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The Cellular Level of Organization

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They perform essential physiological functions and maintain homeostasis at the cellular level. The study of cells is known as cytology, a branch of cell biology.

• Sex cells (germ cells): Reproductive cells such as sperm (male) and oocytes (female).

• Somatic cells: All other body cells except sex cells.

Cell Theory

• All living organisms are composed of cells.

• Cells arise from the division of preexisting cells.

• Cells are the smallest units that perform all vital physiological functions.

• Each cell maintains homeostasis at the cellular level.

Structure of a Typical Cell

Major Components

• Plasma membrane: Separates the cytoplasm from the extracellular fluid and regulates exchange with the environment.

• Cytoplasm: Contains cytosol (intracellular fluid) and organelles.

• Nucleus: The control center of the cell, containing genetic material.

Plasma Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Functions: Physical isolation, regulation of exchange, sensitivity to the environment, and structural support.

• Phospholipid bilayer: Hydrophilic heads face outward; hydrophobic tails face inward, forming a barrier to ions and water-soluble compounds.

• Proteins: Integral (within the membrane) and peripheral (bound to surfaces). Functions include anchoring, recognition, enzymatic activity, receptor binding, carrier transport, and channel formation.

• Carbohydrates: Glycoproteins, glycolipids, and proteoglycans form the glycocalyx, which provides lubrication, protection, anchoring, and cell recognition.

Organelles within the Cytoplasm

Types of Organelles

• Nonmembranous organelles: Direct contact with cytosol (e.g., cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, flagella).

• Membranous organelles: Isolated from cytosol by a membrane (e.g., endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria).

Cytoskeleton

The cytoskeleton provides structural support, determines cell shape, and facilitates movement of organelles and the cell itself.

• Microfilaments: Thin filaments of actin; provide mechanical strength and interact with myosin for muscle contraction.

• Intermediate filaments: Provide durability and stabilize organelle and cell position.

• Microtubules: Hollow tubes of tubulin; anchor organelles, move structures, and form spindle apparatus during cell division.

Surface Extensions

• Microvilli: Increase surface area for absorption; supported by microfilaments.

• Cilia: Move fluids across the cell surface; motile cilia have a 9+2 microtubule arrangement, while primary cilia are nonmotile and act as sensors.

• Flagella: Whip-like extensions for cell movement (e.g., sperm cells).

Ribosomes and Proteasomes

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

• Proteasomes: Contain proteolytic enzymes for breakdown and recycling of damaged or abnormal proteins.

Endoplasmic Reticulum (ER)

The ER is a network of membranous channels involved in synthesis, storage, transport, and detoxification.

• Rough ER (RER): Studded with ribosomes; synthesizes and modifies proteins and glycoproteins.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, steroid hormones, glycerides, and glycogen.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Receives transport vesicles from the ER.

• Produces secretory vesicles, membrane renewal vesicles, and lysosomes.

Lysosomes and Peroxisomes

• Lysosomes: Vesicles containing digestive enzymes; break down damaged organelles, pathogens, and cellular debris. Involved in autolysis (self-destruction of cells).

• Peroxisomes: Vesicles containing enzymes that break down fatty acids and neutralize toxic compounds (e.g., hydrogen peroxide).

Mitochondria

Mitochondria are the powerhouses of the cell, producing ATP through aerobic metabolism.

• Double membrane structure with inner folds (cristae) that increase surface area for metabolic enzymes.

• Site of glycolysis, citric acid cycle (Krebs cycle), and electron transport chain.

• Produce 95% of the cell's ATP.

The Nucleus

Structure and Function

• Largest organelle; surrounded by a double membrane (nuclear envelope) with nuclear pores for communication.

• Contains nucleoplasm, chromatin (DNA + proteins), and nucleoli (sites of rRNA synthesis).

• Controls metabolism, stores and processes genetic information, and directs protein synthesis.

Genetic Material

• DNA: Organized into chromosomes; contains the genetic code (sequence of bases: A, T, C, G).

• Gene: A segment of DNA that codes for a specific protein.

• Triplet code: Three DNA bases specify one amino acid.

Protein Synthesis

Transcription and Translation

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

• RNA processing: Introns are removed, and exons are spliced together before mRNA exits the nucleus.

• Translation: mRNA binds to ribosomes in the cytoplasm, where transfer RNA (tRNA) brings amino acids to build the polypeptide chain according to the mRNA codons.

Membrane Transport Mechanisms

Diffusion and Osmosis

• Diffusion: Movement of molecules from high to low concentration (down a concentration gradient).

• Osmosis: Diffusion of water across a selectively permeable membrane toward higher solute concentration.

• Osmotic pressure: The force required to prevent water movement by osmosis.

• Tonicity: The effect of a solution on cell volume (isotonic, hypotonic, hypertonic).

Carrier-Mediated and Vesicular Transport

• Carrier-mediated transport: Proteins transport ions or molecules across the membrane. Includes facilitated diffusion (passive), active transport (requires ATP), symport, and antiport mechanisms.

• Vesicular transport: Bulk movement of materials via vesicles. Includes endocytosis (receptor-mediated, pinocytosis, phagocytosis) and exocytosis.

Membrane Potential

Membrane potential is the electrical potential difference across the plasma membrane, resulting from the separation of positive and negative charges. The resting membrane potential typically ranges from –10 mV to –100 mV, depending on cell type.

Cell Life Cycle

Phases of the Cell Cycle

• Interphase: Nondividing period; includes G0 (resting), G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitosis (M phase): Division of the nucleus into two identical sets (prophase, metaphase, anaphase, telophase).

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

DNA Replication

• Helicases unwind DNA; DNA polymerase adds complementary nucleotides and forms new DNA strands.

• DNA ligases join DNA fragments on the lagging strand.

Regulation and Cancer

• Cell division is regulated by internal and external factors.

• Uncontrolled cell division leads to tumors (benign or malignant).

• Cancer results from mutations in genes controlling cell growth (oncogenes, mutagens, carcinogens).

• Metastasis is the spread of cancer cells to other tissues.

Cellular Differentiation

All cells contain the same genetic material, but differentiation occurs as cells turn off genes not needed for their specific function. This process allows for the formation of specialized cell types such as liver cells, fat cells, and neurons.