Introduction to Cells: Structure, Types, and Functions

Intro to Cells

Overview

Cells are the fundamental units of life, forming the basis of all living organisms. This section introduces the major types of cells, their structures, and their similarities and differences, providing a foundation for understanding cell biology.

Major Types of Cells

Prokaryotic Cells

Prokaryotic cells are the simplest and most ancient forms of life. They lack membrane-bound organelles and a nucleus.

• Small and simple structure

• No "membrane-bound" regions

• No nucleus

• Domains: Archaea and Bacteria

Example: Escherichia coli (E. coli) is a common prokaryotic bacterium.

Eukaryotic Cells

Eukaryotic cells are larger and more complex than prokaryotic cells. Their interiors are divided into functional compartments by internal membranes.

• Small to relatively large

• Complex structure

• Cell interior divided into organelles

• DNA bound by membrane: nucleus

• Main domain: Eukarya

Example: Animal and plant cells are eukaryotic.

Prokaryotic and Eukaryotic Cells: Similarities

Universal Features of Cells

Despite their differences, all cells share several key features:

• Plasma membrane: Acts as a barrier and controls entry/exit of substances. Composed of phospholipids, other lipids, and proteins.

• Cytoplasm: Semifluid mixture containing cell components. Cytosol refers to just the fluid and dissolved substances.

• DNA: Genetic material. Located in the nucleoid region in prokaryotes and in the nucleus in eukaryotes.

• Ribosomes: Structures that synthesize proteins.

Additional info: All cells also require energy and have mechanisms for growth and reproduction.

Comparison of Prokaryotic and Eukaryotic Cells

Structural Differences

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Organelles	Absent	Present (e.g., mitochondria, ER)
Size	Small (0.1–5 μm)	Larger (10–100 μm)
Domains	Bacteria, Archaea	Eukarya

Additional info: Prokaryotes often have cell walls made of peptidoglycan (bacteria) or pseudopeptidoglycan/proteins (archaea), while eukaryotic plant cells have cellulose-based walls.

Key Cell Structures and Functions

Plasma Membrane

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Composed of: Phospholipid bilayer, proteins, cholesterol (in animal cells)

• Functions: Protection, communication, transport

Cytoplasm and Cytosol

The cytoplasm includes all cellular contents between the plasma membrane and the nucleus. The cytosol is the fluid portion.

• Cytoplasm: Contains organelles and cytosol

• Cytosol: Watery solution of ions, proteins, and other molecules

DNA Location

• Prokaryotes: DNA in nucleoid region (not membrane-bound)

• Eukaryotes: DNA enclosed in nucleus

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Composed of: rRNA and proteins

• Function: Translate mRNA into polypeptides

Example: Free ribosomes in cytosol and bound ribosomes on rough ER in eukaryotes.

Summary Table: Universal Cell Features

Feature	Description
Plasma Membrane	Phospholipid bilayer controlling entry/exit
Cytoplasm	Semifluid matrix with organelles (eukaryotes)
DNA	Genetic material (nucleoid or nucleus)
Ribosomes	Protein synthesis

Key Terms

• Prokaryote: Cell lacking a nucleus and membrane-bound organelles

• Eukaryote: Cell with a nucleus and membrane-bound organelles

• Plasma Membrane: Outer boundary of the cell

• Cytoplasm: Internal region containing cytosol and organelles

• Ribosome: Organelle for protein synthesis

Review Questions

• What features do all cells share?

• How do prokaryotic and eukaryotic cells differ?

• What is the function of the plasma membrane?

• Where is DNA located in prokaryotic vs. eukaryotic cells?