Eukaryotic Plasma Membrane
Eukaryotic plasma membrane/ Cell membrane/ Biomembrane:
i. It is thin, quasi-fluid structure present both extracellularly and intracellularly.
ii. Extracellularly, it is present around protoplast and intracellularly, it is present around most of the cell organelles in eukaryotic cell. It separates cell organelles from cytosol.
iii. Thickness of bio-membrane is about 75Å.
iv. Cell membrane appears trilaminar (made up of three layers) when observed under electron microscope. It shows presence of lipids (mostly phospholipids) arranged in bilayer.
v. Lipids possess one hydrophilic polar head and two hydrophobic non-polar tails. Therefore, phospholipids are amphipathic.
vi. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwiched in between heads. Due to this, tails never come in direct contact with aqueous surrounding.
vii. Cell membrane also shows presence of proteins and carbohydrates.
viii. Ratio of proteins and lipids varies in different cells. For example, in human beings, RBCs show approximately 52% protein and 40% lipids.
Function
i. The significant function of plasma membrane is transport of molecules across it. Plasma membrane is selectively permeable.
ii. Passive transport:
a. Many molecules move across the membrane without spending energy.
b. Some molecules move by simple diffusion along the concentration gradient i.e. from higher to lower concentration.
c. Neutral molecules may move across the membrane by the process of simple diffusion. d. d.Water may also move by osmosis.
iii. Active transport:
a. Few ions or molecules are transported against concentration gradient i.e. from lower to higher concentration.
b. This requires energy, hence ATP is utilized. As such a transport is an energy dependent process in which ATP is utilized, it is called Active transport e.g. Na+/K+ pump.
c. Polar molecules cannot pass through non-polar lipid bilayer. Therefore, they require carrier proteins to facilitate their transport across the membrane.
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