Chapter 3: Cells – The Living Units (Anatomy & Physiology Study Notes)

Cells: The Smallest Living Units

Cell Theory

The cell is the fundamental structural and functional unit of life. The health and function of an organism depend on the activities of its cells, both individually and collectively.

• All living things are composed of cells.

• Cells arise only from preexisting cells through cell division.

• Cell function determines organism function.

Examples: Muscle cells, nerve cells, fat cells, macrophages, sperm cells, etc., each specialized for unique functions.

Generalized Cell Structure

Basic Parts of a Human Cell

Despite specialization, all human cells share three main components:

• Plasma membrane: Flexible outer boundary that separates the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: Control center containing DNA.

Plasma Membrane

Structure and Function

The plasma membrane acts as an active barrier, separating intracellular fluid (ICF) from extracellular fluid (ECF). It controls what enters and leaves the cell, maintaining homeostasis.

• Composed of a phospholipid bilayer with embedded proteins.

• Hydrophilic heads face outward; hydrophobic tails face inward.

• Dynamic and selectively permeable—regulates passage of substances.

Also known as: Cell membrane.

Membrane Proteins and Their Functions

• Transport: Channel and carrier proteins move substances across the membrane. Some use ATP for active transport.

• Receptors for signal transduction: Proteins bind specific molecules (e.g., hormones), triggering cellular responses.

• Enzymatic activity: Membrane proteins may catalyze chemical reactions.

• Cell-cell recognition: Glycoproteins serve as identification tags recognized by other cells.

• Attachment to cytoskeleton and ECM: Helps maintain cell shape and stabilize membrane proteins.

• Cell-to-cell joining: Membrane proteins form junctions between adjacent cells (e.g., tight junctions, desmosomes, gap junctions).

Types of Cell Junctions

• Tight junctions: Prevent leakage of extracellular fluid between cells.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap junctions: Allow direct communication between cells via channels.

Transport Across the Plasma Membrane

Types of Transport

• Passive transport: No energy required. Substances move down their concentration gradient.

• Active transport: Requires energy (usually ATP). Substances move against their concentration gradient.

Passive Membrane Transport

• Simple diffusion: Nonpolar, lipid-soluble substances (e.g., O2, CO2, steroid hormones) diffuse directly through the lipid bilayer.

• Facilitated diffusion: Polar or charged substances move via protein channels or carriers.

• Osmosis: Diffusion of water across a selectively permeable membrane, either through the lipid bilayer or via aquaporins.

Diffusion: Movement from high to low concentration (down the concentration gradient).

Osmosis and Tonicity

• Osmolarity: Measure of solute concentration. Water moves from areas of low solute (high water) to high solute (low water) concentration.

• Tonicity: Ability of a solution to change the shape of cells by altering their internal water volume.

Solution Type	Effect on Cell
Isotonic	No net water movement; cell volume unchanged
Hypertonic	Water leaves cell; cell shrinks (crenation)
Hypotonic	Water enters cell; cell swells and may burst (lysis)

Active Membrane Transport

• Active transport: Uses carrier proteins (pumps) and ATP to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP (e.g., Na+/K+ pump).

• Secondary active transport: Indirect use of ATP; relies on gradients created by primary active transport.

• Vesicular transport: Movement of large particles or fluids via vesicles (endocytosis, exocytosis).

Types of Vesicular Transport

• Endocytosis: Bringing substances into the cell.

  • Phagocytosis: "Cell eating"—engulfing large particles.

  • Pinocytosis: "Cell drinking"—engulfing extracellular fluid.

  • Receptor-mediated endocytosis: Specific uptake of substances via receptors.

• Exocytosis: Expelling substances from the cell via vesicle fusion with the plasma membrane.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It consists of:

• Cytosol: Gel-like solution containing water, proteins, salts, and sugars.

• Organelles: Specialized structures with specific functions, either membranous or nonmembranous.

Membranous Organelles

• Mitochondria: "Powerhouse" of the cell; site of ATP production via aerobic respiration. Double-membraned with inner folds (cristae). Contains its own DNA and ribosomes.

• Endoplasmic reticulum (ER): Network of membranes continuous with the nuclear envelope.

  • Rough ER: Studded with ribosomes; site of protein synthesis for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; involved in lipid metabolism, detoxification, and calcium storage.

• Golgi apparatus: Stacks of flattened membranes; modifies, sorts, and packages proteins and lipids from the ER for delivery.

• Peroxisomes: Contain enzymes for detoxifying harmful substances and breaking down fatty acids.

• Lysosomes: Contain digestive enzymes for breaking down waste, pathogens, and old organelles.

Nonmembranous Organelles

• Ribosomes: Sites of protein synthesis; composed of rRNA and protein. Can be free in cytosol or bound to rough ER.

• Cytoskeleton: Network of protein rods (microfilaments, intermediate filaments, microtubules) providing structural support and facilitating movement.

• Centrioles: Barrel-shaped structures involved in cell division and formation of cilia and flagella.

Nucleus

Structure and Function

The nucleus is the control center of the cell, containing the genetic material (DNA) necessary for protein synthesis and cell division.

• Nuclear envelope: Double membrane surrounding the nucleus.

• Nucleoli: Sites of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA wrapped around histone proteins; condenses to form chromosomes during cell division.

Cell types: Most cells are uninucleate; some (e.g., skeletal muscle) are multinucleate; mature red blood cells are anucleate.

Cell Cycle

Phases of the Cell Cycle

• Interphase: Period of cell growth and normal activity. Subdivided into G1 (growth), S (DNA synthesis/replication), and G2 (preparation for division).

• Mitotic (M) phase: Cell division, consisting of mitosis (nuclear division) and cytokinesis (cytoplasmic division).

DNA Replication

• Occurs during S phase of interphase.

• DNA strands separate at replication forks; each serves as a template for a new complementary strand.

• Key enzymes: DNA polymerase (adds nucleotides), DNA ligase (joins fragments).

Equation:

$\text{DNA}_{\text{parent}} \xrightarrow{\text{replication}} \text{DNA}_{\text{daughter}}$

Control of Cell Division

• Regulated by internal and external signals (e.g., growth factors, hormones, contact inhibition).

• Key proteins: Cyclins and cyclin-dependent kinases (Cdks) regulate progression through the cell cycle.

• Checkpoints ensure proper division; faulty cells may enter G0 (non-dividing state).

Protein Synthesis

Genetic Code and Gene Structure

• Gene: Segment of DNA coding for a polypeptide.

• Triplet code: Three DNA bases specify one amino acid.

• Exons: Coding regions; Introns: Noncoding regions.

Role of RNA

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosome (transcription).

• Ribosomal RNA (rRNA): Structural component of ribosomes.

• Transfer RNA (tRNA): Brings amino acids to ribosome; anticodon pairs with mRNA codon (translation).

Steps of Protein Synthesis

• Transcription: DNA information is copied into mRNA in the nucleus.

• Translation: mRNA is decoded at the ribosome to assemble a polypeptide chain.

Transcription Phases

1. Initiation: RNA polymerase binds to DNA and unwinds the strands.

2. Elongation: RNA polymerase adds complementary RNA nucleotides.

3. Termination: Transcription ends at a termination signal.

Translation Phases

1. Initiation: Ribosome assembles at the start codon of mRNA; initiator tRNA binds.

2. Elongation: tRNAs bring amino acids; peptide bonds form; ribosome moves along mRNA.

3. Termination: Stop codon is reached; polypeptide is released.

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: The notes also reference the endomembrane system, cilia, flagella, and microvilli as specialized structures important for cell function and movement.