All living cells need energy to stay alive. The energy is used to power all the activities of life including digestion, protein synthesis and active transport. A cell’s energy sources are the sugars and other substances derived from nutrients, which can be broken down to release the energy that holds their molecules together.
Cell respiration is the gradual breakdown of nutrient molecules such as glucose and fatty acids in a series of reactions that ultimately release energy in the form of ATP.
Glucose is probably the most commonly used source of energy. Each glucose molecule is broken down by enzymes in a number of stages, which release energy in small amounts as each covalent bond is broken.
If there is insuffi cient glucose available, fatty acids or amino acids can be used instead.
Glycolysis
The fi rst stage in cell respiration is glycolysis. Glucose that is present in the cytoplasm of a cell is broken down by a series of enzymes, to produce two molecules of a simpler compound called pyruvate. As this occurs, there is a net production of two molecules of ATP (Figure 3.19).
glucose → 2 pyruvate + 2 ATP
6-carbon sugar 2 × 3-carbon sugar
Aerobic and anaerobic respiration
The next stage of cell respiration depends on whether or not oxygen is available. In the presence of oxygen, aerobic respiration can take place;
without it, respiration must be anaerobic.
Aerobic respiration is the most effi cient way of producing ATP.
Aerobic respiration is carried out by cells that have mitochondria and it produces a great deal of ATP. Pyruvate molecules produced by glycolysis enter the mitochondria and are broken down, or oxidised, in a series of reactions that release carbon dioxide and water and produce ATP.
The two pyruvate molecules from glycolysis fi rst lose carbon dioxide and become two molecules of acetyl CoA in the link reaction, as Cell respiration the controlled
release of energy in the form of ATP from organic compounds in a cell
ATP (adenosine triphosphate) is the energy currency of a cell. It is needed for every activity that requires energy. Cells make their own ATP in mitochondria. When energy is used, ATP is broken down to ADP (adenosine diphosphate) and inorganic phosphate. This conversion releases energy for use and a cyclic process will reform the ATP during respiration.
during respiration
ADP + Pi ATP + H2O
during metabolic activity
3 THE CHEMISTRY OF LIFE 59 shown in Figure 3.19. Acetyl CoA then enters a stage called the Krebs
cycle and is modifi ed still further, releasing more carbon dioxide.
Finally, products of the cycle react directly with oxygen and the result is the release of large amounts of ATP. The original glucose molecule is completely broken down to carbon dioxide and water so the equation for aerobic respiration is often shown as:
glucose + oxygen → carbon dioxide + water + 38 ATP
Figure 3.19 Summary of glycolysis, the link reaction and the Krebs cycle.
Glycolysis actually uses 2 molecules of ATP to get the process underway, but produces 4 molecules of ATP in total. Thus, we say there is a net production of 2 ATPs.
Lactate is taken by the blood to the liver, where it is converted back to pyruvate. This may either be used as a fuel, producing carbon dioxide and water, or be converted back to glucose using energy.
glycolysis
In cytoplasm
In mitochondria link reaction
Krebs cycle
acetyl CoA glucose
triose phosphate
pyruvate
reduced NAD
reduced NAD reduced
NAD reduced NAD
reduced FAD
CoA reduced NAD
ATP
ATP
ATP
citrate
CO2
Anaerobic respiration occurs in the cytoplasm of cells. In animal cells, the pyruvate produced by glycolysis is converted to lactate (Figure 3.20, overleaf ), which is a waste product and is taken out of the cells. Anaerobic respiration occurs in cases where, for example, a person is doing vigorous exercise and their cardiovascular system is unable to supply suffi cient oxygen for aerobic respiration. One consequence of anaerobic respiration and a build-up of lactate in the muscles is the sensation of cramp.
pyruvate → lactate
In other organisms, such as yeast, anaerobic respiration is also known as fermentation, and produces a diff erent outcome. The pyruvate molecules from glycolysis are converted to ethanol (alcohol) and carbon dioxide (Figure 3.21, overleaf).
pyruvate → ethanol + carbon dioxide
No further ATP is produced by the anaerobic respiration of pyruvate.
Respiration is described in greater detail in Chapter 8.
12 What are the two products of anaerobic respiration in muscles?
13 Where does aerobic respiration take place in a eukaryotic cell?
14 Where in a cell does glycolysis occur?
3.8 Photosynthesis
glucose
triose phosphate
pyruvate lactate
oxidised
NAD oxidised
reduced NAD NAD
glucose
triose phosphate
pyruvate ethanal
CO2
ethanol oxidised
NAD oxidised
reduced NAD NAD
Figure 3.20 Anaerobic respiration in animal cells.
Figure 3.21 Anaerobic respiration in yeast cells.
Assessment statements
•
State that photosynthesis involves the conversion of light energy into chemical energy.•
State the light from the Sun is composed of a range of wavelengths (colours).•
State that chlorophyll is the main photosynthetic pigment.•
Outline the diff erences in absorption of red, blue and green light by chlorophyll.•
State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.•
State that ATP and hydrogen (derived from the photolysis of water) are used to fi x carbon dioxide to make organic molecules.•
Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass.•
Outline the eff ects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis.Photosynthesis means ‘making things with light’. Glucose is the molecule most commonly made.
Photosynthesis and light
The Sun is the source of energy for almost all life on Earth. Light energy from the Sun is captured by plants and other photosynthetic organisms, and converted into stored chemical energy. The energy is stored in
3 THE CHEMISTRY OF LIFE 61 molecules such as glucose, which provide a source of food for organisms
that cannot use light energy directly.
Visible light is composed of a spectrum of colours, which can be separated using a prism (Figure 3.22). A prism bends rays of light and separates the colours because each one has a slightly diff erent wavelength and is refracted (bent) to a slightly diff erent degree. Visible light has a range of wavelengths but the most important regions of the spectrum for photosynthesis are red and blue.
The colour of any object is determined by the wavelength of the light that it refl ects back into our eyes. A blue shirt appears blue because it refl ects blue light, which our eyes can perceive, but the shirt absorbs other wavelengths that fall on it and we do not see those colours. A black object absorbs all wavelengths of light, while something white refl ects them all.
Most plants have green leaves. This tells us that they do not absorb the green part of the spectrum – green light is refl ected and makes the leaf appear green. Looking closely at the structure of plant cells such as those shown in Figure 2.8 (page 23) we can see that the green colour is due to the chloroplasts, which contain a green pigment called chlorophyll.
Chlorophyll is unable to absorb green light, which it refl ects, but it does absorb other wavelengths well. Red and blue light are absorbed particularly well and provide the energy needed for photosynthesis. The top graph in Figure 3.23 (overleaf) shows that the red and blue ends of the visible spectrum are the wavelengths that the photosynthetic pigments in plants absorb most effi ciently. The bottom graph shows that the rate of photosynthesis is highest when plants absorb these wavelengths.
15 If you wanted to make plants grow as well as possible, what colour of light should you shine on them?
16 What would happen to a plant’s growth if it were kept in green light?