Hint: Cell is the basic structural and functional unit of living organisms which is categorized on the basis of its composition as unicellular or multicellular organisms. Plant and animal cells come under multicellular organisms (made up of more than one cell).
Complete Answer: Plastids are the largest cytoplasmic double membraned structure found only in plant cells. In animal cells plastids are absent. It is generally of 3 types: 1. Chromoplast- They are the coloured plastids (in flowers) that play an important role in pollination as they attract pollinating agents and make it easy. 2. Leucoplasts- They are colourless plastids and act as a site of synthesis and storage of food. It lacks pigment such as chlorophyll. 3. Chloroplast- Plastids that contain green coloured pigment chlorophyll. Photosynthesis takes place in chloroplast. They are located in the mesophyll cells on leaves of the plant. Plastids are only present in plant cells because they help to provide green colour to the plants and responsible for storage of food and photosynthesis in plants but animals do not need such type of plastids as they are heterotrophs therefore, not involved in activities like photosynthesis and storage. Also, they do not need any specific or particular colour. So, plastids are absent in animal cells.
Note: Plastids were 1st discovered and named by Ernst Haeckel. It can also store materials like starch. It is also a site of manufacture and storage of many selective chemical compounds. They possess a double stranded DNA molecule that is circular in shape. If plastids are not present in plant cells, then plants are not able to perform the various functions and activities for their survival and it brings them to a stage of death. Also, it affects or disturbs the food chain.