Cell Structure: Prokaryotic Cells

• Biology is the scientific study of life, encompassing a wide range of topics from the molecular mechanisms inside cells to the ecosystems that span the planet.
• Cells are the fundamental units of life, responsible for all structural and functional aspects of organisms.
• Cell structure encompasses the various components within a cell, each with specific roles.
• Cell function refers to the processes carried out by cells to maintain life, such as metabolism, growth, and response to stimuli.
• Cell division is how cells multiply, enabling growth, repair, and reproduction in organisms.

Cell Structure

• Cells are broadly classified into two types: prokaryotic and eukaryotic.
• Prokaryotic cells, like bacteria and archaea, lack a nucleus and other complex organelles.
• Eukaryotic cells, found in plants, animals, fungi, and protists, possess a nucleus and membrane-bound organelles.

Prokaryotic Cell Structure

• Prokaryotic cells are generally smaller and simpler than eukaryotic cells.
• Key components include:
  • Plasma membrane: A barrier that separates the cell's interior from the outside environment.
  • Cytoplasm: The gel-like substance within the cell containing the cell's components.
  • DNA: Genetic material organized in a single circular chromosome located in the nucleoid region.
  • Ribosomes: Structures responsible for synthesizing proteins.
  • Cell wall: A rigid outer layer that provides support and protection.
  • Capsule: A sticky outer layer in some prokaryotes that aids in attachment and protection.
  • Flagella: Long appendages used for movement in some prokaryotes.
  • Pili: Short, hair-like structures used for attachment and genetic exchange in some prokaryotes.

Eukaryotic Cell Structure

• Eukaryotic cells are characterized by their complex organization and membrane-bound organelles.
• Key components include:
  • Nucleus: Contains the cell's DNA organized into chromosomes; the control center of the cell.
  • Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis.
    • Rough ER: Studded with ribosomes for protein synthesis.
    • Smooth ER: Involved in lipid synthesis and detoxification.
  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids.
  • Mitochondria: Powerhouse of the cell, responsible for generating ATP (energy) through cellular respiration.
  • Lysosomes: Contains enzymes for breaking down cellular waste and debris.
  • Peroxisomes: Involved in detoxification and lipid metabolism.
  • Ribosomes: Synthesize proteins; can be free in the cytoplasm or attached to the ER.
  • Plasma membrane: Regulates the movement of substances in and out of the cell.
  • Cytoskeleton: A network of protein fibers (microtubules, intermediate filaments, and microfilaments) that provides structural support and facilitates movement.
  • Cell Wall: Provides support and protection in plant cells (composed of cellulose).
  • Chloroplasts: Site of photosynthesis in plant cells.
  • Vacuoles: Large storage sacs that store water, nutrients, and waste.
  • Centrioles: Involved in cell division (animal cells).

Cell Function

• Cells perform a variety of functions essential for life, including:
  • Metabolism: Chemical reactions that occur within cells to sustain life.
  • Growth: Increase in size and number of cells.
  • Reproduction: Production of new cells or organisms.
  • Response to stimuli: Ability to react to changes in the environment.
  • Homeostasis: Maintaining a stable internal environment.
  • Transport: Movement of substances across the cell membrane.
  • Protein synthesis: Production of proteins based on genetic instructions.

Metabolism

• Metabolism involves two main processes:
  • Catabolism: Breaking down complex molecules into simpler ones, releasing energy.
  • Anabolism: Building complex molecules from simpler ones, requiring energy.
• Enzymes are critical for metabolism as catalysts.

Transport

• Cells transport substances across their membranes through various mechanisms:
  • Passive transport: Movement of substances without energy.
    • Diffusion: Movement of molecules from an area of high concentration to low concentration.
    • Osmosis: Movement of water across a semipermeable membrane from an area of high water concentration to low water concentration.
    • Facilitated diffusion: Movement of molecules across a membrane with transport proteins.
  • Active transport: Movement of substances requiring energy (ATP).
    • Pumps: Transport proteins that move molecules against their concentration gradients.
    • Vesicular transport: Movement of large molecules or bulk substances using vesicles.
      • Endocytosis: Bringing substances into the cell.
      • Exocytosis: Releasing substances out of the cell.

Protein Synthesis

• Protein synthesis involves two main steps:
  • Transcription: DNA sequence is copied into a complementary mRNA molecule.
  • Translation: mRNA sequence is decoded to assemble a specific amino acid chain (polypeptide) at the ribosome.

Cell Division

• Cell division is the process by which cells reproduce, allowing for growth, repair, and reproduction in organisms.
• Two main types of cell division:
  • Mitosis: Produces two genetically identical daughter cells (for growth and repair).
  • Meiosis: Produces four genetically different daughter cells (for sexual reproduction).

Mitosis

• Mitosis is divided into several phases:
  • Prophase: Chromosomes condense and become visible; the nuclear envelope breaks down.
  • Metaphase: Chromosomes line up along the metaphase plate (the middle of the cell).
  • Anaphase: Sister chromatids separate and move to opposite poles of the cell.
  • Telophase: Chromosomes arrive at the poles; the nuclear envelope reforms.
• Cytokinesis: is the division of the cytoplasm.

Meiosis

• Meiosis involves two rounds of cell division: Meiosis I and Meiosis II.
• Meiosis I:
  • Prophase I: Homologous chromosomes pair up and exchange genetic material through crossing over.
  • Metaphase I: Homologous chromosome pairs line up along the metaphase plate.
  • Anaphase I: Homologous chromosomes separate and move to opposite poles.
  • Telophase I: Chromosomes arrive at the poles; the cell divides.
• Meiosis II:
  • Prophase II: Chromosomes condense.
  • Metaphase II: Chromosomes line up along the metaphase plate.
  • Anaphase II: Sister chromatids separate and move to opposite poles.
  • Telophase II: Chromosomes arrive at the poles; the cell divides, resulting in four haploid daughter cells.
• Meiosis results in genetic variation through crossing over during prophase I and independent assortment of chromosomes during metaphase I.