Cell Theory and the Structure of Cells

Cell Theory

Foundations of Cell Theory

The cell theory is a fundamental concept in biology that describes the properties and significance of cells in living organisms. It provides the basis for understanding the structure and function of all living things.

• Cells are the fundamental unit of structure and function in living organisms.

• All living things are made up of cells.

• Cells are basically the same in chemical composition.

• Organisms can be unicellular (composed of a single cell) or multicellular (composed of many cells).

Additional Principles of Cell Theory

• All cells come from pre-existing cells by division.

• Cells contain hereditary information (DNA), which is passed from cell to cell during division.

• All energy flow (metabolism and biochemistry) occurs within cells.

• The activity of an organism depends on the total activity of its independent cells.

Cell Size and Microscopy

Relative Sizes of Cells and Cellular Structures

Cells and their internal structures vary greatly in size, and different tools are required to observe them. The following scale illustrates the range of sizes and the appropriate instruments for viewing:

• Unaided eye: Can see objects down to about 0.1 mm (e.g., chicken egg, frog egg).

• Light microscope: Used to view most plant and animal cells, nuclei, and some bacteria (down to about 0.2 μm).

• Electron microscope: Required to see smaller structures such as viruses, ribosomes, proteins, and molecules (down to about 0.1 nm).

Examples: Human egg (~100 μm), most bacteria (1–10 μm), viruses (20–300 nm), ribosomes (~20 nm), proteins (a few nm).

Metric Units and Prefixes

Understanding cell size requires familiarity with metric units and their prefixes. The following table summarizes common metric prefixes used in biology:

Prefix	Symbol	Value
Giga	G	1,000,000,000
Mega	M	1,000,000
Kilo	k	1,000
Base Unit	-	1
Centi	c	0.01
Milli	m	0.001
Micro	μ	0.000001
Nano	n	0.000000001
Pico	p	0.000000000001

Why Are Cells So Small?

Surface Area to Volume Ratio

Cells are small because their surface area must be large enough relative to their volume to allow efficient exchange of materials with their environment. As a cell grows, its volume increases faster than its surface area, which limits the rate at which substances can enter or leave the cell.

• Cells must interact with their surroundings: Nutrients and gases must be absorbed, and waste products eliminated, all through the cell membrane.

• Internal interactions: Substances within cells must interact efficiently, which is easier in smaller cells.

Surface Area and Volume Calculations:

• For a cube of side length l:

  • Surface area = $6l^2$

  • Volume = $l^3$

  • Surface area to volume ratio = $\frac{6l^2}{l^3} = \frac{6}{l}$

• As l increases, the ratio decreases, making exchange less efficient.

Example: A cell with a side length of 1 mm has a surface area of 6 mm2 and a volume of 1 mm3, giving a ratio of 6:1. If the side length increases to 2 mm, the surface area is 24 mm2 and the volume is 8 mm3, so the ratio drops to 3:1.

Types of Cells: Prokaryotic and Eukaryotic

General Features of All Cells

Despite their diversity, all cells share certain basic features:

• Plasma membrane: Encloses the cell and regulates the passage of materials.

• Cytosol: Semi-fluid substance within the cell.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structure and complexity:

Feature	Prokaryotic Cells	Eukaryotic Cells
Domains	Bacteria, Archaea	Protists, Fungi, Plants, Animals
Nucleus	No true nucleus; DNA in nucleoid	True nucleus, DNA enclosed by nuclear envelope
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Chromosomes	Single, circular	Multiple, linear
Size	1–5 μm	10–100 μm
Examples	Escherichia coli (E. coli)	Human epithelial cell

Additional info: Eukaryotic cells are often compartmentalized with organelles, allowing for specialized functions within the cell.