ZOO 111: Principles of Cell Biology and Genetics Study Notes

Introduction to Cell Biology and Required Literature

  • The course is ZOO 111: Principles of Cell Biology and Genetics, for the 2023/2024 Session.
  • The course lecturer is Dr H.O. Awobode.
  • The recommended text for this course is:
    • Biology: The Dynamic Science (2008), authored by Russell, Wolfe, Hertz, Starr, and McMillan, published by Thomson Books, CA, USA.

Definition and Fundamental Nature of the Cell

  • A cell is defined as the basic unit of structure and function in a living thing.
  • Each individual cell within an organism shares the fundamental characteristics of all living things.
  • The cell is a small, membrane-bound unit filled with an aqueous solution of chemicals.
  • Cells possess the ability to create copies of themselves through the processes of growth and division.
  • A cell is composed of various specialized structures known as organelles.
  • All cells share certain universal common characteristics:
    • They are surrounded by a cell membrane.
    • They contain various organelles.
    • They contain cytoplasm.
    • They contain genetic material.
  • Cells exist in a vast variety of types, shapes, and sizes.
  • A cell generally consists of three major parts:
    • A well-defined boundary known as the cell membrane.
    • A prominent rounded body in the middle called the nucleus.
    • A transparent substance called cytoplasm that surrounds the nucleus and fills the cell.

The History of Cell Discovery and the Cell Theory

  • Robert Hooke (1635–17031635–1703), an English scientist, was the first to record observations of cells.
  • In 16631663, Hooke examined a thin slice of cork using a microscope he built himself.
  • He described the structure of the cork as "little boxes" that resembled a honeycomb.
  • This observation made it clear that all living things are composed of cells and that cells share common features.
  • The collective work of scientists including Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow led to the formulation of the Cell Theory.
  • The Cell Theory is founded on the concept that cells are the basic unit of life and it explains the relationship between cells and living things.
  • The core principles of the Cell Theory include:
    • Cells form the fundamental building blocks of all living things.
    • All living things are composed of one or more cells.
    • Cells arise only from the division of existing cells.
    • Cells contain the hereditary (genetic) information necessary for regulating cell functions and transmitting that information to the next generation.
    • All of an organism's life functions, such as physiological processes and metabolism, occur within cells.
    • Cells are capable of independent existence when provided with suitable conditions.

Classification and Comparison of Cell Types

  • Cells are categorized into two primary types: Prokaryotic and Eukaryotic.
Prokaryotic Cells
  • These cells do not have a nucleus.
  • Their DNA is not contained within a membrane but is bunched up in the center of the cell.
  • They lack membrane-covered organelles.
  • Most are single-celled (unicellular) organisms, though some may form clusters, strings, or multi-cellular structures.
  • They possess a tough protective outer coat called the cell wall.
  • Beneath the cell wall is the plasma membrane, which encloses the cytoplasm.
  • The cytoplasm contains various components including DNA, RNA, and proteins.
  • They are highly adaptable and inhabit a wide variety of environments and ecological niches.
  • Only Bacteria and Archaea are classified as prokaryotic.
  • It is believed that all life on Earth originated from ancient prokaryotic cells.
Eukaryotic Cells
  • These cells contain a nucleus and various membrane-covered organelles.
  • They are approximately ten times larger than prokaryotic cells.
  • They exhibit a much higher level of internal complexity.
  • Their DNA is found securely inside the nucleus.
  • Many classes of eukaryotes form multicellular organisms.
  • This category includes Animals, Plants, Fungi, and Protists.

Structure and Components of the Animal Cell

  • Cell Membrane: Separates the inside of the cell from the outside environment and regulates the passage of materials in and out of the cell.
  • Nucleus: Acts as the control center, managing cell activities and containing hereditary material (DNA).
  • Cytoplasm: Encompasses everything inside the cell membrane except for the nucleus.
  • Mitochondria: They function as the site of energy production, providing most of the cell's energy.
  • Ribosomes: Known as the protein factories of the cell.
  • Endoplasmic Reticulum (ER): A network of passageways for carrying materials from one part of the cell to another.
  • Golgi Body: Receives materials from the ER, packages them, and distributes them to other parts of the cell.
  • Lysosome: Contains chemicals used to break down food particles and worn-out cell parts.
  • Vacuole: Sometimes found in animal cells; used for storing food, water, and wastes.
  • Cytoskeleton: Provides the cell with its shape and structural strength.

Structure and Components of the Plant Cell

  • Cell Wall: Located outside the cell membrane; provides a rigid structure and support.
  • Cell Membrane: Controls the exchange of materials between the cell and its environment.
  • Nucleus: Controls activities and houses DNA.
  • Chloroplasts: Capture energy from sunlight and store it in molecules (photosynthesis).
  • Mitochondria: Produces energy for the cell.
  • Central Vacuole: A large organelle that stores water, food, and waste; provides structural support when full.
  • Ribosomes: Synthesize proteins.
  • Endoplasmic Reticulum (ER): Transports materials throughout the cell.
  • Golgi Body: Processes and packages materials.
  • Lysosome: Involved in digestion and waste removal.
  • Cytoskeleton: Maintains cell shape and strength.

Detailed Analysis of the Cell Membrane

  • The cell membrane is a thin layer separating the internal environment from the exterior.
  • It is selectively permeable, meaning it controls the exchange of materials and keeps cytoplasm inside while allowing waste out.
  • The membrane is approximately 10 mm10\,mm thick and consists of three layers.
  • Chemical composition of the membrane:
    • Protein: 45%45\%
    • Lipids: 45%45\%
    • Carbohydrates: 10%10\%
Lipid and Protein Organization
  • Most of the lipid content consists of phospholipids.
  • A phospholipid consists of a hydrophilic phosphate head and a hydrophobic tail made of two fatty acids.
  • Phospholipids arrange themselves into a bilayer (two molecules thick).
  • The hydrophobic tails point inward (away from water), while the hydrophilic heads face the watery environment both inside and outside the cell.
  • Phospholipids facilitate the passage of certain lipid-soluble substances.
  • Proteins within the membrane vary in structure and function:
    • Intrinsic proteins: Span the entire width of the membrane; some act as carriers for transporting molecules.
    • Extrinsic proteins: Located only on the inner or outer surfaces; they may combine with carbohydrates to form glycoproteins.
    • Glycoproteins and glycolipids act as chemical receptors on the cell surface.
The Fluid Mosaic Model
  • Fluid Aspect: The plasma membrane has the consistency of olive oil at body temperature because of unsaturated phospholipids. Lipids and some proteins can drift laterally. Phospholipids generally do not switch from one layer to the other.
  • Cholesterol Role: Cholesterol acts as a fluidity regulator. At body temperature, it restrains phospholipid movement to lessen fluidity. At colder temperatures, it prevents close packing of phospholipids to maintain fluidity.
  • Mosaic Aspect: Membrane proteins create a collage that differs on each side of the membrane. Hydrophilic regions face outward, while hydrophobic regions face inward.
Internal Membranes
  • These are membranes found inside almost all types of cells, enclosing organelles.
  • Though they appear similar to the plasma membrane under an electron microscope, their specific composition of lipids and proteins differs to reflect their specialized functions.
  • They regulate the traffic of ions and molecules within the cell and isolate chemical reactions.

Organelle Descriptions and Functions

Mitochondria
  • Found in all eukaryotic cells and surrounded by two membranes.
  • The outer membrane is smooth, while the inner membrane is folded into layers called cristae.
  • The matrix is the space within the inner membrane, and the intermembrane space lies between the two membranes.
  • Mitochondria contain their own DNA.
  • Referred to as the ‘powerhouse’ because they produce the energy needed for cell functions.
  • They contain oxidative metabolism enzymes that transfer energy from macromolecules to ATP.
Endoplasmic Reticulum (ER)
  • A system of fluid-filled, flattened, membrane-bound sacs.
  • Primarily involved in the transport of proteins.
  • Rough ER: Covered with bead-like ribosomes.
  • Smooth ER: Lacks ribosomes.
Ribosomes
  • Tiny organelles made of two unequal subunits (one large, one small).
  • They are the site of protein synthesis and release finished proteins into the ER.
  • They can be found either free in the cytoplasm or attached to the ER.
Golgi Bodies (Golgi Apparatus)
  • A stack of flattened, membrane-bound sacs called cisternae.
  • In animals, these stacks form a network.
  • They process and package materials received from the ER.
  • New membrane is added at one end and buds off as vesicles at the opposite end.
  • Functions include the formation of lysosomes and the secretion of substances (e.g., enzymes) via vesicles.
The Nucleus
  • The largest organelle in the cell, often called the "control center."
  • It is enclosed by a nuclear envelope, which is perforated with pores to allow material passage.
  • It houses chromatin (DNA).
  • Euchromatin: Stains lightly; contains active DNA.
  • Heterochromatin: Stains deeply; contains inactive DNA.
  • Contains the Nucleolus, which manufactures ribosomes and acts as a storage area for materials.
Lysosomes
  • Contain enzymes and are involved in autolysis.
  • They break down food, old organelles, and bacteria into reusable molecules.
  • They digest materials ingested by white blood cells.

The Cytoskeleton and Cellular Movement

  • The cytoskeleton is a network of fine protein fibers that provide shape and structure.
  • They can be anchored to the plasma membrane or radiate from near the nucleus.
  • There are three main types of filaments:
    1. Microfilaments:
      • The thinnest filaments (5−6 nm5-6\,nm in diameter).
      • Made of the protein Actin.
      • Involved in movement (e.g., muscle contraction, cytoplasmic streaming, cell migration, and cell division/daughter cell separation).
    2. Microtubules:
      • The thickest filaments; they are straight, hollow, and rigid cylinders of protein.
      • Found in most plant and animal cells.
      • Provide rigidity and are used to construct centrioles, basal bodies, cilia, and flagella.
      • Form Spindles during cell division to pull duplicated chromosomes apart.
    3. Intermediate Filaments:
      • Size is intermediate between microfilaments and microtubules.
      • Provide a supporting framework (e.g., holding the nucleus in position).
      • Found in muscle cells and axons.

Specialized Structures

  • Centrioles: Paired cylinders made of a complex microtubule arrangement; involved in cell division and locomotion (via cilia and flagella).
  • Vacuoles: Large in plants; they store cell sap and provide structural support.
  • Cell Wall: Found in plants; made of cellulose; provides structure and support.
  • Chloroplasts:
    • Large, disc-shaped organelles (5−8 μm5-8\,\mu m diameter ×\times 2−4 μm2-4\,\mu m thickness).
    • Found in plants and some algae; bounded by a double membrane.
    • Contains internal membranes called the stroma (containing phospholipids and proteins).
    • Contains stacks of the green pigment chlorophyll.
    • Produces food molecules and oxygen for mitochondrial use.
    • Contains its own DNA and reproduces by dividing in two.