Biology, Ecology & Bio-tech

Question 0

Explain Carbohydrates

Answer 0

1. Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen atoms.
2. a) In food science and informal contexts, the term carbohydrate often just means any sugary food like chocolate or a starchy food such as bread or pasta.
3. The most basic carbohydrates are simple sugars or monosaccharides such as fructose (C6H12O6), which makes fruit sweet, or ribose (C5H10O5), which forms the backbone of the genetic molecule RNA.
4. a) Monosaccharides, especially glucose, are also the major source of fuel for metabolism. Glucose has the same chemical formula as fructose but a different, typically ring-shaped, molecular structure.
5. b) The larger disaccharide sucrose (C12H22O11)—common table sugar—is formed from fructose and glucose.
6. The most complicated carbohydrates are “polysaccharides,” including starch, which is made up of thousands of glucose units.
7. Plants store their glucose fuel as starch. Many animals, including humans, store glucose as glycogen, a molecule containing a core protein surrounded by many branching glucose units.

Question 1

Explain Protein

Answer 1

1. Proteins are large, complex molecules that play many critical roles in cells.
2. They consist of long chains of hundreds or thousands of smaller simple molecules called amino acids. Some of these are “essential amino acids,” such as phenylalanine, which our bodies don’t synthesize, so it’s essential that we eat plenty of these in our diet.
3. Some proteins act as antibodies, which can prevent disease by targeting foreign particles such as viruses and blocking the sites they use to invade cells.
4. a) Others include receptors and enzymes, which enable thousands of chemical reactions to take place in cells and assist the construction of new proteins by reading the genetic information in DNA.
5. In plants and animals, all proteins are made from different sequences of twenty main amino acids.
6. The exact sequence for a given protein is called its primary structure.
7. When a cell makes a new protein, it forms a linear chain of amino acids that coils into a secondary structure before morphing into a final three-dimensional shape through a process called protein folding.

Question 2

Explain Lipids

Answer 2

1. Lipids are a broad family of molecules including fats, waxes, and some vitamins (including vitamins A, D, E, and K) that are hydrophobic—they repel water and are only soluble in organic solvents such as acetone.
2. Lipids have a wide range of biological functions, including storing energy, maintaining cell membranes and acting as hormones that are able to coordinate complicated processes like fertility
3. Common lipid types include fats, steroids, and phospholipids.

Fats store energy and cushion organs, protecting them from damage, and are composed of fatty acids and glycerol, a sweet-tasting alcohol.

Steroids contain four ring-shaped hydrocarbon molecules and include the dietary fat cholesterol as well as the sex hormones estradiol and testosterone.

4. Phospholipids usually contain two fatty acids and a phosphate group.

In water, they arrange themselves into a two-layered sheet with all their hydrophobic tails lined up in the middle. This layered structure forms cell membranes, which regulate the flow of ions and molecules in and out of a cell.

Question 3

Explain Metabolism

Answer 3

1. Metabolism describes the host of chemical reactions required to keep living organisms alive, generating energy for essential growth and reproduction—for instance, healing injuries and eliminating toxins.
2. Apart from water, most molecules in living organisms are amino acids, the building blocks of proteins, as well as carbohydrates and lipids. Metabolic reactions involving them divide into two categories.

“Anabolism” builds molecules like proteins during construction of new cells and tissues,

while “catabolism” breaks down molecules from food for use as a source of energy.

3. Enzymes also play a vital role in metabolism by acting as catalysts that efficiently transform one chemical into another, helping amino acids join to form proteins, for instance, or breaking down dietary starch into its component sugars.
4. Healthy metabolism in humans depends on good nutrition, plentiful water, and exercise. A lack of any one of these decreases your metabolic rate and can lead to weight gain.

Question 4

Explain Chemosynthesis

Answer 4

1. Chemosynthesis is the process by which some exotic microbes living in hot deep-sea vents derive their energy. It’s similar to photosynthesis, but doesn’t use sunlight.

Instead, the energy comes from the oxidation of inorganic chemicals such as hydrogen sulfide bubbling up from the Earth’s crust.

2. In hydrothermal vents, geothermal heat coming up from fissures on the ocean floor can heat water to more than 212°F (100°C).

Amazingly, some bacteria called extremophiles thrive in these vents at temperatures up to about 250°F (120°C).

There’s no sunlight available, so the bacteria produce their energy by turning available chemicals into sugar. For instance, some bacteria oxidize hydrogen sulfide and use the energy stored in its chemical bonds to make glucose from water and carbon dioxide dissolved in seawater.

Scientists speculate that these bacteria would have been well adapted to the hot conditions on the early Earth, making them a good candidate for one of the earliest types of life.

Question 5

Explain Receptors

Answer 5

1. In biochemistry, a receptor is a protein molecule in the membrane or cytoplasm of a cell onto which signaling molecules such as hormones attach to deliver chemical instructions.

For example, the hormone insulin regulates blood sugar by latching onto a receptor in muscle or liver cells, triggering reactions that speed up sugar absorption.

Signaling molecules effectively target specific receptors because they have the right size, shape, and electric charge distribution to grab hold of them, a bit like a key fitting into a lock.

The binding “unlocks” the cell to cause chemical changes. Many drugs mimic signaling molecules to promote their effect.

For instance, morphine mimics endorphins, naturally occurring feel-good chemicals in the body that relieve pain.

Other drugs lock onto receptors purely in order to block available binding sites, inhibiting the effects of natural signaling molecules.

Examples include antihistamines, which alleviate allergies by inhibiting chemicals called histamines that cause rashes, sneezing, and itching.

Question 6

Explain DNA

Answer 6

1. Deoxyribonucleic acid, or DNA, is the molecule that encodes the genetic instructions for the development and function of all living, self-replicating organisms.
2. Nearly every cell in a person’s body has the same DNA, which is mostly inside the cell nucleus, although some of it resides in mitochondria.

3. The information in DNA is stored as a sequence of four chemical “bases” called 
adenine (A), 
guanine (G), 
cytosine (C), and 
thymine (T). 

3.a) The bases team up (A with T; C with G) to form units called base pairs.

Human DNA consists of about 3.2 billion base pairs. Each base is attached to a sugar molecule called deoxyribose and a phosphate molecule to form a nucleotide.

4. The nucleotides are arranged in two long strands that form a double helix shaped like a spiraling ladder, with the base pairs forming the vertical supports.
5. DNA replicates itself by splitting into single strands that serve as templates for duplicating the sequence of bases.

Question 7

Explain RNA

Answer 7

1. Ribonucleic acid or RNA is a molecule that has some similarities to DNA but does not store genetic information (except in RNA viruses). Instead, RNA performs many different functions in a cell, acting as a temporary copy of genetic information, for instance.
2. Like DNA, RNA molecules have a sequence of four chemical “bases,” in this case uracil (U), adenine (A), cytosine (C), and guanine (G).
3. a) Each base is also attached to a sugar molecule (ribose) and a phosphate molecule to form a nucleotide. The bases sometimes team up with each other (U with A; C with G) to form a double helix structure like DNA,

**but RNA usually occurs as a single strand.

3. “Messenger” RNA (mRNA) is a short-lived molecule that copies a cell’s DNA and carries it to the cell’s protein synthesis machinery, the ribosome, which reads off its information to make the right protein.
4. “Transfer” RNA (tRNA) molecules latch onto individual amino acids and frogmarch them to the ribosome, where they are incorporated into proteins.

Question 8

Explain Genes

Answer 8

1. Genes are segments of DNA that act as a blueprint for the production of specific proteins.
2. b) They exist in alternative forms called alleles, which determine the distinct traits that parents pass on to their offspring according to the laws of inheritance.
3. The entirety of an organism’s genetic information is called the genome.
4. The thirteen-year Human Genome Project, completed in 2003, was a mammoth international effort to identify the sequences of the 3.2 billion base pairs in human DNA and identify its approximately twenty-five thousand genes.
5. a) This showed that the average gene consists of about three thousand base pairs, but sizes vary enormously, with the largest gene having 2.4 million base pairs.
6. b) The Human Genome Project has clarified the role that certain gene sequences play in diseases, including breast cancer and muscular dystrophy.

However, it also revealed that only about 2 percent of the genome actually encodes instructions for protein synthesis, and the role of some of the remaining DNA remains a mystery.

Question 9

Explain laws of inheritance

Answer 9

1. The laws of inheritance are basic rules about how animals and plants pass on traits to their offspring.
2. Gregor Mendel, a nineteenth-century Austrian monk, discovered the laws in pea-breeding experiments, in which he cross-fertilized pea plants and studied traits like flower color and the length of plant stems in subsequent generations.
3. a) Mendel’s experiments revealed that two factors (now known to be genes) determined the traits, one from each parent, and if the two inherited factors are different, the offspring expresses just one of them, the so-called “dominant” trait.
4. b) He also noticed that different traits like flower color and stem length are inherited independently.
5. Today we know that all the human blood groups (A, AB, B, or O) are determined by a single gene. A and B are dominant while O is “recessive,” so a child who inherits A + O or B + O from its parents will have blood types A and B respectively. A and B are said to be “co-dominant,” so that inheriting both A and B gives blood type AB.

Question 10

Explain Prokaryotes

Answer 10

1. Prokaryotes are simple single-celled organisms that don’t have a cell nucleus that houses DNA.
2. a) Instead, their DNA forms a free-floating bundle in the middle of the cell. Like eukaryotic cells, prokaryotes have ribosomes where amino acids are assembled into proteins, and sometimes a flagellum—a rudder-like tail that propels them.
3. The fossil record reveals that prokaryotes evolved on Earth very early, at least 3.5 billion years ago. They reproduce asexually and are typically about 1–10 micrometers (millionths of a meter) wide.
4. They fall into two subcategories: bacteria and archaea.
5. a) Bacteria were discovered in the late 1600s and are ubiquitous in every habitat on Earth. They have a wide range of shapes, including spheres, spirals, and rods.
6. b) Archaea were first classified as a separate group in the late 1970s. Most look similar to bacteria but they have a completely different genetic and biochemical make-up, and often inhabit extreme habitats such as scorching hydrothermal vents on the ocean floor.

Question 11

Explain Eukaryotes

Answer 11

1. Along with prokaryotes, eukaryotes are one of two major cell types—they make up everything from single-celled amoebas to complex animals and plants.
2. A typical size for a eukaryotic cell is roughly 0.01 mm across—about ten or fifteen times wider than a typical prokaryote.
3. The “plasma membrane” forms the outer barrier of a eukaryotic cell, while the cell nucleus houses DNA packaged into chromosomes that vary widely between organisms.
4. Humans have twenty-three pairs of large, linear chromosomes.
   The nucleus is surrounded by a water-rich fluid called cytosol and various organelles that perform different tasks.
5. Mitochondria generate energy, while the endoplasmic reticulum is a network of interconnected membranes dotted with ribosomes, which assemble proteins.
6. Fossil evidence suggests eukaryotes evolved at least 1.7 billion years ago. One possibility is that they arose because some prokaryotic cells engulfed others, which were not digested but lived on as organelles and reproduced.

Question 12

Explain Mitochondria

Answer 12

1. Mitochondria act as the power plants of eukaryotic cells, converting the energy from food into a form that cells can use. A cell may have hundreds or even thousands of mitochondria depending on its energy demands.
2. Mitochondria are like little factories that specialize in using energy from reactions between oxygen and simple sugars to produce molecules of adenosine triphosphate (ATP), the cell’s main energy source.
3. a) ATP is a bit like a charged battery—removal of a phosphate group releases energy to drive complex reactions, leaving “uncharged” adenosine diphosphate (ADP), which returns to mitochondria to be recharged into ATP.
4. Surrounded by two membranes, mitochondria have their own genetic material and reproduce independently of their host cell.
5. Scientists suspect that the ancient ancestors of mitochondria were probably free-living bacteria that somehow became engulfed in other cells. The bacteria thrived in the protective environment of their new host cells, while the hosts came to rely on the bacteria for energy production.

Question 13

Explain Ribosomes

Answer 13

1. In all cells, including those of plants, animals, and bacteria, ribosomes are the mini factories that assemble proteins.
2. Each ribosome is built from RNA molecules and proteins. Some of them are free to roam in the watery cytoplasm of a cell, while others are bound to the endoplasmic reticulum, a complicated network of interconnected membranes inside eukaryotic cells.
3. A strand of messenger RNA (mRNA) brings a copy of genetic information from a cell’s DNA to a ribosome. Meanwhile, transfer RNA (tRNA) molecules latch onto single amino acids and deliver them to the ribosome, to be incorporated into proteins according to the mRNA’s instructions.
4. Cells typically have several thousand ribosomes, but the number can reach several million.
5. The chemical structure of ribosomes is different in bacteria and animal cells, and the differences allow many antibiotic drugs to selectively disrupt the ribosomes of disease-causing bacteria, sabotaging their protein production without making people or animals sick.

Question 14

Explain cell division

Answer 14

1. Cell division is the process by which biological cells multiply.
2. Eukaryotic cells, including human cells, create identical copies of themselves for growth or repair of tissues through a process called mitosis.
3. The double-stranded DNA in the cell nucleus unzips into two strands that each join with nucleotides to form two copies of the original DNA.

In the next step, cytokinesis, the cell splits into two identical copies of the original cell.

4. Meiosis is a type of cell division that creates eggs and sperm for sexual reproduction. The eggs and sperm cells have half the normal number of chromosomes. When sperm fertilizes an egg, they fuse to regain the normal chromosome quota, half from the male and half from the female.
5. Prokaryotic cells such as bacteria usually divide in a process called binary fission.

The single DNA bundle in a prokaryotic cell replicates, then the two copies stick to different parts of the cell membrane, going their separate ways when the cell splits in two.

Question 15

Explain Gametes

Answer 15

1. Gametes are the germ cells of animals and plants that reproduce sexually.
2. In most animals, including humans, the male gametes are called sperm cells and the female ones are called eggs.
3. Gametes form in a cell division process called meiosis, in which a cell splits into four copies instead of the usual two, so that the gametes contain half the normal number of chromosomes—just one of each of the twenty-three pairs normally found in human cells.
4. “Chromosomal crossover” occurs during meiosis, each chromosome pair exchanging genetic material so that the gametes end up with new combinations of genes.
5. During fertilization, an egg and sperm fuse to create a zygote with two copies of each chromosome, one from the male and one from the female.

This single cell develops into an embryo by cell division. In most mammals, the X and Y sex chromosomes determine gender. Inheriting an X chromosome from both parents gives female offspring (XX), while inheriting X from the mother and Y from the father gives males (XY).

Question 16

Explain biological classification

Answer 16

1. The basic classification system for animals, plants, and microbes was first introduced in the early 1700s by the Swedish botanist and zoologist Carl Linnaeus, who grouped species according to shared physical characteristics.

The groupings have been revised since then to take account of new information about evolutionary trees.

2. Today many scientists divide all life into three domains:
   Archaea and
   Bacteria and
   Eukaryota, which includes complex animals and plants. The domains are subdivided into kingdoms, usually six: Animalia, Plantae, Fungi, Protista, Archaea, Bacteria.

Subcategories of the kingdoms multiply into phyla, classes, orders, families, and genera with the final, most specific category being species.

Typically, a species is a group of organisms that are biologically similar enough to each other that they can interbreed to create fertile offspring.

The number of species of life on Earth is impossible to measure—scientists guess that it lies anywhere between about five million and one hundred million.

Question 17

Explain Animals

Answer 17

1. Animals are multicellular, eukaryotic organisms from the kingdom Animalia, and they form a vast group of more than 1.5 million known species including worms, insects, sponges, and people.
2. All animals are heterotrophs—unable to generate some vital organic chemicals internally, they have to eat other organisms to stay alive.

As they develop, their body plans become fixed, although some animals undergo metamorphosis—for instance, when a caterpillar pupates and transforms into a butterfly.

Some animals reproduce asexually, including aphids, which sometimes reproduce independently by effectively cloning themselves. But the vast majority of animals reproduce by sexual reproduction, in which a male and female combine their genetic material to create their offspring.

Animals usually sexually reproduce when an egg and sperm fuse during fertilization to create a zygote with two copies of each chromosome, one from the female and one from the male. This zygote develops into an embryo by cell division.

Question 18

Explain Plants

Answer 18

1. Plants are multicellular eukaryotic organisms that use sunlight to generate energy and organic chemicals through photosynthesis.
2. They include familiar organisms like grasses, bushes, and trees as well as green algae, which are largely aquatic and exist as single cells, colonies, and seaweeds.

There are roughly 350,000 known species of plants.

3. Plants typically have a main stem growing up from a root system in the soil, with branches emerging from “nodes” on the stem.
4. Sexual reproduction in plants often involves male pollen grains fertilizing the ovules in a flower. Protective seed coats form around the fertilized ovules before they disperse, allowing a new generation of plants to germinate.
5. Asexual reproduction doesn’t involve flowers—plants can create genetically identical copies of themselves when bulbs divide, for instance.
6. The first land plants evolved more than 450 million years ago, while forests spread on land around 385 million years ago. Flowering plants evolved roughly 140 million years ago, and since then have become the dominant land plants.

Question 19

Explain Photosynthesis

Answer 19

1. Photosynthesis is the process in which plants, as well as some bacteria and eukaryotic microorganisms, use the energy from sunlight to produce sugars such as glucose.
2. Plant leaves are like solar energy collectors crammed full of photosynthetic cells. These cells combine water and carbon dioxide molecules to create sugars and oxygen.
3. Water enters a land plant through its roots and is then transported up to the leaves. Atmospheric carbon dioxide enters the leaves through tiny pores called stomata, which open and close depending on environmental conditions.

Likewise, oxygen produced during photosynthesis passes back out into the atmosphere through the stomata.

During respiration, plants combine sugar with oxygen to produce carbon dioxide and water, in the process creating adenosine triphosphate, or ATP, the molecule that supplies energy for essential work like protein building.

Plant respiration dominates at night, when the photosynthetic uptake of carbon dioxide and release of oxygen stops.

Question 20

Explain Prokaryotic microbes

Answer 20

1. Microbes made of prokaryotic cells fall into two classes: bacteria and archaea, which are both single-celled.
2. a) These microbes are the most diverse and abundant group of organisms on Earth, accounting for at least half of all biomass, despite their tiny size (they are typically about one-thousandth of a millimeter wide).
3. Bacterial cells come in a wide variety of shapes. Spherical ones are called cocci, while elongated rod-shaped ones are called bacilli.

Sometimes, bacteria come in pairs, where their name is prefixed with “diplo-.” Those that form long chains are prefixed “strepto-,” while some bacteria form triangular groups prefixed “staphylo-.”

Rod-shaped bacteria can divide to form a picket-fence structure called a palisade arrangement.

3. Many bacteria cause diseases. Streptococcus species can cause pneumonia and meningitis, for instance.
4. Archaea look similar to bacteria, but have a completely different biochemical make-up and do not have a known role in disease.

Archaea were possibly the earliest life forms on Earth.

Question 21

Explain Eukaryotic microbes

Answer 21

1. Eukaryotic microbes are a diverse range of organisms so small that they are invisible to the naked eye.
2. They divide into four groups: animals, plants, fungi, and protists.
3. Unlike the simpler prokaryotic microbes, the bacteria and archaea, eukaryotic microbes house their DNA inside a cell nucleus.
4. Micro-animals include dust mites and many nematodes (roundworms) as well as rotifers, tiny filter feeders usually found in fresh water.

Some microscopic, photosynthetic green algae are classed as plants. Fungi have several single-cell species, including baker’s yeast.

Protists are a diverse group of organisms that have little in common except simplicity—they are single-celled or multicellular without specialized tissues.

5. Like bacteria, eukaryotic microbes can cause serious diseases, including malaria, while some fungi present a major health hazard to crops.
6. Finding treatments for these diseases is challenging because any chemical that kills eukaryotic organisms or inhibits their growth is also likely to be toxic to the eukaryotic cells of plants or animals.

Question 22

Explain viruses

Answer 22

1. Viruses are tiny packets of genetic material capable of infecting the cells of living organisms, including animals, plants, and bacteria.
2. Viruses can’t reproduce on their own. Instead, they breed by invading the cells of a host and hijacking their replication machinery.
3. Viruses consist of RNA or DNA protected by a protein coating, and are typically only 10–300 nanometers (billionths of a meter) in size.
4. They infect organisms by penetrating cell membranes, releasing their genetic cargo and forcing the doomed cells to replicate them.
5. New viruses assemble inside each host cell before bursting out, killing the cell.

However, some viruses can remain dormant inside cells for years.

6. Plant viruses are often transmitted between plants by insects that feed on them.
7. Human colds and flu spread through coughing and sneezing, while several viruses are transmitted by sexual contact, including the human immunodeficiency virus (HIV).

Fortunately, our immune systems successfully combat most viral infections, while vaccinations can prevent others.

Question 23

What are the origin of bio-chemicals

Answer 23

1. Scientists can only speculate on how the organic compounds necessary for life emerged on Earth.

They could have arisen spontaneously from chemical reactions of simpler compounds, or they could have arrived from space.

2. A famous experiment at the University of Chicago in 1953 tested whether stormy conditions on the young planet could have triggered reactions between simple chemicals to create the ingredients of life.

Stanley Miller and Harold Urey zapped a mixture of water, methane, hydrogen, and ammonia with electrical discharges to simulate lightning.

Sure enough, this cooked up many organic compounds including amino acids, the building blocks of proteins.

Since then, evidence has emerged that vigorous volcanism on the early Earth enriched the chemical soup with carbon dioxide, nitrogen, and sulfur compounds.

These could have spurred the production of biochemicals.

And astronomers have identified organic molecules, including amino acids in comets, so comet impacts could have delivered off-the-shelf biochemicals.

Question 24

“Replicating life”, what we know?

Answer 24

The early Earth somehow acquired the complex organic chemicals necessary for life.

But how did the jump from inanimate chemicals to living, self-replicating organisms occur?

One puzzle about life’s origins is that all self-replicating organisms today use DNA to store the genetic information needed to build proteins, while protein enzymes are necessary to copy the DNA and spawn new generations.

In other words, DNA and proteins are both vital to life, but the chances that the Earth cooked them up simultaneously seem slim.

An alternative idea is the “RNA world” hypothesis, which proposes that the earliest replicating organisms depended solely on RNA, which is better at multitasking than DNA—it can act as an enzyme as well as a carrier of genetic codes.

However, many scientists are unconvinced that complex RNA molecules could have spontaneously assembled in early-Earth conditions.

Ultimately, it may be impossible to find a convincing theory for how life arose unless a simple “one-pot” experiment repeats it in a very compelling way.

Question 25

What is “panspermia”?

Answer 25

There’s one intriguing alternative solution to the mystery of how life arose on Earth—it didn’t.

A theory called panspermia suggests that comets and meteors from space not only delivered complex organic chemicals to the young Earth, they also delivered bona fide living organisms from which all life evolved.

In that case, we are all descended from aliens.

Proponents of panspermia suggest that simple life is widespread on bodies like comets throughout the solar system, and possibly beyond.

Comets that hit Earth, delivering much of the water in its oceans, could also have brought fully functional living microbes that had gradually evolved in space over billions of years.

That would explain why life “emerged” on Earth so astonishingly quickly after our planet became inhabitable.

What’s more, there is intriguing evidence that some hardy bacteria can survive the harsh conditions of space and even a violent impact on a planetary surface.

But panspermia remains very speculative—although there are hints that bugs could exist in space, there is no direct evidence that they actually do.

Question 26

Explain Evolution

Answer 26

1. Evolution describes the way populations of living organisms change over time due to changes in heritable genetic traits, such as eye color in humans.
2. a) Sometimes the changes are due to environmental pressures—for instance, giraffes may have evolved long necks because those eating plentiful foliage high in the trees survived to produce most offspring.
3. This “natural selection” is one key driver of evolution, but other genetic factors come into play.

Spontaneous, random mutations of genes can sometimes create a beneficial trait that helps an organism produce more offspring, so the mutation persists in the population.

3. “Genetic drift” also plays a role—certain gene variants might flourish simply by chance.
   Sometimes two or more species can evolve through co-evolution, when they have close ecological interactions with each other.

For instance, a plant might evolve thorns to deter herbivores, while the herbivores in turn evolve defenses against thorns that thwart the plant’s strategy.

Question 27

Explain Natural selection

Answer 27

1. Natural selection is one of the basic mechanisms of evolution. The British scientists Charles Darwin and Alfred Russel Wallace independently proposed the theory in 1858.
2. Organisms have different traits, such as size, and these influence whether or not an organism survives long enough to pass on their traits to their offspring.
3. a) Advantageous traits become more common in successive generations. Over time, populations split into different species (“speciation”), and looking back in time, any pair of organisms shares a common ancestor.

For instance, humans shared a common ancestor with chimpanzees about 6 million years ago.

Peppered moths offer an example of rapid natural selection during the industrialization of the United Kingdom. Common pale-colored peppered moths became conspicuous on tree trunks that had darkened with soot, making them easy prey for birds, while darker ones preferentially survived long enough to breed. Eventually, the population became primarily dark, but the process reversed when the clean air standards were enforced.

Question 28

Explain ‘Genetic drift’

Answer 28

1. Genetic drift is one of the driving forces behind evolution. It describes the way that certain traits in a population might flourish or vanish by chance, because individuals with these traits happen to breed most offspring or not breed at all.
2. Genetic drift tends to rapidly reduce genetic diversity in very small populations. For instance, if only two individuals in a population of ten animals carry a certain gene variant and do not breed fertile offspring, that variant will be gone from the population for good.
3. A special case of genetic drift is the “founder effect,” which occurs when a small number of individuals become isolated from the main population.

For example, in the late 1700s, a typhoon on the island of Pingelap in Micronesia left only around twenty human survivors to create future generations. Today, 5–10 percent of the island’s population suffers from a total color blindness disorder that is extremely rare elsewhere, because one of the typhoon survivors must have carried a recessive gene linked to the disorder.

Question 29

“Human origins” what do we know?

Answer 29

1. All people living today are descended from a single female dubbed “mitochondrial Eve” who lived in Africa about 110,000–130,000 years ago.
2. Scientists deduced this from modern genetic analysis of the DNA in human mitochondria, which is inherited via the maternal line.
3. The widely accepted “Out of Africa” theory suggests that modern humans (Homo sapiens) first evolved in Africa roughly two hundred thousand years ago, then migrated all over the world during the last one hundred thousand years.

They reached the Middle East by about seventy thousand years ago, south Asia by sixty thousand years ago, and western Europe around forty thousand years ago.

The dates for North American colonization are unclear—this may have started around thirty thousand years ago or much later.

These human settlers largely replaced indigenous prehuman species, such as the heavy-browed Homo erectus in Asia.

However, genetic evidence hints that some interbreeding took place between anatomically modern humans and Neanderthals, members of the genus Homo that died out 30,000 years ago.

Question 30

Explain “Food webs”

Answer 30

1. Food webs and food chains are graphical representations of the feeding relationships between living organisms in an ecosystem.
2. A food chain usually describes a linear sequence such as “hawks eat snakes, snakes eat toads,” while a food web maps a more complicated network of interconnected chains.
3. Living organisms can be divided into three categories: producers, consumers, and decomposers.

Producers such as green plants can make their own food from energy and simple inorganic compounds.

Consumers eat other organisms. They include herbivores (“primary consumers”) that eat plants, carnivores that eat animals, and omnivores that eat both.

Animals that eat primary consumers (such as snakes that eat mice) are called secondary consumers, while tertiary consumers eat secondary ones, and so on.

Decomposers are organisms that feed on dead animals and plants, such as earthworms or fungus growing on a dead log. They break down materials into the simple inorganic chemicals and nutrients vital for the producers, such as green plants.

Question 31

Explain Carbon cycle, Water cycle, and Nitrogen cycle.

Answer 31

1. The carbon cycle describes the way that carbon, a crucial ingredient for all organic substances, circulates through different components of the Earth’s environment, including the land, oceans, atmosphere, and planetary interior.

Plants absorb carbon dioxide (CO2) during photosynthesis and release carbon as they die and decompose.

Burial and compression over millions of years can transform them into fossil fuels. During respiration, plants and animals release CO2, and burning fossil fuels produces more.

The gas is slightly soluble in water, so lakes and oceans absorb some of it, and organisms such as corals and shellfish convert it into calcium carbonate that accumulates in ocean sediments when they die.

2. Other important cycles include the water cycle, in which warm air passing over the sea surface evaporates water, which rises and condenses into clouds before falling as precipitation.
3. In the nitrogen cycle, nitrogen “fixing” locks up nitrogen in plants as nutrients, while soil bacteria eventually break nitrogen compounds down, returning nitrogen gas to the atmosphere.

Question 32

Explain ‘Bio-diversity’

Answer 32

1. Biodiversity measures how much variety there is between all the different species of life on Earth, from single-celled bacteria through insects to blue whales, the largest known animal ever to have existed.
2. The term also sometimes describes the genetic diversity within a single species, or the diversity of ecosystems like wetlands and forests.
3. Around 1.75 million species of living organisms have been identified on Earth so far, mostly small ones like bacteria and insects, and estimates suggest the true number could be as high as 100 million.
4. But in recent centuries, there has been a rapid increase in the rate of species extinctions due to human activities such as habitat destruction for farming.

Between 1500 and 2009, international organizations documented more than 800 species becoming extinct, including the Javan tiger that died out completely in the 1980s, but the vast majority of disappearances probably go unnoticed.

Conservationists grade the vulnerability of species according to a scale that runs from “extinct” to “least concern.”

Question 33

Explain “Mass extinctions”

Answer 33

1. Mass extinctions occur when environmental conditions change and kill off vast numbers of species of life.

A classic example is the Cretaceous–Tertiary extinction, which wiped out the Earth’s dinosaurs about 65 million years ago.

The consensus is that a large asteroid hit the Earth and altered its climate by filling the atmosphere with dust, blocking sunlight, and making the climate too cool for dinosaurs to survive.

2. The fossil record suggests that many other mass extinctions have occurred in the past, including the Late Ordovician extinction 440 to 450 million years ago.

Scientists suspect that this occurred when inhospitable ice sheets grew across an ancient supercontinent called Gondwana, which existed long before the current familiar continents took shape.

3. Many scientists have speculated that humans are causing a current mass extinction today, by hunting, generating pollution, and destroying habitats, particularly biodiversity hotspots such as tropical forests.

Many species living in these areas will probably become extinct before we even know they existed.

Question 34

What is genetic modification?

Answer 34

1. Genetic modification is the use of modern biotechnology techniques to change the genes of an organism, altering the DNA that instructs its cells how to build proteins.

Many crop plants are genetically engineered to possess desirable traits such as resistance to pests or harsh environments.

In traditional breeding of crops and livestock, farmers pick plants or animals with desirable traits and crossbreed them to create commercially valuable offspring.

2. Genetic modification allows the traits of organisms to be altered in ways that are not possible through traditional breeding.

For example, some cotton plants are modified to carry a gene from soil bacteria. This makes them produce a chemical that kills insect pests, reducing the need for pesticides.

Sometimes, genetic modification turns down or “silences” the activity of genes that an organism already has. This can prevent oilseed rape crops producing unhealthy oils, for instance.

3. Genetically modified animals are often used in experiments to study gene functions, but are not yet bred for commercial agriculture.

Question 35

Explain “Pharming”

Answer 35

1. In biotechnology, pharming (linking “pharmaceutical” and “farming”) means genetically engineering plants or animals so that they produce useful drugs or industrial chemicals.

For instance, plants could be genetically engineered so that their seeds contain rich sources of human antibodies, specialized proteins of the immune system that could fight diseases such as cancer, hepatitis, and malaria.

2. Goats have already been genetically modified for commercial production of a drug called ATryn, which prevents dangerous blood clots. The drug’s active protein is purified from the goats’ milk.

Many plants that have been genetically modified to produce drugs are undergoing tests, but plant-pharmed drugs are not commercially available so far.

3. While supporters say this is a safe and cheap way to manufacture vital vaccines, opponents worry about the plants crossbreeding with natural plants and contaminating the environment and food supplies.

Question 36

Explain “Cloning”

Answer 36

1. Reproductive cloning is the creation of a genetically identical copy of an organism, with exactly the same DNA.
2. a) Scientists created the first cloned mammal, Dolly the sheep, at the Roslin Institute near Edinburgh in Scotland in 1996.
   b) Dolly was created by a technique called “somatic cell nuclear transfer.”
   c) Scientists took a cell from an adult sheep’s mammary gland and transplanted its DNA-containing nucleus into an egg cell from which the nucleus had been removed.
   d) The cell developed into a normal embryo, which was implanted into a surrogate mother sheep who carried the fetus to term and gave birth to Dolly, an exact genetic replica of the adult female that donated the cell nucleus.
3. Since Dolly, researchers have cloned many large and small mammals including horses, goats, cows, mice, pigs, cats, and rabbits.
4. They hope that “therapeutic cloning” (a type of stem cell therapy) may one day provide genetically matched tissues and organs that can be used in transplant operations without any risk of the patient rejecting them.