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Summary IB NOTES U8
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KEY OBJECT TO ACHIEVE 7 IN IB BIOLOGY HL
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8.2 Cell Respiration - Energy is converted to a usable form in cellrespiration.
Nature of Science: Paradigm shift—the chemiosmotic theory led to a paradigm
shift in the field of bioenergetics. (2.3)
∑ - Understandings:
∑ - Cell respiration involves the oxidation and reduction of electron
carriers.
Oxidation involves the loss of electrons from an element through
the gain of oxygen or the loss of hydrogen.
Reduction involves the gain of electrons through the addition of
hydrogen or the loss of an oxygen molecule.
These reactions are called Redox reactions (reduction-oxidation) which
are chemical reactions in which atoms have their oxidation number
changed.
For example the oxidation of glucose to form CO2 and the reduction of
carbon by the addition of hydrogen to yield methane (CH4).
Electron carriers are specific substances that accept and give up
electrons
The main electron carrier in cellular respiration is NAD (nicotinamide
adenine dinucleotide)
During respiration NAD which actually exists as NAD+ accepts 2
electrons and a proton (H+) from the molecule being oxidized (like
pyruvate) to form NADH with one extra H+ leftover as a product. The
reaction is illustrated below.
After the electron carriers are reduced they transport their electrons and
hydrogens to the ETC, where the opposite reaction occurs (oxidation)
, So when NADH is oxidized it donates the electrons and protons to an
electron carrier (complex I) in the inner mitochondrial membrane made
from conjugated proteins (Fe-S core)
This carrier is therefore reduced and will be re-oxidized as it passes the
electrons down the ETC
∑ - Phosphorylation of molecules makes them less stable.
Phosphorylation occurs when a phosphate (PO4^3-) molecule is added
to an organic molecule to make the molecule less stable and more likely
to react
Phosphorylation basically activates the molecule and is an endergonic
reaction
The removal of the phosphate through hydrolysis is an exergonic
reaction
The reactions are coupled together, so one molecule releases the
phosphate and one accepts the phosphate molecule, and are
spontaneous
Example – Phosphorylation of Glucose to Glucose-6-phosphate, coupled
with the hydrolysis of ATP to ADP.
∑ - In glycolysis, glucose is converted to pyruvate in the cytoplasm.
∑ - Glycolysis gives a small net gain of ATP without the use of oxygen.
Glycolysis
In the first stage of glycolysis, 2 ATP molecules are used to
phosphorylate glucose (add phosphates) through a process called
phosphorylation
Immediately following the phosphorylation Fructose 1-6-bisphosphate
splits into 2 G3P (glyceraldehyde-3-phosphate) molecules (lysis)
**Triose phosphate and Glyceraldehyde 3-phosphate (G3P) are the
same molecules. They are sometimes referred to by both names on
different websites**
The 2 G3P molecules formed from the split are then oxidized by the
removal of hydrogen atoms to form NADH + H
Phosphate is added to the G3P’s to briefly form 3 carbon molecules with
2 phosphate groups.
, Next, one phosphate is removed from each to form ATP and glycerate-3-
phosphate (3PG). 3-Phosphoglyceric acid (3PG) is the conjugate acid
of glycerate 3-phosphate (GP).
In the final stages of glycolysis, one more phosphate is removed from
each 3PG to form 2 ATP molecules and 2 pyruvates
Glycolysis occurs in the cytoplasm.
One hexose sugar (glucose) is converted into two three-carbon
compounds(pyruvate) with a net gain of 2 ATP (2 ATP molecules are
used to start the process) and 2 NADH + H+.
∑ - In aerobic cell, respiration pyruvate is decarboxylated and oxidized,
and converted into acetyl compound and attached to coenzyme A to form
acetyl coenzyme A in the link reaction.
Decarboxylation is a chemical reaction that removes a carboxyl group and
releases carbon dioxide (CO2).
Oxidation is the loss of electrons or an increase in oxidation state by
a molecule, atom, or ion. Pyruvate is oxidized by the removal of pairs of
hydrogen atoms (with their electrons), which are passed on the NAD+ and FAD
Link Reaction
Two pyruvate molecules enter the matrix of the mitochondria and are
decarboxylated to form two acetyl groups (removal of carbon as
CO2).
The acetyl group is oxidized and NAD+ is reduced to form NADH.
Each acetyl group combines with CoA (enzyme) producing 2 Acetyl
Coenzyme A molecules.
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