Cellular Level of Organization: Study Notes for Anatomy & Physiology

Chapter 3: The Cellular Level of Organization

Introduction to Cells

The cell is the smallest living unit in the body and forms the foundation of all living organisms. Understanding cell structure and function is essential for studying anatomy and physiology.

• Cell Theory: States that all organisms are composed of cells, and all cells arise from preexisting cells.

• Functions of Cells: Cells carry out essential life functions, including metabolism, growth, and reproduction.

• Types of Cells:

  • Sex cells (gametes): Sperm and oocytes involved in reproduction.

  • Somatic cells: All other body cells.

Plasma Membrane

The plasma membrane surrounds the cell, providing protection, sensitivity, and structural support. It regulates the movement of substances into and out of the cell.

• Structure: Composed of a phospholipid bilayer with embedded cholesterol, proteins (integral and peripheral), and carbohydrates (glycocalyx).

• Functions: Involved in anchoring, recognition, enzymatic activity, receptors, carriers, and channels.

• Example: Receptor proteins bind hormones to trigger cellular responses.

Organelles

Organelles are specialized structures within cells that perform distinct functions necessary for cellular life.

• Nonmembranous Organelles: Cytoskeleton, microvilli, centrioles, cilia, flagella, ribosomes, proteasomes.

• Membranous Organelles: Endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, mitochondria.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, packages, and ships proteins and lipids.

• Lysosomes: Break down cellular debris, fatty acids, and neutralize toxins.

• Peroxisomes: Detoxify alcohol and fatty acids, neutralize toxic compounds.

• Mitochondria: Site of ATP production via cellular respiration.

Nucleus

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

• Contents: DNA, nucleoplasm, nuclear envelope, nucleolus, chromatin/chromosomes.

• Functions: Stores genetic information, controls protein synthesis, and regulates cell cycle and division.

Genetic Control of the Cell

Genetic information stored in DNA is transcribed and translated to produce proteins, which determine cell structure and function.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

• Gene Expression: Not all genes are active in every cell; gene activation determines cell specialization.

Protein Synthesis

Protein synthesis involves two main processes: transcription and translation.

• Transcription: Occurs in the nucleus; DNA is transcribed to mRNA.

• Translation: Occurs in the cytoplasm; ribosomes read mRNA to assemble proteins.

• Mutations: Changes in DNA sequence can alter protein structure and function.

• Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Membrane Transport

Cells must transport substances across their membranes to maintain homeostasis. Transport can be passive or active.

• Passive Transport: Does not require energy; includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Requires energy (ATP); includes carrier-mediated transport, endocytosis, and exocytosis.

• Types of Transport:

  • Diffusion: Movement of molecules from high to low concentration.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

  • Facilitated Diffusion: Transport of substances via carrier proteins.

  • Active Transport: Movement against concentration gradient using energy (e.g., sodium-potassium pump).

  • Vesicular Transport: Includes endocytosis (phagocytosis, pinocytosis) and exocytosis.

• Example: The sodium-potassium pump maintains cellular ion balance by moving Na+ out and K+ into the cell.

Membrane Potential

Membrane potential is the unequal charge distribution across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential: Typically -70 mV in neurons, maintained by ion gradients and selective permeability.

• Equation:

$V_m = V_{inside} - V_{outside}$

Cell Life Cycle

The cell life cycle includes periods of growth, DNA replication, and division. Proper regulation ensures healthy tissue maintenance and repair.

• Interphase: Includes G0 (rest), G1 (growth), S (DNA replication), and G2 (protein synthesis).

• Mitosis: Division of the nucleus; phases include prophase, metaphase, anaphase, and telophase.

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

• Apoptosis: Programmed cell death, important for development and homeostasis.

Regulation of the Cell Cycle

Cell cycle progression is controlled by internal and external signals, ensuring proper cell division and function.

• Growth Factors: Stimulate cell division.

• Inhibitors: Repressor genes or short telomeres can inhibit cell division.

Cancer

Cancer results from uncontrolled cell division due to mutations in genes that regulate the cell cycle.

• Tumor Types: Benign (contained), malignant (invasive), and metastatic (spread to other tissues).

• Causes: Mutations in oncogenes, exposure to mutagens/carcinogens (radiation, chemicals, pathogens).

Differentiation

Differentiation is the process by which cells become specialized in structure and function.

• All cells have the same DNA, but different genes are activated or inactivated to produce specialized cell types.