Cell Biology: Structure, Types, and Organization

Posted on Oct 5, 2024 in Biology

1. The Cell: The Building Block of Life

Organic and inorganic compounds group together to form cells. All living things are composed of one or more living cells. The cell is the smallest unit capable of performing three vital functions: nutrition, interaction, and reproduction.

Human cells are eukaryotic, meaning their genetic material (DNA) is housed within a double-membraned nucleus. The cytoplasm is compartmentalized into numerous organelles, each performing specialized functions. While human cells share a similar basic structure, they vary in shape, size, and function. Most human cells measure a few microns in size. The largest are eggs and some neurons, while the smallest are sperm.

2. The Prokaryotic Cell

  • The Capsule: A rubbery layer that can bind bacteria into colonies or provide protection.
  • The Cell Wall: A rigid framework of murein that prevents osmotic damage and defines cell shape. Common shapes include coccus, bacillus, and spirillum.
  • The Plasma Membrane: A typical phospholipid bilayer.
  • The Mesosomes: Internal folds of the plasma membrane associated with respiration.
  • Photosynthetic Membranes: Surfaces with pigments that absorb light.
  • The Pili: Protein rods involved in cell-to-cell attachment.
  • The Flagellum: Responsible for the motility of many bacteria.
  • The Plasmids: Small, circular pieces of DNA that replicate independently of the cellular genome.
  • The Genetic Material: A ring of double-stranded DNA not enclosed by a nuclear membrane.
  • Ribosomes: Smaller than those of eukaryotes.
  • Food Stores: Commonly lipid or glycogen granules.

3. Ultrastructure of the Animal Cell (Eukaryote)

  • The Plasmalemma: The cell’s surface, controlling contact with its surroundings. It has differential permeability and regulates the movement of solutes.
  • Microvilli: Extensions of the plasma membrane that increase surface area.
  • The Cytoplasm: Composed mostly of water with many solutes, permeated by the cytoskeleton (microtubules and microfilaments).
  • Microtubules: Hollow tubes of tubulin protein involved in intracellular transport, structural support, and components of specialized structures.
  • Microfilaments: Actin protein fibers playing a role in endocytosis and exocytosis.
  • Secretory Vesicles: Carry synthesized products or degradation byproducts to lysosomes.
  • Endocytic Vesicles: Contain molecules or structures too large to cross the membrane directly.
  • The Nucleus: The control center for cellular activities, containing DNA bound to proteins.
  • Free Ribosomes: The site of protein synthesis.
  • Rough Endoplasmic Reticulum: Studded with ribosomes, it compartmentalizes the cell and transports synthesized proteins to the Golgi body.
  • The Golgi Apparatus: A stack of sacs (cisternae) that modifies and packages cellular products, often enclosing them in vesicles for secretion.
  • Lysosomes: Sacs containing digestive enzymes that break down cellular contents.
  • The Peroxisome: Contains oxidative enzymes, important in slowing cellular aging.
  • The Mitochondria: The site of aerobic respiration.
  • The Centrioles: A pair of structures that organize the nuclear spindle.

5. Similarities and Differences Between Prokaryotic and Eukaryotic Cells

Similarities:

  • Both possess DNA as genetic material.
  • Both are surrounded by a plasma membrane.
  • Both possess ribosomes.
  • Both have similar basic metabolism.
  • Both exhibit diverse forms.

Differences: (See table below)

6. Differences Between Animal and Plant Cells

Both plant and animal cells have a cell membrane, but plant cells also have a cellulose cell wall that provides rigidity. Plant cells contain chloroplasts, organelles capable of photosynthesis, making them autotrophic. Animal cells lack chloroplasts. Plant cells have a large central vacuole, while animal cells have multiple smaller vacuoles.

7. Cell Association and Tissues

Cell differentiation is the process by which cells acquire specific shapes and functions during embryonic development, specializing and associating to form tissues. Cell morphology changes significantly during differentiation, but the genome remains unchanged. A tissue is a group of similar cells with a similar structure performing the same function.

Tissue Types:

  • Protection:
    • Integument: Outer covering of the body.
    • Mucosa: Protects internal cavities.
  • Storage: Adipose tissue stores fat as a food reserve.
  • Support: Bone tissue provides rigid support due to mineral content.
  • Secretion: Glandular tissue produces and releases substances.
  • Connection: Connective tissue connects and surrounds organs.
  • Movement: Muscular tissue enables body movement.

9. Organs, Systems, and Apparatuses

Organs: Structures formed by various tissues associated to perform a specific function (e.g., stomach, lungs, heart).

Systems: Organs of the same type performing a similar function (e.g., musculoskeletal, nervous).