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Summary 12 Energy and Respiration
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EVERYTHING you need to know about "Energy and Respiration" for A-level Biology CIE
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Energy and Respiration21 September 2021 12:54
Structure of a Mitochondrion
• Mitochondria are rod-shaped and about 0.5-1µm in diameter with a length of up to 7µm
• They are not rigid and can change their shape
• The number of mitochondria in a cell depends on the activity level of that cell. Liver cells,
which are highly active, have 1000-2000 mitochondria per cell, which may take up about
20% of the total cell volume
• Each mitochondria is surrounded by an envelope of two phospholipid membranes
• The outer membrane is smooth
• The inner membrane is highly folded inwards to form structures called cristae (sing. Crista)
• These cristae provide a huge surface area.
• Although cristae in mitochondria from different cells do differ, a general rule is that
mitochondria from active cells have longer, more densely packed cristae than
mitochondria from less active cells
• Cells can increase both size and number of mitochondria with increased activity level
• The two membranes have different compositions and properties
• The outer membrane is relatively permeable to small molecules
• The inner membrane is less permeable
• The inner membrane is also studded with tiny spheres, about 9nm in diameter, which
attach to their inner membrane by 'stalks'
• Each sphere is an enzyme molecule called ATP synthase
• The inner membrane is also the site of groups of proteins embedded in the membrane,
each of which is known as an electron transport chain (ETC)
Contents of the Mitochondria
• The space between the two membranes (the intermembrane space) usually has a lower
pH than the matrix (the liquid in the centre of the organelle) because it usually contains
large numbers of H+ which are deposited into the intermembrane space by the electron
transport chain
• The matrix of the mitochondria is the site of a number of reactions, including the Link
Reaction and Krebs Cycle
• The matrix contains all the enzymes needed for these reactions to occur
• It also contains 70S ribosomes
• It also contains one or more copies of looped mitochondrial DNA
• The ribosomes and DNA means the mitochondrion can make its own proteins, such as
enzymes for its reactions, and divide independently of the rest of the cell
The Need For Energy in Living Organisms
• All living organisms and all the cells within them require energy in order to survive. They
need this energy in order to carry out work
• Work in a living organism includes:
- The synthesis of complex molecules from simpler ones (anabolic reactions), such as
making polysaccharides from monosaccharides, proteins from amino acids, and DNA and
RNA from nucleotides
- The active transport of particles, against a concentration gradient, such as the sodium
potassium pump needed for many events to occur (eg, nerve impulses)
- Mechanical work, such as muscle contraction, or the movement of cilia and flagella, or
the movement of vesicles within cells
- In some organisms, energy is needed for bioluminescence and electrical discharge
• Energy-requiring reactions must be linked to energy-yielding reactions
• Mammals and birds use the thermal energy released from metabolic reactions to keep
warm (they are endothermic)
• In the complete oxidation of glucose in the presence of oxygen (aerobic respiration) a
large quantity of energy is made available
• Specifically, 2870 kJ of energy is made available during the complete breakdown of a
single glucose molecule
• This is so much energy that the reactions involved in aerobic respiration must occur as a
series of small steps, the cell wouldn't be able to harness the energy usefully if it were all
released in one go (as would be the case in combustion)
• Each step is controlled by a specific enzyme
ATP
• Respiration is a process in which organic molecules act as a fuel
• The organic molecules are broken down in a series of stages to release chemical potential
energy, which is then used to make ATP
• Most cells usually use carbohydrates, specifically glucose, for their fuel, and some cells,
such as those in the brain are only able to metabolise glucose as their fuel source
• However, many cells are able to break down fatty acids (heart cells use fatty acids as their
main respiratory fuel), glycerol and amino acids in respiration
• This can be divided into four stages:
- Glycolysis (which occurs in the cytoplasm)
- The Link Reaction
- The Krebs Cycle
- Oxidative phosphorylation (the last three occur within the mitochondria)
• Technically, the energy released from each of these small steps could go directly to work
done within the cell
• However, it is much more flexible to create an intermediate energy-containing molecule
which could be used for any work needed within the cell rather than just dedicating the
energy to one job
• This intermediate energy containing molecule is called adenosine triphosphate (ATP)
• If the phosphate groups are broken off, change in the potential energy of the system
caused as a result, means lots of energy is made available for work
• ATP contains the purine adenine
• Adenine joins with the sugar ribose (not deoxyribose) to form adenosine
• Adenosine can be combined with one, two, or three phosphate groups
How Does ATP Provide Energy to the Cell?
• Energy cannot be created, it is just changed from one form to another, and during
respiration the energy in the bonds between carbon and hydrogen molecules can be
released and used to form bonds between adenosine and phosphate groups
• These groups can later be broken off and the energy released during that reaction can
power other activities in the cell
• When a phosphate group is removed from ATP, ADP is formed and 30.5 kJmol-1 of energy
is released
• Removal of a second phosphate groups forms AMP and a further 30.5 kJmol-1 of energy is
released
• Removal of the last phosphate group only releases 14.2 kJmol-1 of energy (this final
reaction rarely happens)
• All these reactions are reversible, and it is usually the interconversion between ATP and
ADP that is important in cellular activities
• The rate of interconversion between ADP and ATP is enormous
• It has been estimated that an average human at rest uses about 40 kg of ATP over a 24
hour period, but at any one point in time only contains about 5 kg of ATP
• During strenuous exercise, ATP conversion to ADP may be as much as 0.5 kg per minute
• All the cell's energy yielding reactions are linked to making ATP
• This ATP is then used for all sorts of work, so whatever the type of cell, ATP is the link
between energy yielding and energy requiring reactions
• So ATP is the currency of all cells, rather than each cell using their own different types of
intermediates
• ATP is highly suited to the role
• It is easily hydrolysed, small and water soluble, so easily transported around the cell
• Energy transfer is inefficient
• Whenever energy is transferred, some is lost as thermal energy
• At the different stages in a multi-step process, such as respiration, the energy made
available may not correspond with the energy needed to make ATP
• Any excess energy is lost as heat
• In addition, any reactions that don't require as much energy as that which is released
when ATP loses a phosphate group is also lost as heat
Energy Currency vs Energy Storage Molecules
• ATP is an energy currency molecule. It acts as the immediate donor to an energy requiring
reaction
• An energy storage molecule can be short-term (glucose or sucrose) or long-term (starch,
glycogen, or triglyceride) store of chemical potential energy
Glycolysis
• There are no glucose carriers in the membranes of mitochondria, so glucose is unable to
move into these organelles
• Instead, glucose (a six carbon molecule) must first be split in half to form two smaller
three carbon molecules, each of which are identical, called pyruvate (or pyruvic acid)
• This occurs in the cytoplasm and is a multi-step process, each step being controlled by a
specific enzyme
• Energy from ATP is needed in the first few steps, but energy is released later in the
process and can be used to make ATP again
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