Cells organelles and organisms

Week 9: BPS111 and CSC104 - Cells and Organelles General Overview

This lecture covers the fundamentals of cell biology, focusing on the structure and function of cells and organelles.

Lecture Outcomes:

  • Describe key features of eukaryotic and prokaryotic cells.

  • Explain the major cellular functions of organelles.

  • Understand how cellular physiology integrates with cellular chemistry and structure.

Key Features of Cell Types

Three Domains of Life

  1. Prokaryotes: Simple cells without a nucleus (e.g., bacteria and archaea).

  2. Eukaryotes: Complex cells with a nucleus (e.g., animals, plants, fungi).

  3. Archaea: Similar to bacteria but with distinct biochemistry and genetics.

Types of Biological Cells

  • Examples include Lactobacillus, Archaebacteria, blood cells, fossilized dinosaur eggs, Volvox aureus (green alga), various neuronal cell types, epithelial cells (intestine), and plant cells (vascular tissue).

Prokaryotic vs Eukaryotic Cells

Prokaryotic Cells:

  • Size: Typically 0.1-5 µm in diameter.

  • Nucleus: Lack a defined nucleus; DNA is circular and exists freely in the cytoplasm.

  • Organelles: Absence of membrane-bound organelles. The plasma membrane may lack receptors, limiting interaction with the environment.

Eukaryotic Cells:

  • Size: Generally 5-100 µm in diameter. Categories include animal cells, plant cells, and fungal cells.

  • Nucleus: Contain a clearly defined membrane-bound nucleus where linear DNA is organized around histones.

  • Organelles: Features numerous membrane-enclosed organelles that perform specific functions, along with a plasma membrane equipped with receptors for signaling and cellular communication.

Critical Physiological Processes

  1. Cell Replication:

    • Eukaryotic cells undergo a structured cell cycle (interphase, mitosis, and cytokinesis) for replication to ensure genetic fidelity.

  2. DNA Replication and Protein Synthesis:

    • Crucial for maintaining cellular function and integrity; involves intricate processes like transcription and translation.

  3. Cellular Respiration:

    • A multi-stage process that converts nutrients into ATP, using glycolysis, the Krebs cycle, and oxidative phosphorylation.

  4. Cellular Transport:

    • Mechanisms like endocytosis and exocytosis facilitate moving substances across the plasma membrane, essential for nutrient uptake and waste disposal.

  5. Cell Migration:

    • Essential for processes such as embryonic development and immune responses; involves cytoskeletal rearrangements.

  6. Structural Support:

    • The cytoskeleton, consisting of microtubules, actin filaments, and intermediate filaments, provides rigidity, shape, and organization within the cell.

Organelles in Eukaryotic Cells

  • Plasma Membrane:

    • Selectively permeable membrane that controls molecular entry and exit; involved in cell signaling.

  • Mitochondria:

    • Known as the powerhouse of the cell; generates ATP through oxidative metabolism of glucose and fatty acids and contains its own circular DNA.

  • Lysosomes:

    • Spherical vesicles filled with hydrolytic enzymes that degrade worn-out cellular components and biomaterials (autophagy).

  • Nuclear Envelope:

    • Double-layered membrane that protects the nucleus and is continuous with the rough endoplasmic reticulum (ER).

  • Nucleolus:

    • Dense region within the nucleus where ribosomal RNA (rRNA) is synthesized; becomes more prominent during cell division.

  • Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes for protein synthesis and processing; involved in folding and modifying proteins.

    • Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification of poisons, and calcium ion storage.

  • Golgi Apparatus:

    • Stacked membrane-bound structures responsible for modifying, sorting, and packaging proteins for secretion or delivery to other organelles.

  • Peroxisomes:

    • Small organelles that contain enzymes for fatty acid oxidation and metabolizing harmful substances, such as hydrogen peroxide.

  • Cytoskeleton:

    • Network of protein filaments that maintain cell shape, provide mechanical support, and facilitate intracellular transport.

    • Composed of actin filaments, intermediate filaments, and microtubules.

  • Vacuoles:

    • Membrane-bound sacs primarily found in plant cells; store nutrients and waste products and help maintain turgor pressure.

  • Chloroplasts (in plants):

    • Organelles responsible for photosynthesis, converting sunlight into chemical energy stored in glucose. They contain chlorophyll, the green pigment necessary for capturing light energy.

Additional Topics

Cellular Transport Mechanisms

  • Passive Transport:

    • Movement of molecules across a cell membrane without energy expenditure (e.g., diffusion, osmosis, facilitated diffusion).

  • Active Transport:

    • Requires energy (ATP) to move molecules against their concentration gradient, utilizing transport proteins like pumps, channels and carriers for ions and larger molecules.

The Lysosomes

  • Spherical vesicles containing hydrolytic enzymes crucial for degrading biomaterials.

  • Act in cellular waste disposal and recycling through autophagy, removing damaged organelles and proteins to maintain cell health.

Peroxisomes

  • Involved in fatty acid degradation; generates and neutralizes hydrogen peroxide by oxidative reactions, playing a key role in lipid metabolism.

Endoplasmic Reticulum (ER)

  • Extensive network for macromolecule synthesis.

    • Smooth ER: Lipid synthesis and detoxification of metabolic byproducts.

    • Rough ER: Protein synthesis due to ribosome presence on its cytoplasmic surface.

Golgi Apparatus

  • Essential for processing, packaging, and secretion of proteins and lipids, playing a pivotal role in cell communication and metabolism.

Cytoskeleton

  • Composed of actin filaments, microtubules, and intermediate filaments; supports cellular structure and facilitates organelle organization, cell movement, and intracellular transport.

Evolution of Mitochondria

  • Mitochondria may have originated from engulfed bacteria through endosymbiotic relationships, a theory supported by their double membranes and distinct circular DNA (mtDNA) encoding essential proteins.

Mitochondria in Disease

  • Mitochondrial dysfunction is implicated in various diseases, including neurodegenerative diseases (e.g., Alzheimer’s, Parkinson’s) and metabolic disorders like diabetes, highlighting their crucial role in cellular energy metabolism.

Nucleus Functions

  • Contains and regulates DNA that codes for proteins, orchestrating gene expression and cell behavior.

  • Cell division occurs through mitosis, a process ensuring equal distribution of genetic material to daughter cells.

The Nucleolus

  • Site of rRNA synthesis; reassembles during cell cycle phases and is involved in the formation of ribosomes, the protein synthesis machinery.

Central Dogma of Biology

  • DNA Replication: DNA ➔ DNA (via DNA polymerase).

  • Transcription: DNA ➔ mRNA (via RNA polymerase).

  • Translation: mRNA ➔ Proteins at ribosomes, facilitated by tRNA.

Cellular Organization

  • Cells associate to form tissues, which further form organs and organ systems.

  • Tissues include Epithelial, Connective, Muscle, and Nervous tissues, each serving specific roles in organism development and function.