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The molecules of cells

The genome of the incorporated cell remains separate curved blue line from the host cell genome curved purple line. The incorporated cell may continue to replicate as it exists within the host cell. Over time, during errors of replication or perhaps when the incorporated cell lyses and loses its membrane separation from the host, genetic material becomes separated from the incorporated cell and merges with the host cell genome.

Eventually, the host genome becomes a mixture of both genomes, and it ultimately becomes enclosed in an endomembrane, a membrane within the cell that creates a separate compartment. This compartment eventually evolves into a nucleus. Figure Detail. Besides the nucleus, two other organelles — the mitochondrion and the chloroplast — play an especially important role in eukaryotic cells.

These specialized structures are enclosed by double membranes, and they are believed to have originated back when all living things on Earth were single-celled organisms. At that time, some larger eukaryotic cells with flexible membranes "ate" by engulfing molecules and smaller cells — and scientists believe that mitochondria and chloroplasts arose as a result of this process. In particular, researchers think that some of these "eater" eukaryotes engulfed smaller prokaryotes, and a symbiotic relationship subsequently developed.

Once kidnapped, the "eaten" prokaryotes continued to generate energy and carry out other necessary cellular functions, and the host eukaryotes came to rely on the contribution of the "eaten" cells. Over many generations, the descendants of the eukaryotes developed mechanisms to further support this system, and concurrently, the descendants of the engulfed prokaryotes lost the ability to survive on their own, evolving into present-day mitochondria and chloroplasts.

This proposed origin of mitochondria and chloroplasts is known as the endosymbiotic hypothesis.

Like oil and water

At some point, a eukaryotic cell engulfed an aerobic bacterium, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion. Eukaryotic cells containing mitochondria then engulfed photosynthetic bacteria, which evolved to become specialized chloroplast organelles. In addition to double membranes, mitochondria and chloroplasts also retain small genomes with some resemblance to those found in modern prokaryotes. This finding provides yet additional evidence that these organelles probably originated as self-sufficient single-celled organisms.

Today, mitochondria are found in fungi, plants, and animals, and they use oxygen to produce energy in the form of ATP molecules, which cells then employ to drive many processes. Scientists believe that mitochondria evolved from aerobic , or oxygen-consuming, prokaryotes. In comparison, chloroplasts are found in plant cells and some algae, and they convert solar energy into energy-storing sugars such as glucose.

Chloroplasts also produce oxygen, which makes them necessary for all life as we know it. Scientists think chloroplasts evolved from photosynthetic prokaryotes similar to modern-day cyanobacteria Figure 4. Today, we classify prokaryotes and eukaryotes based on differences in their cellular contents Figure 5. In eukaryotes, however, the DNA takes the form of compact chromosomes separated from the rest of the cell by a nuclear membrane also called a nuclear envelope. Eukaryotic cells also contain a variety of structures and organelles not present in prokaryotic cells.

Throughout the course of evolution, organelles such as mitochondria and chloroplasts a form of plastid may have arisen from engulfed prokaryotes. A paradigm gets shifty. Nature , All rights reserved. Mitochondria — often called the powerhouses of the cell — enable eukaryotes to make more efficient use of food sources than their prokaryotic counterparts.

That's because these organelles greatly expand the amount of membrane used for energy-generating electron transport chains. In addition, mitochondria use a process called oxidative metabolism to convert food into energy, and oxidative metabolism yields more energy per food molecule than non-oxygen-using, or anaerobic , methods.

Eukaryotic cells

Energywise, cells with mitochondria can therefore afford to be bigger than cells without mitochondria. Within eukaryotic cells, mitochondria function somewhat like batteries, because they convert energy from one form to another: food nutrients to ATP. Accordingly, cells with high metabolic needs can meet their higher energy demands by increasing the number of mitochondria they contain.

For example, muscle cells in people who exercise regularly possess more mitochondria than muscle cells in sedentary people.

Mitochondria - Turning on the Powerhouse

Prokaryotes, on the other hand, don't have mitochondria for energy production, so they must rely on their immediate environment to obtain usable energy. Prokaryotes generally use electron transport chains in their plasma membranes to provide much of their energy. The actual energy donors and acceptors for these electron transport chains are quite variable, reflecting the diverse range of habitats where prokaryotes live. In aerobic prokaryotes, electrons are transferred to oxygen, much as in the mitochondria.

The challenges associated with energy generation limit the size of prokaryotes.

Cell Structure and Function - Organelles

As these cells grow larger in volume, their energy needs increase proportionally. However, as they increase in size, their surface area — and thus their ability to both take in nutrients and transport electrons — does not increase to the same degree as their volume.


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As a result, prokaryotic cells tend to be small so that they can effectively manage the balancing act between energy supply and demand Figure 6. This page appears in the following eBook. Aa Aa Aa. Eukaryotic Cells. What Defines an Organelle? Why Is the Nucleus So Important? Why Are Mitochondria and Chloroplasts Special? Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms.

The molecules of cells

In prokaryotes, the DNA chromosome is in contact with the cellular cytoplasm and is not in a housed membrane-bound nucleus. Note that as the radius of a cell increases from 1x to 3x left , the surface area increases from 1x to 9x, and the volume increases from 1x to 27x. Organelles serve specific functions within eukaryotes, such as energy production, photosynthesis, and membrane construction.

Most are membrane-bound structures that are the sites of specific types of biochemical reactions. The nucleus is particularly important among eukaryotic organelles because it is the location of a cell's DNA.


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  4. Two other critical organelles are mitochondria and chloroplasts, which play important roles in energy conversion and are thought to have their evolutionary origins as simple single-celled organisms. Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close. No topic rooms are there.

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