Lecture Notes: Protein Denaturation, Endomembrane System, and Organelle Structure

Notes from Lecture: Cell Structures, Protein Denaturation, and Organelles

  • Protein denaturation and renaturation

    • Denaturation at low pH leads to loss of protein function because the acidic environment disrupts non-covalent interactions that maintain the 3D shape (ionic bonds, hydrogen bonds, hydrophobic interactions).
    • The primary structure (peptide backbone sequence) is typically retained during denaturation, while secondary, tertiary, and quaternary structures unravel.
    • Level of protein structure retained during denaturation: primary structure.
    • Renaturation can restore protein function when conditions return to favorable folding conditions; function is recovered if the protein can refold properly.
    • Practical note from instructor: if you don’t feel confident about an answer, don’t write it—you’ll risk losing points for incorrect statements; write what you know.

  • Administrative and course logistics (Blackboard and syllabus)

    • Dates in the schedule were copied incorrectly; the instructor updated the syllabus and posted the corrected version on Blackboard.
    • One quiz will be on Blackboard at the end of the semester; due dates will be posted and reminded about.
    • The unit sequence for the course will continue; the next unit will build on previous material (e.g., nuclear envelope discussion connects to the following topics).

  • Recap of cell organization: prokaryotes vs eukaryotes

    • Prokaryotic cells do not have a true nucleus; genetic material is localized to the nucleoid.
    • Eukaryotic cells are generally larger and contain membrane-bound organelles; DNA resides in the nucleus bounded by the nuclear envelope.
    • The nucleus is the center where genetic material is contained; the nuclear envelope surrounds it.
    • Eukaryotic cells include animal and plant cells.
    • The lecture emphasizes that you should recognize whether a given diagram shows a prokaryotic vs eukaryotic cell structure, e.g., presence/absence of a true nucleus and membrane-bound organelles.

  • Endomembrane system and protein trafficking

    • Endomembrane system = network of membrane-bound organelles that work together for protein synthesis, modification, and transport.
    • Components (at a high level): nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, vesicles, lysosomes, and other membrane-bound compartments.
    • The ER is continuous with the nuclear envelope and provides a membrane source for these organelles; it contains two distinct regions: rough ER (ribosome-studded, protein synthesis) and smooth ER (lipid synthesis, detoxification).
    • Vesicles are membrane-bound transport bubbles that carry proteins and other molecules between compartments.
    • The ER and cytosol are major sites of protein synthesis for different destinations:
    • Rough ER synthesizes proteins destined for secretion, the plasma membrane, or lysosomes (co-translational translocation into the ER).
    • Cytosol synthesizes soluble proteins that function in the cytoplasm.
    • The nucleus and ER are physically connected via the nuclear envelope; vesicular trafficking moves cargo between compartments.

  • Ribosomes

    • Ribosomes consist of two subunits that come together to form a functional ribosome; they are not membrane-bound organelles.
    • In eukaryotes, ribosomes can translate proteins destined for the cytosol or for secretion/organelles via the rough ER.
    • Bacteria have a different ribosome structure; this difference allows selective targeting by antibiotics without harming human ribosomes.

  • Mitochondria: structure and function

    • Mitochondria are membrane-bound organelles (not part of the endomembrane system) and are the sites of cell respiration (oxidative phosphorylation) producing ATP.
    • Key structural features:
    • Outer membrane
    • Inner membrane with folds called cristae, which increase surface area for ATP synthesis
    • Intermembrane space between the two membranes
    • Matrix inside the inner membrane where many metabolic enzymes reside
    • The cristae increase the surface area available for embedded proteins involved in the electron transport chain and ATP synthesis.
    • Some steps of respiration occur in the mitochondrial matrix, with others associated with the inner membrane.
    • Proton motive force (PMF) forms across the inner membrane due to proton pumping, driving ATP synthesis via ATP synthase.
    • Proton motive force (general formula):
      \Delta p = \Delta \psi - \left(\frac{2.303 RT}{F}\right) \Delta pH
    • where Δp is the proton motive force, Δψ is the membrane potential, R is the gas constant, T is temperature in Kelvin, F is Faraday’s constant, and ΔpH is the pH difference across the membrane.
    • At physiological temperature (about 37°C): \Delta p \approx \Delta \psi - 0.0615 \Delta pH (in volts when Δψ is in volts and ΔpH is unitless).

  • Chloroplasts: structure and photosynthesis

    • Chloroplasts are the sites of photosynthesis in plant cells and some algae.
    • They also have a double membrane similar to mitochondria and contain chlorophyll (the green pigment).
    • Internal membrane system includes thylakoids, which are flattened sacs organized into grana (granum; plural grana).
    • Thylakoids are where the light-dependent reactions occur; grana increase surface area for these reactions.
    • The stroma is the internal fluid surrounding the thylakoids where the Calvin cycle occurs.
    • Chloroplasts, like mitochondria, have their own circular DNA and ribosomes, supporting endosymbiotic origin hypotheses.

  • Cytoskeleton and cell architecture

    • The cytoskeleton is a network of fibers that provides structure and tracks for vesicle movement within the cell.
    • It helps organize the cell’s interior and facilitates transport of vesicles along cytoskeletal tracks.

  • Plant cells vs animal cells: cell wall and extra features

    • Plant cells (and many fungi/bacteria) have a cell wall outside the plasma membrane; plant cell walls are mainly cellulose.
    • The cell wall differs among plants depending on the environment (e.g., desert plants may have adaptations to reduce water loss).
    • The plant cell also contains chloroplasts for photosynthesis and typically a large central vacuole.
    • The cytoskeleton is present in both plant and animal cells, enabling intracellular transport and structural support.

  • Lysosomes, autophagy, and protein turnover

    • Lysosomes are membrane-bound organelles containing hydrolytic enzymes that degrade proteins and other macromolecules.
    • They participate in protein degradation, especially for proteins no longer needed by the cell.
    • Autophagy is the process of recycling and degrading organelles and cellular components by delivering them to lysosomes; this helps regulate protein and organelle turnover and can influence cell growth and cancer risk by preventing excessive cell division.
    • Lysosome-vacuole fusion is a common mechanism for delivering materials to be degraded.

  • Quick connections and exam tips

    • When asked about identifying organelles on diagrams, know the structural hallmarks:
    • Nucleus with a nuclear envelope (present in eukaryotes) vs nucleoid (in prokaryotes).
    • Endomembrane system components: ER, Golgi, vesicles, lysosomes.
    • Mitochondria with outer membrane, inner membrane cristae, matrix, intermembrane space.
    • Chloroplasts with double membrane, thylakoids/grana, stroma.
    • Remember the distinction that bacteria have ribosomes different from eukaryotic ribosomes, which is exploited by some antibiotics.
    • Be able to explain how denaturation affects protein structure and function, and how renaturation can restore function under favorable conditions.

  • Real-world relevance and ethical/philosophical notes (implicit in lecture)

    • Understanding protein folding and stability has implications for disease, biotechnology, and drug design.
    • The endomembrane system’s organization underpins targeted protein processing and secretion, which are central to cell biology and medical applications.
    • Autophagy and lysosome function relate to cellular quality control and cancer biology; disrupting these processes can impact cell proliferation and aging.

  • Key terms to review

    • Nucleoid, nucleus, nuclear envelope, endomembrane system, ribosomes, rough ER, Golgi apparatus, lysosome, autophagy, mitochondrion, cristae, matrix, intermembrane space, chloroplast, chlorophyll, thylakoid, granum, stroma, cytoskeleton, cell wall, prokaryote, eukaryote, vesicle, PMF (proton motive force).