Levels of Organization in Biology

Levels of Organization

• Chemical Level: Atoms (basic building blocks like hydrogen, oxygen, carbon) combine to form molecules (e.g., water, proteins, DNA). Essential for all biological structures and functions.
• Cellular Level: Cells are the smallest functional units of life. Cells contain organelles (e.g., mitochondria, nucleus) working together to maintain life. A muscle cell's contraction, for example, produces movement.
• Tissue Level: Tissues are groups of similar cells with a common function. Four main types: epithelial (cover body surfaces, e.g., skin), connective (support and bind, e.g., bone, blood), muscle (facilitate movement, e.g., skeletal, cardiac, smooth), and nervous (transmit electrical signals, e.g., neurons).
• Organ Level: Organs are composed of different tissue types working together for a specific function. The heart, for example, includes muscle, connective, and nervous tissues.
• Organ System Level: Organ systems are groups of organs working together to perform complex functions. The digestive system includes the stomach, intestines, and liver.
• Organismal Level: The complete human body with organ systems functioning together.

Homeostasis

• Homeostasis: Maintaining a stable internal environment despite external fluctuations.
• Negative Feedback Mechanisms: The body counters changes to return to homeostasis. Example: Thermoregulation. If too hot, sweating and vasodilation occur; if too cold, shivering and vasoconstriction occur.
• Positive Feedback Mechanisms: Intensify a change rather than counteracting it. Example: Childbirth. Oxytocin release intensifies contractions, which further releases oxytocin, continuing the process until delivery.

Membranes and Transport Mechanisms

• Plasma Membrane: Phospholipid bilayer regulating what enters and leaves the cell.
• Selective Permeability: Some substances pass freely (oxygen, carbon dioxide), others are restricted (large molecules, ions).
• Passive Transport: No energy (ATP) required.
  • Diffusion: Movement from high to low concentration (e.g., oxygen entering bloodstream).
  • Osmosis: Water movement across a semipermeable membrane.
• Active Transport: Requires energy (ATP) to move molecules against their concentration gradient.
  • Na+/K+ Pump: Actively transports sodium ions out and potassium ions in, maintaining resting membrane potential.

Protein Synthesis

• Transcription: Copying a gene's DNA sequence into mRNA in the nucleus.
• Translation: mRNA is translated into an amino acid sequence at a ribosome in the cytoplasm. tRNA brings amino acids, and the ribosome builds the protein.

Tonicity

• Hypertonic: Higher solute concentration outside the cell. Water moves out, cell shrinks.
• Hypotonic: Lower solute concentration outside the cell. Water moves in, cell swells.
• Isotonic: Equal solute concentrations inside and outside the cell. No net water movement.

Endocrine vs. Exocrine Glands

• Endocrine Glands: Secrete hormones directly into the bloodstream (e.g., thyroid, pancreas).
• Exocrine Glands: Secrete substances through ducts to an external or internal surface (e.g., sweat glands, salivary glands).

Four Main Tissue Types

• Epithelial Tissue: Functions in protection, secretion, absorption, filtration. Example: stratified squamous epithelium protects the skin. Simple columnar epithelium in intestines aids in absorption.
• Connective Tissue: Functions in support, protection, storage, transportation. Types: Bone, blood, adipose tissue, cartilage. Bone supports, blood transports nutrients & waste, adipose stores energy, and cushions.
• Muscle Tissue:
  • Skeletal: Voluntary, striated, for movement.
  • Cardiac: Involuntary, striated, in the heart.
  • Smooth: Involuntary, non-striated, in internal organs.
• Nervous Tissue: Transmits electrical signals. Composed of neurons (transmit messages) and neuroglia (supporting).

Integumentary System (Skin)

• Skin: Largest organ, composed of epidermis (outer, keratinocytes), dermis (blood vessels, nerves), and hypodermis (fat, insulation).
• Functions: Protection (UV light, pathogens), temperature regulation (sweating), sensation (pain, pressure, temperature), vitamin D synthesis (UV exposure).

Skeletal System

• Axial Skeleton: Skull, vertebral column, rib cage. Supports and protects.
• Appendicular Skeleton: Limbs and girdles (shoulder, pelvic). Attaches limbs.
• Bone Classifications: Based on shape: long (levers, femur), short (stability, carpals), flat (protection, skull), irregular (complex, vertebrae).
• Bone Cells: Osteoblasts build bone, osteoclasts break it down, osteocytes maintain.
• Bone Structure: Compact bone (outer layer, dense), spongy bone (inner layer, lighter, support).
• Bone Homeostasis: PTH (parathyroid hormone) increases blood calcium by stimulating osteoclasts, calcitonin decreases blood calcium by inhibiting osteoclasts.

Muscular System

• Muscle Tissue Types: Skeletal (voluntary, striated), cardiac (involuntary, striated), smooth (involuntary, non-striated).
• Muscle Contraction: Sliding filament theory (actin slides over myosin). ATP required.
• Metabolism: Anaerobic (without oxygen, less ATP, lactic acid), aerobic (with oxygen, more ATP).

Nervous System

• Central Nervous System (CNS): Brain and spinal cord; processes information and sends commands.
• Peripheral Nervous System (PNS): All nerves outside the CNS; carries sensory and motor information.
• Neurons:
  • Structure: Dendrites receive signals, axon transmits to axon terminals (release neurotransmitters).
  • Action Potential: Rapid change in neuronal membrane charge.
  • Resting Potential: Negative inside the neuron.
  • Depolarization: Inside becomes more positive.
  • Repolarization: Returns to resting potential.
• Neurotransmitters: Acetylcholine (muscle contraction), norepinephrine (fight-or-flight).