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Biology OCR A level Respiration Summary Notes
Biology OCR A level Respiration Summary Notes
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1 of 135.2.2 Respiration
(a) The need for cellular respiration
To include examples of why plants, animals and microorganisms need to respire (suitable
examples could include active transport and an outline of named metabolic reactions).
• Respiration is the process that occurs in living cells and
releases the energy stored in organic molecules such as
glucose.
• The energy is then used to synthesise molecules of ATP
from ADP and inorganic phosphate.
• Microorganisms, plants, animals, fungi and protoctists all
respire to obtain energy.
Why do living organisms need energy?
• Energy is the capacity to do work. The energy that is
stored in complex organic molecules- e.g. fats,
carbohydrates and proteins- is potential energy. It is also
chemical energy, converted from light energy during the
process of photosynthesis.
All the chemical reactions that take place within living cells
are known collectively as metabolism or metabolic
reactions.
• Anabolic reactions are metabolic reactions where large
molecules are synthesised from smaller molecules.
• Catabolic reactions are metabolic reactions involving the
hydrolysis of large molecules to smaller ones.
Within living cells atoms, ions and molecules have
kinetic energy, and this allows them to move; for
example, when molecules di use down a concentration
gradient, moving from one place to another, they use
their kinetic energy to do so.
The role of ATP
• ATP is the standard intermediary between energy-
releasing and energy-consuming metabolic reactions
in both eukaryotic and prokaryotic cells.
• The structure of an ATP molecule, is shown by the
diagram, it is a phosphorylated nucleotide.
• Each molecule of ATP consists of adenosine, which is
the nitrogenous base adenine plus the ve-carbon
sugar ribose, and three phosphate (phosphoryl)
groups.
• ATP is relatively stable when in solution in cells, but is
readily hydrolysed by enzyme catalysis. Whilst in
solution, it can easily be moved from place to place within a cell.
• The energy releasing hydrolysis of ATP is coupled with
an energy-consuming metabolic reaction. ATP is the
immediate energy source for this metabolic reaction.
• When ATP is hydrolysed to ADP and Pi, a small
quantity of energy is released for use in the cells.
• Cells can therefore obtain the energy they need for a
process in small manageable amounts that will not
cause damage or be wasteful.
• Some energy is released from the hydrolysis of ATP
as heat. The release of heat, both in respiration and
during ATP hydrolysis, may appear to be ine cient
and wasteful.
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• Heat, however, helps keep living organisms ‘warm’ and enables their enzyme-catalysed
reactions to proceed at or near their optimum rate.
(b) The structure of the mitochondrion
The components of a mitochondrion including inner and outer mitochondrial membranes, cristae,
matrix and mitochondrial DNA.
Key words:
Cristae- inner highly-folded mitochondrial membrane.
Mitochondrial matrix- uid lled inner part of mitochondria.
Mitochondrial structure
• Organelles that are present in all types of eukaryotic cells.
• May be rod-shaped, thread-like or spherical, with diameters of
0.5-1.0μm and lengths of 2-5μm.
• All mitochondria have an inner and an outer phospholipid
membrane making up the envelope.
• The outer membrane is smooth and the inner membrane is
folded into cristae, giving it a large SA.
• Embedded in the inner membrane are proteins that transport
electrons, and protein channels associated with ATP synthase
enzyme that allow protons to di use through them.
• Between the inner and outer mitochondrial membranes of the
envelope, is an intermembrane space.
• The mitochondrial matrix, enclosed by the inner membrane, is
semi-rigid and gel-like; it contains mitochondrial ribosomes,
looped mitochondrial DNA and enzymes for the link reaction
and Krebs cycle.
How the structure of mitochondria enables them to carry out their functions
The matrix
- Is where the link reaction and the Krebs cycle takes place.
It contains:
- Enzymes that catalyse the stages of these reactions.
- Molecules of the coenzymes NAD and FAD.
- Oxaloacetate: the four carbon compound that accepts the acetyl group from the link reaction.
- Mitochondrial DNA: some of which codes for mitochondrial enzymes and other proteins.
- Mitochondrial ribosomes: structurally similar to prokaryotic ribosomes, where these proteins are
assembled.
The outer membrane
- The phospholipid composition of the outer membrane is similar to that of membranes around
other organelles in eukaryotic cells.
- It contains proteins, some of which form channels or carriers that allow the passage of
molecules, such as pyruvate, into the mitochondrion.
The inner membrane
- Lipid composition of the inner membrane di ers from that of the outer membrane.
- This lipid bilayer is less permeable to small ions such as hydrogen ions than is the outer
membrane.
- The folds, cristae, in the inner membrane give a large SA for the electron carriers and ATP
synthase enzymes embedded in them.
- The electron carriers are protein complexes arranged in electron transport chains. Electron
transport chains are involved in the nal stage of aerobic respiration, oxidative phosphorylation.
The intermembrane space
- The intermembrane space between the outer and inner layers of the mitochondrial envelope is
also involved in oxidative phosphorylation.
- The inner membrane is in close contact with the mitochondrial matrix, so the molecules of
reduced NAD and FAD can easily deliver hydrogens to the electron transport chain.
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