A2.2 Cell Structure Study Notes

A2.2 Cell Structure

Guiding Questions

• What are the features common to all cells and the features that differ?
• How is microscopy used to investigate cell structure?

Knowledge and Skills SL and HL

• A2.2.1 Cells as the basic structural unit of all living organisms
• A2.2.2 Microscopy skills
• A2.2.3 Developments in microscopy
• A2.2.4 Structures common to cells in all living organisms
• A2.2.5 Prokaryote cell structure
• A2.2.6 Eukaryote cell structure
• A2.2.7 Processes of life in unicellular organisms
• A2.2.8 Differences in eukaryotic cell structure between animals, fungi, and plants
• A2.2.9 Atypical cell structure in eukaryotes
• A2.2.10 Cell types and cell structures viewed in light and electron micrographs
• A2.2.11 Drawing and annotation based on electron micrographs (HL only)
• A2.2.12 Origin of eukaryotic cells by endosymbiosis
• A2.2.13 Cell differentiation as the process for developing specialized tissues in multicellular organisms
• A2.2.14 Evolution of multicellularity

A2.2.1 Cells as the Basic Structural Unit of All Living Organisms

• Deductive Reasoning: Students should be aware that deductive reasoning can generate predictions from theories.
  • Example: Based on cell theory, a newly discovered organism can be predicted to consist of one or more cells.

Cell Theory

• Definition: Each scientific discipline has central theories that explain phenomena. In biology, cell theory explains life processes by asserting that living things are made up of cells.

Components of Cell Theory

1. All living things are made of cells
2. The cell is the basic unit of life
3. Cells only arise from pre-existing cells

• Focus is primarily on the first two statements.
• Theories provide explanations for observations; for example, plant wilting is linked to cellular conditions.

NOS: Inductive and Deductive Reasoning

• Inductive Reasoning: General conclusions drawn from specific observations.
  • Example: Every living thing viewed under a microscope is made of cells.
• Deductive Reasoning: Specific predictions made based on theory.
  • Example: A pangolin, as a living organism, is probably made of cells.

A2.2.2 Microscopy Skills

• Application of Skills: Students should practice making temporary mounts of cells and tissues, staining, measuring sizes using eyepiece graticules, focusing using coarse and fine adjustments, calculating actual size and magnification, and producing scale bars.
• Quantitative Observation: Measurement using instruments is a form of quantitative observation.

Microscopy Skills

• Light Microscopes: Allow investigation of structures too small to be seen by the naked eye.
• Preparations: Students will learn how to prepare samples for examination.

Calculating Magnification

• Magnification Importance: Biological observations occur at different scales, hence magnification helps visualize size in diagrams.
• Formula to Calculate Magnification:
  \text{Magnification} = \frac{\text{Drawing Size}}{\text{Actual Size}}

Steps for Calculation

1. Determine the actual size using the scale bar label or query text.
2. Measure the drawing size with a ruler.
3. Perform unit conversion for matching units of drawing size and actual size.
4. Plug values into the formula and solve for magnification.

A2.2.3 Developments in Microscopy

• Electron Microscopy: Provides advantages such as high magnification and resolution compared to light microscopy.
• Innovations: Examples include freeze-fracture, cryogenic electron microscopy, fluorescent stains, and immunofluorescence techniques.

Advances in Microscopy

• Technological Prerequisites: Many scientific discoveries arose after appropriate technological advancements in microscopy.
• Light Microscopes’ Limitations: Limited resolution, able to reveal only a few structures (e.g., chloroplasts, nuclei).

Resolution and Scale

• Scale of Biological Structures:
  • Ranges from atoms (1 pm) to eukaryotic cells (100 μm).
  • Types of microscopy scale included:
  • Light Microscope - Eukaryotic Cells
  • Electron Microscope - Organelles, Bacteria, Viruses

Advances in Light Microscopy

• Immunofluorescence Technique: Utilizes fluorescent antibodies for cell structure visualization. For instance, in visualizations, the nucleus may appear blue and cytoskeleton green.

Advantages of Electron Microscopy (EM)

• Concept: Works similarly to light microscopes but uses electrons to form images.
• Advantages:
  • Better magnification and resolution.
• Limitations:
  • Requires dead samples, lacks color, costlier than light microscopes.

A2.2.4 Structures Common to Cells in All Living Organisms

• Cellular Composition: Common features include:
  • DNA as genetic material
  • Water-based cytoplasm enclosed by plasma membrane
• Key Understanding: Students should comprehend reasons for these structures.

Cell Structure Categories

• Prokaryotic Cells:
  • Smaller with no internal membranes. Includes bacteria and archaeans.
• Eukaryotic Cells:
  • Larger with internal membranes surrounding organelles, including nucleus, plants, animals, fungi, and protists.

Main Features Common to All Cells

1. DNA as Genetic Material
2. Water-Based Cytoplasm for Metabolism
3. Ribosomes for Protein Synthesis
4. Plasma Membrane as Outer Boundary

A2.2.5 Prokaryote Cell Structure

• Components Include:
  • Cell wall, plasma membrane, cytoplasm, naked DNA (looped), 70S ribosomes.
• Example: Gram-positive eubacteria like Bacillus and Staphylococcus illustrate typical structures.

Prokaryote Features

• General structure includes:
  • Cell wall for protection
  • Plasma membrane regulating substance exchange
  • Cytoplasm as metabolic reaction medium
  • Looped DNA in nucleoid region
  • 70S ribosomes for protein production.

A2.2.6 Eukaryote Cell Structure

• Typical Features:
  • Compartmentalized cytoplasm, 80S ribosomes, nucleus with chromosomes (DNA + histones), membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus), and a cytoskeleton of microtubules and microfilaments.

Compartmentalization in Eukaryotic Cells

• Eukaryotic lineage includes structural diversity and specific function roles.

Ribosomes in Eukaryotes

• Composed of 80S ribosomes for protein assembly, free-floating or bound to rough ER.

Nucleus Characteristics

• Eukaryotic DNA encased within double-membraned nucleus facilitating mRNA transport out to ribosomes for protein synthesis.

Mitochondria and Endoplasmic Reticulum

• Mitochondria:
  • Functions in aerobic respiration, having a double membrane with folds to maximize surface area for biochemical processes.
• Endoplasmic Reticulum:
  • Rough ER produces membrane-embedded proteins or those for export.
  • Smooth ER synthesizes membrane components and detoxifies substances.

Golgi Apparatus

• Composed of stacked membrane sacs, modifies proteins received from the rough ER and repackages them.

Specialized Structures in Eukaryotic Cells

• Microvilli: High surface area adaptation for cells involved in absorption, e.g., intestinal cells.
• Vesicles, Vacuoles, and Lysosomes:
  • Used for transport, storage, and breakdown of materials.

A2.2.7 Processes of Life in Unicellular Organisms

• Functions Include:
  • Homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli, and reproduction.

Functions of Life (Mr. HM Gren)

• Metabolism: Enzyme-catalyzed reactions (e.g., respiration)
• Response: Interaction with the environment
• Homeostasis: Regulation of internal conditions (e.g., pH)
• Movement: Physical actuation within the environment
• Growth: Increase in overall size/shape
• Reproduction: Offspring production (asexual/sexual)
• Excretion: Waste removal from metabolism
• Nutrition: Energy acquisition via synthesis or absorption.

Specialization in Multicellular Organisms

• In multicellular life, functions are distributed among specialized organs, contrasting with unicellular organisms performing all functions independently.
• Example: Paramecium’s movement towards food exhibits nutrient absorption and metabolic waste excretion.

Homeostasis Example

• Paramecium utilize contractile vacuoles for maintaining water balance in response to environmental conditions.

A2.2.8 Differences in Eukaryotic Cell Structure

• Categories for Comparison: Animals, fungi, and plants.

Summary of Eukaryotic Cell Types

A2.2.9 Atypical Cell Structure in Eukaryotes

• Examples:
  • Use of nuclei variation in skeletal muscle, red blood cells, phloem sieve tubes, and aseptate fungal hyphae.

Specific Atypical Examples

• Red Blood Cells: Lack a nucleus to optimize passage through capillaries.
• Aseptate Fungal Hyphae and Skeletal Muscle: Require multiple nuclei for extensive cell function due to size.
• Phloem Sieve Tube Elements: Originally develop large and are modified to optimize nutrient transport by removing the nucleus.

A2.2.10 Cell Types and Structures in Micrographs

• Skills: Identification of prokaryotic, plant, or animal cells in both light and electron micrographs.
  • Structures to identify include: nucleoid region, prokaryotic cell wall, nucleus, mitochondrion, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum, chromosomes, ribosomes, cell wall, plasma membrane, microvilli.

A2.2.11 Drawing and Annotation Skills from Electron Micrographs

• Biological Drawing Guidelines:
  • Ensure accurate depiction of organelles and structures in cell diagrams.
  • Annotations must include function of each structure.

Drawing Specifications

• Cell wall: Two lines to indicate thickness.
• Cell membrane: Single continuous line, shown as inner line of the wall if applicable.
• Nucleus: Double membrane with pores depicted.
• Vacuoles: Shown as single continuous lines.

A2.2.12 (HL) Origin of Eukaryotic Cells by Endosymbiosis

• Theory Overview: States that eukaryotes evolved from a common unicellular ancestor and that mitochondria developed from endosymbiotic relationships.

Evidence for Endosymbiosis

• Mitochondria and chloroplasts possess traits such as:
  • 70S ribosomes
  • Naked circular DNA
  • Replication through division of pre-existing organelles
  • Double membranes with prokaryotic-like proteins
  • Size comparable to bacteria

Steps to Eukaryotic Evolution

1. Infolding of the cell membrane to create internal structures.
2. Endosymbiosis with larger host cells.

A2.2.14 (HL) Evolution of Multicellularity

• Multicellularity Characteristics: Evolved independently in various eukaryotic groups, providing advantages such as larger organism size and cell specialization.

Comparative Groups in Multicellularity

• Present in all animals, all plants, most fungi, and some algae.

Additional Insights on Multicellularity

• Simple multicellularity may provide defense against predators by cell aggregation.
• Notable examples include the Scenedesmus which can exist as single cells or aggregates based on predation pressures.

A2.2.13 (HL) Cell Differentiation in Multicellular Organisms

• Mechanisms of Differentiation: Driven by distinct patterns of gene expression influenced by environmental changes.

Importance of Differentiation

• Allows multicellular organisms to develop specialized cell types, enhancing functionality and complexity.
• Example: Flowering plants exhibit gene expression changes that lead to blooming based on seasonal cues.