Energy storage in the cell

All biochemical events taking place in the cell are dynamic, i.e. the events by which energy is obtained and consumed. As a living organism, the cell has the ability to manage its own energy, store it or use it again from the stored reserves.

Metabolism
Metabolism is a concept that includes all the enzymatic reactions controlled by a living organism. Metabolism can be further divided into catabolic reactions, defined as reactions that involve breaking down larger molecules into smaller structures, and anabolic reactions, whose task is to build more complex substances from smaller blocks for a certain purpose. Catabolic reactions result in energy gain while anabolic reactions consume energy.

In terms of energy gain, organisms are divided into two large groups:


 * Phototrophic organisms: obtain chemical energy from light (mostly includes plants).
 * Organotrophic organisms: obtain energy from chemical reactions (eg animals).

There is a symbiotic relationship between these two groups of organisms, which could be referred to as the basic concept of metabolism and bioenergetics.

Phototrophic organisms use light energy to construct sugars and oxygen from molecules of water and carbon dioxide. This in turn serves heterotrophic organisms, which oxidize sugars with the help of oxygen through the process of internal respiration, leading to the production of energy. Every living object constantly requires a supply of energy from its surroundings for vital processes. From a bioenergetics perspective, the essence of these processes is the transformation of one type of energy into another. There is a state of balance between the supply and demand of energy.

The balance between anabolic and catabolic processes can vary in both directions over time, and the following cases can occur:


 * The organism consumes energy or there is a demand for an energy source in the organism. Energy is produced as catabolic processes prevail over anabolic ones
 * The organism's energy intake is greater than its output - energy accumulates and the supply is greater than the demand. The organism has to somehow deal with such an amount of excess, in other words - it has to store it (meaning anabolic processes will prevail over catabolic one).

The essence of energy storage in the cell
This concept can be illustrated nicely using the example of food processed by the organism after intake.

The sugars, or glucose, is brought to the liver. About half of the substance is converted into glycogen while the rest passes through the liver into the blood. Glucose enters erythrocytes and the CNS. This is done without the need for insulin and glucose is then further metabolized there to produce energy. The remaining glucose also enters skeletal muscle cells, although it requires the aid of insulin in this case. Here it is either consumed or stored in the form of glycogen as an energy reserve. After the liver's capacity for glycogen synthesis is exceeded, glucose is converted into triacylglycerols and stored as an energy reserve in adipose tissue.

Amino acids (created by the breakdown of proteins) are also transported by the blood to the liver and other tissues. In the liver and other tissues, they serve as basic building blocks for protein synthesis, with others being transported into the blood by binding to specific carriers. There is no organ in the organism that serves as a reservoir of amino acids or nitrogen. Therefore, the excess of amino acids is mainly broken down into glucose, and the nitrogenous residue is excreted as urea.

Lipids enter the blood as chylomicrons via the lymphatic system. When associated with specific proteins, lipids are transported by the blood and absorbed by tissues for metabolic needs or transported to fat tissue, where they are stored.

Metabolismus živin po jídle
Jestliže se po jídle oddáme zažívání vleže a v klidu, potom se největší část živin převedených do organismu přemění na zásobní tuky. Zejména nastává po překročení kapacity pro syntézu glykogenu (v játrech a svalech). Další nadbytečná glukoza (ať již přijatá v čisté formě glukozy nebo látky přeměněné na glukozu sekundárně) se ukládá ve formě triacylglycerolů do tukové tkáně. Děje se to proto, že v buňkách převládá koncentrace ATP nad ADP (nekoná-li se práce, nespotřebovává se ATP) a jak proces glykolýzy, tak proces aerobní fosforylace jsou za tohoto stavu velmi zpomaleny.

Jestliže se však po jídle začne vyvíjet pracovní (motorická) aktivita, pak se způsob přeměny živin podstatně mění. Nastane především využívání živin jako zdroje energie. Tyto zásoby primárně využitelných živin se spotřebují a teprve potom může dojít k odbourávání zásob energie, která si buňka uložila do zásoby především tuků – tedy zásobních triacylglycerolů z tukové tkáně.

Použitá literatura

 * Biochemie v obrazech a schématech; Prof. MUDr. RNDr. Jan Musil, DrSc., Avicenum 1990


 * Harperova biochemie; Murray K. Robert, Daryl K. Granner; nakl. HaH, 2002