Introduction to Cells

The Cell Theory

The cell theory is a fundamental concept in biology that describes the properties of cells, the basic unit of life. It consists of three main principles:

• All living organisms are composed of cells.

• The cell is the basic structural and functional unit of life.

• All cells arise from pre-existing cells.

Exception: Viruses are considered acellular because they lack cellular structure and cannot carry out life processes independently.

Basic Parts of a Cell

Essential Components

All cells, regardless of type, share certain basic structures:

• Plasma Membrane: A selective barrier that surrounds the cell, controlling the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell where organelles are suspended.

• Genetic Material: DNA or RNA that contains the instructions for cellular activities.

Classification of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus:

• Prokaryotic Cells: Do not have a true nucleus; genetic material is not enclosed by a membrane. Example: Bacteria.

• Eukaryotic Cells: Have a true nucleus surrounded by a nuclear membrane. Examples: Plant and animal cells.

Cell Organelles

Main Organelles and Their Functions

Organelles are specialized structures within cells that perform distinct processes:

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Ribosomes: Synthesize proteins from amino acids.

• Endoplasmic Reticulum (ER): Involved in protein and lipid synthesis; can be rough (with ribosomes) or smooth (without ribosomes).

• Golgi Complex: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Contain digestive enzymes to break down waste.

• Centrioles: Involved in cell division in animal cells.

• Vacuoles: Store nutrients, waste products, and help maintain osmotic balance.

Types of Eukaryotic Cells

Plant vs. Animal Cells

• Plant Cells: Have a cell wall, chloroplasts for photosynthesis, and large central vacuoles.

• Animal Cells: Lack a cell wall and chloroplasts, but contain centrioles and smaller vacuoles.

Example: Plant cells can perform photosynthesis due to the presence of chloroplasts, while animal cells cannot.