Chapter 3. Chapter 3: Cells

Cells are the smallest unit of life that can perform all the activities necessary for life, and all living organisms consist of one or more cells.

All prokaryotes share four structural features: (a) a plasma membrane composed of a phospholipid bilayer, which encloses the cellular contents, including the DNA, ribosomes, and cytoplasm; (b) cytoplasm, which is a jelly-like fluid contained within the plasma membrane; (c) DNA organized into one or more circular loops containing the cell’s genetic material; and (d) many ribosomes, which are the sites of protein synthesis, directed by the cell’s genetic material.

Endosymbiosis: According to this theory, mitochondria and chloroplast are thought to have originally been prokaryotes capable of cellular respiration and photosynthesis, respectively, that were engulfed by a cell that developed into a eukaryotic cell. Invagination: In this theory, the cell membrane folded in on itself to form inner compartments that specialized to become the membranous organelles.

Phospholipids have a hydrophilic polar head containing a glycerol molecule attached to a phosphorus-containing molecule, resulting in regions of partial positive and partial negative charge. Phospholipids also have two nonpolar hydrophobic tails, each containing a chain of carbon and hydrogen. As a result of their chemical structure, phospholipids associate to form two layers (bilayers), with the hydrophobic tails facing the center region and the hydrophilic heads facing the outer, water-containing regions. Phospholipids can float around within the same layer, but generally do not mix with the other side of the bilayer. The bilayer is semipermeable, allowing some materials to pass through but not others.

The plasma membrane behaves like a fluid because many membrane molecules can move around within their respective layer of the phospholipid bilayer, and it is described as a mosaic because it is composed of a variety of proteins, lipids, and carbohydrates.

Individuals with cystic fibrosis have a genetic disorder in which faulty transmembrane proteins improperly pump chloride ions across the cell membrane. This occurs primarily in cells lining the respiratory and digestive systems, and results in accumulation of chloride ions in cells and the production of very thick mucus.

Common ways in which HIV can be transmitted from an infected person to a non-infected person include: (a) the transfer of blood; (b) the transfer of semen; (c) the transfer of vaginal fluid; and (d) the transfer of milk, from an infected mother to her nursing child. In each case, to infect exposed cells, the virus must have access to cells that carry the CD4 marker.

None of these methods of transport require the expenditure of additional energy. All are limited to movement of substances down their concentration gradients, from regions of higher concentration to regions of lower concentration.

An isotonic solution has the same concentration of water as another solution—in this case, the contents of the cell. There is no net movement of water between the solution and the cell: the same amount of water that moves into the cell will also move out of the cell.

Energy is required to move substances against their concentration gradient.

The plasma membrane surrounds an object outside the cell, forming a pocket. The pocket and its contents are moved into the cell to form a vesicle.

Cells lining the small intestine have tight junctions between them, preventing the passage of bacteria and nutrients from the intestine into the body cavity.

(a) The nucleus controls the activities of the cell by controlling the type and quantity of molecules produced. (b) The nucleus acts as the cell’s genetic storehouse.

The three types of protein fibers making up the cytoskeleton are: microfilaments, solid rod-like fibers involved in cell contraction and cell division; intermediate filaments, a rope-like system of overlapping proteins that gives the cell great strength; and microtubules, thick, hollow tubes that act as tracks along which molecules and organelles move.

Most of the cell’s energy is produced in the mitochondria, which extract energy from food. This energy conversion requires oxygen.

Lysosomes contain acid and about 50 different enzymes, used to break down certain materials brought into the cell, unwanted materials, and worn-out organelles.

Smooth endoplasmic reticulum contains enzymes involved in detoxifying chemicals. This is one of the many important functions of the liver.

Proteins and lipids.