Like some other organelles, the chloroplasts possess an envelope
consisting of outer and inner membrane. The inner membrane
separates a homogeneous medium (stroma). In the stroma there are
located two kinds of thylakoid membranes — thylakoids of grans
and thylakoids of stroma. The thylakoids of grans are arranged like
coins, one on top of the other, and form chloroplast grans. The
thylakoids of stroma connect the grans one with another or do not get
in touch with them. The thylakoids of grans contain the “miraculous” molecule
chlorophyll (Fig. 2–69), proteins, lipids, carotenoids, etc. that form
peculiar antennae (light-accumulating complexes). These complexes
are fixed to the thylakoid membranes through specific protein
melecules and serve to collect the quantums of light energy, which is
transmitted to the reaction centres. The reaction centres, together with
the donors and acceptors of electrons, are the most important
structural units of the two photosystems.
image
Figure 2–68. Electron micrograph of a giant chloroplast in unicellular alga Chlamydomonas (After De Robertis et al., 1973). 1 — gran; 2 — pyrenoid; 3 — dictyosome; 4 — flagellum; 5 — membrane; 6 — cell wall; 7 — nucleus; 8 — outer membrane of the chloroplasts; 9 — vacuole.
The energy accumulated in themis used for photolysis
of water with oxygen releasing and forming NADP.H₂
and ATP, which participate in the reduction of CO₂ in organic compounds.
The released molecular oxygen (O₂) takes part in the oxidative
phosphorylation in mitochondria, i.e. in the process of respiration, in which
highly energetic molecules are synthetized.
Photosynthesis represented by the general equation
is the greatest creative process in
the living nature. It is not only a unique mechanism of synthesizing organic
compounds, but it has created the prerequisites for the appearance and
development of aerobic metabolism, that has led to increase of the variety
of living organisms. These questions were discussed in more detail in
Chapter 1 (Metabolism).