● 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.")`

● 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.

● 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 their`color{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.")`