Biological Organization Levels and Cell Structure

1. Levels of Organization

Degrees of complexity are called organizational levels. Abiotic levels can be differentiated from biotic levels.

• N. Subatomic: Formed by elementary particles (quarks, leptons, etc.).
• N. Atomic: Atoms.
• N. Molecular: Molecules made up of grouped atoms. The molecules of living things begin here (organic and inorganic).
• N. Cellular: Where we find the cell.
• N. Tissue: A set of specialized cells with the same function and origin.
• N. Organ: Different tissues grouped to form one function.
• N. System: Organs that function together to form one apparatus.
• N. Apparatus: Set of organs with different functions involved in one or more of them.
• N. Organism: The living being as a whole.

2. Variety of Atomic Functional Units and Cells

All living beings are sources of atomic functional units and cells. There exist about 250 types of cells. Characteristics depend on their function, contiguous tissues, and cells.

Cell Shape

Shapes include: cylindrical, cubic, planar, spherical, biconcave, etc. Most human cells are a few micrometers ($\mu$m) in size.

3. Structure of Human Cells

Main Components

• Plasma Membrane: Wraps, protects, and regulates the input and output of nutrients.
• Cytoplasm: Contains the cytoskeleton.
• Nucleus: Contains the nuclear membrane, nucleoplasm, chromatin, and nucleolus.

Organelles

Structures found in the cytoplasm that perform different functions:

• Mitochondria: Cellular respiration.
• Endoplasmic Reticulum:
  • Rough: Stores and transports proteins synthesized by ribosomes.
  • Smooth: Stores and transports lipids.
• Ribosomes: Protein synthesis.
• Golgi Apparatus: Collects substances from the reticulum and secretes these substances to the exterior.
• Vacuoles: Store substances or waste products.
• Lysosomes: Digest complex substances and nutrients.
• Centrosomes: Role in cell division.

4. Cell Association: Tissues

Tissues are all cells of the same type with a very similar structure and the same functions.

Tissue Types

• Epithelial Tissue: Formed by very close cells. Includes glandular epithelium and covering epithelium (revestimiento).
• Connective Tissue: Formed by less specialized cells immersed in an intercellular substance. Includes:
  • Conjunctival fabric.
  • Adipose tissue (T. adiposo).
  • Bone tissue (T. óseo).
• Muscular Tissue: Formed by elongated cells containing contractile protein fibers. Includes:
  • Striated muscle (T. estriado).
  • Smooth muscle (T. liso).
  • Cardiac muscle (T. cardíaco).
• Nervous Tissue: Detects changes in the external and internal environment. Composed of neurons and supporting cells.