Cells: The Structural Units of Life

Introduction to Cells

Cells are the fundamental building blocks of all living organisms. The human body contains approximately 50 to 100 trillion cells, each specialized for particular functions essential to life.

• Definition: A cell is the basic structural and functional unit of life.

• Cell Theory:

  • All living things are composed of cells.

  • The activity of an organism depends on the collective activities of its cells.

  • Principle of Complementarity: The structure (anatomy) of a cell determines its function (physiology).

  • Continuity of life has a cellular basis; new cells arise from pre-existing cells.

Anatomy of a Generalized Cell

Main Regions of a Cell

Most cells share three primary regions, each with distinct roles:

• Plasma Membrane: The outer boundary that regulates entry and exit of substances.

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

• Cytoplasm: The internal environment where cellular activities occur.

The Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that separates the cell from its environment. Its structure is described by the fluid mosaic model.

• Fluid Mosaic Model:

  • Composed of a double layer of phospholipids with embedded proteins and cholesterol.

  • Glycolipids: Sugar groups attached to phospholipids, contributing to cell recognition.

  • Phospholipid Arrangement:

    • Hydrophilic heads: Face the inner and outer surfaces (water-loving).

    • Hydrophobic tails: Face inward, forming a water-repellent interior.

  • This arrangement makes the membrane relatively impermeable to most water-soluble molecules.

Membrane Proteins and Sugars

• Proteins:

  • Act as enzymes, receptors, and transport channels/carriers.

• Glycoproteins: Proteins with attached sugar chains, important for cell recognition and adhesion.

• Glycocalyx: A fuzzy, sugar-rich area on the cell surface, involved in protection and cell interactions.

Cell Junctions

Cells are bound together by specialized junctions:

• Tight Junctions: Impermeable junctions that bind cells into leakproof sheets.

• Desmosomes: Anchoring junctions that prevent cells from being pulled apart under stress.

• Gap Junctions: Communicating junctions allowing direct passage of molecules between cells via connexons.

Nucleus

Structure and Function

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

• Nuclear Envelope: Double membrane with nuclear pores for exchange of materials.

• Nucleolus: Site of ribosome assembly; ribosomes migrate to the cytoplasm for protein synthesis.

• Chromatin: DNA wound around histone proteins; condenses into chromosomes during cell division.

Cytoplasm

Components of the Cytoplasm

The cytoplasm is the site of most cellular activities and contains:

• Cytosol: Fluid containing nutrients and electrolytes.

• Inclusions: Stored nutrients or cell products.

• Organelles: Specialized structures performing specific functions.

Major Organelles

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

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid metabolism and detoxification.

• Golgi Apparatus: Modifies, packages, and sorts proteins and lipids for secretion or use within the cell.

• Lysosomes: Membranous sacs containing digestive enzymes; dispose of bacteria and cell debris.

• Peroxisomes: Contain oxidase enzymes; detoxify harmful substances and break down free radicals.

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

  • Microfilaments (largest)

  • Intermediate filaments

  • Microtubules (smallest)

• Centrioles: Rod-shaped bodies that generate microtubules and direct mitotic spindle formation during cell division.

Cell Surface Extensions

• Cilia: Move materials across the cell surface (e.g., respiratory tract).

• Flagella: Propel the cell; only human example is sperm.

• Microvilli: Increase surface area for absorption (e.g., small intestine).

Membrane Transport

Types of Membrane Transport

The plasma membrane is selectively permeable, allowing some substances to pass while excluding others. Transport occurs via passive or active processes.

• Passive Processes: No energy required; substances move down their concentration gradient.

• Active Processes: Require cellular energy (ATP); substances may move against their concentration gradient.

Passive Processes

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

  • Simple Diffusion: Unassisted movement of lipid-soluble or small molecules.

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

  • Facilitated Diffusion: Movement of larger or charged molecules via protein channels or carriers (e.g., glucose, chloride ions).

• Filtration: Water and solutes are forced through a membrane by hydrostatic pressure; important in kidney function.

Osmosis and Tonicity

• Isotonic Solution: Same solute and water concentration as the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration than the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration than the cell; water moves in, cell swells.

Active Processes

• Active Transport: Uses protein carriers (solute pumps) and ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Vesicular Transport: Bulk movement of substances via vesicles.

  • Exocytosis: Export of substances out of the cell.

  • Endocytosis: Import of substances into the cell.

    • Phagocytosis: "Cell eating"; engulfment of large particles.

    • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

    • Receptor-mediated Endocytosis: Highly selective uptake of specific molecules via receptor proteins.

Cell Division

Cell Cycle Overview

The cell cycle consists of two major periods: interphase (growth and metabolic activity) and cell division (mitosis and cytokinesis).

• Interphase: Cell grows, carries out normal functions, and replicates DNA (G1, S, G2 phases).

• Cell Division:

  • Mitosis: Division of the nucleus, resulting in two identical daughter nuclei.

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

Stages of Mitosis

• Prophase: Chromatin condenses into chromosomes; spindle forms; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Chromatids separate and move to opposite poles.

• Telophase: Chromosomes uncoil; nuclear envelope reforms; nucleoli reappear.

• Cytokinesis: Cleavage furrow forms, dividing the cell into two.

Protein Synthesis

Role of DNA and RNA

Protein synthesis is the process by which cells build proteins based on genetic instructions. DNA serves as the blueprint, but RNA is required to carry out the instructions.

• Gene: DNA segment coding for a specific protein or polypeptide.

• Types of RNA:

  • Messenger RNA (mRNA): Carries genetic instructions from nucleus to ribosome.

  • Ribosomal RNA (rRNA): Forms part of the ribosome structure.

  • Transfer RNA (tRNA): Brings amino acids to the ribosome for protein assembly.

Phases of Protein Synthesis

• Transcription: DNA's base sequence is copied into mRNA in the nucleus.

  • Example: If DNA sequence is AAT-CGT-TCG, mRNA sequence is UUA-GCA-AGC.

• Translation: mRNA sequence is decoded into an amino acid sequence at the ribosome, forming the primary structure of proteins.

Genetic Code and Base Pairing

• Triplet Code: Three DNA bases code for one amino acid.

• Base Pairing Rules:

  • Adenine (A) pairs with Thymine (T)

  • Guanine (G) pairs with Cytosine (C)

Additional info: These notes provide foundational knowledge for understanding cell structure, function, and processes essential to human anatomy and physiology. Mastery of these concepts is critical for further study of tissues, organs, and systems.