The Cytoplasm and Cellular Organelles

Learning Objectives

  • Understand the structure and function of internal components of a human cell.

  • Identify the cytoplasmic compartment and cytosol.

  • Recognize the function of cellular organelles and their roles in cell health.

  • Describe components of the cytoskeleton and their functions.

Structure of a Prototypical Human Cell

  • All cells are enclosed by a cell membrane containing:

    • Cytoplasmic Compartment: Largely composed of cytosol, a jelly-like substance that facilitates biochemical reactions.

    • Nucleus: The most prominent organelle containing DNA, which controls cellular activity.

    • Organelles: Membrane-bound structures each performing unique functions, similar to organs in a body.

Organelles of the Endomembrane System

  • A system including the following organelles:

    • Endoplasmic Reticulum (ER): A series of winding membranes involved in transport, synthesis, and storage.

    • Golgi Apparatus: Sorts, modifies, and packages products from the ER for transport.

    • Lysosomes: Contain enzymes for digestion and breakdown of cellular waste.

Endoplasmic Reticulum (ER)

  • Continuous with the nuclear envelope; serves several critical functions:

    • Rough ER (RER):

      • Studded with ribosomes; site of protein synthesis.

      • Proteins synthesized are modified within ER (glycosylation) and transported to Golgi apparatus.

    • Smooth ER (SER):

      • Lacks ribosomes; synthesizes lipids and steroid hormones.

      • Stores calcium ions (Ca++) and detoxifies substances.

Golgi Apparatus

  • Functions like a post-office:

    • Receives products from RER in vesicles.

    • Modifies, sorts, and packages them for transport to various locations, including secretion outside the cell via exocytosis.

Lysosomes

  • Vesicles containing digestive enzymes:

    • Break down unneeded cellular components and foreign material.

    • Process called autophagy: digestion of the cell's own structures.

    • Important in immune response for digesting pathogens.

Organelles for Energy Production and Detoxification

  • Mitochondria:

    • Bean-shaped organelles considered the "energy transformers" of the cell.

    • Two membranes: an inner membrane (highly folded) and an outer membrane.

    • Site of cellular respiration, producing ATP from glucose; crucial for energy-dependent processes.

  • Peroxisomes:

    • Membrane-bound organelles containing enzymes for metabolic functions such as lipid metabolism and detoxification.

    • Produce hydrogen peroxide (H2O2), which is then broken down into harmless water and oxygen.

Reactive Oxygen Species (ROS)

  • Highly reactive molecules resulting from normal cellular processes.

  • Can cause cellular damage and are implicated in various diseases, including cancer and neurodegenerative disorders.

  • Antioxidants help neutralize ROS.

Free Radical Theory of Aging

  • Suggests that accumulated oxidative damage contributes to aging.

  • Research connects oxidative stress with age-related diseases and lifespan.

  • Calorie restriction may reduce oxidative stress and extend lifespan under certain conditions.

The Cytoskeleton

  • Provides structural support, maintains cell shape, and facilitates cell movement.

  • Composed of three types of protein filaments:

  • Microtubules: Thickest filaments, provide paths for transport and maintain cell shape, involved in cell division, maintain cell shape and structure, resisting compression.

    • Important for positioning organelles within the cell.

    • Main structural components of cilia and flagella:

      • Cilia are found on respiratory epithelial cells.

      • Flagella are present in sperm cells.

    • Serve as paths (like railroad tracks) for chromosome separation during cell division, utilizing ATP and motor proteins.

    • Form tracks for vesicle transport within the cell.

    • Centrioles (two identical microtubule structures) are located near the nucleus, acting as origins for microtubules.

    • Microtubules can dynamically grow (add tubulin subunits) and shrink rapidly.

    • Microfilaments: Composed of actin, important for muscle contraction and cellular movement.

    • Intermediate Filaments: Provide tensile strength, anchoring cells and organelles.

Function of Cytoskeletal Components

  • Microtubules: Critical for chromosome segregation during cell division, facilitate movement via cilia and flagella.

  • Microfilaments: Involved in muscle contraction and forming the cleavage furrow during cytokinesis.

  • Intermediate Filaments: Support cell integrity during mechanical stress, critical for maintaining position within tissues.

Cytoplasm and Cellular Organelles: Overview of components including organelles (endoplasmic reticulum, mitochondria, lysosomes) and structural components (cytoskeleton).

  • Cytoskeleton:

    • Provides structural support to cells.

    • Critical for cell motility, reproduction, and transportation of substances.

    • Comprised of three types of filaments:

      • Microtubules:

        • Thickest filaments, consisting of tubulin dimers.

        • Maintain cell shape and resist compression.

        • Role in positioning organelles and forming tracks for vesicle movement.

        • Main component of cilia and flagella.

        • Dynamic structure: can rapidly grow and shrink.

      • Cilia and Flagella:

        • Cilia beat to move materials in respiratory system; flagella aid in sperm locomotion.

        • Both consist of an axoneme structure (9+2 arrangement of microtubules).

      • Microfilaments:

        • Composed of actin, involved in muscle contraction and cellular movement.

        • Dynamic, allowing cell shape changes and migration (e.g., white blood cells).

      • Intermediate Filaments:

        • Provide tensile strength, composed of various proteins (e.g., keratin).

        • Important for maintaining cell structure and linking cells together.

  • Endomembrane System:

    • Includes endoplasmic reticulum (smooth and rough), Golgi apparatus, lysosomes, and vesicles.

    • Functions together to produce, package, and export cellular products.

    • Smooth ER: Lipid synthesis, detoxification; abundant in liver cells.

    • Rough ER: Ribosome-studded, protein synthesis and modification.

    • Golgi Apparatus: Sorting and packaging proteins; compared to a post office.

    • Lysosomes: Recycling centers with digestive enzymes; digest cellular components and foreign materials (autophagy, apoptosis, phagocytosis).

  • Ribosomes: Sites of protein synthesis, composed of proteins and ribosomal RNA.

    • Found free in cytosol or attached to rough ER.

    • Synthesize proteins via translation from mRNA.

  • Mitochondria: Powerhouses of the cell, site of ATP production through aerobic respiration.

    • Consist of outer and inner membranes; cristae increase surface area for ATP synthesis.

  • Peroxisomes: Contain enzymes for lipid metabolism and detoxification.

    • Produce and break down hydrogen peroxide; abundant in liver cells.

  • Reactive Oxygen Species: Products of normal cellular processes that can cause damage.

    • Free radicals linked to various diseases including cancer.

    • Defense mechanisms include antioxidant enzymes (e.g., catalase).

  • Nucleus: Control center of the cell containing genetic information (DNA).

    • Surrounded by nuclear envelope; contains nucleolus for ribosomal RNA production.

    • Genes transcribed to mRNA for protein synthesis.