Lesson 8 - Cell Structure and Function – Comprehensive BIOL 1107 Notes

LO8.1: Principles of the cell theory and fundamental units of life

  • Cell theory (Cell Theory LO8.1):

    • All living things are made of cells – the smallest unit of life.

    • Cells are the basic units of structure, function, and reproduction in organisms.

    • All cells arise from preexisting cells.

  • Structure determines function concept:

    • Internal parts (organelles) give cells their power and capabilities.

  • Today’s mission (from Lesson 08):

    • Understand what all cells have in common.

    • Explore why cells are small.

    • Compare prokaryotic and eukaryotic cells.

    • Discover how plant and animal cells are similar and different.

  • Factory analogy summarized:

    • Nucleus = blueprint room storing DNA instructions.

    • Ribosomes = production room that manufactures proteins.

    • Cell membrane = factory walls separating interior from environment.

  • Endnote: Plants may have lytic vacuoles acting like lysosomes in animal cells; plants have microtubules and secretory vesicles (not always labeled); cell membrane and plasma membrane are two names for the same structure.

LO8.2: Structural features shared by all cells; surface area–to–volume and cell size

  • Four common components shared by all cells:

    • Plasma membrane (cell membrane) — encloses the cell.

    • Cytoplasm (cytosol) — interior fluid where cellular components reside.

    • DNA — genetic material.

    • Ribosomes — protein synthesis machines.

  • Relationship to size:

    • Surface Area to Volume (SA:V) ratio constrains cell size and exchange efficiency with the environment.

    • As a cell grows, volume increases faster than surface area, reducing the SA:V ratio and transport efficiency.

    • Small cells have higher SA:V and exchange nutrients and wastes more effectively; large cells must develop folds or compartments to compensate.

  • Prokaryotic cells emphasize diffusion across the plasma membrane due to lack of internal membranes.

  • Illustrative size comparisons (conceptual):

    • Examples include a cube with side lengths 1 mm, 2 mm, and 4 mm showing SA and V growth patterns:

    • 1 mm cube: SA = 6, V = 1;
      extSA=6extmm2,extV=1extmm3ext{SA} = 6 ext{ mm}^2, ext{ V} = 1 ext{ mm}^3

    • 2 mm cube: SA = 24, V = 8;
      extSA=24extmm2,extV=8extmm3ext{SA} = 24 ext{ mm}^2, ext{ V} = 8 ext{ mm}^3

    • 4 mm cube: SA = 96, V = 64;
      extSA=96extmm2,extV=64extmm3ext{SA} = 96 ext{ mm}^2, ext{ V} = 64 ext{ mm}^3

  • Practical takeaway: Smaller cells with larger SA:V ratios enable efficient transport and rapid response to environmental changes.

LO8.3: Prokaryotic vs. eukaryotic cells

  • Prokaryotic cell features:

    • No membrane-enclosed internal compartments (no true nucleus).

    • Nucleoid region contains the chromosome; no membrane around DNA.

    • Ribosomes are free-floating in the cytoplasm; 70S type.

    • Plasma membrane usually surrounded by a rigid cell wall (often peptidoglycan).

    • May possess capsules, pili, and flagella; some species have these structures.

    • Common domains: Archaea and Bacteria.

  • Eukaryotic cell features:

    • Internal compartmentalization with membrane-bound organelles (e.g., nucleus, ER, Golgi, mitochondria, chloroplasts).

    • DNA organized into chromosomes within a nucleus; larger genome complexity and sophisticated transport systems.

    • Typically larger than prokaryotes; use intracellular transport networks.

  • Shared and distinguishing elements:

    • Both have plasma membranes, cytoplasm, DNA, and ribosomes, but organization and complexity differ.

  • Endosymbiotic theory (context for organelles like mitochondria and chloroplasts):

    • Internal mutualistic incorporation of prokaryotes into ancestral eukaryotic cells.

    • Evidence includes: mitochondria and chloroplasts with their own DNA and ribosomes; self-replication; double membranes; size similar to prokaryotes; supported by Lynn Margulis’ work.

  • Plant-specific note:

    • Plants may contain lytic vacuoles, microtubules, secretory vesicles (not always labeled).

  • Endomembrane system connection:

    • Endomembrane components (nuclear envelope, ER, Golgi, lysosomes, vesicles, plasma membrane) coordinate synthesis, modification, packaging, and transport of lipids and proteins.

LO8.4: Functional advantages of membrane-bound organelles in eukaryotic cells

  • Key advantages of compartmentalization:

    • Specialized, simultaneous processes (protein synthesis, energy production, waste removal) can occur without interference.

    • Membranes create distinct chemical environments, enabling optimized conditions for specific reactions.

    • Internal transport systems enable larger cell size and multicellularity with division of labor.

  • Eukaryotic cells are typically ~10x larger than prokaryotes, necessitating intracellular transport networks.

  • Conceptual figure reference: A typical eukaryotic cell contains nucleus, ER, Golgi, mitochondria, etc., with specialized functions.

  • Endosymbiosis remains a core explanation for mitochondria and chloroplast origins within this context.

LO8.5: Plant cells vs. animal cells; distinct structures and functions

  • Shared: Both are eukaryotic and share nucleus, mitochondria, ER, Golgi, ribosomes, etc.

  • Plant-specific structures and roles:

    • Cell wall (cellulose) providing structural support and protection.

    • Chloroplasts for photosynthesis and energy capture.

    • Large central vacuole for water balance and storage.

    • Plasmodesmata for intercellular communication.

    • Plastids for pigment storage and other molecules.

  • Animal-specific structures and roles:

    • Centrosome (includes centrioles) organizing microtubules, spindle formation during division.

    • Lysosomes for waste and debris breakdown.

    • Microtubule organizing center (MTOC) role of centrosome.

  • Centrosomes/centrioles details:

    • Centrosome acts as MTOC in many animal cells; contains two centrioles at right angles; centriole composed of nine triplets of microtubules; held together by non-tubulin proteins.

  • Plant-specific features:

    • Cell wall construction and central vacuole contribute to turgor pressure and plant rigidity.

  • Practical implication: Differences reflect specialized roles in multicellular life – movement and signaling in animals vs photosynthesis and structural support in plants.

  • Note on labeling:

    • Figures illustrate animal cells (e.g., lysosomes, centrosome) and plant cells (cell wall, chloroplasts, central vacuole).

LO8.6: Cytoplasm, ribosomes, and cytoskeletal elements; intracellular transport, support, movement

  • Cytoplasm:

    • Gel-like region between plasma membrane and nuclear envelope.

    • Composition: cytosol (watery solution) + organelles + cytoskeleton.

    • Water content ~70 ext{-}80 ext{ \,%}; semi-solid due to proteins.

  • Ribosomes:

    • Site of protein synthesis.

    • Size difference: prokaryotic 70S; eukaryotic 80S.

    • Subunits: 60S60S (large) + 40S40S (small).

    • Not membrane-bound; located free in cytoplasm or attached to rough ER.

  • Cytoskeleton:

    • Dynamic network of protein filaments providing shape, anchoring organelles, and facilitating transport.

    • Major components and roles:

    • Microfilaments: made of actin; ~7extnm7 ext{ nm}; resist tension; drive cell crawling and muscle contraction; assist cytokinesis; move organelles.

    • Intermediate filaments: made of keratin/lamin; ~8ext–12extnm8 ext{–}12 ext{ nm}; provide mechanical strength; anchor nucleus and organelles; some types support nuclear envelope or connect cells.

    • Microtubules: made of extα−tubulinext{α-tubulin} and extβ−tubulinext{β-tubulin}; hollow tubes ~25extnm25 ext{ nm}; resist compression; serve as intracellular tracks; enable movement via cilia/flagella; drive chromosome separation during division; position organelles and vesicles.

  • The cytoskeleton functions like a combination of scaffolding, conveyor belt, and steering system for the cell.

  • Nonmembranous organelles highlighted: ribosomes and cytoskeleton.

LO8.7: Endomembrane system; protein and lipid synthesis, modification, and transport

  • Endomembrane system definition:

    • A coordinated network of membranes and organelles within eukaryotic cells that modify, package, and transport lipids and proteins.

  • Core components (within the endomembrane system):

    • Nuclear envelope

    • Endoplasmic reticulum (ER): rough and smooth

    • Golgi apparatus

    • Lysosomes

    • Vesicles

    • Plasma membrane

  • Ribosome (protein synthesis):

    • On RER, ribosomes synthesize secreted or membrane proteins; others in cytosol.

  • Mitochondria and chloroplasts noted (in context of energy and photosynthesis) as separate organelles with endosymbiotic origin; not part of the endomembrane system proper, but often discussed alongside.

  • Nucleus:

    • Houses DNA; transcription occurs here; RNA transcripts exit via nuclear pores to ER.

  • Endoplasmic Reticulum (ER):

    • Extensive, interconnected membrane network continuous with nuclear envelope.

    • Rough ER (RER): studded with ribosomes; synthesizes proteins for secretion or membrane insertion; proteins often go to Golgi for modification.

    • Smooth ER (SER): lacks ribosomes; synthesizes lipids (phospholipids, steroids); detoxifies drugs/poisons (especially in liver); stores calcium ions in muscle cells.

  • Golgi apparatus:

    • Series of flattened sacs (cisternae) near ER.

    • Functions: receives proteins/lipids from RER; modifies (e.g., adds carbohydrates to form glycoproteins); sorts and packages for delivery to final destinations (plasma membrane, lysosomes, secretion).

    • Cis face = receiving side; Trans face = shipping side.

  • Vesicles:

    • Transport sacs that move materials between organelles or to/plasma membrane.

  • Nucleus, ER, Golgi, vesicles, and plasma membrane work together to synthesize, modify, and transport cellular products.

  • Additional note: secretory pathway involves proteins entering RER lumen, folding, packaging into transport vesicles to Golgi for further processing and sorting.

LO8.8: Lysosomes, peroxisomes, vacuoles, and vesicles

  • General roles:

    • Store and transfer chemicals; segregate contents with membranes; may store nutrients or wastes.

  • Lysosomes:

    • Membrane-bound sacs with acidic hydrolases (~pH 5).

    • Break down damaged organelles and ingested particles; enable intracellular recycling; fuse with vesicles during phagocytosis or autophagy.

  • Peroxisomes:

    • Contain enzymes that detoxify harmful compounds (e.g., hydrogen peroxide H₂O₂).

    • Detoxification and oxidation reactions; lipid metabolism; contribute to myelin synthesis in nerves.

    • Example enzymatic arsenal includes catalase: 2H<em>2O</em>2<br>ightarrow2H<em>2O+O</em>22 H<em>2O</em>2 <br>ightarrow 2 H<em>2O + O</em>2.

  • Vacuoles:

    • Storage compartments for water, nutrients, or wastes; especially prominent in plant cells (central vacuole).

  • Vesicles:

    • Small transport sacs shuttling materials between organelles or to the plasma membrane.

  • Endocytosis and exocytosis:

    • Vesicles participate in endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis (secretory vesicles fusing with plasma membrane to release contents).

  • Role of membrane trafficking:

    • Essential for intracellular digestion, detoxification, storage, and targeted delivery of products to destinations.

LO8.9: Mitochondria and chloroplasts; endosymbiotic theory

  • Mitochondria:

    • Function: aerobic respiration; generates most ATP; also involved in apoptosis, calcium storage, and heat production.

    • Structure: double membrane; outer membrane smooth; inner membrane folded into cristae; matrix contains enzymes, 70S ribosomes, and circular DNA.

    • Endosymbiotic evidence:

    • Self-replicating, divide independently of the cell.

    • Contain their own circular DNA and 70S ribosomes, similar to bacteria.

    • Size and double-membrane structure support ancestry as endosymbionts.

  • Chloroplasts:

    • Function: photosynthesis; light energy converted to ATP and sugars.

    • Structure: double membrane; thylakoid membranes organized into grana; stroma contains enzymes, circular DNA, and 70S ribosomes.

    • Endosymbiotic evidence:

    • Self-replicating; chloroplast DNA and ribosomes resemble prokaryotic counterparts; similar size to bacteria.

  • Endosymbiotic theory core:

    • Eukaryotes evolved through a union of primitive aerobic prokaryotes with larger anaerobic host cells.

    • Internalized aerobes became mitochondria; photosynthetic cyanobacteria evolved into chloroplasts.

  • Historical note:

    • Proponent: Lynn Margulis; theory explains the presence of circular DNA, 70S ribosomes, and double membranes in these organelles.

LO8.10: Predicting functional consequences of altering organelles or cellular processes

  • General predictive framework:

    • To predict function of a cell: identify organelles present; analyze function of each; infer overall cell function from presence and abundance of organelles.

    • To predict structure of a cell: identify cell’s function; analyze requirements; predict structures/organelles likely present.

  • Examples:

    • Large number of mitochondria indicates high energy demand.

    • A cell specialized for protein synthesis and secretion is expected to have rough ER and Golgi apparatus.

  • Five-step approach to predict outcome when altering vesicles, Golgi, or ER (LO8.10):
    1) Define the element or process being changed.
    2) Understand the normal function of that element.
    3) Consider potential effects and what could go wrong.
    4) Make theoretical predictions about outcomes.
    5) Evaluate and refine predictions with new information.

  • Four-step approach to predicting consequences of altering a single organelle (alternative framing):
    1) Identify the organelle.
    2) Understand its function.
    3) Predict the impact on the cell (adapt or fail).
    4) Consider secondary effects on other systems.

  • Case study: Altering the ER (partial deactivation):

    • Predicted consequences:

    • Fewer properly folded proteins → accumulation of misfolded proteins → stress.

    • Less lipid synthesis → membrane formation problems.

    • Disrupted detox and calcium storage in SER.

    • Secondary effects: delays in Golgi trafficking, membrane defects, impaired signaling.

    • Bottom line: The ER is essential for maintaining flow of materials and cellular homeostasis; its failure affects the entire cell.

Table and additional references

  • Comparative analysis (Table 3.5, pages 62–63): Prokaryotic vs. Eukaryotic organelles — core functions and presence/absence of membranous organelles; highlights:

    • Ribosomes: protein synthesis; present in all, with 70S in prokaryotes and 80S in eukaryotes.

    • Cytoskeleton: important for shape and transport; present in all; prokaryotes have some cytoskeletal elements; eukaryotes have fully developed cytoskeleton.

    • Nucleus: present in eukaryotes; absent in prokaryotes.

    • Endomembrane system: present in eukaryotes; absent in prokaryotes.

    • Lysosomes and peroxisomes: present in eukaryotes; typically absent in prokaryotes.

    • Vacuoles and vesicles: storage and transport in eukaryotes; absent in prokaryotes.

  • Endomembrane system overview image reference: nucleus, ER, Golgi, lysosomes, vesicles, plasma membrane.

Quick connections and real-world relevance

  • Cellular organization underpins multicellularity and division of labor—why eukaryotes can be more complex and larger.

  • Endosymbiosis as a foundational concept for energy metabolism (mitochondria) and photosynthesis (chloroplasts) in eukaryotes, influencing our understanding of evolution.

  • Understanding SA:V constraints explains why many cells remain small or develop internal membranes and organelles to increase surface area and internal transport efficiency.

  • Endocytosis and exocytosis describe fundamental cellular traffic that underlies immune responses, secretion of hormones/enzymes, and nutrient uptake.

Key terms recap (selected from Transcript)

  • Cell theory, Prokaryote, Eukaryote, Endomembrane system, Cytoskeleton, Microfilaments, Intermediate Filaments, Microtubules, Nucleus, Nuclear envelope, Nucleolus, Chromatin, Ribosome, ER, Rough ER, Smooth ER, Golgi apparatus, Lysosome, Peroxisome, Vacuole, Vesicle, Mitochondrion, Chloroplast, Endosymbiosis, Endosymbiotic Theory, SA:V ratio, Plasmodesmata, Centrosome, Centriole, Cilia, Flagella, Taxis, Run, Tumble, β-lactam antibiotics, Peptidoglycan, Gram-positive, Gram-negative, Secretory vesicles, Photosynthesis, Grana, Stroma, Thylakoid, Matrix, Cristae, 70S vs 80S ribosomes, Lysosomal digestion, Autophagy, Phagocytosis, Endocytosis, Exocytosis

Note: All mathematical expressions and relevant numbers have been formatted in LaTeX where appropriate, including SA:V discussions, ribosome sizes, and organelle dimensions where stated.