Cell Organelles: Mitochondria and Ribosomes

Ribosomal Subunits and Ribosomes

• Ribosomal subunits are synthesized in the nucleolus of the nucleus.
• The main function of ribosomes is to facilitate protein synthesis by translating messenger RNA (mRNA) into polypeptide chains.

Energy and Apoptosis

• The mitochondrion is referred to as the 'powerhouse' of the cell due to its role in ATP production through cellular respiration.
• Mitochondria play a key role in apoptosis by releasing cytochrome c, which activates the apoptotic pathway.

Cellular Structures and Functions

• Ribosomal subunits pass from the nucleus to the cytoplasm after being formed, where they assemble with mRNA and tRNA to synthesize proteins.
• The endoplasmic reticulum (ER) is an interconnected network of tubules, vesicles, and cisternae involved in protein and lipid synthesis.
• The plasma membrane functions as a protective barrier that regulates the movement of substances in and out of the cell.

Cell Membrane Composition

• Phospholipids make up approximately 45% of the cell membrane structure.
• The cell membrane is primarily composed of a phospholipid bilayer, along with proteins, cholesterol, and carbohydrates.

Phospholipids

• The hydrophilic (water-attracting) part of a phospholipid molecule is the phosphate group.
• The hydrophobic (water-repelling) tail of the membrane is made up of fatty acid chains.

Organelle Functions

• Organelles in a cell serve specific functions, including energy production, protein synthesis, and waste disposal.
• Lysosomes function as the 'digestive system of the cell,' breaking down waste material and cellular debris.

Lysosomes and Cellular Maintenance

• Cells with high metabolic activity, such as macrophages and certain glandular cells, are likely to have abundant lysosomes.
• During autophagy, lysosomes help break down damaged organelles, misfolded proteins, and other cellular components.
• Lysosomes are sometimes referred to as 'cellular garbage disposals' because they digest and remove waste from the cell.

Membrane Components and Proteins

• Cholesterol maintains the fluidity and stability of the plasma membrane.
• Integral membrane proteins extend through the entire thickness of the membrane, while peripheral proteins are attached to the surface.

Membrane Structure and Function

• The hydrophobic tail of membrane phospholipids prevents the free passage of water-soluble substances.
• Membrane proteins do not directly perform energy production; their roles include transport, signaling, and structural support.
• Peripheral proteins differ from integral proteins as they are not embedded within the lipid bilayer.

Role of Linker Proteins

• Linker proteins in the cell membrane connect cytoskeletal elements to membrane proteins, contributing to cell shape and structure.

Specialized Cellular Structures

• The rough endoplasmic reticulum (RER) is prominent in cells that specialize in protein secretion, characterized by ribosomes on its surface.
• The smooth endoplasmic reticulum (SER) in liver cells functions in the detoxification of drugs and the metabolism of carbohydrates and lipids.
• The Golgi apparatus is essential for modifying, sorting, and packaging proteins for secretion in secretory cells.

Calcium Management in Muscle Cells

• In muscle cells, the sarcoplasmic reticulum is responsible for the release and recapture of calcium ions, crucial for muscle contraction and relaxation.

Structural Differences in ER

• Rough endoplasmic reticulum (RER) has ribosomes attached to its cytoplasmic surface, making it appear 'rough' under a microscope, while smooth endoplasmic reticulum (SER) lacks these ribosomes and appears smooth.

Learn about the vital functions of mitochondria as the 'powerhouses' of the cell and the role of ribosomes in protein synthesis. Understand how mitochondria play a key role in apoptosis, while ribosomes are essential for creating proteins.