Chapter 3: The Cellular Level of Organization - Practice Flashcards

Parts of a Cell

  • The cell is subdivided into three main parts: plasma membrane (plasmalemma), cytoplasm (cytosol + organelles), and nucleus (chromosomes + genes).
  • Cytoplasm includes cytosol and organelles; cytoskeleton provides structure and enables movement.

The Plasma Membrane

  • Flexible yet sturdy barrier surrounding the cytoplasm.
  • Separates the cell’s interior from the exterior; regulates entry/exit of materials.
  • Facilitates communication between cells and the external environment.

The Plasma Membrane Structure

  • Fluid mosaic model: a dynamic, moving sea of lipids with embedded proteins.
  • Two types of membrane proteins:
    • Integral (transmembrane) proteins: extend into/through the lipid bilayer; firmly embedded.
    • Peripheral proteins: attached to polar heads or outer surface; not embedded.

Membrane Proteins – Functions

  • Membrane proteins perform diverse roles; their types determine membrane functions (e.g., transport, signaling, enzymes, receptors, anchors).

Membrane Fluidity

  • Membrane lipids and many proteins move easily within the bilayer.
  • Cholesterol stabilizes the membrane and reduces fluidity.

Membrane Permeability

  • Lipid bilayer is always permeable to small, nonpolar, uncharged molecules.
  • Transmembrane proteins (channels/transporters) increase permeability for other substances.
  • Macromolecules cross via vesicular transport.

Gradients Across the Plasma Membrane

  • Concentration gradient: difference in chemical concentration across the membrane.
  • Electrical gradient: difference in ion concentrations across the membrane.
  • Electrochemical gradient: combination of chemical and electrical gradients.

Knowledge Check (Key Concepts)

  • Plasma membrane composition: lipids + proteins + cholesterol influences function and permeability.
  • Proteins determine most membrane functions and transport capabilities.

Transport Across the Plasma Membrane

  • Passive processes: move substances down a gradient using kinetic energy (no cellular energy).
  • Active processes: move substances up a gradient using ATP or vesicles.
  • Vesicular transport includes endocytosis, exocytosis, and transcytosis.

Passive Transport: Diffusion

  • Diffusion is influenced by:
    • Gradient steepness: larger difference → faster rate.
    • Temperature: higher → faster.
    • Mass of diffusing substance: larger → slower.
    • Surface area: more area → faster.
    • Diffusion distance: greater distance → slower.

Passive Transport: Simple Diffusion vs Facilitated Diffusion

  • Simple diffusion: solutes move freely through the lipid bilayer; nonpolar molecules (O2, CO2, steroid hormones, fats).
  • Facilitated diffusion: relies on membrane proteins for polar/hydrated solutes.
    • Channel-mediated: ion channels for specific ions (e.g., K⁺, Na⁺, Cl⁻).
    • Carrier-mediated: solute binds a carrier; transport maximum (saturation) occurs when all carriers are occupied.

Passive Transport: Channel-Mediated Facilitated Diffusion

  • Channel proteins form pores; gates control entry/exit of ions.
  • Example: K⁺ channel; gate states regulate ion flow.

Passive Transport: Carrier-Mediated Facilitated Diffusion

  • A solute binds a carrier and is transported across the membrane.
  • Transport maximum (saturation) occurs when all carriers are engaged.

Passive Transport: Osmosis

  • Net movement of solvent (water) across a selectively permeable membrane from high to low water concentration.
  • Water moves via:
    • Simple diffusion through phospholipids.
    • Aquaporins (water-specific channels).

Tonicity

  • Isotonic: normal cell shape/volume; water moves in/out equally.
  • Hypotonic: lower solute concentration outside; water enters cells → swelling or bursting.
  • Hypertonic: higher solute concentration outside; water exits cells → crenation/shrinkage.

Active Transport: Primary vs Secondary

  • Primary active transport: ATP directly drives the pump to move substances against their gradient.
  • Secondary active transport: energy stored in a gradient (e.g., Na⁺ or H⁺) drives transport of other substances.
    • Symporters: two substances move in the same direction.
    • Antiporters: two substances move in opposite directions.

Vesicular Transport

  • Receptor-mediated endocytosis: uptake of specific extracellular substances via receptor binding.
  • Phagocytosis: "cell eating"; ingestion of large particles/bacteria; digestion in residual body via exocytosis.
  • Bulk-phase endocytosis (pinocytosis): "cell drinking"; uptake of small droplets of extracellular fluid and solutes.
  • Exocytosis: release of contents to the extracellular space via vesicles fusing with the plasma membrane.
  • Transcytosis: combination of endocytosis and exocytosis to move substances across a cell.

Cytoplasm and Organelles

  • Cytoplasm comprises:
    • Cytosol: intracellular fluid; site of metabolic reactions.
    • Organelles: specialized structures with distinct functions.
    • Cytoskeleton: microfilaments, intermediate filaments, microtubules; provides shape, organization, and movement.

Centrosome and Cilia/Flagella

  • Centrosome: consists of pericentriolar material and a pair of centrioles; tubulins aid microtubule formation and spindle growth for cell division.
  • Cilia and Flagella: motile surface projections with microtubules; cilia move fluids over surfaces; flagella propel the cell.

Ribosomes and the Endomembrane System

  • Ribosomes: ribosomal RNA + proteins; free or attached to rough ER; synthesize proteins for export or for membranes.
  • Rough Endoplasmic Reticulum (RER): studded with ribosomes; synthesizes glycoproteins and phospholipids; transfers to organelles, membranes, or secreted via exocytosis.
  • Smooth Endoplasmic Reticulum (SER): lipid synthesis; detoxification; calcium storage/release in muscle cells.
  • Golgi Complex: modifies, sorts, and packages proteins; has cis (entry) and trans (exit) faces; secretory vesicles.
  • Lysosomes: digestive enzymes; digest contents of endosomes, phagosomes; autophagy/autolysis.
  • Peroxisomes: oxidize fatty acids and detoxify harmful substances; produce hydrogen peroxide and convert to water and oxygen via catalase; form from preexisting peroxisomes.
  • Proteasomes: degrade damaged or unneeded proteins into peptides.
  • Mitochondria: ATP production via aerobic respiration; cristae increase surface area; matrix contains enzymes; involved in apoptosis.

Nucleus and Genetic Material

  • Nucleus: nuclear envelope with pores; nucleolus; chromosomes (chromatin during interphase).
  • Genes organized on chromosomes; govern cellular structure and function.
  • Nuclear pores regulate transport between nucleus and cytoplasm.

Nucleus – Key Structures

  • Nuclear envelope: double membrane with openings (pores).
  • Nuclear pores: control movement of substances between nucleus and cytoplasm.
  • Nucleolus: ribosome synthesis.
  • Chromatin: DNA + proteins; becomes visible chromosomes during division.

Protein Synthesis

  • Proteins determine structural and functional characteristics of cells; gene expression comprises transcription and translation.

Transcription (in the nucleus)

  • DNA is copied into messenger RNA (mRNA) by RNA polymerase.
  • Roles of RNA types:
    • mRNA directs protein synthesis.
    • rRNA forms ribosomes with ribosomal proteins.
    • tRNA carries amino acids and helps assemble them at the ribosome.

Translation (in the cytoplasm, at the ribosome)

  • Ribosome reads the mRNA sequence to determine the amino acid sequence of the protein.
  • Ribosome structure involves large and small subunits; codons on mRNA are matched by tRNA anticodons.
  • Key steps involve initiation at start codon, elongation with successive aminoacyl-tRNAs, peptide bond formation, ribosome translocation, and termination at stop codon.

Ribosome and Translation – Visual Cues

  • A site: aminoacyl-tRNA entry.
  • P site: growing peptide chain; tRNA with growing protein.
  • E site: exiting tRNA.

Protein Synthesis Summary

  • Gene expression via transcription (DNA → RNA) and translation (RNA → protein).

Cell Division

  • Cell division allows replication and distribution of genetic material.
  • Terms:
    • Somatic cell: any body cell (not a germ cell).
    • Germ cell: destined to become a gamete.
    • Reproductive cell division: meiosis; produces gametes.
    • Somatic cell division: mitosis; produces two daughter cells.

Somatic Cell Division (Mitosis) and Interphase

  • Interphase (preparation): G1 (growth and organelle duplication), S (DNA replication), G2 (protein synthesis and final prep).
  • Mitosis: division of the nucleus into two nuclei; followed by cytokinesis.
  • Mitosis stages: Prophase, Metaphase, Anaphase, Telophase.
  • Cytokinesis: cytoplasm divides; forms two separate cells.

Mitosis – Stage Highlights

  • Prophase: chromatin condenses into chromosomes; nuclear envelope disappears; centrosomes move to opposite poles.
  • Metaphase: chromosomes align at the metaphase plate.
  • Anaphase: sister chromatids separate and move toward opposite poles.
  • Telophase: spindle dissolves; chromosomes de-condense; nuclear envelope reforms.

Meiosis (Reproductive Cell Division)

  • Meiosis I: reduces chromosome number by half; includes Prophase I, Metaphase I, Anaphase I, Telophase I.
    • Prophase I: tetrads form; crossing over occurs; homologous chromosomes pair (synapsis).
    • Metaphase I: tetrads align at the metaphase plate.
    • Anaphase I: homologous chromosomes separate to opposite poles (sister chromatids remain together).
    • Telophase I and Cytokinesis: two haploid cells formed.
  • Meiosis II: similar to mitosis but with haploid cells; Prophase II, Metaphase II, Anaphase II, Telophase II.
  • End result: four haploid gametes that are genetically different from the original cell.

Mitosis vs Meiosis – Quick Comparison

  • Mitosis: somatic cells, diploid (2n) starting with replicated chromosomes; produces two diploid daughter cells; no crossing over.
  • Meiosis: germ cells, results in four haploid (n) gametes; includes crossing over (genetic recombination) and two rounds of division (Meiosis I and II).

Quick Recap

  • Key components of the cellular level: plasma membrane, cytoplasm (cytosol + organelles), nucleus.
  • Major organelles and their roles: ER (rough vs smooth), Golgi, lysosomes, peroxisomes, mitochondria, ribosomes, cytoskeleton, centrosome, cilia/flagella, nucleus.
  • Core processes: diffusion/osmosis, active transport, vesicular transport, protein synthesis (transcription and translation), and cell division (mitosis and meiosis).