Structure of Eukaryotic Cells

• Mitochondria are the primary sites of aerobic respiration, where chemical energy from organic molecules is converted into ATP (adenosine triphosphate). They feature a double membrane, with the inner membrane folded into cristae to maximize the surface area for electron transport chain enzymes.

• Chloroplasts, found in plants and algae, capture light energy to drive the synthesis of organic compounds through photosynthesis. They contain stacks of thylakoids called grana, which house chlorophyll, and a fluid-filled stroma containing enzymes for the Calvin cycle.

• Both organelles are semi-autonomous, possessing their own circular DNA and $70\text{S}$ ribosomes, which supports the endosymbiotic theory of their evolutionary origin from engulfed prokaryotes.

• The Endoplasmic Reticulum (ER) is an extensive network of membranes; the Rough ER is studded with ribosomes for protein synthesis and transport, while the Smooth ER lacks ribosomes and specializes in lipid synthesis and detoxification.

• The Golgi apparatus consists of flattened membrane sacs called cisternae that modify, sort, and package proteins and lipids received from the ER into vesicles. These vesicles then transport their cargo to specific cellular destinations or the cell surface for secretion.

• Lysosomes are specialized vesicles containing hydrolytic enzymes that function at an acidic pH. They are essential for digesting worn-out organelles, breaking down macromolecules, and destroying pathogens engulfed by the cell.

Plant vs. Animal Cells

• While both share many organelles, distinct structures define their functional capabilities and physical constraints.

• The cell wall provides mechanical strength and prevents osmotic lysis, while the large central vacuole maintains turgor pressure against the wall to support the plant's structure.

• Identify by Micrograph: Practice identifying organelles from electron micrographs; look for the double membrane and cristae of mitochondria versus the stacked cisternae of the Golgi apparatus.

• Link Structure to Function: If a question describes a cell with many mitochondria, conclude it has high energy demands (e.g., muscle cells); if it has extensive Rough ER, it is specialized for protein secretion (e.g., pancreatic cells).

• Check the Ribosomes: Remember that eukaryotic cells contain $80\text{S}$ ribosomes in the cytoplasm, but their mitochondria and chloroplasts contain $70\text{S}$ ribosomes, similar to prokaryotes.

• Avoid Generalizations: Never state that plant cells 'only' have chloroplasts; they also contain mitochondria for respiration, especially in non-photosynthetic tissues like roots.