Overview
This lecture covers the cellular level of organization, including cell theory, membrane structure, cell transport mechanisms, organelles, the cell cycle, and the process of protein synthesis.
Cell Theory and Cell Types
- The cell is the smallest structural and functional living unit.
- Humans have over 200 different types of cells varying in size, shape, and function.
- All cells originate from the division of a fertilized ovum (zygote).
- Differentiation is the process by which cells develop specialized structures and functions.
- Four main tissue types arise from cell differentiation: epithelial, connective, muscle, and neural tissue.
Plasma Membrane Structure and Function
- The plasma membrane separates the cell from the extracellular fluid (ECF).
- It is a phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward.
- Cholesterol stabilizes membrane fluidity; proteins serve as channels, carriers, enzymes, receptors, and provide structural support.
- Glycolipids are lipids with attached carbohydrate groups on the membrane surface.
Membrane Transport Mechanisms
- Membrane permeability can be freely permeable, selectively permeable, or impermeable.
- Passive processes (no ATP): simple diffusion, facilitated diffusion, and osmosis.
- Simple diffusion: movement from high to low concentration (for small, nonpolar molecules).
- Facilitated diffusion: carrier or channel proteins allow specific molecules to cross.
- Osmosis: diffusion of water through aquaporins from low to high solute concentration.
- Tonicity: isotonic (no net movement), hypertonic (cell shrinks), hypotonic (cell swells).
- Active processes (require ATP): primary and secondary active transport, vesicular transport.
- The sodium-potassium pump (primary active transport) moves Na+ out and K+ in.
- Secondary active transport uses ion gradients for co-transport (symport/antiport).
- Vesicular transport: exocytosis (out), endocytosis (in), transcytosis (across); includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Cellular Organelles and Structures
- Rough ER: protein synthesis and modification; smooth ER: lipid and carbohydrate synthesis.
- Golgi apparatus: modifies, packages, and ships proteins/lipids.
- Lysosomes: contain digestive enzymes for waste processing.
- Peroxisomes: detoxify harmful substances and neutralize free radicals.
- Mitochondria: produce ATP via aerobic respiration; have their own DNA/RNA.
- Cytoskeleton: microfilaments (cell movement), microtubules (shape, organelle distribution), intermediate filaments (resist tension).
- Cilia and flagella: motile cell extensions; cilia move substances, flagella move cells.
- Ribosomes: synthesize proteins; found free in cytoplasm or bound to rough ER.
- Nucleus: contains genetic material; nucleolus produces rRNA and ribosome subunits.
Protein Synthesis and Genetic Code
- Transcription: DNA to messenger RNA (mRNA) in the nucleus.
- Translation: mRNA is decoded at the ribosome to build proteins from amino acids.
- Genetic code: three-base (codon) sequences on mRNA specify amino acids.
- DNA uses thymine (T); RNA uses uracil (U) instead.
- Transcription uses RNA polymerase; translation uses mRNA, tRNA, and rRNA.
Cell Cycle and Mitosis
- Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).
- Mitosis: prophase (chromosomes visible, spindle forms), metaphase (chromosomes align), anaphase (chromatids separate), telophase (nuclear envelope reforms), cytokinesis (cell splits).
- Errors in the cell cycle can produce cancer; benign tumors remain localized, malignant tumors metastasize.
Key Terms & Definitions
- Differentiation — process where cells become specialized in structure and function.
- Phospholipid bilayer — double layer forming the plasma membrane.
- Osmosis — movement of water across a membrane from low to high solute concentration.
- Tonicity — ability of a solution to change cell volume.
- Primary active transport — direct use of ATP to move substances against gradients.
- Secondary active transport — uses energy stored in gradients from primary transport.
- Exocytosis — vesicular transport of substances out of a cell.
- Endocytosis — vesicular transport of substances into a cell.
- Transcription — synthesis of RNA from a DNA template.
- Translation — assembly of proteins using mRNA at the ribosome.
- Codon — three-base sequence on mRNA specifying an amino acid.
- Mitosis — process of cell division producing two identical cells.
Action Items / Next Steps
- Review diagrams of cell structures and transport mechanisms.
- Study the detailed steps of transcription, translation, and the cell cycle.
- Complete assigned readings on cellular organelles and membrane transport.