Cells Study Notes

Introduction to Cells

  • Historical Context

    • The 1600s:

    • Robert Hooke: Observed cork through a microscope and coined the term "cellulae" for the tiny compartments he viewed.

    • Anton van Leeuwenhoek: Used his own constructed microscope to observe "little animalcules," discovering and describing diverse protists, sperm cells, and bacteria.

The Cell Theory

  • Definition: By the mid-19th century, three fundamental generalizations emerged from microscopic observations, constituting the cell theory:

    • All organisms are composed of one or more cells.

    • The cell is the basic structural and functional unit of all living organisms.

    • Cells arise only from the division of preexisting cells.

Basic Features of Cell Structure and Function

  • Essential Processes of Life:

    • Cells, as the basic units of life, carry out all essential processes, including:

    • Containing DNA and RNA, which carry hereditary information and direct the manufacture of cellular molecules.

    • Using energy, responding to environmental changes, reproducing, and passing on hereditary information.

Visualization of Cells

  • Microscopy:

    • Most cells are too small to see with the naked eye, typically ranging from about 0.5 μm (bacteria) to several hundred micrometers (plant cells).

    • Light Microscopes: Use light to illuminate specimens.

    • Electron Microscopes: Use electrons for higher resolution imaging of cells and their structures.

Cell Structure

  • Plasma Membrane:

    • All cells are bounded by the plasma membrane, a bilayer composed of phospholipids interspersed with protein molecules.

    • The phospholipid bilayer serves as a hydrophobic barrier against water-soluble substances.

    • Selected substances can penetrate cell membranes via transport protein channels, facilitating selective transport of ions and molecules, thus maintaining specialized internal environments.

Internal Organization of Cells

  • Central Region: Contains DNA molecules that store hereditary information (genes).

  • Cytoplasm: Lies between the plasma membrane and the central region, consisting of:

    • Cytosol: An aqueous solution of ions, organic molecules, and organelles.

    • Cytoskeleton: Maintains cell shape and plays critical roles in cell division and chromosome segregation.

Types of Cells

Prokaryotic Cells

  • Characteristics:

    • Found in organisms of the domains Bacteria and Archaea.

    • The nucleoid region is not bound by a membrane.

    • Typical of species with few internal membranes.

  • Specific Features:

    • Common shapes include spherical, rod-like, and spiral forms.

    • DNA (located in the nucleoid) is usually a single, circular molecule known as the prokaryotic chromosome.

    • Information from DNA is transcribed into messenger RNA (mRNA) and carried to ribosomes in the cytoplasm for protein synthesis.

    • The plasma membrane is encased by a rigid external cell wall coated with polysaccharides (glycocalyx).

    • When loosely associated, termed a slime layer; when firmly attached, referred to as a capsule.

    • Plasma membrane systems metabolize food into ATP (adenosine triphosphate).

  • Cellular Structures:

    • The prokaryotic cytoskeleton maintains shape and assists in cell division.

    • Flagella: Long, whip-like structures permit motility. In bacteria, the flagellum rotates and propels the cell through liquid media.

    • Pili: Hairlike structures that facilitate attachment to surfaces and other cells; includes a special sex pilus for bacteria mating.

Eukaryotic Cells

  • Characteristics:

    • More complex than prokaryotic cells, with a true nucleus enclosed by membranes.

    • The cytoplasm contains a network of membranous organelles specialized for various functions.

  • Functions of Organelles:

    • The cytosol participates in energy metabolism and molecular synthesis while also supporting the cell and allowing for motility.

    • Researchers utilize cell fractionation to isolate and study cell organelles.

Ribosomes

  • Types of Ribosomes:

    • Some are freely suspended in the cytosol, while others are membrane-bound.

    • Proteins made on free ribosomes: may remain in the cytosol, move into the nucleus, or become parts of mitochondria, chloroplasts, the cytoskeleton, or other structures.

    • Proteins made on membrane-bound ribosomes: follow a pathway to the endoplasmic reticulum (ER).

Endoplasmic Reticulum (ER)

  • Definition: An extensive interconnected network of membranous channels and cisternae.

  • Forms of ER:

    • Rough ER: Characterized by ribosomes on its outer surface.

    • Proteins synthesized enter the ER lumen for folding and modification, including adding carbohydrate groups to form glycoproteins.

    • Proteins are delivered to other cell regions (e.g., Golgi complex) via vesicles.

    • Smooth ER: Lacks ribosomes and is responsible for lipid synthesis and detoxification.

    • In the liver, it converts drugs and toxins into less harmful substances.

Golgi Complex

  • Structure: Composed of stacked, flattened membranous sacs called cisternae.

  • Function:

    • Proteins from the ER enter the complex at the cis face, chemically modified, and exit at the trans face via vesicles.

    • Acts as a sorting center or “tags” proteins for their destinations.

    • Vesicles transporting proteins to be secreted from the cell undergo exocytosis upon fusing with the plasma membrane.

Lysosomes

  • Description: Small, membrane-bound vesicles containing hydrolytic enzymes that digest complex molecules; recycle subunits.

  • Formation: Budding from the Golgi complex, with enzymes synthesized in the rough ER.

  • Functionality:

    • Maintain an acidic environment (pH ~ 5) for enzymatic activity.

    • Digest food, worn-out organelles (via autophagy), and breakdown debris (via phagocytosis).

    • In lysosomal storage diseases, specific hydrolytic enzymes are absent.

Mitochondria

  • Function: Membrane-bound organelles wherein cellular respiration takes place.

  • Process:

    • Energy-rich food molecules are broken down, yielding water and carbon dioxide; energy is captured in ATP.

  • Requirement: Mitochondria necessitate oxygen, which humans collect through breathing.

The Cytoskeleton

  • Definition: An interconnected system of protein fibers and tubes that maintains cell shape and internal organization.

  • Components:

    • Includes microtubules, intermediate filaments, and microfilaments.

    • Microtubules serve as tracks for vesicle movement and aid in chromosome separation during cell division.

Specialized Structures of Plant Cells

  • Plant cells possess certain structures not found in animal cells:

    • Chloroplasts: Organelles involved in photosynthesis.

    • Central Vacuole: Large storage compartment.

    • Cell Walls: Provide structural support.

    • Also present in algae and fungi.

Chloroplasts

  • Structure: Enclosed by an outer and inner membrane, surrounding an inner compartment (stroma).

  • Thylakoids: Membranes within the stroma organized into stacks (grana) which contain chlorophyll for light absorption.

Central Vacuoles

  • Functions:

    • Store various substances, such as salts, organic acids, and pigments.

    • Produce colors in flowers through pigments concentrated in vacuoles.

    • Contain enzymes for breaking down molecules and provide chemical defense against pathogens.

Cell Walls

  • Composition: Made of cellulose fibers, providing support, and protecting the cell from bacteria and fungi.

  • Types:

    • Primary Cell Wall: Soft and flexible.

    • Secondary Cell Wall: Additional cellulose and carbohydrate layers for reinforcement, contains lignin in woody plants.