Cellular Respiration The importance of ATP Glycolysis The link reaction The Kreb’s cycle The electron transport chain

What is respiration? The gradual release of energy in a number of small steps from the breakdown of glucose Enzymes are required for the breakdown of glucose forming a metabolic pathway Small quantities of energy are released at each stage which are carried by ATP, ‘the universal energy currency in living organisms’

All cells need energy Cells need energy to undertake a variety of metabolic functions: Active transport Secretion Synthesis of large molecules from smaller ones e.g. making proteins from amino acids Replication e.g. DNA, organelles Muscle contraction

The structure of ATP Phosphates ATP is the energy currency of cells. It is a nucleotide with high energy bonds between the terminal phosphate groups.

How does ATP function? Energy is stored in the bond between the second and third phosphate group. When the bond is broken, energy is released and ADP is formed. Adenine Ribose ATP ADP + iP kJmol -1

How ATP is made and used Energy is released when ATP is broken down (exergonic reaction). The hydrolysis is catalysed by an enzyme called ATP ase. The hydrolysis liberates 30.66kJmol -1 of free energy. Energy is needed to combine ADP and iP to form ATP (endergonic reaction). The addition of phosphate to ADP is called phosphorylation (making ATP) There are two kinds of phosphorylation: Oxidative phosphorylation (respiration) Photophosphorylation. (photosynthesis)

Occurs in the cells and uses an enzyme ATPase and water Occurs in the mitochondria and uses an enzyme ATP synthetase, water is made Energy released is used in anabolic reactions Energy made from catabolic reactions. e.g respiration Exergonic reaction Endergonic reaction

ATP is a means of transferring free energy from respiration to cells. It is known as the energy currency of the cell

Advantages of ATP as an intermediate energy carrier Only one enzyme is required to release energy whilst many are required for the release of energy from glucose. It releases energy in small amounts unlike glucose – so it can be released only when required. A common source of energy that increases efficiency and control by the cell.

ATP is not stored but synthesised as required. The rate of synthesis keeps pace with demand. A metabolically active cell may require 2.0 X 10 6 molecules of ATP per second!

There are four main stages in aerobic respiration Glycolysis (cytoplasm) Link reaction (mitochondrial matrix) Krebs cycle (mitochondrial matrix) Electron transport chain (stalked particles on the cristae of mitochondria)

Where does cellular respiration occur? Mitochondrion Golgi apparatus Cytoplasm Nucleolus Cytoskeleton Lysosome Cell Membrane Chromatin Nuclear envelope Centrioles VacuoleRough ER Smooth ER

The organelle of aerobic respiration A site separate from the rest of the cytoplasm where enzyme- catalysed reactions take place to convert ADP and iP into ATP.

How are mitochondria adapted for respiration? Double membrane with inner membrane highly folded to form cristae This arrangement provides a large surface area for chemical reactions to take place The cristae are lined with stalked particles with contain the enzyme ATP synthetase

Glycolysis Literally means “sugar breakdown”. Occurs in the cytoplasm of ALL cells. Hexose sugars (such as glucose) are the usual substrates. Net production of 2 X ATP molecules. The end product is pyruvic acid (pyruvate) and two molecules of reduced NAD

Two molecules of triose phosphate Dehydrogenation

Glucose (6C hexose) is phosphorylated to make it bigger and more reactive. The phosphate is obtained from ATP. glucose biphosphate (6C hexose biphosphate) is formed. The 6C glucose biphosphate is cleaved to form two triose phosphates (3C). Each has one phosphate group from the glucose biphosphate. Each triose phosphate is converted to pyruvic acid (3C) (pyruvate) In the final stage enough energy is made to yield four molecules of ATP. This is called substrate level phosphorylation. The breakdown of one molecule of glucose yields two molecules of ATP, two molecules of reduced NAD and two molecules of pyruvate

The link reaction This is the link between Glycolysis (in the cytoplasm) and Kreb’s Cycle (in the matrix of the mitochondrion). The two pyruvate molecules produced from glycolysis diffuse from the cytoplasm to the mitochondrial matrix Here the two molecules of pyruvate are converted to two molecules of 2-carbon acetate with the formation of two molecules of reduced NAD and the loss of two molecules of carbon dioxide The acetate combines with coenzyme A to from acetyl coenzyme A

The link reaction Pyruvate (3C) Acetyl C o. A (2C) 2HCO 2 NAD + NADH.H + Dehydrogenation Decarboxylation

Kreb’s Cycle The function of the Kreb’s cycle is to oxidise the acetyl group of the acetyl CoA to two molecules of carbon dioxide. Produce one molecule of ATP NAD and FAD remove hydrogen atoms and deliver hydrogen to the electron transport system (next stage)

Stage of the kreb’s cycle The stages involve decarboxylation-removal of carbon as carbon dioxide. The stages involve dehydrogenation-removal of hydrogen atoms which are collected by carriers NAD and FAD which are reduced to NADH/H+ and FADH 2 For each turn of the cycle the overall production is: One ATP Three reduced NAD One reduced FAD Two molecules of carbon dioxide

The electron transport chain At the end of the kreb’s cycle we are left with reduced NAD and reduced FAD which are carrying hydrogen atoms These enter the electron transport chain which is on cristae of the mitochondria The Hydrogen will be used to make lots of ATP

Matrix Intermembrane space

The electron transport system The electron transport chain is a series of proton pumps and electron carriers Reduced NAD and reduced FAD donate hydrogen atoms. The carriers become re oxidised in the process (due to loss of hydrogen) and return to glycolysis, link reaction or the krebs cycle to collect more hydrogen The hydrogen atoms split into protons (H + ) and electrons. (occurs in the matrix) The electrons are transported along a series of carriers embedded in the inner mitochondrial membrane Each electron carrier is at a lower energy level that the one before As electrons flow along the electron transport chain, energy is released and used to pump the protons from the matrix into the intermembrane space using special protein pumps. This creates an electrochemical gradient with a build up of protons in the intermembrane space. Can also be called proton gradient and a pH gradient The electron carriers become reduced then oxidised as the electron is past along the chain, which are a series of redox reactions. Animation to watch

The chemiosmotic theory

The protons cannot flow back through the phospholipid membrane but can diffuse through special protein channels which are an ATP synthetase complex (facilitated diffusion) It is the flow of protons back into the matrix via the ATP synthase (stalked particles) which drives the synthesis of ATP The flow provides the energy to attach P to ADP These protons then recombine in the matrix with the electrons to from hydrogen atoms The hydrogen atoms combine with oxygen to form water This reaction is catalysed by the enzyme oxidase. Oxygen is therefore the final electron acceptor The chemiosmotic theory

How is ATP generated here?

How many ATP’s are formed? If NADH is initial acceptor, each pair of hydrogen atoms releases three molecules of ATP If FADH replaces NADH as the initial acceptor then only two molecules of ATP are produced

GlycolysisLink reactionKrebs 2NADH 2ATP 2NADH6NADH 2FADH 2ATP In total, respiration produces: 10 molecules of NADH in total produces 30 ATP’s 2 molecules of FAD in total produces 4 ATP’s 4 molecules of ATP are made by substrate level phosphorylation In total 38 molecules of ATP from one molecule of glucose

Respiration of proteins and lipids Proteins Amino acids 3C 5C 4C Lipids Fatty acids Glycerol as Triose Sugars in Glycolysis 2C Acetyl CoA-can be produced by respiration of carbohydrates, proteins and fats

Respiration of fats and proteins Fats and proteins can be used as respiratory substrates Fats are hydrolysed into fatty acids and glycercol Glycerol converted into 3C sugar (triose phosphate) which enters respiration Fatty acids are split into 2C’s and enter the Kreb’s cycle as acetyl coA (the length of the carbon chain determines the number of ATP produced because the hydrogen ions released are fed into the electron transport chain) fats carry a lot of energy Protein is only used during starvation Protein is hydrolysed to amino acids which are deaminated in the liver The amino group is converted to urea and excreted The other component is converted to a Kreb’s cycle intermediate

Deamination of amino acids The remaining keto group can be used in respiration either converted to acetyl CoA, pyruvic acid or a krebs intermediate

The liver removes the amino group (deamination) and produces ammonia which is toxic and needs to excreted as urea Carbon dioxide reacts with ammonia as part of the ornithine cycle to produce urea ammonia +carbon dioxide=urea and water 2NH 3 +CO 2 =CO(NH 2 ) 2 +H 2 0 Deamination of amino acids