Cell Anatomy (Fig. 1-4)

Cell Membrane

1. Each cell is surrounded by a lipid bilayer that acts as a permeability barrier, allowing the cell to maintain a cytoplasmic composition different from the extracellular fluid (Table 1-2).

2. Lipid bilayers are nearly impermeable to water-soluble substances, such as ions and glucose. Conversely, fat-soluble substances (steroids) and gases readily cross cell membranes.

Transfer of Molecules through Cell Membranes

1. Diffusion. Oxygen, carbon dioxide, and nitrogen move through cell membranes by simple diffusion through the lipid bilayer (Table 1-3).

2. Endocytosis and exocytosis transfer molecules such as nutrients across cell membranes without the molecule actually passing through the cell membrane (Fig. 1-5). Neurotransmitters are ejected from cells by exocytosis, a process that requires calcium ions and resembles endocytosis in reverse.

3. Sodium-potassium ATPase (sodium-potassium pump) is an ATP-dependent sodium and potassium transporter on the cell membrane that ejects three sodium ions from the cell in exchange for the import of two potassium ions (Fig. 1-6).

4. Ion channels are transmembrane proteins that generate electrical signals in the brain, nerves, heart, and skeletal muscles (Fig. 1-7).

5. Some channels are highly specific with respect to ions allowed to pass (sodium, potassium), whereas other channels allow all ions below a certain size to pass (Table 1-4).

6. Genes encoding the protein ion channels may be defective, leading to diseases such as cystic fibrosis (chloride channel defects) and long Q-T interval syndrome (mutant potassium or, less commonly, sodium channels).

Nucleus is primarily made up of the 46 chromosomes, except the nucleus of the egg cell, which contains 23.

1. Structure and Function of DNA and RNA (Fig. 1-8)

   1. The genetic message is determined by the sequence of nucleotides.

   2. The human genome is composed of 20,000 to 25,000 genes (the protein encoding genes account for only 1% to 2% of our DNA).

2. Cytoplasm consists of water; electrolytes; and proteins, including enzymes, lipids, and carbohydrates; and also contains numerous organelles.

   1. Mitochondria are the power-generating units of cells containing both the enzymes and substrates of the tricarboxylic acid cycle (Krebs cycle) and the electron transport chain.

   2. Endoplasmic reticulum is a complex lipid bilayer that folds and creates vesicles in the cytoplasm. The portion of the membrane containing these ribosomes is known as the rough endoplasmic reticulum. The part of the membrane that lacks ribosomes is the smooth endoplasmic reticulum. This smooth portion of the endoplasmic reticulum membrane functions in the synthesis of lipids, metabolism of carbohydrates, and other enzymatic processes. The sarcoplasmic reticulum is found in muscle cells, where it serves as a reservoir for calcium.