BSC1010C: General Biology 1 - Cell Structure (Module 4 - Part 2 Notes)

General Biology 1: Cell Structure

Learning Objectives - Part 2

This module covers key aspects of eukaryotic cells:

  • Describing the structure and function of nuclear transport.
  • Describing the structure and function of the endomembrane system.
  • Comparing the roles of microfilaments, intermediate filaments, and microtubules, and describing the structure and function of the cytoskeleton.
  • Comparing and contrasting cilia and flagella.

Life's Properties: Collaboration of Internal Structures

  • Life's properties emerge from the intricate collaboration of internal structures within a cell.
  • Understanding cells involves examining their parts (prokaryotic and eukaryotic structures) and how these parts fit into a cohesive whole.
  • Key areas of focus include nuclear transport, the endomembrane system, and the dynamic cytoskeleton.

Putting the Parts into a Whole

Cytology: The Study of Cells
  • Cytology is the study of cells, combining microscopy and biochemical analysis.
  • The structure of each cell component directly correlates with its function.
  • Size and number of different types of organelles: This varies depending on the cell's specialized function.
  • Examples:
    • Fat cells: Rounded, globular structures optimized for lipid storage.
    • Cardiac muscle cells: Long and tapered, specialized for contraction.
  • Variation in organelle content is a key indicator of cell specialization.
Cell Fractionation: Separating Cell Components
  • Cell fractionation is a technique that uses cell lysis (breaking open cells) and differential centrifugation to separate cellular components based on their size and density.
  • Goal: To take cells apart and isolate specific organelles for individual study.
  • Centrifuge: Used to separate cellular and molecular components.
    • Can separate components by size and density, often employing gradients for finer separation.
  • Ultracentrifuges: Highly powerful centrifuges that can spin at speeds up to 130,000 revolutions per minute (rpm) and generate forces up to 1,000,000 \times G (relative centrifugal force).
    • These are used to separate even smaller cellular particles.
The Dynamic Cell
  • Techniques: Differential centrifugation and fluorescent tags are used by researchers to isolate cell components and analyze their chemical composition.
  • Cellular Activity Examples:
    • The body's cells utilize approximately 10\text{ million} ATP molecules per second.
    • Cellular enzymes can catalyze over 25,000 reactions per second.
    • Each membrane phospholipid can traverse the breadth of its organelle or the entire cell in less than a minute.
    • Hundreds of trillions of mitochondria are completely replaced approximately every 10 days, highlighting the dynamic nature of cellular components.

Cell Systems I: Nuclear Transport

The Nucleus: Information Center
  • The nucleus serves as the information center of eukaryotic cells.
  • Function: Genetic information stored in DNA is decoded and processed here.
  • Large suites of enzymes interact within the nucleus to produce RNA messages.
  • Nucleolus: Functions as the primary site of ribosome assembly.
    • Ribosomal RNA (rRNA) binds with proteins to form ribosomes.
    • Messenger RNA (mRNA) carries genetic information from DNA to synthesize proteins in the cytoplasm.
Structure and Function of the Nuclear Envelope
  • Nuclear envelope: A double membrane that separates the nucleus from the rest of the cell (cytosol).
  • Nuclear pore complexes: Openings that perforate the nuclear envelope.
    • They connect the inside of the nucleus directly with the cytosol.
    • Each complex consists of about 30 different proteins.
  • Traffic into the nucleus (Inbound traffic): The nucleus selectively imports large molecules.
    • Nucleoside triphosphates (building blocks for DNA/RNA).
    • Proteins responsible for DNA copying (replication).
    • Proteins responsible for RNA synthesis (transcription).
    • Proteins needed for assembling ribosomes.
  • A typical cell imports over 500 molecules through 2000 to 5000 nuclear pores every second, demonstrating high transport activity.
Molecular Transport into the Nucleus
  • Selectivity: Import of large molecules into the nucleus is highly selective.
  • Nuclear pores: Act as dynamic gates, controlling passage through the nuclear envelope.
  • Nuclear Localization Signal (NLS):
    • Nuclear proteins contain a specific amino acid sequence called a Nuclear Localization Signal (NLS).
    • This NLS serves as a recognized tag that allows import receptor proteins to bind to the protein and facilitate its transport through the nuclear pore complex into the nucleus.
  • Export of molecules:
    • mRNA and tRNA molecules, which are synthesized in the nucleus, are actively exported to the cytosol to participate in protein synthesis.
    • Ribosomal subunits, assembled in the nucleolus, are also exported to the cytoplasm.

Cell Systems II: The Endomembrane System

  • The endomembrane system is a collection of membranes inside and surrounding the eukaryotic cell, functionally interconnected either directly or through the transfer of membrane segments as vesicles.
  • Components: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
  • Functions: protein synthesis, modification, and transport; lipid synthesis; detoxification of poisons.
Endoplasmic Reticulum (ER)
  • A network of membranes and sacs called cisternae that extends throughout the cytoplasm.
    • Rough ER: Studded with ribosomes.
    • Involved in the synthesis of proteins destined for secretion, insertion into membranes, or delivery to organelles such as lysosomes and the Golgi apparatus.
    • Folds and modifies proteins.
    • Smooth ER: Lacks ribosomes.
    • Site of lipid synthesis (e.g., steroids, phospholipids).
    • Metabolism of carbohydrates.
    • Detoxification of drugs and poisons, particularly in liver cells.
    • Storage of calcium ions.
Golgi Apparatus
  • Consists of flattened membranous sacs called cisternae, typically arranged in parallel.
  • Functions:
    • Modification: Modifies, sorts, and packages proteins and lipids synthesized in the ER.
    • Cis face: Receiving side, usually located near the ER.
    • Trans face: Shipping side, buds off vesicles that travel to other sites.
    • Adds molecular tags (e.g., phosphate groups) to vesicles to direct them to the correct destinations.
Lysosomes
  • Membranous sacs of hydrolytic enzymes that can digest macromolecules.
  • Functions:
    • Phagocytosis: Engulfing and digesting food particles.
    • Autophagy: Recycling the cell's own organic material, breaking down damaged organelles or cellular debris.
    • Work best in acidic environments.
Vacuoles
  • Large vesicles derived from the ER and Golgi apparatus.
  • Types and functions:
    • Food vacuoles: Formed by phagocytosis; store food.
    • Contractile vacuoles: Pump excess water out of freshwater protists, maintaining water balance.
    • Central vacuole (in plant cells): Stores water, nutrients, and waste products; maintains turgor pressure against the cell wall.
Peroxisomes
  • Specialized metabolic compartments bounded by a single membrane.
  • Functions:
    • Contain enzymes that transfer hydrogen atoms from various substrates to oxygen, producing hydrogen peroxide (H2O2).
    • Break down fatty acids into smaller molecules that are transported to mitochondria for cellular respiration.
    • Detoxify alcohol and other harmful compounds in the liver.
    • H2O2 is then converted to water by other enzymes (e.g., catalase).

Cell Systems III: The Cytoskeleton

  • A network of fibers extending throughout the cytoplasm that organizes the cell's structures and activities.
  • Functions: Mechanical support, cell motility, anchors organelles, guides vesicle movement.
Microfilaments (Actin Filaments)
  • Solid rods, about 7 nm in diameter, made of intertwined strands of actin protein.
  • Functions:
    • Maintain cell shape: Bear tension.
    • Muscle contraction: Interact with myosin filaments.
    • Cell motility: Form pseudopodia (false feet) for amoeboid movement.
    • Cytoplasmic streaming (in plant cells):
Intermediate Filaments
  • Fibrous proteins supercoiled into thicker cables, 8-12 nm in diameter.
  • Functions:
    • Maintain cell shape: Bear tension.
    • Anchor organelles: Form a sturdy cage for the nucleus.
    • Keratin: Found in skin cells, hair, and nails.
    • More permanent than microfilaments and microtubules.
Microtubules
  • Hollow rods, about 25 nm in diameter, made of tubulin dimers.
  • Functions:
    • Maintain cell shape: Resist compression.
    • Cell motility: Form the core of cilia and flagella.
    • Chromosome movement in cell division: Form spindle fibers.
    • Organelle movement: Act as tracks for motor proteins (e.g., kinesin, dynein) carrying vesicles.
    • Centrosomes and Centrioles: In animal cells, microtubules grow out from a centrosome; within the centrosome are a pair of centrioles arranged in a "9+0" pattern of microtubule triplets.

Cell Systems IV: Cilia and Flagella

  • Locomotive appendages that extend from the surface of many eukaryotic cells.
  • Composed of microtubules arranged in a distinctive pattern.
Cilia
  • Short, hair-like appendages, typically numerous on the cell surface.
  • Function: Usually move fluids over the cell surface (e.g., lining of trachea) or propel single-celled organisms.
  • Structure: "9+2" arrangement of microtubules (nine doublets surrounding a central pair).
Flagella
  • Long, whip-like appendages, usually one or a few per cell.
  • Function: Propel cells (e.g., sperm cells, some protists).
  • Structure: Also "9+2" arrangement of microtubules, similar to cilia but typically much longer.
  • Movement: Generates force parallel to the flagellum's axis.
Basal Body
  • The base of a cilium or flagellum, structurally identical to a centriole, with a "9+0" arrangement of microtubule triplets.
  • Anchors the cilium or flagellum within the cell.
Motor Proteins (Dyneins)
  • Play a crucial role in the bending movement of cilia and flagella.
  • Dynein arms, attached to one microtubule doublet, walk along the adjacent doublet, using ATP to create