A2.2 Cell Structure

Unity & Diversity

Cells

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?

A2.2.1 Cells as Basic Structural Units

• Cells are the fundamental building blocks of all living organisms.

• Guidance:

  • Nature of Science (NOS): Students should recognize that deductive reasoning helps in forming predictions based on theories.

  • Cell theory can predict that a newly discovered organism may consist of one or more cells.

• Characteristics of cells:

  • Cells are extremely small and can only be visualized using a microscope.

  • Unicellular organisms, such as protists, consist of a single cell.

  • Multicellular organisms consist of many cells, such as animals and plants.

• Cell Theory:

  1. Cells can only arise from pre-existing cells.

  2. Living organisms are composed of cells.

  3. Cells perform all functions essential for life.

A2.2.2 Microscopy Skills

• Nature of Science (NOS):

  • Measurement using instruments constitutes a form of quantitative observation.

• Skills to Master:

  • Create temporary mounts of cells and tissues.

  • Use staining techniques to visualize cellular structures better.

  • Measure sizes using an eyepiece graticule; focus using both coarse and fine adjustments.

  • Calculate actual size and magnification based on observed values.

  • Produce scale bars for photographs taken.

A2.2.3 Developments in Microscopy

• Advantages of modern microscopy techniques include:

  • Electron Microscopy: Provides significantly higher magnification and resolution.

  • Freeze Fracture: A technique used to reveal membrane structures.

  • Cryogenic Electron Microscopy: Produces high-resolution images while preserving specimens.

  • Fluorescent Stains & Immunofluorescence: Used for visualizing specific cellular components.

A2.2.4 Structures Common to Cells

• Typical cells share the following structural features:

  • DNA serves as genetic material.

  • Cytoplasm, primarily water-based, is enclosed by a plasma membrane composed of lipids.

  • Both plant and animal cells possess these structures for particular functions.

A2.2.5 Prokaryote Cell Structure

• Components of prokaryotic cells:

  • Cell Wall: Provides structural support and protection.

  • Plasma Membrane: Encloses cell contents.

  • Cytoplasm: Contains cell components and metabolic activities.

  • DNA: Present as a single loop (naked DNA) located in the nucleoid region.

  • Ribosomes: 70S type involved in protein synthesis.

• Example organisms:

  • Gram-positive eubacteria like Bacillus and Staphylococcus.

A2.2.6 Eukaryote Cell Structure

• Common features of eukaryotic cells:

  • A plasma membrane surrounds a compartmentalized cytoplasm containing 80S ribosomes.

  • A nucleus contains DNA organized into chromosomes, which are surrounded by a double membrane having nuclear pores.

  • Membrane-bound organelles include mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and a cytoskeleton.

A2.2.7 Processes of Life in Unicellular Organisms

• Key life functions performed by unicellular organisms:

  • Homeostasis: Maintenance of stable internal conditions.

  • Metabolism: Chemical reactions that sustain life.

  • Nutrition: Uptake and utilization of nutrients.

  • Movement: Physical relocation or cellular motility.

  • Excretion: Removal of metabolic waste products.

  • Growth: Increase in size or cell number.

  • Response to Stimuli: Reacting to environmental changes.

  • Reproduction: Cell division or sexual reproduction processes.

A2.2.8 Differences in Eukaryotic Cell Structure

• Major differences in structure between animals, fungi, and plants include:

  • Presence of cell walls and their composition.

  • Size and function variations of vacuoles.

  • Presence of chloroplasts in plants (phototrophic activities) and other organelles.

• Organelles present in specific cell types:

  • Centrioles assist in cell division in animal cells.

  • Cilia and flagella are involved in motility.

A2.2.9 Atypical Cell Structures in Eukaryotes

• Examples of atypical structures include:

  • Multinucleate conditions in certain fungal hyphae, muscle cells, red blood cells, and phloem sieve tube elements.

• Functionality is linked to their structural adaptations.

A2.2.10 Cell Types and Structures in Micrographs

• Application of skills involves identifying cell types (prokaryotic, plant, or animal) in micrographs.

• Structures to recognize include nucleoid region, cell wall, nucleus, various organelles, and cell membranes in light and electron microscopy images.

A2.2.11 Drawing and Annotation Based on Micrographs

• Essential skills involve drawing and annotating diagrams of cellular organelles and structures based on their appearance in micrographs, including detailed functions in annotations.

A2.2.12 Origin of Eukaryotic Cells by Endosymbiosis

• The endosymbiotic theory presents the hypothesis that eukaryotic cells evolved from a prokaryotic common ancestor.

• Mitochondria and chloroplasts are thought to have originated from engulfed prokaryotic cells, exhibiting features such as double membranes, 70S ribosomes, and circular DNA that suggest prokaryotic ancestry.

A2.2.13 Cell Differentiation as the Process for Developing Specialized Tissues

• Cell differentiation is guided by gene expression patterns influenced by environmental signals.

• Specialized cells are organized into tissues and organs, each performing distinct functions necessary for the organism's survival and efficiency.

A2.2.14 Evolution of Multicellularity

• Multicellularity has independently arisen in various organisms, including fungi, algae, and all plants and animals.

• Advantages gained include larger body sizes, longer lifespans, and specialized cell functions, enhancing survival and adaptability.

Summary

• Understanding microscopy and the underlying principles of cell structure and function is critical in studying both prokaryotic and eukaryotic cells.

• Commonalties reflect unity among organisms, while differences illustrate diversity in cellular structure and function, supporting complex tissue formation.

• Notable Reference Links:

  • Cell Theory Timeline

  • [Experimental Techniques in Drawing Microorganisms](https://open.lib.umn.edu/app/uploads/sites/276/2021/04/5.1 Leafcrosssectionmicrolabeled.jpg)

  • Understanding Eukaryotic Origins and the Endosymbiotic Theory

Video Resources

• Light Microscopy & Resolution: Watch here

• Eyepiece Graticule & Stage Micrometer: Watch here

• Magnification & Memory Diagrams: Watch here

Acknowledgments

C.J. Clegg and Andrew Davis (2023)