Structure and function of cell

All organisms are composed of cells, a cell is the structural and functional unit of life of every organism. some organisms are made up of single-cell and some consist of many cells. For example organisms like bacteria, protozoans, and some algae are made up of a single cell and higher fungi, plants and animals, and humans are composed of many cells.

Robert Hooke in 1665 observed a piece of cork under the microscope and saw small compartments and named it ‘cell’. After that In 1672, Leeuwenhoek observed bacteria, sperms, and red blood corpuscles, as cells. Robert brown found a centrally positioned body and called it ‘nucleus’ these are found in all cells. Cells vary considerably, in shapes and sizes for example Nerve cells of animals have long extensions, and the Egg of the ostrich is the largest cell (75 mm).

The cell theory

M.J. Schleiden and Theodore Schwann formulated the “cell theory in 1838. the cell theory was based on three observation as-

  • all organisms are composed of cells.
  • the cell is the structural and functional unit of life. 
  • cells arise from pre-existing cells.
The Cell

The cell may be called the unit of protoplasm and bound by a cell membrane that has a nucleus. They may be microscopic and also can be seen by nude eyes, for example, we cannot see bacteria from nude eyes but we can see a hen’s egg. It is interesting to know that the Human body has trillions of cells that vary in shapes and sizes. Different groups of cells perform a variety of functions. They are of two types on the basis of numbers as- multicellular and unicellular.

Unicellular-Organism made of a single cell. The single-celled organisms are called unicellular (uni: one; cellular: cell) organisms. Eg. amoeba, paramecium, etc.

Multicellular-Organisms made of more than one cell are called multicellular (multi: many; cellular: cell) organisms. Eg. plants, animals, etc.

The shape of the cell – the shape of the cell varies as they may be spherical, round, and elongated in shape.

size of cell size of the cell varies as it has categories of function. The smallest cell is 0.1 to 0.5 micrometer of bacteria. The largest cell 170 mm ×130 mm, egg of an ostrich.

Components of cell-Cell membrane, cytoplasm, and nucleus are the basic components of a cell. The cytoplasm and nucleus are enclosed within the cell membrane also known as the plasma membrane. This membrane act as a porous membrane which allows passing the material from both the outer and inner side. In the center of the cell, a dense round body is found called the nucleus. The nucleus is separated from the cytoplasm by the nuclear membrane. The cytoplasm is a jelly-like substance between the nucleus and the cell membrane. The cytoplasm contains small particles called organelles.

( Click here: read cell organelles)

In addition to the cell membrane, an outer thick layer is found in cells of plants known as the cell walls. Cell wall helps in protection of plants.

The cells having nuclear material without a nuclear membrane are called prokaryotic cells. prokaryotes (pro: primitive; karyon: nucleus). Examples are bacteria and blue-green algae.

structure of prokaryotic cell

fig- a prokaryotic cell

Cells having a well-organized nucleus with a nuclear membrane are termed

eukaryotic cells. (EU: true; karyon: nucleus). These cells possessing a clearly defined nucleus enclosed within a nuclear membrane. 

Ex. all organisms except bacteria and blue-green algae

Structure of eukaryotic cell

fig- eukaryotic cell
Differences between Eukaryotic and Prokaryotic cells
Eukaryotic cell

1.Nucleus is distinct, with a well-formed nuclear membrane.

2.Double-membraned cell organelles(chloroplasts, mitochondria, nucleus)and single membraned (Golgi apparatus, lysosomes, vacuole, endoplasm reticulum)are present.

3.Ribosomes – 80 S is found

4.Distinct compartments in the cell are found i.e. the cytoplasm and the nucleus.

5.Each chromosome has one linear double-stranded DNA complex with histones

6.Each chromosome has one centromere that divides a chromosome into two arms. However, if the centromere is terminal, the chromosome would have only one arm

Prokaryotic cell

1.Nucleus not distinct

2.Single-membraned cell bodies like mesosomes present. The endoplasmic reticulum, plastids, mitochondria microbodies like lysosomes, and Golgi body is absent

3.Ribosomes – 70 S is found. The chromosome is circular and remains attached to the cell membrane at one point

4.The chromosome having a single double-stranded circular DNA molecule and is not associated with histones.

5.The chromosome without a centromere.

Plant cell

  1. The cell wall presents the outer side of the cell membrane which is made up of cellulose in plant cells.
  2. In plants, cell Vacuoles are found generally large.
  3. Plastids are present in a plant cell.
  4. Golgi body present in the form of units termed as dictyosomes in plants.
  5. The centriole is absent.

plant cell

Animal cell
  • The cell wall is not found, the outermost structure is the cell membrane or plasma membrane.
  • Generally, vacuoles are absent in animals cells and if present, they are very small.
  • Plastids are absent in animal cells.
  • Golgi body is well developed having two cisternae
  • The centriole is present in animal cell

animal cell

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