A student investigated the oxygen consumption of isolated mitochondria under different conditions． Mitochondria were isolated by crushing liver cells in ice cold buffer and filtering to remove cell debris．The filtrate was centrifuged to separate the mitochondria．The mitochondria were suspended in ice cold buffer．（a）State one reason for keeping the mitochondria in cold buffer solution during isolation． . ［1］ Fig． 2.1 shows the apparatus used to measure the oxygen consumption of the isolated mitochondria． During this procedure the oxygen concentration in the chamber was measured continuously and displayed as a trace on the computer screen as shown in Fig. 2.2. Using the figures from the trace, the computer calculated the rate of oxygen consumption following each addition. This procedure was repeated three more times using fresh samples of mitochondria. Table 2.1 shows the rates of oxygen consumption for each of the four trials.

Biology

IGCSE&ALevel

CAIE

Exam No：9700_s25_qp_54 Year：2025 Question No：2(a)

Answer:

Knowledge points:

1.2.1.1 cell surface membrane

1.2.1.10 Cilia

1.2.1.11 microvilli

1.2.1.12 chloroplasts (including small circular DNA)

1.2.1.13 cell wall

1.2.1.14 plasmodesmata

1.2.1.15 large permanent vacuole and tonoplast of plant cells

1.2.1.2 nucleus, nuclear envelope and nucleolus

1.2.1.3 rough endoplasmic reticulum

1.2.1.4 smooth endoplasmic reticulum

1.2.1.5 Golgi body (Golgi apparatus or Golgi complex)

1.2.1.6 mitochondria (including small circular DNA)

1.2.1.7 ribosomes (80S in the cytoplasm and 70S in chloroplasts and mitochondria)

1.2.1.8 lysosomes

1.2.1.9 centrioles and microtubules

1.2.2 describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells

1.2.3 compare the structure of typical plant and animal cells

1.2.4 state that cells use ATP from respiration for energy-requiring processes

1.2.5.1 unicellular

1.2.5.2 generally 1–5 µm diameter

1.2.5.3 peptidoglycan cell walls

1.2.5.4 circular DNA

1.2.5.5 70S ribosomes

1.2.5.6 absence of organelles surrounded by double membranes

1.2.6 compare the structure of a prokaryotic cell as found in a typical bacterium with the structures of typical eukaryotic cells in plants and animals

1.2.7 state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids

12.2.1.1 glycolysis in the cytoplasm

12.2.1.2 link reaction in the mitochondrial matrix

12.2.1.3 Krebs cycle in the mitochondrial matrix

12.2.1.4 oxidative phosphorylation on the inner membrane of mitochondria

12.2.10 outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)

12.2.11 explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions (a detailed account of the total yield of ATP from the aerobic respiration of glucose is not expected)

12.2.12 explain how rice is adapted to grow with its roots submerged in water, limited to the development of aerenchyma in roots, ethanol fermentation in roots and faster growth of stems

12.2.13 describe and carry out investigations using redox indicators, including DCPIP and methylene blue, to determine the effects of temperature and substrate concentration on the rate of respiration of yeast

12.2.14 describe and carry out investigations using simple respirometers to determine the effect of temperature on the rate of respiration

12.2.2 outline glycolysis as phosphorylation of glucose and the subsequent splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules (3C), which are then further oxidised to pyruvate (3C), with the production of ATP and reduced NAD

12.2.3 explain that, when oxygen is available, pyruvate enters mitochondria to take part in the link reaction

12.2.4 describe the link reaction, including the role of coenzyme A in the transfer of acetyl (2C) groups

12.2.5 outline the Krebs cycle, explaining that oxaloacetate (4C) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (6C), which is converted back to oxaloacetate in a series of small steps

12.2.6 explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of the coenzymes NAD and FAD

12.2.7 describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane

12.2.8.1 hydrogen atoms split into protons and energetic electrons

12.2.8.2 energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected)

12.2.8.3 the released energy is used to transfer protons across the inner mitochondrial membrane

12.2.8.4 protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected)

12.2.8.5 oxygen acts as the final electron acceptor to form water

12.2.9 describe the relationship between the structure and function of mitochondria using diagrams and electron micrographs

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