From Cells to Organ Systems: Structure and Function in Human Biology

Introduction: Unicellular and Multicellular Life

Unicellular Organisms

Unicellular organisms are the earliest forms of life, consisting of a single cell. They rely entirely on their immediate external environment, which can vary greatly.

• Definition: Organisms made up of only one cell.

• Dependence: Their survival depends on environmental conditions.

• Example: Bacteria and protozoa.

Multicellular Organisms

Multicellular organisms consist of many cells, allowing for larger size and specialization. Cells work together, each contributing to the well-being of the organism.

• Definition: Organisms composed of multiple cells.

• Specialization: Cells differentiate to perform specific functions.

• Example: Humans, plants, animals.

Tissues: Groups of Cells with a Common Function

Definition and Types

Tissues are groups of specialized cells similar in structure and function. Several tissue types combine to form organs.

• Epithelial tissue: Covers surfaces and lines cavities.

• Connective tissue: Supports and connects body parts.

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Transmits impulses.

Epithelial Tissues Cover Body Surfaces and Cavities

Structure and Function

Epithelial tissues are organized as sheets of cells, forming one or more layers. They line body cavities, cover surfaces, and protect underlying tissues.

• Glandular epithelia: Specialized to form glands.

• Exocrine glands: Secrete products into ducts or hollow organs.

• Endocrine glands: Secrete hormones into the blood.

Classification by Shape

• Squamous: Flattened cells; found in skin, blood vessels, lungs.

• Cuboidal: Cube-shaped; found in kidney tubules, glands.

• Columnar: Tall, rectangular; found in digestive tract, reproductive organs.

Classification by Layers

• Simple: Single layer; adapted for diffusion.

• Stratified: Multiple layers; provides protection.

The Basement Membrane Provides Structural Support

The basement membrane is a noncellular layer beneath epithelial tissue, composed of proteins. It provides structural support and attaches the epithelial layer to underlying connective tissue.

Cell Junctions—Hold Adjacent Cells Together

Types of Cell Junctions

• Tight junctions: Seal plasma membranes; found in digestive tract, bladder.

• Adhesion junctions (desmosomes): Allow movement; found in skin.

• Gap junctions: Protein channels for material movement; found in liver, heart.

Connective Tissue Supports and Connects Body Parts

General Functions

Connective tissue supports organs, connects body parts, stores fat, and produces blood cells. Cells are embedded in a nonliving matrix that provides strength.

Types of Connective Tissue

• Fibrous connective tissue: Provides strength and elasticity.

• Specialized connective tissue: Includes cartilage, bone, blood, adipose tissue.

Fibrous Connective Tissue

Structure and Components

• Ground substance: Gel-like material with water, proteins, polysaccharides.

• Cells: Fibroblasts, macrophages, lymphocytes, neutrophils, fat cells, mast cells.

• Fibers: Collagen (strong), elastic (stretchy), reticular (framework).

Types of Fibrous Connective Tissue

• Loose (areolar): Surrounds organs, muscles, blood vessels.

• Dense: Forms tendons, ligaments, skin layers.

• Elastic: Surrounds organs that change shape (stomach, bladder).

• Reticular: Internal framework for soft organs (liver, lymph nodes).

Specialized Connective Tissues Serve Special Functions

Cartilage

• Function: Transitional tissue for bone development, maintains shape, cushions joints.

• Structure: Dense collagen fibers, ground substance produced by chondroblasts, high water content.

• Healing: Slow due to lack of blood vessels.

Bone

• Matrix: Calcium phosphate, few living cells, many blood vessels.

Blood

• Plasma: Fluid matrix.

• Cells: Red blood cells (oxygen transport), white blood cells (immune defense), platelets (clotting).

Adipose Tissue

• Function: Fat storage, insulation, protection.

• Location: Under skin, around organs.

• Weight loss: Reduces cell size, not number.

Table: Types of Connective Tissue

Type	Main Function	Location
Loose (Areolar)	Support, elasticity	Surrounds organs, blood vessels
Dense	Strength	Tendons, ligaments, skin
Elastic	Stretch, flexibility	Stomach, bladder
Reticular	Framework	Liver, lymph nodes
Cartilage	Shape, cushioning	Nose, ears, joints
Bone	Support, protection	Skeleton
Blood	Transport	Blood vessels
Adipose	Insulation, energy	Under skin, around organs

Muscle Tissue Contracts to Produce Movement

Types of Muscle Tissue

• Skeletal muscle: Moves body parts, voluntary, multiple nuclei.

• Cardiac muscle: Heart only, involuntary, gap junctions for coordination.

• Smooth muscle: Surrounds hollow organs, involuntary, tapered cells.

Nervous Tissues Transmit Impulses

Structure and Function

Nervous tissue forms a rapid communication network. Neurons generate and transmit electrical impulses, while glial cells support and nourish neurons.

• Neuron components: Cell body, dendrites (receive signals), axon (transmits impulses).

• Glial cells: Protect and provide nutrients.

Organs and Organ Systems Perform Complex Functions

Organs

• Definition: Structures composed of two or more tissue types.

• Example: Heart (muscle, nervous, connective, epithelial tissues).

Organ Systems

• Definition: Groups of organs serving broad functions.

• Example: Digestive system (mouth, stomach, intestines, liver, pancreas).

• Multiple functions: Lymphatic system (defense, circulation, transport).

Body Cavities

Main Cavities

• Anterior cavity: Thoracic (lungs, heart), abdominal, pelvic.

• Posterior cavity: Cranial, spinal.

• Serous membranes: Line and lubricate cavities.

Tissue Membranes Line Body Cavities

Types of Tissue Membranes

• Serous membranes: Lubricate internal cavities.

• Mucous membranes: Line airways, digestive, reproductive tracts.

• Synovial membranes: Line movable joints.

• Cutaneous membrane: Skin.

Describing Body Position or Direction

Body Planes

• Midsagittal: Equal left and right sides.

• Parasagittal: Unequal left and right sides.

• Frontal (coronal): Front and back.

• Transverse: Top and bottom.

Directional Terms

• Anterior: Front

• Posterior: Back

• Proximal: Nearer to trunk

• Distal: Farther from trunk

• Superior: Above

• Inferior: Below

The Skin Is an Organ of the Integumentary System

Functions

• Protection: Against dehydration, injury, microorganisms.

• Regulation: Body temperature.

• Vitamin D: Synthesis.

• Sensation: Touch, temperature, pain.

Skin Structure: Epidermis and Dermis

Epidermis

• Outer layer: Stratified squamous epithelial cells.

• Keratinocytes: Produce keratin for waterproofing.

• Basal cells: Dividing keratinocytes at base.

• Melanocytes: Produce melanin for UV protection.

Dermis

• Dense connective tissue: Collagen, elastic, reticular fibers.

• Cells: Fibroblasts, mast cells, white blood cells, fat cells.

Accessory Structures of Dermis

• Hair: Shaft and root, follicle supports root.

• Smooth muscle: Raises hair.

• Sebaceous glands: Secrete sebum for moisture.

• Sweat glands: Regulate temperature, antimicrobial.

• Blood vessels: Nutrient supply, waste removal, temperature regulation.

• Sensory nerve endings: Detect stimuli.

• Vitamin D synthesis: Cholesterol-like molecule converted in skin.

Homeostasis in Multicellular Organisms

Definition and Importance

Homeostasis is the relative constancy of internal conditions. The interstitial fluid surrounding cells must remain stable for life.

• Interstitial fluid: Source of nutrients, repository for wastes.

• Metabolic activities: Maintain homeostasis.

Homeostasis Is Maintained by Negative Feedback

Negative Feedback Control System

• Controlled variable: Property that must be regulated.

• Sensor: Monitors variable, sends info to control center.

• Control center: Compares value to set point, signals effector.

• Effector: Corrects imbalance.

Example: Core Body Temperature

• Sensors: Temperature sensors in skin and organs.

• Control center: Hypothalamus in brain.

• Effectors: Blood vessels, sweat glands, skeletal muscles.

Positive Feedback Amplifies Events

Positive feedback is less common and amplifies changes rather than maintaining homeostasis. Example: childbirth.

• Not a mechanism for homeostasis.