Harvesting Energy: Glycolysis and CellularGlycolysis has two stages: energy investment and energy harvesting 1) The energy investment steps of glycolysis are energy requiring Glucose

Harvesting Energy:

Glycolysis and Cellular Respiration

Introduction to Life Processes - SCI 102 1

Lesson 5

How Cells Obtain Energy

• Cells require a constant flow of energy

Most cellular energy is stored in adenosine triphosphate (ATP)

• Photosynthesis is the ultimate source of cellular energy

Photosynthetic organisms capture and store the energy of

sunlight in sugar and other organic molecules

The chemical equation for glucose formation during

photosynthesis is essentially the reverse of the equation of

glucose breakdown by glycolysis and cellular respiration


How Cells Obtain Energy

• Glucose is a key energy-storage molecule

All cells metabolize glucose for energy

Glucose breakdown occurs in stages

• Glycolysis in the cytoplasm begins the process

• If oxygen is present, cellular respiration occurs

• If oxygen is absent, fermentation occurs


Glycolysis

• Glycolysis breaks down glucose to pyruvate, releasing chemical

energy

Glycolysis has two stages: energy investment and energy harvesting

1) The energy investment steps of glycolysis are energy requiring

Glucose is converted to fructose bisphosphate, a 6-C glucose with two phosphate

groups

Fructose bisphosphate is unstable and high in energy

Glucose activation “costs” two ATP

This is an endergonic reaction

2) The energy harvesting steps yield ATP and NADH

Fructose bisphosphate splits into two 3-C molecules of G3P

Each G3P molecule undergoes a series of steps to be converted to pyruvate

Energy-harvesting steps produce two NADH and four ATP

Glycolysis produces a net two ATP and two NADH (high-energy electron

carriers) for each molecule of glucose converted to two pyruvate


Glycolysis

• Glycolysis does not require oxygen to occur

If a cell (ex: bacteria) shifts from an environment with oxygen

to one without, it will need to increase its rate of glycolysis in

order to have energy

• In an environment with oxygen, the bacteria can perform cellular

respiration which produces much more energy than glycolysis

• Metabolic poison can interfere with glycolysis when the

poison has a structure which is very similar to glucose

but is unable to be metabolized


Cellular Respiration

• In most organisms, if oxygen is present, cellular

respiration occurs

Cellular respiration in eukaryotic cells occurs in

mitochondria in three stages

A mitochondrion has two membranes that produce two

compartments: the matrix and the intermembrane space



• Stage 1of cellular respiration: pyruvate is broken down

First, pyruvate is broken down in the mitochondrial matrix,

releasing energy and CO2

• In the mitochondrial matrix, pyruvate reacts with a molecule of

coenzyme A to produce acetyl-CoA and one CO2 and one NADH

• Each acetyl-CoA combines with a 4-C molecule to produce 6-C

citrate, releasing coenzyme A

• Citrate goes through a series of rearrangements in a cycle of

reactions called the Krebs cycle

• The end products of the Krebs cycle per molecule of pyruvate are

two CO2, one ATP, one FADH2, and three NADH; the 4-C molecule

is regenerated



• Stage 2 of cellular respiration: high-energy electrons

travel through the electron transport chain

From glycolysis and the mitochondrial matrix reactions, the

cell has accumulated 4 ATP, 10 NADH, and 2 FADH2

The electron carriers NADH and FADH2 release their electrons

to the electron transport chains located in the inner

mitochondrial membrane

• Energy released by these electrons is used to pump hydrogen ions

from the matrix to the intermembrane space to produce ATP by

chemiosmosis

• At the end of the electron transport chain (ETC), the energy-

depleted electrons are transferred to oxygen, forming water



• Stage 3 of cellular respiration: chemiosmosis

generates ATP

During chemiosmosis, the flow of hydrogen ions provides

enough energy to produce 32 to 34 ATP

The ATP diffuses out of the mitochondria to the

cytoplasm through the outer membrane, which is

permeable to ATP



• A summary of glucose breakdown in eukaryotic cells

Glycolysis occurs in the cytoplasmic fluid

This process produces two pyruvate molecules, two ATP molecules, and two

NADH molecules

Cellular respiration breaks down the two pyruvates during the Krebs cycle

• This process produces NADH and FADH2 and a small amount of ATP

• Electrons from NADH and FADH2 are donated to the electron transport chain, producing 32

or 34 ATP through chemiosmosis

• Cellular respiration can extract energy from a variety of molecules

Cellular respiration can extract energy from sugars, fats, and amino acids

• Cyanide poisoning

Occurs because cyanide inhibits an enzyme in the electron transport pathway

• This becomes deadly because ATP can no longer be produced by chemiosmosis


Fermentation

• Fermentation allows NAD+ to be recycled when

oxygen is absent

Under aerobic conditions, most organisms use cellular

respiration, regenerating NAD+ from the ETC

Under anaerobic conditions, cellular respiration does not

occur, so NAD+ must be regenerated another way to

allow glycolysis to occur


Fermentation

• Some cells ferment pyruvate to form lactate

Muscle cells undergo lactate fermentation during

vigorous exercise when not enough oxygen is available

As soon as oxygen is available, lactate will be converted

back to pyruvate in the liver, and cellular respiration will

resume


Fermentation

• Some cells ferment pyruvate to form alcohol and

carbon dioxide

Many microorganisms, including yeast, convert pyruvate

to ethanol and carbon dioxide

Alcoholic fermentation can be used to produce alcoholic

beverages and bread


Fermentation

• Pyruvate in the cytosol is converted into lactate or

ethanol and carbon dioxide

Lactic acid fermentation produces lactic acid from pyruvate

Alcoholic fermentation produces alcohol and CO2 from

pyruvate

• Does not produce ATP

• Fermentation is needed to convert the NADH produced

during glycolysis back to NAD+, which needs to be

continuously available for glycolysis to happen


Fermentation

• Lactate fermentation

When muscles are deprived of oxygen, they do not stop

working immediately

• During vigorous activity, muscles become sufficiently low on

oxygen and perform glycolysis to produce two ATP molecules

per glucose

This provides a brief burst of speed

• The muscle cells ferment the resulting pyruvate molecules to

lactate, using electrons from NADH and hydrogen ions


Fermentation

• Lactate fermentation

Example: Joe bicycles up a hill during the neighborhood

biking race. As pedals up the hill, he “feels the burn” in his

legs. His muscles are shifting away from cellular respiration

due to the lack of oxygen and shifting towards lactic acid

fermentation to produce energy in the leg muscles.

Example: Bacteria in the mouth feed off of sugars that we

eat. As they ferment the sugar, they produce lactic acid which

causes cavities in the teeth.


Fermentation

• Alcohol fermentation

Pyruvate is converted into ethanol (an alcohol) and

carbon dioxide

• This releases NAD+, which is then able to accept more high-

energy electrons during glycolysis

Many microorganisms use alcoholic fermentation when

they are in anaerobic conditions

• Example: yeast


Fermentation

• Fermentation of yeast:

When yeast ferments, it produces carbon dioxide gas

which causes bread dough to “rise” (the carbon dioxide

gas takes up space and pushes the dough to expand)

If a single yeast cell undergoes alcohol fermentation and

uses 50 molecules of glucose, it will only generate 100

molecules of ATP (for every molecule of glucose, 2 ATPs

are produces)

• This is much less energy than in cellular respiration


Documents

Harvesting Energy: Glycolysis and CellularGlycolysis has two stages: energy investment and energy harvesting 1) The energy investment steps of glycolysis are energy requiring Glucose