Cell Organelles

Following are the Important Cell-organelles:

• Endoplasmic Reticulum (ER)

• Golgi Apparatus

• Lysosomes

• Peroxisomes

• Ribosomes

• Centrosome

• Mitochondria

• Plastids

Endoplasmic Reticulum (ER)

This is a complex network of tubes, the lumen of which is filled with fluid. Two types of endoplasmic reticula are seen. They are:

• Tubes with a smooth surface are called smooth endoplasmic reticula. They secrete lipids.

• Tubes with spherical bodies (ribosomes) attached are known as rough endoplasmic reticula.

The functions of the endoplasmic reticulum are to form the skeletal framework of the cell, to provide a pathway for the distribution of nuclear material from one cell to the other and to synthesize fats, steroids and cholesterol with the help of enzymes secreted by the cell.

Golgi Apparatus

Also known as Golgi Complex or Golgi Bodies, they consist of tiny, elongated, flattened sacs (cisternae), which are stacked parallel to one another along with some vacuoles and clusters of vesicles.

The function of the golgi body is to secrete certain hormones and enzymes. It also forms lysosomes and peroxisomes. The golgi body is usually found close to the nucleus.

Lysosomes

These are tiny, spherical, sac-like structures scattered all over the cytoplasm. Their main function is digestion. They contain powerful destructive enzymes capable of digesting all organic material, and hence called “digestive bags”.

Lysosomes present in white blood cells are capable of digesting bacteria and viruses. During starvation, lysosomes digest proteins, fats and glycogen in the cytoplasm, and supply energy to the cell. They are also capable of digesting worn out cell organelles, or even digesting the entire damaged cell containing them. Hence, “suicide bag” is a sobriquet that is often used for Lysosomes.

Peroxisomes

These organelles are found in the liver and kidney cells. They are small, membrane-bound sacs, and contain powerful oxidative enzymes.

Their chief function is to remove toxic substances.

Ribosomes

These are spherical, granular particles which occur freely in the matrix or remain attached to the rough endoplasmic reticulum.

Ribosomes contain RNA (ribonucleic acid) and proteins. Their function is to provide the surface for protein synthesis.

Centrosome

This is found in the cytoplasm near the outer surface of the nucleus and contains two cylinders called centrioles.

The centrosome is found only in the animal cell. The centrosome and the centrioles play an important role by forming the poles of the spindle during cell division.

Mitochondria

These may be cylindrical, rod-shaped or spherical and distributed in the cytoplasm. Each mitochondrion is bound by a double membrane. The inner membrane is folded into ridges called cristae, which increase the surface area of the membrane.

It is in the mitochondria that the sugar is finally burnt during cellular respiration. The energy thus released is stored as high-energy chemicals called ATP (adenosine triphosphate). Hence, mitochondria are termed as the “power house” or the “power plant” of the cell. The body cells use the energy stored in ATP for synthesis of new chemical compounds, the transport of these compounds and for mechanical work.

Plastids

These organelles are found only in plant cells.

Plastids are of three types:

• Chloroplasts

• Chromoplasts

• Leucoplasts

Chloroplasts

They are green and found in leaves. The green colour is due to the presence of chlorophyll.

Chromoplasts

They are yellow, orange and red, and found in flowers and fruits.

Leucoplasts

They are colourless and found in roots, seeds and underground stems.

The function of the chloroplast is to trap solar energy for photosynthesis. Chromoplasts impart colour to flowers to attract insects for pollination. Leucoplasts store food in the form of carbohydrates, fats and proteins.

Nucleus

This is a prominent, spherical or oval structure found at the centre of the cell. It is the controlling centre of all cell activities and has been described as the brain of the cell. It regulates all metabolic and hereditary activities of the cell.

The nucleus is composed of the following structures:

• Nuclear Membrane

• Nucleoplasm

• Nucleolus

• Chromatin network

Nuclear Membrane

This is a double-layered membrane which separates the nucleoplasm from the cytoplasm. The nuclear membrane has minute pores which allow the selective transfer of material between the nucleoplasm and the cytoplasm.

Nucleoplasm

Within the nuclear membrane, completely filling up the space, is a clear, semi-solid, granular substance or matrix called the nucleoplasm. The nucleolus and the chromatin network lie suspended in the nucleoplasm.

Nucleolus

This dense, spherical granule found in the nucleus contains RNA (ribonucleic acid) which is responsible for protein synthesis in the cytoplasm.

Chromatin Network

These are very fine thread-like, coiled filaments uniformly distributed in the nucleoplasm. At the time of cell division, the chromatin becomes thick and ribbon like and are known as chromosomes. The chromosomes contain genes, which are composed of DNA (deoxy-ribonucleic acid). Genes are responsible for storing and transmitting hereditary characteristics from one generation to another. A gene is the functional unit of a chromosome. Genes are arranged in single linear order along the chromosome. One gene may be responsible for a single characteristic, or a single characteristic may be transmitted by a set of genes.

A Typical Animal Cell

A Typical Plant Cell

Study the structures of a typical animal cell and a typical plant cell as seen under an electron microscope.

A Generalised Animal Cell as observed under an Electron Microscope

The main differences between a typical animal cell and a typical plant cell are summarized below:

Animal cell	Plant cell
Usually smaller in size	Usually larger in size
Cell wall is absent. Cellulose in any form is absent	Cell wall made up of cellulose is present
Cytoplasm is denser, more granular and occupies most of the space in the cell	Cytoplasm is pushed to the periphery and forms a thin lining against the cell wall. The central space in the cell may be occupied by a large, single vacuole
Vacuoles absent. If present, they are small, temporary and concerned with excretion or secretion	Vacuoles are large and prominent. May be one or more
Plastids are absent	Plastids are usually present
Centrosome is present	Centrosome is absent. Instead two small clear areas called polar caps are present
Prominent and highly complex Golgi bodies present near nucleus	Contain several sub units of Golgi apparatus called dictyosomes