Biology Chapter on Cell Structure and Function

Unit 2: Structure of Cell & Tissue – Its Components and Their Functions

• Cells are the fundamental structural and functional units of the body.
• Cytology is the branch of science dealing with the study of cells.
• Robert Hooke observed and named cells in 1665 from cork slices under a microscope.
• Some organisms are unicellular (composed of a single cell), while others are multicellular.
• The average diameter of a cell ranges from 2-120 micrometers (µm).

Cell Theory

• Proposed by Matthias Schleiden and Theodor Schwann in 1839.
• All living organisms are composed of cells and cell products.
• Cells are the basic units of structure and function of all living organisms.
• All cells come from the division of pre-existing cells.
• An organism's activities and interactions are a result of the combined actions of all its cells.

Organelles (Detailed Diagram Included)

• Nucleus: The control center of the cell. Contains DNA, responsible for inherited traits.
• DNA: Contains coded information for inherited characteristics.
• Mitochondria: Generate energy for cellular processes through oxidative metabolism.
• Golgi Complex: Collects, packages, and distributes molecules within the cell.
• Endoplasmic Reticulum: Smooth and rough networks moving and storing materials.
• Lysosomes: Digest cellular nutrients and waste materials.
• Cytoplasm: Jelly-like fluid between the nucleus and cell membrane. Contains organelles.
• Ribosomes: Tiny protein-producing factories.
• Vacuoles: Storage areas for fats and other substances.
• Peroxisomes: Contain enzymes for particular reactions, such as detoxifying harmful molecules.
• Cell Membrane: Double-layered fatty material that controls the passage of materials into and out of the cell. Its thickness is about 4.5 nanometers (nm)
• Centrioles: Aid in cell division.

Plasma Membrane

• A thin outer layer that maintains the integrity of the cell.
• Primarily composed of phospholipids, cholesterol, glycolipids, and carbohydrates.
• Functions:
  • Separates the cytoplasm from the extracellular fluid.
  • Separates one cell from another.
  • Provides a surface for chemical reactions to occur.
  • Regulates the movement of materials into and out of the cell.

Movement Across Cell Membranes

• Passive Transport: Movement of substances across the membrane without energy expenditure.
  • Diffusion: Random movement of molecules from high to low concentration. (Examples: gases/air in alveoli, and other substances)
  • Facilitated Diffusion: Larger molecules use protein channels to pass through the membrane. (e.g. amino acids)
  • Osmosis: Special type of diffusion referring to the passage of water across a selectively permeable membrane.
  • Filtration: Small molecules pass through a membrane due to pressure. (Kidney function example)
• Active Transport: Movement of substances across the membrane that requires energy expenditure.
  • Utilizes transport proteins that bind to the transported substances.
  • Most importantly, ions move against the concentration gradient.
    • Primary Active Transport: Energy comes directly from ATP hydrolysis(e.g., Na+-K+ pump).
    • Secondary Active Transport: Driven by energy stored in the concentration gradient of ions.

Vesicular Transport

• Fluids containing large particles and macromolecules are transported across cellular membranes inside bubble-like, membranous sacs called vesicles.
• Types include endocytosis (moving into the cell) and exocytosis (moving out of the cell). Details about specific types (phagocytosis, pinocytosis, receptor-mediated endocytosis, etc.) are given in the document.

Cytoplasm

• A semi-transparent, elastic fluid containing suspended particles, tubules, and filaments (cytoskeleton).
• Primarily water (75-90%).
• Contains proteins, carbohydrates, lipids, and inorganic substances.
• The inorganic component is in solution form.

Organelles (Continued)

• Specialized compartments with specific shapes, playing roles in cell growth, maintenance, repair, and control. (Various types are covered within the provided text, along with illustrations)

Nucleus (Continued)

• Oval-shaped, largest structure in a cell.
• Contains the cell's hereditary material (DNA).
• Controls cell activity and structure.
• Separated from other cell structures by a double membrane called the nuclear membrane.
• Has pores to facilitate communication with the cytoplasm.
• Contains cytoplasm-like fluid (nucleoplasm), the genetic material (chromosomes), and a non-membrane-bound mass of material called the nucleolus.
• DNA, RNA, and protein assists ribosome construction.

Endoplasmic Reticulum (Continued)

• A double-membrane channel that is continuous with the nuclear membrane.
• Involved in intracellular material exchange with the cytoplasm.
• Transports materials between different parts of the cell.
• Acts as a storage area for synthesized molecules.
• Together with the Golgi complex, involved in synthesis and packaging.

Ribosomes

• Tiny granules made of rRNA.
• Site of protein synthesis.

Two types of Endoplasmic Reticulum (ER)

• Smooth ER: No ribosomes. Site of lipid and steroid hormone synthesis. Present mainly in lipid-forming cells (adipocytes, interstitial cells, etc.)
• Rough ER: Contains ribosomes. Site of protein synthesis, processing, and packaging. Mainly present in protein-forming cells (pancreatic acinar cells, plasma cells, etc.)

Golgi Complex (Continued)

• Network of membranous sacs. Processes, sorts, packages, and delivers proteins to various parts of the cell.

Mitochondria (Continued)

• Small, spherical, rod-shaped or filamentous organelles generating energy.
• Possess two membranes, the inner membrane being folded into cristae
• The central cavity of a mitochondrion is the matrix.
• Vital for maintaining proper calcium ion concentrations within the cell.
• Facilitates energy transduction through respiration (cellular respiration), and thermogenesis (heat production).

Lysosomes (Continued)

• Membrane-enclosed spheres formed from Golgi complexes.
• Single membrane.
• Diameter (250-750nm).
• Contain digestive enzymes.
• Important for digesting worn-out cell components, external bacteria, and other material.
• Also involved in programmed cell death (autolysis).

Some Other Organelles

• Cytoskeleton: A complex network of filaments (microfilaments, microtubules, and intermediate filaments) that provides structural support and facilitates movement within the cell.
• Centrosome: Contains centrioles. Involved in cell division.
• Cilia/Flagella: Thread-like appendages that enable movement. Found mainly in reproductive organs and the respiratory tract.

Tissues

• Groups of similar cells working together to perform specific functions.
• Histology: The study of tissues.
• Four major tissue types: Epithelial, connective, muscle, and nervous tissue. (Details about each tissue type are provided in separate sections)

Epithelial Tissue

• Covers body surfaces, lines body cavities and ducts, forms glands.

  • Covering and lining epithelium: Forms outer coverings, lines internal organs, and is part of sensory organs (smell, hearing, touch, etc.) and reproductive organs.
  • Glandular epithelium: Makes and releases substances such as mucus, digestive juices, etc. (Located in organs like breast, lungs, stomach, etc.)

• Classification of Epithelial Tissue based on layers and cell shape:

  • Simple Epithelium: Single layer (absorption, filtration).
  • Stratified Epithelium: Multiple layers (protection).
  • Pseudostratified Epithelium: Appears layered but is really single (secretion, absorption)
    • Shape Classification:
      • Squamous: Flattened cells (e.g. lining of lungs)
      • Cuboidal: Cube-shaped cells (e.g. kidney tubules)
      • Columnar: Tall, column-shaped cells (e.g. intestines)

Connective Tissue

• Characterized by cells that are more widely spaced and have abundant intercellular matrix (fibers in a ground substance).
• Embryonic Connective Tissue: Mesenchyme (tissue all other connective tissues evolve from), mucous connective tissue
• Adult Connective Tissue: Broadly categorized into connective tissue proper, cartilage, bone, blood/vascular tissue
• Functions: Binding and structural support, protection, transport, and insulation.
• Components: Fibers (collagen, elastic, reticular), ground substance, cells (e.g., fibroblasts, adipocytes, chondrocytes, osteocytes)

Muscle Tissue

• Specialized contractile cells (fibers) that enable movement.
• Three types: Skeletal muscle (voluntary movement), smooth muscle (automatic movements such as peristalsis), and cardiac muscle (heart contractions)

Nervous Tissue

• Composed of neurons and supporting cells.
• Function: Transmit electrical signals throughout the body.
• Found in the brain, spinal cord, and nerves.

Test your knowledge on cell structure, the plasma membrane, tissue types, and cellular processes with this quiz. Explore key concepts related to biological membranes, tissue classification, and the characteristics of different cell types. Perfect for students studying biology at any level.