Chapter 6 The Structure of Living Organisms
Introduction
We are surrounded by living and non-living things. All animals and plants are living things and biology is the study of these living things.
A cat, playing with a ball, is obviously living. A pigeon flying from tree to tree is also a living thing.
There are seven characteristics of living things: feeding, movement, breathing or respiration, excretion, growth, sensitivity and reproduction.
Some non-living things may show one or two of these characteristics but living things show all seven characteristics.
Non-living things can be divided into two groups, those which were once part of a living thing (Organic) and those which were never part of a living thing (Inorganic).
Some non-living things show one or two of the seven characteristics of living things. Machines, such as washing machines, can create movements of their parts.
The car needs to be fed with petrol in order to move. Sand, wood and glass are all non-living things. None of them shows any of the characteristics listed above.
They were never part of a living thing. The second group is those who were once part of living things. Coal is a good example. It was formed when trees died and sank into the soft ground.
This happened many millions of years ago when the Earth was covered with forests. Paper is non-living but it is also made from trees. Jam is also non-living but it was made from the fruit of a plant.
Read And Learn More WBBSE Notes For Class 8 School Science
The Seven Characteristics of Living Things
1. Feeding: All living organisms need to take substances from their environment to obtain energy, grow and stay healthy.
2. Movement: All living organisms show the movement of one kind or another. All living organisms have internal movement, which means that they have the ability to move substances from one part of their body to another.
Some living organisms show external movement as well—they can move from place to place by walking, flying or swimming.
3. Breathing or Respiration: All living things exchange gases with their environment. Animals take in oxygen and breathe out carbon dioxide.
4. Excretion: Excretion is the removal of waste from the body. If this waste is allowed to remain in the body it could be poisonous.
Humans produce liquid waste called urine. We also excrete waste when we breathe out. All living things need to remove waste from their bodies.
Growth: When living things feed they gain energy. Some of this energy is used for growth. Living things become larger and more complicated as they grow.
Sensitivity: Living things react to changes around them. We react to touch, light, heat, cold and sound, as do other living things.
Reproduction: All living things produce young. Humans make babies, cats produce kittens and pigeons lay eggs. Plants also reproduce. Many make seeds which can germinate and grow into new plants.
Differences Among Living And Non-Living
An attempt should now be made to distinguish living things from lifeless nonliving or inanimate objects. There is no difficulty in recognising a coconut palm or guinea pig as living objects and rocks in the field or the sand grains in the seashore as non-living bodies.
It may be mentioned here that viruses are an intermediate stage between living and nonliving forms of matter.
The principal points of difference between living and nonliving objects are described in tabular form below:
| Living | Non-living | ||
| 1 | Each kind of plant and animal has a definite form and size, which may vary within very narrow limits in different individuals of the same kind. | 1 | Non-living objects, such as masses of clouds or collections of stones have neither a definite size nor any precise form |
| 2 | A living body is organized of cells, tissues and organs with the division of labour. | 2 | No such organisation exists |
| 3 | Life is an external manifestation of metabolic activities like nutrition, respiration, secretion, circulation, excretion etc. | 3 | None of the metabolic activities is detected in nonliving objects. |
| 4 | The living body increases in bulk by wedging in new particles in between already existing cellular matter. | 4 | Growth may occur occasionally by deposition of particles only on the outer surface of the body |
| 5 | A living body can reproduce its own kind and thus perpetuate its race. | 5 | There is no power to reproduce its own kind |
| 6 | A living body has a definite life cycle. | 6 | No life cycle is observed. The period of duration is infinite and there is no death. |
Cell-The Unit of an Organism
Your body, as a whole, is one organism. However, many, many parts make up that whole. First, you notice the entire body. Next, you see that the entire body is made up of parts and organs, and each of those organs is made up of a variety of tissues.
And if, as a pathologist does, you examine a magnified sample of one of the human body’s tissues under a microscope, millions of cells become visible.
Yet you can turn up the magnification for an even closer look, Cells contain molecules that are made up of even smaller components called atoms.
Atoms, molecules, cells, tissues, organs, and organ systems are the body’s building blocks. ‘Schleiden and Schwann together proposed the cell theory in 1839.
Cell theory states that “all plants and animals are composed of cells and cellular products.” Thus, the cell is the basic unit of life.
- Salient points of cell theory:
- All living things are composed of cells and their products.
- All cells arise from pre-existing cells.
- AII cells are basically alike in chemical composition and metabolic activities.
- The function of an organism as a whole is the outcome of the activities and interaction of the constituent cells of which the organism is built.
Therefore, all living beings are made up of cells which are the smallest structural and functional unit of the body. Some of them are made up of only one cell and others have many cells.
All living organisms are made up of individual and identifiable cells, whose number, together with their size and type, ultimately defines the structure and functions of an organism.
While the total cell number of lower organisms is often known, it has not yet been defined in higher organisms. In particular, the reported total cell number of a human being is about 37 trillion (one trillion =1,000,000,000,000).
How a Cell can be seen?
Cells got their name from an Englishman named Robert Hooke in the year 1665. He first saw and named “cells” while he was experimenting with a new instrument we now call a “microscope.
“For his experiment, he cut very thin slices from cork. He looked at these slices under a microscope.
He saw tiny box-like shapes. These tiny boxes reminded him of the plain small rooms that monks lived in called “cells”. The smallest objects that the unaided human eye can see are about 0.1 mm long.
A magnifying glass can help you to see them more clearly, but they will still look tiny. Cells are not visible under ordinary magnifying glasses.
Smaller cells are easily visible under a light microscope. Light microscopes use a system of lenses to magnify an image.
The power of a light microscope is limited by the wavelength of visible light, which is about 500 nm. The most powerful light microscopes can resolve bacteria but not viruses.
Light microscopes (optical microscopes) that are commonly used in schools are of two types – compound microscopes and simple or stereo microscopes (also known as dissecting or binocular microscopes).
Left: Stereo microscope; Center: Compound microscope with a binocular head; Right: Compound microscope with a monocular head is greater resolving power than light microscopes, so we can use them to see even more detail than is visible under a light microscope.
To see anything smaller than 500 nm, you will need an electron microscope Electron microscopes shoot a high-voltage beam of electrons onto or through an object, which deflects and absorbs some of the electrons.
Resolution is still limited by the wavelength of the electron beam, but this wavelength is much smaller than that of visible light. The most powerful electron microscopes can resolve molecules and even individual atoms.
Some of the specialized types of electron microscopes are – Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Reflection Electron Microscope (REM), and Scanning Transmission Electron Microscope (STEM).
Cell Diversity
Living organisms include unicellular organisms living as only one cell, and multicellular organisms whose bodies are made of many cells. Unicellular organisms exist, from bacteria such as the 1-2 pm long Escherichia coli (E. coli) to protists such as the 200 pm long paramoecia.
Multicellular organisms, e.g., humans, are made of a great variety of cells. Cells have various shapes, including flat cells (e.g., skin epithelial cells), disc-shaped cells (e.g., red blood cells),
And long, narrow, extended cells (e.g., nerve cells). Cell sizes vary from a diameter of about 7 pm in red blood cells to a dendrite of length up to 1m in nerve cells.
Plant cells also have a variety of shapes and sizes, including rectangular compartmentalized cells (e.g., cork cells), cells resembling jigsaw puzzle pieces (e.g., spongy cells), and cells elongated from a few millimetres to several dozen centimetres in length (e.g., pollen tube cells).
- Several different types of cells.
- The protist Giardia lamblia,
- a plant cell,
- a budding yeast cell,
- a red blood cell,
- a fibroblast cell, a eukaryotic nerve cell, and
A retinal rod cell In the human body, the heart and the brain are made of different types of cells having different functions. Different organs are made of more than one type of cell for proper body functions.
Types of cell
There are two distinct types of cells: prokaryotic and eukaryotic. Prokaryotic cells are much simpler than eukaryotic cells. Prokaryotic cells do not possess membrane-bound organelles.
The primitive nucleus (nucleoid) lies freely in the cytoplasm. Bacteria and cyanobacteria (Blue-green algae) are prokaryotes.
A cell which possesses membrane-bound organelles like the nucleus, mitochondria, chloroplasts etc is called a eukaryotic cell. The eukaryotic cells further show diversities in plant cells and animal cells.
Cells and their sizes
Prokaryotic cells are the smallest cells, typically example, an Escherichia coli (often referred to as E. coli) bacterium is typically a few PMs long.
Mammalian red blood cells are among the smallest eukaryotic cells. They are typically about 8 pm in diameter and have a distinctive bi-concave shape in humans.
It is interesting to note that the shape can be very different. For example, the red blood cells of sheep are nearly spherical and those of camels are elliptical.
The Amoeba is a relatively large single-celled organism, typically 10 to 100 pm across. Technically, the yolk of an ostrich egg is one cell though it is rather specially adapted.
All the nucleotide material is contained in a small volume at the edge of the yolk. The bulk is a supply of ‘raw material’ that the cells of the growing embryo use to assemble the cells of the ostrich chick.
Single giraffe nerve cells can be the length of the giraffe, from head to toe, several metres! Of course, these cells are still extremely narrow and the diagram shown here is very schematic.
Sclerenchyma fibre cells (1m long) are the longest plant cells and Acetabularia (10 cm) is the single largest plant cell.
Specialised Cells
The table below shows examples of some specialised animal and plant cells, with their functions and special features.
Levels of organisation:
The Organisation of Cells, Tissues, Organs, Systems and the Organism
From the least complex to the most complex, the organisms are made up of—
- Atom – The smallest piece of matter that still has physical and chemical properties of matter.
- Molecule – The smallest piece of a compound that still has the physical and chemical properties of that substance (e.g., water). The molecule is composed of atoms.
- Cell – The smallest piece of life (some arguments remain about viruses). It is the basic unit of all living beings.
- Tissue – Collection of cells that work together.
- Organ – Collection of tissues that work together.
- Organ System – Collection of organs that work together.
- Organism – Collection of organ systems that makes up a single life form.
Two or more kinds of tissues together form each of our organs, like the heart or lungs. Many organs together form an organ system, like the digestive or respiratory system.
All the organ systems together form an organism, such as a person, dog or blue whale. The body’s organisation goes from cells to tissues to organs to organ systems to a whole working organism. In unicellular (single-celled) organisms, the single cell performs all life functions.
It functions independently. However, multicellular (many-celled) organisms have various levels of organization within them. Individual cells may perform specific functions and also work together for the good of the entire organism.
The cells become dependent on one another. Multicellular organisms have the following 5 levels of organization ranging from simplest to most complex:
Level-1 Cells
Are the basic unit of structure and function in living things. May serve a specific function within the organism.
Examples – are blood cells, nerve cells, bone cells, etc.
Level-2 Tissues
Made up of cells that are similar in structure and function and which work together to perform a specific activity.
Examples – are blood, nervous, bone, etc. Humans have 4 basic tissues: connective, epithelial, muscle, and nervous.
Level-3 Organs
Made up of tissues that work together to perform a specific activity. Examples – heart, brain, skin, etc.
Level-4 Organ Systems
Groups of two or more tissues that work together to perform a specific function for the organism.
The Human body has 11 organ systems – circulatory, digestive, endocrine, excretory (urinary), immune (lymphatic), integumentary, muscular, nervous, reproductive, respiratory, and skeletal.
Level -5 Organisms
Entire living things can carry out all basic life processes. Meaning they can take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce.
Usually made up of organ systems, but an organism may be made up of only one cell such as bacteria or protists.
Examples – are bacteria, Amoeba, mushrooms, sunflowers, and humans.
The levels of organization in the correct order are:
cells ⇒ tissues⇒ organs ⇒ organ systems ⇒ organisms
Levels of the body from smallest to largest: Atoms, molecules, cells, tissues, organs, and organ systems.
What Are Tissues
A group of cells having a common origin, similar structure and performing a definite function is called a tissue. Tissues are found in plants and animals.
Plants and animals are made up of many different kinds of tissues. For example, groups of bone cells form bone tissues and muscle cells form muscle tissue.
Different types of tissues have distinctive architectures best suited for what they do. We will see that plant tissues are different from animal tissues in many ways.
Every organism, whether its body is unicellular or multicellular, is capable of performing all vital functions such as respiration, ingestion, excretion and reproduction.
In multicellular organisms, because of the increase in body size, it is difficult for each cell to efficiently cope with the vast variety of physiological needs of the organism. So cells group together to perform definite functions.
Plant Tissues
Plant tissues can be grouped into two basic types: meristematic and permanent tissues.
Meristematic tissue: This tissue is in a state of cell division and produces new cells. It is found in various growing organs of plant-like roots, shoots, leaves buds and flowers.
Permanent tissue: This tissue is composed of mature cells that have lost the power of division and have definite form and shape. These tissues are responsible for photosynthesis, storage of food, secretion and mechanical functions. Permanent cells always derive their origin from the meristematic cells.
Animal Tissues
The smallest units of the body-the cells-are are individually too small to be seen by the naked eye. Yet when many like cells are together, they form a tissue. Tissues are groups of cells with a common structure and function.
There are four main tissues in the body – epithelium, muscle tissue, connective tissue and nervous tissue.
Epithelium (or epithelial tissue) is found all over the body with several functions. In the skin, it protects us from the outside world, in the stomach and intestines it absorbs. In the kidney, it filters and in the glands it secretes.
Muscle tissue is responsible for body movement, moves blood, food, and waste through the body’s organs, and is responsible for mechanical digestion.
Connective tissue wraps around, supports, cushions and protects organs. It stores nutrients and gives the skin strength. As tendons and ligaments, it protects joints and attaches muscles to bone and each other. It includes some specialized tissues including cartilage, bone and blood.
Nervous tissue conducts impulses to and from body organs via neurons. It makes up the brain, spinal cord and nerves.
Difference between Animal and Plant Tissues
| Animal Tissue | Plant Tissue |
| Since animals are mobile so they require more energy, hence more living tissues are required. | Since plants are stationary so they do not require much energy. Hence more living tissues are not required. |
| Animals move from one place to another in search of food, shelter etc., hence they need more energy and there more tissues are living. | In plants, most tissues provide structural strength. Most of these tissues are dead, can provide mechanical strength as easily as the living ones and need less maintenance. |
| Cell growth is uniformly distributed. | Growth is limited to certain regions. |
| The structural organisation of organs and organ systems is more specialised and complex. | The structural organisation of organs is comparatively less complex. |
Structure and Function of a typical cell
Cell organelles and their structure and function
An organelle is any specialized structure based on the analogy that cells have their cell, especially one for which a specific organelle is just as complex as a multicellular function can be assigned.
As we mentioned organisms have their livers, kidneys, and stomachs earlier, the name means “little organ” and in other organs.
The Three Main Components of any Plant or Animal Cell:
1. Plasma Membrane / Cell Membrane
Structure – It is a bilipid membraneous layer containing proteins and lipids. There are two outer layers of protein and a middle layer of phospholipid. It is called a unit membrane and is semifluid and dynamic in nature.
Function – The cell membrane separates the cell from its external environment, and is selectively permeable (controls what gets in and out). It protects the cell and provides stability.
Proteins are found embedded within the plasma membrane, with some extending all the way through in order to transport materials.
Carbohydrates are attached to proteins and lipids on the outer lipid layer.
2. Cytoplasm
Structure – It is a colourless, semisolid and jelly-like substance composed of main water and found between the cell membrane and nucleus.
The cytoplasm makes up most of the “body” of a cell and is constantly streaming. The transparent fluid part of the cytoplasm is called cytosol.
Function – Organelles are found here and substances like salts may be dissolved in the cytoplasm. It helps in the exchange of materials between cell organelles and is the active site for metabolic processes like sugar, protein and fatty acid synthesis.
3. Nucleus
Structure – The largest organelle in the cell. It is dark and round and is surrounded by a double membrane called the nuclear envelope/ membrane.
In spots, the nuclear envelope fuses to form pores which are selectively permeable. The nucleus contains genetic information (DNA) on special strands called chromosomes.
The colourless dense sap present inside the nucleus is called the nucleoplasm. One or more round bodies present in the nucleoplasm are called nucleoli. A network of dark-staining fibre in the nucleus is called chromatin.
Function – The nucleus is the “control centre” of the cell, for cell metabolism and reproduction. It regulates the cell cycle and is the storehouse of genes.
2. The Organelles found in both Plant and Animal cells:
1. “ER” or Endoplasmic Reticulum
The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They carry materials throughout the cell. The ER is the “transport system” of the cell.
There are two types of ER: rough ER (RER) and smooth ER. (SER)Rough Endoplasmic Reticulum is lined with ribosomes and is rough in appearance and smooth endoplasmic reticulum contains no ribosomes and is smooth in appearance.
ER gives support to the cytoplasm. It forms an intracellular transport system. SER plays a crucial role in detoxifying many poisons and drugs. It also helps in the formation of proteins and lipids.
2. Ribosomes
Ribosomes are small particles which are found individually in the cytoplasm and also line the membranes of the rough endoplasmic reticulum. Ribosomes produce protein. They could be thought of as “factories” in the cell.
3. Golgi Body / Apparatus
Golgi bodies are stacks of flattened membranous sacs, (they look like pancakes). The Golgi Body temporarily stores protein which can then leave the cell via vesicles pinching off from the Golgi.
In plants, the Golgi apparatus is scattered in the cytoplasm and found in the diffused form. These are known as Dictyosomes.
Secretion is the main function of the Golgi complex. It mainly performs the function of packaging.
The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. It is directly involved in the formation of lysosomes.
4. Lysosomes
Lysosomes are small sac-like structures surrounded by a single membrane and containing strong digestive enzymes which when released can break down worn-out organelles or food.
The lysosome is also known as a suicide sac. This help to keep the cell clean by digesting any foreign material and worn-out cell organelles.
5. Mitochondria (Singular = Mitochondrion)
The mitochondria are round “tube-like” organelles that are surrounded by a double membrane, with the inner membrane being highly folded.
The mitochondria are often referred to as the “powerhouse” of the cell. The mitochondria release food energy from food molecules to be used by the cell.
This process is called respiration. Some cells ( muscle cells) require more energy than other cells and so would have many more mitochondria.
6. Vacuoles
Vacuoles are fluid-filled organelles enclosed by a membrane. They can store materials such as food, water, sugar, minerals and waste products.
3. Animal Cell Organelles not found in Plant Cells
1. Cilia and Flagella
Both cilia and flagella are hair-like organelles which extend from the surface of many animal cells. The structure is identical in both, except that flagella are longer and whip-like and cilia are shorter.
There are usually only a few flagella on a cell, while cilia may cover the entire surface of a cell. The function of cilia and flagella include locomotion for one-celled organisms and moving substances over cell surfaces in multi-celled organisms.
4. Organelles and other features found only in Plant Cells
1. Cell Wall
The cell wall is a rigid organelle composed of cellulose lying just outside the cell membrane. The cell wall gives the plant cell its box-like shape. It also protects the cell. The cell wall contains pores which allow materials to pass to and from the cell membrane.
2. Plastids
Plastids are double membrane bound organelles. It is in plastids that plants make and store food. Plastids are found in the cytoplasm and there are two main types:
Leucoplasts – Colourless organelles which store starch or other plant nutrients, (for example – starch stored in a potato).
Chromoplasts – Contain different coloured pigments. The most important type of chromoplast is the chloroplast, which contains the green pigment chlorophyll. This is important in the process of photosynthesis.
3. Central Vacuole
The central vacuole is a large fluid-filled vacuole found in plants.
Components of animal and plant cells
Differences between plant and animal cells:
| Point | Plant cell | Animal cell |
| Cell wall Plastids | Present | Absent |
| Centrosome | Present | Absent |
| Vacuoles | Absent | Present |
| Size | Large, central and permanent large | Small, many and temporary Smaller |
Types of Habitat
1. Habitat: Habitat is the immediate surroundings of an organism. It is the place that is natural for the life and growth of an organism. Animals and plants adapt themselves to their different habitats.
2. Terrestrial habitat: All the deserts, mountains and forests and grasslands are included under terrestrial habitat.
3. Desert: Camels show different adaptations to live in deserts. They possess long legs, long eyelashes, covered nostrils, special fat cells, and modified cells in the stomach as adaptations. Snakes and rats live in burrows and come out only during the night when it is cool.
Desert plants are called xerophytes and they exhibit some typical adaptations to live in desert conditions. Leaves are reduced to spines.
Stems are green and fleshy to carry out photosynthesis and store water. Roots grow deep into the soil to absorb water. The reduced leaf and the thick waxy layer of the stem minimise the rate of transpiration.
4. Cold region: Most of the trees in cold mountains are cone-shaped. The leaves are very thin and needle-shaped to slide off the snow and rainwater immediately.
Most deciduous plants shed their leaves and remain dormant during the intense cold. Plants may hold on to layers of dead leaves for thermal insulation.
Animals in cold areas have long thick skin and fur to protect themselves from cold climates. They possess strong hooves to run easily and large furry feet to distribute the load uniformly as well as to increase the grip on ice.
They have a small body surface area to volume ratio to minimise heat loss. Certain vertebrates in cold regions produce antifreeze proteins (AFPs), a class of polypeptides that bind to small ice crystals to inhibit the growth and recrystallization of ice. This permits survival in subzero temperatures.
RBC count increases abruptly in animals living in high altitudes and cold climates. There is a substantial increase in the number of mitochondria and myoglobin content in muscle cells.
Haemoglobin amount also increases with a proportionate increase in RBC count to combat the less oxygenated environment.
5. Grasslands: Lions living in the forests possess skin colour so as to match their environment. They hide in grasslands to catch their prey. They possess strong claws and teeth to tear food. They have sharp eyesight.
Deer possess long ears to have sharp hearing sense. Deers possess eyes on the side of its head which makes them look in all directions. They have long legs to run fast.
Aquatic Habitat: All the fresh water and marine water bodies are included under aquatic habitat. Fish possess streamlined bodies, which reduce friction and allow them to move freely in the water.
Sea animals like the octopus and squid do not possess a streamlined bodies as they stay deep inside the ocean. They have the ability to make their body streamlined when they move in the water.
The cells of these animals have a large number of mitochondria. The level of calcium is relatively high in cells of the endoskeleton to give additional energy and strength.
Aquatic animals possess gills that help them to absorb the dissolved oxygen from water. Dolphins and whales possess blowholes to breathe in air from the atmosphere directly.
Aquatic plants have much smaller roots which are mostly free-floating. Stems are long and light. Leaves in submerged plants possess ribbon—like leaves which allow the plants to bend themselves in the direction of the flowing water. Plants have aerenchyma tissue that helps in floatation.
Frogs are amphibious in nature, i.e., a frog can live both in water and on land. Frogs have strong hind legs to hop on land and webbed feet to swim in the water.
Frogs also have a protective membrane called the nictitating membrane on their eyes.
6. Harsh environment: Bacterial cells are adapted to anaerobic respiration using mesosomes in a less oxygenated environment.
They can carry out chemosynthesis at high temperatures and high sulphur-containing environments.
The thermophile bacteria can survive very high temperatures even as high as 235°C. Acidophilic bacteria can tolerate high levels of acidity (pH 5.5 or below). The organisms growing in an environment rich in decaying organic compounds usually can tolerate high levels of acidity.