d. Nucleus. Within the cell is the nucleus. This structure has a nuclear
membrane separating it from the cytoplasm. Within the nucleus is the chromatin
material, made up of the protein deoxyribonucleic acid (DNA). At the time of cell
division, this chromatin material is aggregated into individual structures known as
chromosomes. Each chromosome has a set of specific genes, which determine all of
the physical and chemical characteristics of the body, which represent its structure and
a. We mentioned in lesson 1 that the human body depended upon external
sources for energy. Plants use solar radiation to make glucose and other nutrients.
The human body takes glucose and other nutrients directly or indirectly from plants.
The body receives oxygen from the air. The energy that was once derived by plants
from solar radiation is released within human cells by the process of metabolic
oxidation. This involves the combination of glucose and other nutrients with oxygen,
releasing the stored energy.
b. The mitochondria of the cells use this released energy to form ATP molecules
from ADP molecules. Adenosine diphosphate is converted to ATP by the addition of a
"part of a molecule" called a phosphate radical. The binding of this phosphate radical
requires a large quantity of energy, which can be released later when the phosphate
radical is separated off. Adenosine triphosphate provides energy for cellular processes
such as active transport of substances across membranes, synthesis of chemical
compounds for the body, and mechanical work (such as muscle contraction). When an
Adenosine triphosphate molecule provides energy for such a process, it loses a
phosphate radical and becomes ADP. Then, the cycle begins again as ADP is
converted into ATP within the mitochondria.
c. Certain cells, such as muscle cells and nerve cells, require great amounts of
energy. Such cells have well-developed mitochondria.
Section II. BODY FLUIDS
Approximately 56 percent of the human body consist of fluids. Soft tissues
consist almost completely of fluids. These body fluids are composed largely of water.
Thus, water is the major component of living substances.