The Cell The Basic Unit of Life

Just as atoms are the units of chemistry, cells are the building blocks of life. Three statements constitute the cell theory:

► Cells are the fundamental units of life.

► All organisms are composed of cells.

► All cells come from preexisting cells.

Cells are composed of water molecules and the small and large molecules we examined in the previous two chapters. Each cell contains at least 10,000 different types of molecules, most of them present in many copies. Cells use these molecules to transform matter and energy, to respond to their environment, and to reproduce themselves.

The cell theory has three important implications. First, it means that studying cell biology is in some sense the same as studying life. The principles that underlie the functions of the single cell in a bacterium are similar to those governing the 60 trillion cells of your body. Second, it means that life is continuous. All those cells in your body came from a single cell, the fertilized egg, which came from the fusion of two cells, a sperm and an egg from your parents, whose cells came from their fertilized eggs, and so on. Finally, it means that the origin of life on Earth was marked by the origin of the first cells.

Cells may have come from stable bubbles

Isolation from the general environment can be achieved in the laboratory within aggregates produced from molecules made in prebiotic synthesis experiments. Called protobionts, these aggregates cannot reproduce, but they can maintain internal chemical environments that differ from their sur-

Protobionts

90 nm

4.1 Protobionts These aggregates, made by agitating a solution of macromolecules, are chemical compartments, can perform some metabolic reactions, and can exchange materials with their environ-ment.They are a model of how cells may have originated.

90 nm

4.1 Protobionts These aggregates, made by agitating a solution of macromolecules, are chemical compartments, can perform some metabolic reactions, and can exchange materials with their environ-ment.They are a model of how cells may have originated.

roundings. Under the microscope, they look a lot like tiny cells (Figure 4.1).

In the 1920s, Russian scientist Alexander Oparin mixed a large protein and a polysaccharide in solution. When he agitated this mixture, bubbles formed. He could also do this with other polymers. The interiors of these bubbles had much higher concentrations of the macromolecules than their surroundings. Moreover, they catalyzed chemical reactions, and they had some control over what left them and crossed the boundary into the environment. In other words, they were protobionts. Later, other researchers showed that if lipids are mixed in an aqueous environment, they spontaneously arrange themselves into droplets surrounded by a bilayer.

Taken together with the prebiotic chemistry models and RNA world hypothesis described in Chapter 3, these experiments suggest a bubble theory for the origin of cells.

Cell size is limited by the surface area-to-volume ratio

Most cells are tiny. The volume of cells ranges from 1 to 1,000 |im3 (Figure 4.2). The eggs of some birds are enormous exceptions, to be sure, and individual cells of several types of algae and bacteria are large enough to be viewed with the unaided eye. And although neurons (nerve cells) have a volume that is within the "normal" cell range, they often have fine projections that may extend for meters, carrying signals from one part of a large animal to another. But by and large, cells are minuscule. The reason for this relates to the change in the surface area-to-volume ratio (SA/V) of any object as it increases in size.

As a cell increases in volume, its surface area also increases, but not to the same extent (Figure 4.3). This phenomenon has great biological significance for two reasons:

► The volume of a cell determines the amount of chemical activity it carries out per unit of time.

► The surface area of a cell determines the amount of substances the cell can take in from the outside environment and the amount of waste products it can release to the environment.

As a living cell grows larger, its rate of waste production and its need for resources increase faster than its surface area. This explains why large organisms must consist of many small cells: Cells are small in volume in order to maintain a large surface area-to-volume ratio.

In a multicellular organism, the large surface area represented by the multitude of small cells that make up the organism enables it to carry out the multitude of functions required for survival. Special structures transport food, oxygen, and waste materials to and from the small cells that are distant from the external surface of the organism.

This scale is logarithmic. Each unit is ten times bigger than the previous unit.

0.1 nm 1 nm 10 nm 100 nm 1 |m 10 |m 100 |m 1 mm 1 cm 0.1 m 1 m III. I I I.MM.ll . I I I I I I I I I I ■ ■■ ■■■ I I

Light microscope

Electron microscope

Atoms

Lipids

T2 phage

Chloroplast

Protein

Small molecules r»f

Most bacteria r»f

Most bacteria

Plant and animal cells

4.2 The Scale of Life This scale shows the relative sizes of molecules, cells, and multicellular organisms.

Most cell diameters are in the range of 1-100 |m.

4.2 The Scale of Life This scale shows the relative sizes of molecules, cells, and multicellular organisms.

1-mm cube

2-mm cube

4-mm cube

2-mm cube

4-mm cube

Surface area

6 sides x l2

6 sides x 22

6 sides x 42

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Responses

  • Sirpa Sievinen
    Who is credited with the statement, "cells are the fundamental units of life."?
    8 years ago

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