Chapter 3.  How Cells are Put Together, part one 

I. Introduction & Historical Perspective 

A.  Early concepts – late 1600’s.  Anton van Leeuwenhoek was a Dutch shopkeeper, discovered microscopic life.  Robert Hooke was an English scientist who coined term “cell” for empty compartments in cork.  Spontaneous generation, now an outdated theory that microbes (i.e. cells) ‘generate’ or arise from nonliving matter, was a widely accepted explanation for the occurrence of certain forms of life (especially microbes).  The theory of spontaneous generation stated that a vital force was required to create life from nonliving matter. 

B.     The Cell Theory – 1800’s, three Germans:  Schleiden (botanist) and Schwann (zoologist) state that all animals and plants are composed of cells; Virchow states that cells come from preexisting cells, no vital force required, the mechanical process of cell division explains the occurrence of new cells.  [Science can only investigate the nature of life from the philosophical viewpoint that only mechanical processes as measured by empirical data are accepted as explanations.  Life is reduced to chemical reactions.  “The brain secretes thought as the kidney secretes urine.”  Advancements in our understanding of thought will indeed be made through continued chemical investigation of brain physiology.  Mechanism and reductionism are characteristics of science.]     

• All life is composed of cells

• The smallest living unit is the cell

• Cells arise from preexisting cells

II.  Cell  Images, shapes, & form.

·         Living cells have at least 3 parts:  plasma membrane, cytoplasm, and genetic material (DNA).  Cells are the fundamental unit of life, they are the smallest structures that are alive.

·         Microscopes produce micrographs (images taken through microscopes).  Scanning electron mircrographs (SEM’s) show surface detail while transmission electron micrographs (TEM’s) show cytoplasmic detail.  See Fig. 3.3, p. 40.

·         Cells vary tremendously in shape.  Flat cells line you mouth.  Cuboidal cells line the intestines.  Nerve cells are long and branched.  Red blood cells are doughnut shaped.  White blood cells can be amoeboid.

·         Similar cells joined together form tissues (epithelial tissue lines the mouth).  Histology is the study of tissues and cell types.

·         Many cells function when dead, e.g. out skin cells, epithelial check cells, and human hair are made of dead cells.  Heartwood in a tree trunk is made of dead cells.

·         Cells secrete materials for external support and protection, e.g. plant, fungi, and bacteria cells secrete the chemicals that collect outside of the plasma membrane and form the cell wall.  In fungi chitin is secreted and forms the cell wall.  In plants cellulose is secreted and forms a strong yet absorbent cell wall.  Some plant cell walls have an added material called lignin that makes the wall hard as in wood from a tree. Bone and cartilage have scattered living cells in a non-cellular matrix.  Seashells are chemical deposits secreted by cells. 

III.  Why are cells small?

·         Cells are small because they must be efficient in the uptake and removal of chemicals.  Larger bodies are less efficient in exchange rates because they have less surface area relative to their volume.  Small bodies have more surface area relative to their volume.  This is expressed mathematically as the fractional relationship of surface area (SA) / volume (V) or SA/V.  Given the same shape, a smaller object will have a larger SA/V and than a larger object.

·         Many cells have modified shapes that further increase their surface area, for example, cells that line your intestines have  elongate, finger-like projections that greatly increase their surface area available for nutrient absorption.