Invention of microscope brought many advantages in study of science, such as regarding microscopic animals and internal structure of various animals and plants. Microscope is an instrument which makes an object look many times bigger than its original size. It has one eye piece , draw tube, body tube, objective lens, stage and mirror as main parts. Tiny structures are kept on stage under objective lens. Light is adjusted by mirror . Then structure is watched through eye piece . Much bigger structure is visible. So students can easily and clearly understand the microscopic structures.

(a)

(b)

The nuclear structure of plant and animal cells is same but the nucleus of bacterial cell is different. It is not well organised. It and does not have nuclear membrane and nucleolus. The genetic material is present in special region. Therefore, bacteria is called prokaryotes while plants and animals are eukaryotes.

1. The additional layer surrounding the cell membrane of plant cells is called cell wall.
2. The plants do not show physical movement and cannot escape from extreme climatic conditions, therefore cell wall protects the plant cells from temperature variation, high wind speed, atmospheric moisture, etc.

(a) Mitochondria is called powerhouse of the cell.
(b) The energy from the food is released into the mitochondria, from where it is used by cell for various activities.
(c) Supriya is intelligent, attentive and has scientific attitude and knowledge

Spindle-shaped muscle cells are

Functions Muscle cells are specially adapted for movement. They have a special property to contract and relax and thus brings about movement of body parts.

In an animal cell, nucleus is located in the centre of the cell. On the contrary, there is a large cavity called the vacuole which occupies the centre in a plant cell. This means that the cytoplasm is pushed towards the edge of the cell. The nucleus is usually found in this layer (i.e. close to periphery).

It is said that the nucleus controls all the metabolic activities as well as formation of various cell organelles. Thus, if the nucleus is removed, it will adversely affect the organism. In organism such as Amoeba, removal of nucleus may even lead to death.

Cells were discovered by Robert Hooke in 1665. He observed a thin slice of cork (part of bark of a tree) under a simple microscope and saw that it was made up of small box-like structures. These boxes appeared like a honeycomb, with one box separated from other by a partition. Hooke coined the term 'cell' for each box

(a) The above diagram represents an animal cell because cell is bounded by cell membrane and cell wall is absent.
(b) The above diagram represents a eukaryotic cell as it has a well-organised nucleus and other cell organelles in it.

Difference between plasma membrane and cell wall are:

The above-mentioned statement is wrong. The unicellular organisms also respire, reproduce and carry all metabolic functions like multicellular organisms.
The difference is that all the functions are performed by cell organelles of a single cell.

All living organisms are made up of similar or different types of cells. A single cell is capable of performing basic functions and activities and acts as unicellular organism, e.g. Amoeba, bacteria, etc.
Group of cells performing similar function is called tissue, group of tissues that performs a specialised function form an organ which collectively form organ system. Thus, cell is the basis of all levels of organisation and is called structural and functional unit of living organisms.

(a) Cytoplasm It is a jelly-like substance, present between the cell membrane and the nucleus. It encloses tiny structures called cell organelles that perform different functions. It also contains water, sugar, proteins, minerals, etc.
(b) Nucleus of a cell It is a spherical component of cell commonly located in the centre of cell. It is covered by porous nuclear membrane and contains genetic material like RNA and DNA. It also contains thread-like structures called chromosomes which carry genes. Nucleus controls various activities of the cell and play role in inheritance.