UNIT 3.1 Cell Structure, Organelles, and Cell Junctions

Cell Structure, Organelles and Cell Junctions

Lecture Objectives

  • Describe the major difference between Prokaryotic and Eukaryotic cells

    • Understand the fundamental distinction that Eukaryotic cells possess a nucleus and membrane-bound organelles, while Prokaryotic cells lack these features.

  • Understand the complementary structure and function of the cells in the human body

    • Recognize how the structure of different cell types contributes to their specific functions in the body.

  • Describe the importance of the cell membrane

    • Learn the role of the cell membrane in protecting cellular contents and facilitating communication and transport.

  • Differentiate between rough and smooth endoplasmic reticulum

    • Distinguish between the functions of rough ER (involved in protein synthesis) and smooth ER (involved in lipid metabolism).

  • Be able to describe the importance of different organelles in the cell

    • Understand the specific roles of cellular organelles in maintaining cellular health and function.

  • Define the different types of cell junctions

    • Identify and explain the roles of tight junctions, desmosomes, and gap junctions in tissue formation and integrity.

Eukaryotic vs. Prokaryotic Cells

  • Eukaryotes

    • Defined as any single-celled or multicellular organisms with cells containing a distinct, membrane-bound nucleus.

    • Examples include:

    • Animals

    • Plants

    • Fungi

    • Eukaryotic cells contain organelles such as:

    • Nuclei

    • Endoplasmic Reticulum

  • Prokaryotes

    • Defined as unicellular organisms that lack membrane-bound structures such as the nucleus and endoplasmic reticulum.

    • Example: Bacteria

Cell Diversity & Specialization

  • Types of specialized cells in the human body:

    1. Epithelial Cells: Connect body parts, form linings, and transport substances.

    2. Erythrocytes (Red Blood Cells): Transport gases throughout the body.

    3. Fibroblasts: Provide support and structure to tissues.

    4. Skeletal Muscle Cells: Enable movement by contracting.

    5. Fat Cells: Store nutrients and energy.

    6. Macrophages: Fight disease by engulfing pathogens.

    7. Nerve Cells: Gather information and control body functions.

    8. Sperm Cells: Involved in reproduction.

Basic Structure of Eukaryotic Cells

  • Components of Human Cells

    • Plasma Membrane: Flexible outer boundary that regulates the entry and exit of substances.

    • Cytoplasm: Intracellular fluid containing organelles and other components.

    • Nucleus: DNA-containing control center that manages cell functions.

The Cell (Plasma) Membrane

  • Structure

    • Composed of membrane lipids forming a flexible lipid bilayer.

    • Includes specialized membrane proteins that float within this fluid membrane, facilitating dynamic changes in structure.

    • Surface sugars form a glycocalyx that assists in cell recognition and adhesion.

  • Functions of Cell Membrane

    • Physical Barrier: Encloses the cell and separates the cytoplasm from extracellular fluid via the phospholipid bilayer.

    • Selective Permeability: Regulates which substances can enter or exit the cell based on size, charge, or solubility.

    • Communication: Plasma membrane proteins interact with specific chemical messengers, relaying signals to the cell's interior.

    • Cell Recognition: Cell surface carbohydrates and proteins facilitate interactions between cells, aiding adhesion and communication.

The Cytoplasm

  • Definition

    • All cellular material located between the plasma membrane and the nucleus.

  • Components

    • Cytosol: Gel-like solution made of water and soluble molecules (proteins, salts, sugars).

    • Inclusions: Insoluble molecules that vary with cell type (e.g., glycogen granules, pigments, lipid droplets, vacuoles, crystals).

    • Organelles: Structures with specialized functions; can be either membranous or nonmembranous.

Cytoplasmic Organelles

  • Membranous Organelles:

    • Mitochondria

    • Endoplasmic Reticulum (Rough and Smooth)

    • Golgi Apparatus

    • Peroxisomes

    • Lysosomes

  • Nonmembranous Organelles:

    • Ribosomes

    • Cytoskeleton

    • Centrioles

Mitochondria

  • Known as the "power plant" of cells due to their role in producing most of the cell's energy molecules (usually ATP).

  • Key structures include:

    • Outer Mitochondrial Membrane

    • Inner Mitochondrial Membrane

    • Cristae: Infoldings that increase surface area for energy production.

    • Matrix: Contains enzymes involved in energy production.

Ribosomes

  • Definition

    • Nonmembranous organelles that serve as the site of protein synthesis.

    • Composed of protein and ribosomal RNA (rRNA).

Endoplasmic Reticulum (ER)

  • Smooth Endoplasmic Reticulum (SER):

    • Network of looped tubules continuous with rough ER.

    • Involved in lipid metabolism, synthesis of cholesterol and steroid-based hormones, and making lipids for lipoproteins.

  • Rough Endoplasmic Reticulum (RER):

    • Network of looped tubules that is continuous with the nuclear envelope.

    • Site for synthesis of proteins intended for secretion out of the cell.

Golgi Apparatus

  • Comprises stacked and flattened membranous cisterns.

  • Functions to modify, concentrate, and package proteins and lipids received from the RER.

Lysosomes

  • Definition

    • Membranous sacs containing hydrolytic enzymes.

  • Operate in an acidic environment (pH 4.5-5).

    • Types of acid hydrolases include:

    • Proteases

    • Nucleases

    • Lipases

    • Phospholipases

    • Phosphatases

    • Sulfatases

  • Principal site for intracellular digestion.

Peroxisomes

  • Surrounded by a single lipid bilayer that encloses a dense protein matrix.

  • Function as powerful detoxificants and perform various metabolic functions including:

    • Beta-oxidation of fatty acids

    • Oxidative reactions using enzymes known as oxidases.

    • Decomposition of hydrogen peroxide (H2O2) using catalase.

  • Collaborate with other organelles (mitochondria, ER) for functional processes.

  • Significant in infection response, immune responses, aging, and cancer.

Lysosomes vs. Peroxisomes

  • Both structures carry out metabolic functions that involve:

    • Breaking down large molecules into smaller, usable components.

    • Recycling cellular debris and old cell parts.

    • Hydrolytic enzymes for digesting materials (lysosomes).

    • Detoxification processes (peroxisomes).

  • New roles in signaling and immune responses are being studied.

Cell Junctions

  • Definition

    • Cell junctions are structures that provide adhesion between cells, vital for forming tissues and organs.

  • Some cells remain free (not bound to others), such as blood cells and sperm cells.

  • Cells can be connected through three primary junctions:

    1. Tight Junctions: Prevent leakage between cells.

    2. Desmosomes: Provide mechanical strength to tissues.

    3. Gap Junctions: Facilitate communication between adjacent cells.

Types of Cell Junctions

  1. Tight Junctions:

    • Form continuous seals around cells, creating impermeable barriers.

    • Prevent molecules from passing between cells.

    • Example location: Bladder.

  2. Desmosomes:

    • Rivet-like junctions formed by linker proteins (cadherins).

    • Provide tensile strength to tissues by binding adjacent cells together.

    • Example location: Skin, heart.

  3. Gap Junctions:

    • Formed by transmembrane proteins called connexons, creating channels that allow small molecules to pass directly from one cell to another.

    • Important for rapid communication, especially in cardiac and smooth muscle cells.

Key Takeaways

  • Eukaryotic cells like human cells possess membrane-bound organelles, enabling compartmentalization and specialization of functions.

  • The plasma membrane is a dynamic lipid bilayer critical for regulating transport, communication, and adhesion via junctions.

  • Key organelles include:

    • Mitochondria: ATP generation

    • Ribosomes: Protein synthesis

    • ER: Protein and lipid production

    • Golgi: Modifying and shipping proteins

    • Lysosomes & Peroxisomes: Cleanup and detoxification tasks.

  • Three main types of cell junctions (tight junctions, desmosomes, and gap junctions) are essential for maintaining tissue integrity and enabling coordinated cellular functions.