The cell

It is the basic structural and functional unit of organism.

Cells have 3 main Parts: –              

  • Plasma membrane
  • The cytoplasm
  • The nucleus


  • Plasma membrane forms a membrane which separates the cell’s internal environment from external environment.
  • It acts as a selective barrier that regulates the flow of material into and out of the cell and helps to maintain the appropriate environment from normal cellular activities.


  • Lipid bilayer
  • It acts as a selective barrier that regulates the flow of material into and out of the cell and helps to maintain the appropriate environment from normal cellular activities.
  • It has basic structural framework of lipid bilayer made up of three types lipid molecule i.e. phospholipid (75%), cholesterol (20%) and glycolipid (5%).


The lipid molecules in the cells are amphipathic molecule which have both polar and nonpolar end. The head of the lipid molecule are arranged outwards and have hydrophilic nature while the tail points inwards and are hydrophobic in nature.

Arrangement of protein in membrane: –

Membrane proteins are classified as integral and peripheral protein.


It is a passive process in which the random mixing of particles in the solutions occurs because of the particles kinetic energy.
The solute molecules move from higher concentration to lower concentration.

Factors affecting diffusion :-
1.Steepness of the concentration gradient
The greater the difference in concentration between two sides of membrane, the higher the rate of diffusion concentration gradient.
The higher the temperature, faster the rate of diffusion. All of the diffusion process in the body occur more rapidly in a person with fever.
3.Amount of diffusing substance
More the substance, more the diffusion.
4.Surface area
The larger the surface area of the diffusion membranes, faster the diffusion.
5.Diffusion distance
The greater the distance over which the diffusion take place, longer the diffusion occurs.


A passive process in which substance more freely through the bilipid layer of the plasma membrane without the help of transport protein.
For example-O2,CO2,N2, Fatty acid, steroids, fat soluble vitamins[A,D,E,K], Water area can pass through the cell membrane by simple diffusion.


In this process an integral membrane protein access a specific substance across the cell membrane. This is of two types:
1.Channel mediated facilitated diffusion
2.carrier mediated facilitated diffusion


In this process, solute moves down its concentration gradient through a membrane channels. These channels are mostly ion channels and selective in nature. i.e, only selective ions can more through these ion channels. For example-K ion channel, CL ion channel. This process is slightly slower than simple diffusion.


A carrier or transporter is use to move a solute along its concentration gradient. The solute finds with the carrier on one side and is related to the other side of the membrane along its concentration gradient.
The rate of movement depends upon steepness of the concentration.
Availability of carries protein in plasma membrane. For example-glucose, fructose , galactose ,and some vitamin.


It is a type of diffusion in which solvent moves from its higher concentration to lower concentration through a semi permeable membrane.
During osmosis water molecules pass through plasma membrane by two ways. simple diffusion
2.through aquaporins (water channel)


Active process is the process which utilizes energy for transporting the solute across the membrane, energy is required for carrier protein to move solute across the ,membrane against the concentration gradient.
Energy sources for cellular transport:-
1.It obtained by hydrolysis of adenosine triphosphate (primary source).
2.Energy stored in ionic concentration gradient (secondary). Ions such as Na+, K+ ,H+ ,Ca++,Cl-, amino acid and monosaccharides travel by active transport.

When a cell membrane moves molecules as ions uphill against a concentration gradient the process is called active transport. Na+, K+, H+, Ca++, Cl-, different sugars and most of the amino acid prevail across cell membrane by active transport.
Active transport is divided into two types according to the source of the energy is used to cause the transport primary active transport and secondary active transport.


Energy is derived from directly breakdown of ATP or some other high energy phosphate compound.


Energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondarily molecule or ionic substance between the five sides of cell membrane.

Primary active transport:-

Sodium-potassium pump– Na-K pump is the transport process that pumps Na+ ions outside and at the same time K+ ions inside this pump is responsible for maintenance of Na-K concentration gradient as well as negative electrical voltage across the cell membrane. This pump has three receptors sides for binding of Na+ ions and two receptors sides for K+ ions.
The inside portion of this protein has ATP as activity.
When two K+ ions bind on the outside of the protein and three Na+ ion bind on the inside of the ATPase function of the protein becomes activated. This then cleaves one molecules of ATP splits it into ADP and high amount of energy is liberated. This energy then cause chemical and conformational change in the protein molecules transporting sodium ion outside the cell and 2K+ inside the cell.

Secondary active transport:-

a.k.a. co- transport- When sodium ion are transported outside of the cell by primary active transport, a large concentration gradient develops across the cell membrane. This gradient is a store house of the energy because excess sodium ion attempts to diffuse inside the cell under appropriate condition, the diffusion energy of sodium ion can pull other substances along with the sodium through the cell membrane. This phenomena is called co- transport.


Vesicles are small spherical sac through which substance are transported across cell types.


Movement of material into a cell though vesicle. Endocytosis require energy supplied by ATP hence transport in vesicles in an active process.

The three types are:-

1.Receptor mediated endocytosis
3.Bulk phase endocytosis (Pinocytosis)
1.Receptor mediated endocytosis- It is highly selectively type of endocytosis by which cell takes up specific ligand.


1.Binding-On the extra cellular site of plasma membrane, a ligand attacks to a specific receptor to form a receptor ligand complex. This cause the membrane to fold inward.
2.vesicle formation-The invaginated edge of membrane from a small piece of membrane that pinches off and forms a vesicles.
3.uncoating-Almost immediately after the formation of vesicles the protein coated vesicles looses its pose.
4.Fusion with endosome-The uncoated vesicles quickly fuses with another vesicles known as endosome and within this vesicles ligand separates from the receptors.
5.Recycling of receptors to plasma membrane-Most of the receptors accumulate in elongated perfusion of the endosome. These pinch off forming a transport vesicles that returns the receptors to the plasma membrane.
6.Degradation in lysosome-Other transport vesicles that contain ligands molecules but of endosome and attaches with lysosome. Lysosome contain digestive enzymes. These enzymes breaks down the vesicles and particle is released into the cytosol of the cell.


In this process cell engulf large solid particles such as worn out cell from a bacteria or a virus. Only few body cells are able to carry out phagocytosis. For example-microphase and neutrophils
This process begins when a particles binds to a plasma membrane receptor on the phagocyte causing to form pseudopodia. i.e. projection of plasma membrane and cytoplasm.
Pseudopodia surrounds the particles outside the cell and membrane fuse to form a vesicles called phagosome. This phagosome enters the cytoplasm.
Phagosome is then fused with lysosome which break down the ingested material. Phagocytosis is the vital defense mechanism that help protecting the body from disease.


Also called bulk phase endocytosis. In this process tiny droplets of extracellular fluids are taken out. No receptor proteins are involved.
During this process, the plasma membrane fold inwards and form a vesicles containing droplets. The vesicle then detached and enter the cytosol. It is then fused with lysosome where enzymes degrades the vesicles resulting in dispensing of small molecule such as amino acid and fatty acid with cytosol.