Fundamentals of Biology: Cells, Tissues, and Organ Systems

Posted on Nov 30, 2025 in Biology

Seven Characteristics of Life

  1. All living things are highly organized entities composed of one or more cells.
  2. Use energy for their metabolic activities.
  3. Respond to environmental stimuli.
  4. Regulate the internal environment to maintain relatively constant internal conditions (homeostasis).
  5. Grow and develop.
  6. Pass genetic material through reproduction.
  7. Adapt to their environment through evolution.

Biological Levels of Organization

  • Cell (e.g., human blood cell)
  • Tissue (e.g., human skin tissue)
  • Organ and Organ System (e.g., stomach and intestine make up the human digestive system)
  • Organism-Population-Community:
    • Organism: Each individual (e.g., a pine cone).
    • Population: All individuals of one species (e.g., all pines in a forest).
    • Community: All living things in the forest.
  • Ecosystem
  • Biosphere (All ecosystems on Earth)

Growth and Reproduction

  • Growth: Increase in body size, cell number, and/or cell size.
  • Development: Change in form over time.
  • Asexual Reproduction: Involves one parent; offspring are genetically identical.
  • Sexual Reproduction: Involves two parents; offspring inherit a mix of DNA from both parents.

Biodiversity and Species Distribution

Key Biodiversity Concepts

  • Species Richness: Describes the number of different species present in an area.
  • Species Evenness: Describes the relative abundance of different species in an area (similar abundance equals more evenness).

Global Species Data

  • Known Number of Species: 1.8 million.
  • Estimated Total Number of Species: 10–100 million (with about 10,000 new species discovered every year).
  • Group with Most Species: Insects.

Megadiversity Spots

Spots that house the largest indices of biodiversity, including a large number of endemic species.

Tropical Rainforests

The hot and wet climate provides ideal conditions for many plant and animal species to thrive. Nutrients are rapidly recycled, speeding up plant growth. These areas are defined by geographic location and amount of precipitation, housing approximately 50% of all species.

Cell Structure and Types

Prokaryotes vs. Eukaryotes

  • Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells do not.
  • Eukaryotes store genetic information in the nucleus. In prokaryotes, DNA is bundled in the nucleoid region, but it is not stored within a membrane-bound nucleus.
  • Eukaryotes have many membrane-bound organelles; prokaryotes have none.
  • Eukaryote DNA consists of multiple molecules of double-stranded linear DNA. Prokaryote DNA is typically double-stranded and circular.

Components Common to All Cells

  • Plasma Membrane: Membrane found in all cells that separates the interior of the cell from the outside environment.
  • Cytoplasm: Gelatinous liquid that fills the inside of a cell.
  • DNA: Complex molecule that contains all the information necessary to build and maintain an organism.
  • Ribosomes: Particles consisting of RNA and associated proteins that function to synthesize proteins.

Prokaryotic Cell Components

Always Present:

  • Cell Membrane
  • Ribosome
  • Chromosome (DNA)
  • Nucleoid Region: Irregularly shaped region of the cell that contains most or all genetic material.

Often Present:

  • Cell Wall: Protective layer that surrounds some cells, giving them shape and rigidity.

Sometimes Present:

  • Capsule: Sticky outermost layer.
  • Pili: Short, hair-like structures on the cell surface.
  • Flagellum: Hair-like structure that works as an organelle of locomotion.

Eukaryotic Organelles

  • Cytoskeleton: Made of filamentous proteins; provides mechanical support to the cell.
  • Peroxisome: Oxidative organelle.
  • Mitochondrion: Membrane-bound organelle that generates large quantities of energy (ATP).
  • Rough Endoplasmic Reticulum (RER): Series of connected flattened sacs, often involved in protein synthesis and modification.
  • Nucleus: Houses the cell’s DNA.
  • Smooth Endoplasmic Reticulum (SER): Meshwork of fine, disk-like tubular membranes, involved in lipid synthesis and detoxification.
  • Golgi Apparatus: Series of flattened, stacked pouches (cisternae), involved in modifying, sorting, and packaging proteins and lipids.
  • Lysosome: Subcellular organelle containing digestive enzymes.

Specialized Eukaryotic Structures

Plant Cells:

  • Large Central Vacuole (LCV): Surrounded by its own membrane; contains water and dissolved substances.
  • Tonoplast: Membrane that surrounds the LCV.
  • Chloroplast: Organelle that carries out photosynthesis.
  • Plasmodesmata: Microscopic channels connecting plant cells.

Animal Cells:

  • Centrioles: Paired, barrel-shaped organelles involved in cell division.

Other Cell Types:

  • Multinucleate Cell: Can be formed by cell fusion or by nuclear division not being followed by cytokinesis.
  • Unicellular Protist Locomotion: Achieved using flagella, cilia, and pseudopods.

The Four Major Types of Tissue

  1. Connective: Supports other tissues and binds them together.
  2. Epithelial: Provides a covering or lining.
  3. Muscle: Facilitates movement (e.g., moves the skeleton).
  4. Nerve: Transmits electrical impulses for communication.

Epithelial Tissue Subtypes

  • Simple Squamous Epithelium: Single layer of flat cells; allows small molecules to pass quickly through membranes.
  • Simple Columnar Epithelium: Single layer of columnar cells attached to the basement membrane; lines most organs of the digestive tract.
  • Transitional Epithelium: Stratified tissue made of multiple cell layers; stretches when filled with fluid and is unstretched when fluid pressure is low.
  • Simple Cuboidal Epithelium: Consists of a single layer of cuboidal cells; often involved in secretion and absorption (e.g., prevention of water loss).
  • Stratified Squamous Epithelium: Flattened cells arranged in layers upon a basal membrane.

Connective Tissue Subtypes (CT)

  • Loose CT: Most common; holds organs in place.
  • Adipose: Consists of lipid-rich cells; stores energy.
  • Fibrous CT: Parallel bundles of collagen fibers; provides shock absorption.
  • Cartilage: Resilient, smooth, elastic tissue (like rubber); protects the ends of long bones.
  • Bone
  • Blood

Muscular Tissue Subtypes

  • Skeletal: Striated, multinucleate, voluntary.
  • Cardiac: Striated, uninucleate, involuntary.
  • Smooth: Non-striated, uninucleate, involuntary.

Plant Tissues

  • Xylem: Composed of tracheids and vessel members (with thick secondary cell walls); transports water and dissolved minerals.
  • Phloem: Composed of sieve-tube members (with primary cell walls); transports dissolved food (sugars).

General Organ Terminology

  • Parenchyma: Tissue that carries out the primary function of an organ.
  • Stroma: Supportive framework of an organ.

Organ Structure Examples

Heart Layers

  • Endocardium: Covers the inner surface of the heart.
  • Myocardium: Muscular wall of the heart; consists of cardiac muscle.
  • Epicardium: Covers the outer surface of the heart.

Small Intestine Layers (4 layers)

  1. Mucosa
  2. Submucosa
  3. Muscularis
  4. Serosa

Leaf Structure

  • Upper and Lower Epidermis
  • Mesophyll
  • Veins (Vascular Bundles)

Major Organ Systems

  • Respiratory System: Lungs, diaphragm.
  • Digestive System: Oral cavity, intestines, stomach.
  • Excretory System: Kidneys, bladder, urethra.
  • Reproductive System: Ovaries, uterus, vagina, testes, penis.
  • Nervous System: Brain, spinal cord.
  • Endocrine System: Pituitary, hypothalamus.
  • Muscular System: Skeletal, cardiac, and smooth muscle.
  • Skeletal System: Bones, tendons, ligaments.
  • Integumentary System: Skin, hair, nails.
  • Circulatory System: Heart, blood vessels.
  • Lymphatic System: Nodes, ducts.