PHOTOSYNTHESIS
(a) The interrelationship between the process of photosynthesis and respiration
● To include the relationship between the raw materials and products of the two processes.
Oxygen and respiration are antagonistic processes: respiration using glucose and oxygen as
reactants and producing water and carbon dioxide as products, releasing energy;
photosynthesis, uses water and carbon dioxide as reactants, whilst producing glucose and
oxygen, and trapping energy in the process. This plays a role in the energy transfer through
ecosystems, with the chemical energy stored in photosynthesis by autotrophs being released in
respiration by heterotrophs. It also means the processes play a role in the relative
concentrations of oxygen and carbon dioxide in the atmosphere. In plants, oxygen is a
competitive inhibitor to RuBisCo, the enzyme that fixes carbon dioxide, leading to
photorespiration consuming products of the calvin cycle, as well as leading to the production of
toxic phosphoglycolate instead of GP, which must be converted using ATP to other organic
molecules. However, RuBisCo has a higher affinity for CO2.
(b) The structure of a chloroplast and the sites of the two main stages of photosynthesis.
● The components of a chloroplast including outer membrane, lamellae, grana, thylakoid,
stroma and DNA.
Chloroplasts are disc-shaped and about 2-10 um long. Each is surrounded by an a double
membrane, the envelope, with an intermembrane space of width 10-20nm between the inner and
outer membrane. The outer membrane is highly permeable to allow the the diffusion of CO 2 and
O2 across the membrane. Highly folded thylakoid membranes create flattened, membrane-bound
sacs, which form stacks of grana, containing about one-hundred grana, joined by intergranal
lamellae. This is the location of the light-independent reactions and provide a large surface area
for the distribution of photosystems that contain photosynthetic pigments that harvest light and
electron carriers in and ATP synthase needed to perform chemiosmosis, generating ATP.
Proteins embedded in the membrane hold the photosystems in place. This is surrounded by the
stroma—the fluid-filled matrix, containing the enzymes of photosynthesis, as well as starch
grains, lipid droplets, and DNA containing genes for some of the proteins needed for
photosynthesis, which are translated on 70S ribosomes found in the stroma.
(c) (i) The importance of photosynthetic pigments in photosynthesis.
● To include reference to light harvesting systems and photosystems.
● (ii) Practical investigations using thin layer chromatography (TLC) to separate
photosynthetic pigments.
Within the thylakoid membranes are funnel-shaped protein antenna complexes, that contain
photosynthetic pigments, making up a photosystem. Pigments absorb a particular wavelength of
light and funneling the energy through resonance energy transfer down to the primary pigment
reaction centre. The primary pigment is chlorophyll a—P680 in photosystem II and P700 in
photosystem I—consisting of a porphyrin group, containing a magnesium cofactor, and a long
hydrocarbon chain. Appearing blue-green, chlorophyll a has a peak absorption of 680/700, but
also absorbs blue light with a wavelength ~440 nm. The reaction centre is surrounded by light
harvesting complexes enhance the absorption of a wider range of wavelengths. The antenna
complex contains many accessory pigments which absorb other frequencies of light.
Chlorophyll b absorbs light of wavelengths of 400-500 nm and around 640 nm, and appear
yellowy green; Carotenoids absorb blue light of wavelengths between 400-500 nm; and
xanthophylls absorb blue and green light of wavelengths 375-550 nm and reflect yellow light.
