Question

Where in the eukaryotic cells we find membranes.
Mention at least four membrane structures?

What kind of function does the cell membrane have?

Mention any types of molecules that can penetrate directly through the lipid bilayer, which is really not permeable?

How do larger molecules and charged ions come through
membranes?

Answer 1

A eukaryotic cell is covered externally by the plasma membrane or the cell membrane. In plant cells outer to the plasma membrane there is a cellulosic cell wall. Other membrane structures are present in the cytoplasm of the eukaryotic cell.

In the cytoplasm of the eukaryotic cell there are many intracelluar membrane structures. This includes the inner and outer membranes of mitochondria, membrane of endoplasmic reticulum, membrene of Golgi, membrane of lysosomes, membrene of peroxisomes and glyoxisomes and the inner and outer nuclear membranes.

Within the nucleusof the eukaryotic cell there are no membrane structures.

The cell membrane separates the intracellular components from the extracellular environment.The microtubles and filaments that make up the cytoskeleton and provide a definite form and shape to each cell type is anchored to the cell membrane. Adjacent cells in a tissue connect and communicate each other with the help of the cell membrane. The plasma membrane is selectively permeable allowing the influx and outflux of only certain molecules and ions. This is important in the establisment of blood-brain barrier, maintenence of the tonicity of cytoplasm, maintenence of membrane potential and transmision of electrical signal through neurons etc. The cell membrane contains various types of antigens (like the blood antigens), recptors for hormones and various other ligands, many transport proteins, ion channels etc. The membrane can also transport water and various molecules and ions across it by mechanisms like osmosis, simple diffusion, facilitated diffusion, primary and secondary active transport, co-transport and counter transport.  

Water can freely move through the plasma membrane. The movement of water through the membrane occurs by osmosis and the direction of movement (enosmosis/exosmosis) depends on the concentration of various solutes in the cytoplasm. Water can also pass through the membrane through channel proteins called aquaporins. Uncharged substances such as Oxygen and Carbon dioxide can also pass through the plasma membrane and the movement occurs by simple diffusion in a pressure gradient. Lipid derivatives like steroid hormones and lipophilc drugs and other lipophilic molecules can pass freely through the plasmam membrane.

Larger molecules are transported across the membrane with the help of carreir moleules or transpoters. These are proteins present in the lipid bilayer of the plasma membrane. The transport of larger molecules with the help of carriers is called facilitated diffusion. For example transport of Glucose, transport of amino acids etc.Transport of these molecules may occur either in a concentration gradient (psssive transport) or against a concentration gradient (active transport).  Plasma membrane can also transport materials in bulk by endocytosis and exocytosis. Various ions are transported across the membrane through the ion channels. These are pores in the membrane proteins. The flow of ions through these channels can be controlled by using various gating mechanisms like ligand or voltage gating that decides whether the channel is open or closed.