Page 324 - Biology_F5

Gas exchange and respiration

Importance of the Kreb’s cycle proton and electron acceptor are known
Kreb’s cycle is an economical way of as anaerobes. In anaerobes, other less-
turning food components in the cell into oxidising substances such as sulphate
-
usable energy. Only the acetyl groups are ion (SO ), nitrate ion (NO ), sulphur,
2-
4
3
destroyed in the cycle. The seven enzymes and fumarate are used. These terminal
FOR ONLINE READING ONLY
that carry out the various reactions and the electron acceptors have smaller reduction
intermediate compounds on which these potentials than oxygen. This means
enzymes act can be used again and again. that less energy is released per oxidised
molecule. Generally, anaerobic respiration
is less efficient in energy production than
Moreover, many of the intermediate
compounds produced in the Kreb’s cycle aerobic respiration. It is mainly used by
prokaryotes that live in environment
are of value as starting materials for the devoid of oxygen. Many anaerobic
synthesis of amino acids, carbohydrates, organisms are obligate anaerobes as they
and other cellular products. Pyruvates are can only respire anaerobically and they
broken down to carbon dioxide. Thus, the die in the presence of oxygen. Muscles

Kreb’s cycle degrades macromolecules do also respire anaerobically whenever
into simpler molecules. The Kreb’s cycle there is deficiency or lack of oxygen at
releases ATP, NADH and FADH. The a particular tissue point. This creates
ATP is directly utilised by the cellular the basis for categorising anaerobic
activities while the NADH and FADH are respiration into alcoholic fermentation
metabolites for oxidative phosphorylation and lactic acid fermentation.
of the electron transport system in which
ATP are produced. Alcoholic fermentation
Alcoholic fermentation is also called
The fate of pyruvic acid under ethanol fermentation, which occurs in
anaerobic condition cells, such as plant and yeast cells (Figure
Task 6.7 6.11). This biological process converts
Carry out library and internet search two pyruvates into two acetaldehydes
to find out if the bacteria and yeast producing two carbondioxide molecules
can form alcohols in the absence of as a waste product. The two acetaldehydes
oxygen or they possess a metabolic are then converted into two ethanol
pathway that does not involve oxygen. molecules by using hydrogen ions from
Write down your findings.
NADH; converting NADH back into
+
NAD . Ethanol fermentation has many
In the absence of oxygen, the principal
product of glycolysis (pyruvate) enters uses, including producing alcoholic
a fermentation process. The organisms beverages, ethanol fuel, as well as raising
that do not use oxygen as the final reagent in baking bread.

Form Five Student’s Book
317