Cells: The Living Units – Study Notes for Anatomy & Physiology

Chapter 03: Cells – The Living Units

Introduction to Cells

Cells are the fundamental structural and functional units of life. The health and function of an organism depend on the combined activities of its cells. Cellular structure and function are closely related, and the continuity of life is maintained through cellular reproduction.

• Cell Theory: All living organisms are composed of cells; cells are the basic unit of structure and function; cells arise only from preexisting cells.

• Generalized Cell Structure: All human cells share common features:

  • Plasma membrane: Flexible outer boundary.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: DNA-containing control center.

Extracellular Materials

Substances found outside cells play important roles in tissue structure and function.

• Extracellular fluids: Include interstitial fluid (bathes cells), blood plasma, and cerebrospinal fluid.

• Cellular secretions: Such as saliva and mucus.

• Extracellular matrix: Acts as a glue to hold cells together.

Plasma Membrane

Functions of the Plasma Membrane

The plasma membrane separates the cell from its environment and regulates interactions.

• Physical barrier: Encloses the cell, separating cytoplasm from extracellular fluid.

• Selective permeability: Controls entry and exit of substances.

• Communication: Membrane proteins interact with chemical messengers.

• Cell recognition: Surface carbohydrates allow cells to recognize each other.

Structure of the Plasma Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins.

• Phospholipids: Have polar hydrophilic heads facing water inside and outside the cell, and nonpolar hydrophobic tails facing inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

Membrane Transport

Types of Membrane Transport

Substances move across the plasma membrane by passive or active mechanisms.

• Passive transport: No energy required.

• Active transport: Requires energy (ATP).

Passive Transport Mechanisms

• Diffusion: Movement of molecules from high to low concentration due to kinetic energy.

• Osmosis: Movement of water across a selectively permeable membrane.

Osmosis occurs through the lipid bilayer and aquaporins. Water moves from areas of low solute concentration to high solute concentration.

Tonicity and Its Effects on Cells

Tonicity describes the ability of a solution to change cell shape by altering water volume.

Solution Type	Osmolarity	Effect on Cell
Isotonic	Same as cell	No change
Hypertonic	Higher than cell	Cell shrinks (crenation)
Hypotonic	Lower than cell	Cell swells (lysis)

Active Transport Mechanisms

• Sodium-potassium pump: Moves Na+ out and K+ into the cell using ATP.

• Vesicular transport: Moves large particles and fluids via vesicles (endocytosis and exocytosis).

Types of Endocytosis

Type	Description	Specificity
Phagocytosis	Engulfs large particles via pseudopods	Specific (receptors)
Pinocytosis	Gulps extracellular fluid	Nonspecific
Receptor-mediated	Ingests specific substances bound to receptors	Highly specific

Cytoplasmic Organelles

Types of Organelles

Organelles perform specialized functions within the cell.

• Membranous: Mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes, lysosomes.

• Nonmembranous: Ribosomes, cytoskeleton, centrioles.

Mitochondria

• "Power plant" of the cell; produces ATP via aerobic respiration.

• Double membrane with inner folds (cristae).

• Contains its own DNA, RNA, and ribosomes.

Ribosomes

• Sites of protein synthesis; composed of protein and rRNA.

• Free ribosomes synthesize soluble proteins; membrane-bound ribosomes synthesize proteins for membranes or export.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Peroxisomes

• Contain enzymes for detoxification (oxidase and catalase).

• Break down fatty acids and neutralize free radicals.

Lysosomes

• Contain digestive enzymes (acid hydrolases).

• Break down ingested substances, nonfunctional organelles, and release stored molecules.

Cytoskeleton

The cytoskeleton is a network of protein rods that provide structural support and facilitate movement.

• Microfilaments: Thinnest; composed of actin; support cell shape.

• Intermediate filaments: Rope-like; resist pulling forces.

• Microtubules: Hollow tubes; determine cell shape and organelle distribution; involved in movement (e.g., cilia, flagella).

Cellular Extensions

• Cilia and flagella: Aid in movement of cell or materials across cell surface.

• Microvilli: Increase surface area for absorption.

Nucleus and Genetic Material

Nucleus Structure

• Nuclear envelope: Double membrane surrounding nucleus.

• Chromatin: DNA and proteins; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosome synthesis.

Chromatin and Chromosomes

• Chromatin consists of DNA, histone proteins, and RNA.

• Nucleosomes: DNA wrapped around histones.

• Chromosomes: Condensed chromatin for cell division.

Cell Cycle and Division

Phases of the Cell Cycle

• Interphase: Cell grows and carries out normal functions; DNA replication occurs.

• Mitotic phase: Cell divides (mitosis and cytokinesis).

Mitosis Stages

• Prophase

• Metaphase

• Anaphase

• Telophase

DNA Replication

DNA replication ensures each daughter cell receives a complete set of genetic information.

• Main function of DNA: Dictates protein synthesis.

Protein Synthesis

Genetic Code and Information Flow

Protein synthesis is directed by DNA through transcription and translation.

• Gene: Segment of DNA coding for a polypeptide.

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

• Exons: Coding regions; Introns: Noncoding regions.

Role of RNA

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

• Ribosomal RNA (rRNA): Forms part of ribosome structure.

• Transfer RNA (tRNA): Brings amino acids to ribosome during translation.

• RNA uses uracil instead of thymine and ribose sugar.

Steps in Protein Synthesis

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

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

Genetic Code Table (Described)

There are 64 possible mRNA codons, each consisting of three bases. Some amino acids are specified by more than one codon, providing redundancy and protection against transcription errors.

Key Equations

• Diffusion Rate:

• $J = -D \frac{dC}{dx}$

• Osmosis (Water Potential):

• $\Psi = \Psi_s + \Psi_p$

• Sodium-Potassium Pump:

• $3\text{Na}^+_{\text{out}} : 2\text{K}^+_{\text{in}}$ per ATP hydrolyzed

Example:

During translation, the mRNA codon GGC codes for the amino acid glycine. If the DNA triplet is CCG, the mRNA codon will be GGC.

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