5. Cell Structure And Organization - part 04 - Cell membrane / Plasma membrane

 

5. Cell Structure And Organization - part 04 - Cell membrane / Plasma membrane


Cell membrane / Plasma membrane/ biomembrane :

  • It is thin, quasifluid structure present both extracellularly and intracellularly.
  • 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.
  • Thickness of biomembrane is about 75 A0.
  • Under electron microscope, cell membrane appears trilaminar (made up of three layers). 
  • It shows presence of lipids (mostly phospholipids) arranged in bilayer. 
  • Lipids posses one hydrophilic polar head and two hydrophobic non-polar tails. So, phospholipids are amphipathic. 
  • Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwitched in between heads. 
  • Due to this, tails never come in direct contact with aqueous surrounding.
  • Cell membrane also shows presence of -
  1. proteins and 
  2. carbohydrates. 
  • Ratio of proteins and lipids varies in different cells. 
  • For example in human beings, RBCs show approximately 52% protein and 40% lipids.

Fluid mosaic model :


  • It is most accepted model of cell membrane. 
  • It was proposed by Singer and Nicholson in 1972.
  • According to this model, it is made up of - 
  1. phospholipid bilayer and 
  2. proteins. 
  • Proteins are like icebergs in the sea of lipids. Proteins can change their position. 
  • Some proteins are intrinsic i.e. occur at different depths of bilayer.
  • They span the entire thickness of the membrane. So, they are called transmembrane proteins. 
  • They form channels for passage of water. 
  • Extrinsic or peripheral proteins are found on two surfaces of the membrane.
  • Quasifluid nature of lipid enables lateral movement of proteins. This ability to move within the membrane is measured as fluidity.
  • Main function of plasma membrane is transport of molecules across it. 
  • This membrane is selectively permeable. 
  • During passive transport, many molecules move across the membrane without spending energy. 
  • Some solutes move by simple diffusion along the concentration gradient (from higher to lower concentration). 
  • Neutral solutes may move across the membrane by the process of simple diffusion. This is called the passive transport.
  • Water may also move by osmosis.
  • During active transport, few ions or molecules are transported against concentration gradient (from lower to higher concentration).It requires energy. So, 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. 
  • Polar molecules cannot pass through non-polar lipid bilayer. So, they require carrier proteins.

Comments

Post a Comment

Popular posts from this blog

6. Biomolecules - part 01 - Carbohydrates

Image
  6. Biomolecules - part 01 - Carbohydrates Introduction : Our planet is having a wide diversity of living organisms that are classified as - unicellular  (consisting of a single cell; including bacteria and yeast) or  multicellular  having many cells (e.g. plants and animals).  living organisms have cell as the basic structural and functional unit.  The cells have protoplasm containing numerous chemical molecules, the biomolecules. Biochemistry  : Biochemistry is  biological chemistry  that provides us the idea of the chemistry of living organisms and molecular basis for changes taking place in plants, animals and microbial cells.  It develops the foundation or understanding  all biological processes  and communication within and between cells as well as chemical basis of inheritance and diseases in animals and plants. Chemical analysis of all living organisms indicates presence of the most common elements like  carbon, h...
Read more

6. Biomolecules - part 02 - Lipids

Image
  6. Biomolecules - part 02 - Lipids B. Lipids : These are group of substances with greasy consistency with  long hydrocarbon chain containing carbon, hydrogen and oxygen. In lipids, hydrogen to oxygen ratio is  greater than 2:1  (in carbohydrates it is always 2:1). Lipid is a broader term used for fatty acids and their derivatives.  They are soluble in organic solvents (non-polar solvents).  Fatty acids are organic acids which are composed of hydrocarbon chain  ending in carboxyl group (-COOH).  They can be  saturated fatty acids  with no double bonds between the carbon atoms of the hydrocarbon chain.  Palmitic and stearic acids  found in all animal and plant fats are examples of saturated fatty acids. Unsaturated Fatty Acids  are with one or more double bonds between the carbon atoms of the hydrocarbon chain.  Oleic acid  found in nearly all fats and  linoleic acid  found in many seed oils are example...
Read more

6. Biomolecules - part 05 - Enzymes

Image
  6. Biomolecules - part 05 - Enzymes Enzymes : Thousands of different chemical reactions take place automatically at a given time in a tiny living cell. The reactions take place at the body temperature.  If these enzymes were not present in the cell, either the reactions would not occur or if they occur they would occur at a  very very slow rate . Know the scientists : German chemist  Edward Buchner  discovered enzymes by accident.  Buchner discovered that living cells were not necessary but that  yeast  extract could bring about fermentation.  He then coined the term Enzyme (Gk. En = in, zyma = yeast i.e. in yeast). This term is now commonly used for all biocatalysts. Each enzyme catalyzes a small number of reactions, specifically perhaps only one. Substrate :  The substance upon which an enzyme acts is termed as the substrate.  Endo-enzymes :  The enzymes which act within the cell in which they are synthesizedare known as en...
Read more