Cellular Organelles and Structures Practice Flashcards

Introduction to Cellular Internal Components

Conceptual Overview: A prototypical human cell contains various internal structures and organelles that function harmoniously, much like the organs in a human body, to maintain health and perform vital functions.
Cytoplasmic Compartment: All living cells in multicellular organisms contain an internal cytoplasmic compartment and a nucleus.
Cytosol: * Definition: The jelly-like substance within the cell. * Etymology: "Cyto" means cell. * Function: Provides the fluid medium necessary for biochemical reactions.
Cytoplasm: Composed of the organelles and cytosol taken together.
Organelle: * Definition: "Little organ"; one of several different types of structures in the cell, each performing a unique function.
Nucleus: The cell’s central organelle which contains the cell’s DNA.

Summary Table of Cellular Organelles and Structures

Cell Membrane: Membranous (Yes). Function: Separate extracellular and intracellular fluids, regulate transport of material into and out of the cell, and regulate cell interactions.
Cytosol: Membranous (No). Function: Suspension of internal structures, providing for biochemical reactions.
Cytoskeleton (Microtubules, Intermediate Filaments, & Microfilaments): Membranous (No). Function: Cell shape, cell structure, organelle placement, internal transport.
Centrosome: Membranous (No). Function: Production of microtubules during cell division to divide DNA equally.
Ribosome: Membranous (No). Function: Protein synthesis.
Rough Endoplasmic Reticulum (RER): Membranous (Yes). Function: Protein folding and modification.
Smooth Endoplasmic Reticulum (SER): Membranous (Yes). Function: Lipid synthesis.
Golgi Apparatus: Membranous (Yes). Function: Protein sorting, modification, and shipping.
Lysosome: Membranous (Yes). Function: Break down of internal components or external material.
Peroxisome: Membranous (Yes). Function: Lipid metabolism and chemical detoxification.
Mitochondria: Membranous (Yes). Function: ATP production.
Nucleus: Membranous (Yes). Function: Storage of DNA (genetic code).

1a. Cytoskeleton and Centrosome

The Cytoskeleton: * Definition: A group of fibrous proteins that provide structural support for cells as well as support for cell motility, cell reproduction, and transportation of substances within the cell. * Function: Helps cells maintain structural integrity, similar to how the bony skeleton supports the human body. * Structure: A complex thread-like network consisting of three types of protein-based filaments.
Microtubules: * Composition: Structural filaments composed of subunits of a protein called tubulin. * Properties: The thickest of the three cytoskeletal components. * Functions: * Maintain cell shape and structure. * Resist compression of the cell. * Position organelles within the cell. * Create pathways for transport (acting like "railroad tracks") for vesicles and DNA. * Make up cellular appendages for motion: cilia and flagella.
Centrosome and Centrioles: * Structure: A centrosome consists of two short, identical microtubule structures called centrioles. * Location: Found near the nucleus. * Function: Serve as the cellular origin point for microtubules extending outward during cell division to ensure DNA is divided equally. * Growth: Microtubules grow from the centrosome by adding tubulin subunits, similar to adding links to a chain.
Microfilaments: * Composition: Primary component is actin, a protein that forms chains of single actin subunits. * Properties: A thinner type of cytoskeletal filament. * Functions: * Anchor the cytoskeleton to membrane proteins. * Thicken the consistency of the cytosol. * Form extensions of the cell membrane such as microvilli. * During cell division, they play a role in creating the division that splits the cell down the middle to form two new cells.
Intermediate Filaments: * Composition: Made up of long fibrous subunits of a protein called keratin wound together like threads in a rope. * Properties: Intermediate in thickness between microtubules and microfilaments. * Functions: * Maintain cell shape and structure in concert with microtubules. * Resist tension (forces that pull apart cells), such as when skin cells are compressed or tugged. * Anchor organelles together within the cell. * Link cells to other cells by forming special cell-to-cell junctions.

1b. Ribosomes

Definition: A non-membranous organelle that serves as the site of protein synthesis.
Composition: Two ribosomal RNA subunits combined with proteins.
Function: Functions to produce a chain of amino acids (proteins) based on the sequence of nucleotides in an RNA transcript.
Types Based on Location: * Free Ribosomes: Float in the cytosol unattached to any structure. * Attached Ribosomes: Attached to the external surface of the endoplasmic reticulum.

1c. Endoplasmic Reticulum (ER)

Structure: * A system of channels continuous with the nuclear membrane (envelope). * Composed of the same lipid bilayer material as the nuclear envelope. * Analogy: Described as a series of winding thoroughfares similar to the waterway canals in Venice or the catacombs of Paris. * Surface Area: The winding structure provides a large membranous surface area to support functions.
Rough Endoplasmic Reticulum (RER): * Appearance: Dotted with externally embedded ribosomes, giving it a bumpy appearance. * Function: Primary job is the synthesis and modification of proteins. * Process: Proteins are synthesized in the ribosome, released into the RER channel, and transported via vesicles to the Golgi apparatus. * Visual Data: Electron micrograph (EM) magnification is approximately $\times 110,000$ .
Smooth Endoplasmic Reticulum (SER): * Appearance: Lacks ribosomes. * Functions: * Synthesis of lipids, including phospholipids and steroid hormones. * Metabolizes some carbohydrates. * Breaks down certain toxins. * Sequestration and regulation of cellular $Ca^{2+}$ (calcium) concentration, which is critical in nervous system cells for triggering neurotransmitter release. * Biological Variation: Cells that produce high quantities of steroid hormones, such as female ovaries and male testes, contain large amounts of SER. * Visual Data: Electron micrograph (EM) magnification is approximately $\times 110,510$ .

1d. The Golgi Apparatus

Structure: A membranous organelle shaped like a stack of flattened discs (compared to oddly shaped pancakes).
Analogy: Operates much like a post office.
Functions: Sorting, modifying, and shipping off products received from the rough ER.
Operational Mechanism: * Receiving Side: One side receives products in vesicles. * Processing: Products are sorted and repackaged into new vesicles. * Shipping Side: Released from the opposite side. * Exocytosis: If products are exported from the cell, vesicles migrate to the cell surface, fuse with the membrane, and secrete cargo.
Additional Roles: Produces new organelles called lysosomes.

1e. Lysosomes

Definition: A membranous organelle containing enzymes that break down and digest unneeded cellular components.
Analogy: Similar to a "wrecking crew" that takes down old and unsound buildings.
Origin: Formed from enzyme-containing vesicles released by the Golgi apparatus which may form new lysosomes or fuse with existing ones.
Processes: * Autophagy: "Self-eating"; the process of a cell digesting its own structures, such as a damaged organelle. * Phagocytosis: Breaking down foreign material. For example, when immune cells phagocytize bacteria, the bacterial cell is transported into a lysosome and digested. * Autolysis/Apoptosis: A "self-destruct" mechanism where lysosomes release digestive enzymes into the cytoplasm to kill damaged or unhealthy cells, facilitating controlled cell death.

1f. Peroxisomes, ROS, and Oxidative Stress

Definition: A membrane-bound cellular organelle containing mostly enzymes for lipid metabolism and chemical detoxification.
Chemical Function: Enzymes transfer hydrogen atoms from molecules to oxygen to produce hydrogen peroxide ( $H_2O_2$ ).
Analogy: Miniature sewage treatment plants that neutralize toxins.
Organ Specificity: Liver cells contain an exceptionally high number of peroxisomes because the liver is primarily responsible for detoxifying blood.
Reactive Oxygen Species (ROS): * Definition: Highly reactive products of normal cellular processes, including ATP production. * Examples: Hydroxyl radical ( $OH$ ), hydrogen peroxide ( $H_2O_2$ ), and superoxide ( $O_2$ ). * Positive Functions: Used in cell signaling processes and immune responses. * Negative Impact: Free radicals contain unpaired electrons and can oxidize other molecules, causing cellular damage, mutations, and cell death. * Linked Pathologies: Cancer, coronary artery disease, Alzheimer’s disease, cardiovascular diseases, diabetes, Parkinson’s disease, arthritis, Huntington’s disease, and schizophrenia.
Defense Mechanisms: * Peroxisomes contain enzymes that convert toxic $H_2O_2$ into water and oxygen. * Antioxidants: Found in fruits and vegetables; they work by being oxidized themselves to halt destructive reaction cascades.
Oxidative Stress: * Definition: Damage to cellular components caused by ROS when they accumulate beyond the capacity of cellular defenses. * Mechanism: Chain reactions where electrons are removed from lipids, proteins, carbohydrates, and nucleic acids.
Mutation: * Definition: A change in the nucleotide sequence in a gene within a cell’s DNA, potentially altering the protein coded by that gene.

1g. Mitochondria

Definition: A membranous, bean-shaped organelle described as the "powerhouse of the cell."
Function: Produces the vast majority (approximately $95\%$ ) of adenosine triphosphate (ATP) required for cellular energy.
Structure: * Outer Membrane: A lipid bilayer. * Inner Membrane: A lipid bilayer highly folded into winding structures called cristae to create large surface area.
Cellular Respiration: Biochemical reactions occurring along the inner membrane that convert energy from nutrient molecules (like glucose) into ATP. This process requires oxygen molecules.
Abundance Variation: * Muscle Cells: Packed with mitochondria to sustain muscle contraction. * Nerve Cells: A single neuron may contain over $1,000$ mitochondria to run sodium-potassium pumps. * Bone Cells: Less metabolically active; may only have a couple hundred.
Unique Characteristics: * They consist of two membranes. * They contain their own set of DNA separate from the nucleus.
Evolutionary Origin (Endosymbiotic Theory): Long ago, mitochondria were independent single-cell organisms. They were transported into other cells by phagocytosis. Instead of being digested, they provided ATP while the host cell provided protection, creating an evolutionary advantage.
Maternal Lineage: During human sexual reproduction, all mitochondria in a child come from their biological mother. Mitochondrial DNA matches the biological mother, maternal grandmother, and so on.

Scientific Perspectives on Aging and the Cell

Free Radical Theory on Aging: * Proposed in the 1950s. * Core Premise: Accumulated cellular damage from oxidative stress contributes to the physiological and anatomical effects of aging. * Version A: The aging process itself is a result of oxidative damage. * Version B: Oxidative damage causes age-related diseases and disorders (more widely accepted).
Research Findings: * Reducing oxidative damage increased lifespan in yeast, worms, and fruit flies. * Increasing oxidative damage shortened lifespan in mice and worms. * Calorie Restriction: Moderately restricting caloric intake increased lifespan in some lab animals, likely due to reduced oxidative stress. However, a long-term study on primates showed no increase in lifespan.

Source and Terms Attribution

License: CC Attribution 4.0 International.
Adaptation: This tutorial has been adapted from OpenStax "Anatomy and Physiology 2e."