would like to learn more about by using a Search Engine.


Cytoplasm 

Cytoplasm (a type of protoplasm) is a semifluid, gelatinous, nutrient matrix. Within the cytoplasm are found insoluble storage granules and various cytoplasmic organelles, including endoplasmic reticulum, ribosomes, Golgi complexes, mitochondria, centrioles, microtubules, lysosomes, and other membrane-bound vacuoles. Each of these organelles has a highly specific function, and all of the functions are interrelated to maintain the cell and allow it to properly perform its activities. The cytoplasm is where most of the cell’s metabolic reactions occur. The semifluid portion of the cytoplasm, excluding the granules and organelles, is sometimes referred to as the cytosol.


Endoplasmic Reticulum 

The endoplasmic reticulum (ER) is a highly convoluted system of membranes that are interconnected and arranged to form a transport network of tubules and flattened sacs within the cytoplasm. Much of the ER has a rough, granular appearance when observed by transmission electron microscopy and is designated as rough endoplasmic reticulum (RER). This rough appearance is caused by the many ribosomes attached to the outer surface of the membranes. ER to which ribosomes are not attached is called smooth endoplasmic reticulum (SER).

Ribosomes 

Eukaryotic ribosomes are 18 to 22 nm in diameter. They consist mainly of rRNA and protein and play an important part in the synthesis (manufacture) of proteins. Clusters of ribosomes (called polyribosomes or polysomes), held together by a molecule of messenger RNA (mRNA), are sometimes observed by electron microscopy.

Within a cell, ribosomes are the sites of protein synthesis. Each eukaryotic ribosome is composed of two subunits a large subunit (the 60S subunit) and a small subunit (the 40S subunit)—that are produced in the nucleolus. The subunits are then transported to the cytoplasm where they remain separate until such time as they join together with an mRNA molecule to initiate protein synthesis. When united, the 40S and 60S subunits form an 80S ribosome. (The “S” refers to Svedberg units, and 40S, 60S, and 80S are sedimentation coefficients. A sedimentation coefficient expresses the rate at which a particle or molecule moves in a centrifugal field; it is determined by the size and shape of the particle or molecule.)

Most of the proteins released from the ER are not mature. They must undergo further processing in an organelle known as a Golgi complex before they are able to perform their functions within or outside of the cell.

Golgi Complex 

A Golgi complex, also known as a Golgi apparatus or Golgi body, connects or communicates with the ER. This stack of flattened, membranous sacs completes the transformation of newly synthesized proteins into mature, functional ones and packages them into small, membrane-enclosed vesicles for storage within the cell or export outside the cell (exocytosis or secretion). Golgi complexes are sometimes referred to as “packaging plants.”

Lysosomes and Peroxisomes 

Lysosomes are small (about 1-μm diameter) vesicles that originate at the Golgi complex. They contain lysozyme and other digestive enzymes that break down foreign material taken into the cell by phagocytosis (the engulfing of large particles by amebas and certain types of white blood cells called phagocytes). These enzymes also aid in breaking down worn out parts of the cell and may destroy the entire cell by a process called autolysis if the cell is damaged or deteriorating. Lysosomes are found in all eukaryotic cells.

Peroxisomes are membrane-bound vesicles in which hydrogen peroxide is both generated and broken down. Peroxisomes contain the enzyme catalase, which catalyzes (speeds up) the breakdown of hydrogen peroxide into water and oxygen. Peroxisomes are found in most eukaryotic cells, but are especially prominent in mammalian liver cells.

Mitochondria 

The energy necessary for cellular function is provided by the formation of high-energy phosphate molecules such as adenosine triphosphate (ATP). ATP molecules are the major energy-carrying or energy-storing molecules within cells. Mitochondria (sing., mitochondrion) are referred to as the “power plants,” “powerhouses,” or “energy factories” of the eukaryotic cell, because this is where most of the ATP molecules are formed by cellular respiration. During this process, energy is released from glucose molecules and other nutrients to drive other cellular functions. The number of mitochondria in a cell varies greatly depending on the activities required of that cell. Mitochondria are about 0.5 to 1μm in diameter and up to 7 μm in length. Many scientists believe that mitochondria and chloroplasts arose from bacteria living within eukaryotic cells (see “The Origin of Mitochondria and Chloroplasts” on the point.

Plastids 

Plant cells contain both mitochondria and another type of energy-producing organelle, called a plastid. Plastids are membrane-bound structures containing  various photosynthetic pigments; they are the sites of photosynthesis. Chloroplasts, one type of plastid, contain a green, photosynthetic pigment called chlorophyll. Chloroplasts are found in plant cells and algae. Photosynthesis is the process by which light energy is used to convert carbon dioxide and water into carbohydrates and oxygen. The chemical bonds in the carbohydrate molecules represent stored energy. Thus, photosynthesis is the conversion of light energy into chemical energy.

Cytoskeleton 

Present throughout the cytoplasm is a system of fibers, collectively known as the cytoskeleton. The three types of cytoskeletal fibers are microtubules, microfilaments (actin filaments), and intermediate filaments. All three types serve to strengthen, support, and stiffen the cell, and give the cell its shape. In addition to their structural roles, microtubules and microfilaments are essential for various activities, such as cell division, contraction, motility (see the section on flagella and cilia), and the movement of chromosomes within the cell. Microtubules are slender, hollow tubules composed of spherical protein subunits called tubulins.

Cell Wall 

Some eukaryotic cells contain cell walls—external structures that provide rigidity, shape, and protection (Fig. 3). Eukaryotic cell walls, which are much simpler in structure than prokaryotic cell walls, may contain cellulose, pectin, lignin, chitin, and some mineral salts (usually found in algae). The cell walls of algae contain a polysaccharide—cellulose—that is not found in the cell walls of any other microorganisms. Cellulose is also found in the cell walls of plants. The cell walls of fungi contain a polysaccharide—chitin—that is not found in the cell walls of any other microorganisms. Chitin, which is similar in structure to cellulose, is also found in the exoskeletons of beetles and crabs.
Figure.3 : Presence or absence of a cell wall in various types of cells. Mycoplasma is a genus of bacteria.
Figure.3 : Presence or absence of a cell wall in various types of cells. Mycoplasma is a genus of bacteria.

Flagella and Cilia 

Some eukaryotic cells (e.g., spermatozoa and certain types of protozoa and algae) possess relatively long, thin structures called flagella (sing., flagellum). Such cells are said to be flagellated or motile; flagellated protozoa are called flagellates. The whipping motion of the flagella enables flagellated cells to “swim” through liquid environments; flagella are said to be whip-like. Flagella are referred to as organelles of locomotion (cell movement). Flagellated cells may possess one flagellum or two or more flagella. Cilia (sing., cilium) are also organelles of locomotion, but they tend to be shorter (more hair-like), thinner, and more numerous than flagella. Cilia can be found on some species of protozoa (called ciliates) and on certain types of cells in our bodies (e.g., the ciliated epithelial cells that line the respiratory tract). Unlike flagella, cilia tend to beat with a coordinated, rhythmic movement. Eukaryotic flagella and cilia, which contain an internal “9 + 2” arrangement of microtubules (Fig. 4), are structurally more complex than bacterial flagella.
Figure.4 : Cilia. A transmission electron micrograph showing cross sections of mouse respiratory cilia. Note the 9 + 2 arrangement of microtubules within each cilium: two single microtubules in the center, surrounded by nine doublet microtubules. (Courtesy of Louisa Howard and remf.dartmouth. edu/images.)
Figure.4 : Cilia. A transmission electron micrograph showing cross sections of mouse respiratory cilia. Note the 9 + 2 arrangement of microtubules within each cilium: two single microtubules in the center, surrounded by nine doublet microtubules. (Courtesy of Louisa Howard and remf.dartmouth. edu/images.)

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