Energy & The CellGlycolysis, Cellular

Respiration & Fermentation

Energy• All life requires energy• Therefore cells require energy

– for growth, active transport, synthesis of carbohydrates, lipids, & proteins

• The source of energy for cells is the energy stored in chemical bonds of organic molecules– these molecules = food molecules,

especially carbohydrates (also lipids)– most common is glucose

Recycling Energy

ATP

• Cells store energy in the chemical bonds of sugar, but cannot use it directly

• To use this energy, the cell must transfer the energy in sugar molecules to ATP

• ATP = adenosine triphosphate

Structure of ATP• The base, adenine• The sugar, ribose• Ribose is bound to

a chain of 3 phosphate molecules connected by high energy bonds

Phosphorylation forms ATP

Releasing Energy from ATP

• If the cell needs energy it breaks the last phosphate bond, releasing energy

ATP ADP + P + energy

• Almost all energy requiring processes in cells use ATP as the energy source

Fermentation• So, where does ATP come from?• Fermentation = breakdown of glucose,

yielding ATP, without O2

• The first living organisms were single cells that existed without O2

– Anaerobic– Lack the enzymes needed to break down

energy molecules with O2

• Many types of bacteria and other single celled organisms still use anaerobic processes to convert energy

Types of Fermentation

• 2 kinds:• Alcoholic fermentation:

– occurs in micro-organisms such as yeast

• Lactic acid fermentation: – occurs in bacteria and animal cells

Fermentation in the Cytosol

• Fermentation occurs in the cytosol

• It produces lactic acid or alcohol

• Fermentation begins with the process of glycolysis, which is also part of aerobic respiration.

Glycolysis• Occurs in the cytoplasm of the cell• One molecule of glucose is split into two

molecules of a three carbon compound called pyruvic acid

• 2 molecules of ATP provide the energy to split the glucose molecule

• When glucose splits, it releases enough energy to form 4 molecules of ATP from ADP + P

• Therefore 2 molecules of ATP are gained

Energy of Glycolysis

The Role of NAD in Glycolysis

• During the conversion of glucose to pyruvic acid, hydrogen is released

• This hydrogen is picked up by a coenzyme, nicotinamide adenine dinucleotide (NAD)

• NAD is a hydrogen acceptor• When it accepts hydrogen, becomes

NADH2

Summary of Glycolysis - Investment

Summary of Glycolysis - Payoff

Energy of Fermentation

• As a result of fermentation, each molecule of glucose yields 2 molecules of ATP

• These ATP molecules come from glycolysis– Fermentation produces no ATP beyond

glycolysis• It removes pyruvic acid, and recycles

NAD, which allows glycolysis to continue, producing ATP

Alcoholic Fermentation• Pyruvic acid from glycolysis combines

with H from NADH2 to produce ethyl alcohol

•  2CH3COCOOH + 2NADH2 2CH3CH2OH + 2CO2 + 2NAD

– CO2 is a waste product

Alcoholic Fermentation Pathway

Lactic Acid Fermentation

• Pyruvic acid combines with H from NADH2 to produce lactic acid:

2CH3COCOOH + 2NADH2 2CH3CHOHCOOH + 2NAD

• Unlike alcoholic fermentation, no CO2 is given off

• Occurs in human cells when there is not enough O2 available for aerobic respiration

Lactic Acid Fermentation Pathway

Uses of Lactic Acid Fermentation

• During strenuous exercise glycolysis occurs at a high rate

• Pyruvic acid is produced rapidly• Muscle cells may not receive enough O2 to

process pyruvic acid through aerobic respiration

• Therefore muscles produce lactic acid which permits glycolysis to continue to supply ATP to your muscles

• When lactic acid builds up, your muscles ache• O2 you take in from heavy breathing helps

convert lactic acid back to pyruvic acid

Cellular Respiration

• Most cells produce ATP by breaking the energy containing bonds of glucose in the presence of oxygen

• Production of ATP this way = Respiration

• Uses O2 to break sugars down to CO2 & H2O– Not the same as breathing

– provides O2, but otherwise quite different

• This process occurs in the many mitochondria of each cell

The Process of Cellular Respiration

• C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

(sugar) (ATP)

• Two stages of Cellular Respiration:– Anaerobic

• without oxygen

– Aerobic • with oxygen

Cellular Respiration Overview

Anaerobic Stage• The anaerobic stage of cellular respiration is

glycolysis, the same pathway used in fermentation

• This part of cellular respiration occurs in the cytoplasm

• Recall the energy budget for glycolysis:– One molecule of glucose is split into two

molecules of a three carbon compound called pyruvic acid

– 2 molecules of ATP provide the energy to split the glucose molecule

– When glucose splits, it releases enough energy to form 4 molecules of ATP from ADP + P

– Therefore 2 molecules of ATP are gained

Energy of Glycolysis

Aerobic Stage

• After glycolysis, the chemical bonds of pyruvic acid are broken down in a series of chemical reactions

• These occur in the mitochondria and require O2

• The aerobic stage has two parts:– The Citric Acid Cycle– The Electron Transport Chain

Pyruvate Forms Acetyl CoA

The Citric Acid Cycle• Steps to break down pyruvic acid:• In the presence of O2, pyruvic acid breaks

down to acetic acid and CO2 – CO2 is released as waste

• Acetic acid combines with coenzyme A acetyl CoA– This step also forms NADH2 from NAD

• Acetyl CoA enters the citric acid cycle and combines with a 4 carbon compound to produce citric acid

• As the cycle continues, citric acid is broken down in a series of steps, back to the original 4 carbon compound

Energy from the Citric Acid Cycle

• For each molecule of acetyl CoA that enters the cycle, 8 atoms of H are released.

• These hydrogen atoms are trapped by NAD, forming NADH2.

• Therefore, each turn of the cycle yields 4 NADH2

The Electron Transport Chain

• NADH2 releases the hydrogen atoms trapped during glycolysis & the citric acid cycle– Therefore NADH2 becomes NAD again

• Electrons contained in the H atoms pass through a series of coenzymes which are electron acceptors.

• Each time an electron moves from one acceptor to another, an electron is released

• The electron released is used to form molecules of ATP from ADP + P

• This whole process = electron transport chain

Oxygen & The Electron Transport Chain

• The last part of the chain is the electron acceptor, oxygen

• Electrons combine with oxygen & hydrogen to form H2O, which is released as a byproduct

Chemiosmosis

• The process of formation of ATP during the ETS of aerobic respiration as the result of a pH gradient across the membrane of the cristae in the mitochondria = chemiosmosis

• Steps:– H+ ions from the matrix are pumped into the space

between the cristae and the outer membrane.– A H+gradient develops between the inside and

outside of the cristae– This pH differential creates free energy– H+ pass back across the membrane through F1

– O2 is the final H+/ electron acceptor producing H2O

Picturing Chemiosmosis

Cellular Respiration Summary• Thus for every molecule of glucose that is

broken down by glycolysis and respiration, 38 molecules of ATP are formed– Used 2 ATP to begin the process Therefore 36 ATP

gained