The nucleus is surrounded by a double membrane known as the nuclear envelope. What is the function of this membrane? To prevent any substances from entering or leaving the nucleus. To act as the site of RNA synthesis. To allow the attachment of the nucleus to the cell membrane. Or to separate the contents of the nucleus from the cytoplasm.
Let’s approach this question by using information we already know about the function of the nucleus, the nuclear envelope, and other cellular components. The nuclear envelope is described in the question as a membrane. A plasma membrane is a semipermeable barrier which surrounds cells and some eukaryotic organelles, separating them from their external environment. Membranes usually also control what enters and leaves a certain area or cell. This is a very simple diagram of a eukaryotic cell, which is one that contains a nucleus. You can see the cell surface membrane which surrounds the cell labeled here. The nucleus, like all organelles, is connected to the cell surface membrane via the cytoskeleton.
Let’s take a closer look at the nucleus so we can better understand structures within it and their different functions. The nuclear envelope is a specialized version of a plasma membrane which surrounds the nucleus of most eukaryotic cells. The nuclear envelope prevents certain larger substances, such as DNA molecules pictured here, from entering and leaving the nucleus. The nuclear envelope contains small gaps called nuclear pores. These pores allow molecules like RNA to pass into and out of the nucleus easily. This is because it is a smaller genetic molecule than DNA. So, RNA is able to fit through the nuclear pores while DNA cannot.
The nuclear envelope is also useful as it compartmentalizes reactions occurring in the nucleus and elsewhere in the cell. Compartmentalization is the formation of separate membrane-bound areas within a cell to allow incompatible reactions to occur within different parts of the cell. This means that the reactants, products, and enzymes involved in different reactions in the nucleus and in the cytoplasm can be kept separate. Within the nucleus, there is a dense sphere called the nucleolus. The nucleolus is the area of the nucleus where RNA, specifically the rRNA that ribosomes are made of, is synthesized.
Now that we know some key information about the nucleus and its components, let’s review our different options to decide which is correct. First, let’s look at the option stating that the nuclear envelope functions to prevent any substances from entering or leaving the nucleus.
We know that one of the roles of a membrane is to control what enters and leaves a certain area or cell. But there will be no use if they blocked all substances from entering or leaving. The nuclear pores in the nuclear envelope might prevent larger substances from entering and leaving the nucleus. But smaller molecules like the RNA that we spoke about here do pass through the nuclear envelope using these pores. Therefore, this option is incorrect as it states all substances are prevented from passing through the membrane, while we know that some substances like RNA, which is smaller, can.
Let’s look at the option referring to RNA synthesis. If we look at our diagram, we can see that the nucleolus is the area in the nucleus where the rRNA that ribosomes are made of is synthesized. This means that this option is also not describing the function of the nuclear envelope, so is incorrect.
Let’s look at the option referring to the attachment of the nucleus to the cell membrane. As we mentioned earlier, the nucleus is connected to the cell surface membrane via the cytoskeleton. Therefore, this attachment is not a direct result of the nuclear envelope being present. So, this option is also incorrect.
Let’s look at our final option. This answer states that the function of the membrane is to separate the contents of the nucleus from the cytoplasm. This is describing compartmentalization, as you’ll recall. So, it does describe one of the key functions of all membranes separating the contents of something from something else outside it so the different chemical reactions can occur in each section. This makes our best option for the function of the nuclear envelope to separate the contents of the nucleus from the cytoplasm.