Living things show the characteristics of (1) metabolic processes, (2) generative
processes, (3) responsive processes, (4) control processes, and (5) a
unique structural organization. The concept of the cell has developed over
a number of years. Initially, only two regions, the cytoplasm and the nucleus,
could be identified. At present, numerous organelles are recognized
as essential components of both major cell types, prokaryotic and eukaryotic.
The structure and function of some of these organelles are compared in table 20.3. This table also indicates whether the organelle is unique to prokaryotic or eukaryotic cells or found in both.
The cell is the common unit of life. We study individual cells and
their structures to understand how they function as individual living organisms
and as parts of many-celled beings. Knowing how prokaryotic and
eukaryotic cell types resemble or differ from each other helps physicians
control some organisms dangerous to humans.
In the process of respiration, organisms convert foods into energy
(ATP) and waste materials (carbon dioxide and water). Aerobic cellular
respiration uses oxygen (O2) in this biochemical pathway. This energyreleasing
process is composed of three stages: (1) glycolysis, (2) Krebs cycle,
and (3) the electron transport system. Plants use the products of
respiration in the photosynthesis pathway. Photosynthesis is comprised of
three stages: (1) light-capturing events, (2) light-dependent reactions, and
(3) light-independent reactions. Photosynthetic organisms carry out both
biochemical pathways. There is also a constant cycling of materials between
plants and animals. Sunlight supplies the essential initial energy
for making the large organic molecules necessary to maintain the forms
of life we know.All cells come from preexisting cells as a result of cell division. This
process is necessary for growth, repair, and reproduction. Eukaryotic cells
go through a cell cycle that includes cell division (mitosis and cytokinesis)
and interphase. Interphase is the period of growth and preparation for
division. Mitosis is divided into four stages: prophase, metaphase,
anaphase, and telophase. During mitosis, two daughter nuclei are
formed from one parent nucleus. These nuclei have identical sets of
chromosomes and genes that are exact copies of those of the parent. Although the process of mitosis has been presented as a series of phases,
you should realize that it is a continuous, flowing process from prophase
through telophase. Following mitosis, cytokinesis divides the cytoplasm,
and the cell returns to interphase.
