Cellular Level of Organization – Fundamentals of Anatomy & Physiology (Chapter 3 Study Notes)

Introduction to Cells

Cell Theory and Cellular Homeostasis

The cell is the fundamental unit of life in all organisms. Cell theory provides the foundation for understanding biological structure and function.

• Cell Theory: States that all living things are composed of cells, all cells arise from preexisting cells, and cells are the smallest units that perform vital physiological functions.

• Homeostasis: Each cell maintains its own internal environment, contributing to the overall homeostasis of the organism.

Types of Cells

Cells are classified based on their roles in reproduction and bodily function.

• Sex Cells (Germ Cells): Specialized for reproduction. Includes sperm (male) and oocytes (female, which develop into ova).

• Somatic Cells: All other body cells except sex cells.

Anatomy of a Model Cell

Cell Structure Overview

A typical animal cell contains various organelles, each with specialized functions essential for cellular activity.

• Plasma Membrane: Separates the cell from its external environment and regulates the movement of substances.

• Cytosol: The fluid component inside the cell, containing dissolved nutrients, ions, proteins, and waste products.

• Organelles: Structures within the cytoplasm that perform specific functions. These are divided into nonmembranous (e.g., ribosomes, cytoskeleton) and membranous (e.g., endoplasmic reticulum, Golgi apparatus, mitochondria) organelles.

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier that maintains cellular integrity and mediates communication with the environment.

• Physical Isolation: Acts as a barrier between the cell and its surroundings.

• Regulation of Exchange: Controls entry and exit of ions, nutrients, and waste products.

• Sensitivity: Detects changes in the extracellular environment.

• Structural Support: Provides anchoring for cells and tissues.

Membrane Components

• Phospholipid Bilayer: Composed of hydrophilic heads facing outward and hydrophobic tails facing inward, forming a barrier to water-soluble substances.

• Proteins: Integral (embedded within the membrane) and peripheral (attached to the surface) proteins serve as stabilizers, identifiers, enzymes, receptors, carriers, and channels.

• Carbohydrates: Proteoglycans, glycoproteins, and glycolipids form the glycocalyx, which provides lubrication, protection, anchoring, and cell recognition.

Organelles within the Cytoplasm

Nonmembranous Organelles

• Cytoskeleton: Provides structural support and shape. Includes microfilaments (actin), intermediate filaments, and microtubules (tubulin).

• Centrioles: Involved in cell division, forming the spindle apparatus.

• Ribosomes: Sites of protein synthesis; can be free in the cytosol or fixed to the endoplasmic reticulum.

• Proteasomes: Degrade damaged or unneeded proteins.

• Microvilli, Cilia, Flagella: Surface extensions that increase absorption (microvilli), move fluids (cilia), or enable cell movement (flagella).

Membranous Organelles

• Endoplasmic Reticulum (ER): Network of membranes involved in synthesis, storage, transport, and detoxification. Rough ER has ribosomes and synthesizes proteins; Smooth ER synthesizes lipids and carbohydrates.

• Golgi Apparatus: Modifies, packages, and distributes proteins and lipids.

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

• Peroxisomes: Break down fatty acids and neutralize toxic compounds.

• Mitochondria: Produce ATP through aerobic respiration.

Cell Nucleus

Structure and Function

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

• Nuclear Envelope: Double membrane surrounding the nucleus, with nuclear pores for communication.

• Nucleoplasm: Contains the nuclear matrix and nucleoli (sites of rRNA synthesis).

• Chromatin and Chromosomes: DNA is organized around histone proteins as chromatin (loose) or chromosomes (condensed during cell division).

• Genetic Code: DNA sequence of bases (A, T, C, G) in triplets encodes amino acids for protein synthesis.

Protein Synthesis

Role of DNA and RNA

Protein synthesis is the process by which cells build functional polypeptides, determining cell structure and function.

• Gene Activation: DNA is uncoiled and histones are temporarily removed.

• Transcription: RNA polymerase synthesizes messenger RNA (mRNA) from the DNA template strand.

• mRNA Processing: Introns (noncoding regions) are removed, and exons (coding regions) are spliced together.

• Translation: mRNA binds to ribosomes; transfer RNA (tRNA) brings amino acids, matching mRNA codons with tRNA anticodons. Peptide bonds form between amino acids, creating a polypeptide.

Genetic Code Table

DNA Triplet	mRNA Codon	tRNA Anticodon	Amino Acid
TTT	UUU	AAA	Phenylalanine
TTA	UUA	AAU	Leucine
TGT	UGU	ACA	Cysteine
GTT	GUU	CAA	Valine
ATG	AUG	UAC	Methionine
AGC	AGC	UCG	Serine
CCG	GGC	CCG	Proline
GCC	GCC	CGG	Alanine

Diffusion and Osmosis

Membrane Transport Mechanisms

Cells exchange materials with their environment through various transport processes.

• Diffusion: Movement of substances from high to low concentration. Influenced by distance, molecule size, temperature, concentration gradient, and electrical forces.

• Osmosis: Diffusion of water across a selectively permeable membrane toward higher solute concentration.

• Osmotic Pressure: The force required to prevent water movement by osmosis.

• Tonicity: Describes the effect of a solution on cell volume:

  • Isotonic: No net water movement; cell remains unchanged.

  • Hypotonic: Water enters cell; cell may swell and burst (hemolysis).

  • Hypertonic: Water leaves cell; cell shrinks (crenation).

Carrier-Mediated and Vesicular Transport

Carrier-Mediated Transport

• Specificity: Each carrier protein transports specific substrates.

• Saturation Limits: Transport rate depends on protein and substrate availability.

• Regulation: Activity can be affected by cofactors such as hormones.

• Symport: Two substances move in the same direction.

• Antiport: Substances move in opposite directions.

• Facilitated Diffusion: Passive transport of large molecules (e.g., glucose) via carrier proteins.

• Active Transport: Requires energy (ATP) to move substances against concentration gradients. Includes ion pumps and exchange pumps.

• Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump: $\text{3 Na}^+ \text{out},\ 2 K^+ \text{in}$ per ATP).

• Secondary Active Transport: Uses gradients established by primary active transport to move other substances.

Vesicular (Bulk) Transport

• Endocytosis: Import of materials via vesicles. Includes receptor-mediated endocytosis, pinocytosis (cell drinking), and phagocytosis (cell eating).

• Exocytosis: Export of materials by fusion of vesicles with the plasma membrane.

Membrane Potential

Origin and Significance

Membrane potential is the electrical potential difference across the plasma membrane due to separation of charges.

• Resting Membrane Potential: Typically ranges from 10 mV to -100 mV, depending on cell type.

Cell Life Cycle

Phases of the Cell Cycle

• Interphase: Nondividing period with G1, S, and G2 phases; cell grows and DNA replicates.

• Mitosis: Division of the nucleus into two identical sets of chromosomes. Stages include prophase, metaphase, anaphase, and telophase.

• Cytokinesis: Division of the cytoplasm, producing two daughter cells.

• Mitotic Rate: Rate of cell division; varies by cell type (e.g., skin cells divide rapidly, neurons rarely divide).

Regulation of the Cell Life Cycle

Control of Cell Division

• Balance: Cell division is regulated to balance cell loss.

• Stimulation: Internal and external factors (growth factors) can promote division.

• Inhibition: Repressor genes and telomere shortening inhibit division; faulty repressors can lead to cancer.

Cell Division and Cancer

Neoplasms and Cancer Progression

• Tumor (Neoplasm): Mass of abnormal cell growth. Benign tumors are contained; malignant tumors invade surrounding tissues.

• Cancer: Results from mutations in genes regulating cell growth (oncogenes). Mutagens and carcinogens can cause these mutations.

• Metastasis: Spread of cancer cells to other areas, beginning with invasion of nearby tissues.