Chapter 4: A Tour of the Cell

Overview: The Fundamental Units of Life

Cells are the basic structural and functional units of all living organisms. Despite their diversity, all cells share certain common features and are related by descent from earlier cells.

• All organisms are made of cells

• The cell is the simplest collection of matter that can be alive

• Cells are related by their descent from earlier cells

• Cells can differ substantially from one another but share common features

Concept 4.1: Biologists Use Microscopes and the Tools of Biochemistry to Study Cells

Most cells are too small to be seen by the unaided eye. Microscopes and biochemical techniques are essential for studying cell structure and function.

• Microscopy allows visualization of cell structures at various scales

• Light microscopes can magnify objects up to about 1000 times

• Electron microscopes provide much higher resolution, revealing ultrastructure

Figure 4.2: The Size Range of Cells

Cells and their components vary greatly in size, from small molecules to large eggs.

• Most plant and animal cells: 10–100 μm

• Bacteria: 1–5 μm

• Viruses: 20–300 nm

• Proteins and lipids: 1–10 nm

Concept 4.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cells are classified as either prokaryotic or eukaryotic. Internal membranes in eukaryotic cells create compartments for specialized functions.

• Prokaryotic cells: Domains Bacteria and Archaea

• Eukaryotic cells: Protists, fungi, animals, and plants

Comparing Prokaryotic and Eukaryotic Cells

All cells share certain basic features, but differ in organization and complexity.

• Basic features of all cells:

  • Plasma membrane

  • Semifluid substance called cytosol

  • Chromosomes (carry genes)

  • Ribosomes (make proteins)

• Eukaryotic cells:

  • Most DNA in the nucleus (double membrane-bound organelle)

  • Membrane-bound organelles

  • Cytoplasm between plasma membrane and nucleus

• Prokaryotic cells:

  • No nucleus

  • DNA in an unbound region called the nucleoid

  • No membrane-bound organelles

  • Cytoplasm bound by plasma membrane

Table: Comparison of Prokaryotic and Eukaryotic Cells

Additional info:

• Surface area-to-volume ratio is critical for cell function; as cell size increases, volume grows faster than surface area, limiting efficient exchange of materials.

• Biological membranes are composed of a double layer of phospholipids, forming selective barriers.