The Unit of Biological Organization: The Cell

Genetic Continuity and the Genetic Program

Cells are the fundamental units of life, and their continuity is maintained through a genetic program. This program ensures that organisms are genetically continuous with their ancestors and one another.

• Genetic Program: The set of instructions encoded in DNA that governs cellular structure and function.

• Proteins: Macromolecules coded by genes, synthesized from amino acids connected in a specific sequence.

• Amino Acids: There are 20 naturally occurring amino acids, each represented by a unique three-letter and single capital letter code.

Example: The first 30 amino acids of β-hemoglobin protein are conserved among vertebrates, showing homology.

Additional info: Sequence divergence among species demonstrates evolutionary adaptation and limited homology.

Sequence Divergence and Evolution

Proteins and genes that change at a relatively fast rate demonstrate life's ability to adapt and promote survival.

• Sequence Divergence: Variation in genetic or protein sequences among species.

• Homology: Similarity in sequence due to shared ancestry.

• Evolutionary Adaptation: Changes in genetic program to improve survival.

Molecular Tree of Life

Classification Based on rRNA Sequence

The molecular tree of life classifies cellular life into three domains based on genetic similarity:

• Bacteria (Prokaryotes): Most life forms are microbial.

• Archaea: Share more similarities with eukaryotes than bacteria.

• Eukaryotes: More complex, likely originated from multiple endosymbiotic events.

Endosymbiosis: The process by which organelles like mitochondria and chloroplasts originated from free-living bacteria.

Common Features of All Cell Types

• Phospholipid Bilayer: Separates chemical reactions inside the cell from the external environment.

• Self-Assembly: Phospholipids spontaneously form bilayers in water, creating a barrier for polar molecules.

Cellular Compartmentalization and Seclusion

Physical and Conditional Isolation of Reactions

Cell structures, such as cell walls and membranes, allow for the isolation and regulation of metabolic reactions.

• Seclusion: Reactions can be isolated and promoted conditionally, occurring only when needed.

• Compartmentalization: Lipid bilayers prevent diffusion of polar molecules, maintaining distinct internal environments.

Homeostasis and Steady State

Maintaining Internal Stability

Cells and organisms maintain a steady state or homeostasis, resisting equilibrium to keep conditions stable and constant.

• Homeostasis: Regulation of the internal environment to maintain stable conditions.

• Negative Feedback: Mechanism that accelerates response toward homeostatic condition, preventing overshoot.

Blood Glucose Regulation Example

Blood glucose levels are regulated by hormones and organ responses to maintain homeostasis.

• Hyperglycemia: High blood glucose triggers insulin secretion and glucose uptake/storage.

• Hypoglycemia: Low blood glucose triggers glucagon secretion and glucose release.

Cellular Energy and Thermodynamics

Energy Requirement for Life

Life requires and processes energy to maintain order and complexity, dissipating available energy into the environment.

• Energy: The capacity to do work; cells use energy to maintain homeostasis and drive metabolic reactions.

• Thermodynamics: Principles governing energy flow and transformation in biological systems.

Example Equation:

Where is the change in free energy, is the change in enthalpy, is temperature, and is the change in entropy.

Regeneration and Renewal in Cells

Autopoiesis: Making Oneself from Within

Cells constantly interact with their environment, selecting, disposing, and renewing molecules to maintain function.

• Selection: Maintain concentrations of necessary molecules inside the cell.

• Disposal: Remove or turn over damaged or unnecessary parts.

• Renewal: Synthesize new molecules for function or renewal from internal materials.

Additional info: This process is termed autopoiesis, a key property of living systems.

Summary Table: Key Properties of Living Cells