How molecules transport across the Cell Membrane?

 Cell membrane is made up of phospholipid bilayer. It provide a path for material or molecule to go inside the cell or outside the cell. Cell membrane is a Semipermeable membrane. That ensures only useful material go inside the cytoplasm.

There are two main processes for transporting of material inside or outside the cell.  

I) Physical process 

II) Biological process 

I) Physical Process-   in this process the molecules are move across the cell membrane by diffusion, osmosis, filtration and dialysis. 

i) Diffusion- is the process in which the solute molecules move from higher concentration to the lower concentration. This process does not require semi-permeable membrane. Diffusion molecules may be solid, liquid or gas. It can take place in any medium.  

Example- Take a beaker and fill it with water. Now place a cube of sugar in the beaker. The concentration of solute is higher than the concentration of solvent. Slowly the molecules of sugar dissolve in water and spread in all water. This is known as diffusion. 

Also, a one easier example. when we spray a perfume in one place but it spread and give fragrance on another place, that known as gas diffusion. 

The movement of molecules obey the Brownian movement. 

ii) Osmosis- is the process in which the movement of solvent from higher water concentration to lower water concentration through a semi permeable membrane. This process required semi-permeable membrane. Osmosis occurs in liquid medium only.  

Example- Take a beaker and separate them with a semi-permeable membrane. Give this separation name (A-Sided) and (B-Sided). Add water in the beaker. Now the concentration of both sided water is equal. There is constant flow of water molecules. Then, we add little amount of sugar in A-Sided water. Now due to adding the sugar the concentration of water is low and the semi-permeable membrane did not allow the sugar molecules to pass through it. In this case, the higher concentrated water molecules(solvent)from side-B moves to lower concentration of water that is side-A. This is known as OSMOSIS.   

Osmosis occur in two types- Ex-osmosis and Endosmosis. 

Ex-osmosis- Exo mean outside. when the surrounded solution of cell is hypertonic, on that time the water comes out of the cell is known as Ex-osmosis. 

Endosmosis- Endo means inside. When the surrounded solution of cell is hypotonic, on that time water enter inside the cell is known as endosmosis. 

Solute- is the substance like sugar, salt that dissolve in the solvent. 

Solvent- is the substance that dissolve the solute. like water. 

Solution- the mixture of solute and solvent is known as solution.  

iii) Filtration- is the process in which the movement of molecules occur through semi-permeable membrane with hydrostatic pressure.  

Example- in our body the kidney is the organ who filtrate our blood and wash out our waste material. This filtration of blood is occurred due to high blood pressure in glomerular capillaries and this cause high filtration pressure. High filtration pressure moves the substance through semi-permeable membrane.  

iv) Dialysis- an the process in which the small dissolved molecules move from one solution to another solution through semi-permeable membrane. Its movement is vice-versa. Dialysis is a slow process and it does not allow big molecules to pass out through the semi-permeable membrane. It is just only for small molecules.  

Example- Take a beaker and partial it with semipermeable membrane. Then add water in beaker. After that add sugar in one side of beaker. In few minutes, sugar dissolve in it and move on other side. The movement of molecule is vice-versa. So that, the concentration of both sided solution is same. This is known as dialysis.

DIALYSIS

II) Biological process - in which the movement of material across the membrane with the help of cell itself. There are two types of biological process.

1) Transport by membrane protein 

2) Internalization of macromolecules, fluid droplet and large particles.  

1) Transport by membrane protein- Selective molecules are transported through membrane protein. These include small ions, amino acids and glucose. Each transport protein has specific chemically similar substance. There are three different ways for transport the molecules by membrane protein. 

a) Passive transport by channel protein- transport of molecules on the basis of concentration gradient. The molecules are transport from higher to lower concentration gradient by protein channel. When this channel opens the molecule enter inside the cytoplasm.  

b) Passive transport by carrier protein- In this the molecules are entered into the cell with the help of a carrier. This carrier carries a selective molecule from outside of the cell membrane and transport the molecule into the cytoplasm through itself. They do not use cellular energy. Concentrated gradient energy use for this process. This passive transport by carrier protein is also known as facilitated diffusion.  

c) Active transport by carrier protein- In active transport the carrier protein requires energy because it move the substance against the concentration gradient. Energy is provided by ATP. The carrier protein molecules have site for ATP binding. When ATP molecule bind with carrier protein, it hydrolyses and convert into ADP. This releasing energy bring the substrate close to the carrier and bind with substrate binding site. This makes a carrier- substrate complex. Now this substrate slowly transports inside the cytoplasm. After the whole process carrier protein comes in their original form and restart the process.  

2) Internalization of macromolecules, fluid droplets and large particles - large size of molecules are not able to transport through the cell membrane. so that there are three types of process.  

a) Endocytosis- Endocytosis means taking up molecules inside the cell. In this the macromolecules and fluid droplets are taken up inside the cell. The endocytosis are two types- Pinocytosis and receptor- mediated endocytosis. 

i) Pinocytosis- In this the extracellular fluid is taken up by plasma membrane. It is also known as cell drinking. The cell membrane enfold itself from inside and filled with extracellular fluid. Then this enfolded area converts into Pino-some and extracellular fluid get inside the cell cytoplasm.  

ii) Receptor- mediated Endocytosis- In this, the plasma membrane bear protein receptor. These receptors recognize a specific macromolecule called ligand present in extracellular fluid. The Ligand bind with receptor and make a ligand-receptor complex. This complex enfolds and closes as a pinocytotic vessel. Then this release into the cytoplasm. Receptor- mediated endocytosis is for specific uptake of macromolecules otherwise it cannot be able to pass inside the cell.  

b) Phagocytosis - in this the solid particle intake by a cell. It is also known as cell eating. In case of amoeba, the cell membrane engulfs the food particles by making pseudopodium. Then the food particles come inside the cell cytoplasm by phagosome. Phagocytosis occurs in many unicellular organisms. 

c) Exocytosis- mean outside the cell. In this the cell secretion stored in exocytotic vesicle. These vesicles attached with cell membrane from the side of cytoplasm and fused with inside cell membrane. After this the vesicle discharge the cell secretion outside the cell. It is also known as cellular vomiting. Undigested food left in food vacuoles are also eliminate through exocytosis.  

Some Important questions.

Q1) Which process required energy for cell transportation?

Ans. Energy required in active transport by carrier protein process. In this energy release by converting ATP to ADP.

Q2) What is the Full form of ATP and ADP?

Ans. ATP stands for Adenosine triphosphate and ADP stands for Adenosine Diphosphate.