Cellular Organelles: Peroxisomes, Mitochondria, and Chloroplasts
Peroxisomes and Glyoxysomes
Peroxisomes: These organelles are similar to lysosomes but contain oxidase enzymes instead of hydrolases. Key enzymes include peroxidase and catalase. They facilitate oxidation reactions, similar to those in mitochondria, but the released energy is dissipated as heat. Peroxisomes also detoxify harmful substances.
Glyoxysomes: A specialized type of peroxisome found in plants, containing enzymes for the glyoxylic acid cycle. This cycle enables the synthesis of carbohydrates from lipids.
Mitochondria
Mitochondria: These are the powerhouses of the cell, where cellular respiration occurs. This process generates energy stored as ATP. Mitochondria are present in all eukaryotic cells. A group of mitochondria is called a chondriome.
Structure
Mitochondria vary in shape and size, typically ranging from 1-4 microns by 0.3-0.8 microns. They have a double membrane: an outer membrane (highly permeable) and an inner membrane (almost impermeable, with many proteins to regulate transport). The inner membrane folds into cristae, which house ATP synthase. The space between the membranes is the intermembrane space. The inner membrane’s lipid composition resembles that of bacterial membranes. The matrix, enclosed by the inner membrane, contains ribosomes similar to prokaryotes, circular double-helix DNA, and various enzymes.
Function
Cellular Respiration: The respiratory chain, located in the inner membrane, produces ATP. The Krebs cycle occurs in the mitochondrial matrix. Other functions include fatty acid oxidation, protein synthesis in ribosomes, and DNA replication and transcription.
Chloroplasts
Chloroplasts: Found only in plants, these organelles contain chlorophyll, enabling photosynthesis. This process converts light energy into chemical energy stored in ATP.
Structure
Chloroplasts vary in shape and are green due to chlorophyll. Algae and protozoa have few chloroplasts, while plants have 20-40. They are larger than mitochondria and have a double membrane: a highly permeable outer membrane and an almost impermeable inner membrane with permeases. These membranes lack chlorophyll and cholesterol. The stroma contains circular double-helix DNA, plastoribosomes (similar to bacterial ribosomes), and enzymes for CO2 reduction, DNA replication, transcription, and translation. The stroma also contains starch and lipid inclusions. Thylakoids, sac-like structures, are found within the stroma. They can be spread throughout the stroma or stacked into grana. Thylakoid membranes contain photosynthetic pigments and enzymes for light capture, electron transport, and ATP formation.
Function
Photosynthesis: Chloroplasts are responsible for carrying out photosynthesis.