Introduction to Cells

Basic Functions and Components of Cells

All living organisms are composed of cells, which perform essential functions necessary for life. Despite their diversity, most cells share several fundamental characteristics and structures.

• Cell Metabolism: The sum of all chemical reactions that occur within a cell, including anabolic (building) and catabolic (breaking down) processes.

• Transport: Movement of substances into, out of, and within the cell.

• Communication: Cells communicate with each other and their environment through chemical and electrical signals.

• Reproduction: Cells reproduce to maintain tissue health and organismal growth.

• Three Basic Components:

  • Plasma Membrane: Separates the cell from its environment, creating the extracellular fluid (ECF) and intracellular fluid (ICF) compartments.

  • Cytoplasm: Includes the cytosol (fluid), organelles, and cytoskeleton.

  • Nucleus: Contains most of the cell’s DNA and regulates cellular activities.

• Cell Diversity: Variations in size and structure allow cells to perform specialized functions.

Structure of the Plasma Membrane

Composition and Function

The plasma membrane is a dynamic barrier that controls the movement of substances and facilitates communication between the cell and its environment.

• Phospholipid Bilayer: Composed of hydrophilic phosphate heads (facing water) and hydrophobic fatty acid tails (facing inward).

• Fluid Mosaic Model: Describes the membrane as a flexible, fluid structure with proteins and other molecules moving laterally within the bilayer.

• Membrane Proteins:

  • Integral Proteins: Embedded within the membrane; transmembrane proteins span the entire membrane.

  • Peripheral Proteins: Attached to one side of the membrane.

  • Functions: Channels, carriers, enzymes, receptors, and structural support.

Transport Across the Plasma Membrane

Selective Permeability and Transport Mechanisms

The plasma membrane is selectively permeable, allowing certain substances to cross while restricting others. Transport can be passive or active.

• Passive Transport: No ATP required; substances move down their concentration gradient.

  • Simple Diffusion: Movement of small, nonpolar solutes directly through the bilayer.

  • Facilitated Diffusion: Movement of polar or ionic solutes via protein channels or carriers.

  • Osmosis: Movement of water from low to high solute concentration across a selectively permeable membrane.

• Tonicity: Describes the effect of ECF on cell volume.

  • Isotonic: No net water movement; cell volume remains constant.

  • Hypertonic: Water leaves the cell; cell shrinks.

  • Hypotonic: Water enters the cell; cell swells.

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

  • Primary Active Transport: Direct use of ATP to pump substances (e.g., Na+/K+ pump).

  • Secondary Active Transport: Uses energy from a concentration gradient established by primary active transport.

• Membrane Potential: Separation of charges across the membrane creates an electrical potential (resting value: ).

• Vesicular Transport: Movement of large substances via vesicles.

  • Endocytosis: Uptake of materials into the cell.

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

    • Pinocytosis: "Cell drinking"; ingestion of fluid and dissolved substances.

    • Receptor-Mediated Endocytosis: Specific uptake via receptor-ligand binding.

  • Exocytosis: Release of substances from the cell via vesicle fusion with the plasma membrane.

Cytoplasmic Organelles

Structure and Function of Major Organelles

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions.

• Mitochondria: Site of ATP production via oxidative catabolism.

• Peroxisomes: Detoxify harmful substances and metabolize fatty acids; synthesize certain phospholipids.

• Ribosomes: Non-membranous organelles responsible for protein synthesis; composed of two subunits.

• Endomembrane System: Network of organelles involved in synthesis, modification, and transport of cellular materials.

  • Rough Endoplasmic Reticulum (RER): Studded with ribosomes; modifies and folds proteins.

  • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids, stores calcium, detoxifies substances.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

  • Lysosomes: Contain digestive enzymes for breakdown of ingested materials and worn-out organelles.

The Cytoskeleton

Types and Functions of Cytoskeletal Elements

The cytoskeleton is a network of protein filaments that provides structural support, facilitates movement, and organizes cellular components.

• Actin Filaments (Microfilaments): Thin filaments involved in cell shape, movement, and muscle contraction (with myosin).

• Intermediate Filaments: Rope-like filaments providing mechanical strength and stability.

• Microtubules: Largest filaments; maintain cell shape, transport organelles, form cilia and flagella; originate from the centrosome.

• Specialized Extensions:

  • Microvilli: Increase surface area for absorption.

  • Cilia: Move substances across the cell surface.

  • Flagella: Propel the cell (e.g., sperm cell).

The Nucleus

Structure and Genetic Function

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

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Chromatin: DNA-protein complex; condenses into chromosomes during cell division.

• Chromosomes: Human cells have 23 pairs (46 total); one set from each parent.

• Nucleoli: Sites of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Protein Synthesis

From DNA to Functional Protein

Protein synthesis is the process by which genetic information is used to build proteins, essential for cell structure and function.

• Gene: Segment of DNA coding for a specific protein.

• Genetic Code: Specifies which amino acid corresponds to each mRNA codon.

• Introns and Exons: Introns are noncoding regions; exons are coding regions.

• Transcription: DNA is copied into messenger RNA (mRNA) in three stages: initiation, elongation, termination.

• RNA Processing: Pre-mRNA is modified before leaving the nucleus.

• Translation: mRNA is decoded on ribosomes to assemble amino acids into a polypeptide chain, with transfer RNA (tRNA) as the adaptor.

  • Initiation: Assembly of ribosome, mRNA, and initiator tRNA at the start codon.

  • Elongation: Sequential addition of amino acids.

  • Termination: Release of the completed polypeptide at a stop codon.

• Posttranslational Modification: Polypeptides are folded and modified to become functional proteins.

The Cell Cycle

Phases and Regulation

The cell cycle is the series of events that cells go through as they grow and divide, ensuring proper replication and distribution of genetic material.

• Interphase: Period of cell growth and DNA replication; includes:

  • G1 Phase: Cell growth and normal functions.

  • S Phase: DNA synthesis (replication); each chromosome duplicates.

    • Semiconservative Replication: Each new DNA double helix contains one old and one new strand.

  • G2 Phase: Preparation for cell division.

• M Phase: Includes mitosis (division of genetic material) and cytokinesis (division of cytoplasm).

  • Mitosis Stages:

    1. Prophase

    2. Metaphase

    3. Anaphase

    4. Telophase

  • Cytokinesis: Cytoplasm divides, forming two daughter cells; occurs alongside anaphase and telophase at the cleavage furrow.

• Cell Cycle Control: Checkpoints regulate progression, ensuring balance between cell formation and cell death.

Example: The Na+/K+ Pump Equation

• The sodium-potassium pump is a classic example of primary active transport, moving 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed:

Table: Comparison of Passive and Active Transport

Additional info: The above notes expand on the original content by providing definitions, examples, and a comparison table for clarity and completeness.