Complete Cell Organelles & Functions Cheatsheet: Structure, Function & Interactions

Introduction: Understanding Cell Organelles

Cell organelles are specialized structures within cells that perform specific functions necessary for cellular survival, growth, and reproduction. Like organs in the human body, each organelle has a unique role in maintaining cellular homeostasis. These microscopic compartments enable cells to efficiently carry out thousands of biochemical reactions simultaneously by creating specialized microenvironments. Understanding cell organelles is fundamental to comprehending how cells function as the basic units of life.

Prokaryotic vs. Eukaryotic Cells

Major Membrane-Bound Organelles

Nucleus

Structure:

• Double membrane (nuclear envelope) with nuclear pores
• Contains nucleoplasm, chromatin (DNA + proteins)
• Houses nucleolus (ribosome production site)
• Surrounded by nuclear lamina (structural support)

Functions:

• Houses genetic material (DNA)
• Controls cellular activities through gene expression
• Site of DNA replication
• Regulates movement of materials between nucleus and cytoplasm
• Nucleolus synthesizes ribosomal RNA and assembles ribosome subunits

Key Components:

• Nuclear envelope: Double membrane with nuclear pores
• Nuclear pores: Allow selective transport between nucleus and cytoplasm
• Chromatin: DNA + histone proteins
• Nucleolus: Site of ribosomal RNA synthesis
• Nuclear lamina: Protein network providing structural support

Endoplasmic Reticulum (ER)

Rough Endoplasmic Reticulum (RER)

• Structure: Network of membrane-bound sacs with attached ribosomes
• Functions:
  • Protein synthesis and folding
  • Quality control of newly synthesized proteins
  • Transport of proteins to Golgi apparatus
  • Synthesis of phospholipids for cell membrane
  • Initial glycosylation of proteins

Smooth Endoplasmic Reticulum (SER)

• Structure: Network of membrane-bound tubules without ribosomes
• Functions:
  • Lipid synthesis (steroids, phospholipids)
  • Detoxification of drugs and harmful substances
  • Calcium storage and release
  • Carbohydrate metabolism
  • Steroid hormone production (in certain cells)

Golgi Apparatus (Golgi Complex)

Structure:

• Stack of flattened membrane-bound sacs (cisternae)
• Polarized structure: cis face (receiving) and trans face (shipping)
• Associated vesicles for transport

Functions:

• Modification, sorting, and packaging of proteins and lipids
• Processing of proteins received from ER
• Further glycosylation of proteins (adding sugar groups)
• Production of lysosomes
• Formation of secretory vesicles
• Synthesis of cell wall components in plant cells

Key Regions:

• Cis Golgi network: Receives vesicles from ER
• Medial Golgi: Intermediate processing compartment
• Trans Golgi network: Sorts and packages molecules for specific destinations

Lysosomes

Structure:

• Membrane-bound spherical vesicles
• Acidic interior (pH ~4.5)
• Contains hydrolytic enzymes (acid hydrolases)

Functions:

• Intracellular digestion of macromolecules
• Breakdown of old/damaged organelles (autophagy)
• Destruction of phagocytosed particles
• Recycling of cellular components
• Role in programmed cell death (apoptosis)
• Defense against bacteria and viruses

Special Features:

• Proton pumps in membrane maintain acidic environment
• Membrane protects cytoplasm from digestive enzymes
• Particularly abundant in phagocytic cells

Peroxisomes

Structure:

• Single membrane-bound vesicles
• Granular matrix
• Formed by growth and division of existing peroxisomes or budding from ER

Functions:

• Breakdown of long-chain fatty acids
• Detoxification of hydrogen peroxide (H₂O₂)
• Metabolism of alcohol and other toxins
• Photorespiration in plant cells
• Synthesis of bile acids and plasmalogens
• Role in cellular respiration

Key Enzymes:

• Catalase: Breaks down hydrogen peroxide into water and oxygen
• Oxidases: Generate hydrogen peroxide during metabolic reactions

Mitochondria

Structure:

• Double membrane organelle
• Outer membrane (smooth)
• Inner membrane (folded into cristae)
• Matrix (gel-like interior)
• Own circular DNA (mitochondrial DNA)
• Own ribosomes (70S)

Functions:

• ATP production through cellular respiration
• Oxidative phosphorylation
• Citric acid cycle (Krebs cycle)
• Beta-oxidation of fatty acids
• Calcium storage and signaling
• Heat production in brown fat
• Role in apoptosis (programmed cell death)
• Oxygen and nitrogen metabolism

Special Features:

• Semiautonomous organelles with own DNA and ribosomes
• Can divide independent of cell cycle (fission)
• Maternally inherited in most species
• “Powerhouse of the cell”

Vacuoles

Structure:

• Large, fluid-filled, membrane-bound sacs
• Single membrane (tonoplast in plants)

Functions in Plant Cells:

• Storage of nutrients, waste products, and pigments
• Maintenance of turgor pressure
• pH regulation
• Storage of defensive compounds
• Degradation of macromolecules

Functions in Animal Cells:

• Smaller and multiple (vs. large central vacuole in plants)
• Food vacuoles in protists
• Contractile vacuoles in freshwater protists (osmoregulation)

Special Features:

• Can occupy up to 90% of cell volume in mature plant cells
• Tonoplast contains aquaporins and ion channels for transport

Plant-Specific Organelles

Chloroplasts

Structure:

• Double membrane organelle
• Outer membrane (smooth)
• Inner membrane
• Intermembrane space
• Stroma (fluid-filled interior)
• Thylakoid membrane system with grana stacks
• Own circular DNA (chloroplast DNA)
• Own ribosomes (70S)

Functions:

• Photosynthesis (light reactions in thylakoid membranes)
• Carbon fixation (Calvin cycle in stroma)
• Synthesis of fatty acids
• Storage of starch
• Amino acid synthesis
• Secondary metabolite production

Key Components:

• Thylakoids: Membrane sacs containing photosynthetic pigments
• Grana: Stacks of thylakoids
• Stroma: Fluid matrix where Calvin cycle occurs
• Chlorophyll: Primary photosynthetic pigment

Cell Wall

Structure:

• Rigid layer outside cell membrane
• Primary cell wall: cellulose, hemicellulose, pectin
• Secondary cell wall (in woody plants): added lignin
• Middle lamella: connects adjacent cells

Functions:

• Structural support and protection
• Maintenance of cell shape
• Prevention of excessive water uptake
• Resistance to turgor pressure
• Cell-to-cell communication via plasmodesmata
• Defense against pathogens

Components by Organism:

• Plants: Cellulose, hemicellulose, pectin, lignin
• Fungi: Chitin, glucans
• Bacteria: Peptidoglycan, teichoic acids
• Archaea: Various (pseudopeptidoglycan, glycoprotein)
• Algae: Cellulose, alginic acid, carrageenan (varies by type)

Plasmodesmata

Structure:

• Channels through cell walls connecting adjacent plant cells
• Lined by plasma membrane
• Central desmotubule (from ER)

Functions:

• Cell-to-cell communication
• Transport of small molecules, ions, and some proteins
• Coordination of plant growth and development
• Distribution of nutrients
• Propagation of signaling molecules

Non-Membrane-Bound Organelles

Ribosomes

Structure:

• Composed of ribosomal RNA (rRNA) and proteins
• Two subunits: large and small
• Eukaryotic ribosomes: 80S (60S + 40S subunits)
• Prokaryotic ribosomes: 70S (50S + 30S subunits)

Functions:

• Protein synthesis (translation)
• Decoding mRNA information
• Formation of peptide bonds between amino acids
• Quality control of protein synthesis

Locations:

• Free ribosomes: In cytoplasm, synthesize soluble proteins
• Bound ribosomes: Attached to ER, synthesize membrane/secretory proteins
• Mitochondrial/chloroplast ribosomes: Synthesize organelle-specific proteins

Cytoskeleton

Microtubules

• Structure: Hollow tubes made of tubulin protein dimers
• Functions:
  • Maintenance of cell shape
  • Intracellular transport (via motor proteins)
  • Cell division (mitotic spindle)
  • Cell movement (cilia and flagella)
  • Organization of cell contents

Microfilaments (Actin Filaments)

• Structure: Twisted strands of actin proteins
• Functions:
  • Cell shape and support
  • Muscle contraction
  • Cell motility
  • Cytoplasmic streaming
  • Cell division (cytokinesis)
  • Cell adhesion

Intermediate Filaments

• Structure: Fibrous proteins specific to cell type
• Functions:
  • Structural support
  • Resistance to mechanical stress
  • Anchoring of nucleus and other organelles
  • Formation of nuclear lamina
  • Cell-to-cell junctions

Centrioles

Structure:

• Cylindrical structures composed of nine triplets of microtubules
• Found in pairs oriented perpendicular to each other
• Located in centrosome near nucleus

Functions:

• Organization of microtubules
• Formation of mitotic spindle during cell division
• Formation of basal bodies for cilia and flagella
• Role in cell division and separation of chromosomes

Note: Absent in most plant cells and some fungal cells

Nucleolus

Structure:

• Dense region within nucleus (not membrane-bound)
• Composed of DNA, RNA, and proteins
• Forms around nucleolar organizing regions (NORs)

Functions:

• Synthesis of ribosomal RNA (rRNA)
• Assembly of ribosomal subunits
• Processing of pre-rRNA
• Response to cellular stress

Animal-Specific Structures

Lysosomes

Structure:

• Single membrane-bound vesicles
• Acidic interior (pH ~4.5)
• Filled with digestive enzymes

Functions:

• Breakdown of waste materials, foreign particles
• Recycling of old organelles (autophagy)
• Role in programmed cell death
• Defense against pathogens

Cilia and Flagella

Structure:

• Membrane-bound extensions containing microtubules
• 9+2 arrangement of microtubules
• Basal body (derived from centriole) at base

Functions:

• Cilia: Short, numerous extensions for moving materials across cell surface
• Flagella: Long, whip-like structures for cell movement
• Sensory roles in some specialized cells

Specialized Cell Organelles and Structures

Melanosomes

• Membrane-bound organelles in melanocytes
• Synthesize and store melanin pigment
• Function in skin/hair color and UV protection

Secretory Vesicles

• Membrane-bound sacs containing molecules for secretion
• Transport proteins from Golgi to cell membrane or extracellular space
• Function in regulated secretion (hormones, neurotransmitters, digestive enzymes)

Hydrogenosomes

• Found in certain anaerobic eukaryotes
• Alternative to mitochondria in low-oxygen environments
• Produce hydrogen gas and ATP through fermentation

Glyoxysomes

• Specialized peroxisomes in plant seeds
• Convert stored lipids to carbohydrates during germination
• Contain enzymes of glyoxylate cycle

Synaptic Vesicles

• Small membrane-bound sacs in nerve cells
• Store neurotransmitters
• Release contents into synaptic cleft during nerve signaling

Organelle Interactions and Cellular Pathways

Protein Synthesis and Transport Pathway

Cellular Respiration Pathway

Photosynthesis Pathway

Autophagy Pathway

Cell Type Comparison: Organelle Distribution

Common Dysfunctions and Related Diseases

Study Techniques for Cell Organelles

Microscopy Methods

Cell Fractionation

• Technique to isolate specific organelles by differential centrifugation
• Based on size, density, and sedimentation coefficients of organelles
• Used for biochemical and functional studies of isolated organelles

Resources for Further Learning

Textbooks

• “Molecular Biology of the Cell” by Alberts et al.
• “Cell Biology” by Pollard, Earnshaw, Lippincott-Schwartz, and Johnson
• “Molecular Cell Biology” by Lodish et al.

Online Resources

• Cell Image Library: Repository of cellular imaging
• iBiology: Video lectures on cell biology
• HHMI BioInteractive: Animations and resources on cell structure

Interactive Tools

• CellsAlive!: Interactive cell models
• CellAnim: Cell and organelle animations
• Cell Biology by the Numbers: Database of cellular measurements and calculations

Research Journals

• “Journal of Cell Biology”
• “Nature Cell Biology”
• “Current Opinion in Cell Biology”
• “Trends in Cell Biology”