An Introduction to Molecular Biotechnology. Группа авторовЧитать онлайн книгу.
3.18). Apart from the surrounding inner and outer biomembranes, the chloroplast contains an extensively folded membrane system, known as thylakoids. These contain chlorophyll, as well as the proteins and enzymes required for photosynthesis, to enable the plants to turn sunlight into energy in the form of ATP and NADPH (Figure 3.19). The electron transport between photosystem II and I and the production of NADPH are explained in Figure 3.19b. The light reaction leads to the buildup of a proton gradient, which is then used by ATP synthase to produce ATP. During the subsequent CO2fixation process (Calvin cycle), CO2 is first bound to ribulose‐1,5‐biphosphate, which is then cleaved into two C3 units (3‐phosphoglycerate). 3‐Phosphoglycerate is transformed into glycerol aldehyde‐3‐phosphate, which is used for the regeneration of ribulose‐1,5‐biphosphate and for building glucose, fatty acids, and amino acids. A plant cell can generate additional ATP from glucose for the energy supply of the cell. This makes plants autotrophic and a suitable basic nutrient for heterotrophic animals that live on organic matter.
Figure 3.18 Structure of a chloroplast. (a) Electron microscope photo of a chloroplast.
Source: Courtesy of T. Elliot Weier.
(b) Schematic representation.
Source: Voet et al. (2016). Adapted with permission of John Wiley and Sons.
Figure 3.19 Essential steps in photosynthesis. (a) Overview of photosynthetic reactions in chloroplasts. (b) Electron transport between photosystem II and I across the thylakoid membrane, resulting in NADPH production. ATP synthase uses the proton gradient for the production of ATP. Q, plastoquinone; FD, ferredoxin; PS I, photosystem I.
Like mitochondria, chloroplasts contain their own ring‐shaped DNA (cpDNA) as well as independent replication, transcription, and protein biosynthesis. The chloroplast genome has a size of 120–200 kb (Figure 3.20); it encodes 120 genes and is present at 20–300 copies in a single chloroplast. As a plant cell contains up to 40 chloroplasts, the total number of cpDNA copies is between 800 and 3200 per cell. The analysis of nucleotide sequences from chloroplast genes has become an important tool in systematics to establish phylogenies and to define species.
Figure 3.20 Overview of the arrangement of genes in chloroplast genomes.
For chloroplasts, too, it is assumed that there is an endosymbiotic origin (Table 3.6). The nucleotide sequences in chloroplast genes and the amino acid sequences of the corresponding proteins are more closely related to those of cyanobacteria than to the respective genes in the plant cell nucleus. Figure 3.21 gives a schematic overview of the presumptive origin of chloroplasts. Similar to the intake of mitochondria, an early eucyte seems to have taken up photosynthetic bacteria through phagocytosis and tamed them to develop an endosymbiosis. It is thought that the acquisition of chloroplasts happened several times in the phylogeny of photosynthetically active algae and plants.
Table 3.6 Prokaryotic properties of plastids and mitochondria.
| Genome | Mostly circular DNA adhesive to biomembrane without histones and nucleosomes, several copies concentrated in nucleoids; gene arrangement more or less prokaryotic (operon structure); repetitive sequences rare or nonexistent |
| Ribosomes | 70S‐type |
| Translation | No Cap structure at the 5′ end of mRNAs; prokaryotic complement of initiation factors |
| Tubulin, actin | Not found in organelles; FtsZ, a bacterial, tubulin‐homologous cell division protein is involved in the division of plastids |
| Plastid fatty acid synthesis | As in bacteria, using acyl carrier proteins |
| Cardiolipin | Membrane lipid found in many bacteria. Not present in eukaryotic membranes except the inner mitochondrial membrane |
Figure 3.21 Development of chloroplasts through phagocytosis of cyanobacteria.
Mitochondria and chloroplasts never emerge de novo, but replicate through division. When the cell divides, mitochondria are distributed over the daughter cells. Mitochondria can also fuse with each other. Both mitochondria and mostly also chloroplasts are inherited maternally. Mitochondria in sperm cells are not incorporated into the fertilized egg. Although replication, transcription, and protein biosynthesis still happen in the same way in mitochondria and chloroplasts, they have become organelles and are no longer autonomous. They import most of their proteins from the cytoplasm. These proteins carry signaling sequences that bind to receptors on the organelles (see Chapter 5), and through complex transport mechanisms, they finally reach their working place inside the mitochondria and chloroplasts. The corresponding genes used to be part of the endosymbionts but have increasingly been moved into the nucleus. Only a relatively small set of genes has remained in mitochondria and chloroplasts. While this applies mostly to protein‐coding genes, tRNA and rRNA genes have remained in the organelles.
3.1.4 Cytoplasm
The cytoplasm or cytosol of a eukaryotic cell is what is left when all membrane systems and organelles have been removed. In most cells, this is the largest compartment. In bacteria, it is the only existing compartment. It contains a multitude of low‐molecular‐weight compounds and proteins, including hundreds of regulatory proteins that are interlinked and communicate through complex interaction, such as phosphorylation and dephosphorylation of proteins, modulation by the binding of GTP or GDP, and conformational changes (cell biologists coined the term cross talk for protein interaction). They can pick up signals and pass them on (signal transduction), and it will require extensive research to understand these processes in detail.
At