Chapter 3: Cells – The Living Units (Bio 230: Intro to Anatomy) Study Notes

Cells – The Living Units

The Big Picture: How Cells Work

Cells are the fundamental units of life, functioning as integrated systems where organelles collaboratively execute essential processes. The Central Dogma (DNA → RNA → Protein) is central to cellular function, with protein synthesis linking most organelles. Energy (ATP from mitochondria) powers all cellular work, and disease often originates at the cellular level due to organelle dysfunction or failures in homeostatic regulation.

• Central Dogma: DNA → RNA → Protein

• Protein synthesis is the core process linking most organelles

• Cells are fluid-filled "little bags" with organelles working collaboratively

• Energy (ATP from mitochondria) powers all cellular work

• Disease frequently begins at the cellular level; understanding cells is crucial for understanding health

Cell Theory & Cell Types

The cell theory outlines the foundational principles of biology, emphasizing the universality and essential nature of cells.

• Principle 1: All living things are made of cells

• Principle 2: Cells are the basic structural and functional unit of life

• Principle 3: All cells come from pre-existing cells (no spontaneous generation)

• Principle 4: Structure determines function (Principle of Complementarity)

Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	No (nucleoid region)	Yes (membrane-bound)
DNA	Circular, in cytoplasm	Linear, in nucleus
Membrane-bound organelles	No	Yes
Ribosomes	Yes (smaller)	Yes (larger)
Examples	Bacteria	Human cells, plants, animals

Plant vs Animal Cells – Key Differences

Feature	Animal Cell	Plant Cell
Cell wall	No	Yes (cellulose)
Energy source	Mitochondria only	Mitochondria + chloroplasts
Shape	Irregular	Fixed, rectangular
Chloroplasts	No	Yes (photosynthesis)
Central vacuole	Small/temporary	Large central
Centrioles	Yes (cell division)	No
Lysosomes	Yes	Yes (but fewer)

Cell Membrane & Cytoplasm

The cell membrane is a selective barrier composed of a phospholipid bilayer, maintaining the internal environment and controlling the movement of substances. The cytoplasm includes all fluid and contents inside the cell, outside the nucleus, and contains organelles suspended in cytosol.

• Phospholipid bilayer: Hydrophilic heads face outward/inward; hydrophobic tails face each other

• Membrane proteins: Channels, receptors, glycoproteins, cholesterol

• Cytoskeleton: Provides structure and anchors organelles

Nucleus & Nucleolus

The nucleus is the command center, containing genetic material and controlling transcription. The nucleolus produces ribosomes. Chromatin is the most common form of DNA during the cell's life, condensing into chromosomes during cell division.

• Transcription: DNA is transcribed into messenger RNA (mRNA)

• Nucleolus: Makes ribosomes

• Chromatin: DNA + proteins, loose form

• Chromosomes: Condensed chromatin, visible during mitosis/meiosis

Ribosomes & Protein Synthesis

Ribosomes are the protein builders, reading mRNA instructions and assembling amino acids into proteins. Protein synthesis follows the Central Dogma: DNA → mRNA → Ribosome → Protein.

• Translation: Ribosome reads mRNA, links amino acids into polypeptide chains

• LEGO Metaphor: mRNA = instruction manual, ribosome = builder, amino acids = LEGO bricks, protein = finished structure

• tRNA: Brings amino acids to ribosome

• rRNA: Makes up ribosome, catalyzes peptide bonds

Key Codons

• AUG: Start codon (Methionine)

• UAA, UAG, UGA: Stop codons (no amino acid)

Endoplasmic Reticulum (ER)

Rough ER (RER)

The rough ER modifies proteins through folding, glycosylation, quality control, and destination tagging.

• Initial folding: Proteins folded into correct 3D structure

• Quality control: Misfolded proteins retained or destroyed

• Glycosylation: Addition of sugars to proteins

• Destination tagging: Molecular address labels for protein destination

Smooth ER (SER)

The smooth ER synthesizes lipids and carbohydrates, using enzymes produced via the protein synthesis pathway.

• Lipid synthesis: Fats, phospholipids, steroids

• Carbohydrate synthesis: Enzymes required, which are proteins

Golgi Apparatus

The Golgi apparatus further modifies proteins, packages them into vesicles, and ships them to their final destinations.

• CIS face: Receiving side (closest to ER)

• TRANS face: Shipping side (facing cell membrane)

• Protein destinations: Cell membrane, outside cell (exocytosis), other organelles (e.g., lysosomes)

Mitochondria & Cellular Respiration

Mitochondria are the powerhouses of the cell, producing ATP through cellular respiration. ATP powers all cellular work, including protein synthesis, cell division, and active transport.

• Cellular Respiration Equation:

$C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + ATP$

• Inputs: Glucose, Oxygen

• Outputs: Carbon dioxide, Water, ATP

Lysosomes & Peroxisomes

Lysosomes

Lysosomes contain digestive enzymes, breaking down cellular waste, old organelles, and invaders. They can initiate apoptosis (programmed cell death).

• Enzymes: Follow the protein synthesis pathway (DNA → mRNA → ribosome → rough ER → Golgi → lysosome)

• Apoptosis: Cell deliberately destroys itself when damaged, infected, or during development

Peroxisomes

Peroxisomes detoxify harmful substances, breaking down hydrogen peroxide into water and oxygen.

• Function: Oxidizes or neutralizes toxins

Vacuoles

Vacuoles store chemicals and materials. Plant cells have one large central vacuole for water and turgor pressure; animal cells have multiple small vacuoles.

• Plant cells: Large central vacuole

• Animal cells: Small, multiple vacuoles

Cytoskeleton

The cytoskeleton provides internal structure, holds organelles in place, and is composed of proteins. It consists of microtubules, intermediate filaments, and microfilaments.

• Microtubules: Long, hollow fibers; connect organelles; form spindle fibers during cell division

• Intermediate filaments: Rope-like strength; provide overall cell structure

• Microfilaments: Made of actin; provide structure near cell membrane; involved in cell movement

Centrosome & Cell Division

The centrosome organizes microtubules and is essential for cell division. It duplicates, moves to opposite ends, forms spindle fibers, attaches to chromosomes, and ensures each daughter cell receives complete DNA.

• Composed of: Two centrioles (animal cells only)

• Function: Forms spindle fibers, pulls chromosomes apart during mitosis

Cell Cycle & Mitosis

Phase	What Happens
G₁	Cell growth, protein synthesis, organelle duplication
S	DNA replication (each chromosome becomes 2 sister chromatids)
G₂	Prepares for division, checks DNA, synthesizes proteins for mitosis
Prophase	Chromosomes condense, nuclear envelope breaks down, spindle fibers form
Metaphase	Chromosomes line up at metaphase plate, spindle fibers attach
Anaphase	Sister chromatids separate, pulled to opposite poles
Telophase	Nuclear envelopes reform, chromosomes decondense
Cytokinesis	Cytoplasm divides, two daughter cells form

Cell Surface Structures: Microvilli, Cilia, Flagella

Cell surface structures serve specialized functions. Microvilli increase surface area for absorption, cilia move substances across the cell surface, and flagella propel the cell forward.

Feature	Microvilli	Cilia	Flagella
Movement?	No	Yes	Yes
Microtubules inside?	No	Yes	Yes
Function	Increase surface area	Move substances	Propel cell
Location	Small intestine	Lungs (bronchioles)	Sperm cell

Membrane Transport & Tonicity

Cells use various transport mechanisms to move substances across membranes. Tonicity describes the effect of extracellular solution concentration on cell volume.

Transport Type	Energy?	Membrane Protein?	Direction	Examples
Simple Diffusion	No	No	High → Low	O₂, CO₂, lipids
Facilitated Diffusion	No	Yes	High → Low	Glucose, ions
Osmosis	No	Aquaporins	High water → Low water	H₂O
Active Transport (Primary)	Yes (ATP)	Yes (pump)	Low → High	Na⁺/K⁺ pump, Ca²⁺ pump
Active Transport (Secondary)	Yes (indirect)	Yes (co-transporter)	Low → High	Na⁺-glucose co-transport
Vesicular Transport	Yes (ATP)	Vesicles	Into/out of cell	Endocytosis, exocytosis

Tonicity Effects

Solution	Solute Outside vs Inside	Water Movement	Animal Cell Result	Plant Cell Result	Clinical Example
Isotonic	Same concentration	Equal in and out	Normal	Flaccid (normal)	0.9% saline, 5% dextrose
Hypotonic	Lower outside (more water)	Water enters cell	Swells → Lysis	Turgid (good!)	Distilled water
Hypertonic	Higher outside (less water)	Water leaves cell	Shrinks → Crenation	Plasmolyzed (wilts)	10% saline, concentrated sugar

Clinical Connections

Cellular dysfunction can lead to various diseases. Examples include:

• Cilia dysfunction: Kartagener's syndrome (immotile cilia → chronic lung infections, infertility)

• Protein folding diseases: Alzheimer's, Parkinson's, Cystic fibrosis (misfolded proteins accumulate)

• Lysosomal storage disorders: Tay-Sachs disease (missing enzymes → toxic buildup)

• Mitochondrial diseases: ATP production fails, affects high-energy tissues

• Apoptosis dysfunction: Too little → cancer; too much → neurodegeneration

• Cell cycle regulation: Cancer (checkpoints fail, p53 mutated)

• Tonicity: IV fluids must be isotonic to blood to prevent RBCs from bursting or shriveling

Concept Map – The Connected Cell

All organelles are interconnected through the protein synthesis pathway and energy production. The nucleus contains DNA instructions for all proteins; ribosomes make those proteins; ER and Golgi modify and ship them; lysosomal enzymes, cytoskeleton proteins, membrane proteins all originate from this pathway; mitochondria provide ATP to power the entire process.

Summary Table: Organelle Functions

Organelle	Function
Nucleus	Contains DNA; site of transcription
Nucleolus	Makes ribosomes
Ribosome	Reads mRNA; assembles proteins
Rough ER	Modifies proteins (folding, glycosylation, tagging)
Smooth ER	Synthesizes lipids and carbohydrates
Golgi	Packages proteins into vesicles
Mitochondria	Produces ATP
Lysosome	Digests waste; apoptosis
Peroxisome	Detoxifies harmful substances
Cytoskeleton	Provides structure; holds organelles in place

Main Takeaway

Cells are integrated systems where organelles collaborate to execute life processes. The Central Dogma and protein synthesis pathway connect all organelles, and energy from mitochondria powers every cellular function. Understanding these connections is essential for comprehending health and disease at the cellular level.