Cellular Structures: Endoplasmic Reticulum, Golgi, Lysosomes, and More

Posted on Jan 4, 2025 in Geology

Cellular Structures

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of membranes extending throughout the cytoplasm from the nuclear envelope to the cell periphery. It consists of flattened sacs (cisternae) and interconnected tubules.

Types of ER

  • Smooth ER: A network of smooth tubules without ribosomes. Functions include lipid synthesis, detoxification of toxic substances, and regulation of intracellular calcium levels.
  • Rough ER: Composed of flattened sacs with ribosomes attached to the cytosolic face of the membrane. Functions include synthesis of secreted proteins, glycosylation, and protein folding.

Golgi Apparatus

The Golgi apparatus consists of 5-8 flattened sacs (cisternae) with dilated ends, connected to each other. These cisternae have three parts:

  • Cis face: The part closest to the nucleus, receiving proteins and lipids from the ER.
  • Medial: Where metabolic activities occur.
  • Trans face: The part closest to the plasma membrane, where proteins and lipids are sorted for their final destinations.

Functions of the Golgi

The Golgi is involved in end-glycosylation of proteins, synthesis of glycolipids and sphingomyelin, synthesis of complex polysaccharides for the plant cell wall, and directing the distribution of proteins for export.

Lysosomes

Lysosomes are membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion of biological molecules. These enzymes are acid hydrolases, functioning optimally at an acidic pH (around 5). The lysosome maintains this pH through an ATP-dependent proton pump.

Classification of Lysosomes

  • Primary lysosomes: Newly formed vesicles from the Golgi, containing only enzymes.
  • Secondary lysosomes: Formed when primary lysosomes fuse with vesicles containing materials to be digested.
  • Residual bodies: Secondary lysosomes that contain undigested waste.

Peroxisomes

Peroxisomes are small vesicles containing enzymes called oxidases, which use oxygen to oxidize organic substrates, producing hydrogen peroxide (H2O2). Catalase then degrades the toxic H2O2.

Functions of Peroxisomes

Peroxisomes are involved in the synthesis of phospholipids, such as plasmalogens, and the oxidation of fatty acids.

Mitochondria

Mitochondria are organelles found in aerobic eukaryotic cells, responsible for cellular respiration. They have two membranes:

  • Outer membrane: Contains porins that form channels for the passage of small molecules.
  • Inner membrane: Folded into cristae, increasing its surface area. It contains many proteins and the lipid cardiolipin. The inner side contains F1F0 ATP synthase complexes.

The space between the membranes is similar to the cytosol, and the mitochondrial matrix contains the mitochondrial genome, a double helix of DNA.

Specific Structures in Plant Cells

Chloroplasts

Chloroplasts are plastids containing chlorophyll. They have an outer membrane with porins, an inner membrane with transporters, and thylakoids, which are stacked into grana. Chloroplasts have three compartments: the intermembrane space, the stroma, and the thylakoid space.

Cell Wall

The plant cell wall is composed of cellulose and can have three layers:

  • Middle lamella: The outermost layer, shared between adjacent cells, composed of proteins, pectin, and calcium.
  • Primary wall: A thicker layer composed of cellulose and hemicellulose.
  • Secondary wall: Present in some cells, can be impregnated with various substances.
Functions of the Cell Wall

The cell wall provides mechanical support, protection against osmotic effects, defense against pathogens, and participates in cell communication through plasmodesmata. It also directs cell growth.

Formation of the Cell Wall

The cell wall is formed by the deposition of materials. Proteins, pectins, and hemicellulose are secreted by the Golgi, while cellulose is synthesized at the plasma membrane by cellulose synthase enzymes.

Vacuoles

Vacuoles are membrane-bound compartments that store substances. Plant cells often have a large central vacuole, occupying up to 90% of the cytoplasm. The vacuole membrane, called the tonoplast, has active transport systems, including ion pumps and proton pumps, which maintain an acidic pH.

Functions of Vacuoles

Vacuoles are involved in nutrient storage, defense, pigment accumulation, and skeletal support.

Special Peroxisomes

Special peroxisomes are found in oilseed cells, where they transform sugars into fatty acids, and in leaves, where they participate in photorespiration.