Cell Structure, Function, and Bone Anatomy: Comprehensive Study Notes

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Cell Theory and Structure

Cell Theory

The cell theory is a fundamental concept in biology, describing the properties and significance of cells in living organisms.

All living things are made of cells: Cells are the basic structural and functional units of all organisms.
The cell is the basic unit of life: All vital functions occur within cells.
Cells come from preexisting cells: New cells arise only from existing cells through cell division.

Plasma (Cell) Membrane

The plasma membrane is a dynamic structure that separates the cell's interior from its external environment.

Fluid Mosaic Model: Describes the membrane as a flexible layer made of phospholipids, proteins, carbohydrates, and cholesterol.
Phospholipid bilayer: Hydrophilic heads face water; hydrophobic tails avoid water.
Functions: Selectively permeable, protection, communication.
Cholesterol: Stiffens and stabilizes membrane, decreases water permeability.
Carbohydrates: Serve as cell identity markers and aid in cell recognition.
Proteins: Channel, carrier, and receptor proteins determine membrane function.

Membrane Transport Mechanisms

Simple Diffusion

Simple diffusion is the passive movement of molecules from areas of high concentration to low concentration.

No ATP required.
Examples: Oxygen (O2), Carbon dioxide (CO2), Steroids.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

Water moves toward higher solute concentration and lower water concentration.

Facilitated Diffusion

Facilitated diffusion uses channel or carrier proteins to move substances from high to low concentration without ATP.

Active Transport

Active transport moves substances against their concentration gradient, requiring energy (ATP).

Sodium-Potassium Pump: For each ATP, 3 Na+ move out, 2 K+ move in.

Filtration

Filtration is the movement of substances across membranes due to pressure differences.

Examples: Kidneys, capillaries.

Cellular Organelles and Functions

Nucleus

The nucleus is the control center of the cell, housing genetic material.

Stores genetic information (DNA).
Controls protein synthesis.
Humans have 46 chromosomes.

Ribosomes

Ribosomes are responsible for protein synthesis.

Can be free or attached to rough ER.

Endoplasmic Reticulum (ER)

Rough ER: Has ribosomes; synthesizes and transports proteins.
Smooth ER: No ribosomes; involved in lipid metabolism, steroid synthesis, and detoxification.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport.

Forms transport vesicles.

Mitochondria

Mitochondria are the site of cellular respiration and ATP production.

Known as the powerhouse of the cell.
Has a double membrane.

Cytoskeleton

Microfilaments (actin): Cell shape and movement.
Microtubules (tubulin): Intracellular transport, cilia/flagella structure, mitosis spindle fibers.

Centrioles

Centrioles organize spindle fibers during cell division.

Cellular Extensions

Cilia

Numerous, short extensions.
Move substances across cell surface.
Example: Respiratory tract.

Flagella

Long, whiplike extension.
Moves the entire cell.
Example: Sperm cell.

Microvilli

Increase surface area for absorption.
Examples: Small intestine, kidney tubules.

Cytoplasm and Extracellular Fluid

Cytoplasm: Watery environment inside the cell.
Extracellular Fluid (ECF): Watery environment outside the cell.

Protein Synthesis

Transcription

Transcription is the process of copying DNA into messenger RNA (mRNA) in the nucleus.

DNA → mRNA

Translation

Translation is the process of building proteins from mRNA at the ribosomes.

mRNA → Protein

Cellular Respiration

Cellular respiration is the process by which cells produce ATP from glucose and oxygen.

Occurs in mitochondria.
Equation:

Cell Cycle and Division

Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide.

Interphase

G1: Cell growth.
S Phase: DNA replication (all DNA copied).
G2: Final preparations for division.

Mitosis

Mitosis produces two genetically identical daughter cells, each with 46 chromosomes.

Prophase: Chromosomes condense, nuclear envelope disappears, spindle forms.
Metaphase: Chromosomes line up in the middle.
Anaphase: Sister chromatids separate.
Telophase: Nuclear envelopes reform.
Cytokinesis: Cytoplasm divides.

Mnemonic: P-M-A-T (Prepare → Middle → Apart → Two)

Meiosis

Meiosis produces four nonidentical gametes (sperm or eggs), each with 23 chromosomes.

Purpose: Sexual reproduction.

Phagocytosis

Phagocytosis is the process by which cells engulf large particles ('cell eating').

Example: Macrophages.

Bone Structure and Function

Functions of Bones

Bones perform several essential functions in the body.

Support
Protection
Anchorage for muscles
Mineral storage (calcium and phosphorus)
Blood cell formation (hematopoiesis)
Fat (triglyceride) storage

Skeleton Divisions

The human skeleton is divided into two main parts.

Axial Skeleton: Skull, vertebral column, ribs, sternum.
Appendicular Skeleton: Arms, legs, pectoral girdle, pelvic girdle.
Total bones: 206

Classification of Bones by Shape

Long Bones: Femur, humerus.
Short Bones: Carpals, tarsals.
Flat Bones: Sternum, ribs, skull.
Irregular Bones: Vertebrae, hip bones.

Compact vs Spongy Bone

Compact Bone: Dense, strong, organized into osteons.
Spongy Bone: Contains trabeculae, lighter, contains red marrow.

Long Bone Anatomy

Diaphysis: Shaft of long bone.
Epiphysis: Ends of long bone.
Medullary Cavity: Hollow center, contains yellow marrow in adults.
Articular Cartilage: Hyaline cartilage covering joint surfaces, reduces friction.
Periosteum: Covers outer bone surface, contains blood vessels and nerves, site of tendon and ligament attachment.
Endosteum: Inner lining of bone.

Bone Marrow

Red Marrow: Produces blood cells.
Yellow Marrow: Stores fat.

Bone Cells

Osteoprogenitor Cells: Stem cells that produce osteoblasts.
Osteoblasts: Build bone, secrete osteoid. (BLAST = BUILD)
Osteocytes: Mature bone cells, maintain bone matrix, sense stress.
Osteoclasts: Break down bone, bone resorption. (CLAST = CRASH)

Osteon (Haversian System)

The osteon is the structural unit of compact bone.

Central Canal: Contains blood vessels and nerves.
Lamellae: Rings of bone matrix.
Lacunae: Small spaces containing osteocytes.
Canaliculi: Tiny channels connecting osteocytes.

Chemical Composition of Bone

Organic Portion (Osteoid): Collagen fibers, glycoproteins; provides flexibility and tensile strength.
Inorganic Portion: Calcium phosphate, hydroxyapatite crystals; provides hardness and compression resistance.

Ossification (Bone Formation)

Ossification is the process of bone formation.

Intramembranous Ossification: Forms skull bones and clavicle from fibrous membrane.
Endochondral Ossification: Forms most bones by replacing hyaline cartilage.

Endochondral Ossification Steps:

Bone collar forms
Cartilage calcifies
Periosteal bud enters
Primary ossification center develops
Secondary ossification centers form
Epiphyseal plate remains

Growth Plate (Epiphyseal Plate)

Function: Lengthwise growth of bone.
Closure: Bone stops growing when plate closes.

Bone Growth

Length Growth: Occurs at epiphyseal plates.
Width Growth: Occurs by appositional growth.

Hormones and Bone Growth

Growth Hormone (GH): Stimulates childhood growth.
Testosterone and Estrogen: Cause growth spurt at puberty, eventually close growth plates.

Bone Remodeling

Bone remodeling is the process of removing old bone and depositing new bone, occurring throughout life.

Osteoclasts: Resorb bone.
Osteoblasts: Deposit new bone.

Calcium Homeostasis

Calcium levels in the blood are regulated by hormones.

Condition	Hormone	Effect
Low Blood Calcium	PTH (Parathyroid Hormone)	Stimulates osteoclasts, releases calcium into blood, increases blood calcium
High Blood Calcium	Calcitonin	Inhibits osteoclast activity, promotes calcium storage in bone, lowers blood calcium

Wolff’s Law

Bone remodels according to the stress placed on it.

Weightlifting: Stronger bones.
Bed rest: Weaker bones.

Osteoporosis

Osteoporosis is a condition where bone resorption exceeds bone formation.

Decreased bone mass.
Fragile bones.
Spongy bone affected first.

Summary Table: High-Yield Exam Questions

Question	Answer
Which organelle makes ATP?	Mitochondria
Which organelle makes proteins?	Ribosomes
Which organelle packages proteins?	Golgi apparatus
Which organelle synthesizes lipids?	Smooth ER
What happens during S phase?	DNA replication
How many chromosomes in body cells?	46
How many chromosomes in gametes?	23
What does mitosis produce?	2 identical cells
What does meiosis produce?	4 nonidentical cells
What molecules use simple diffusion?	O2, CO2, steroids
Sodium-potassium pump?	3 Na+ out, 2 K+ in
Who builds bone?	Osteoblasts
Who breaks down bone?	Osteoclasts
Who maintains bone?	Osteocytes

Must-Know Topics for Exam Preparation

Cell organelles and their functions
Phases of mitosis (PMAT)
Mitosis vs meiosis
Transcription vs translation
Sodium-potassium pump (3 Na+ out, 2 K+ in)
Diffusion, osmosis, active transport
Cellular respiration equation
6 functions of bone
Axial vs appendicular skeleton
Classification of bones (long, short, flat, irregular)
Compact vs spongy bone
Long bone anatomy (diaphysis, epiphysis, periosteum, endosteum)
Bone cells (osteoblast, osteocyte, osteoclast)
Osteon structures (central canal, lacunae, canaliculi)
Organic vs inorganic bone matrix
Intramembranous vs endochondral ossification
Epiphyseal plate and bone growth
PTH vs calcitonin
Wolff’s Law
Osteoporosis