Cell Structure, Organelles, and Microscopy: Study Notes for General Biology

Cell Structure and Microscopy

Microscopes and Viewing Cells

Microscopes are essential tools in biology for visualizing cells and their components. Understanding magnification, resolution, and contrast is key to interpreting microscopic images.

• Magnification: The process of enlarging the appearance of an object. It is expressed as a ratio (e.g., 125x means the image is 125 times larger than the actual object).

• Resolution: The ability to distinguish two close points as separate entities. Higher resolution allows for clearer, more detailed images.

• Contrast: The difference in light intensity between the object and the background, which helps in distinguishing structures.

• Types of Microscopy:

  • Light Microscopy (LM): Uses visible light to illuminate specimens. Suitable for viewing live cells and general cell structure.

  • Electron Microscopy (EM): Uses beams of electrons for much higher resolution. Includes Transmission EM (TEM) and Scanning EM (SEM).

• Example: Calculating the actual size of a cell from a micrograph: If a cell appears 125 mm long in an image with 125x magnification, the actual size is $\frac{125\,\text{mm}}{125} = 1\,\text{mm}$.

Comparing LM and EM

Light and electron microscopes differ in their principles, capabilities, and applications.

• LM: Lower resolution, can view live cells, uses glass lenses and light.

• EM: Much higher resolution, cannot view live cells, uses electromagnetic lenses and electron beams.

• Additional info: TEM provides internal cell details; SEM gives 3D surface images.

Cell Organelles and Their Functions

Genetic Information Pathways

Cells use various organelles to carry out genetic instructions and protein synthesis.

• Ribosomes: Sites of protein synthesis, translating mRNA into polypeptides.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

  • Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Pathway Example: DNA → mRNA (transcription in nucleus) → Ribosome (translation) → ER (modification) → Golgi (packaging) → Destination (e.g., cell membrane).

Protein Modification and Transport

Proteins synthesized in the ER may require further modification in the Golgi apparatus before becoming functional.

• Trans face of the Golgi: The side where modified proteins exit the Golgi apparatus.

• Additional info: Proteins destined for secretion are often glycosylated (addition of sugar groups) in the Golgi.

Energy Organelles: Mitochondria and Chloroplasts

Structure and Function

Mitochondria and chloroplasts are specialized organelles responsible for energy conversion in eukaryotic cells.

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP (energy currency of the cell).

• Chloroplasts: Found in plant cells; site of photosynthesis, converting light energy into chemical energy (glucose).

• Membrane Structure: Both have double membranes and their own DNA, supporting the endosymbiotic theory.

• Example: Chloroplasts contain thylakoid membranes where light-dependent reactions occur.

Cell Membrane and Cytoskeleton

Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model, which explains its structure and dynamic nature.

• Phospholipid Bilayer: Provides a semi-permeable barrier between the cell and its environment.

• Proteins: Embedded in the membrane, serving as channels, receptors, and enzymes.

• Cholesterol: Modulates membrane fluidity.

Microtubules and Cytoskeleton

The cytoskeleton provides structural support and facilitates movement within the cell.

• Microtubules: Hollow tubes made of tubulin proteins; involved in cell shape, transport, and division.

• Functions: Form the mitotic spindle, serve as tracks for organelle movement, and maintain cell structure.

• Composition: Primarily alpha and beta tubulin.

• Additional info: Other cytoskeletal elements include microfilaments (actin) and intermediate filaments.

Organelle Identification and Diagrams

Animal and Plant Cell Diagrams

Recognizing organelles and their functions is essential for understanding cell biology.

• Animal Cells: Lack cell walls and chloroplasts; have centrioles.

• Plant Cells: Have cell walls, chloroplasts, and large central vacuoles.

• Key Organelles: Nucleus, mitochondria, ER, Golgi apparatus, lysosomes, peroxisomes.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Function: Breakdown of macromolecules, cellular debris, and foreign substances.

• Example: Lysosomes fuse with vesicles containing material to be degraded.

Whole Membrane System Working Together

Coordination of Organelles

The endomembrane system includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles, working together for synthesis, modification, and transport of cellular products.

• Pathway: Proteins and lipids are synthesized in the ER, modified in the Golgi, and transported via vesicles.

• Importance: Ensures proper delivery and function of biomolecules within the cell.

Table: Comparison of LM and EM

Feature	Light Microscopy (LM)	Electron Microscopy (EM)
Source of Illumination	Visible light	Electron beam
Resolution	~200 nm	~0.1 nm
Can view live cells?	Yes	No
Image Type	Color (with stains)	Black and white
Applications	General cell structure	Detailed ultrastructure

Test Your Understanding

Review questions (e.g., #1-5, 10) help reinforce key concepts about cell structure, organelles, and microscopy.