Metabolic Pathways & Their Control

Cell Metabolism

Cells in a living organism carry out a never-ending sequence of chemical reactions and at any given time, all of the pathways that are a part of these reactions must be balanced. To achieve this, cells organise reactions into metabolic pathways. There are two types of metabolic pathway: anabolic and catabolic. Anabolic reactions build complex molecules from simpler ones and require energy (e.g. phosphorylation), whereas catabolic reactions break down complex molecules into smaller, simpler ones and require energy.

b40072cf-51e9-4057-8844-2288d289f4de.gif (image/gif)

Caption: : Each stage in a metabolic pathway is controlled by an enzyme, with the product of one reaction becoming the substrate of the next. (e.g. Product D can only be produced if enzymes 1, 2 and 3 are present)

Membranes

Cell membranes are to separate the internal contents from the external environment of a cell and controls both the absorption and secretion of substances through selective communication. Membranes form compartments to localise the metabolic activity of a cell. The high surface to volume ratio of these compartments allows high concentrations and high reaction rates.Structures found within cells known as organelles are bound by membranes, some organelles also have internal membranes.

membranes separate contents from environment membranes control absorption and secretion high concentrations and high reaction rates in membranes' compartments due to high surface to volume ratio

Plasma Membrane

The plasma membrane is made up of two components: phospholipid molecules and proteins. The fluid mosaic model proposes that the plasma membrane consists of a fluid phospholipid bilayer (with a hydrophillic head and hydrophobic tail) which are constantly moving. The phospholipids form a boundary around the cell in which protein molecules are embedded; hence forming a mosaic appearance.This membrane's function is secretion and absorption. 

comprised of proteins and phospholipid molecules fluid bilayer made of phospholipids form a boundary for control of absorption/secretion

Na/K Pump

The Sodium/Potassium pump is an active transport mechanism in which an ATP molecule provides energy for a sodium ion to bind to a protein channel; causing the channel to change shape and open to the outside. One phosphate group from ATP remains bound with the channel and sodium ions are released outside.The new channel shape has a high affinity for potassium ions and hence two bind to the channel. This causes a shape change again and so the channel now attract sodium ions and a new cycle is started.

b9cee0fd-2789-4d2e-8543-be9ef67c2037 (image/jpeg)

Activation Energy

Regardless of the reaction being anabolic or catabolic, energy is required to initially break the bonds in the reactions to form an unstable compound (called activation energy). When bonds are made to form the product, energy is released. Catalysts lower the activation energy required for a reaction to be carried out. 

48361b17-a124-4970-9375-c093f642ae57.gif (image/gif)

Enzymes

When an enzyme meets a substrate, it may not be an exact physical match; its shape may vary slightly from the active site. As the two start to bind, the active site moulds around the substrate until they fit - called induced fit.When the reaction involves two or more substrates, the shape of the active site determines the orientation of the reactants. This ensures that they are held together in such a way that the reaction can proceed. First, the active sites holds the two reactants closely in an induced fit, then it acts on them to weaken chemical bonds that must be broken during a catabolic reaction. This process reduces the activation energy needed by the reaction to reach transition state. Once the reaction has taken place, the products have a low affinity for the active site and are released.

substrate and active site do not perfectly fit induced fit allows active site to mould around substrate if more than one substrate is used, they are arranged by the shape of the active site held in induced fit, then bonds are 

Control by Regulation

There are two kinds of enzyme inhibitor: competitive or non-competitive.Competitive inhibitors are similar in shape to the normal substrate and compete with it for the active site of the enzyme. If the active site is blocked by this inhibitor then the enzyme cannot work and reaction rate is reduced. This can be reversed by increasing concentration of substrate.Non-competitive inhibitors inhibit enzymes by combining with the allosteric site. This results in the shape of the active site being changed so the normal substrate no longer fits. The enzyme can no longer perform its action and is denatured. This cannot be reversed by increasing substrate concentration.

0e25d217-4791-4cbf-99b3-b80a256b90c7 (image/png)