Chapter 3: Cells – The Living Units Learning Objectives

Cells: The Living Units

Definition and Functions of Cells

Cells are the fundamental structural and functional units of all living organisms. Each cell is the smallest unit that exhibits all the characteristics of life, including metabolism, growth, response to stimuli, and reproduction.

• Cell Metabolism and Energy Use: Cells carry out chemical reactions to obtain and use energy necessary for survival and function.

• Synthesis of Molecules: Cells synthesize a wide variety of molecules, including proteins, lipids, nucleic acids, and carbohydrates, essential for structure and function.

• Communication: Cells communicate by producing and receiving electrical and chemical signals, allowing coordination within tissues and organs.

• Reproduction and Inheritance: Each cell contains DNA, the hereditary material. Some cells are specialized as gametes (sperm and egg) for sexual reproduction.

Cell Theory

The cell theory is a foundational concept in biology, emphasizing the central role of cells in life processes.

• Structural and Functional Unit: A cell is the basic structural and functional unit of living organisms.

• Organism Function: The overall function of an organism depends on the individual and collective activities of its cells.

• Complementarity of Structure and Function: The biochemical functions of cells are determined by their shapes and specific subcellular structures.

• Continuity of Life: All cells arise from preexisting cells, ensuring the continuity of life.

Cell Diversity

Human bodies contain over 200 different types of cells, each specialized for particular functions. These differences in size, shape, and subcellular components enable the wide variety of physiological roles necessary for life.

• Examples of Specialized Cells:

  • Fibroblasts, Erythrocytes, Epithelial cells: Connect body parts, form linings, or transport gases.

  • Skeletal muscle cells, Smooth muscle cells: Move organs and body parts.

  • Fat cells: Store nutrients.

  • Macrophages: Fight disease.

  • Nerve cells: Gather information and control body functions.

  • Sperm cells: Involved in reproduction.

Generalized Cell Structure

Despite their diversity, all eukaryotic cells share certain structural features. Human cells, as eukaryotes, have three main components:

1. Plasma Membrane: A flexible outer boundary that separates the cell's internal environment from the external environment.

2. Cytoplasm: The intracellular fluid containing organelles, which are specialized structures that perform distinct cellular functions.

3. Nucleus: The control center of the cell, containing DNA and directing cellular activities.

Additional info: Later sections of this chapter typically cover the detailed structure and function of each of these components, including the plasma membrane's composition, organelle functions, and the role of the nucleus in gene expression and cell division.