Which metabolic process is most efficient

Both Vitamin A and E are toxic at high dosages. Now that you understand how cells get energy from the sun photosynthesis by green plant cells or from food non-photosynthetic cells, including yours , let's discuss energy flow through a multi-celled organism, which is an assemblage of many cells working together.

Let's use your body as an example. In a multi-celled organism, cells are organized into tissues. Tissue are combined into organs and organs cooperate in "organ systems". One of these is the digestive system of your body.

The digestive system includes the digestive tract, a pathway through your body that is taken by the food you eat. The digestive tract includes mouth, esophagus, stomach, and intestines small and large. The liver and pancreas are part of the digestive system also. The liver stores blood sugar glucose as glycogen animal starch and produces bile salts for the digestion of fats in the small intestine.

The pancreas secretes digestive enzymes and bicarbonate buffer to neutralize stomach acid into the small intestine. The pancreas also releases insulin and glucagon into the blood.

These are two hormones that act to maintain a stable concentration of glucose in the blood. Adipose tissue can also be considered part of the digestive system. It stores fat for later conversion to glucose, if necessary. All of your cells need glucose and oxygen to perform aerobic respiration. The different cells and organs of your body coordinate to provide glucose and oxygen to all while taking into account the constraints of gathering and eating the food that provides the glucose. After eating, the different components of your food contribute to cell respiration in different ways:.

Starches and sugars are readily converted to glucose by enzymes in your mouth and stomach. The glucose is taken up by your blood in your intestine.

All of your body's cells can use the glucose to make ATP but some do other things with it. Nerve cells use only glucose for aerobic respiration. Unlike other cells, they cannot take up fatty acids from the blood as an alternative. Cells harness the energy of this proton gradient to create three additional ATP molecules for every electron that travels along the chain.

Overall, the combination of the citric acid cycle and oxidative phosphorylation yields much more energy than fermentation - 15 times as much energy per glucose molecule! Together, these processes that occur inside the mitochondion, the citric acid cycle and oxidative phosphorylation, are referred to as respiration , a term used for processes that couple the uptake of oxygen and the production of carbon dioxide Figure 6.

The electron transport chain in the mitochondrial membrane is not the only one that generates energy in living cells. In plant and other photosynthetic cells, chloroplasts also have an electron transport chain that harvests solar energy. Even though they do not contain mithcondria or chloroplatss, prokaryotes have other kinds of energy-yielding electron transport chains within their plasma membranes that also generate energy.

When energy is abundant, eukaryotic cells make larger, energy-rich molecules to store their excess energy. The resulting sugars and fats — in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are large enough to be visible in electron micrographs. Animal cells can also synthesize branched polymers of glucose known as glycogen , which in turn aggregate into particles that are observable via electron microscopy.

A cell can rapidly mobilize these particles whenever it needs quick energy. Athletes who "carbo-load" by eating pasta the night before a competition are trying to increase their glycogen reserves. Under normal circumstances, though, humans store just enough glycogen to provide a day's worth of energy.

Plant cells don't produce glycogen but instead make different glucose polymers known as starches , which they store in granules. In addition, both plant and animal cells store energy by shunting glucose into fat synthesis pathways. One gram of fat contains nearly six times the energy of the same amount of glycogen, but the energy from fat is less readily available than that from glycogen. Still, each storage mechanism is important because cells need both quick and long-term energy depots. Fats are stored in droplets in the cytoplasm; adipose cells are specialized for this type of storage because they contain unusually large fat droplets.

Humans generally store enough fat to supply their cells with several weeks' worth of energy Figure 7. Figure 7: Examples of energy storage within cells. A In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell G , surrounding the nucleus N.

B In this cross-section of a plant cell, starch granules st are present inside a chloroplast, near the thylakoid membranes striped pattern. C In this amoeba, a single celled organism, there is both starch storage compartments S , lipid storage L inside the cell, near the nucleus N.

Qian H. Letcher P. A Bamri-Ezzine, S. All rights reserved. This page appears in the following eBook. Aa Aa Aa. Cell Energy and Cell Functions. Figure 3: The release of energy from sugar. Compare the stepwise oxidation left with the direct burning of sugar right. Figure 5: An ATP molecule. ATP consists of an adenosine base blue , a ribose sugar pink and a phosphate chain. Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation.

Glycolysis takes place in the cytoplasm. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.

Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides starch and glycogen and lipids. Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Lynen did it become clear that the molecule acetyl-CoA donated its acetyl group to oxaloacetate.

Until this time, the TCA cycle was seen as a pathway to carbohydrate oxidation only. Most high school textbooks reflect this period of biochemistry knowledge and do not emphasize how the lipid and amino acid degradation pathways converge on the TCA cycle. The cell is depicted as a large blue oval. A smaller dark blue oval contained inside the cell represents the mitochondrion.

The mitochondrion has an outer mitochondrial membrane and within this membrane is a folded inner mitochondrial membrane that surrounds the mitochondrial matrix. The entry point for glucose is glycolysis, which occurs in the cytoplasm. Glycolysis converts glucose to pyruvate and synthesizes ATP. Pyruvate is transported from the cytoplasm into the mitochondrial matrix. In the TCA cycle, acetyl-CoA reacts with oxaloacetate and is converted to citrate, which is then converted to isocitrate.

Isocitrate is then converted to alpha-ketoglutarate with the release of CO 2. Then, alpha-ketoglutarate is converted to succinyl-CoA with the release of CO 2.

Succinyl-CoA is converted to succinate, which is converted to fumarate, and then to malate. Malate is converted to oxaloacetate.

The ETC is represented by a yellow rectangle along the inner mitochondrial membrane. Fatty acids are transported from the cytoplasm to the mitochondrial matrix, where they are converted to acyl-CoA. Amino acids are transported from the cytoplasm to the mitochondrial matrix. Then, the amino acids are broken down in transamination and deamination reactions. The products of these reactions include: pyruvate, acetyl-CoA, oxaloacetate, fumarate, alpha-ketoglutarate, and succinyl-CoA, which enter at specific points during the TCA cycle.

In the s, a series of experiments verified that the carbon atoms of fatty acids were the same ones that appeared in the acids of TCA cycle. Holmes, F. Lavoisier and the Chemistry of Life. Madison: University of Wisconsin Press, Krebs, H. Nobel Prize Lecture Kresge, N. Journal of Biological Chemistry , e18 Lusk, G. The Elements of the Science of Nutrition , 4th ed.

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