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Study Notes: Cell Membrane, Transport, Organelles, Metabolism, Mitosis, and Genetics

Cell Membrane (Plasma Membrane): Structure, Synonyms, and Baseline

  • Image shows a phospholipid bilayer. Key points:
    • It is a bilayer: two layers of phospholipids (phospholipid = phosphate-containing lipid; lipid = fat).
    • It forms the barrier between inside and outside of the cell, common to all cell types (heart, blood, liver, brain, etc.).
    • The membrane contains channels and carriers (proteins) embedded in/through the wall to move substances in and out.
  • Terminology you’ll hear:
    • Cell membrane, plasma membrane, and cell wall are used as synonyms in this context.
    • The barrier’s job is to maintain homeostasis by regulating what enters and leaves, maintaining balanced water, pH, and electrical charge.
    • The membrane is sensitive to the outside environment via receptors, which detect changes (e.g., Na⁺ concentration, temperature, water balance, pH) and trigger responses.
  • Structural role:
    • Provides a scaffold by attaching to the cytoskeleton to help maintain cell shape and stability.
  • Why membranes matter:
    • Without a barrier, nutrients cannot enter and wastes cannot exit; with no selective permeability, balance cannot be maintained.
  • Quick takeaway:
    • The membrane is a selective barrier, a site of transport, and a structural anchor for the cell.

The Membrane and Its Proteins: Types and Functions

  • Major categories of membrane proteins (named by function):
    • Anchoring proteins: anchor the cytoskeleton to the membrane to maintain shape.
    • Recognition proteins: recognize self (identifying what belongs) vs. foreign.
    • Enzymes: catalyze reactions at the membrane; many enzymes end in the suffix -ase (e.g., lipase).
    • Receptor proteins: receive information from the outside environment to trigger a response.
    • Transport proteins: help move substances across the membrane (solutes, ions, large molecules).
    • Cell adhesion proteins: help cells stick together or form barriers (cell-to-cell attachment).
  • Quick clarifications:
    • Cell adhesion proteins = cell-to-cell binding.
    • Receptor proteins = binding to external signals or substances (could be to other proteins or foreign matter).
    • A single membrane image may not show all protein types; some can be indistinct, but function is what matters.
  • Visual cue in images:
    • A front-protein shown as a transport channel; other proteins could be receptors or carriers, but you won’t be asked to identify exact proteins in an image—focus on function.
  • Permeability context:
    • The membrane is selectively permeable: some things pass, others are blocked to maintain homeostasis.

Permeability Concepts and Gradient Basics

  • Selective permeability: cells allow some substances to pass while blocking others to maintain internal balance.
  • Visual metaphors used:
    • Impermeable barrier (brick wall): nothing passes.
    • Freely permeable (grass): many things pass readily.
    • Selectively permeable (brick with holes): some things pass depending on size/shape/energy.
  • Concentration gradient:
    • Definition: the difference in concentration of a substance inside vs. outside the cell.
    • Tendency: substances move from high concentration to low concentration unless energy or barriers block them.
  • Key transport modes (passive, no energy):
    • Simple diffusion: small particles move directly through the membrane without energy.
    • Facilitated diffusion: requires help but no energy; uses channels or carriers.
    • Channel-mediated diffusion: diffusion through membrane channels (type of facilitated diffusion).
    • Carrier-mediated diffusion: diffusion via a carrier molecule (like a bus) to carry a solute across.
    • Osmosis: diffusion of water across a semipermeable membrane toward higher solute concentration (water moves to balance concentrations).
  • Quick explanations:
    • Simple diffusion examples: water, O₂, CO₂ (small molecules that cross easily without energy).
    • Facilitated diffusion examples: ions or larger molecules that cannot pass alone and need a channel or carrier; still no energy used.

Osmosis, Isotonic, Hypertonic, and Hypotonic Concepts

  • Osmosis specifics:
    • Movement of water only, across a membrane that is permeable to water but not to solutes.
    • Water moves toward higher solute concentration (lower water concentration) to equalize concentrations.
    • Outcome: equal concentrations (not necessarily equal volumes) on both sides.
  • Isotonic, hypertonic, hypotonic (definitions and implications):
    • Isotonic: equal water movement in and out, balanced; cell size remains stable.
    • Hypertonic: outside solution has higher solute concentration; water leaves the cell; cell shrinks (crenation in red blood cells, shrinkage).
    • Hypotonic: outside solution has lower solute concentration; water enters the cell; cell swells and may burst (lysis).
  • Illustrative analogy used in class:
    • Brick wall = impermeable (no water through).
    • Grass = freely permeable to water (lots of holes).
    • Partially permeable (mixed brick and holes) mirrors selective permeability.
  • Practical takeaway:
    • Cells aim for isotonic conditions to maintain stability; deviations lead to swelling or shrinkage.

Active Transport: Moving Against the Gradient with Energy

  • Active transport moves substances from low to high concentration (against the gradient) and requires energy.
  • Primary energy source:
    • Adenosine triphosphate (ATP).
    • ATP: adenosine with three phosphates; energy released when a phosphate group is removed.
    • Formula: ATP → ADP + P_i (energy released).
  • Na⁺/K⁺ pump as the canonical example of active transport:
    • Function: Na⁺ is pumped out of the cell; K⁺ is pumped into the cell.
    • Stoichiometry (per cycle): three Na⁺ out and two K⁺ in, powered by one ATP.
    • Representation: 3 \, ext{Na}^+{ ext{out}} \ 2 \, ext{K}^+{ ext{in}} \text{per ATP} with energy conversion:
    • ATP hydrolysis: ext{ATP}
      ightarrow ext{ADP} + P_i
  • Why this matters: helps maintain the cell’s electrical charge and homeostasis; imbalance affects membrane potential and cell function (a preview of nerve and muscle physiology).

Vesicular Transport: Endocytosis and Exocytosis

  • Vesicular transport is a mechanism to move large particles or bulk materials via vesicles.
  • Endocytosis: import into the cell; vesicle forms from the plasma membrane to engulf material.
  • Exocytosis: export from the cell; vesicle fuses with the membrane to release contents outside.
  • Conceptual note on terminology:
    • Endocytosis = inside (endo- means inside).
    • Exocytosis = outside (exo- means outside).
    • The suffix -osis denotes a process or condition.
  • Relationship: endocytosis and exocytosis are reverse processes; one brings material in, the other releases it.

Key Cell Organelles and Their Roles (High-Level Overview)

  • Nucleus:
    • Control center; houses DNA; DNA provides instructions for cell identity and function (e.g., liver vs. bladder cells).
  • Mitochondria:
    • Powerhouse that generates ATP (energy) for the cell.
    • Glucose input leads to ATP production via aerobic respiration when oxygen is available, and via anaerobic pathways when oxygen is limited.
  • Endoplasmic Reticulum (ER):
    • Rough ER and smooth ER are involved in synthesis; rough ER has ribosomes and synthesizes proteins; smooth ER synthesizes lipids.
    • Producers of proteins and lipids, respectively.
  • Golgi apparatus:
    • The “post office” of the cell; packages, processes, and sorts proteins and lipids for transport or use inside the cell.
  • Lysosomes:
    • Cleaning crew; degrade waste products and potentially harmful materials.
  • Ribosomes:
    • Protein synthesis machines; translate genetic information into proteins.
    • Link to transcription/translation processes.

Mitochondrial Energy Production: Glucose to ATP

  • Overview: Energy production begins with glucose entering the cell and mitochondria.
  • Glycolysis (in cytosol):
    • Breaks down glucose into a smaller amount of ATP; net gain of 2 \,\mathrm{ATP} per glucose.
  • Aerobic respiration (with oxygen):
    • If oxygen is available, the breakdown continues through the mitochondria via the Krebs cycle and oxidative phosphorylation.
    • Net ATP yield per glucose: 36\,\mathrm{ATP} (as described in lecture; often stated as ~30-38 depending on shuttle systems, but the transcript emphasizes 36).
    • Byproducts: water (H₂O) and carbon dioxide (CO₂).
  • Anaerobic respiration (without oxygen):
    • ATP yield is much lower; lactic acid may accumulate in muscles during rapid exercise when oxygen is limited.
  • Key point: Aerobic respiration is more efficient than anaerobic respiration; oxygen availability drives higher ATP yield.
  • Quick clarification from the lecture:
    • Pyruvate is an intermediate product of glucose breakdown; in aerobic conditions it proceeds to the Krebs cycle; in anaerobic conditions, pyruvate is redirected to lactate production in some tissues.
  • Practical note: The liver, muscles, and other tissues handle oxygen availability and waste removal differently, affecting ATP yield and byproducts.

Transcription and Translation: From DNA to Protein

  • General idea:
    • Transcription and translation are two linked processes that produce proteins from genetic information.
  • Transcription:
    • What happens: DNA is used to synthesize RNA (specifically, mRNA).
    • Location: Nucleus (DNA resides in the nucleus; transcription occurs there).
    • Outcome: RNA (messenger RNA, mRNA) that carries the genetic message to the ribosome.
  • Translation:
    • What happens: mRNA is translated into a protein.
    • Location: Cytoplasm (at ribosomes); can occur on free ribosomes or ribosomes attached to rough ER.
    • Key players:
    • mRNA: the message (the recipe) that encodes the protein.
    • tRNA: transfer RNA; the translator; brings amino acids to the ribosome and matches them to codons on mRNA.
    • rRNA: ribosomal RNA; a component of the ribosome that helps link amino acids together.
  • Specifics about where translation occurs:
    • Translation can occur on ribosomes in the cytoplasm or on ribosomes attached to rough ER; animation emphasizes cytoplasmic (free ribosome) translation in the shown example, but either location is possible.
  • How the components interact:
    • The ribosome (rRNA + proteins) catalyzes the formation of peptide bonds between amino acids carried by tRNA.
    • The tRNA anticodon pairs with the mRNA codon, ensuring correct amino acid incorporation.
    • The mRNA sequence determines the amino acid sequence of the protein.
  • Quick conceptual recap:
    • mRNA = messenger; tRNA = translator; rRNA = ribosome.
    • Transcription starts with DNA in the nucleus to produce RNA; translation occurs in the cytoplasm at the ribosome to build a protein.
    • The process is highly context-dependent and is taught in more depth in lab simulations and animations.

Mitosis: Duplication of the Cell’s DNA and Formation of Two Daughter Cells

  • Context:
    • Mitosis is the cellular process that copies the cell’s DNA to duplicate a cell.
    • Interphase (before mitosis) is when the cell is functioning and not actively dividing; most of a cell’s life is spent in interphase.
  • Stages of mitosis (as described in the lecture):
    • Prophase: chromosomes condense and become visible; spindle apparatus begins to form.
    • Metaphase: chromosomes line up along the middle equator of the cell.
    • Anaphase: sister chromatids are pulled apart and pulled toward opposite poles; cell elongates.
    • Telophase: chromosomes arrive at poles; nuclear envelope begins to reform around each set of chromosomes; two nuclei form.
  • Outcome:
    • After mitosis, two daughter cells are produced; the original cell is effectively divided and no longer exists as a single entity.
    • The process ensures genetically identical copies of the original cell.
  • Additional notes from the lecture:
    • If DNA is damaged during replication, the daughter cells may carry mutations, which can lead to abnormal growth or cancer-like outcomes.
    • Aging and cellular lifespan: all cells have predetermined lifespans; over time, duplicated cells can become less efficient, contributing to aging.
  • Contextual takeaway:
    • Mitosis is a fundamental duplication mechanism; DNA replication fidelity is crucial for healthy cell function and organismal health.

Rationale, Review, and Connections to Practice

  • The lecture emphasizes the connection between membrane function, transport, and cellular homeostasis, with downstream implications for physiology (nerve impulses, muscle contraction, kidney function, etc.).
  • The content links to real-world relevance:
    • Pathogens can trick receptor systems or damage membranes, impacting homeostasis and immune responses (to be covered in immune system modules).
    • Potassium and sodium gradients underlie electrical activity in nerves and muscles; the Na⁺/K⁺ pump is foundational to maintaining those gradients.
    • Cellular energy production (glycolysis and aerobic respiration) fuels all cellular processes, including active transport and mitosis.
  • Terminology and language cues:
    • Words ending in -ase indicate enzymes (e.g., lipase).
    • Endo- means inside; exo- means outside; -osis denotes a process or condition (e.g., endocytosis, exocytosis, osmosis).
  • Summary of key takeaways:
    • Cells maintain homeostasis via a selectively permeable plasma membrane and a set of transport mechanisms (passive and active).
    • Diffusion, facilitated diffusion, and osmosis govern passive transport; the Na⁺/K⁺ pump exemplifies active transport.
    • Vesicular transport (endocytosis and exocytosis) handles bulk material movement.
    • Organelles coordinate energy, synthesis, packaging, and waste management to support cellular function.
    • Mitosis orchestrates DNA duplication and division into two genetically identical daughter cells.
    • Transcription and translation convert genetic information into functional proteins, with distinct cellular compartments (nucleus for transcription; cytoplasm for translation).
  • Quick Q&A reminders:
    • Where does transcription occur? In the nucleus, from DNA to RNA.
    • Where does translation occur? In the cytoplasm, at ribosomes (free in cytoplasm or on rough ER).
    • What is the Na⁺/K⁺ pump’s stoichiometry? 3\,\mathrm{Na^+}{\text{out}}, 2\,\mathrm{K^+}{\text{in}}\; \text{per} \; \mathrm{ATP}; ATP hydrolysis provides the energy: \text{ATP} \rightarrow \text{ADP} + P_i.