JX

Lesson 05: The Structure of Cells

5.1 Cellular Components and Properties

  1. Organizational Hierarchy of Life

    • Atoms \rightarrow molecules \rightarrow macromolecules \rightarrow subcellular structures (organelles) \rightarrow cells.

  2. Organelles

    • Discrete internal macromolecular structures with specialized functions.

    • Assembled from macromolecules.

  3. Cell Classification

    • Commonly classified as either prokaryotic or eukaryotic.

  4. Microscopy for Visualization

    • Human Eye: Can image objects as small as \approx 200 \mu m.

    • Light Microscope: Illuminates with visible light, images objects as small as 200 nm. Can image all cells and many subcellular structures (e.g., mitochondria, chloroplasts).

    • Electron Microscope: Illuminates with an electron beam, images even smaller objects (e.g., virus particles, macromolecules). Provides better perspective of organelle structure.

  5. Prokaryotic Cells

    • Comprise two of the three domains of life: Archaea and Bacteria.

    • Unicellular and generally smaller than eukaryotic cells.

    • Lack the internal compartments characteristic of eukaryotic cells.

    • Cell Walls: Surround the plasma membrane, composed of carbohydrates and proteins (specific molecules differ between Archaea and Bacteria, explaining antibiotic specificity).

    • Capsule: Some produce a polysaccharide coating.

    • Pili and Flagella: Some produce protein-based extensions for cell interaction and swimming.

  6. Fundamental Similarities between Prokaryotes and Eukaryotes

    • Plasma Membrane: Separates the interior (cytoplasm) from the exterior environment.

    • Genetic Material: DNA serves as the genetic material.

    • Gene Expression: DNA is translated into proteins to carry out cellular functions, a process tightly controlled.

    • Energy Metabolism: Generate and consume energy (e.g., glycolysis, cellular respiration, photosynthesis).

    • Ribosomes: Present in both, large RNA-protein complexes that synthesize proteins by converting nucleotide sequence information into amino acid sequence information.

      • Exist as two discrete units: large and small subunits, which assemble during polypeptide synthesis.

      • Found in the cytoplasm of both prokaryotes and eukaryotes.

      • Also found inside mitochondria and chloroplasts of eukaryotic cells.

  7. Eukaryotic Cell Features

    • Have membrane-bound organelles that compartmentalize different functions.

    • Comparison of Animal and Plant Cell Features:

      • Both contain: Nucleus, Endoplasmic Reticulum (ER), Golgi apparatus, Peroxisomes, Mitochondria.

      • Lysosomes: Present in animal cells, presence in plant cells is debated.

      • Only Plant Cells contain: Chloroplasts, Vacuole, Cell Wall.

  8. Eukaryotic Cell Interior Terminology

    • Cytoplasm: Everything inside the plasma membrane; applies to both prokaryotes and eukaryotes.

    • Cytosol: All cellular material enclosed by the plasma membrane but outside of organelle membranes.

      • Gel-like consistency, filled with water-soluble molecules.

    • Lumen: The interior of each organelle, separated from the surrounding cytosol by the organelle membrane.

5.2 Eukaryotic Organelles: Nucleus to Vacuoles

  1. Nucleus

    • Typically the largest and most visible compartment in eukaryotic cells.

    • Interior: Mostly chromatin (a complex of proteins and DNA).

    • Nuclear Envelope: Consists of an inner and outer membrane, separating the nucleus from the cytosol.

    • Nuclear Pores: Large openings in the nuclear envelope for protein and RNA movement between the nucleus and cytosol.

    • Nucleolus: A distinctive structure within the nucleus that synthesizes the RNA components of the ribosome.

    • Functions: Protects DNA from damage; separates RNA synthesis (nucleus) from protein synthesis (cytosol), providing more control over protein synthesis.

  2. Endoplasmic Reticulum (ER)

    • A network of membranes with a continuous lumen; accounts for \approx 10 \% of cell volume and 50 \% of total cell membrane.

    • Critical Functions: Protein synthesis, lipid synthesis, calcium ion storage, detoxification.

    • Rough ER (RER):

      • Named for the presence of ribosomes on its cytosolic surface.

      • Synthesizes roughly one-third of all types of cellular proteins, which enter the endomembrane system.

    • Smooth ER (SER):

      • Lacks ribosomes.

      • Site of lipid synthesis and detoxification reactions.

    • Transport: Proteins and lipids produced by the ER move to other parts of the endomembrane system via transport vesicles.

  3. Golgi Apparatus

    • Consists of a polarized stack of flattened membranes.

    • Functions: Enzymes modify proteins and lipids arriving from the ER, preparing them for final destinations.

    • Transport Pathway: Vesicles from the ER arrive at the cis (entry) face, contents flow through the Golgi towards the trans (exit) face.

    • Sorting: At the trans face, vesicles form and travel, sorting proteins to their correct destinations.

  4. Exocytosis

    • The outward movement of proteins and lipids from the ER \rightarrow Golgi, and then to the plasma membrane, extracellular environment, or lysosomes.

    • A transport vesicle fuses with the plasma membrane, releasing its inner contents (e.g., secreted proteins) and incorporating its phospholipids into the plasma membrane.

  5. Lysosomes

    • Contain digestive enzymes that break down molecules, allowing the cell to recycle building blocks.

    • Enzyme Pathway: Synthesized on the RER, travel through the endomembrane pathway via the Golgi, and are sorted to the lysosomes.

    • Degradation: Degrade old or damaged organelles and material ingested by the cell.

    • Internal Environment: Highly acidic (pH \approx 5), which facilitates macromolecule degradation.

  6. Peroxisomes

    • Enclose a variety of enzymatic reactions that could damage molecules in the cytosol.

    • Reactions: Oxidation of fatty acids, various biosynthetic reactions, and detoxification of compounds (e.g., hydrogen peroxide).

    • Often include an internal crystal-like structure of digestive and detoxifying enzymes.

  7. Vacuoles (Plants and Fungi)

    • Several types identified (e.g., central vacuole in plants).

    • Functions: Maintaining water balance, storage of small molecules, and storage of waste products.

    • Some also function in macromolecule degradation, analogous to lysosomes.

5.3 Mitochondria and Chloroplasts: The Endosymbiont Theory

  1. Theory of Endosymbiosis

    • Origin: Scientists theorize that mitochondria and chloroplasts originated from bacteria engulfed by ancestral eukaryotes.

      • Mitochondria: From bacteria that performed oxidative metabolism.

      • Chloroplasts: From bacteria that performed photosynthesis.

    • Evidence: Both organelles contain their own circular DNA, ribosomes, and divide within the eukaryotic cell via binary fission (characteristics typically seen in prokaryotes).

  2. Mitochondria

    • Found in both animal and plant cells.

    • Function: Synthesize ATP from sugars; often called the "powerhouse of the cell."

    • Reproduction: By binary fission.

    • Structure: Two membranes (outer membrane interacts with the cytosol; inner membrane is where ATP synthesis reactions occur).

    • Genetic Material: Contain circular DNA molecules that code for some mitochondrial proteins, but most are coded by nuclear DNA.

    • Ribosomes: Present within the matrix to facilitate DNA expression.

  3. Chloroplasts

    • Found in plant cells and other photosynthetic eukaryotes.

    • Function: Use light energy to synthesize ATP and sugar molecules.

    • Reproduction: By binary fission.

    • Structure: Three membranes (unlike mitochondria's two).

    • Genetic Material: Contain circular DNA and ribosomes.

    • Pigment: Readily visible due to the presence of light-absorbing chlorophyll.

5.4 The Cytoskeleton and Extracellular Structures

  1. Eukaryotic Cytoskeleton

    • A complex and dynamic internal structure, constantly changing.

    • Filament Types: Comprised of three main types of protein filaments:

      • Actin filaments

      • Microtubules

      • Intermediate filaments

    • Note: The term cytoskeleton is conventionally reserved for eukaryotic structures only, as prokaryotic flagella and pili, though functionally similar, are molecularly different.

  2. Actin Filaments

    • Composition: Assembled from actin monomers (globular proteins).

    • Roles:

      • Muscle Contraction: Work with myosin filaments to generate the sliding mechanism.

      • Non-muscle Cells: Control cell shape and involved in cell crawling (e.g., amoeba engulfing yeast).

  3. Microtubules

    • Composition: Assembled from tubulin dimers (globular proteins).

    • Roles:

      • Cell Organization: Form a network throughout the cell, acting as a transport system for vesicles (similar to highways).

      • Cell Division: Anchor to DNA and help pull DNA apart during mitosis.

      • Cilia and Flagella: Hair-like appendages on the outside of the cell that support cell swimming.

        • Structure: Comprised of multiple microtubules bundled in a distinctive 9+2 arrangement (two central, nine pairs in a circle).

        • Movement: Driven by motor proteins (e.g., dynein) along the microtubules, requiring ATP. (Eukaryotic flagella differ molecularly and in powering mechanism from bacterial flagella).

      • Internal Vesicle Movement: Motor proteins also drive the movement of internal cellular vesicles (e.g., neurotransmitter transport, pigment changes in animals).

  4. Intermediate Filaments

    • Composition: Formed from a variety of different types of fibrous subunits (not globular).

    • Roles: Serve to support and protect the cell.

    • Examples:

      • Keratin: Major component of skin cells, hair, and nails, providing a waterproof barrier.

      • Lamins: Form a layer of intermediate filaments surrounding the nucleus.

    • Structure: Arranged in an overlapping pattern, similar to burlap cloth.

  5. Extracellular Structures (Eukaryotic)

    • Secreted by cells, providing support from the outside.

    • Synthesized in the ER and sent to the cell surface via the endomembrane pathway.

    • Animal Cells: Extracellular Matrix (ECM)

      • A meshwork of secreted proteins and carbohydrates.

      • Has a fibrous nature and is particularly abundant in connective tissue.

    • Plant Cells (and Fungal Cells): Cell Wall

      • Located outside the plasma membrane, providing protective and structural support.

      • Chemically and structurally distinct from prokaryotic cell walls.

      • Plant Cell Walls: Main structural component is the carbohydrate cellulose.

      • Fungal Cell Walls: Main component is chitin.

      • Material is produced and secreted by the cells.