Leaving Certificate Biology: Cell Structure and Diversity Complete Study Guide

Fundamentals of Cell Biology and Microscopy

• Cellular Nature of Life

  • All living organisms are constructed from fundamental biological units called cells.
  • Cells are microscopic structures that cannot be perceived by the naked eye and require magnification tools for observation.
  • Unicellular Organisms: Organisms composed of a single cell that performs all vital life processes (e.g., bacteria, certain protists).
  • Multicellular Organisms: Organisms composed of many specialized cells organized into functional units (e.g., plants, animals, fungi).

• Classification of Cells: Eukaryotic vs. Prokaryotic

  • Eukaryotic Cells:
    • Contain a distinct, membrane-bound nucleus housing genetic material.
    • Possess membrane-bound organelles that compartmentalize specific biochemical functions.
    • Organelles are specialized sub-cellular structures that carry out distinct physiological roles.
    • Genetic material (DNA) is structured into linear chromosomes within the nucleus.
    • Examples: Cells of all animals, plants, fungi, and protists.
  • Prokaryotic Cells:
    • Lack a membrane-bound nucleus; genetic material floats freely in the cytoplasm.
    • Lack internal membrane-bound organelles.
    • Examples: Organisms belonging to Domain Bacteria and Domain Archaea.

• Microscopy and Cell Ultrastructure

  • Light Microscopy:
    • Standard laboratory compound light microscopes utilize visible light and optical lenses to magnify specimens up to $400\times$ magnification.
    • Higher-grade research light microscopes can achieve magnifications up to $2,000\times$.
  • Electron Microscopy:
    • Uses beams of accelerated electrons rather than light rays to achieve vastly superior resolution and magnification, up to $1,000,000\times$ ($1\text{ million}\times$).
    • Cell Ultrastructure: The fine detail of cell internal architecture and sub-cellular organelles as revealed under an electron microscope.
    • Transmission Electron Microscope (TEM): Transmits electrons through an ultra-thin specimen section to reveal internal sub-cellular structures.
    • Scanning Electron Microscope (SEM): Scans an electron beam across the surface of a specimen to yield detailed three-dimensional surface images.
    • Electron Micrograph: A digital image or photograph captured through an electron microscope.

Basic Structure of Animal and Plant Cells

• Basic Animal Cell Architecture

  • Cell (Plasma) Membrane: The outer limiting boundary enclosing the internal cell contents.
  • Cytosol: The fluid liquid portion of the cytoplasm, consisting predominantly of water with dissolved solutes.
  • Cytoplasm: The total cellular contents within the plasma membrane, encompassing the cytosol and all suspended organelles, but excluding the nucleus.
  • Nucleus: The prominent central membrane-bound organelle containing the genetic instructions (DNA).

• Basic Plant Cell Architecture

  • Shares common core components with animal cells: a cell membrane, cytosol, cytoplasm, and nucleus.
  • Contains distinct additional structures:
    • Cell Wall: A rigid, protective layer surrounding the cell membrane.
    • Large Central Vacuole: A prominent internal fluid-filled storage compartment.
    • Chloroplasts: Specialized green plastids wherein photosynthesis takes place.

• Comparison of Animal and Plant Cell Features

  • Cell Wall:
    • Animal Cells: Do not have a cell wall.
    • Plant Cells: Do have a rigid cell wall composed of cellulose.
  • Chloroplasts:
    • Animal Cells: Do not have chloroplasts.
    • Plant Cells: Do have chloroplasts in photosynthetic tissues.
  • Vacuoles:
    • Animal Cells: Contain small, temporary, non-permanent vacuoles.
    • Plant Cells: Contain large, permanent central vacuoles.

Animal Cell Ultrastructure

• Plasma (Cell) Membrane
  • Phospholipid Bilayer: Constructed of two opposing layers of amphipathic phospholipid molecules.
  • Embedded Proteins: Proteins span or sit upon the bilayer, providing structural integrity and creating selective channel proteins for transport.
  • Antigen Markers: Surface glycoproteins and proteins that serve as immunological identification markers, allowing the body's immune system to recognize the cell as "self" and prevent immune destruction.
  • Cholesterol: Embedded within the lipid bilayer to regulate fluidity and structural stability.
  • Permeability Properties:
    • Semi-permeable: Passively permits small molecules (e.g., water $H_2O$, oxygen $O_2$) to diffuse freely across the lipid membrane.
    • Selectively permeable: Actively controls and regulates which larger or charged molecules are permitted to enter or exit the cell.

• The Nucleus
  • Functioning as the information and control centre of the cell, storing all primary genetic code.
  • Nuclear Envelope (Membrane): A double phospholipid bilayer membrane enclosing the nuclear content.
  • Nuclear Pores: Specialized microscopic protein channels spanning the nuclear envelope that permit selective macromolecular transport (e.g., messenger RNA) between the nucleus and cytoplasm.
  • Nucleolus: A dense, non-membrane-bound sub-region inside the nucleus dedicated to synthesizing ribosomal RNA ($rRNA$).
  • Chromatin vs. Chromosomes:
    • DNA strands are wrapped around structural proteins termed histones.
    • Chromatin: Uncoiled, extended, thread-like DNA-protein complexes visible when the cell is in a non-dividing state.
    • Chromosomes: Tightly condensed, organized rod-like structures visible during cell division.
  • Genes: Specific segments of DNA residing along chromosomes containing encoded nucleotide sequences for synthesizing specific functional proteins.

• Mitochondria
  • Sites of aerobic cellular respiration, responsible for breaking down nutrients to liberate usable metabolic energy.
  • ATP (Adenosine Triphosphate): The primary energy currency of the cell. Metabolic energy released from food breakdown is stored in $ATP$ molecules; cellular chemical reactions consume $ATP$ to perform biological work.
  • Structural Composition:
    • Outer Membrane: Smooth boundary layer encapsulating the organelle.
    • Inner Membrane: Highly invaginated inner membrane folded into deep ridges called cristae, which vastly increase surface area for respiratory enzymes.
    • Matrix: The internal semi-fluid compartment enclosed within the inner membrane containing specialized enzymes.

• Ribosomes
  • Organelles responsible for protein synthesis (translation).
  • Composed of ribosomal RNA ($rRNA$) and enzymatic proteins; lack an outer bounding membrane.
  • Consist of two distinct functional subunits: a large subunit and a small subunit.
  • Synthesize polypeptide chains by translating genetic sequence codes delivered on messenger RNA ($mRNA$) molecules exported from the nucleus.

• Endoplasmic Reticulum (ER) [Higher Level]
  • An extensive network of membrane-bound flattened sacs and interconnected tubes termed cisternae extending throughout the cytoplasm.
  • Serves as a primary synthesis factory for cellular lipids (phospholipids, cholesterol), steroids, and hormones, as well as protein transport.
  • Rough Endoplasmic Reticulum (Rough ER): Outer membrane surface is heavily studded with attached ribosomes; primary site for protein synthesis, folding, and intracellular transport.
  • Smooth Endoplasmic Reticulum (Smooth ER): Lacks attached ribosomes; functions in lipid synthesis, phospholipid manufacture, steroid production, and cellular detoxification.

• Golgi Apparatus (Golgi Complex) [Higher Level]
  • Consists of stacks of flattened, membrane-bound sacs named cisternae that are physically separate and not interconnected with one another.
  • Functions to collect, modify, package, and sort proteins and lipids synthesized in the ER, dispatching them via outgoing transport vesicles to specific intracellular organelles or secreting them outside the cell.
  • Present in both animal and plant cells. In plant cells, Golgi apparata synthesize structural carbohydrates required for cell wall assembly.

Plant Cell Ultrastructure

• Structural Overview

  • Plant cells contain all basic eukaryotic organelles found in animal cells: cell membrane, nucleus, mitochondria, ribosomes, endoplasmic reticulum, and Golgi apparata.
  • They additionally possess three defining structures: a cell wall, chloroplasts, and a large central vacuole.

• Cell Wall

  • A rigid layer surrounding the plasma membrane providing mechanical strength, structural support, and protection against osmotic rupture.
  • Composition: Composed primarily of cellulose, a tough structural polysaccharide (complex carbohydrate).
  • Permeability: Fully permeable to water and dissolved molecules of all sizes.
  • Middle Lamella: A pectin-rich intercellular layer located between adjacent primary plant cell walls that glues neighboring plant cells together.
  • Fungal Comparison: Fungal cell walls are composed of chitin, rather than cellulose.

• Chloroplasts
  • Specialized organelles responsible for conducting photosynthesis, converting light energy into chemical potential energy.
  • Contain the green photosynthetic pigment chlorophyll.
  • Enclosed by a double membrane system and contain an internal loop of semi-autonomous DNA.
  • Internal Anatomy:
    • Stroma: The internal fluid-filled matrix surrounding the inner membrane system.
    • Thylakoids: Green, flattened, disc-like membrane sacs containing light-capturing complexes.
    • Grana (singular: Granum): Stacks of disc-like thylakoids interconnected by stroma lamellae.
  • Tissue Distribution: Highly concentrated in green plant tissues exposed to light (e.g., leaf mesophyll); absent in non-photosynthetic structures like root cells or epidermal layers.

• Vacuoles
  • Large, permanent, membrane-enclosed fluid compartments in mature plant cells.
  • Filled with cell sap, an aqueous solution containing dissolved sugars, mineral salts, metabolic amino acids, and waste materials.
  • Turgor Pressure: Pushes the cytoplasm outward against the cell wall, maintaining mechanical rigidity, tissue turgidity, and plant posture.

Microscopy Components and Practical Investigations

• Components and Functions of the Compound Light Microscope
  • Eyepiece lens: Magnifies the image generated by the objective lens; the lens through which the specimen is observed.
  • Objective lens: Primary magnifying lens positioned directly over the stage.
  • Stage: Mechanical platform supporting the glass slide.
  • Slide: Glass slide holding the biological specimen.
  • Coverslip: Thin glass square covering the specimen; prevents cell drying, holds specimen in place, flattens sample, and protects objective lens from liquid stains.
  • Coarse focus wheel: Moves stage up and down rapidly to bring specimen into general, rough focus.
  • Fine focus wheel: Provides precise vertical movements to bring image into sharp focus.
  • Light source: Provides illumination passing up through the specimen.
  • Iris diaphragm: Regulates the aperture size, controlling the quantity and brightness of light illuminating the slide.

• Specified Investigation 6A: Preparation and Microscopic Examination of Animal Epithelial Cells (Cheek Cells)
  • Equipment Required: Light microscope, cotton bud, dropper, methylene blue stain, pointing needle, tissue paper, glass slide, coverslip.
  • Step-by-step Procedure:
    1. Place a drop of clean water on a clean glass slide using a dropper.
    2. Gently scrape the inner cheek lining with a clean cotton bud or gloved finger to harvest buccal epithelial cells.
    3. Smear the collected cells into the water drop on the glass slide.
    4. Add 1 to 2 drops of methylene blue stain onto the cell smear using a dropper.
    5. Allow dye to penetrate for a short duration, then blot off excess stain.
    6. Angle a coverslip at $45^\circ$ to the slide surface and lower it slowly onto the sample using a mounted pointing needle to prevent trapping air bubbles.
    7. Alternative Irrigation Method: Lower coverslip onto specimen first, apply a drop of methylene blue stain along one edge of the coverslip, and draw stain across cells using tissue/filter paper placed at the opposite edge.
    8. Place slide on stage, focus under low-power objective first, then switch to high power.
  • Results: The methylene blue stain selectively colors nucleic acids, causing the central nucleus to stain dark blue against a lighter cytoplasm.

• Specified Investigation 6B: Preparation and Microscopic Examination of Plant Cells (Onion Epidermal Cells)
  • Equipment Required: Light microscope, onion bulb, scalpel, paintbrush, dropper, iodine solution, pointing needle, tissue paper, glass slide, coverslip.
  • Step-by-step Procedure:
    1. Using forceps, peel a thin, transparent layer of epidermis (one cell thick) from an inner fleshy onion leaf scale and place it in a dish of water.
    2. Place a drop of clean water on a slide using a dropper.
    3. Transfer the onion tissue onto the water drop using a fine paintbrush, flattening any wrinkles.
    4. Add 2 to 3 drops of iodine solution stain using a dropper.
    5. Lower a coverslip at a $45^\circ$ angle slowly using a mounted needle to eliminate air bubbles.
    6. Blot away excess iodine stain using tissue paper.
    7. Mount slide on microscope stage and observe under low-power focus before rotating to high-power magnification.
  • Results: Iodine stain reacts with plant tissue structures, turning the rigid cell wall and central nucleus distinct brown/amber against translucent cytoplasm.

Cell Diversity, Stem Cells, and Biological Organisation

• Cell Diversity and Structural Organisation

  • Cell Diversity: The presence of many structurally distinct cell types within a multicellular organism. Higher multicellular complexity correlates with greater cell diversity.
  • Specialized Cells: Cells modified structurally to perform dedicated biological functions.
  • Levels of Biological Hierarchy:
    • Cell: Fundamental unit of structure and function.
    • Tissue: A group of similar specialized cells performing a specific function (e.g., muscle tissue, vascular tissue).
    • Organ: A structure composed of multiple different tissues working together to carry out a complex physiological function (e.g., heart, leaf).
    • Organ System: A collection of distinct organs working co-operatively to perform major bodily operations (e.g., digestive system, circulatory system).
    • Organism: An individual living entity composed of integrated organ systems.

• Stem Cells

  • Unspecialized, undifferentiated biological precursor cells capable of perpetual self-renewal and potential transformation into specialized cell lineages.
  • Plant Stem Cells: Located perpetually within specialized growing zones termed meristems at root tips, shoot tips, and leaf margins.

• Categories of Stem Cells

  • Pluripotent Stem Cells:
    • Possess the capability to differentiate into almost every somatic cell type comprising the human body.
    • Primary Example: Embryonic stem cells harvested from early blastocyst-stage embryos.
  • Multipotent Stem Cells (Adult Stem Cells):
    • Possess restricted differentiation capacity; restricted to generating specific lineages within a given tissue family.
    • Primary Example: Haematopoietic stem cells in bone marrow, which exclusively give rise to blood lineages (erythrocytes, leukocytes, platelets).
  • Artificial Cloning Stem Cells:
    • Stem cells generated artificially through laboratory procedures.
    • Somatic Cell Nuclear Transfer (SCNT): Produced by extracting the nucleus of a fully differentiated somatic (body) cell and inserting it into an enucleated egg cell (whose original nucleus was removed), creating a cloned embryo identical to the somatic donor.
  • Induced Pluripotent Stem Cells (iPSCs):
    • Mature adult somatic cells (e.g., skin or muscle cells) genetically reprogrammed back into a pluripotent embryonic-like stem cell state by scientists.

• Therapeutic Applications in Medicine

  • Transplantation into damaged tissues to replace degenerate cell populations.
  • Potential curative treatment for neurodegenerative diseases (e.g., Parkinson's disease), metabolic conditions (e.g., type 1 diabetes), and spinal cord injury trauma.
  • Pluripotent stem cells, cloning stem cells, and iPSCs offer maximum therapeutic potential due to their unrestricted capacity to generate any organ cell type.

• Ethical Considerations of Stem Cell Research and Therapy

  • Ethics: The branch of philosophy dealing with moral principles and standards governing conduct.
  • Key ethical questions surrounding stem cell technologies:
    1. Does an early human embryo possess full moral status and human rights equal to a born person?
    2. Is it morally permissible to create or destroy human embryos explicitly for medical research?
    3. Should binding international legislative frameworks govern stem cell research and therapeutic cloning?
    4. How can stem cell therapies be rendered equitably accessible to all socio-economic strata?
    5. Is capital allocation for complex stem cell research justified relative to funding immediate global treatments for preventable diseases?