Scientists believe that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them. We also know that mitochondria and chloroplasts have DNA and ribosomes, just as bacteria do. Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. It is also beneficial for the microbes because they are protected from other organisms and are provided a stable habitat and abundant food by living within the large intestine. This relationship is beneficial for us because we are unable to synthesize vitamin K. Microbes that produce vitamin K live inside the human gut. Endosymbiotic relationships abound in nature. Endosymbiosis ( endo-= within) is a relationship in which one organism lives inside the other. Symbiosis is a relationship in which organisms from two separate species live in close association and typically exhibit specific adaptations to each other. Have you wondered why? Strong evidence points to endosymbiosis as the explanation. We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Their photosynthetic pigments are located in the thylakoid membrane within the cell itself. Some bacteria also perform photosynthesis, but they do not have chloroplasts. Like plant cells, photosynthetic protists also have chloroplasts. The chloroplasts contain a green pigment called chlorophyll, which captures the energy of sunlight for photosynthesis. The fluid enclosed by the inner membrane and surrounding the grana is called the stroma. Each stack of thylakoids is called a granum (plural = grana). Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked, fluid-filled membrane sacs called thylakoids (Figure 3). This is the major difference between plants and animals: Plants (autotrophs) are able to make their own food, like glucose, whereas animals (heterotrophs) must rely on other organisms for their organic compounds or food source. In photosynthesis, carbon dioxide, water, and light energy are used to make glucose and oxygen. Chloroplasts function in photosynthesis and can be found in photoautotrophic eukaryotic cells such as plants and algae. Like mitochondria, chloroplasts also have their own DNA and ribosomes. This simplified diagram of a chloroplast shows the outer membrane, inner membrane, thylakoids, grana, and stroma.
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