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Structure and Function of Cells

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Structure and Function of Cells - Lesson Summary

Cell is the structural and functional unit of all living organisms. Every cell in the body is meant to have a specific function. e.g. Alveolar cell is an example of single cell.
A group of cells that have similar structure and function to perform a specific activity constitute a tissue. e.g. A group of alveolar cells form alveolar tissue. 

Tissues aggregate to form an organ. Tissues which are same at their structure and perform similar activities form a complete organ. e.g. Alveolar tissues aggregate to form lungs, the organ.

Many organs together constitute organ system. Organs made up of a group of tissues organise themselves to perform specific activity. e.g. Nostrils, nasal cavity, trachea, bronchi, bronchioles, lungs together form the respiratory system, the organ system.

Organ systems aggregate to form an organism. A human being is a complete organism made up of 11 organ systems. e.g. Digestive system, respiratory system, circulatory system, immune system, muscular system, skeletal system, nervous system, endocrine system, integumentary system, excretory system, reproductive system etc. work together to form a complete organism. 

Types of cells

1) Based on the presence of nuclear membrane, cells can be of two types – Prokaryotic cells and eukaryotic cells.

Prokaryotic cells are the cells which do not possess definite nucleus but the nuclear material is in the form of a nucleiod dispersed in the protoplasm of the cell. Membrane bound organelles are absent in these cells e.g. Bacterial cell, Blue-green algal cell.

Eukaryotic cells are the cells with well defined nucleus and distinct nuclear membrane. Organelles dispersed in the cytoplasm are membrane bound organelles. e.g. Animal cells, Plant cells.

2) Based on the type of organisms they are present in , cells are of two types namely plant cells and animal cells.

Plants are made up of plant cells. Plant cells are eukaryotic cells with well distinct nucleus and membrane bound organelles. Genetic material is in the form of DNA present in chromosomes. Organelles are specific in their functions. Characteristic features of animal cells are that the cells are enclosed in a protective layer called as cell wall.


Animals are made up of animal cells. Animal cells are eukaryotic cells with well developed nucleus enclosed in a nuclear membrane. Animal cells are protected by semi-permeable membrane called as cell membrane. Animal cells are characterised by possessing some special structures like centrioles, many lysosomes, cilia and flagella.  

Differences between plant cells and animal cells
                     PLANT CELLS           ANIMAL CELLS Plant cells are almost straight that is quadrangular or hexagonal in shape. Animal cells are round in shape. Plant cells possess distinct cell walls which are protective in function. Animal cells do not have any cell walls. Plant cells possess special structures called as plastids. Plastids help plants to synthesise and store their food. Animal cells do not have any plastids. Lysosomes are absent or scanty in plant cells. Lysosomes are many in animal cells. In plant cells, vacuole is mostly one and large in size   In animal cells, vacuoles are many and smaller in size. Cilia and flagella are absent in plant cells. Cilia and flagella are present in animal cells. Plasmodesmata provide connections between adjacent plant cells. Strands of connective tissue provide connections between animal cells. Higher plants do not possess centrioles in their cells. Animal cells contain centrioles which help in cell division process. 

Structures of the cell
A distinct cell is made up of cell wall, cell membrane, cytoplasm and the cell organelles.

a) Cell wall:
       •  This is the structure present only in plant cells.
       •  The cell wall gives rigidity to the cell. It provides toughness and strength to the plant body which stands erect in different environmental conditions.
       •  It is made up of cellulose and pectin.

CELL WALL CELL MEMBRANE It is present only in plant cells and some bacterial cells. It is present in all living cells. It exists outside the cell membrane. It exists around the protoplasm of all the living cells. It is thick and permeable layer. It is thin and selectively permeable layer. It is mainly made up of cellulose and pectin. It is made up of lipoproteins. It is protective in functions. It also provides mechanical strength to the cells. It regulates the exchange of materials between the cell and the environment.

b) Cell membrane:
       •  The cell membrane is a biological membrane that separates the interior of all cells from the outside environment.
       •  The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells.
       •  It consists of the phospholipid bilayer with embedded proteins.

c) Cytoplasm:
       •  It is a jelly-like fluid present between the nucleus and the cell membrane.
       •  In a prokaryotic cell, the organelles are dispersed in the cytoplasm.
       •  In a eukaryotic cell, the contents of the nucleus are separated from the cytoplasm. Organelles like chloroplasts, mitochondria, ribosomes, lysosomes, Golgi apparatus etc. are dispersed in the cytoplasm.
       •  Cytoplasm is the site of protein synthesis. Various metabolic reactions take place in the cytoplasm.
       •  The movement exhibited by the cytoplasm is called as cyclosis.

PROTOPLASM CYTOPLASM NUCLEOPLASM It lies inside the cell membrane of prokaryotic cells. It lies between the cell membrane and the nuclear membrane. It strictly lies within the nuclear membrane. It includes both cytoplasm and nucleoplasm. It includes cell organelles and provides an environment for metabolic processes of the cell. It plays a main role during cell division.It includes nucleolus and chromatin in it.

d) Cell organelles:
       •  Cell organelles are the living structures which float in the cytoplasm of the cell.
       •  Different structures present inside the cell are called as cell organelles.
       •  Cell organelles include nucleus, chromosomes, plastids, vacuole, Golgi apparatus, lysosomes, mitochondria, ribosomes etc.

i. Nucleus 
       •  The nucleus is the control centre of the cell as it controls all its activities.
       •  The nucleus is a membrane-enclosed organelle found in eukaryotic cells.
       •  The nucleus contains the genetic material of the cell in the form of chromosomes.

ii. Chromosomes
       •  Chromosomes are organised structures of DNA and protein found in cells. They are single pieces of coiled DNA containing many genes.
       •  These are considered to be the vehicles of heredity as they carry genes. Genes are small points on the chromosome responsible for one character of an individual.
       •  Genes on chromosomes help in the inheritance or transfer of characteristics from the parents to the offspring.
       •  Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

iii. Plastids 
These are the organelles especially present only in plants and certain bacteria like cyanobacteria. Three types of organelles are chloroplasts, chromoplasts and leucoplasts.
       •  Chloroplast is a type of plastid involved in the process of photosynthesis in plants. These are specialised organelles found in all higher plant cells. The green colour of these organelles is due to the presence of pigment chlorophyll. Chlorophyll absorbs energy from the sunlight required for the process of photosynthesis.
       •  Chromoplasts are the coloured plastids which provide colours for different parts of the plant like flowers and fruits. Chromoplasts synthesise and accumulate carotenoid pigments. Chromoplasts are rich in carotenoid pigments responsible for imparting the colour. Chromoplasts are angular in shape.
       •  Leucoplasts are colourless plastids found in the storage organs of the plant. These occur in different shapes. They do not have any coloured pigments. They occur as amyloplasts, aleuronoplasts and elaioplasts.

iv. Mitochondria
       •  Mitochondria are double membrane bound organelles which are termed to be ‘power houses of the cell’.
       •  These are the sites of ATP synthesis. ATP is considered to be an energy coin.
       •  ATP molecules are synthesised by aerobic respiration.
       •  Mitochondria have their own DNA which helps mitochondria to replicate by themselves.

v. Vacuoles
       •  These are membrane bound transparent organelles present in both plant and animal cells offers buoyancy to the cell.
       •  Vacuoles are filled with water containing inorganic and organic molecules, including enzymes, in solution.
       •  Vacuoles maintain turgidity of the cell.
       •  Vacuoles also store waste materials in them.
       •  Vacuoles are small and many in animal cells but mostly large and one in plant cells.

vi. Golgi apparatus
       •  These are membrane bound sacs present in the cytoplasm of the cell.
       •  They are secretory in function.
       •  They are responsible for processing and packing of macromolecules synthesised in the cell.
       •  Golgi bodies in the plant cells are termed to be dictyosomes.

vii. Lysosomes
       •  These are membrane enclosed organelles containing hydrolytic bags.
       •  These bags are rich in hydrolytic enzymes which have the ability to digest cellular components.
       •  They burst themselves when necessary to kill the foreign substances entering the cells.
       •  Hence, these organelles are termed to be 'suicidal bags'.
       •  Two types of digestion exhibited by these hydrolytic enzymes are heterophagy (lysis of foreign particles), autophagy (lysis of own cell in case of infection).


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