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2.1 The Components of Cells

Categories of Cells
1. There are three broad varieties of cells -- Eubacteria (the more common forms of bacteria), Archaea (the extreme bacteria), and Eukaryotes (higher cells) -- based on cellular complexity.
2. The Archaea and Eubacteria are similar in that they are single-celled organisms, but they differ in certain features of their RNA and membranes. These cells lack nuclei and other organelles and therefore are categorized as prokaryotes.
3. Eukaryotic cells are complex, with abundant and diverse organelles that compartmentalize biochemical reactions. Human cells are therefore eukaryotic.

Chemical Constituents of Cells
1. The human body is composed of trillions of cells that share common features.
2. Cells are constructed from numerous small molecules and macromolecules.
3. The building blocks of cells include carbohydrates (simple sugars and polysaccharides), amino acids and proteins, lipids (fats and oils), and nucleic acids (nucleotides, DNA, RNA).
4. Enzymes are proteins that catalyze the multitude of biochemical reactions that occur in the cell.
5. The proteome is the set of proteins that a particular cell type can produce.
6. Expression of different subsets of genes in different cells (and therefore production of different proteomes) gives rise to over 250 specialized cell types.

Organelles
1. Organelles represent the compartments (and unique microenvironments) in the cell and are involved in a variety of functions (division of labor).
2. The nucleus (the storehouse of the majority of DNA in the cell) has a double membrane and nuclear pores, which allow macromolecular traffic in and out of the nucleus.
3. The rough endoplasmic reticulum (ER), smooth ER, and Golgi body function as a membrane network for the synthesis of proteins and lipids that are targeted for delivery to the plasma membrane, organelles, or for secretion. Secretions bud off in vesicles and leave the cell.
4. A mitochondrion has a double membrane whose inner folds carry enzymes that catalyze reactions that extract energy from nutrients.
5. Lysosomes contain enzymes that degrade cellular debris.
6. Peroxisomes house enzymes that detoxify certain substances, break down lipids, and synthesize bile acids.

The Cell Membrane
1. The cell membrane (composed of a bilayer of phospholipids) and the underlying cytoskeleton (microtubules and microfilaments) give a cell its specific architecture.
2. The proteins (or glycoproteins) residing in the cell membrane function as enzymes, signal transduction receptors, transport proteins, and cell adhesion proteins.

The Cytoskeleton
1. The major cytoskeleton components include microtubules (tubulin), microfilaments (actin), and intermediate filaments (various families of intermediate filament proteins).
2. Microtubules also serve at the foundation of cilia and flagella.
3. Spherocytosis is a heredity defect in the cytoskeleton of the red blood cell. An abnormal cytoskeletal protein, ankyrin, lies beneath the cell membrane, and causes the red blood cells to balloon out, blocking narrow blood vessels in organs.

2.2 Cell Division and Death

The Cell Cycle
1. The cell cycle consists of interphase, when a cell is not dividing, and mitosis.
2. During interphase, proteins, lipids, and carbohydrates are produced in the G1 phase; DNA and proteins are made during S phase; and more proteins are produced in G2. Replicated chromosomes have two sister chromatids attached at their centromeres.
3. In mitotic prophase, replicated chromosomes condense, a spindle forms, and the nuclear membrane breaks down. In metaphase, chromosomes align down the center of the cell (equator or metaphase plate). In anaphase, centromeres part, one chromatid from each pair pulled to opposite ends of the cell. In telophase, the cell pinches in the middle (cytokinesis), and the two new cells separate.
4. The cell cycle is tightly controlled and regulated at several "checkpoints."
5. A cellular clock that limits the number of divisions is based on shrinking telomeres.
6. Crowding, hormones, and growth factors are extracellular influences on mitosis.
7. Within cells, kinases and cyclins activate the genes whose products carry out mitosis.
8. Non-dividing cells may become arrested during interphase and enter a quiescent phase (G0).

Apoptosis
1. Mitosis (cell division) and apoptosis (cell death) are continuous processes that occur in a series of steps and are both initiated by signals in the extracellular environment.
2. The balance between cell division and death maintains tissues in growth, development, and repair.
3. In prenatal development coordination of these processes sculpts body form. After birth, mitosis and apoptosis protect and maintain the body.
4. Disruption of the balance between cell division and cell death can lead to cancer or other disorders.

2.3 Cell-Cell Interactions

Signal Transduction
1. Cellular activities, including secretion, signal transduction, cell division, cell adhesion, and the function of the cytoskeleton, affect each other.
2. In signal transduction, cell surface receptors get information from extracellular first messengers and pass the signals to second messengers inside the cell, which responds.
3. Neurofibromatosis type I (NF1) is caused by faulty signal transduction. Nerve cells beneath the skin inappropriately transmit a growth factor signal, triggering cell division. A tumor forms.

Cell Adhesion
1. Cell adhesion is a precise sequence of interactions between cell surface proteins that join cells.
2. In inflammation, cell adhesion molecules (CAMs) guide white blood cells to injury sites.
3. Leukocyte-adhesion deficiency and cancer are disorders that can result from abnormal cell adhesion.

2.4 Stem Cells and Cell Specialization
1. Cellular differentiation requires differential gene expression and results in the formation of specialized cells with distinctive characteristics and functions (i.e. erythrocytes, lymphocytes, heart muscle cell, fat cell, nerve cell, etc.).
2. The zygote or fertilized egg is totipotent, capable of producing any cell type.
3. Cells of the early embryo are pluripotent, meaning they can differentiate into some but not all types of cells.
4. Later in development, pluripotent stem cells give rise to progenitor cells that are committed to a particular pathway.
5. Stem cells are non-specialized cells that retain the potential to differentiate and enable a tissue to grow or repair itself.
6. Regenerative medicine uses stem cells to replace or rejuvenate injured or diseased tissue.

2.5 Viruses and Prions-Not Cells, but Infectious

Viruses
1. Viruses are infectious agents that disrupt normal cellular function.
2. A virus contains genetic material in the form of nucleic acid (DNA or RNA) wrapped in a protein coat.
3. Viruses use the metabolic and protein-synthetic machinery of cells to replicate.

A Prion - One Protein that Takes Two Forms 1. Prions do not contain genetic material but are abnormally folded glycoproteins that cause a rare type of disease called a spongiform encephalopathy.
2. A normally folded form of the prion protein (PrP) naturally occurs in the brain of mammals.
3. Prions cause disease by interacting with normal prion proteins and coaxing them to adopt the infectious (abnormally folded) form.





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