Why does nucleus have a double membrane

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin Figure 2 ; chromatin describes the material that makes up the chromosomes both when condensed and decondensed.

We will focus on chromatin and chromosomes in greater detail later. Figure 2. We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Improve this page Learn More. Skip to main content. Module 4: Cellular Structure. Search for:. When we divide the organisms that live on this planet, we make a distinction between those that have a nucleus, that are called eukaryotes, and those that don't have a nuclei, which we call prokaryotes.

The nucleus contains all of the genetic material for a eukaryotic cell, but this genetic material needs to be protected. And it's protected by the nuclear membrane, which is a double membrane that encloses all the nuclear genetic material and all the other components of the nucleus. There are some small holes or pores that are in the nuclear membrane that allow the messenger RNA and the proteins to move between the nucleus and the cytoplasm. 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. 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. 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.

Figure 6: The relationship between the radius, surface area, and volume of a cell 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. This page appears in the following eBook. Aa Aa Aa. Eukaryotic Cells.

Figure 1: A mitochondrion. Figure 2: A chloroplast. What Defines an Organelle? Why Is the Nucleus So Important? Why Are Mitochondria and Chloroplasts Special?

Figure 4: The origin of mitochondria and chloroplasts. Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms. Figure 5: Typical prokaryotic left and eukaryotic right cells. In prokaryotes, the DNA chromosome is in contact with the cellular cytoplasm and is not in a housed membrane-bound nucleus. Figure 6: The relationship between the radius, surface area, and volume of a cell.

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. 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.

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