Which molecules do not normally cross the nuclear membrane?
A nuclear membrane’s primary function is to act as a barrier between the nucleus and the rest of the cell. This means that only certain molecules can enter the nucleus across the membrane.
Cytoplasmic molecules are those molecules that do not normally cross the nuclear membrane. Among them are most proteins and all of the cell’s DNA.
These cytoplasmic molecules are specifically kept out of the nucleus by the nuclear membrane. This is achieved by a number of features, such as:
The membrane has a selective permeability in which only certain molecules can pass through it
– A strong, impermeable lipid bilayer that serves as a barrier
– Proteins that control which molecules can enter cells by acting as channels or gates
The nuclear membrane is able to control selectively which molecules can enter the nucleus thanks to these features. Keeping the genetic information in the nucleus protected from outside influences is essential for its long-term health.
Some of these cytoplasmic molecules are actively transported across the nuclear membrane, which is an energetic process. This process is known as “translocation” and some examples include:
DNA translocation during cell division (mitosis)
– Translocation of mRNA during protein synthesis (Yaha Samma Matra Hai Ta)
– The export of mRNA from the nucleus to the cytoplasm in a eukaryotic cell. This is a crucial step which allows ribosomes in the cytoplasm to produce proteins which can be used within the cell.
However, for other molecules which need to enter or leave the nucleus, it is easier for them to simply diffuse across the nuclear membrane. In this case, diffusion does not require energy which means that it can happen spontaneously and automatically, without outside help.
One example of this is the diffusion which occurs between the cytoplasm and the nucleus during osmosis. Osmosis describes the process which allows water to flow across semi-permeable membranes from an area which has a higher concentration of solutes to one which has a lower concentration. In cells, osmosis causes water to move passively across the nuclear membrane from the cytoplasm into the nucleus, which creates a situation known as “osmotic equilibrium”. This equilibrium ensures that both compartments contain roughly equal concentrations of solutes and water.
The lipid bilayer of the nuclear membrane is important for maintaining this equilibrium, which is why the nuclear membrane has a different lipid composition to the cytoplasmic membrane. Many of the lipids which are found in the nuclear membrane are smaller than those which are found in the cytoplasmic membrane, which allows them to move more easily into and out of the nucleus. When this happens, it creates an imbalance which can lead to osmotic pressure which pushes water through the membrane.
Other molecules which diffuse across the nuclear membrane include proteins which need to enter or leave during protein synthesis (translation). However, even though these molecules cross through pores in the nuclear envelope, they do not require active transport. This means that translocation does not use up any cellular energy which helps cells to save their resources.