Chapter 2: Cells – The Living Units

2.1 Overview of Cells

Cells are the fundamental units of life, forming the basis of all living organisms. This section introduces the history of cell theory, the scientists involved, and the three major regions common to all cells.

• Definition of a Cell: The smallest structural and functional unit of an organism, capable of performing all the basic activities necessary for life.

• Cell Theory: Developed by scientists such as Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, stating that all living things are composed of cells, and all cells arise from pre-existing cells.

• Three Major Regions of a Cell:

  • Plasma Membrane: The outer boundary that separates the cell from its environment.

  • Cytoplasm: The internal fluid and organelles between the plasma membrane and nucleus.

  • Nucleus: The control center containing genetic material (DNA).

• Basic Activities of Cells: Metabolism, growth, response to stimuli, reproduction, and homeostasis.

Example: Human muscle cells contract to produce movement, demonstrating specialized cell function.

2.2 The Plasma (Cell) Membrane

The plasma membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell. Its structure and function are closely related, reflecting the principle that anatomy determines physiology.

• Composition:

  • Lipid Bilayer: Primarily phospholipids, with embedded cholesterol and glycolipids.

  • Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

  • Carbohydrates: Attached to lipids (glycolipids) and proteins (glycoproteins), contributing to cell recognition.

• Functions:

  • Physical barrier separating internal and external environments.

  • Regulates transport of substances (selective permeability).

  • Facilitates communication via receptors.

  • Supports cell adhesion and structural integrity.

• Membrane Transport Mechanisms:

  • Passive Transport: Does not require energy; includes diffusion (simple and facilitated) and osmosis.

  • Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Vesicular Transport: Involves endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.

Key Concepts:

• Transport is described by direction (into or out of cell), energy requirement, and involvement of membrane proteins.

• Concentration gradients drive passive transport; active transport moves substances against these gradients.

Example: Sodium-potassium pump (Na+/K+ ATPase) actively transports sodium out and potassium into the cell.

Additional info: The four basic macromolecules in the membrane are lipids, proteins, carbohydrates, and nucleic acids (though nucleic acids are not a major membrane component).

2.3 The Cytoplasm

The cytoplasm is the cellular region between the plasma membrane and the nucleus, containing cytosol, organelles, and inclusions. Each organelle has a unique structure and function essential for cell survival.

• Cytosol: The fluid portion containing dissolved ions, nutrients, and proteins.

• Cytoplasmic Organelles:

  • Ribosomes: Sites of protein synthesis; can be free or bound to rough ER.

  • Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

    • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

  • Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

  • Mitochondria: Powerhouse of the cell; site of ATP production via aerobic respiration.

  • Peroxisomes: Break down fatty acids and detoxify harmful substances.

  • Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structural support and facilitating movement.

  • Centrosome and Centrioles: Organize microtubules and play a role in cell division.

• Cytoplasmic Inclusions:

  • Glycosomes: Store glycogen and enzymes for its breakdown.

  • Lipid Droplets: Store neutral fats for energy reserves.

Example: Liver cells contain abundant smooth ER for detoxification and glycogen storage in glycosomes.

2.4 The Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities such as growth, metabolism, and reproduction.

• Nuclear Envelope: Double membrane with nuclear pores, regulating entry and exit of materials.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Chromatin: DNA-protein complex; exists as extended (active, accessible for transcription) or condensed (inactive, tightly packed) forms.

• Chromosomes: Condensed chromatin visible during cell division.

• Functions:

  • Controls gene expression and mediates the replication of DNA during the cell cycle.

  • Separates genetic material from the cytoplasm, protecting DNA integrity.

Key Concepts:

• DNA serves two main purposes: replication (for cell division) and transcription (for gene expression).

• Condensed chromatin is associated with cell division; extended chromatin is associated with active gene expression.

Example: Muscle cells have multiple nuclei to support high protein synthesis demands.

2.5 The Cell Life Cycle

The cell life cycle encompasses the sequence of events from one cell division to the next, including periods of growth, DNA replication, and division.

• Phases of the Cell Life Cycle:

  • Interphase: Period of cell growth and DNA replication; subdivided into G1 (growth), S (DNA synthesis), and G2 (preparation for division).

  • Mitotic Phase (M Phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Key Events:

  • Prophase: Chromatin condenses into chromosomes; spindle apparatus forms.

  • Metaphase: Chromosomes align at the cell's equator.

  • Anaphase: Sister chromatids separate and move to opposite poles.

  • Telophase: Nuclear envelopes reform; chromosomes decondense.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Example: Skin cells undergo frequent cell division to replace lost or damaged cells.

Additional info: In-depth details such as kinetochore microtubules are more relevant to advanced genetics courses.

2.6 Developmental Aspects of Cells

Cell differentiation is the process by which cells become specialized in structure and function, despite all cells containing the same DNA. This diversity arises from selective gene expression.

• Central Dogma of Biology: DNA → RNA → Protein; different proteins are produced in different cell types due to gene activation.

• Functional Cell Groups: Cells are classified into groups based on their function, such as epithelial, connective, muscle, nervous, reproductive, and immune cells.

• Relationship of Shape to Function: Cell shape is closely related to its specialized role (e.g., red blood cells are biconcave for gas exchange; neurons have long processes for signal transmission).

Example: Neurons have long axons to transmit electrical impulses over distances.

Table: Major Cell Organelles and Their Functions