My Science School

Life on Earth cannot go on forever because it depends on the Sun and, like all stars, our Sun will eventually die. However, that will happen billions of years in the future. In the meantime, we need to be concerned about the way in which we are using our planet now, so that it will continue to provide a home for all the living things that share it with us in the next century and beyond.

The biological and geological future of Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the rate of cooling of the plant’s interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun’s luminosity. An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. The current Holocene extinction is being caused by technology and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will result in a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at carbon dioxide concentrations as low as 10 parts per million. However, the long-term trend is for plant fe to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.

In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end and with them the entire carbon cycle. Following this event, in about 2–3 billion years, the planet's magnetic dynamo may cease, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth's surface temperature will cause a runaway greenhouse effect, heating the surface enough to melt it. By that point, all life on the Earth will be extinct. The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet's current orbit.

In many parts of the world, life expectancy — the number of years that a person can expect to live — is increasing. A thousand years ago, 40 might have seemed a good age for an adult to reach. Now we expect to live twice as long. Of course, these are just averages. Since records began there have been exceptional people who lived to 80 and beyond, but for most people, the dangers of dying of disease, accident, war or starvation were very high. Childhood in particular was a dangerous time. A woman might give birth to more than 10 children, none of them living to adult-hood. We must not forget that there are parts of the world where this is still true, and billions of people still die each year from lack of food or medical care.

Demographic research suggests that at the beginning of the 19th century no country in the world had a life expectancy longer than 40 years. Every country is shown in red. Almost everyone in the world lived in extreme poverty, we had very little medical knowledge, and in all countries our ancestors had to prepare for an early death.

Over the next 150 years some parts of the world achieved substantial health improvements. A global divide opened. In 1950 the life expectancy for newborns was already over 60 years in Europe, North America, Oceania, Japan and parts of South America. But elsewhere a newborn could only expect to live around 30 years. The global inequality in health was enormous in 1950: People in Norway had a life expectancy of 72 years, whilst in Mali this was 26 years. Africa as a whole had an average life expectancy of only 36 years, while people in other world regions could expect to live more than twice as long.

The decline of child mortality was important for the increase of life expectancy, but as we explain in our entry on life expectancy increasing life expectancy was certainly not only about falling child mortality – life expectancy increased at all ages.

Such improvement in life expectancy — despite being exclusive to particular countries — was a landmark sign of progress. It was the first time in human history that we achieved sustained improvements in health for entire populations. After millennia of stagnation in terrible health conditions the seal was finally broken.

Now, let’s look at the change since 1950. Many of us have not updated our world view. We still tend to think of the world as divided as it was in 1950. But in health — and many other aspects — the world has made rapid progress. Today most people in the world can expect to live as long as those in the very richest countries in 1950. The United Nations estimate a global average life expectancy of 72.6 years for 2019 – the global average today is higher than in any country back in 1950. According to the UN estimates the country with the best health in 1950 was Norway with a life expectancy of 72.3 years.

The three maps summarize the global history of life expectancy over the last two centuries: Back in 1800 a newborn baby could only expect a short life, no matter where in the world it was born. In 1950 newborns had the chance of a longer life if they were lucky enough to be born in the right place. In recent decades all regions of the world made very substantial progress, and it were those regions that were worst-off in 1950 that achieved the biggest progress since then. The divided world of 1950 has been narrowing.

Globally the life expectancy increased from less than 30 years to over 72 years; after two centuries of progress we can expect to live much more than twice as long as our ancestors. And this progress was not achieved in a few places. In every world region people today can expect to live more than twice as long.

The global inequalities in health that we see today also show that we can do much better. The almost unbelievable progress the entire world has achieved over the last two centuries should be encouragement enough for us to realize what is possible.

Human beings are mammals, which mean that their young develop inside the mother until they are ready to be born. This development takes place inside the womb or uterus, where the baby gains the nutrients and oxygen it needs for growth from its mother’s own blood, supplied through the umbilical cord.

A woman’s ovaries usually release one egg each month. As it travels through the fallopian tube towards the uterus, it may be fertilized by a sperm that has enter her bady during sexual intercourse.

As soon as it is fertilized, the egg call begins to divide, until it becomes a ball of cells called a blastocyst. This ball then implants itself in the wall of the uterus.

After four weeks, the blastocyst has become an embryo. Its brain, spin and limbs are already forming and its heart will soon begin to beat.

At 12 week, the embryo is now called a foetus. All its organs are formed. For the rest of the time before it is born, it simply has to grow.

From 38 weeks onwards, the baby is ready to be born. It moves down into the pelvis. At birth, the cervix gradually opens and the baby is born through the vagina.

The characteristics of individual human beings are passed from one generation to the next in their chromosomes. Each of our parents gives us 23 chromosomes, making 46 in all. That means that we have two versions of each of our genes, but one is often dominant. We see the effect of the dominant gene, but the other (recessive) gene is still there and can be passed to our children.

The Law of Inheritance – Mendel’s Law, is significant in comprehending how characteristics or traits are genetically passed from one generation to the next. Heredity is the process through which a new individual acquires traits from its parents during the event of reproduction.

Every individual has 23 pairs of chromosomes, each of which comes from the father and the mother. As genes are present on chromosomes, we receive two copies of each gene from paternal and maternal side respectively and one pair of sex chromosomes from each parent to form 46 chromosomes on the whole.

Traits acquired through inheritance are determined by rules of heredity. These traits are coded in our DNA and hence can be passed to the offspring (eye color, hair color, height etc.). Thus for each trait, there are two versions in a child. During the cell division process, genetic information (DNA structure) containing chromosomes are transferred into the cell of the new individual, therefore, passing traits to the next generation.

Gestation is the length of time between conception — the fertilization of an egg by a sperm — and the birth of the baby that grows from the fertilized egg. The length of gestation varies according to the species.

Gestation, in mammals, the time between conception and birth, during which the embryo or fetus is developing in the uterus. This definition raises occasional difficulties because in some species (e.g., monkeys and man) the exact time of conception may not be known. In these cases the beginning of gestation is usually dated from some well-defined point in the reproductive cycle (e.g., the beginning of the previous menstrual period).

The length of gestation varies from species to species. The shortest known gestation is that of the Virginian opossum, about 12 days, and the longest that of the Indian elephant, about 22 months. In the course of evolution the duration of gestation has become adapted to the needs of the species. The degree of ultimate growth is a factor, smaller animals usually having shorter periods of gestation than larger ones. Exceptions are the guinea pig and related South American rodents, in which gestation is prolonged (averaging 68 days for the guinea pig and 111 days for the chinchilla). The young of these species are born in a state of greater maturity than are those of the rat with its period of 22 days. Another factor is that, in many species with restricted breeding seasons, gestation is adjusted so that birth coincides with the period when food is most abundant. Thus the horse, a spring breeder with 11 months’ gestation, has its young the following spring, as does the sheep, a fall breeder with a five months’ gestation. Animals that live in the open tend to have longer gestations and to bear young that have reached a state of greater maturity than do animals that can conceal their young in underground burrows or in caves. Marsupials generally have short gestations—e.g., 40 days for the largest kangaroos. The young, born in an extremely immature state, transfer to the pouch in which gestation may be said to continue.

Embryos of some species experience an arrest in development that greatly prolongs gestation. This is especially true of the fur-bearing carnivores the martens and weasels. Embryos of the European badger and American marten, which breed in July and August, develop for a few days, and then lie dormant in the uterus, being implanted in January. Birth occurs in March. Of the total gestation period of 250 days, growth occurs during only 50. Delayed implantation also occurs in mice and other small rodents that become pregnant while they are still suckling a litter.

Either a single factor or a great number of minor factors, all culminating at or near one date, determine the length of gestation. Several minor variations are known: in man, gestation for males is three to four days longer than that for females; and in cattle, bulls are carried about one day longer than heifers. In both species gestation of twins is five to six days less than for singlet’s. In animals such as the rabbit or pig, which bear many young at a time, gestation is shorter for larger litters than for smaller ones. Heredity also influences gestation; in cattle the mean gestation period for Holstein-Friesians is 279 days; for Brown Swiss, 290 days; other breeds fall between these extremes. When hybrids are produced by crossing two species with different gestation periods, the hybrid is carried for a period lying somewhere between those of the two parents and tending toward the mother’s species. Thus a mare carries a mule foal (fathered by a jackass) about 10 days longer than the normal period for the horse (about 337 days). For human gestation, see pregnancy.

Human beings are far from being the longest-living animals. The giant tortoise can reach 150 years, while several aquatic creatures, such as the killer whale and some species of sea anemone, can survive for well over 80 years. At the other end of the scale, the adult mayfly lives for less than two days. The plant kingdom has far longer-living species. Several trees, such as the yew and giant sequoia, live for thousands of years.

There are tortoises alive today that were 25 to 50 years old when Charles Darwin was born. There are whales swimming the oceans with 200-year-old ivory spear points embedded in their flesh. There are cold-water sponges that were filter-feeding during the days of the Roman Empire. In fact, there are a number of creatures with life spans that make the oldest living human seem like a spring chicken in comparison.

Greenland shark: This shark lives in Arctic waters and slowly grows to an average length of 16 feet. It scavenges for its food and is attracted to the smell of rotting meat in the ocean. It's also known to primarily live in deeper ocean depths compared to other sharks. A group of scientists conducted radiocarbon testing on the eye lens of 28 female sharks and determined its life span to reach at least 272 years. They concluded that the Greenland shark is the longest-living vertebrae known to man.

Geoducks: These large saltwater clams that are native to the Puget Sound and have been known to live for at least 160 years. They are characterized by their long 'necks', or siphons, which can grow to more than 1 meter long.

Tuatara: The word "dinosaur" is commonly used to describe an old person, but when it refers to tuataras, the term is perfectly metaphorical. The two species of tuatara alive today are the only surviving members of an order that flourished about 200 million years ago — they are living fossils. They are also among the longest-lived vertebrates on Earth, with some individuals living for between 100 and 200 years.

Lamellibrachia tube worms: These colorful deep sea creatures are tube worms (L. luymesi) that live along hydrocarbon vents on the ocean floor. They have been known to live 170 years, but many scientists believe there may be some that have lived for more than 250 years.

Red sea urchins: The red sea urchin or Strongylocentrotus franciscanus is found only in the Pacific Ocean, primarily along the West Coast of North America. It lives in shallow, sometimes rocky, waters from the low-tide line down to 90 meters, but they stay out of extremely wavy areas. They crawl along the ocean floor, using their spines as stilts. If you discover one, remember to respect your elders — some specimens are more than 200 years old.

Bowhead whales: Also known as the Arctic whale, the bowhead is by far the longest living mammal on Earth. Some bowhead whales have been found with the tips of ivory spears still lodged in their flesh from failed attempts by whalers 200 years ago. The oldest known bowhead whale was at least 211 years old.

Koi: Koi are an ornamental, domesticated variety of the common carp. They are common in artificial rock pools and decorative ponds. Amazingly, some varieties are capable of living more than 200 years. The oldest known koi was Hanako, a fish that died at the age of 226 on July 7, 1977.

Tortoises: Tortoises are considered the longest living vertebrates on Earth. One of their oldest known representatives was Harriet, a Galápagos tortoise that died of heart failure at the age of 175 years in June 2006 at a zoo owned by the late Steve Irwin. Harriet was considered the last living representative of Darwin's epic voyage on the HMS Beagle. An Aldabra giant tortoise named Adwaita died at the rumored age of 250 in March 2006.

Two things affect the way in which living things grow and age. The first is their genetic make-up — the genes that they have inherited from their parents. The DNA in their chromosomes controls the way that cells divide to cause the growth of the young organism, its coming to maturity and its aging. The other important factor is the environment and conditions that the organism experiences — how much of the right kind of food it eats, where it lives, the climate and the kinds of events and accidents that happen to it.

Every living organism begins life as a single cell. Unicellular organisms may stay as one cell but they grow too. Multicellular organisms add more and more cells to form more tissues and organs as they grow.

The Growth and development of living organisms are not the same things. Growth is the increase in size and mass of that organism. Development involves the transformation of the organism as it goes through the growth process.

Think of a newly born baby. It has all the features of a fully-grown adult, but they are very tiny. As the years go by, they become big and become a young person like you, and later on, into a fully grown adult, maintaining all the features that they are born with. This is growth. But in their mummy’s tummy, they started off as a single cell and transformed into a zygote and into a foetus before transforming into a tiny baby.

In some organisms, growing involves drastic transformation. Think of a butterfly for instance. It starts off as a cell (egg). Then it transforms into a caterpillar, then into a pupa (chrysalis), and then pops out as a beautiful butterfly.

Plants often start from a tiny seed, and grow into a big tree. One thing common to all organisms is that they grow or develop to look just like their parent species, even though there may be some slight variations resulting from the mixing of cells by the parents. 

Cell growth and development include its repair. As cells grow old, they wear off. Sometimes they suffer injury and bruises, but they are able to repair themselves by growing new cells in a process called Mitosis.

As living things grow, they undergo a process called aging (age). As they get close to the end of their lifespan, their ability to carry out life functions reduces. Eventually, they die to end the process of life.

DNA is an abbreviation of the name of a chemical: deoxyribonucleic acid. It is DNA that contains the instructions for making and controlling every living thing. Inside the nucleus of a cell, the DNA forms chromosomes. Living things have different numbers of chromosomes. Human beings have 46, arranged in 23 pairs. Each of us has inherited one half of each chromosome pair from our father and the other half from our mother. A gene is a small part of the DNA molecule that can make one of the proteins that the living organism needs.

Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). Mitochondrial are structures within cells that convert the energy from food into a form that cells can use.

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

Living things are said to be animate. Inanimate things are not living. Metal, plastic and glass, for example, are inanimate. All animate things are able to do six things that inanimate things cannot.

Although seemingly diverse, living things, or organisms, share certain essential characteristics. The most recent classification system agreed upon by the scientific community places all living things into six kingdoms of life, ranging from the simplest bacteria to modern-day human beings. With recent innovations such as the electron microscope, scientists peered inside cells and began to understand the intracellular processes that defined life.

Composition

Cells compose all life, performing the functions necessary for an organism to survive in its environment; even the most primitive of life forms, bacteria, consists of a single cell. While peering through a microscope at slices of cork tissue in the late 17th century, scientist Robert Hooke discovered numerous tiny compartments which he coined “cells.” After several developments regarding cell structure and function, Robert Virchow compiled a book, “Cellular Pathology,” describing the nature of cells in relation to life. He formed three conclusions: cells form the basis of all life, cells beget other cells and cells can exist independent of other cells.

Energy Use

All processes occurring within organisms, whether single-celled or multicellular, expend energy. The method of procuring that energy, however, differs between organisms. Organisms called autotrophs make their own energy while heterotrophs must feed to obtain their energy needs. Autotrophs such as plants and some bacteria produce their own food by converting carbon dioxide and water into sugar with the aid of the sun’s energy via photosynthesis. Other autotrophic bacteria use chemicals such as sulfur to make energy in a process called chemosynthesis. The energy organisms need comes in the form of a molecule called ATP, or adenosine triphosphate. Living things make ATP by breaking down glucose.

Response

Organisms use their senses to obtain information from and have the capability of reacting to stimuli in their environments. Even unicellular organisms such as bacteria and seemingly immobile plants can respond to stimuli. Plants such as sunflowers can sense heat and light, so they turn toward the sun’s rays. Predators such as cats can track their prey with keen senses of vision, smell and hearing and then hunt them down with superior agility, speed and strength.

Growth

Living things grow and change through the process of cell division, or mitosis. In organisms composed of more than one cell, mitosis either repairs damaged cells or replace older ones that have died. Additionally, multicellular organisms grow larger in size by increasing the number of cells in their bodies. Unicellular organisms take in nutrients and enlarge. They grow to a certain point and then must divide into two new daughter cells. The process of mitosis takes place in four phases. Certain signals trigger cells to divide. The cell replicates its genetic information, resulting in two exact copies of the gene-bearing structures called chromosomes. Cellular structures separate the chromosome copies, moving them to different sides of the cell. The cell then pinches itself down the middle, creating a new barrier to separate the two new cells.

Reproduction

For a species or organism to continue existing, members of the species must reproduce, either asexually or sexually. Asexual reproduction produces offspring that exactly resemble the parent organism. Certain members in each of the kingdoms of life can reproduce asexually. Bacteria from Kingdoms Archaebacteria and Eubacteria, amoeba of the Kingdom Protista and yeast of Kingdom Fungi use binary fission to simply divide in two, resulting in two identical daughter cells. Worms called planaria can break off a segment that grows into a new organism. Plants such as potatoes form buds which, when cut off and planted, will produce a new potato plant. Sexual reproduction, which allows a mixing of genes from two individuals of a species, evolved from asexual reproduction because the benefits of sex outweigh its costs.

Adaptation

Since the beginning of life, organisms have adapted and evolved to survive according to their environments. Those individuals unable to adapt to changing conditions will die or be unable to pass on much of their genes to the next generation. Many times in the history of the earth, entire species, including many dinosaur groups, have died out when they failed to respond appropriately to environmental changes such as droughts or cooling climates. The environment selects for those individuals best acclimated to live under specific conditions; these creatures have the best selections of mates and will contribute to a greater percentage of descendants.

All cells have a cell wall, hut in plant cells this is made of a stiff, tough layer of cellulose. Cellulose is made of tiny fibres, layered together to form a strong sheet. Most plant cells also contain organelles called chloroplasts. It is in these that photo-synthesis takes place.

Animal cells and plant cells are similar in that they are both eukaryotic cells. These cells have a true nucleus, which houses DNA and is separated from other cellular structures by a nuclear membrane. Both of these cell types have similar processes for reproduction, which include mitosis and meiosis. Animal and plant cells obtain the energy they need to grow and maintain normal cellular function through the process of cellular respiration. Both of these cell types also contain cell structures known as organelles, which are specialized to perform functions necessary for normal cellular operation. Animal and plant cells have some of the same cell components in common including a nucleus, Golgi complex, endoplasmicreticulum, ribosomes, mitochondria, peroxisomes, cytoskeleton, and cell (plasma) membrane. While animal and plant cells have many common characteristics, they are also different.

Size

Animal cells are generally smaller than plant cells. Animal cells range from 10 to 30 micrometers in length, while plant cells range from 10 and 100 micrometers in length.

Shape

Animal cells come in various sizes and tend to have round or irregular shapes. Plant cells are more similar in size and are typically rectangular or cube shaped.

Energy Storage

Animal cells store energy in the form of the complex carbohydrate glycogen. Plant cells store energy as starch.

Proteins

Of the 20 amino acids needed to produce proteins, only 10 can be produced naturally in animal cells. The other so-called essential amino acids must be acquired through diet. Plants are capable of synthesizing all 20 amino acids.

Differentiation

In animal cells, only stem cells are capable of converting to other cell types. Most plant cell types are capable of differentiation.

Growth

Animal cells increase in size by increasing in cell numbers. Plant cells mainly increase cell size by becoming larger. They grow by absorbing more water into the central vacuole.

Cell Wall

Animal cells do not have a cell wall but have a cell membrane. Plant cells have a cell wall composed of cellulose as well as a cell membrane.

Centrioles

Animal cells contain these cylindrical structures that organize the assembly of microtubules during cell division. Plant cells do not typically contain centrioles.

Cilia

Cilia are found in animal cells but not usually in plant cells. Cilia are microtubules that aid in cellular locomotion.

Cytokinesis

Cytokinesis, the division of the cytoplasm during cell division, occurs in animal cells when a cleavage furrow forms that pinches the cell membrane in half. In plant cell cytokinesis, a cell plate is constructed that divides the cell.

Glyoxysomes

These structures are not found in animal cells but are present in plant cells. Glyoxysomes help to degrade lipids, particularly in germinating seeds, for the production of sugar.

Lysosomes

Animal cells possess lysosomes which contain enzymes that digest cellular macromolecules. Plant cells rarely contain lysosomes as the plant vacuole handles molecule degradation.

Plastids

Animal cells do not have plastids. Plant cells contain plastids such as chloroplasts, which are needed for photosynthesis.

Plasmodesmata

Animal cells do not have plasmodesmata. Plant cells have plasmodesmata, which are pores between plant cell walls that allow molecules and communication signals to pass between individual plant cells.

Vacuole

Animal cells may have many small vacuoles. Plant cells have a large central vacuole that can occupy up to 90% of the cell's volume.

Prokaryotic Cells

Animal and plant eukaryotic cells are also different from prokaryotic cells like bacteria. Prokaryotes are usually single-celled organisms, while animal and plant cells are generally multicellular. Eukaryotic cells are more complex and larger than prokaryotic cells. Animal and plant cells contain many organelles not found in prokaryotic cells. Prokaryotes have no true nucleus as the DNA is not contained within a membrane, but is coiled up in a region of the cytoplasm called the nucleoid. While animal and plant cells reproduce by mitosis or meiosis, prokaryotes propagate most commonly by binary fission.

Other Eukaryotic Organisms

Plant and animal cells are not the only types of eukaryotic cells. Protists and fungi are two other types of eukaryotic organisms. Examples of protists include algae, euglena, and amoebas. Examples of fungi include mushrooms, yeasts, and molds.

Mitochondria are organelles that break up food materials to make energy. Other important organelles are ribosomes, which make proteins, and endoplasmic reticulum. This is a structure, made of double membranes, that is linked to the nucleus and to the cell wall, so that chemicals can be carried around the cell. The cell wall itself is said to be semi-permeable. That means that some chemicals can pass through it into the cell but none can pass out.

Mitochondria are specialized structures unique to the cells of animals, plants and fungi. They serve as batteries, powering various functions of the cell and the organism as a whole. Though mitochondria are an integral part of the cell, evidence shows that they evolved from primitive bacteria. 

All living organisms are built with one fundamental brick: the cell. In some cases, a single cell constitutes an entire organism. Cells contain genetic material (DNA and RNA), and they carry out essential functions, such as metabolism and protein synthesis. Cells are also capable of self-replicating. However, the level of organization varies within the cells of different organisms. Based on these differences, organisms are divided into two groups: eukaryotes and prokaryotes. 

Plants, animals and fungi are all eukaryotes and have highly ordered cells. Their genetic material is packaged into a central nucleus. They also have specialized cellular components called organelles, each of which executes a specific task. Organelles such as the mitochondria, the rough endoplasmic reticulum and the Golgi serve respectively to generate energy, synthesize proteins and package proteins for transport to different parts of the cell and beyond. The nucleus, as well as most eukaryotic organelles, is bound by membranes that regulate the entry and exit of proteins, enzymes and other cellular material to and from the organelle.

Prokaryotes, on the other hand, are single-celled organisms such as bacteria and archaea. Prokaryotic cells are less structured than eukaryotic cells. They have no nucleus; instead their genetic material is free-floating within the cell. They also lack the many membrane-bound organelles found in eukaryotic cells. Thus, prokaryotes have no mitochondria.

Cells certainly are the building blocks of life, but they are very busy building blocks! Inside each cell thousands of chemical reactions are going on, so that the cell can carry out its tasks. A typical cell has a cell wall or membrane surrounding a kind of watery jelly called cytoplasm. Within the cell there are a number of parts called organelles. These do all the work that the cell is designed to do. The nucleus is a particularly important organelle. It controls all the activities of the cell.

A cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells.

All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular).  Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy. Cells are complex, and their components perform various functions in an organism. They are of different shapes and sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes.

Cells are the lowest level of organisation in every life form. From organism to organism, the count of cells may vary. Humans have the number of cells compared to that of bacteria. Cells comprise several cell organelles that perform specialised functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells.

Trees in a forest, fish in a river, horseflies on a farm, lemurs in the jungle, reeds in a pond, worms in the soil — all these plants and animals are made of the building blocks we call cells. Like these examples, many living things consist of vast numbers of cells working in concert with one another. Other forms of life, however, are made of only a single cell, such as the many species of bacteria and protozoa. Cells, whether living on their own or as part of a multicellular organism, are usually too small to be seen without a light microscope.

Cells share many common features, yet they can look wildly different. In fact, cells have adapted over billions of years to a wide array of environments and functional roles. Nerve cells, for example, have long, thin extensions that can reach for meters and serve to transmit signals rapidly. Closely fitting, brick-shaped plant cells have a rigid outer layer that helps provide the structural support that trees and other plants require. Long, tapered muscle cells have an intrinsic stretchiness that allows them to change length within contracting and relaxing biceps.

Everything in the universe is mare of atoms, arranged in different ways. But living things, unlike rocks or metal, have larger building blocks called cells. Some living things have only one cell, while others contain millions. Each cell has a job to do, but they all work together to make a living organism.

Living organisms are made up of cells. Cells are the structural and functional units of a living organism. In 1665, Robert Hooke discovered the existence of cells using a microscope, which further paved way for the discovery of various other microscopic organisms. Some organisms consist of a single cell, for example, the amoeba. Other organisms are multicellular, having millions of cells.

A single cell is able to produce many cells through a process known as cell division. Different organisms have different kinds of cells. A human body alone shows various kinds of cells such as – blood cells, nerve cell, fat cell etc. Shapes and sizes of cells depend upon the functions they perform. Amoeba has an ever-changing shape as it changes form to locomote. Some cells have a fixed shape and perform a specific function, such as nerve cells, which are usually shaped like trees.

An organism is any being that consists of a single cell or a group of cells, and exhibit properties of life. They have to eat, grow and reproduce to ensure the continuation of their species. Organ systems collectively work together for the proper functioning of a living organism, failure of even one of these systems has an impact on our lives.