B2.2 - Organelles and Compartmentalization

• Organelles are separate subunits in cells, and include the nuclei, chloroplasts, mitochondria, vesicles, ribosomes, and the plasma membrane
• The only organelles common in both eukaryotic and prokaryotic cells are the cell membrane and ribosomes
• The cell wall, cytoskeleton, and cytoplasm are not considered organelles
• The nuclear membrane keeps DNA separate from other parts of the cell, which protects it from many cellular reactions that occur in the cytoplasm
• Separating the nucleus and cytoplasm allows gene transcription and translation to be kept separate
• The sections of genes that carry meaningful info (code for amino acids) are called exons. The non coding sequences that intervene interruptions are called introns
• Some eukaryotic genes contain no introns at all.
• An enzyme catalyzed reaction removed introns as soon as the mRNA has been formed. The production of this enzyme is also under the cntrol of a gene. The short lengths of 'nonsense; transcribed into the RNA sequence bases are removed and this is known as RNA splicing.
• The new, sliced mRNA is what passes out of the nucleus and into the cytoplasm, to go to the ribosomes where it is involved in protein synthesis.
• Variants of a protein can be produced from a single gene, due to gene splicing, which is an advantage in eukaryotes
• Because prokaryotes don't have a nucleus, the mRNA immediately meets the ribosomes and therefore prokaryotes do not have introns or enzymes to carry out splicing.
• The nuclear pores in the nuclear envelope control the exit of mRNA from the nucleus and the entry and exit of proteins.
• Nuclear pores are made of specific proteins forming a specific structure. The pores are tiny, but there are many of them so they make up about 1/3 of the nuclear membrane's surface area.
• Because of nuclear pores, it suggests that communication between the nuclear and the cytoplasm is important. The selective movement of proteins and RNA through the nuclear pores not only establishes the internal composition of the nucleus, but also plays a crucial role in regulating eukaryotic gene expression.
• Nuclear pores transport proteins in their folded form, which differentiates it from other forms of protein transport, as proteins must unfold to cross the membranes of the mitochondria and chloroplasts
• RNA molecules that are synthesized in the nucleus must be exported to the cytoplasm efficiently, where they function in protein synthesis.
• Ribosomal RNA is assembled with ribosomal proteins in the nucleolus, and ribosomal subunits are then transported into the cytoplasm.
• Export of RNA through nuclear pore complexes is an active process that requires energy.
• Each organelle has a different function, carrying out a specific biological process and a different set of chemical reactions.
• Biochemical reactions within organelles are what sustain life.
• Compartmentalization (the division of the interior cell into separate, discrete areas) allows the correct concentration of metabolites to be present for specific metabolic processed and the appropriate enzymes to be present. Incompatible biochemical processes can be separated.
• Lysosomes are vesicles bound by a single membrane. They contain digestive enzymes in an acidic environment, which are only active in the lysosome's acidic interior. The pH of the cell is neutral to slightly alkaline, so the enzymes acid dependent activity protects the cell from self - digestion in case of a leakage.
• Phagocytosis is a process used by specific cells to capture and ingest foreign particles.
• Phagocytic white blood cells engulf 'foreign' material, such as bacteria, then digest and destroy it
• Steps of Phagocytosis

  1. recognition of the foreign particles by receptors, ingestion of the foreign particle in a phagocytic vacuole.
  2. fusion of the phagocytic vacuole with a lysosome, creating a phagolysosome
  3. final destruction of the ingested particle
• The phagocytic vacuole is a temporary organelle formed by the process of phagocytosis, from the outer membrane phagocyte. The fusion with the lysosome provides the digestive enzymes to break down and destroy the foreign particle.
• Respiration begins in the cytoplasm, in a process called glycolysis. The process is anaerobic and results in the production of pyruvate. This molecules diffuses into the mitochondria where aerobic respiration takes place.
• The mitochondria are formed from two membranes, an inner and outer membrane.
• The outer membrane of mitochondria contains transport proteins, which move pyruvate into the mitochondrion. In the mitochondrion, carbon is removed from pyruvate and this process is known as decarboxylation. Hydrogen is also removed by an oxidation reaction.
• Electrons pass down a sequence of proteins in the inner membrane of the mitochondria, know as the electron transport train (ETC), releasing energy that is used to pump the protons into the space between the two membranes (called the intermembrane space)
• The inner membrane of the mitochondria contains a transmembrane multiprotein assembly, which generates ATP.
• The inner membrane of mitochondria is highly folded into structures called cristae, which increases the surface area for ETC and ATP synthase, increasing the production of ATP
• The process where hydrogen and carbon atoms are removed from the molecules of glucose in a sequence of enzyme controlled stages is called the Krebs cycle and it occurs in the matrix of the mitochondria, bc it has the pH and the right enzymes.
• Chloroplasts are the organelles where the reactions of photosynthesis occur.
• Chloroplasts contain photosynthetic pigments (chlorophyll), along with the enzymes and electron transport proteins for the reduction of CO2 to glucose and for ATP formation using light energy.
• There is a double membrane around the chloroplast and a third inner membrane that folds extensively at various points to form a system of branching membranes. These membranes are called thylakoids.
• Thylakoid membranes are organized into flat, compact, circular piles called grana (singular granum), almost like stack of coins.
• Between the grana in a chloroplast, there are loosely arranged tubular membranes suspended in water stroma.
• Separate grana are connected to lamella.
• Chlorophyll exists in the grana. They are grouped together in structures called photosystems, help in the thylakoid membranes of the grana by proteins.