The Fundamental Unit of Life

Some organisms have only one cell while some have more than one cell. Thus, based on the number of cells, organisms can be divided into two groups viz. Unicellular organisms and Multicellular organisms.

Unicellular Organisms

Organisms having only one cell are called Unicellular Organisms. Here "Uni" means "one or single".

Unicellular organisms are made up of only one cell. Example: Amoeba, Chlamydomonas, Paramoecium, and Bacteria. All functions that require for the life processes are performed by only a single cell in unicellular organisms.

Multicellular Organisms

Organisms having more than one cell are called Multicellular Organisms, for example, fungi, plants, and animals.

Here, "Multi" means "many", and thus "Multicellular Organisms" means organisms made up of many cells.

In Multicellular Organisms, many cells group together in a single body and assume different functions in it to form various body parts.

Multicellular Organisms come from a single cell and whole the organism is made after multiplying the cell.

The shape and size of cells

Cells of some organisms have different sizes and shapes.

Example:

In the human body, some particular cells are different in shape and size.

Nerve cells are cylindrical in shape with a tail. One end of the nerve cell has a spherical shape around which some hair like branches protrudes. The end of the tail of the nerve cell also has a plant root like structure.

Nerve cells are the longest cell in the human body.

Blood cells are spherical in shape.

Ovum cells are spherical in shape but bigger than blood cells.

Bone cells have star like shape.

Smooth muscle cells have long cylindrical shapes pointed at both ends.

Amoeba, which is a Unicellular Organism, changes its shape.

Function of Cell

Some characteristics of living organisms make them different from non-living ones. For example, growth, reproducing their own kind, respiration, etc. These basic functions of living organisms are performed at the cellular level. This means each living cell is capable to perform certain basic functions that are characteristics of all living forms.

Different parts of the cell perform different tasks. These parts of a cell are called Organelles and these organelles together form a cell. All cells have the same organelles regardless of the organisms in which the cell is found.

Structure of a Cell

Plasma Membrane

Plasma membrane is the outer covering of a cell. Plasma membranes are known as Cell Membrane also.

Structure of the Plasma Membrane

The plasma membrane is the covering of the cell similar to a thin film. The plasma membrane is made up of a double layer of protein and lipids. This double layer of the plasma membrane is called phospholipid bilayer.

Function of the Plasma Membrane

The plasma membrane separates a cell and its components from its external environment. The plasma membrane is a semi permeable membrane.

The exchanges of substances from inside to outside and vice versa take place from a cell through the plasma membrane by the process called Diffusion and Osmosis.

Diffusion

The Diffusion is a process in which the movement of substance takes place from a region of high concentration to a region of low concentration.

Example: if a favourite food is being cooked in the kitchen, we can feel its fragrance by sitting far in the room. The fragrance of the food reaches our nostrils because of diffusion.

In the cell, carbon dioxide is a waste produced during respiration and needs to be expelled. The expelling of carbon dioxide out from the cell takes place by the process of diffusion.

When the concentration of carbon dioxide becomes higher compared to the outer environment of the cell, it moves to the region of low concentration through the process of diffusion and is finally expelled out through exhalation.

Similarly, when the concentration of oxygen decreases inside the cell, it moves into the cell by the process of diffusion through the plasma membrane.

Osmosis

The movement of water in and out through a semi permeable membrane like plasma membrane is called Osmosis.

Osmosis is a process in which the molecules of a liquid enter through a semi permeable membrane from a high concentration to a low concentration.

Plasma membrane allows required substances especially liquid, such as water to enter and expel from a cell using a process called Osmosis.

Since water is a liquid, thus the molecules of water pass through the semi permeable membrane. The semi permeable membrane is called a selectively permeable membrane also.

Since osmosis takes place from the region of high concentration to the region of low concentration, thus osmosis is a special case of diffusion through a selectively permeable membrane.

Because of osmosis, the root of plants absorbs water from the soil.

Endocytosis

An Amoeba ingests (takes in) food by changing its shape.

The flexibility of the plasma membrane enables amoeba to ingest the food by changing its shape.

This process of taking other materials from the external environment by a cell, like Amoeba, is known as Endocytosis.