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Adenosine Diphosphate: Adenosine 5'-(trihydrogen diphosphate). An adenine nucleotide containing two phosphate groups esterified to the sugar moiety at the 5'-position.

Cross-bridge Cycle

JoVE 10870

As muscle contracts, the overlap between the thin and thick filaments increases, decreasing the length of the sarcomere—the contractile unit of the muscle—using energy in the form of ATP. At the molecular level, this is a cyclic, multistep process that involves binding and hydrolysis of ATP, and movement of actin by myosin.

When ATP, that is attached to the myosin head, is hydrolyzed to ADP, myosin moves into a high energy state bound to actin, creating a cross-bridge. When ADP is released, the myosin head moves to a low energy state, moving actin toward the center of the sarcomere. Binding of a new ATP molecule dissociates myosin from actin. When this ATP is hydrolyzed, the myosin head will bind to actin, this time on a portion of actin closer to the end of the sarcomere. Regulatory proteins troponin and tropomyosin, along with calcium, work together to control the myosin-actin interaction. When troponin binds to calcium, tropomyosin is moved away from the myosin-binding site on actin, allowing myosin and actin to interact and muscle contraction to occur. As a regulator of muscle contraction, calcium concentration is very closely controlled in muscle fibers. Muscle fibers are in close contact with motor neurons. Action potentials in motor neurons cause the release of the neurotransmitter acetylcholine in the vicinity of muscle fibers. This ge

 Core: Biology


JoVE 10743

Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives many processes in living cells. Oxidative phosphorylation involves two processes—electron transport and chemiosmosis. During electron transport, electrons are shuttled between large complexes on the inner mitochondrial membrane and protons (H+) are pumped across the membrane into the intermembrane space, creating an electrochemical gradient. In the next step, protons flow back down their gradient into the mitochondrial matrix via ATP synthase, a protein complex embedded within the inner membrane. This process, called chemiosmosis, uses the energy of the proton gradient to drive the synthesis of ATP from adenosine diphosphate (ADP). The electron transport chain is a series of complexes that transfer electrons from electron donors to electron acceptors via simultaneous reduction and oxidation reactions, otherwise known as redox reactions. At the end of the chain, electrons reduce molecular oxygen to produce water. The shuttling of electrons between complexes is coupled with proton transfer, whereby protons (H+ ions) travel from the mitochondrial matrix to the intermembrane space against their concentration gradient. Eventually, the high concentration of protons in the interm

 Core: Biology

Energy-releasing Steps of Glycolysis

JoVE 10739

While the first phase of glycolysis consumes energy to convert glucose to glyceraldehyde 3-phosphate (G3P), the second phase produces energy. The energy is released over a sequence of reactions that turns G3P into pyruvate. The energy-releasing phase—steps 6-10 of glycolysis—occurs twice, once for each of the two 3-carbon sugars produced during steps 1-5.

The first energy-releasing step—considered the 6th step of glycolysis overall—consists of two concurrent events: oxidation and phosphorylation of G3P. The electron carrier NAD+ removes one hydrogen from G3P, oxidizing the 3-carbon sugar and converting (reducing) NAD+ to form NADH and H+. The released energy is used to phosphorylate G3P, turning it into 1,3-bisphosphoglycerate. In the next step, 1,3-bisphosphoglycerate converts ADP to ATP by donating a phosphate group, thereby becoming 3-phosphoglycerate. The 3-phosphoglycerate is then converted into an isomer, 2-phosphoglycerate. Subsequently, 2-phosphoglycerate loses a water molecule, becoming the unstable molecule 2-phosphoenolpyruvate, or PEP. PEP easily loses its phosphate group to ADP, converting it into a second ATP molecule and becoming pyruvate in the process. The energy-releasing phase releases two molecules of ATP and one molecule of NADH per converted sugar. Because

 Core: Biology

Hydrolysis of ATP

JoVE 10732

The bonds of adenosine triphosphate (ATP) can be broken through the addition of water, releasing one or two phosphate groups in an exergonic process called hydrolysis. This reaction liberates the energy in the bonds for use in the cell—for instance, to synthesize proteins from amino acids.

If one phosphate group is removed, a molecule of ADP—adenosine diphosphate—remains, along with inorganic phosphate. ADP can be further hydrolyzed to AMP—adenosine monophosphate—by the removal of a second phosphate group. ATP consists of an adenine base, a ribose sugar, and three phosphate groups, with the latter attached to each other through high-energy phosphoanhydride bonds.

 Core: Biology

Photosystem II

JoVE 10751

Photosystem II is a multi-protein complex embedded within the thylakoid membrane where it harvests light energy. Chlorophyll molecules transfer energy to a specific pair of chlorophyll a molecules in the reaction center of Photosystem II. Here, the chlorophyll a molecules lose an electron (oxidation), transferring it to a primary electron acceptor. The donated electrons pass through the electron transport chain into Photosystem I. Splitting a water molecule releases one oxygen atom, two protons (H+) and two electrons. The electrons replace the donated electrons of the two chlorophyll a molecules in the reaction center. The oxygen atom immediately reacts with another oxygen atom, producing O2 that is released into the atmosphere. The protons accumulate and create a concentration gradient across the thylakoid membrane that drives ATP synthesis in a process called chemiosmosis. The multi-protein complex Photosystem II harvests photons and transfers energy through its bound pigments chlorophyll a and b, and carotenoids. Carotenoids have a protective function as they help dissipate the vast amount of energy taken in that could otherwise damage the plant tissue. Energy travels from chlorophyll molecule to chlorophyll molecule until it reaches a pair of specialized chlorophyll a molecules in a region called the re

 Core: Biology

The Calvin Cycle

JoVE 10753

Oxygenic photosynthesis converts approximately 200 billion tons of carbon dioxide (CO2) annually to organic compounds and produces approximately 140 billion tons of atmospheric oxygen (O2). Photosynthesis is the basis of all human food and oxygen needs.

The photosynthetic process can be divided into two sets of reactions that take place in different regions of plant chloroplasts: the light-dependent reaction and the light-independent or “dark” reactions. The light-dependent reaction takes place in the thylakoid membrane of the chloroplast. It converts light energy to chemical energy, stored as ATP and NADPH. This energy is then utilized in the stroma region of the chloroplast, to reduce atmospheric carbon dioxide into complex carbohydrates through the light-independent reactions of the Calvin-Benson cycle. The Calvin-Benson cycle represents the light-independent set of photosynthetic reactions. It uses the adenosine triphosphate (ATP) and nicotinamide-adenine dinucleotide phosphate (NADPH) generated during the light-dependent reactions to convert atmospheric CO2 into complex carbohydrates. The Calvin-Benson cycle also regenerates adenosine diphosphate (ADP) and NADP+ for the light-dependent reaction. At the start of the Calvin-Benson cycle, atmospheric CO2 enters the leaf throug

 Core: Biology

The Citric Acid Cycle

JoVE 10741

The citric acid cycle, also known as the Krebs cycle or TCA cycle, consists of several energy-generating reactions that yield one ATP molecule, three NADH molecules, one FADH2 molecule, and two CO2 molecules.

Acetyl CoA is the point-of-entry into the citric acid cycle, which occurs in the inner membrane (i.e., matrix) of mitochondria in eukaryotic cells or the cytoplasm of prokaryotic cells. Prior to the citric acid cycle, pyruvate oxidation produced two acetyl CoA molecules per glucose molecule. Hence, the citric acid cycle runs twice per glucose molecule. The citric acid cycle can be partitioned into eight steps, each yielding different molecules (italicized below). With the help of catalyzing enzymes, one acetyl CoA (2-carbon) reacts with oxaloacetic acid (4-carbon), forming the 6-carbon molecule citrate. Next, citrate is converted into one of its isomers, isocitrate, through a two-part process in which water is removed and added. The third step yields α-ketoglutarate (5-carbon) from oxidized isocitrate. This process releases CO2 and reduces NAD+ to NADH. The fourth step forms the unstable compound succinyl CoA from α-ketoglutarate, a process that also releases CO2 and reduces NAD+ to NADH. The fifth

 Core: Biology

What is Glycolysis?

JoVE 10737

Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts “food energy” (from the chemical bonds of macromolecules) into chemical energy in the form of adenosine triphosphate (ATP). The first step of this tightly regulated and intricate process is glycolysis. The word glycolysis originates from Latin glyco (sugar) and lysis (breakdown). Glycolysis serves two main intracellular functions: generate ATP and intermediate metabolites to feed into other pathways. The glycolytic pathway converts one hexose (six-carbon carbohydrate such as glucose), into two triose molecules (three-carbon carbohydrate) such as pyruvate, and a net of two molecules of ATP (four produced, two consumed) and two molecules of nicotinamide adenine dinucleotide (NADH). Did you know that glycolysis was the first biochemical pathway discovered? In the mid-1800s, Louis Pasteur determined that microorganisms cause the breakdown of glucose in the absence of oxygen (fermentation). In 1897, Eduard Buchner found that fermentation reactions can still be carried out in cell-free yeast extracts, achieved by breaking open the cell and collecting the cytoplasm which contains the soluble molecules and organelles. Shortly thereafter in 1905, Arthur Harden and William Young discovered that the rate of fermentation decreases wit

 Core: Biology

Cellular Respiration- Concept

JoVE 10567

Autotrophs and Heterotrophs

Living organisms require a continuous input of energy to maintain cellular and organismal functions such as growth, repair, movement, defense, and reproduction. Cells can only use chemical energy to fuel their functions, therefore they need to harvest energy from chemical bonds of biomolecules, such as sugars and lipids. Autotrophic organisms, namely…

 Lab Bio

Laser Microirradiation to Study In Vivo Cellular Responses to Simple and Complex DNA Damage

1Department of Biological Chemistry, School of Medicine, University of California, Irvine, 2Beckman Laser Institute and Medical Clinic, University of California, Irvine, 3Department of Developmental and Cell Biology, School of Biological Sciences, University of California, Irvine, 4Department of Biomedical Engineering and Surgery, University of California, Irvine

JoVE 56213

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