Understanding the Key Features and Levels of Living Things

Posted on Mar 25, 2025 in Biology

Key Features of Living Things

  • Complex Organization: Molecules interact in complex internal and external functions.
  • Cellular Structure: Can be unicellular or multicellular.
  • Feeding and Metabolism: Capture and use energy and matter for growth, development, and maintenance.
  • Responsiveness: Detect external stimuli and respond appropriately.
  • Reproduction: Ability to create new individuals, similar or different from themselves.

Levels of Biological Organization

Subatomic Level

Composed of neutral protons and electrons.

Atomic Level

Consists of atoms.

Molecular Level

Composed of molecules, which can be:

  • Functional: Containing C, H, O, N, S, P atoms.
  • Inorganic: Not consisting of C and H atoms, but may contain O, N, S, P.

Molecular Sub-Level

  • Macromolecules: Polymers formed by the union of many organic molecules (monomers).
  • Supramolecular Complexes: Binding of various macromolecules.
  • Cellular Organelles: Union of supramolecular complexes.

Cellular Level

A cell is a structure of living matter containing cytoplasm and genetic material that dictates structure and function.

  • Prokaryotic Cells: Lack a differentiated nucleus; genetic material is dispersed in the cytoplasm.
  • Eukaryotic Cells: Possess a differentiated nucleus enclosed by a nuclear membrane.

Multicellular Level

Composed of more than one cell.

Sub-Levels:

  • Thallus: Cells are similar and perform all functions (e.g., algae, fungi, mosses, ferns).
  • Tissues: Groups of similar cells with a common embryological origin and function (e.g., muscle tissue).
  • Organs: Structural and functional units performing specific movements, composed of different tissues (e.g., heart).
  • Systems: Sets of similar organs formed by the same tissues, but with independent movements (e.g., muscular system).
  • Apparatus: Sets of different organs coordinated for specific functions (e.g., digestive system).

Chemical Bonds in Biological Systems

Ionic Bond

Atoms with fewer than 4 electrons tend to donate them, becoming electropositive (metallic). Atoms with more than 4 electrons tend to attract electrons, becoming electronegative (non-metallic).

Covalent Bond

Occurs between atoms with high and similar electronegativity. It is a strong bond. If electronegativity is similar, molecules are apolar; if different, polar molecules (dipoles) form.

Hydrogen Bond

Occurs between dipoles of hydride molecules due to the small size of hydrogen, allowing close proximity to other atoms.

Van der Waals Forces

Arise between apolar molecules due to random, asymmetric electron distribution causing temporary dipoles.

Bioelements

Primary Bioelements

Essential for forming organic biomolecules (carbohydrates, lipids, proteins, nucleic acids): O, C, H, N, P, S.

Secondary Bioelements

May be essential (Ca, Na, K, Mg, Cl, Fe, S) or variable (Br, Zn, Ti).

Water (H2O)

Exists in three forms: circulating, interstitial, and intracellular. In humans, circulating water accounts for 8% of body weight, interstitial water 15%, and intracellular water 40%. Water molecules are dipolar.

Characteristics of Water

  • High Cohesion: Essential for liquid water properties.
  • High Surface Tension: Resists breaking the surface.
  • High Specific Heat: Requires significant energy to raise temperature, acting as a thermal stabilizer against sudden temperature changes.