Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A.N. Ogurtsov 2011

Fundamentals of Cell Theory
Cell Structure

Despite the wide variety of Cells, they all share a common structural plan. The Structure of animal and plant cells is shown in Figure 17, and the Main Components of a Introduction/5.html">Eukaryotic Cell are presented in Figure 18.

Its primary constituent parts are The Cell membrane, The Nucleus, and the Cytoplasm.

The cell envelope consists of: (1) The Plasma Membrane,

(2) the supramembrane complex (glycocalyx or Cell wall), and

(3) the submembrane cytoskeletal-contractile apparatus.

These three functionally unified components provide three universal Functions common to all types of cell envelopes: (1) barrier, (2) transport, and (3) receptor.

The nucleus is the largest organelle of a eukaryotic cell (about 10 µm in diameter) visible under a Light Microscope.

The nucleus performs critical metabolic and genetic functions, as it houses the Chromosomes containing DNA molecules. The size, shape, and structure of nuclei vary depending on the functional state of the cells, rapidly responding to changing external conditions.

The nucleus typically moves passively within the cell along with the flow of the surrounding cytoplasm, but occasionally it is capable of independent movement via amoeboid-type locomotion.

The nucleus is separated from the rest of the cell by an envelope consisting of inner and outer nuclear membranes (Figure 19).

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Figure 18 - Diagram of a eukaryotic cell: 1 - nucleus; 2 - chromosomes; 3 - nuclear pores; 4 - Ribosomes; 5 - rough Endoplasmic reticulum; 6 - mitochondrion; 7 - Golgi apparatus; 8 - centriole; 9 - cilia; 10 - flagella

The region between the two nuclear membranes is called the perinuclear space. The outer nuclear membrane is studded with ribosomes and continuous with the rough endoplasmic reticulum. The inner nuclear membrane is lined with specialized Proteins (such as lamins) that serve to anchor nuclear structures (the nuclear lamina).

Almost all of the cell's DNA is located in the nucleus. This DNA carries Genetic information and serves as the primary site for its Replication and expression.

During interphase (the phases between cell divisions), most of the nuclear DNA is present as heterochromatin, which is tightly packed DNA associated with RNA and proteins.

Figure 19 - Diagram of the Cell Nucleus

Less tightly packed DNA is called euchromatin; this is the site of active METABOLISM/31.html">Transcription of DNA into RNA.

The nucleus frequently contains a nucleolus, and sometimes several nucleolii. During Cell Division, the nuclear structure breaks down. Chromatin organizes into chromosomes, which are highly condensed forms of DNA molecules visible under a light microscope.

The exchange of macromolecules, such as proteins and RNA, between the nucleus and the cytoplasm occurs through nuclear pores (approximately 7 nm in diameter) formed by a protein complex. These pores regulate Transport Across the nuclear membranes. Peptides and small proteins, such as Histones, can easily enter the nucleus. Larger proteins (over 40 kDa) can pass through the nuclear membrane only if they carry a specific signal sequence.

The cytoplasm is the internal content of the cell. It is permeated by a dense network of protein fibers that form the Cytoskeleton, and it contains Organelles and a membranous vacuolar system. Interaction among them is mediated by the semi-fluid component of the cytoplasm—the Cytosol—a unique colloidal system containing 75-80% Water, 10-12% PROTEINS AND AMINO acids, 4-6% CARBOHYDRATES, 2-3% Lipids, and 1% other substances. The cytosol accounts for about 50% of the total volume of a typical cell.

Cytoplasmic organoids (organelles) are essential Structural elements of a eukaryotic cell; in their absence, the cell loses viability.

The primary cytoplasmic organelles are Mitochondria, Plastids, and ribosomes.