The Human Cell
Cells are the very essence of life.
Cells may be defined as the basic living, structural, and functional
units of the human body. They vary greatly in size, chemical
composition, and function, but each one is a remarkable miniaturization
of human life. Cells move, grow, ingest food and excrete wastes, react
to their environment, and even reproduce. This chapter provides a brief
review of the basics of a cell, including cellular components,
communication, energy, and transport. An overview of the natural life
span of a typical cell is provided because of its importance in
nutrition and disease.
multicellular organisms
Cells of all multicellular organisms are called eukaryotic cells (from
the Greek eu, meaning “true,” and karyon, “nucleus”). Eukaryotic cells
evolved from simpler, more primitive cells called prokaryotic cells
(from the Greek meaning “before nucleus”).The major distinguishing
feature between the two cell types is that eukaryotic cells possess a
defined nucleus, whereas prokary-otic cells do not. Also, eukaryotic
cells are larger and much more complex structurally and functionally
than their ancestors.
cell specialization
Specialization among cells is a necessity for the living, breathing
human, but cells in general have certain basic similarities. All human
cells have a plasma membrane and a nucleus (or have had a nucleus), and
most contain an endoplasmic reticulum, Golgi apparatus, and
mitochondria. Considering the relationship between the normal
functioning of a typical cell and the health of the total organism the
human being brings to mind the old rule: “A chain is only as strong as
its weakest link.”
Components/Parts of the human cell
Cell membrane or plasma membrane
Cells are surrounded by a phospholipid bilayer that contains embedded
proteins, carbohydrates, and lipids. Membrane proteins act as receptors
sensitive to external stimuli and channels that regulate the movement
of substances into and out of the cell
Golgi apparatus
The Golgi apparatus is a series of membrane sacks that process and
package proteins after they leave the rough endoplasmic reticulum.
Lysosome
Contains digestive enzymes that break up proteins, lipids, and nucleic acids. They also remove and recycle waste products.
Cytoplasm
The cytoplasm is the gel-like substance inside cells. Cytoplasm
contains cell organelles, protein, electrolytes, and other molecules.
Nucleus
The nucleus contains the DNA in the cell. Molecules of DNA provide coded instructions used for protein synthesis.
Mitochondrion
Organelles that produce most of the energy (ATP) used by cells.
Rough endoplasmic reticulum
A series of membrane sacks that contain ribosomes that build and process proteins.
Smooth endoplasmic reticulum
Region of the endoplasmic reticulum involved in lipid synthesis. Smooth
endoplasmic reticula do not have ribosomes and are not involved in
protein synthesis.
