Cell Structure and Organelles — Comprehensive Notes

Basic Concepts of Cells

  • Cells are the smallest living units of an organism and share three things in common:

    • A cell membrane that separates the inside of the cell from its environment
    • Cytoplasm, a jelly-like fluid
    • DNA, the cell's genetic material

  • Two broad categories of cells:

    • Eukaryotic cells: have organelles (including the nucleus) and are found in plants and animals; they are more advanced/complex
    • Prokaryotic cells: lack a nucleus and membrane-enclosed organelles; contain genetic material not contained within a nucleus; typically unicellular (one-celled organisms) such as bacteria

  • What are organelles?

    • Organelles mean "little organs" and are specialized parts of a cell with unique jobs to perform

The Nucleus: The Cell’s Control Center

  • The nucleus is the control center of the cell and contains DNA (genetic material)
  • DNA dictates what the cell is going to do and how it's going to do it
  • Chromatin is the tangled, spread-out form of DNA inside the nuclear membrane
  • When a cell is ready to divide, DNA condenses into chromosomes
  • The nucleus also contains a nucleolus, a structure where ribosomes are made
  • After ribosomes leave the nucleus, they synthesize (make) proteins

Cytoplasm, Ribosomes, and the Endoplasmic Reticulum

  • Outside the nucleus, ribosomes and the rest of the organelles float in cytoplasm (the jelly-like substance)
  • Ribosomes can be free-floating in the cytoplasm or attached to the endoplasmic reticulum (ER)
  • Endoplasmic reticulum (ER) has two types:
    • Rough ER: with ribosomes attached
    • Smooth ER: lacks ribosomes
  • The ER is a membrane-enclosed passageway for transporting materials, such as proteins synthesized by ribosomes
  • Proteins and other materials exit the ER in small vesicles, which are received by the Golgi apparatus (Golgi body)

Golgi Apparatus: Protein Processing and Packaging

  • As proteins move through the Golgi body, they are customized into usable forms for the cell
  • The Golgi body folds proteins into usable shapes and can add other materials to them, such as lipids or carbohydrates

Vesicles, Vacuoles, and Lysosomes

  • Vesicles: small sacs that transport proteins and other materials from the ER to the Golgi and to their final destinations
  • Vacuoles: sac-like structures that store different materials
    • In plant cells, the central vacuole stores water
  • Lysosomes (in animal cells): garbage collectors that take in damaged or worn-out cell parts; filled with enzymes that break down cellular debris

Mitochondria: The Powerhouse

  • Mitochondrion is the powerhouse of the cell (in both animal and plant cells)
  • During cellular respiration, mitochondria produce ATP molecules that provide energy for cellular activities
  • Cells that require more energy have more mitochondria

Cytoskeleton: Maintaining Shape and Structure

  • The cytoskeleton helps maintain cell shape and structure
  • It includes:
    • Microfilaments: thread-like structures made of protein
    • Microtubules: thin hollow tubes

Chloroplasts and Photosynthesis (Plant Cells)

  • Some plants are photoautotrophic, meaning they capture sunlight for energy
  • Plant cells contain chloroplasts, where photosynthesis occurs
  • Chloroplasts are green due to chlorophyll pigment

Plant Cell Walls vs Animal Cell Membranes

  • Plant cells have a cell wall outside the cell membrane that shapes, supports, and protects the cell
  • Animal cells do not have a cell wall

Other Unique Structures

  • Cilia: found in many cells lining the human respiratory tract; microscopic hair-like projections that move in waves to trap inhaled particles and expel them when coughing
  • Flagella: a tail-like structure that helps some cells move
    • Some bacteria have flagella
    • A human cell with a flagellum is the sperm cell

Summary and Key Takeaways

  • Eukaryotic cells (plant and animal) have a nucleus and membrane-bound organelles
  • Prokaryotic cells are unicellular and lack a nucleus and membrane-bound organelles
  • All cells have a cell membrane, cytoplasm, and genetic material
  • Chloroplasts are found only in plant cells; mitochondria are present in both plant and animal cells
  • Major organelles and components discussed: nucleus, chromatin, chromosomes, nucleolus, ribosomes, rough ER, smooth ER, Golgi apparatus, vesicles, vacuoles, lysosomes, mitochondria, cytoskeleton (microfilaments and microtubules), chloroplasts, chlorophyll, cell wall, cilia, flagella, sperm cell

Connections to Bigger Concepts (Contextual Notes)

  • This content aligns with the cell theory: cells are the basic units of life; all living things are composed of cells; cells arise from pre-existing cells (implied by the division of DNA into chromosomes and the role of the nucleus in cell function).
  • Structure relates to function: organelles perform specific tasks (protein synthesis, energy production, transport, storage, and defense).
  • Energy flows within the cell through processes like cellular respiration (mitochondria) and, in plants, energy capture via photosynthesis (chloroplasts).
  • The presence or absence of organelles like a nucleus, chloroplasts, or a cell wall distinguishes major cell types and influences organismal biology and physiology.