Cell Biology: Structure, Functions, and Tissue Organization

Posted on Jul 29, 2025 in Biology

The Cell: Basic Unit of Life

The cell is the structural and functional unit of all living beings. The Cell Theory states that:

  • All living organisms are composed of cells.
  • The cell is the basic unit of structure and function in all living organisms.
  • All cells come from pre-existing cells through the process of cell division.

Cell Morphology: Types of Cells

There are two main types of cells:

  • Prokaryotic Cells: These are simple cells without a true nucleus or membrane-bound organelles (except ribosomes). Organisms belonging to the Kingdom Monera (like bacteria and archaea) are prokaryotes.
  • Eukaryotic Cells: These are more complex cells with a true nucleus, cytoplasm, and various membrane-bound organelles. They form unicellular or multicellular organisms belonging to other kingdoms (Protista, Fungi, Plantae, Animalia).
    • Plant Cells: Characterized by a cell wall (made of cellulose), chloroplasts (for photosynthesis), and a large central vacuole. Examples include algae and plants.
    • Animal Cells: Typically lack a cell wall and chloroplasts. They may have small vacuoles. Examples include protozoa, many fungi, and animals.

Eukaryotic Animal Cell: Structure and Organelles

A typical eukaryotic animal cell consists of three main parts:

  1. Plasma Membrane: Formed by lipids and proteins, it regulates the exchange of substances between the cell’s interior and exterior.
  2. Nucleus: Enclosed by a double membrane (nuclear envelope) with pores, it allows the exchange of substances between the nucleus and cytoplasm.
    • Nucleolus: The site of ribosome synthesis.
    • DNA: Carries genetic information and controls cellular activity.
  3. Cytoplasm: The jelly-like substance filling the cell, containing:
    • Cytoskeleton: A network of protein fibers that provides structural support and enables cell movement.
    • Cellular Organelles:
      • Ribosomes: Small organelles where proteins are synthesized.
      • Endoplasmic Reticulum (ER): A network of membranes connected to the nuclear envelope.
        • Rough Endoplasmic Reticulum (RER): Has ribosomes attached and is involved in protein synthesis and modification.
        • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes and is involved in lipid synthesis and detoxification.
      • Golgi Apparatus: A stack of flattened sacs (cisternae) and vesicles that modifies, sorts, and packages substances for secretion or delivery to other organelles.
      • Lysosomes: Vesicles containing digestive enzymes for intracellular digestion.
      • Vacuoles: Vesicles that store substances. Animal cells typically have small, temporary vacuoles.
      • Centrioles: Two small, cylindrical structures involved in cell division, ensuring equal distribution of chromosomes.
      • Mitochondria: Organelles with a double membrane (outer and inner folded into cristae) where cellular respiration occurs, producing energy (ATP) for the cell.

Essential Cell Functions

Cells perform several vital functions:

  • Nutrition Function: Cells incorporate nutrients and synthesize biomolecules.
  • Cellular Metabolism: All chemical reactions occurring within the cell.
  • Cellular Respiration: A metabolic process occurring in the mitochondria where organic matter combines with oxygen (from air) to produce CO2, H2O, and ATP (energy).
  • Relationship Function: Cells are capable of detecting stimuli and developing responses, which allows for movement.
    • Flagella: Long, whip-like extensions, usually one per cell, enabling movement.
    • Cilia: Short, numerous hair-like extensions that move in a synchronized, rapid manner.
    • Pseudopods: Temporary, irregular extensions of the cell membrane used for movement and engulfing particles.
  • Reproduction Function: The formation of daughter cells from parent cells.
    • In unicellular organisms, reproduction ensures the continuation of the species.
    • In multicellular organisms, it allows for growth, repair, and regeneration of tissues.

Cell Life Cycle Stages

The cell life cycle consists of two main stages:

  • Interphase: The stage where the cell performs its normal functions, grows, and prepares for division. It includes processes related to nutrition and metabolism.
  • Cell Division Stage: The stage where a parent cell divides to form two identical daughter cells. This involves two main processes:
    • Nuclear Division (Mitosis): The process of nuclear division where DNA chromosomes condense, and each chromosome forms two identical sister chromatids, ensuring their equal distribution to daughter cells.
    • Cytoplasmic Division (Cytokinesis): The division of the cytoplasm and organelles, resulting in two separate daughter cells.

Organization of Multicellular Organisms

Organisms can be:

  1. Unicellular Organisms: Composed of a single cell (e.g., some algae, fungi, bacteria, protozoa).
  2. Multicellular Organisms: Composed of many cells that work in a coordinated manner (e.g., plants, animals, most fungi).

In multicellular organisms, cells are organized into higher levels:

  • Tissues: A group of specialized cells that work together to perform a specific function.
  • Organs: An association of different tissues working together to perform a specific function.
  • Organ Systems: A group of organs that cooperate to perform major functions in the body.

Primary Tissue Types in Animals

There are four primary types of animal tissues:

  • Epithelial Tissue: Composed of closely packed cells with little intercellular substance. It can be:
    • Lining/Covering Epithelium: Forms protective linings on external surfaces (like the epidermis of the skin) and internal cavities (like mucous membranes lining body cavities).
    • Glandular Epithelium: Forms glands that produce secretions.
  • Connective Tissue: Characterized by abundant intercellular substance. It includes:
    • Loose Connective Tissue: Fills spaces and connects organs.
    • Adipose Tissue: Specialized for fat storage.
    • Cartilage Tissue: Provides support and protection for certain body parts.
    • Bone Tissue: Provides structural support and protection for the body.
  • Muscle Tissue: Responsible for movement through contraction and relaxation.
    • Voluntary Muscles: Such as skeletal muscles in limbs.
    • Involuntary Muscles: Such as smooth muscles in internal organs.
  • Nervous Tissue: Produces and transmits electrical impulses.
    • Neurons: Cells that process and transmit nerve impulses.
    • Glial Cells: Support, protect, and nourish neurons.