Cell Biology 1. Explain the Warburg effect and why researchers think that cancer

Question

Cell Biology

1. Explain the Warburg effect and why researchers think that cancer cells use this form of glucose metabolism.

The following questions are in regard to the importation of proteins that are nuclear encoded and synthesized to completion on cytosolic ribosomes but function within mitochondria.

2a)Explain in detail the process by which a protein is imported from the cytosol into the mitochondrial matrix.

2b) Explain in detail the process by which a protein is imported from the cytosol and ends up in the mitochondrial inner membrane.

3.) Explain how photosynthetic pigments are thought to function in energy transfer, including the three possible ways in which light energy absorbed by pigments can be handled by a photosystem. Indicate the two main types of these pigments found in higher plants.

Explanation / Answer

Otto Warburg postulated that the most common cause of cancer is that most cancer cells prefer to produce energy by a high rate of glycolysis in the cytosol, rather than by a comparatively low rate of glycolysis in mitochondria as in most normal cells.

This is a consequence of one of the following:

It is now accepted that glycolysis provides cancer cells with the most abundant extracellular nutrient, glucose, to make ample ATP metabolic intermediates, such as ribose sugars, glycerol and citrate, nonessential amino acids, and the oxidative pentose phosphate pathway, which serve as building blocks for cancer cells and hence cancer cells mainly use glycolysis for energy production.

Once proteins are released from the ribosomes in to the cytosol, they are immediately taken up by the mitochondria. Hence, mitochondrial proteins are first fully synthesized as precursor proteins in the cytosol and then translocated into mitochondria. These precursor proteins have a signal sequence at their N terminus that is rapidly removed in the mitochondrial matrix. These signal sequences help in recognition and import of the proteins These sequences have the tendency to fold into an amphipathic helix. This configuration is then recognized by specific receptor proteins that initiate protein translocation.

B. Explain in detail the process by which a protein is imported from the cytosol and ends up in the mitochondrial inner membrane

Multi-subunit protein complexes that function as protein translocators play an important role in protein translocation across mitochondrial membranes. These complexes contain some components that act as receptors for mitochondrial precursor proteins and other components that form the translocation channel. The TOM complex imports all nucleus-encoded mitochondrial proteins. The TIM23 complex then distributes the different proteins by transporting some of these proteins into the matrix space, while helping to insert transmembrane proteins into the inner membrane. The TIM22 complex mediates the insertion of a subclass of inner membrane proteins, including the carrier protein that transports ADP, ATP, and phosphate. Insertion of inner membrane proteins that are synthesized within the mitochondria is mediated by a third protein translocator in the inner mitochondrial membrane, the OXA complex.

There are several proteins, pigments and other co-factors assembled together in a photosynthetic reaction centre. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along a series of protein-bound co-factors. The most important co-factors are light-absorbing molecules (pigments) such as chlorophyll and phaeophytin. The energy of the photon is used to promote an electron to a higher molecular energy level of a pigment. The free energy created is then used to reduce a chain of nearby electron acceptors, which have subsequently higher redox-potentials. These electron transfer steps are the initial phase of a series of energy conversion reactions, ultimately resulting in the production of chemical energy during photosynthesis.

The three ways in which light energy can be handled by a photosystem are:

The two main types of pigments found in higher plants are chlorophylls (a and b) and carotenoids.

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