The Human Body Cell

Basic Structure

• Guyton and Hall Textbook of Medical Physiology
• Histology: A Text and Atlas, Histology

Detailed Diagram of Organelles

Major Organelles and Their Functions

1. Cell Membrane (Plasma Membrane)

• A lipid bilayer 7.5-10 nanometers thick
• Composition: ~55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, 3% carbohydrates
• Hydrophilic phosphate heads face outward (toward water), hydrophobic fatty acid tails face inward
• Proteins embedded in the membrane act as channels, receptors, and enzymes
• Fat-soluble substances (O₂, CO₂) cross freely; ions and glucose require protein channels

2. Nucleus

• The control center - contains the genome (DNA) and enzymes for DNA replication and RNA transcription
• Surrounded by a double nuclear membrane with nuclear pores
• Contains the nucleolus - a dense region of RNA that is the site of ribosome assembly
• The nucleoplasm (fluid inside) contains chromatin (DNA + histone proteins)

3. Mitochondria

• The powerhouses of the cell - produce ATP via oxidative phosphorylation
• Double-membraned; the inner membrane is folded into cristae to maximize surface area
• Contain their own DNA (inherited maternally) - evidence of their evolutionary origin as bacteria
• Loss of mitochondrial function eliminates >95% of the cell's energy supply

4. Endoplasmic Reticulum (ER)

• Rough ER (rER) - studded with ribosomes; site of protein synthesis and modification
• Smooth ER (SER) - lacks ribosomes; involved in lipid synthesis, detoxification (liver), and calcium storage (muscle)

5. Ribosomes

• Nonmembranous organelles composed of rRNA and protein
• Site of protein synthesis; found free in cytoplasm or attached to rough ER
• Assembled in the nucleolus

6. Golgi Apparatus

• The post office of the cell - receives proteins from rER, modifies, packages, and sorts them
• Produces secretory vesicles, lysosomes, and membrane components

7. Lysosomes

• Membrane-bound sacs containing ~40 types of hydrolytic enzymes
• Digest worn-out organelles, bacteria, and cellular debris (autophagy and phagocytosis)
• Their enzyme-resistant membrane is critical - if ruptured, the enzymes would digest the entire cell

8. Peroxisomes

• Contain oxidative enzymes (catalase, oxidases)
• Break down fatty acids and detoxify harmful substances like hydrogen peroxide (H₂O₂ → H₂O + O₂)

9. Cytoskeleton

• Microtubules - hollow tubes built from tubulin; form cilia, flagella, the mitotic spindle, and axonal transport tracks
• Intermediate filaments - structural support; vary by cell type (e.g., keratin in skin cells, vimentin in connective tissue)
• Actin filaments (microfilaments) - involved in cell shape, movement, and muscle contraction

10. Centrioles

• Paired structures that organize the mitotic spindle during cell division
• Also form the basal bodies of cilia and flagella

Cell Membrane - Molecular Detail

• Phospholipids - most abundant; amphipathic (hydrophilic head + hydrophobic tail)
• Sphingolipids - protective functions, signal transmission, adhesion sites
• Cholesterol - regulates membrane fluidity and permeability

Chemical Composition of a Typical Cell

Types of Human Cells (~200 types)

Two Categories of Organelles

• Membranous organelles (enclosed by a lipid bilayer): plasma membrane, rough ER, smooth ER, Golgi apparatus, lysosomes, peroxisomes, mitochondria, nucleus
• Nonmembranous organelles (self-assembled protein polymers): ribosomes, centrioles, cytoskeletal filaments