Essential Requirements for Life: Homeostasis, Metabolism, and Cellular Processes

Essential Requirements for Life

  • Maintenance of Boundaries (Homeostasis)
  • Movement
  • Responsiveness
  • Digestion
  • Excretion
  • Metabolism
  • Growth
  • Reproduction

Necessary for Survival

  • Nutrients
  • Oxygen
  • Water (70%)
  • Normal Body Temperature (98.6°F or 37°C)


– The body’s ability to maintain a relatively stable internal environment

Components of Homeostasis

  • Receptor – A sensor that monitors the internal and external environments for stimuli and sends input to the brain
  • Control Center – The brain: determines a response to the information and triggers a response
  • Effector – Provides the response to the stimuli (sweating, release of needed hormones)

Types of Feedback Systems

  • Negative Feedback System: The process restores homeostatic balance
  • Positive Feedback Loop: The effector response accelerates or increases the reaction (e.g., labor or blood clotting)

Anatomical Position

– Body Upright – Palms face-up – Toes straight

Directional Terms

  • Superior: Toward the head end or upper part of the body
  • Inferior: Away from the head end or toward the lower part of the body
  • Anterior (ventral): Toward or on the front of the body
  • Posterior (dorsal): Toward or on the back of the body
  • Medial: Toward or at the midline (vertical center) of the body
  • Lateral: Away from the midline of the body
  • Proximal: Closer to the origin point of the body or attachment point of a limb
  • Distal: Farther from the origin point of a body part or attachment point of a limb
  • Superficial: Toward or at the body surface
  • Deep: Away from the body surface, internal

Body Divisions

  • Axial – Head, neck, and trunk
  • Appendicular – Limbs (legs and arms)

Body Cavities

  • Dorsal Body Cavity: Protects the brain and spinal cord
  • Ventral Body Cavity: Protects visceral organs and structures. Divided into Thoracic and Abdominopelvic

Chemical Foundations


– Different forms of an element, distinguished by differing numbers of protons

Chemical Bonds

  • Covalent Bonds: Share electrons to create a mutually stable relationship
    • Non-polar covalent bonds: Mostly neutral, very stable, share electrons evenly
    • Polar covalent bonds: A molecule with charged regions (polarized)
  • Hydrogen Bonds: Form when one weakly positive atom in a molecule is attracted to a negative atom in another molecule

Water Properties

  • Cohesion – Water’s ability to “stick” to itself
  • Adhesion – Water’s ability to “stick” to other substances
  • Hydrophilic – “Water loving” – Molecules (polar and ionic) that easily dissolve in water
  • Hydrophobic – “Water fearing” – Nonpolar molecules that do not


– A solvent that dissolves a solute – Homogeneous – Molecules are evenly distributed – Solute – A compound that is dissolved in a solvent

Organic Molecules

– Consist of carbon atoms covalently bonded to oxygen, hydrogen, and some other elements (commonly nitrogen and phosphorus)

Essential Organic Compounds

  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic Acids


– Composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio

Types of Carbohydrates
  • Monosaccharides
    • Hexose sugars (6 carbons): Glucose, fructose, & galactose
    • Pentose sugars (5 carbons): Ribose, deoxyribose
  • Disaccharides
    • A pair of monosaccharides
    • In the body – Sucrose (table sugar) and lactose (milk sugar)
  • Polysaccharides
    • Starches – Polymer of glucose, plant-based foods
    • Glycogen – Polymer of glucose, stored in animal tissue (humans)
    • Cellulose – Cell wall component of green plants, “fiber”, is not digestible but is important for digestion


– Compounds made mostly of hydrocarbons (h-c bonds). The non-polar hydrocarbons make lipids hydrophobic.

Types of Lipids
  • Triglycerides: Most common dietary lipid group. Commonly referred to as fat.
  • Phospholipids: A bond between the glycerol part of a lipid and a phosphorus molecule
  • Steroids (a sterol compound) – Ring compound with a 4 hydrocarbon base combined with other atoms. Cholesterol is most important
  • Prostaglandins: Sensitize nerves to pain


– An organic molecule made up of amino acids. They contain hydrogen, oxygen, and carbon with the addition of nitrogen. – 20 amino acids = protein

Nucleic Acids

– Formed by a phosphate group, a pentose sugar, and a nitrogen base (adenine, cytosine, guanine, thymine, uracil)

Types of Nucleic Acids
  • RNA – Nucleotide that creates genetic code as protein
  • DNA – Nucleotide that stores genetic information


Cell Membrane

– Made up of lipid bilayer phospholipids, cholesterol, and glycolipids

Cell Junctions

  • Tight Junctions: Integral proteins on adjacent cells fuse to form an impermeable junction that encircles the whole cell and prevents fluids and most molecules from moving in between cells
  • Desmosomes: Rivet-like cell junction formed when linker proteins (cadherins) of neighboring cells interlock like the teeth of a zipper
  • Gap Junctions: Transmembrane proteins form tunnels that allow small molecules to pass from cell to cell

Passive Transport

(no energy) •Diffusion- movement of molecules down their concentration gradients (from high to low) until equilibrium is reached Simple diffusion–Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer Facilitated diffusion –Certain hydrophobic molecules are transported passively down their concentration gradient by carrier-mediated and channel mediated •Osmosis –Movement of solvent, such as water, across a selectively permeable membrane  Water moves by osmosis from areas of low solute (high water) concentration to high areas of solute (low water) concentration •Hydrostatic pressure: pressure of water inside cell pushing on membrane •Osmotic pressure: tendency of water to move into cell by osmosis – the more solutes inside a cell, the higher the osmotic pressure •Tonicity-Ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume  •Isotonic solution has same osmolarity as inside the cell, so volume remains unchanged  •Hypertonic solution has higher osmolarity than inside cell, so water flows out of cell, resulting in cell shrinking –Crenation: shrinking of cells  Hypotonic solution has lower osmolarity than inside cell, so water flows into cell, resulting in cell swelling –Lysing: Bursting of cells  •Active transport-Moves solutes against their concentration gradient (from low to high) •Endocytosis- Phagocytosis(cell eating) Pinocytosis(cell drinking) Exocytosis(Material exits cell