Biology CELL MEMBRANE

KEY TOPICS

star Eukaryotic cells
star Cell membrane
star Functions of Cell membrane

EUKARYOTIC CELLS

● The color{brown}("eukaryotes") include all the color{violet}("protists, plants, animals") and color{violet}("fungi.")

● In color{violet}("eukaryotic cells") there is an color{brown}("extensive compartmentalisation") of color{violet}("cytoplasm") through the presence of membrane color{violet}("bound organelles.")

● color{violet}("Eukaryotic cells") possess an organised nucleus with a color{violet}("nuclear envelope.")

● In addition, color{violet}("eukaryotic cells") have a variety of complex color{brown}("locomotory and cytoskeletal") structures.

● Their genetic material is organised into color{brown}("chromosomes.")

● All color{violet}("eukaryotic cells") are not identical.

● color{violet}("Plant") and color{violet}("animal cells") are different as the former possess cell walls, plastids and a large central vacuole which are absent in color{violet}("animal cells.")

● On the other hand, color{violet}("animal cells") have centrioles which are absent in almost all color{violet}("plant cells.")

CELL MEMBRANE

● The detailed color{violet}("structure of the membrane") was studied only after the advent of the color{violet}("electron microscope") in the 1950s.

● Meanwhile, chemical studies on the color{violet}("cell membrane,") especially in color{brown}("human red blood cells") (RBCs), enabled
the scientists to deduce the possible structure of color{violet}("plasma membrane.")

● These studies showed that the color{violet}("cell membrane") is composed of lipids that are arranged in a color{brown}("bilayer.")

● Also, the color{violet}("lipids") are arranged within the membrane with the polar head towards the outer sides and the
color{violet}("hydrophobic tails") towards the inner part.

● This ensures that the color{violet}("nonpolar tail") of color{violet}("saturated hydrocarbons") is protected from the aqueous environment.

● The lipid component of the membrane mainly consists of color{brown}("phosphoglycerides.")

● Later, color{violet}("biochemical investigation") clearly revealed that the cell membranes also possess color{violet}("protein and carbohydrate. ")

● The color{violet}("ratio of protein") and color{violet}("lipid varies") considerably in different cell types.

● In human beings, the color{brown}("membrane of the erythrocyte") has approximately 52 per cent protein and 40 per cent lipids.

● Depending on the ease of extraction, color{violet}("membrane proteins") can be classified as color{brown}("integral or peripheral.")

● color{violet}("Peripheral proteins lie") on the surface of membrane while the integral proteins are partially or totally buried in the membrane.

● An improved model of the color{violet}("structure of cell membrane ") was proposed by color{brown}("Singer and Nicolson (1972)") widely accepted as color{brown}("fluid mosaic model")

● According to this, the color{brown}("quasi-fluid nature") of lipid enables lateral movement of color{violet}("proteins") within the overall color{violet}("bilayer.")

● This ability to move within the membrane is measured as its color{brown}("fluidity.")

● The color{violet}("fluid nature") of the membrane is also important from the point of view of functions like color{violet}("cell growth,") formation of intercellular junctions, secretion, endocytosis, cell division etc.

FUNCTIONS OF CELL MEMBRANE

● One of the most important functions of the color{violet}("plasma membrane") is the color{brown}("transport of the molecules") across it.

● The membrane is color{brown}("selectively permeable") to some molecules present on either side of it.

● color{violet}("Many molecules") can move briefly across the membrane without any requirement of color{violet}("energy and thisis") called the color{brown}("passive transport.")

● color{brown}("Neutral solutes") may move across the membrane by the process of color{violet}("simple diffusion") along the color{brown}("concentration gradient"), i.e., from higher concentration to the lower.

● Water may also move across this membrane from color{violet}("higher to lower ") concentration.

● Movement of water by color{violet}("diffusion") is called color{brown}("osmosis.")

● color{brown}("As the polar molecules") cannot pass through the color{violet}("nonpolar lipid bilayer,") they require a carrier protein of the membrane to facilitate theircolor{violet}(" transport across the membrane.")

● A color{brown}("few ions or molecules") are transported across the membrane against their color{brown}("concentration gradient"), i.e., from lower to the higher concentration.

● Such a color{violet}("transport") is an energy dependent process, in which ATP is utilised and is called color{brown}("active transport"), e.g., color{brown}(Na^+//K^+ "Pump.")