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CELL STRUCTURE AND FUNCTION

Prepared by: Benjo R. De Guzman Lecturer, Biological Sciences Department, School of Science and Technology, Centro Escolar University Manila

Levels of Biological Organization

  • Biosphere: Global sum of all ecosystems.

  • Ecosystem: Community of living organisms and their environment.

  • Community: Group of organisms living together.

  • Population: Group of individuals of the same species.

  • Organism: Individual living entity.

  • Organ system: Group of organs that work together.

  • Organ: Structure made up of tissues.

  • Tissue: Group of similar cells.

  • Cell: Basic unit of life.

  • Organelle: Specialized subunit within a cell.

  • Increasing complexity: From atom to biosphere.

  • Macromolecule: Large molecules made from smaller subunits.

  • Molecule: Two or more atoms bonded together.

  • Atom: Smallest unit of matter.

Cytology

  • Definition: Branch of biology that deals with cell structure, function, and pathology.

  • Cell: The basic structural and functional unit of an organism.

Historical Figures in Cell Biology

  • Robert Hooke (1665): Published Micrographia, observed "cells" in cork.

Cell Theory

  • Classical/Unified Version:

    1. All organisms are made up of cells.

    2. Cells are the basic unit of structure and function in organisms.

    3. Cells arise from pre-existing cells.

  • Modern (Expanded Version): 4. DNA is passed between cells during division. 5. Cells in similar species are mostly the same structurally and chemically. 6. Energy flow occurs within cells.

  • Contributors: Schwann (1839), Virchow (1858), Schleiden (1838).

Essential Functional Characteristics of Cells

  1. Metabolism:

    • Set of chemical reactions that maintain life, involving metabolic pathways.

    • Divisions:

      • Anabolism: Constructive processes that build molecules.

      • Catabolism: Breakdown of complex molecules that provide energy.

  2. Growth:

    • Accumulation of mass leading to an increase in size, occurring when cellular biosynthesis exceeds degradation.

  3. Reproduction:

    • Cells duplicate their contents and divide, yielding cells with similar contents.

Prokaryotic vs Eukaryotic Cells

Prokaryotic Cells

  • Characteristics:

    • Unicellular, small size (0.2-2.0 μm), no true nucleus, smaller ribosomes, circular DNA.

    • Examples: Bacteria and Archaea.

    • Asexual reproduction via binary fission.

Eukaryotic Cells

  • Characteristics:

    • Unicellular or multicellular, larger size (10-100 μm), true nucleus, larger ribosomes, linear DNA.

    • Examples: Plant and animal cells.

    • Reproduction via mitosis and meiosis.

Structures Present in All Cells

  • Protoplasm: Living content of the cell.

  • Plasma Membrane: Membrane enclosing the cell.

  • Genetic Material: DNA that carries genetic instructions.

Plasma Membrane Structure

  • Fluid Mosaic Model: Describes the plasma membrane as a dynamic structure made of a phospholipid bilayer with embedded proteins.

  • Components:

    • Hydrophilic head and hydrophobic tail of phospholipids.

    • Integral and peripheral proteins, cholesterol, carbohydrates.

Major Functions of Plasma Membrane Proteins

  1. Transport: Movement of substances across the membrane.

  2. Enzymatic Activity: Catalyzing chemical reactions.

  3. Signal Transduction: Communication of signals through receptors.

  4. Cell-Cell Recognition: Identification of cells.

  5. Intercellular Joining: Linking adjacent cells.

  6. Attachment to Cytoskeleton and ECM: Structural support.

Endoplasmic Reticulum (ER)

  • Smooth ER: Synthesis of lipids, detoxification, calcium storage.

  • Rough ER: Protein synthesis and modification using ribosomes.

Golgi Apparatus

  • Structure: Stacks of flattened membranous sacs with polarity (cis and trans).

  • Function: Modifies and sorts proteins and lipids received from the ER before distribution.

Lysosomes

  • Structure: Membranous sacs containing hydrolytic enzymes.

  • Function: Digestion of macromolecules, recycling of cellular components.

Peroxisomes

  • Structure: Membrane-bound compartments.

  • Function: Contain enzymes for breaking down fatty acids and detoxifying.

Cytoskeleton

  • Structure: Network of fibers (microtubules, microfilaments, intermediate filaments).

  • Function: Provides support, shape, and facilitates movement within the cell.

Cell Junctions

  • Types:

    • Tight Junctions: Seal between cells.

    • Desmosomes: Anchoring junctions providing strength.

    • Gap Junctions: Channels for communication between cells.

Extracellular Matrix (ECM)

  • Composition: Fibers embedded in a gel-like substance.

  • Function: Support, adhesion, and regulation of cell behavior.

Nucleus Structure and Function

  • Components: Double membrane (nuclear envelope), DNA (chromatin), nucleolus.

  • Function: Repository of genetic information; control center of the cell.

Mitochondria

  • Function: Sites of oxidative phosphorylation, have their own DNA (mtDNA).

  • Unique features, including maternal inheritance and high mutation rate.

Comparison: Nuclear DNA vs. Mitochondrial DNA

  • Nuclear DNA:

    • Encodes most proteins, linear structure, repair mechanisms, inherited from both parents.

  • Mitochondrial DNA:

    • Circular structure, maternally inherited, fewer genes, higher mutation rates.

Alterations in Nuclear Morphology and Disease

  • Laminopathies: Genetic disorders arising from mutations in nuclear lamina genes; examples include progeria.

Conclusion

Understanding cell structure and function is crucial in biology as it lays the foundation for comprehending the complexity of life.