Chapters 3 and 9: Enzymes and Cellular Respiration

Energy – the ability to do work or bring about a change Cells need energy to maintain their organization Cells need energy to carry out reactions used to grow, develop, and reproduce

Forms of energy: Kinetic energy – energy of motion Ex: you raise your arm Potential energy – stored energy; capable of producing energy, but not being used yet Ex: food we eat has potential energy Chemical energy – composed or organic molecules such as carbohydrates Ex: food we eat, ATP

First law of thermodynamics (the law of conservation of energy) – energy cannot be created or destroyed, but it can be changed from one form to another Energy flows; it does not cycle As materials change from one form of energy to another, some energy is given off as heat (a form of energy)

Second law of thermodynamics – energy cannot be changed from one form to another without a loss of usable energy Heat given off through the conversion of chemical energy to kinetic energy is not a usable form of energy For this reason, living things are dependent upon an outside source of energy – the sun

Enzymes – speed up the rate of chemical reactions Enzymes – speed up the rate of chemical reactions Substrates – molecules which react with enzymes Only one small part of an enzyme, called the active site, reacts with the substrate(s). Active site may undergo a slight change in shape in order to fit with the substrate The enzyme is not changed by the reaction (active site returns to its original state), and it is free to act again. Animation – How enzymes work

Activation energy (Ea) – the minimum amount of energy required to start a chemical reaction Enzyme lowers the amount of energy required for reaction to occur Enzymes allow reactions to take place at lower temperatures – otherwise, reactions would not be able to occur at normal body temperatures

Energy of activation (Ea) When no enzyme is present – more energy required When an enzyme is added – less energy required

Induced fit model Because the enzyme must undergo a slight change in shape to fit with the substrate, this is known as the induced fit model.

Enzymatic reaction Substrates are combined into a larger product Substrate is broken down into smaller products Substrates are combined into a larger product

Every reaction in a cell requires a specific enzyme. Enzymes are named for their substrates: Substrate Enzyme Lipid Lipase Urea Urease Maltose Maltase Ribonucleic acid Ribonuclease

Factors Affecting Enzymatic Speed Factors Affecting Enzymatic Speed Temperature pH Substrate concentration Enzyme concentration

Temperature: As the temperature rises, enzyme activity increases. If the temperature is too high, enzyme activity slows rapidly because the enzyme is denatured. When enzyme is denatured, its shape changes and it can no longer attach to the substrate. Each enzyme has an ideal temperature at which the rate of reaction is highest.

pH: Each enzyme has an ideal pH at which the rate of reaction is highest. Change in pH can change the structure of the enzyme, and can eventually cause enzyme to denature.

Rate of an enzymatic reaction as a function of temperature and pH

Rates and concentration: Reaction rate depends on the number of enzyme-substrate complexes that can be formed. When all available enzymes and active sites are filled, the rate of activity cannot increase further.

Substrate concentration Enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme. Enzyme concentration Enzyme activity increases as enzyme concentration increases because there are more collisions between substrate molecules and the enzyme.

Overview of Cellular Respiration Makes ATP molecules Releases energy in 4 reactions Glycolysis, Transition reaction, Citric acid cycle (Kreb’s cycle), and Electron transport system An aerobic process that requires O2 If oxygen is not available (anaerobic), glycolysis is followed by fermentation

The four phases of complete glucose breakdown

Cellular respiration takes the potential chemical energy in the bonds of glucose and transforms it into the potential chemical energy in the bonds of ATP. ATP molecules store usable chemical energy to drive life processes through coupled reactions.

ATP (adenosine triphosphate) The energy currency of cells. A nucleotide made of the following: Adenine (a base) Ribose (a sugar) Three phosphate groups Constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell. Pneumonic devices – ATP – a triple phosphate - ADP – a double phosphate http://www.stolaf.edu/people/giannini/flashanimat/metabolism/atpsyn2.swf

The ATP cycle

Oxidation is the loss of electrons; hydrogen atoms are removed from glucose. Reduction is the gain of electrons; oxygen atoms gain electrons. Remember OIL RIG (oxidation is loss, reduction is gain)

Enzymes involved: NAD+ Nicotinamide adenine dinucleotide Accepts H+ to become NADH FAD Flavin adenine dinucleotide (sometimes used instead of NAD+) Accepts 2H+ to become FADH2

The NAD+ cycle

Structure of mitochondria: Has a double membrane, with an intermembrane space between the two layers. Cristae are folds of inner membrane The matrix, the innermost compartment, which is filled with a gel-like fluid.

Where does each step occur? Outside the mitochondria Step 1 - Glycolysis Inside the mitochondria Step 2 - Transition reaction (matrix) Step 3 – Citric acid cycle (matrix) Step 4 – Electron transport system (cristae)

Occurs in the cytoplasm (outside the mitochondria) Step 1. Glycolysis Occurs in the cytoplasm (outside the mitochondria) Glucose  2 pyruvate molecules. Universally found in all organisms Does not require oxygen (anaerobic). http://www.science.smith.edu/departments/Biology/Bio231/glycolysis.html

Glycolysis summary Inputs: Outputs: Glucose 2 NAD+ 2 ATP 4 ADP + 2 P Inputs: Glucose 2 NAD+ 2 ATP 4 ADP + 2 P Outputs: 2 pyruvate 2 NADH 2 ADP 2 ATP (net gain) When oxygen is available, pyruvate enters the mitochondria, where it is further broken down If oxygen is not available, fermentation occurs

Step 2 - Transition Reaction Requires oxygen (aerobic) Occurs in the matrix of the mitochondria Pyruvate (made during glycolysis) is converted to acetyl CoA, and CO2 is released NAD+ is converted to NADH The transition reaction occurs twice per glucose molecule.

Transition reaction summary Inputs: 2 pyruvate 2 NAD+ Outputs: 2 acetyl groups 2 CO2 2 NADH http://www.science.smith.edu/departments/Biology/Bio231/krebs.html

Step 3 - Citric Acid Cycle (aka Kreb’s Cycle) Occurs in the matrix of the mitochondria. Requires oxygen (aerobic) Cycle occurs twice (each of the following occurs twice) C2 acetyl group is converted to a C6 citrate. NAD+ accepts electrons 3 times FAD accepts electrons once. Produces four CO2 Results in a gain of one ATP

Citric acid cycle summary Inputs: 2 acetyl groups 6 NAD+ 2 FAD 2 ADP + 2 P Outputs: 4 CO2 6 NADH 2 FADH2 2 ATP http://www.science.smith.edu/departments/Biology/Bio231/krebs.html

Step 4 - Electron Transport System (ETS) Requires oxygen (aerobic) Located in the cristae of mitochondria NADH and FADH2 carry electrons picked up during glycolysis, transition reaction, & citric acid cycle and enter the ETS. The ETS consists of: protein complexes that pump H+ mobile carriers that transport electrons ATP synthase complex - H+ flow through it, making ATP H+ flow through from high to low concentration For every 3 H+ that flow through, one ATP is made

http://vcell.ndsu.nodak.edu/animations/atpgradient/movie.htm http://www.sp.uconn.edu/%7Eterry/images/movs/synthase.mov http://www.science.smith.edu/departments/Biology/Bio231/etc.html http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter9/animations.html

Energy Yield from Glucose Metabolism Energy Yield from Glucose Metabolism Per glucose molecule: 10 NADH take electrons to the ETS  3 ATP from each 2 FADH2 take electrons to the ETS  2 ATP from each

Accounting of energy yield per glucose molecule breakdown

Occurs when oxygen is not available. Follows glycolysis Fermentation Occurs when oxygen is not available. Follows glycolysis Pyruvate formed by glycolysis is reduced to alcohol and CO2, or to lactate. Fermentation uses NADH and regenerates NAD+, which can be used during glycolysis. Occurs in anaerobic bacteria, fungus, & human muscle cells. http://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/GLYCOLYSIS.HTML

Glycolysis and Fermentation Summary Inputs (all into glycolysis): Glucose 2 ATP 4 ADP + 2 P Outputs: 2 lactate or 2 alcohol & 2 CO2 2 ADP (glycolysis) 2 ATP (net gain) (glycolysis)

Advantages and Disadvantages of Fermentation Fermentation can provide a rapid burst of ATP in muscle cells, even when oxygen is in limited supply. For bacteria, glycolysis and fermentation is the main energy source Lactate, however, is toxic to cells. Initially, blood carries away lactate as it forms; eventually lactate builds up, lowering cell pH, and causing muscles to fatigue. Oxygen debt occurs, and the liver must reconvert lactate to pyruvate.

Efficiency of Fermentation Two ATP produced during fermentation are equivalent to 14.6 kcal; complete oxidation of glucose to CO2 and H2O represents a yield of 686 kcal per molecule of glucose. Thus, fermentation is only 2.1% efficient compared to cellular respiration (which is 39% efficient). (14.6/686) x 100 = 2.1%