An Introduction to Cells — Key Terms (Vocabulary)

3-1 Plasma Membrane

  • Extracellular fluid (interstitial fluid): A watery medium that surrounds a cell.

  • Plasma membrane (cell membrane) separates cytoplasm from the extracellular fluid.

  • Functions of the plasma membrane:

    • Physical isolation: Barrier

    • Regulation of exchange with the environment: Ions and nutrients enter; wastes eliminated and cellular products released

    • Sensitivity to the environment: Extracellular fluid composition and chemical signals

    • Structural support: Anchors cells and tissues

  • Membrane lipids:

    • Phospholipid bilayer: Hydrophilic heads face outward toward watery environments; hydrophobic fatty-acid tails face inward.

    • Barrier to ions and water-soluble compounds.

  • Membrane proteins:

    • Integral proteins: Within the membrane

    • Peripheral proteins: Bound to inner or outer membrane surfaces

  • Membrane proteins (functions):

    • Anchoring proteins (stabilizers): Attach to inside or outside structures.

    • Recognition proteins (identifiers): Label cells as normal or abnormal.

    • Enzymes: Catalyze reactions.

    • Receptor proteins: Bind and respond to ligands (ions, hormones).

    • Carrier proteins: Transport specific solutes through the membrane.

    • Channels: Regulate water flow and solutes; gated channels open or close to regulate passage.

  • Membrane carbohydrates:

    • Proteoglycans, glycoproteins, and glycolipids extend outside the membrane to form the glycocalyx.

    • Functions of the glycocalyx:

    • Lubrication and protection

    • Anchoring and locomotion

    • Specificity in binding (receptors)

    • Recognition (immune response)

  • Figure reference (general): The plasma membrane consists of a phospholipid bilayer with integral and peripheral proteins, cholesterol, glycolipids, and glycocalyx on the extracellular surface.

3-2 Organelles within the Cytoplasm

  • Cytoplasm = all materials inside the cell, excluding the nucleus.

    • Cytosol (intracellular fluid): contains dissolved nutrients, ions, proteins, and waste products; high protein and potassium; low carbohydrate, lipid, amino acid, and sodium levels.

    • Organelles: structures with specific functions.

  • Organelles are categorized as:

    • Nonmembranous organelles: No membrane; direct contact with cytosol. Examples: cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, and flagella.

    • Membranous organelles: Isolated from cytosol by a plasma membrane. Examples: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and mitochondria.

  • Inclusions: masses of insoluble materials (e.g., glycogen or lipid droplets).

  • Cytoskeleton: structural proteins for shape and strength, including:

    • Microfilaments (actin): provide mechanical strength; interact with myosin for muscle contraction; adjust cytosol consistency.

    • Intermediate filaments: durable; strengthen cell and maintain shape; stabilize organelle positions.

    • Microtubules: large hollow tubes of tubulin; attach to centrosome; anchor and move organelles; change cell shape; form spindle apparatus for chromosome distribution; form centrioles and cilia.

  • Microvilli: extensions of the plasma membrane containing microfilaments; increase surface area for absorption.

  • Centrioles: form spindle apparatus during cell division.

  • Centrosome: cytoplasm next to the nucleus that surrounds centrioles.

  • Cilia and flagella:

    • Cilia: slender extensions with microtubules; primary cilium is nonmotile and acts as a sensor; motile cilia move materials across cell surfaces.

    • Basal body: anchoring structure for cilia.

    • Flagellum: whip-like extension (e.g., sperm tail).

  • Ribosomes: synthesize proteins; composed of small and large subunits with rRNA.

    • Free ribosomes: proteins enter cytosol.

    • Fixed ribosomes: attached to rough endoplasmic reticulum (RER); proteins enter ER for packaging.

  • Proteasomes: contain proteolytic enzymes; disassemble damaged proteins for recycling.

  • Endoplasmic reticulum (ER): network of membranous channels (cisternae) with distinct regions:

    • Rough ER (RER): surface studded with ribosomes; active in protein and glycoprotein synthesis; folds proteins; packages them into transport vesicles for Golgi.

    • Smooth ER (SER): no ribosomes; synthesizes phospholipids, cholesterol, steroid hormones, glycerides, and glycogen storage.

  • Golgi apparatus (Golgi complex): stacked cisternae where vesicles enter on the forming (cis) face and exit on the maturing (trans) face.

    • Functions:

    • Modifies and packages secretions (hormones, enzymes) for release.

    • Adds or removes carbohydrates to/from proteins.

    • Renews/modifies the plasma membrane.

    • Packages enzymes into lysosomes for cytoplasmic use.

  • Lysosomes:

    • Primary lysosomes: contain inactive enzymes.

    • Secondary lysosomes: formed when primary lysosomes fuse with damaged organelles; enzymes activated.

    • Functions: destroy bacteria, break down molecules, recycle damaged organelles.

  • Autolysis: self-destruction of damaged or inactive cells; lysosomal membranes break down and digestive enzymes are released; materials recycled.

  • Peroxisomes: small enzyme-containing vesicles; produced by division of existing peroxisomes.

    • Functions: breakdown of fatty acids; production of hydrogen peroxide (H2O2); catalase converts H2O2 to water and oxygen.

  • Mitochondria:

    • Structure: smooth outer membrane; inner membrane with cristae; matrix inside cristae.

    • Function: powerhouse of the cell; produce ATP via oxidative metabolism (aerobic respiration).

    • Energy production pathways:

    • Glycolysis occurs in cytosol (glucose to pyruvic acid).

    • Pyruvate enters mitochondria; Citric acid cycle (Krebs/TCA) occurs in matrix.

    • Electron transport chain on the inner mitochondrial membrane.

    • Aerobic metabolism (cellular respiration): uses O2 to convert glucose into ATP; approximately 95% of ATP is produced this way.

    • Overall equation (conceptual): Glucose+O<em>2+ADP→CO</em>2+H2O+ATP.\text{Glucose} + \text{O}<em>2 + \text{ADP} \rightarrow \text{CO}</em>2 + \text{H}_2\text{O} + \text{ATP}.

  • Membrane flow (membrane trafficking): continuous exchange of membrane segments by vesicles; involves all membranous organelles except mitochondria; allows adaptation and change.

3-3 Cell Nucleus

  • Nucleus: largest organelle; the cell’s control center.

  • Nuclear envelope: double membrane around nucleus; perinuclear space between the two membranes;

  • Nuclear pores serve as communication passages.

  • Contents of the nucleus:

    • Nuclear matrix in the nucleoplasm provides support filaments.

    • Nucleoli: sites of rRNA synthesis and assembly of ribosomal subunits.

    • Nucleosome: DNA coiled around histones; loosely coiled chromatin in non-dividing cells; tightly coiled chromosomes form before division.

    • Chromatin: DNA packaged with proteins; organized into chromosomes during division.

  • Genetic information storage:

    • Genetic code: triplet code; sequence of bases (A, T, C, G) forms codons; three bases code for one amino acid.

    • Gene: DNA instructions for a single protein; functional unit of heredity.

  • Protein synthesis control: DNA directs synthesis of proteins; extracellular signals can influence intracellular signaling pathways and gene expression.

3-4 Protein Synthesis

  • Protein synthesis overview: assembling functional polypeptides in the cytoplasm.

  • Gene activation: DNA uncoiling and temporary histone removal.

  • Transcription (DNA to RNA):

    • Synthesis of RNA from a DNA template; all RNA (including mRNA) is formed via transcription.

    • Two DNA strands: coding strand specifies amino acid sequence; template strand used for mRNA production.

    • Process:

    1. RNA polymerase binds to DNA promoter region.

    2. RNA polymerase links nucleotides to form mRNA in codons (three-base sets).

    3. Transcription ends at stop signal; RNA polymerase and mRNA detach.

  • RNA processing: mRNA editing before leaving the nucleus; introns removed, exons spliced together.

  • Translation (RNA to protein):

    • mRNA binds to ribosomal subunits in the cytoplasm.

    • Each codon translates to one amino acid; amino acids delivered by tRNA.

    • tRNA anticodon pairs with complementary mRNA codon.

    • Enzymes join amino acids via peptide bonds.

    • Translation terminates at a stop codon; ribosomal subunits dissociate.

  • Genetic code examples and start signal:

    • Translation begins with the start codon AUG, which encodes methionine (Met) for initiation.

    • The process involves large and small ribosomal subunits with E (Exit), P (Polypeptide), and A (Arrival) sites on the ribosome.

  • Protein processing and trafficking (summary from Golgi/ER interaction): proteins synthesized in RER are modified and packaged into transport vesicles, then processed in the Golgi for final delivery (secretory vesicles, lysosomes, membrane renewal).

  • Important note: DNA controls cell structure and function by directing protein synthesis; environmental cues can influence transcription and translation.

3-5 Diffusion and Osmosis

  • Plasma membrane permeability regulates what enters and leaves; membranes can be impermeable, freely permeable, or selectively permeable.

  • Selective permeability depends on:

    • Size

    • Electrical charge

    • Molecular shape

    • Lipid solubility

  • Transport types:

    • Passive processes: diffusion and osmosis; carrier-mediated diffusion; vesicular transport not requiring energy.

    • Active processes: requiring energy (ATP).

  • Diffusion: net movement from high to low concentration; molecules are in constant random motion; concentration gradient drives diffusion.

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

  • Effects of diffusion/osmosis:

    • Factors affecting rate: gradient size, molecular size, temperature, concentration gradient steepness, electrical forces.

    • Water-soluble molecules diffuse through membrane channels; lipid-soluble molecules diffuse directly through the bilayer.

  • Osmolarity and tonicity:

    • Osmolarity: total solute concentration in a solution.

    • Tonicity: effect of a solution on cell volume; isotonic, hypotonic, and hypertonic conditions.

    • Isotonic solution: no net water movement; cells retain shape.

    • Hypotonic solution: water enters cells; risk of hemolysis in RBCs.

    • Hypertonic solution: water leaves cells; cells crenate.

  • Aquaporins: water channels that facilitate rapid osmosis; water movement is faster than solute diffusion.

3-6 Carriers and Vesicles

  • Carrier-mediated transport: specific for particular substrates; proteins transport ions or organic substrates across the membrane.

    • Specificity: one carrier protein for a defined substrate set.

    • Saturation: rate depends on the availability of carriers and substrates.

    • Regulation: cofactors (e.g., hormones) can affect transporter activity.

  • Carrier mechanisms:

    • Symport (cotransport): two substances move in the same direction.

    • Antiport (countertransport): one moves in while another moves out.

    • Facilitated diffusion: passive; carrier protein transports large or insoluble molecules; molecule binds to receptor site on the carrier causing shape change to release on the other side.

    • Active transport: moves solutes against their concentration gradient; requires ATP.

    • Primary active transport: uses ATP directly (e.g., Na+/K+ exchange pump).

      • Na+/K+ pump: 3 Na+ out, 2 K+ in per ATP hydrolyzed.

      • Equation: 3 Na+ out,  2 K+ in per ATP.3\,Na^+ \text{ out}, \; 2\,K^+ \text{ in} \text{ per } \mathrm{ATP}.

    • Secondary active transport: uses established gradient (e.g., Na+ gradient) to drive transport of another solute; ATP used to maintain the gradient.

  • Vesicular transport (bulk transport): transport of materials via vesicles; energy-dependent (ATP).

    • Endocytosis: import of extracellular materials packaged in vesicles.

    • Receptor-mediated endocytosis: ligands bind receptors; pits form and internalize in clathrin-coated vesicles; ligands released inside; receptors recycled.

    • Pinocytosis: cell drinking; endosomes take up extracellular fluid.

    • Phagocytosis: cells engulf large particles via pseudopodia forming phagosomes.

    • Exocytosis: vesicles fuse with the plasma membrane to release contents outside the cell; secretory vesicles and membrane-renewal vesicles.

  • Receptor-mediated endocytosis specific steps (summary from figure): ligand binding, clathrin-coated pit formation, endosome formation, lysosomal fusion, ligand release, receptor recycling.

3-8 Cell Life Cycle

  • Cell life cycle: cell division as cellular reproduction; a cell divides to form two daughter cells; apoptosis is programmed cell death.

  • Interphase: nondividing period; G0, G1, S, G2 phases.

  • G1 phase: normal cell functions + cell growth; duplication of organelles; centriole replication begins and may continue to G2; duration varies (8–12 hours in fast-dividing cells).

  • S phase: DNA replication; synthesis of histones and other nuclear proteins; chromosomes duplicated.

  • G2 phase: short (2–5 hours); final protein synthesis and centriole replication completed.

  • G0 phase: nondividing state; some mature cells (skeletal muscle, neurons) remain in G0; stem cells may divide with brief interphase and not enter G0.

  • M phase (mitosis) and cytokinesis:

    • Mitosis: duplication of chromosomes and their separation into two identical sets; stages include prophase, metaphase, anaphase, and telophase.

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

  • DNA replication details (summary): helicases unwind DNA; DNA polymerase synthesizes new strands; leading strand is continuous; lagging strand formed in segments (Okazaki fragments) with ligases joining fragments; cell completes replication and proceeds to mitosis.

  • Mitosis stages (quick references):

    • Prophase: chromatin condenses into visible chromosomes; spindle apparatus forms.

    • Metaphase: chromosomes align at the metaphase plate.

    • Anaphase: sister chromatids separate and move to opposite poles.

    • Telophase: nuclear membranes reform; chromosomes decondense.

  • Cytokinesis completes cell division.

  • Mitotic rate considerations:

    • Slower mitotic rate implies longer cell life.

    • Some cells (muscle, neurons) rarely divide; exposed cells (skin, digestive tract) divide frequently to replenish.

    • Energy requirement: ATP for division.

3-11 Cellular Differentiation

  • All cells contain the same chromosomes and genes.

  • Differentiation occurs by turning off genes not needed by a given cell type.

  • This selective gene expression allows the formation of diverse cell types (e.g., liver cells, fat cells, neurons) despite identical genetic content.

Connections and Practical Relevance

  • Cell theory foundation: All organisms are composed of cells; cells arise from preexisting cells; cells are the basic units of life and homeostasis.

  • Understanding organelles and membranes helps explain how drugs affect cells (e.g., how membrane transporters influence drug uptake; how lysosomal pathways relate to diseases).

  • Energy production in mitochondria is central to cellular metabolism and many diseases show mitochondrial dysfunction.

  • Diffusion, osmosis, and transporter activity underpin many medical treatments (e.g., IV fluids, osmotic therapies, electrolyte management).

  • The cell cycle and differentiation are foundational to development and cancer biology; dysregulation can lead to uncontrolled proliferation or failure to differentiate.

Key Formulas (LaTeX)

  • Sodium–potassium exchange pump stoichiometry: 3 Na+ out,2 K+ in per ATP3\,Na^+ \text{ out}, \quad 2\,K^+ \text{ in} \text{ per } \mathrm{ATP}

  • Mitochondrial energy production (overall): Glucose+O<em>2+ADP→CO</em>2+H2O+ATP\text{Glucose} + \text{O}<em>2 + \text{ADP} \rightarrow \text{CO}</em>2 + \text{H}_2\text{O} + \text{ATP}

  • Aerobic metabolism yields about: ≈95% of cellular ATP\approx 95\% \ \text{of cellular ATP}

  • Translation initiation codon: AUG encodes Methionine (start codon for protein synthesis).

  • Facilitated diffusion (conceptual): a substrate binds to a carrier protein causing a conformational change to move the substrate across the membrane.

  • Primary active transport (sodium–potassium pump): see above for ATP usage and ion exchange.