A2.2.10 Cell types and cell structures viewed in light and electron micrographs Notes

Identifying Cells and Organelles in Light and Electron Micrographs

When looking through a microscope, the key to distinguishing between an animal, a plant, and a fungal cell lies in identifying the cell’s unique features and organelles.

Recognizing Prokaryotic, Plant, and Animal Cells

1. Cells fall into two broad categories: prokaryotic and eukaryotic.
2. Within eukaryotic cells, plant and animal cells have distinct structural differences.

Prokaryotic Cells

1. Prokaryotic cells, like bacteria, are structurally simpler than eukaryotic cells.
2. To identify a prokaryotic cell, look for:
   1. Small size: Typically less than 5 µm in diameter.
   2. Cell wall: Composed of peptidoglycan, providing structural support.
   3. Nucleoid region: A lighter area in the cytoplasm where circular DNA is located, not enclosed by a membrane.
   4. No membrane-bound organelles: Prokaryotes lack organelles like a nucleus, mitochondria, or chloroplasts.
   5. 70S ribosomes: Smaller ribosomes compared to the 80S ribosomes of eukaryotic cells.

Example

In an electron micrograph of Escherichia coli, you might notice a dense cytoplasm with scattered ribosomes, a nucleoid region, and a visible cell wall forming the outer boundary.

Plant Cells

1. Cell wall: Made of cellulose, giving the cell rigidity and support.
2. Chloroplasts: Organelles containing stacks of thylakoids (grana) where photosynthesis occurs.
3. Large central vacuole: A prominent organelle used for storage and maintaining cell turgor pressure.
4. Regular shape: Plant cells often appear rectangular or box-like due to the rigid cell wall.

Animal Cells

1. No cell wall: Animal cells have a plasma membrane only, resulting in a more irregular shape.
2. No chloroplasts: Animal cells do not perform photosynthesis.
3. Small vacuoles: If present, they are much smaller and temporary compared to the large central vacuole in plant cells.
4. Centrioles: Organelles involved in cell division, present in animal cells but absent in most plant cells.

Identifying Organelles in Electron Micrographs

1. Electron micrographs provide detailed images of cellular structures, enabling the identification of specific organelles.
2. Below are the key organelles to recognize and their distinguishing features:

1. Nucleus

1. Appearance: A large, spherical structure with a double membrane (nuclear envelope) and visible nuclear pores.
2. Function: Stores genetic material (linear DNA) and controls cell activities.
3. Distinction: Found only in eukaryotic cells.

2. Mitochondria

1. Appearance: Oval-shaped with a double membrane; the inner membrane forms folds called cristae.
2. Function: Site of aerobic respiration and ATP production.
3. Distinction: Present in both plant and animal cells.

3. Chloroplast

1. Appearance: Oval-shaped with a double membrane; contains stacks of thylakoids (grana) and a fluid-filled stroma.
2. Function: Site of photosynthesis.
3. Distinction: Found exclusively in plant cells.

4. Ribosomes

1. Appearance: Small, dense granules in the cytoplasm or attached to the rough endoplasmic reticulum (RER).
2. Function: Protein synthesis.
3. Distinction: 70S ribosomes in prokaryotes; 80S ribosomes in eukaryotes.

5. Endoplasmic Reticulum

1. Rough ER: Appears as a network of membranes with ribosomes attached. Involved in protein synthesis.
2. Smooth ER: Lacks ribosomes and is involved in lipid synthesis and detoxification.

6. Golgi Apparatus

1. Appearance: A stack of flattened, membrane-bound sacs.
2. Function: Modifies, sorts, and packages proteins and lipids for transport.

7. Vacuole

1. Appearance: A large, central organelle in plant cells; smaller and temporary in animal cells.
2. Function: Stores substances and maintains turgor pressure in plant cells.

8. Plasma Membrane

1. Appearance: A thin, dark line enclosing the cell.
2. Function: Regulates the movement of substances in and out of the cell.

9. Cell Wall

1. Appearance: A thick outer layer surrounding the plasma membrane.
2. Function: Provides structural support and protection.

10. Villus

1. Appearance: Finger-like projections of the plasma membrane.
2. Function: Increase surface area for absorption.

Key points for Interpreting Micrographs

1. Identify the scale: Look for a scale bar to determine the magnification and size of structures.
2. Focus on key features: Check for a cell wall, chloroplasts, or nucleus to classify the cell type.
3. Examine organelles: Look for unique features such as cristae in mitochondria or grana in chloroplasts.
4. Use context clues: Consider the cell’s shape, size, and arrangement to aid identification.