Cell Biology: Structure and Function of Cells

Cell Size

Overview of Cell Size

Cells are the fundamental units of life, and their size varies greatly among different organisms. Most cells are microscopic and cannot be seen with the naked eye.

• Smallest cell: The bacterium Mycoplasma (~0.2 μm).

• Largest cell: Bird egg (e.g., ostrich egg).

• Relative size: Plant and animal cells are typically about 10 times larger than bacterial cells.

Restrictions on Cell Size

• Cells must be large enough to contain all necessary DNA, proteins, and internal structures for survival and reproduction.

• Cells must be small enough to efficiently exchange nutrients and wastes with their environment.

Surface Area to Volume Ratio

• The surface area to volume ratio is a key factor limiting cell size.

• Large cells have a smaller surface area relative to their volume compared to small cells.

• This ratio affects the rate of material exchange; smaller cells are more efficient at exchanging substances.

Example Table: Cell Size Comparison

Cell Type	Approximate Size
Bacterium (Mycoplasma)	0.2 μm
Typical Bacterial Cell	1-10 μm
Plant/Animal Cell	10-100 μm
Bird Egg	Several centimeters

Prokaryotic Cells vs Eukaryotic Cells

Classification of Cells

Cells are classified into two main groups based on their structural complexity and size: prokaryotes and eukaryotes.

• Prokaryotes: Includes bacteria and archaea.

• Eukaryotes: Includes protists, fungi, plants, and animals.

Common Features

• Both cell types are surrounded by a plasma membrane.

• Both contain chromosomes composed of DNA.

• Both have ribosomes for protein synthesis.

Key Differences

• Eukaryotes: Have a membrane-bound nucleus and membrane-bound organelles.

• Prokaryotes: Lack a nucleus and membrane-bound organelles.

Prokaryotic Cells

Structure and Features

Prokaryotic cells are generally much smaller and simpler than eukaryotic cells.

• Size: 1-10 μm in diameter (about 1/10 the size of a typical eukaryotic cell).

• Nucleoid: Region where DNA is located; not surrounded by a membrane.

• Chromosome: Usually a single, circular DNA molecule.

• Ribosomes: Smaller and structurally different from eukaryotic ribosomes; targeted by certain antibiotics.

Cell Wall and Capsule

• Cell wall: Provides protection and maintains cell shape; chemically complex.

• Capsule: Additional protective layer; helps bacteria adhere to surfaces (e.g., tissues, medical devices).

Surface Structures

• Pili: Surface projections aiding in attachment to surfaces and other bacteria.

• Flagella: Longer structures for motility; may be one or more per cell.

Example Table: Prokaryotic vs Eukaryotic Cell Features

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present (double membrane)
Organelles	Absent	Present (membrane-bound)
Chromosome Shape	Circular	Linear
Ribosome Size	Smaller (70S)	Larger (80S)

Eukaryotic Cells

Structure and Features

Eukaryotic cells are found in plants, animals, fungi, and protists. They are larger and more complex than prokaryotic cells.

• Nucleus: Contains DNA; surrounded by a double membrane; includes a nucleolus (site of ribosome synthesis).

• Chromosomes: Multiple, linear DNA molecules.

• Organelles: Specialized, membrane-bound structures with distinct functions (e.g., mitochondria, endoplasmic reticulum).

Plant vs Animal Cells

• Plant cells: Have a rigid cell wall, chloroplasts (site of photosynthesis), and a large central vacuole; lack lysosomes and centrioles.

• Animal cells: Lack a cell wall and chloroplasts; have lysosomes and centrioles.

Example Table: Plant vs Animal Cell Features

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large	Small or absent
Lysosomes	Absent	Present
Centrioles	Absent	Present

Additional info:

• Surface area to volume ratio formula: $\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Ribosome sizes: Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits), eukaryotic ribosomes are 80S (composed of 60S and 40S subunits).

• Antibiotics can target bacterial ribosomes due to structural differences, minimizing harm to human (eukaryotic) cells.