Plasma Membrane and Associated

Structures 50

I. The structure of the cell

(plasma) membrane is described by a fluid-mosaic model.

A. The membrane is composed predominately of a double layer of phospholipids.

B. The membrane also contains proteins, most of which span its entire width.

II. Some cells move by extending pseudopods; cilia and flagella protrude from the cell membrane of some specialized cells.

III. In the process of endocytosis, invaginations of the plasma membrane allow the cells to take up molecules from the external environment.

A. In phagocytosis, the cell extends pseudopods that eventually fuse together to create a food vacuole; pinocytosis involves the formation of a narrow furrow in the membrane, which eventually fuses.

B. Receptor-mediated endocytosis requires the interaction of a specific molecule in the extracellular environment with a specific receptor protein in the cell membrane.

C. Exocytosis, the reverse of endocytosis, is a process that allows the cell to secrete its products.

Cytoplasm and Its Organelles 55

I. Microfilaments and microtubules produce a cytoskeleton that aids movements of organelles within a cell.

II. Lysosomes contain digestive enzymes and are responsible for the elimination of structures and molecules within the cell and for digestion of the contents of phagocytic food vacuoles.

III. Mitochondria serve as the major sites for energy production within the cell. They have an outer membrane with a smooth contour and an inner membrane with infoldings called cristae.

IV. Ribosomes are small protein factories composed of ribosomal RNA and protein arranged into two subunits.

V. The endoplasmic reticulum is a system of membranous tubules in the cell.

A. The granular endoplasmic reticulum is covered with ribosomes and is involved in protein synthesis.

B. The agranular endoplasmic reticulum provides a site for many enzymatic reactions and, in skeletal muscles, serves to store Ca2+.

VI. The Golgi complex is a series of membranous sacs that receive products from the endoplasmic reticulum, modify those products, and release the products within vesicles.

Cell Nucleus and Gene Expression 61

I. The cell nucleus is surrounded by a double-layered nuclear envelope. At some points, the two layers are fused by nuclear pore complexes that allow for the passage of molecules.

II. Genetic expression occurs in two stages: transcription (RNA synthesis) and translation (protein synthesis).

A. The DNA in the nucleus is combined with proteins to form the threadlike material known as chromatin.

B. In chromatin, DNA is wound around regulatory proteins known as histones to form particles called nucleosomes.

C. Chromatin that is active in directing RNA synthesis is euchromatin; the highly condensed, inactive chromatin is heterochromatin.

III. RNA is single-stranded. Four types are produced within the nucleus: ribosomal RNA, transfer RNA, precursor messenger RNA, and messenger RNA.

IV. Active euchromatin directs the synthesis of RNA in a process called transcription.

A. The enzyme RNA polymerase causes separation of the two strands of DNA along the region of the DNA that constitutes a gene.

B. One of the two separated strands of DNA serves as a template for the production of RNA. This occurs by complementary base pairing between the DNA bases and ribonucleotide bases.

Protein Synthesis and Secretion 65

I. Messenger RNA leaves the nucleus and attaches to the ribosomes.

II. Each transfer RNA, with a specific base triplet in its anticodon, binds to a specific amino acid.

A. As the mRNA moves through the ribosomes, complementary base pairing between tRNA anticodons and mRNA codons occurs.

B. As each successive tRNA molecule binds to its complementary codon, the amino acid it carries is added to the end of a growing polypeptide chain.

III. Proteins destined for secretion are produced in ribosomes located on the granular endoplasmic reticulum and enter the cisternae of this organelle.

IV. Secretory proteins move from the granular endoplasmic reticulum to the Golgi complex.

A. The Golgi complex modifies the proteins it contains, separates different proteins, and packages them in vesicles.

B. Secretory vesicles from the Golgi complex fuse with the plasma membrane and release their products by exocytosis.

DNA Synthesis and Cell Division 69

I. Replication of DNA is semiconservative; each DNA strand serves as a template for the production of a new strand.

A. The strands of the original DNA molecule gradually separate along their entire length and, through complementary base pairing, form a new complementary strand.

B. In this way, each DNA molecule consists of one old and one new strand.

II. During the G1 phase of the cell cycle, the DNA directs the synthesis of RNA, and hence that of proteins.

III. During the S phase of the cycle, DNA directs the synthesis of new DNA and replicates itself.

Cell Structure and Genetic Control

IV. After a brief time gap (G2), the cell begins mitosis (the M stage of the cycle).

Mitosis consists of the following phases: interphase, prophase, metaphase, anaphase, and telophase.

In mitosis, the homologous chromosomes line up single file and are pulled by spindle fibers to opposite poles.

C. This results in the production of two daughter cells, each containing forty-six chromosomes, just like the parent cell.

V. Meiosis is a special type of cell division that results in the production of gametes in the gonads. A. The homologous chromosomes line up side by side, so that only one of each pair is pulled to each pole.

This results in the production of two daughter cells, each containing only twenty-three chromosomes, which are duplicated.

The duplicate chromatids are separated into two new daughter cells during the second meiotic cell division.

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