- 1 What is the difference between Prokaryotic and Eukaryotic cells? and Explain Levels of Organization of Living Things
Evolution, a key concept in Biology Topics, explains the development of species over time.
What is the difference between Prokaryotic and Eukaryotic cells? and Explain Levels of Organization of Living Things
Prokaryotic cells are more primitive than eukaryotic cells and have a simple structure. The nucleus of prokaryotic cells is not well organised. There is no nuclear membrane around the nuclear material in the prokaryotic cells. The cells having nuclear material without a nuclear membrane around it, are called prokaryotic cells [see Figure (a)]. The nuclear material in a prokaryotic cell is in direct contact with the cytoplasm. In fact, prokaryotic cells have no real nucleus. The organisms made of prokaryotic cells are called prokaryotes, (pro = primitive ; karyon = nucleus). All the prokaryotes are simple, unicellular organisms. Thus, the simple unicellular organisms whose cells do not possess a nucleus bound by a nuclear membrane are called prokaryotes.
Prokaryotes have no real nucleus in their cells. Prokaryotes are relatively simple organisms. Bacteria and Blue-green algae are prokaryotes. Bacterium is a prokaryote because its cell has no real nucleus [see Figure (a)]. The nuclear material in a bacterium cell is not surrounded by a nuclear membrane. The nuclear material in a bacterium cell is in direct contact with the cytoplasm. Thus, the nucleus of a bacterium cell is not well organised like the nucleus of cells of multicellular organisms. Prokaryotes were probably the first living things to evolve on the earth.
The cells having nuclear material enclosed by a nuclear membrane are called eukaryotic cells [see Figure (b)]. Eukaryotic cells have a proper, well organised nucleus. The nuclear material in eukaryotic cells is not in direct contact with cytoplasm, it is separated from cytoplasm by the nuclear membrane. Eukaryotic cells are more advanced than prokaryotic cells. Amoeba cell is a eukaryotic cell because it has a proper nucleus bound by a nuclear membrane [see Figure (b)], The onion peel cells and cheek cells are also eukaryotic cells. The organisms made of eukaryotic cells are called eukaryotes (eu = true ; karyon = nucleus). In other words, the organisms whose cells possess a nucleus bound by a nuclear membrane are called eukaryotes.
All the organisms other than bacteria and blue-green algae are eukaryotes. For example, plants, animals, fungi and protozoa, etc., are all eukaryotes. Amoeba is a eukaryote because it has a distinct nucleus which is bound by a nuclear membrane [see Figure (b)], A human being, a lion, tiger, dog, cat, birds, rose plant, neem tree and mushroom, etc., are all eukaryotes. We will now discuss the variety in number, shape and size of cells present in living organisms.
Organisms Show Variety In Cell Number, Cell Shape And Cell Size
There are millions of living organisms all of which are made of tiny ‘cells’. The various organisms, however, differ in the number of cells which make up their body ; they differ in the shapes of cells in their body ; and they also differ in the size of cells in their body.
- Different organisms have different number of cells in their bodies. This is called variety in the number of cells (or variety in ‘cell number’).
- The cells in multicellular organisms (multicellular plants and animals) have many different shapes. This is called variety in shape of cells (or variety in ‘cell shape’).
- The cells in multicellular organisms can have many different sizes. This is called variety in size of cells (or variety in ‘cell size’).
We will now describe the variety in ‘number of cells’, ‘shape of cells’ and ‘size of cells’ in organisms in detail. Before we do that we should keep in mind that : A million is a thousand thousand (which is 1000,000) ; a billion is a thousand million ; and a trillion is a thousand billion.
1. Variety in Number of Cells
All the living organisms are made up of cells. But the number of cells varies from organism to organism. Some organisms (plants and animals) are made up of just ‘one cell’ (single cell) while other organisms are made up of ‘a large number of cells’. This is called variety in the number of cells. Depending on the number of cells in the body of an organism, an organism is called unicellular or multicellular.
The simplest living organisms have only ‘one cell’ in their body The organisms which are made up of only ‘one cell’ are called unicellular organisms (uni = one ; cellular = cell). One cell is also called ‘single cell’. So, the unicellular organisms are actually single-celled organisms. Some of the examples of unicellular organisms (or single-celled organisms) are : Amoeba, Paramecium, Euglena, Chlamydomonas and Bacteria (see Figure). The single cell of all these organisms behaves as a ‘complete organism’ (or individual).
A unicellular organism can perform all the necessary life functions with the help of just one cell (which the multicellular organisms perform with the help of many groups of specialised cells). For example, Amoeba is a tiny animal which consists of only one cell [see Figure 11(a)], All the basic functions of life like taking food (or capturing food), digestion, respiration, movement, response to environmental changes, removal of waste (excretion), and reproduction, etc., are performed by the single cell of Amoeba. Similar functions in multicellular organisms are performed by various organs (or organ systems) which are made up of many, many different types of cells.
The organisms which are made up of many cells are called multicellular organisms (multi = many ; cellular = cell). Depending on its size, a multicellular organism may be made up of millions, billions or trillions of cells joined together. Most of the plants and animals around us are multicellular organisms. For example, a rose plant, a neem tree, a rat, a human being (man) and an elephant are all multicellular
organisms which are made up of many, many cells joined together (see Figure). A big and tall tree, a huge animal like an elephant and the body of a human adult has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions.
A rat is a small organism whereas an elephant is a big organism. So, a rat has a smaller number of cells in its body than that of an elephant. The smaller number of cells in a rat does not affect the functioning of a rat. In general we can say that: The number of cells being less in small organisms does not in any way affect the functioning of the small organisms. In fact, even an organism made of millions, billions or trillions of cells begins its life as a single cell called ‘fertilised egg cell’ (or ‘zygote’). The fertilised egg cell (or zygote) divides and multiplies due to which the number of cells increases as the development of the organism proceeds.
2. Variety in Shape of Cells
There are many types of cells in the bodies of multicellular organisms. These cells differ in shapes. For example, the shape of a nerve cell in animals is very different from the shape of a muscle cell [see Figures (a) and (b)].
A nerve cell is long and branched (having thread-like projections) whereas a muscle cell is pointed at both ends and has a spindle shape. Due to this, a nerve cell looks very different from a muscle cell. The different shapes of cells are related to their functions which they have to perform in the body of an animal (or plant). For example, nerve cells receive and transmit messages between brain and other body parts whereas muscle cells bring about movement in body parts. Nerve cells and muscle cells are actually specialised animal cells because they perform specific functions in the bodies of animals. Some of the specialised animal cells and plant cells are shown in Figure.
Cells are different in shapes and sizes so that they can perform different functions. We say that the cells are specialised (to do different jobs). All the cells shown in Figure 13 are specialised cells. Each one of these cells is adapted to perform some specific function. For example, some cells protect the body parts, some cells help in moving body parts, some cells help in sending messages from brain to body parts (or from body parts to brain), and so on. The human body is made up of about 20 different types of cells and each type of cell performs different functions. Some of the examples of animal cells (or human body cells) are: Nerve cell (or Neuron), Muscle cell, Epithelial cell, Red blood cell, White blood cell, Bone cell and Cartilage cell [see Figures (a) to (g)].
The shape of a cell helps in its functioning. For example, a nerve cell is very long and has wire-like projections coming out of it [see Figure (a)], The large length of nerve cell makes it carry messages over long distances in the body. And the wire-like projections of the nerve cell help it to make many contacts with other nerve cells so that messages from the brain can be sent to all the parts of the body. Thus, the nerve cells are long and have projections so that they can make contacts with many other nerve cells and carry messages over long distances (between brain and other parts of the body). And we say that the nerve cells are specially adapted to transmit messages. Since nerve cells receive and transfer messages, they help to control and coordinate the working of different parts of the body.
Let us take the example of muscle cells now. Muscle cells are specially adapted for movement. This happens as follows: The muscle cells have a special property that they can contract (and relax). When the muscle cells contract, they shorten in length. So, the contraction of muscle cells moves the body part (to which they are attached). And when these contracted muscle cells relax, they expand (increase in length), so that the body part comes back to its original position. Thus, muscle cells bring about the movement of body parts by contracting and relaxing.
Epithelial cells are rectangular in shape. The epithelial cells form a thin layer over the body parts and protect the cells below them from injury. Red blood cells are spherical in shape. Red blood cells carry oxygen around the body. White blood cells have irregular shape. White blood cells eat up or kill bacteria which enter the blood and save us from many diseases. We will study more about the animal cells in higher classes.
Some of the important plant cells are: Epidermal cells, Xylem cells, Phloem cells and Photosynthetic cells [see Figures (h) to (k)]. The epidermal cells form a layer around the plant organs and protect the cells below from injury. Xylem cells are the tube-like plant cells having thick and strong walls which carry water and mineral salts from the roots of the plant to its leaves.
Phloem cells are also tube-like plant cells having thin walls which carry the food made by leaves to all other parts of the plant. The photosynthetic cells of the plant contain chlorophyll and prepare food by photosynthesis. The mesophyll cells of leaf are the photosynthetic plant cells. These cells in the leaf of a plant are specially adapted for making food by photosynthesis.
All the cells of different shapes which we have described above are those which are present in the bodies of multicellular animals and plants. They are not capable of independent existence. We will now describe the shape of a cell called Amoeba which is a complete organism in itself and capable of independent existence. The shape of an Amoeba cell is irregular [see Figure (a)]. In fact, the Amoeba cell has no fixed shape. The Amoeba cell keeps on changing its shape continuously. The shape of Amoeba changes because Amoeba can make its cytoplasm flow in any direction it wants to.
The Amoeba cell has finger-like projections of varying lengths protruding out of its body which are called pseudopodia (pseudopodia means false feet). Amoeba can produce pseudopodia on any side by pushing the cytoplasm in that side. The pseudopodia appear and disappear when Amoeba ‘moves’ or ‘feeds’. For example, Amoeba moves very slowly with the help of pseudopodia (which keep on appearing and disappearing to make it move). Amoeba also uses pseudopodia to catch (or engulf) the food particles from the water in which it lives. Thus, Amoeba derives two advantages by changing its shape : The changing of shape due to the formation of pseudopodia helps Amoeba
- in movement; and
- in capturing food.
We have just studied that Amoeba is a single cell which can change its shape. A white blood cell (WBC) present in human blood is another example of a single cell which can also change its shape. The difference between Amoeba cell and white blood cell is that while Amoeba cell is a full fledged organism capable of independent existence, white blood cell is merely a cell of human blood which is not a full fledged organism and hence cannot exist independently. It can exist only inside the blood.
Before we describe the variation in the size of cells, we should know the very small unit of measuring length called micrometre (or micron). A micrometre (or micron) is one-millionth of a metre (which is 10”6 metre). Let us discuss the sizes of cells now.
3. Variety in Size of Cells
The cells are of many different sizes. The cells in living organisms may be as small as ‘a millionth of a metre’ (micrometre or micron), or it may be as large as ‘a few centimetres’. However, most of the cells are extremely small in size (or microscopic) and hence cannot be seen with the naked eyes. Most of the cells found in living organisms have to be enlarged (or magnified) by using a microscope before they can be seen by us. Bacteria cells are extremely small (see Figure). The bacteria cells have a length of 0.1 micrometre to 0.5 micrometre.
In fact, bacteria Mycoplasma is the smallest cell having the size (or length) of only 0.1 micrometre. Even the long cells are so thin that they can be seen only with a microscope. For example, the muscle cells in animals are a few centimetres long and the nerve cells are more than a metre long, but they are so thin that they can only be seen with the help of a microscope.
Some of the cells are, however, big and can be seen easily with naked eye. The bird’s eggs are very large cells (see Figure). Each egg of the bird is a single cell. For example, the hen’s egg is a single cell. The hen’s egg is quite big. We can see the single cell called hen’s egg with naked eyes (without using a microscope).
The biggest cell (which can be seen with the naked eye) is the ostrich egg. The ostrich egg can be as much as 17 centimetres long (which is 170 millimetres long or 170,000 micrometres long). In the human body, some blood cells are the smallest and the nerve cells are the longest.
The sizes (or lengths) of some of the common cells are given below. We are giving the lengths of these cells in millimetres (mm) as well as in micrometres (microns) just for the sake of comparison.
We have just said that the biggest cell is the egg of an ostrich bird (see Figure). The ostrich egg is actually 170 millimetres long and 130 millimetres wide. So, we can say that the biggest cell is the egg of an ostrich and it measures 170 mm × 130 mm. If we use the unit of centimetres, we can say that an ostrich egg measures 17 cm × 13 cm. The size of cells has no relation with the size of the body of an animal (or plant).
Now, elephant is a very big animal whereas rat is a quite small animal. It is not necessary that the cells in the elephant be much bigger than those in a rat. The size of a cell is related to its function. For example, the nerve cells, both in elephant and rat are long and branched. They perform the same function of transmitting messages (in the form of electrical impulses).
Cells, Tissues, Organs, Organ Systems And Organism
A cell is the smallest unit of life which has a definite structure and performs a specific function. All the cells of a multicellular organism are not similar. They are of many different shapes and sizes. Most of the cells are specialised to perform particular functions. They are called ‘specialised cells’. For example, in animals, muscle cells are specialised to contract (and relax) so that they can bring about movement in body parts (see Figure). In plants, photosynthetic cells are specialised to carry out photosynthesis and make food. There are many types of specialised cells in animals and plants which perform different functions.
A multicellular organism (animal or plant) is made up of millions of cells. The cells in a multicellular organism do not work as single cells, they work in groups of similar cells. The group of similar cells which work together to perform a particular function is called a tissue. For example, in animals, muscle tissue is a group of muscle cells joined together which is specialised to contract (and relax) so as to move body parts (see Figure).
Thus, muscle tissue brings about movement in the body parts of animals. In plants, photosynthetic tissue is a group of photosynthetic cells joined together which is specialised to do photosynthesis and make food. There are many different types of tissues in both, animals as well as in plants. In fact, there are as many types of tissues as there are cells.
The bodies of animals and plants are made up of different types of tissues. The different tissues in the body of an animal or a plant do not work separately. The different tissues combine together to form ‘organs’. These organs perform different tasks for the animal or the plant. We can now say that : An organ is a collection of different tissues which work together to perform a particular function in the body of an organism. The multicellular organisms are made up of many different organs which do different jobs for the organism. Some of the organs which are found in the bodies of animals (including human beings) are: Heart, Stomach, Brain, Lungs, Kidneys, Liver, Intestine, Mouth, Eyes and Hands, etc. Some of the
human organs are shown in Figure. The function of heart is to pump blood around the body. The function of stomach is to digest food. The function of brain is to control all the parts of the body. Lungs are the organs of breathing. The function of lungs is to take in oxygen and give out carbon dioxide.
The body of a plant also contains many organs. The various organs found in the body of a plant are: Roots, Stem, Leaves, Flowers and Fruit. Some of the plant organs are shown in Figure. Each of these organs performs a particular job for the plant. For example, roots absorb water and dissolved mineral salts from the soil. Stem carries water and minerals from the roots to the leaves, and the prepared food
from the leaves to other parts of the plant. It also holds branches and leaves. The leaves prepare food for the plant by the process of photosynthesis. The flowers are reproductive organs which lead to the formation of fruits and seeds. The fruit protects the seeds.
The various organs in an organism (animal or plant) do not work separately. The organs work in groups. A group of interconnected organs which works together to do a big job for the organism, is called an organ system (or just a ‘system’). All the multicellular animals and plants have many organ systems in their bodies to carry out various life processes. For example, the various organ systems of animals (including human beings) are : Digestive system, Respiratory system, Circulatory system, Nervous system, Excretory system (or Urinary system), Reproductive system, Muscular system and Skeletal system.
Each organ system performs a specific function for the body. For example, the function of digestive system is to break down the food into simple substances which can be absorbed by the body. The main organs of the digestive system are : Mouth, Oesophagus (Food pipe), Stomach, Small intestine, Large intestine, Rectum and Anus. Each organ of the digestive system performs a different function such as ingestion, digestion, absorption, assimilation and egestion, etc. The plants have two main organ systems : Root system and Shoot system. The root system consists of various types of roots whereas shoot system is made up of organs like stem, leaves, flowers and fruits, etc. Different organs of the ‘organ systems’ of plants perform specific functions for the plants. For example, roots help in the absorption of water and minerals. And leaves are responsible for food making by photosynthesis.
An organism is an animal or a plant which can exist on its own. An organism is made up of many different organ systems which work together to perform all the functions necessary for maintaining life. For example, the human being is an organism. The human being is made up of many different organ systems like digestive system, respiratory system, circulatory system, nervous system, excretory system, reproductive system, muscular system and skeletal system.
All these organ systems work together to make man (or human being) a living organism. A cat, dog, bird, neem tree, mango tree, and rose plant, etc., are all organisms which are made of many different organ systems working together.
From all the above discussion, we come to the conclusion that multicellular organisms (animals and plants) are built like this : cells make up tissues, tissues make up organs; organs make up organ systems and finally organ systems make up an organism. The organisation within the living body can now be depicted as follows :