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Worksheet 8.1
Chapter 8: Cell respiration and photosynthesis – fifteen
summary facts
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Cellular metabolism is the sum of all chemical reactions carried out by an organism. These
reactions involve catabolic and anabolic pathways. Cellular respiration is an example of a
catabolic pathway. Photosynthesis is an example of an anabolic pathway.
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One general type of chemical reaction is oxidation. Oxidation involves loss of electrons and/or
hydrogen and gain of oxygen; it results in a compound with lower potential energy. Reduction
involves gain of electrons and/or hydrogen and loss of oxygen; it results in a compound with
higher potential energy.
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Glycolysis occurs in the cytosol; the link reaction, the Krebs cycle, electron transport chain and
chemiosmosis all occur within the mitochondrion.
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Glycolysis allows a net gain of two ATPs, two molecules of NADH, and two molecules of
pyruvate. This metabolic process is controlled by enzymes. If ATP levels are high in the cell,
feedback inhibition blocks the first enzyme of the pathway. This slows or stops the process.
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Once pyruvate enters the matrix of the mitochondrion, decarboxylation occurs to form a 2-carbon
acetyl group. The acetyl group is then oxidized and combines with coenzyme A to form acetyl
CoA. CoA can be produced from a number of original compounds including carbohydrates, lipids,
and amino acids. The preferred molecule of the cell for the complete respiration process is the
hexose called glucose.
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The Krebs cycle begins and ends with oxaloacetate. Oxidation of the 6-carbon compound
produced by the combination of acetyl CoA and oxaloacetate then occurs in the Krebs cycle to
produce one ATP, three NADH molecules, one FADH2 molecule and two carbon dioxide
molecules. For one molecule of glucose, two molecules of acetyl CoA go through the Krebs cycle
thus doubling all the products just mentioned.
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The electron transport chain is where most of the ATPs from cellular respiration are produced.
This reaction occurs in the inner mitochondrial membrane and the membranes of the cristae.
Carriers having different electronegativities allow multiple transfers of electrons down an energy
gradient. The molecules NADH and FADH2 from the previous stages of cellular respiration
provide the electrons (hydrogen) for this chain to occur. Oxygen is the final electron (hydrogen)
acceptor forming ‘water of metabolism’.
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Chemiosmosis involves the movement of protons (hydrogen ions) so that phosphorylation,
formation of ATP may occur. This is called oxidative phosphorylation. Substrate-level
phosphorylation occurs in glycolysis and does not involve an electron transport chain.
9
All reactions of photosynthesis occur within the chloroplast and include the light-dependent
reaction and the light-independent reaction. Photosynthesis only occurs within autotrophs and
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requires the presence of light.
10 The light-dependent reaction occurs in the thylakoids (grana) of chloroplasts. Pigments occur in
photosystems and allow the absorption of light energy. Modern plants have two types of
photosystem: photosystem I and photosystem II.
11 Photosystem II begins the process, allowing the formation of free oxygen, energized electrons that
are de-energized through an electron transport chain to produce ATP and free hydrogens.
Photosystem I then occurs with the formation of NADPH.
12 ATP and NADPH from the light-dependent reaction then pass to the light-independent reaction.
The light-independent reaction then occurs within the stroma of the chloroplast with the ultimate
formation of glucose (complex carbohydrate).
13 The light-independent reaction involves the Calvin cycle and begins and ends with the compound
ribulose bisphosphate (RuBP). Once ATP and NADPH have delivered their products to this
reaction, their de-energized forms are delivered back to the light-dependent reaction so that they
may become re-energized.
14 The wavelengths of light that plants actually use in the light-dependent reaction make up the
action spectrum and include mainly blue and red light. The absorption spectrum refers to the
wavelengths of light actually absorbed. There is a positive correlation between the absorption and
the action spectra.
15 Photosynthesis has several limiting factors. They include: temperature, light intensity, and carbon
dioxide concentration.
© Pearson Education Ltd 2010. For more information about the Pearson Baccalaureate series please visit
www.pearsonbacc.com
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