Cellular Respiration

Dr. Mark A. McGinleyHonors College and Department of Biological

SciencesTexas Tech University

Biological Work• Most of the energy used to do biological work

comes from ATP• ATP breaks down and releases energy that is

used to do biological work

Energetics in a Nutshell

• Photosynthesis converts light energy to potential energy stored in chemical bonds of glucose

• Cellular Respiration converts potential energy in glucose to potential energy stored in ATP– ATP releases energy used to do work

• Glucose links the two processes

Breaking Down Glucose to Release Potential Energy

• Starts with the process of glycolysis• Followed by either– Fermentation (in anaerobic environments)– Citric Acid Cycle (Krebs Cycle) + electron transport

(in aerobic environments)

Glycolysis

• Glucose broken down into two molecules of pyruvate– Occurs in the cytosol

• Breaking down ATP requires the input of energy from 2 molecules of ATP but releases energy in 4 molecules of ATP

• Thus, net gain of energy of 2 ATPs in glycolysis

Glycolysis

• Glucose + 2ATP => 2 pyruvate + 4 ATP + H+

Glycolysis

• Glycolysis breaks down glucose to release energy in two ATPs– ATPs can release energy to do biological work

Problem Facing the Cell

• Glucose <= => 2 pyruvate + H+

• This reaction will continue to break down glucose to release ATP until the reaction reaches an equilibrium

• Once equilibrium is reached, glycolysis will stop, so no more ATP is released

Solution

• In order to allow glycolysis to continue cells must maintain the concentration gradient by removing pyruvate and H+ from the cell.

• H+ picked up by NAD+ => NADH

• Eventually NAD+ gets saturated

Ultimate Solution

• H+ must be removed from NADH in order to allow glycolysis to continue

• Key Point- How this happens depends on whether or not there is oxygen in the environment

Anaerobic Environment

• When there is no oxygen in the environment then pyruvate and H+ are removed from the cell by fermentation

• Several patterns of fermentation including– Alcohol fermentation– Lactic acid fermentation

Alcohol Fermentation

• Pyruvate and H+ => acetaldehyde => ethanol• Ethanol becomes the ultimate “hydrogen

acceptor”

Advantages and Disadvantages of Alcohol Fermentation

• Benefit– End products of glycolysis are removed from the

cell so glycolysis can continue• Disadvantage– Alcohol can be poisonous to cells– Pyruvate used to help remove H+ from the cell• Still lots of potential energy stored in pyruvate • Can’t break down pyruvate to release energy

Lactic Acid Fermentation

• Pyruvate + H+ => lactate• Lactate becomes the ultimate hydrogen

acceptor

Advantages and Disadvantages of Lactic Acid Fermentation

• Benefit– End products of glycolysis are removed from the

cell so glycolysis can continue• Disadvantage– lactate can be poisonous to cells– Pyruvate used to help remove H+ from the cell• Still lots of potential energy stored in pyruvate • Can’t break down pyruvate to release energy

Review in Anaerobic Environments

• Glucose broken down by glycolysis and fermentation

• For each glucose molecule broken down there is a net gain of two ATPs

Aerobic Environments

• When oxygen is present – O2 + H+ => H20

• Water becomes the ultimate hydrogen acceptor– Benefit• Water is non-toxic and in fact is beneficial• Pyruvate can be broken down to release more stored

energy

Energy From Pyruvate

• Glycolysis occurs in the cytosol– NADH and pyruvate move into the mitochondria

• In the mitochondria pyruvate is broken down to release ATP in two processes– Citric acid cycle (Krebs Cycle)– Electron transport

Pyruvate Links Glycolysis and Citric Acid Cycle

Citric Acid Cycle

• The details of the Citric Acid Cycle are well know– Not super important for this course

• Key Points– Inside of the mitochondrion pyruvate breaks down to

produce CO2 + Acetyl CoA– Acetyl CoA enters Citric Acid Cycle

• Acetyl CoA + oxaloacetate = > citrate

– CO2 released– 1 ATP produced for each Acetyl CoA that enters the cycle

• Thus, 2 ATPs per glucose

Electron Transport

• In a process very similar to what we talked about in cyclic electron flow in photosynthesis– An excited electron moves down an electron

transport chain (located in inner membranes of mitochondria)• Energy released used to actively transport H+ • H+ concentration gradient powers Chemiosmosis

– Releases lots of ATP– 26 or 28 ATP/glucose

Electron Transport

Review in Aerobic Environments

• Glucose broken down by glycolysis, citric acid cycle, and electron transport

• For each glucose molecule broken down there is a net gain of 30 - 32 ATPs– 2 per glucose from glycolysis– 2 per glucose from citric acid cycle– 26 – 28 per glucose from electron transport

Advantages of Breaking Down Glucose in Aerobic Environments

• Benefit– End products of glycolysis are removed from the

cell so glycolysis can continue– Ultimate hydrogen acceptor (water) is beneficial

to cells– Pyruvate can be broken down to release much

more energy