Cell Organelles and Functions

Cell Organelles

• Cell organelles are the different compartments within a cell, each performing specific functions
• Examples include the Golgi apparatus, endoplasmic reticulum, nucleus, mitochondria, cytoplasmic membrane

Mitochondria

• Mitochondria are organelles considered the "powerhouse" of the cell
• They are found in both animal and plant cells (except red blood cells), but not in bacteria.
• The number of mitochondria varies depending on the cell's metabolic needs.
• Algae and fungi may have only one or a few mitochondria per cell.
• Liver cells contain between 500 - 1000 mitochondria.
• Mitochondria are located dispersed in the cytoplasm, concentrated in areas requiring the most energy.
• Mitochondria are often near muscle fibers.

Mitochondrial Structure

• Mitochondria have various shapes, including spherical, oval, rod-like and are elongated
• Typically 0.5 - 1 μm in diameter and 1 - 2 μm in length
• They are similar in size to a bacterium.
• They are double-membraned organelles
  • Outer membrane
  • Inner membrane
  • Intermembrane space
  • Cristae (folds in inner membrane)
  • Matrix (space within inner membrane)

Mitochondrial Function

• Mitochondria are the site of aerobic respiration.
• They use three main pathways to generate energy from glucose and other molecules:
  • Glycolysis
  • Krebs Cycle
  • Electron Transport Chain

Endoplasmic Reticulum (ER)

• The ER is a network of membranes within the cytoplasm
• It originates from the outer nuclear membrane.
• It is a critical part of the cell, acting as a warehouse and manufacturing unit.
• Eukaryotic cells have ER
  • Rough ER (RER) studded with ribosomes responsible for protein synthesis and folding
  • Smooth ER (SER) involved in lipid synthesis and calcium ion storage.

Rough Endoplasmic Reticulum (RER)

• RER are flattened, interconnected sacs continuous with the nuclear membrane.
• Found throughout the cell, but ribosome density is high near the nucleus and Golgi apparatus.
• Ribosomes are attached to the outer surface of the RER to synthesize proteins.

Smooth Endoplasmic Reticulum (SER)

• SER forms an interconnecting network of curved, tubular membranes throughout the cytoplasm.
• Provides an area for synthesizing different types of lipids, including steroid hormones
• Lipid composition in SER differs from other membranes, with a high abundance of phosphatidyl choline.
• Having a high concentration of unsaturated fatty acids makes the SER very fluid and disordered.

Functions of ER

• Folding and modification of proteins
• Synthesis of phospholipids and steroids
• Storing calcium ions, releasing them upon cellular signal.
• Carbohydrate metabolism

Golgi Apparatus

• The Golgi apparatus is a complex collection of membranes with a cis and trans face (polarity)
• Prominent in cells with secretion functions (e.g. epithelial cells).
• Modifies proteins and lipids previously synthesized in the ER.
• Produces vesicles that secrete molecules from the trans face (maturing face).

Golgi Apparatus—Vesicle Movement

• Vesicle fusion is the process where vesicles move molecules, lipids and proteins from the donor to the acceptor compartment.

Inside the Golgi Apparatus

• Modifies glycoproteins by replacing sugar groups.
• Modifies phospholipids' acylgroups and head groups.
• Produces vesicles to transport molecules for cell-surface secretion

Lysosomes

• Lysosomes are membrane-bound organelles involved in endocytosis, phagocytosis, and autophagy
• They consist of hydrolytic enzymes enclosed in a membrane.
• Enzymes operate at a pH of 4-5.
• Originate from the ER but are activated in the Golgi.
• Function as cellular garbage disposals, degrading and recycling cellular components.

Cytoskeleton

• The cytoskeleton, composed of fibrous proteins, gives the cell its shape and allows for movement of molecules.
• It includes three types of filaments:
  • Microfilaments
  • Intermediate filaments
  • Microtubules

Functions of the Cytoskeleton

• Maintain cell shape and provide structural support.
• Supporting cell movement.
• Cell division
• Involved in motility, transport of molecules within the cell, whole-cell movements, and signal transduction.

Exosomes

• Exosomes are vesicles released by cells to communicate with other cells.
• They are created in the multivesicular bodies and move through the cytoplasm before being released from the cell.

Summary

• Mitochondria are involved in generating energy; ER processes proteins and synthesizes specific lipids; Golgi apparatus modifies and sorts proteins and lipids.
• Cytoskeleton maintains cell shape and enables cell movement.
• Lysosomes are involved in cellular recycling and degradation.

Explores the roles of key cell organelles such as the mitochondria, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes. Covers their structure, function, and importance in cellular processes and waste management.