Chapter 6

Carvings from ancient Egypt show barley being crushed and mixed with water (left) and then put into closed vessels (centre) where airless conditions are suitable for the production of alcohol by yeast cells residing on the vessels’ walls. The beer is then ready for consumption (right).

Hammurabi – Ancient laws

regarding alcohol. Workers

were rationed 2 litres a day,

civil servants 3 litres, and 5

litres for highest priest.

Better than water!

This is an example of cellular respiration, which can be used to make beer and wine using different metabolic pathways

For these reasons we call this unit ‘Metabolism’

You can use these pathways to make bread, fermenting cabbage for kimchi or sauerkraut, and culturing cheese

For example, making bread uses yeast, yeast ferments sugars, producing carbon dioxide, which causes the bread to rise

Bacteria ferment the sugars in cabbage and produce lactic acid, which gives sauerkraut and kimchi their tangy flavours

Cheese is produced by bacteria that use lactose(milk sugar) to produce lactic acid, lactic acid causes milk to curdle

Why does fresh air inhibit the formation of alcohol by yeast cells?

Catabolism or the beer sugar is a cellular process, so yeast cells must be present

With air, yeasts can use aerobic metabolism to fully oxidize glucose to CO2

Without air, yeasts use alcohol fermentation, producing ethanol, less CO2, and less energy (slower growth)

6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism

6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy

6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

6.4 Catabolic and Anabolic Pathways Are Integrated

6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy

6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates

6.1

Crash Course - Metabolism

In Chapter 2 & 3 we introduced the general concepts of energy, enzymes, and metabolism.

Energy is stored in chemical bonds and can be released and transformed by metabolic pathways.

Chemical energy available to do work is termed free energy (G).

There are five general principles that govern metabolic pathways:

1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway.

2. Each reaction is catalyzed by a specific enzyme.

3. Most metabolic pathways are similar in all organisms.

4. In eukaryotes, many metabolic pathways occur inside specific organelles.

5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

Chemical energy available to do work is termed free energy(G)

According to the laws of thermodynamics, a biochemical reaction may change the form of energy but not the net amount

Recall, a exergonic reaction releases energy and an endergonic reaction requires energy to the reactants

In cells, energy-transforming reactions are often coupled:

An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction.

Two coupling molecules are the coenzymes ATP and NADH.

Synthesis of ATP

from ADP and Pi is

endergonicHydrolysis of

ATP to ADP

and Pi is

exergonic

Cells use adenosine triphosphate (ATP) as a kind of ‘energy currency’

ATP can release energy in exergonic reaction after there has been an endergonic reaction between adenosine diphosphate and an inorganic phosphate

The ATP molecule harnesses this energy (currency) to use at a different location or process by bring hydrolysed

BOZEMAN - ATP

An active cell requires the production of millions of molecules of ATP per second to drive its biochemical machinery

Some of these activities include

Active transport across a membrane (more detail later)

Condensation reactions that use enzymes to form polymers (2.2)

Motor proteins that move vesicles along microtubules (4.4 – later)

The ATP molecule consists of the nitrogen-containing base adenine bonded to ribose (sugar), which is attached to a sequence of three phosphate groups

Hydrolysis of ATP is exergonic:

ATP + H2O ADP + Pi + free energy

ΔG is about –7.3 kcal/mol

A molecule of ATP can also be hydrolyzed to adenosine monophosphate (AMP) resulting in more energy being released

®

Energy is released as a result of ATP hydrolysis because the P-O bonds in a free hydrogen phosphate (Pi) molecule are stronger and more stable than a relatively weak P-O bonds between the phosphate groups in ATP

The free energy of the bond between phosphate groups is much higher than the energy of the O—H bond that forms after hydrolysis.

StrongerWeaker

In 6.2 we will see glycolysis in action - break down glucose and form pyruvate (make more ATP) with the production of two molecules of ATP

In Glycolysis, ATP is formed by substrate level phosphorylation – where there is an enzyme mediated direct transfer of phosphate from another molecule (the substrate) to ADP.

Another way of transferring energy in chemical reactions is to transfer electrons.

A reaction in which one substance transfers one more electrons to another substance is called a reduction-oxidation reaction or redox reaction

• Reduction is the gain of one or more electrons.

Oxidation is the loss of one or more electrons.

Oxidation and reduction always occur together, as one chemical is oxidized, the electrons it loses are transferred to another chemical reducing, thus some chemicals are called reducing agents

We say that oxidation and reduction is defined in traffic of electrons but in reality, gain or loss of H+ atoms.

These transfers of H+ involve the transfer of electrons (H = H+ +e-)

So when a molecule loses an hydrogen atom, it becomes oxidized.

In general, the more reduced a molecule is, the more energy it has stored in in its covalent bonds

Energy is transferred in a redox reaction.

Energy in the reducing agent is transferred to the reduced product.

Coenzyme NAD is a key electron carrier in redox reactions.

NAD+ (oxidized form)

NADH (reduced form)

Cells use the coenzyme nicotinamide adenine dinucleotide as an electron carrier in a redox reaction.

It occurs in two forms

Oxidized (NAD+)

Reduced (NADH)

Bozeman – Coupled Reactions

Reduction of NAD+ is highly endergonic:

NAD+ + H+ + 2 e– NADH

Oxidation of NADH is highly exergonic:

NADH + H+ + ½ O2 NAD+ + H2O

Energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH.

Energy for anabolic processes is supplied by ATP.

Most energy-releasing reactions produce NADH, but most energy-consuming reactions require ATP.

Oxidative phosphorylation transfers energy from NADH to ATP.

Cellular Respiration - Bozeman