Cellular Structure

Biological systems are essentially an assembly of molecules where water, amino acids, carbohydrates (sugar), fatty acids, and ions account for 75-80%

TABLE 3.2. Hierarchical Buildup of a Living Organism

Cells

Cell differentiation and association

Tissues

Organization to perform a function if

Organs

Integration of various functions

Organism of the matter in cells. The remainder of the cell mass is accounted for by macro-molecules, also called polymers (or biopolymers in the present case), which include peptides/proteins (formed from amino acids), polysaccharides (formed from sugars), DNA (dioxyribonucleic acid, formed from nucleotide bases and dioxyribose sugar), RNA (ribonucleic acid, formed from nucleotide bases and ribose sugar), and phospholipids (formed from fatty acids). These macromol-ecular polymers organize to form cells. To contain these molecules, a semipermeable membrane (phospholipid bilayer) surrounds them to form a cell. Within this biological universe, two types of organized cells exist, as shown in Table 3.1. Prokaryotic cells (bacteria) are cells with little internal structure and no defined nucleus. Eukaryotic cells have a significantly more complex internal architecture including a defined, membrane-bound nucleus. The smallest organized particle is a virus. The smallest self-replicating cells are bacteria. Eukaryotic cells, for the most part, organize to form complex living organisms. From a single pluripotent cell (a cell with the capacity to differentiate into several cell types) arises tissues and organs, and finally a complex living organism as shown in Table 3.2.

It is speculated that living organisms evolved from the prebiotic conditions in existence during the first one billion years. Although still speculative, it is hypothesized that simple organic molecules (those containing carbon) were formed during the violent electrical discharges in a heated atmosphere containing methane, carbon dioxide, ammonia, and hydrogen. These molecules formed the primordial soup from which primitive proteins and nucleotides were born. From this soup arose the first self-replicating membrane-bound organism.

The ability to self-replicate endows an organism with the ability to evolve. According to the Darwinian principle, organisms vary randomly and only the fittest survive. In living systems, genes (discussed below) define the cell constituents, structures, and cellular activities. Alteration in structure and organi zation nurtures the evolutionary changes necessary for survival of an organism. Recent progress in determining the nucleotide sequence for a variety of organisms has revealed that subtle, not drastic, changes are responsible for the evolutionary process.

The structure of a cell—specifically, eukaryotic cells—can be described in terms of the various subcellular compartments and the constituent chemical species they contain. The main structural components of a cell are:

• Plasma membrane, which defines the outer boundary of a cell. This is present in all cells.

• Cell wall, which exists in the prokaryotic cells as well as in the eukaryotic cells of plants but not animals.

• Cytoplasm, which represents everything within a cell, except the nucleus.

• Cytosol, which is the fluid of the cytoplasm.

• Organelle, which is the name used for a subcellular compartment in a cell where a specific cellular function takes place.

• Nucleus, which contains the chromosomes (genetic information).

Figure 3.1 compares the schematic representation of a eukaryotic cell versus a prokaryotic cell. It is readily seen that the prokaryotic cell, or bacteria, has a much less complex internal structure compared to the eukaryotic cell. In addition, a complex outer wall structure exists in most bacteria and is composed of unique outer membrane and inner membrane structures between which are sandwiched a rigid unique polysaccharide cell wall. This wall consists of a macromolecule known as peptidoglycan that enables an organism to survive in changing environments.

Organelles are like little organs of a cell that perform various cellular functions, just like organs perform various tasks in a living system. Organelles are intracellular (or subcellular) structures: specifically, nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, cytoskeleton, lysosomes, and peroxi-somes. In the case of plant cells, other organelles are plastids, chloroplasts, vacuole, and cell wall (already listed above). The following describes some structural aspects of various cellular components and functions they perform (Audesirk et al., 2001).

Plasma Membrane. This forms a semipermeable outer boundary of both prokaryotic and eukaryotic cells. This outer membrane, about 4-5 nm thick, is a continuous sheet of a double layer (bilayer) of long-chain molecules called phospholipids. A phospholipid molecule has a long tail of alkyl chain, which is hydrophobic (repels water), and a hydrophilic head (likes water) which carries a charge (and is thus ionic). Phospholipid molecules spontaneously orient (or self-organize) to form a bilayer in which the hydrophobic tails are pointed inwards (shying away from the outer aqueous environment). The hydrophilic, ionic head groups are in the exterior and are thus in contact with

Nucleoid DNA Septum Mesosome

Periplasmic space and cell wall

Nucleoid DNA Septum Mesosome

Inner (plasma) membrane Cell wall

- Periplasmic space

- Outer membrane

Inner (plasma) membrane Cell wall

- Periplasmic space

- Outer membrane

Periplasmic space and cell wall

Outer membrane Inner (plasma) Nucleoid a5 mm membrane

(a) Prokaryotic cell

Nuclear membrane

Golgi vesicles Mitochondrion Peroxisome Lysosome

Rough endoplasmic reticulum

Nuclear membrane

Plasma membrane

Golgi vesicles Mitochondrion Peroxisome Lysosome

Endoplasmic reticulum

Golgi vesicles

Lysosome Ó

Mitochondrion 1 mm

Rough endoplasmic reticulum

Secretory vesicle

(b) Eukaryotic cell

Figure 3.1. Left: Drawings; right: Electron micrographs. Comparison of prokaryotic and eukaryotic cells. (Reproduced with permission from Lodish et al., 2000).

the surrounding aqueous environment. This structure is shown in detail in Figure 3.2. The membrane derives its rigidity by inclusion of cholesterol molecules, which are interdispersed in the phospholipid bilayer. Also embedded are membrane proteins (receptors, pores, and enzymes) that are important for a number of cell activities including communication between the intracellular and extracellular environments. The plasma membrane controls the transport of food, water, nutrients, and ions such as Na+, K+, and Ca2+ (through so-called ion channels) to and from the cell as well as signals (cell signaling) necessary for proper cell function.

Cytoplasm. As indicated above, cytoplasm represents everything enclosed by the plasma membrane, with the exclusion of the nucleus. It is present in all cells where metabolic reactions occur. It consists mainly of a viscous fluid medium that includes salts, sugars, lipids, vitamins, nucleotides, amino acids, RNA, and proteins which contain the protein filaments, actin microfilaments, microtubules, and intermediate filaments. These filaments function in animal and plant cells to provide structural stability and contribute to cell movement. Many of the functions for cell growth, metabolism, and replication are carried out within the cytoplasm. The cytoplasm performs the functions of energy pro-

Figure 3.2. Schematics of the phospholipid membrane bilayer structure. (Reproduced with permission from Lodish et al., 2000.)

duction through metabolic reactions, biosynthetic processes, and photosynthesis in plants. The cytoplasm is also the storage place of energy within the cell. Cytosol, a subset of cytoplasm, refers only to the protein-rich fluid environment, excluding the organelles.

Cytoskeleton. The cytoskeleton structure, located just under the membrane, is a network of fibers composed of proteins, called protein filaments. This structure is connected to other organelles. In animal cells, it is often organized from an area near the nucleus. These arrays of protein filaments perform a variety of functions:

• Establish the cell shape

• Provide mechanical strength to the cell

• Perform muscle contraction

• Control changes in cell shape and thus produce locomotion

• Provide chromosome separation in mitosis and meiosis (these processes are discussed below)

• Facilitate intracellular transport of organelles

Nucleus. The nucleus is often called the control center of the cell. It is the largest organelle in the cell, usually spherical with a diameter of 4-10 mm, and is separated from the cytoplasm by an envelope consisting of an inner and an outer membrane. All eukaryotic cells have a nucleus. The nucleus contains DNA distributed among structures called chromosomes, which determine the genetic makeup of the organism. The chromosomal DNA is packaged into chromatin fibers by association with an equal mass of histone proteins. The nucleus contains openings (100 nm across) in its envelope called nuclear pores, which allow the nuclear contents to communicate with the cytosol.

Outer membrane Inner membrane

Nucleoplasm Nucleolus

Chromatin

Nuclear envelope

Pore in nuclear envelope

Nuclear envelope

Pore in nuclear envelope

http://wing-keung.tripod.com/cellbiology.htm.)"/>
Figure 3.3. Schematics of the structure of the nucleus. (Reproduced with permission from http://wing-keung.tripod.com/cellbiology.htm.)

Figure 3.3 shows a schematic of a nucleus. The inside of the nucleus also contains another organelle called a nucleolus, which is a crescent-shaped structure that produces ribosomes by forming RNA and packaging it with ribosomal protein. The nucleus is the site of replication of DNA and transcription into RNA. In a eukaryotic cell, the nucleus and the ribosomes work together to synthesize proteins. These processes will be discussed in a later section.

Mitochondria. Mitochondria are large organelles, globular in shape (almost like fat sausages), which are 0.5-1.5mm wide and 3-10mm long. They occupy about 20% of the cytoplasmic volume. They contain an outer and an inner membrane, which differ in lipid composition and in enzymatic activity. The inner membrane, which surrounds the matrix base, has many infoldings, called cristae, which provide a large surface area for attachment of enzymes involved in respiration. The matrix space enclosed by the inner membrane is rich in enzymes and contains the mitochondrial DNA. Mitochondria serve as the engine of a cell. They are self-replicating energy factories that harness energy found in chemical bonds through a process known as respiration, where oxygen is consumed in the production of this energy. This energy is then stored in phosphate bonds. In plants, the counterpart of mitochondria is the chloro-plast, which utilizes a different mechanism, photosynthesis, to harness energy for the synthesis of high-energy phosphate bonds.

Endoplasmic Reticulum. The endoplasmic reticulum consists of flattened sheets, sacs, and tubes of membranes that extend throughout the cytoplasm of eukaryotic cells and enclose a large intracellular space called lumen. There is a continuum of the lumen between membranes of the nuclear envelope. The rough endoplastic reticulum (rough ER) is close to the nucleus, and is the site of attachment of the ribosomes. Ribosomes are small and dense structures, 20 nm in diameter, that are present in great numbers in the cell, mostly attached to the surface of rough ER, but can float free in the cytoplasm. They are manufactured in the nucleolus of the nucleus on a DNA template and are then transported to the cytoplasm. They consist of two subunits of RNA (a large, 50S, and a small, 30S) that are complexed with a set of proteins. Ribo-somes are the sites of protein synthesis. The process of protein synthesis using a messenger RNA template is described below. The rough ER transitions into a smooth endoplastic reticulum (smooth ER), which is generally more tubular and lacks attached ribosomes. The smooth ER is the primary site of synthesis of lipids and sugars and contains degradative enzymes, which detoxify many organic molecules.

Golgi Apparatus. This organelle is named after Camillo Golgi, who described it. It consists of stacked, flattened membrane sacs or vesicles, which are like shipping and receiving departments because they are involved in modifying, sorting, and packaging proteins for secretion or delivery to other organelles or for secretion outside of the cell. There are numerous membrane-bound vesicles (<50 nm) around the Golgi apparatus, which are thought to carry materials between the Golgi apparatus and different compartments of the cell.

Lysosomes. These are bags (technical term: vesicles) of hydrolytic enzymes that are 0.2-0.5 mm in diameter and are single-membrane bound. They have an acidic interior and contain about 40 hydrolytic enzymes involved in intra-cellular digestions.

Peroxisomes. These are membrane-bound vesicles containing oxidative enzymes that generate and destroy hydrogen peroxide. They are 0.2-0.5 mm in diameter.

Chloroplast. This cell organelle exists only in plants. It contains pigments, called chlorophylls, which harvest light energy from the sun. The chloroplast is the site of photosynthesis, where light energy from the sun is converted into chemical energy to be utilized by the plant cell (synthesis of ATP).

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