A eukaryotic cell equipped with chloroplasts has no need to chase after other cells as prey; it is nourished by the captive chloroplasts it has inherited from its ancestors. Correspondingly, plant cells, although they possess the cytoskeletal equipment for movement, have lost the ability to change shape rapidly and to engulf other cells by phagocytosis. Instead, they create around themselves a tough, protective cell wall. If the first eukaryotic cells were predators on other organisms, we can view plant cells as cells that have made the transition from hunting to farming.
Fungi represent yet another eukaryotic way of life. Fungal cells, like animal cells, possess mitochondria but not chloroplasts; but in contrast with animal cells and protozoa, they have a tough outer wall that limits their ability to move rapidly
Figure 1–28 A mitochondrion. (A) A cross section, as seen in the electron microscope.
(B) A drawing of a mitochondrion with part of it cut away to show the three-dimensional structure (Movie 1.2). (c) A schematic eukaryotic cell, with the interior space of a mitochondrion, containing the mitochondrial dnA and ribosomes, colored. note the smooth outer membrane and the convoluted inner membrane, which houses the proteins that generate ATP from the oxidation of food molecules. (A, courtesy of daniel s. Friend.)
Figure 1–29 The origin of mitochondria.
archaeon) is thought to have engulfed the bacterial ancestor of mitochondria, initiating a symbiotic relationship. clear evidence of a dual bacterial and archaeal inheritance can be discerned today in the genomes of all eukaryotes.
or to swallow up other cells. Fungi, it seems, have turned from hunters into scavengers: other cells secrete nutrient molecules or release them upon death, and fungi feed on these leavings—performing whatever digestion is necessary extracellularly, by secreting digestive enzymes to the exterior.
The genetic information of eukaryotic cells has a hybrid origin—from the ancestral anaerobic archaeal cell, and from the bacteria that it adopted as symbionts. Most of this information is stored in the nucleus, but a small amount remains inside the mitochondria and, for plant and algal cells, in the chloroplasts. When mitochondrial DNA and the chloroplast DNA are separated from the nuclear DNA and individually analyzed and sequenced, the mitochondrial and chloroplast genomes are found to be degenerate, cut-down versions of the corresponding bacterial genomes. In a human cell, for example, the mitochondrial genome consists of only 16,569 nucleotide pairs, and codes for only 13 proteins, 2 ribosomal RNA components, and 22 transfer RNAs.
Figure 1–30 Chloroplasts. These organelles capture the energy of sunlight in plant cells and some single-celled eukaryotes. (A) A single cell isolated from a leaf of a flowering plant, seen in the light microscope, showing the green chloroplasts (Movie 1.3 and see movie 14.9). (B) A drawing of one of the chloroplasts, showing the highly folded system of internal membranes containing the chlorophyll molecules by which light is absorbed. (A, courtesy of Preeti dahiya.) early eukaryotic cell eukaryotic cell capable of photosynthesis
Many of the genes that are missing from the mitochondria and chloroplasts have not been lost; instead, they have moved from the symbiont genome into the DNA of the host cell nucleus. The nuclear DNA of humans contains many genes coding for proteins that serve essential functions inside the mitochondria; in plants, the nuclear DNA also contains many genes specifying proteins required in chloroplasts. In both cases, the DNA sequences of these nuclear genes show clear evidence of their origin from the bacterial ancestor of the respective organelle.