Endomembrane System and Cellular Respiration: A Comprehensive Overview

Posted on Nov 13, 2024 in Biology

Endomembrane System

Functions

The endomembrane system participates in the synthesis of various macromolecules.

Components

Clathrin

Clathrin is assembled from protein subunits.

Coatomer

Coatomer consists of six protein chains and is formed from COP (Coat Proteins).

Endoplasmic Reticulum (ER)

The ER occupies 10% of the total cell volume.

Rough Endoplasmic Reticulum (RER)

The RER is covered with ribosomes concentrated in the core. Its membrane is thinner (7nm) compared to the plasma membrane, with 70% protein and 30% lipids. It contains less cholesterol, sphingomyelin, but more phosphatidylcholine.

• Function: Protein production, membrane formation, cell secretion.

Glycosylation

• Function: The majority of proteins synthesized in the RER incorporate a carbohydrate chain. This process involves adding a branched chain of 12 monosaccharides.

Smooth Endoplasmic Reticulum (REL)

The REL consists of interconnected tubules. Its membrane is similar to the RER but contains more cholesterol and sphingomyelin. It lacks ribosomes attached to its outer membrane.

• Functions:
  1. Protein synthesis
  2. Lipid synthesis and metabolism
  3. Calcium level regulation
  4. Detoxification

Golgi Apparatus

The Golgi apparatus has two sides:

• Cis (forming/proximal) face: Communicates with the ER.
• Trans (maturing/distal) face: Molecules exit the Golgi to different targets.

Lysosomes

Lysosomes originate from the Golgi apparatus and are membrane-bound sacs containing hydrolytic enzymes. They are not present in plant cells. Their membrane is rich in cholesterol and sphingomyelin.

• Functions:
  • Primary lysosomes: Do not participate in intracellular digestion. They can receive contents originating outside the cell.
  • Secondary lysosomes: Involved in molecule digestion. There are three types:
    • Endolysosome:
    • Phagolysosome: Digests phagosomes, defending the cell against pathogens or toxic substances.
    • Autophagolysosome: Involved in intracellular digestion, obtaining nutrients for the cell.

Peroxisomes

Peroxisomes are single-membrane organelles containing enzymes. The main enzyme is catalase. In plant cells, glyoxysomes are found.

• Function: Cells use H₂O₂ to oxidize toxic substances (detoxification). Catalase decomposes H₂O₂ into H₂O and O₂, which are harmless.

Vacuoles

Vacuoles originate from small vesicles formed by the Golgi apparatus. They are bounded by the tonoplast. Their pH is between 3 and 6. They are large in plant cells and small in animal cells.

• Function: Facilitate exchange with the external environment, regulate cell turgor, cell digestion, and accumulate substances (reserves).

Nucleic Acids

Nucleic acids are composed of C, H, O, N, and P. They contain hereditary material.

Nucleotides

Nucleotides are the monomers of nucleic acids. There are two types: DNA and RNA. Each is composed of a sugar, a nitrogenous base, and a phosphate group.

• Sugar + nitrogenous base = Nucleoside
• Nucleoside + phosphate group = Nucleotide

Nucleotides are molecules that store energy in phosphate bonds.

Cellular Messengers

Cyclic AMP (cAMP) is formed by specific enzymes and triggers different responses in each cell type.

DNA

DNA is the most important molecule in humans. In eukaryotes, it is located in the nucleus, with small amounts in mitochondria and chloroplasts. In prokaryotes, there can be other small DNA molecules called plasmids.

• Right-handed helix: The DNA strands coil to the right.
• Coaxial strands: The strands face each other and are linked by hydrogen bonds between bases.

RNA

RNA transcribes the genetic message from DNA and translates it into protein. It contains uracil instead of thymine. There are three types of RNA:

• Messenger RNA (mRNA): Consists of 300-500 ribonucleotides, representing 5% of total RNA. Carries genetic information from the nucleus to ribosomes.
• Transfer RNA (tRNA): Consists of 70-80 nucleotides, representing 15% of total RNA. Captures activated amino acids in the cytoplasm, transfers them to ribosomes, and places them according to the mRNA sequence.
• Ribosomal RNA (rRNA): Consists of 3000-5000 ribonucleotides, representing 80% of total RNA. Forms part of the ribosome structure.

When the temperature increases above 90°C, DNA strands separate (denaturation). This is reversible by keeping it at 64°C for several hours, allowing new hydrogen bonds to form (renaturation).

Mitochondria

Mitochondria are the site of cellular respiration, where food molecules are degraded to produce energy (ATP). They were described by Altman in 1884. They produce 95% of the cell’s ATP. Their number ranges from 1,000 to 300,000.

Structure and Chemical Composition

Mitochondria are mobile and plastic organelles with a diameter of 0.5 to 1 μm. They are surrounded by two membranes:

• Outer membrane: Contains 60% protein and 40% lipid. It is permeable due to porin channels.
• Inner membrane: Highly folded to form cristae. It delimits the mitochondrial matrix. It contains 80% protein and 20% lipid, no cholesterol, and cardiolipin, which makes it impermeable to ions. Contains three types of proteins:
  1. Proteins involved in oxidation and reduction reactions in the respiratory chain.
  2. ATP synthase complex.
  3. Transport proteins.

Mitochondrial matrix: Contains circular double-stranded DNA, RNA, and some proteins synthesized by the inner membrane.

Function

• Oxidative respiration: The goal is ATP production.
• Anaerobic pathways:
  • Alcoholic fermentation
  • Lactic acid fermentation

These pathways regenerate NAD+, allowing glycolysis to continue and produce a small amount of ATP.

NADH and NADPH

These are electron carriers that transport high-energy electrons and hydrogen atoms to participate in redox reactions. The most important is NAD+.

Aerobic Respiration

Releases CO₂ and H₂O in two steps:

• Krebs cycle: Occurs in the mitochondrial matrix. Pyruvic acid is oxidized before entering the cycle.
• Electron transport chain and oxidative phosphorylation: Involves cytochrome electron carriers.