Cell: Structure and Functions

Cell: Structure and Functions

Table of Contents

What is a cell? / what is a cell in human body?

Cell is the smallest and the functional unit of life. It is capable of independent existence. A single cell has the capability to give rise to a new individual.

Cell theory

There were three scientists who proposed cell theory. Matthias Schleiden, Theodor Schwann and Rudolf Virchow. Matthias Schleiden was a botanist who proposed that all plants are made up of cells. Then, Theodor Schwann who was a zoologist proposed that all animals are made up of cells. Later, Rudolf Virchow come into picture and proposes that all cells arise from the pre-existing cells. So, cell theory comprises of-

  • Cell is the structural and functional unit of life.

  • All plants are made up of cells.

  • All animals are made up of cells.

  • All cells arise from the pre-existing cells.

Cell structure/ Animal cell structure and functions /what is the structure of the cell?

Cell membrane structure and functions / Cell membrane is the outermost covering of the cell. It acts as a barrier that allows only certain molecules to pass through it. So, cell membrane is known as selectively permeable membrane. It is mainly composed of lipid bilayer with proteins. This model is known as Fluid Mosaic Model, which is proposed by Singer and Nicholson.

Structure of the cell membraneFig. 1. Structure of the cell membrane

Nucleus controls most of the activities of a cell. It is a double membrane structure consisting of outer and inner nuclear membrane. Nuclear membrane contains nuclear pores that allows only certain molecules to pass through. The genetic component that is, DNA is present inside the nucleus of the cell. It is found only in eukaryotic cell. Instead of nucleus, prokaryotes contain nucleoid not bounded by nuclear membrane.

Structure of the nucleus Fig. 2. Structure of the nucleus

Endoplasmic Reticulum consists of network of tubules that are continuous with the nuclear membrane. It is a site where proteins, lipids etc. Synthesized. ER is of two types - Smooth ER and Rough ER. Rough ER contain ribosomes on its surface that helps in protein synthesis, whereas smooth ER do not contain ribosomes and participates in lipid biosynthesis and detoxification of xenobiotics.

Structure of the ER Fig. 3. Structure of the ER

Golgi Body or Golgi Complex participates in packaging of materials and its transport to the target. Proteins which are synthesized in ER are packed in vesicles and fuses with ER to reach the target site.

Structure of the golgi body

Fig. 4. Structure of the golgi body

Ribosomes is a single membrane structure present in the cytoplasm of the cell as well as on ER. They participate in protein synthesis. Ribosome consists of small subunit and large subunit. The prokaryotic ribosome is 70S whereas eukaryotic ribosome is 80 S.

Structure of the ribosomesFig. 5. Structure of the ribosomes

Mitochondria is a double membrane structure which is site of ATP synthesis. The outer membrane is similar to plasma membrane and inner membrane is folded into cristae. The inner membrane consists if F0-F1 complex that synthesizes ATP, the energy currency of the cell. It has its own DNA and ribosomes.

Structure of mitochondriaFig. 6. Structure of mitochondria

Lysosomes is a single membrane structure found in eukaryotic cell. It is known as Suicidal Bags due to its ability to degrade old or weakened cells. It contains acid hydrolases that degrades the waste material.

Structure of the lysosomesFig. 7. Structure of the lysosomes

Vacuole is a single membrane structure found in both plant and animal cells. The membrane of the vacuole is known as Tonoplast. it stores food, waste, water etc. it should not be confused with contractile vacuole found in Amoeba.

Structure of the vacuoleFig. 8. Structure of the vacuole

Plant cell structure and functions

Cell wall is the outermost covering of the plant cells. It provides strength and rigidity to the cell. It is mainly composed of cellulose in case of plant cells.

Chloroplast is a double membrane structure found in plant cells. It is site of photosynthesis in plant cells. The green pigment of the plant, chlorophyll is present in chloroplast. Inner membrane encloses the stroma of the chloroplast where dark reactions occurs. Stroma contain stack of thylakoid known as Grana which are a site for light reactions.

Structure of the chloroplastFig. 9. Structure of the chloroplast

Glyoxysomes are found in plant cells. It participates in storage of fats, photorespiration, breakdown of fatty acids etc.

Rest of the cell organelles are same in plant cells as found in animals cells.

Transport across the cell membrane

Transport across the membrane occurs in a following ways:

  • Diffusion is the most common type of membrane transport that occurs down the concentration gradient. This process does not require energy for transport. Most of the gases such as oxygen, hydrogen etc. passes into or out of the cell through diffusion.

Diffusion across the membrane

Fig. 10. Diffusion across the membrane

  • Facilitated Diffusion is a membrane transport that occurs via certain carrier molecules such as permeases that transport the molecules without the expenditure of the energy. For Example, glucose transport occurs through facilitated diffusion.

Facilitated diffusion
Fig. 11. Facilitated diffusion

  • Active Transport required expenditure of energy to transport certain molecules against the concentration gradient. There are two types of transport: Primary Active Transport and Secondary Active Transport. Primary active transport uses energy to transport the molecules into or out of the cell whereas secondary active transport uses energy produced due to electrochemical gradient formed due to pumping of ions in or out of the cell.

Active transportFig. 12. Active transport


Biomolecule is an organic molecule that is produced by living organism. They act as building block of life and perform important functions in living organisms. They are primarily composed of carbon, hydrogen, nitrogen, oxygen, phosphorous and Sulphur. The four common biomolecules are: ProteinsNucleic Acid, Carbohydrates, and Lipids.


Any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms.

Structure of amino acids

Fig. 13. Structure of amino acids

The building blocks of proteins are known as amino acids. There are 22 naturally occurring amino acids found in nature. Amino acids are made up of carbon, hydrogen, oxygen, and nitrogen. So, single amino acid is made up of amino group, carboxyl group, hydrogen atom and distinctive side chain, all bonded to alpha-carbon.

Nucleic Acids

Nucleic Acid was first discovered by Friedrich Miescher from the nuclei of pus cells. There two types of Nucleic Acids: Deoxyribonucleic Acids and Ribonucleic Acids.

The monomeric unit of nucleic acid is known as Nucleotides. Nucleotide is made up of three components: A Nitrogenous Base, A Five-Carbon Sugar and Phosphoric Acid.

Structure of nucleotide

Fig. 14. Structure of nucleotide

Nitrogenous bases are heterocyclic, aromatic molecules. There are basically two types of Nucleic Acids: Purines and Pyrimidines.

Purines are of two types- adenine and guanine whereas pyrimidines can be thymine, cytosine, or uracil.

The 5- carbon sugar found in DNA is Deoxyribose and for RNA is Ribose. Nucleotide without phosphate is known as Nucleoside.


They are defined as the polyhydroxy aldehydes or polyhydroxy ketones. They consist of hydrogen, carbon, and oxygen. The simplest carbohydrates are known as Monosaccharides, such as glucose. Two monosaccharides joined to form disaccharides. Polymers of 2 to 10 units of monosaccharides is known as Oligosaccharides. When hundreds to thousands of monosaccharides are joined, they are known as Polysaccharides.

Structure of aldose and ketose sugarFig. 15. Structure of aldose and ketose sugar

Monosaccharides with aldehyde group is known as Aldoses. And monosaccharides with ketone group is known as Ketoses. Trioses are the simplest monosaccharides.


They are insoluble or poorly insoluble in water. They are soluble in non-polar solvents such as ether, chloroform or benzene.

Biological functions of lipids

  • They serve as storage form of energy.
  • They are major component of membranes.
  • They are protective in function such as in bacteria, plants, insects, and vertebrates.

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