Biology (Holt)

The Cell Theory

It took scientists more than 150 years to fully appreciate the discov-
eries of Hooke and Leeuwenhoek. In 1838, the German botanist
Mattias Schleiden concluded that cells make up not only the stems
and roots but every part of a plant. A year later, the German zoolo-
gist Theodor Schwann claimed that animals are also made of cells.
In 1858, Rudolph Virchow, a German physician, determined that
cells come only from other cells. The observations of Schleiden,
Schwann, and Virchow form the ,which has three parts:

1.All living things are made of one or more cells.

2. Cells are the basic units of structure and function in organisms.
   3.All cells arise from existing cells.

Cell Size

Small cells function more efficiently than large cells. There are
about 100 trillion cells in the human body, most ranging from 5 μm
to 20 μm in diameter. What is the advantage of having so many tiny
cells instead of fewer large ones? All substances that enter or leave
a cell must cross that cell’s surface. If the cell’s surface area–to-
volume ratio is too low, substances cannot enter and leave the cell in
numbers large enough to meet the cell’s needs. Small cells can
exchange substances more readily than large cells because small
objects have a higher surface area–to-volume ratio than larger
objects, as shown in Table 2.As a result, substances do not need to
travel as far to reach the center of a smaller cell.

cell theory

Cell Features

SECTION 2 Cell Features 55

Section 2

Objectives

Listthe three parts of the

cell theory.

Determinewhy cells must

be relatively small.

Comparethe structure of

prokaryotic cells with that

of eukaryotic cells.

Describethe structure of

cell membranes.

Key Terms

cell theory

cell membrane

cytoplasm

cytoskeleton

ribosome

prokaryote

cell wall

flagellum

eukaryote

nucleus

organelle

cilium

phospholipid

lipid bilayer

Side length Surface area Volume Surface area/volume ratio

1 mm 6 mm^2 1 mm^3 6:1

2 mm 24 mm^2 8 mm^3 3:1

4 mm 96 mm^2 64 mm^3 3:2

1 mm

2 mm

4 mm

Table 2 Relationship Between Surface Area and Volume

4A 4B

4B

4B

4B