Cellular Structures and Organelles: Anatomy & Physiology I Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Cell

Introduction to Cell Structure

Cells are the fundamental units of life, characterized by their complex organization and chemical composition. They are primarily composed of carbon, hydrogen, nitrogen, oxygen, and trace amounts of other elements. The diversity in cell shape reflects their specialized functions within the body.

Cell Shape Variability: Cells may be spherical (fat cells), disk-shaped (red blood cells), branching (nerve cells), or cubelike (kidney tubule cells).
Function and Shape: The shape of a cell is closely related to its function. For example, flat, tile-like epithelial cells form protective barriers.
Composite Cell Concept: While no single cell type is identical to all others, most share common structural and functional features.

Cellular Structures

Cytoplasm

The cytoplasm is the cellular material inside the plasma membrane and outside the nucleus. It is the site where most cellular activities occur.

Cytosol: The viscous, semitransparent fluid environment of the cytoplasm, containing suspended elements. It is a complex mixture with both colloidal and true solution properties.
Organelles: Specialized compartments within the cell, each engineered to carry out specific functions. Organelles are the metabolic machinery of the cell.
Inclusions: Non-functioning units, such as stored nutrients (glycogen granules, lipid droplets) and pigments (melanin), present depending on cell type.

Cell Organelles

Mitochondria

Mitochondria are bean-shaped or sausage-shaped organelles known as the "powerhouses" of the cell, responsible for producing most of the cell's ATP (adenosine triphosphate).

Structure: Surrounded by two lipid bilayer membranes. The outer membrane is smooth; the inner membrane has folds called cristae that increase surface area for energy production.
Function: Enzymes in the matrix and cristae break down glucose and other nutrients, capturing energy to attach phosphate groups to ADP, forming ATP.
Cellular Respiration: The process is aerobic, requiring oxygen.
Genetic Material: Mitochondria contain their own DNA and RNA and are self-replicating. Mitochondrial DNA is inherited maternally.

Ribosomes

Ribosomes are minute granules composed of proteins and ribosomal RNA (rRNA). They are the sites of protein synthesis.

Structure: Each ribosome consists of two subunits (large and small) that fit together to form a functional ribosome.
Types: Free ribosomes synthesize proteins for cellular use; membrane-bound ribosomes (attached to the ER) synthesize proteins for membranes or export.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is an extensive system of interconnected membranes enclosing fluid-filled cavities (cisternae). It is continuous with the nuclear and plasma membranes.

Rough ER: Studded with ribosomes; involved in protein synthesis and modification. Proteins are assembled and threaded into the ER, where they are modified and transported.
Smooth ER: Lacks ribosomes; involved in lipid metabolism, synthesis of phospholipids and cholesterol, steroid hormone synthesis, detoxification, and storage of glycogen and calcium.

Golgi Apparatus

The Golgi apparatus consists of flattened membranous sacs and is the principal traffic director for cellular proteins.

Function: Modifies, concentrates, and packages proteins and lipids for delivery to specific destinations.
Protein Tagging: Proteins are tagged for delivery to lysosomes, the plasma membrane, or secretory granules.

Lysosomes

Lysosomes are spherical membranous bags containing hydrolytic enzymes, synthesized by the rough ER and packaged by the Golgi apparatus.

Function: Digestion of particles (bacteria, viruses, toxins), worn-out organelles, and non-useful tissues. Breakdown of stored glycogen and release of thyroid hormone.
Membrane Adaptation: Contains proton pumps to maintain low pH and retains acid hydrolases for safe digestion.
Autolysis: Lysosomal rupture can lead to self-digestion of the cell (autolysis), hence lysosomes are sometimes called "suicide sacs".

Peroxisomes

Peroxisomes are membranous sacs containing enzymes called peroxidases, which detoxify harmful substances and disarm free radicals.

Function: Convert hydrogen peroxide () to water via the enzyme catalase.
Location: Numerous in liver and kidney cells, active in detoxification.

Vaults

Vaults are large, octagonal barrel-shaped organelles, three times the size of ribosomes. Their function is not fully understood but may involve transport of molecules such as mRNA or ribosomal subunits between the nucleus and cytoplasm.

Structure: Hollow interior, possibly acting as cellular "trucks" for molecular transport.

Cytoskeleton

Microfilaments

Microfilaments are thin strands of the contractile protein actin, involved in cell motility and shape changes.

Function: Support cell movement, division, and contraction (especially in muscle cells).

Intermediate Filaments

Intermediate filaments are tough, insoluble protein fibers with high tensile strength, providing structural support and resisting mechanical stress.

Role: Most stable cytoskeletal elements; involved in desmosome formation.

Microtubules

Microtubules are long, hollow tubes formed of tubulin proteins, acting as the cell's internal scaffolding and organizers.

Function: Maintain cell shape, facilitate intracellular transport, and form the mitotic spindle during cell division.
Centrosome: Microtubules are anchored at the centrosome, which contains paired centrioles (each with a 9 triplet microtubule arrangement).

Cilia and Flagella

Cilia and flagella are motile cellular projections formed from centrioles.

Cilia: Hairlike extensions that move substances across the cell surface (e.g., respiratory tract).
Flagella: Longer projections; in humans, the sperm cell is the only example, using its flagellum for motility.

Nucleus

Structure and Function

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

Nuclear Membrane: Double membrane barrier with nuclear pores for molecular transport.
Nucleolus: Spherical body within the nucleus, site of ribosomal RNA synthesis and ribosome assembly.
Chromatin: Fine, threadlike DNA-protein complex that condenses into chromosomes during cell division.

Functions of Enzymes

Role in Cellular Processes

Enzymes are proteins that catalyze chemical reactions, increasing their rate and efficiency. They are essential for digestion, metabolism, growth, and cellular respiration.

Definition: Biological catalysts that lower activation energy for reactions.
Examples: ATP synthase (produces ATP), digestive enzymes (break down nutrients).
Equation for Enzyme Action: (Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product)

Summary Table: Major Cell Organelles and Their Functions

Organelle	Main Function	Key Features
Mitochondria	ATP production (cellular respiration)	Double membrane, cristae, own DNA
Ribosomes	Protein synthesis	Free or membrane-bound, two subunits
Endoplasmic Reticulum (ER)	Rough: Protein synthesis; Smooth: Lipid metabolism	Interconnected membranes, rough has ribosomes
Golgi Apparatus	Protein modification and packaging	Stacked membranous sacs
Lysosomes	Digestion of macromolecules	Hydrolytic enzymes, acidic interior
Peroxisomes	Detoxification, breakdown of peroxides	Enzymes (catalase, peroxidase)
Vaults	Molecular transport (possible)	Octagonal barrel shape
Nucleus	Genetic control center	Double membrane, nucleolus, chromatin

All Cells Have Three Major Parts

Nucleus: Control center, usually centrally located.
Cytoplasm: Surrounds the nucleus, packed with organelles.
Plasma Membrane: Encloses the cytoplasm, forming the external boundary.

Plasma Membrane Structure

Fluid Mosaic Model

The plasma membrane is a dynamic lipid bilayer composed of phospholipids, proteins, and cholesterol. It acts as a selective barrier and is involved in cell recognition, transport, and communication.

Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails face inward.
Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.
Carbohydrates: Glycoproteins and glycolipids contribute to cell recognition.

Specializations of the Plasma Membrane

Microvilli: Fingerlike projections that increase surface area for absorption.
Cell Junctions: Structures that connect cells, including tight junctions, desmosomes, and gap junctions.

Cell Junctions

Junction Type	Function	Location
Tight Junctions	Prevent passage of substances between cells	Epithelium (e.g., intestinal lining)
Desmosomes	Anchor cells together, resist mechanical stress	Skin, cardiac muscle
Gap Junctions	Allow passage of ions and small molecules	Cardiac and smooth muscle

Additional info: These notes expand on the original slides by providing definitions, examples, and academic context for each organelle and structure, as well as including relevant equations and tables for comparison.