Reading Exercise: What are Alleles?

Chromosomes are found inside the nucleus of cells and consist of long strands of DNA. Each human cell has 46 chromosomes (23 pairs), apart from gametes (sex cells) which have only 23.

You can think of chromosomes as a set of books. Each book (chromosome) contains a set of instructions (genes). All of the books together contain all the instructions needed to make a certain organism (a living thing), for example a human or a flower.

There are always two copies of the same gene in an organism because chromosomes come in pairs. These two versions of the same gene are called alleles.

The two alleles for one characteristic do not have to be the same. They can be different. If both alleles for one gene are the same, they are homozygous (from homo = the same). If the alleles are different, they are heterozygous (from hetero = different).

For example, everyone has two alleles of the gene that decides eye colour. If someone has brown eyes and is homozygous for that gene, they will have two alleles for brown eyes. We can see this for individual B in the middle of the image..

A person could also have brown eyes and be heterozygous for that gene. They have one allele for brown eyes and another allele for blue eyes. We can see this for individual A to the left in the image. The reason that this person has brown eyes is that alleles can be either dominant or recessive.

The gene for brown eyes is dominant and we will always see this characteristic, no matter what other gene is present. The gene for blue eyes is recessive and we only see it when the dominant gene for brown eyes is not present. We can see this for individual C to the right in the image.

Characteristics that we see on the outside are called the phenotype. The characteristics in our genes are called the genotype. Phenotype and genotype can be slightly different. In our example we have seen that someone can have brown eyes as their phenotype, but both brown and blue eyes in their genotype.

Questions

  1. What are chromosomes and where are they found?
  2. What are alleles?
  3. What is meant by homozygous?
  4. What is meant by heterozygous?
  5. What is meant by dominant?
  6. What is meant by recessive?
  7. Why does a person have brown eyes even when they have genes for both brown and blue eyes?
  8. What is meant by phenotype?
  9. What is meant by genotype?
  10. What is the phenotype (= the colour seen on the outside) of a rose that is homozygous and has two dominant alleles for red colour?
  11. What is the phenotype of a rose that is homozygous and has two recessive alleles for white colour?
  12. What is the phenotype of a rose that is heterozygous and has one dominant allele for red colour and one recessive allele for white colour?

Reading Exercise: The Structure of DNA

DNA is found inside the nucleus of each cell. One very long, coiled up molecule of DNA is called a chromosome. Human cells have 46 chromosomes in total.

Each DNA molecule contains two strands that are connected by a pair of substances called bases. It looks like a ladder, where the bases form the rungs. In addition, the ladder is wound and looks a bit like a spiral staircase. We call this wound-ladder structure of DNA a “double helix”.

This double helix structure of DNA was discovered by the British scientists James Watson and Francis Crick who received the Nobel Prize for their work in 1962.

There are four bases in DNA, adenine, thymine, cytosine and guanine, normally just called A, T, C and G. When forming pairs to make the rungs of the ladder, A always pairs with T and C with G. We call this complementary base pairs. The base pairs are held together by a weak attraction called hydrogen bonding.

Furthermore, each base is attached to a sugar which in turn bonds to a phosphate group. The sugars and phosphate form the backbone of the DNA strands.

A gene is one section of DNA that codes for one single characteristic or protein. We all have very small differences in our genes caused by slightly different orders of the bases in our DNA. This means that everyone’s DNA is unique. It allows scientists to match DNA from cells to specific people. For example, it helps scientists to find out how people are related or it can be used by forensic scientists to identify criminals.

Questions

  1. Where is DNA found?
  2. What is a chromosome? How many chromosomes do humans have?
  3. What does the structure of DNA look like? What do we call it?
  4. Who discovered the structure of DNA?
  5. Name the four bases in DNA.
  6. What is complimentary base pairing?
  7. What holds the base pairs together?
  8. What is a gene?
  9. Why is everyone’s DNA unique?
  10. How can the knowledge that everyone’s DNA is unique help scientists?

Where can we find acids and alkalis in nature?

Image: Richard Bartz, 2007. Bee stings contain formic acid and are slightly acidic. Wasp stings, on the other hand, are slightly alkaline.

What are acids and bases?

Before we delve into the different acids and bases found in nature, we need to be clear about what they actually are. In their simplest definition acids are solutions that have a pH below 7 and react with bases in neutralization reactions which means that the acid effect is cancelled out by the base. A more advanced explanation would add that acids release in hydrogen ions (H+) in water.

Bases are the chemical opposite of acids. Their pH values are above 7 and they react with acids in neutralization reactions. You may also have heard the word alkali being used for bases. Alkalis are bases that are soluble in water and release hydroxide ions (OH-). In this article we will use the word alkalis.

Food and digestion

There is one strong acid that you are carrying around with you all the time, the hydrochloric acid in you stomach. Your stomach acid is quite strong with a pH of 2. Its job is to break down food and kill pathogens that enter the digestive system. Pathogens are microorganisms like bacteria and viruses that can cause diseases.

Most of you will have heard that there is a lot of acid in citrus fruit like lemon. They contain an acid called citric acid which also gives them their sour taste. Lemon juice has a very acidic pH between 2 and 3. However, other fruits and vegetables contain acids too. For example, there is malic acid in apples. Tomatoes and pears contain citric acid as well as malic acid.

A sour taste will tell us if food or drinks are acidic. We can also tell from the taste if they are alkaline. The give-away for alkalis is a bitter taste. Examples of alkaline foods are leafy green vegetables like kale, spinach and parsley.

Insect bites and stings

So far we have only talked about beneficial acids and alkalis in food, drinks or our stomach. However, there are some unpleasant acids and alkalis to be found in nature as well.

Bee and ant venoms contain formic acid making their stings or bites slightly acidic contributing to the pain they cause. In fact, formic acid was first extracted from ants which lead to it being named after the Latin word for ant ”formica”. Nevertheless, we need to be aware that insect poisons are a mixture of different unpleasant substances that work together in a sting. Bees and ants do not rely on the formic acid in their venom alone.

Wasp venom, on the other hand, is slightly alkaline. Just like in bees and ants wasp poison too is a cocktail of various chemicals which contribute to the effect of a sting. Also wasps need other substances apart from the alkali in their venom.

Reading exercise: Acids and Bases

Acids

There are acids in the laboratory, for example hydrochloric acid, sulphuric acid and nitric acid. But there are also natural acids in vinegar, sour fruits like lemon and even your stomach! When an acid is dissolved in lots of water, it is called dilute. The more concentrated an acid, the less water it is dissolved in.

Concentrated acids are corrosive. This means they burn through skin, other living tissue and metal.

Bases and Alkalis

Bases are the chemical opposite of acids. They react strongly with acids in neutralization reactions. In such a neutralization reaction, bases neutralize acids which means that they cancel out the acid effect. If a base is soluble in water it is called an alkali.

Alkalis in concentrated solutions are corrosive like acids. This powerful chemical action is often used in bath and oven cleaners.

Neutralization

Although neutralization may seem like something that only happens in the lab, it is also used at home. Stomach indigestion is usually caused by too much acid in the stomach. The pain from indigestion can be removed if the acid is neutralized. This can be done by swallowing a weak alkali like bicarbonate of soda or milk of magnesia.

Neutralization also occurs when treating wasp or bee stings. A wasp sting is an alkali and can be treated with vinegar which is a weak acid. The vinegar neutralizes the alkali wasp venom. A bee sting is an acid and can be neutralized using baking soda which is a weak alkali.

Farmers use neutralization when too much acid is in the soil is causing it to be “sour”. They normally add lime to their fields. Lime is an alkali that dissolve in rain water and neutralizes the acid in the soil. Crops can now grow better.

Questions

  1. Name three laboratory acids.
  2. Name three acids you have in the home.
  3. What is meant by dilute?
  4. What is meant by concentrated?
  5. What is meant by corrosive?
  6. What are bases? What are alkalis?
  7. What happens in a neutralization reaction?
  8. Where are alkalis used in the home?
  9. How can you treat stomach indigestions? Why does it work?
  10. How can you treat bee and wasp stings? Why does it work?
  11. Why do farmers sometimes add lime to their fields?

Challenge: Why is it important to use a weak alkali when treating stomach indigestion and bee stings?

Why do you have to use a weak acid to treat wasp stings?

Reading Exercise: The Halogens

Elements in group 7 of the periodic table are called the halogens. They share some similar properties (= features) and show a pattern in the way their properties change throughout the group.

All halogens exist as diatomic molecules where two atoms are held together by a chemical bond. They are non-metals which makes them poor conductors of heat and electricity. In addition, all halogens are poisonous and corrosive which means they burn through metals and living tissue like skin.

Physical properties

When going down group 7 melting points, boiling points and densities of the halogens increase. At room temperature, fluorine is a pale yellow gas and chlorine a green gas. Bromine is a brown liquid that evaporates easily. Iodine is a purple/black solid.

Chemical properties

Most halogens react with metals to form salts called metal halides, for example:

chlorine + magnesium -> magnesium chloride

fluorine + sodium -> sodium fluoride

The halogens become less likely to react in chemical reactions when going down the group. Their reactivity decreases. Fluorine is most reactive.

Displacement reactions

The order of the reactivity of halogens can be shown using displacement reactions where a more reactive element takes the place of a less reactive element in a salt.  For example, chlorine displaces bromine from sodium bromide in solution because chlorine is more reactive than bromine.

chlorine + sodium bromide à sodium chloride + bromine

However, bromine cannot displace chlorine from sodium chloride because it is less reactive.

Uses

All halogens can be used in disinfectants and bleaches because they kill microorganisms like bacteria and remove colour from materials. Chlorine is used as a disinfectant in swimming pools and in bleaches. Iodine is used to clean wounds.

Test for chlorine

If a damp, blue litmus paper is placed in chlorine gas, it first turns red then bleaches white.

Questions

  1. In which group of the periodic table are the halogens found?
  2. Halogens exist as diatomic molecules. What is meant by “diatomic”?
  3. Halogens are also corrosive. What is meant by “corrosive”?
  4. Describe what fluorine, chlorine, bromine and iodine look like at room temperature.
  5. What is formed when halogens react with metals?
  6. Chlorine reacts with iron. What is the name of the product formed in this reaction? Write the word equation.
  7. What is the trend in reactivity for halogens when going down the group?
  8. What happens in a displacement reaction?
  9. Why can chlorine displace bromine from sodium bromide?
  10. Give two uses for chlorine and one use for iodine.
  11. Describe the chemical test for chlorine.
  12. Fluorine is mixed with lithium iodide Can fluorine displace iodine from lithium iodide? Why? Write the word equation.

Reading Exercise: Absorbing Water and Mineral Ions

Image: Osmosis. The process plant roots use to take in water.

Plants use their roots to absorb (= take up) water and mineral ions from the soil. The water absorbed by plant roots is used for:

  • Transporting dissolved mineral ions
  • Keeping cells rigid (= stiff), so the plant stays upright
  • Cooling the leaves (when the water evaporates from them)
  • Photosynthesis

Root hair cells

The outside of roots is covered with root hair cells. The “root hair” is an extension of the cell and provides a larger surface area, so that water and mineral ions can be absorbed faster. In addition, root hair cells have very thin cell walls to improve the flow of water into the cells.

Diffusion and osmosis

Diffusion is the movement of particles from a place of high concentration to a place of low concentration. We say that the particles move along a concentration gradient.

Osmosis is the diffusion of water. Water particles move from a place with a high water concentration (lots of water) to a place with a low water concentration (less water). This happens across a semi-permeable membrane which lets small molecules like water pass, but keeps back larger molecules like sugar. The semi-permeable membrane acts like a sieve.

Root hair cells take in water from the soil using osmosis. The water moves from the soil with lots of water into the root with less water. Cell wall and cell membrane act as the semi-permeable membrane.

Active transport

Mineral ions are ionic compounds or salts that are found naturally in soil. Plants need these ions to build important substances like proteins.

The concentration of mineral ions inside the root is higher than outside. Mineral ions cannot diffuse from a lower concentration outside to a higher concentration inside the root. Therefore, the cell membrane needs to actively pump mineral ions inside. This process requires energy and is called active transport.

Questions

  1. What is the water absorbed by plant roots used for?
  2. What process do plants need carbon dioxide, water and sun light for?
  3. Give two adaptations (= special features) that root hair cells have to let water enter the cells faster.
  4. What is the difference between diffusion and osmosis?
  5. How does water enter root hair cells? What is the name of the process?
  6. What is a semi-permeable membrane?
  7. What are mineral ions?
  8. What is active transport?
  9. Why do mineral ions not diffuse into root hair cells?

Reading Exercise: Gas Pressure

In a gas, the particles move around at high speed. By doing this they collide with the sides of their container and produce a force. This is the force that keeps balloons and tires inflated. We call it gas pressure.

There are two ways to increase gas pressure. The first is to add more gas particles. More gas particles can collide with the walls more often which increases the pressure. This is what happens when you blow up a balloon or pump up your bike tire. You add more air particles.

The second way to increase gas pressure is to increase the temperature. When it is warmer, gas particles gain more energy and can move around faster. At a higher speed the particles collide with the walls more often and the pressure increases.

If you reduce the temperature, the opposite happens. The gas particles move slower and collide with the walls less frequently. This decreases the gas pressure.

Questions:

  1. What causes gas pressure?
  2. How can you increase gas pressure? Describe both ways.
  3. How can you decrease gas pressure?
  4. What would happen if I put a party balloon into the sun on a hot day? Why?
  5. What would happen if I put a party balloon in a freezer? Why?

Reading Exercise: The Dangers of Radioactivity

Radioactive radiation

Radioactive radiation is dangerous because it is ionizing. This means that it has enough energy to knock electrons out of their shells and thus turn atoms into ions.

A large amount of ionizing radiation can cause radiation burns (skin burns) and radiation sickness. Small amounts of ionizing radiation can damage DNA. This is called mutation and can later lead to cancer. If mutations occur in gametes (sex cells), they can be passed on to the next generation and can cause birth defects in newborns.

Protection from radiation

We are exposed to background radiation all the time. However, the amount is too small to harm us. But people who work with radioactive materials are exposed to more radiation and need to take precautions.

There are three ways to limit the effects of ionizing radiation: keeping a distance from the source, shielding and a time limit to exposure. For example, radioactive samples are always handled with tongs to keep a distance and stored in containers with thick lead walls for shielding.

Ionizing radiation is also used in hospitals to detect and treat cancer. Patients can be exposed to small amounts of radiation during their treatment. However, this is only done if the benefits are greater than the risks.

Irradiation and contamination

Sometimes there are accidents in nuclear power stations which can lead to radioactive materials escaping to the environment. This can lead to the irradiation and contamination of plants, animals and people. It is important to understand the difference between irradiation and contamination.

Someone is irradiated if they are close to a radioactive material and exposed to its radiation. Once the person moves away the irradiation stops.

Someone is contaminated if they get particles of radioactive material on their skin or inside their body, for example by eating or breathing. This person continues to be exposed to the radiation until all the material has decayed inside their body.

Things to do

  1. What is ionizing radiation?
  2. Give four effects of ionizing radiation on the human body.
  3. Why is background radiation not dangerous for us?
  4. State three ways to protect yourself from radiation.
  5. Why are radioactive samples handled with tongs and kept in lead containers with thick walls?
  6. What is the difference between irradiation and contamination?
  7. In case of a nuclear accident, what is worse? Irradiation or contamination? Why?
  8. Do you know any examples of nuclear accidents that have happened around the world? If yes, which do you know?

Active reading exercise: Isotopes

Isotopes

In 1932 James Chadwick, a British scientist, discovered the neutron. His discovery explains how isotopes are formed. Isotopes have the same number of protons, but different numbers of neutrons.

We can also say that the atomic number is the same, but the mass numbers are different. Isotopes are the same element because their atomic number does not change.

We refer to an isotope by adding its mass number to the element’s name. The isotope in the diagram below is called carbon-12.

carbon

Carbon isotopes

Carbon can occur as three different isotopes. They are called carbon-12, carbon-13 and carbon-14.

Carbon dating is used to identify the age of very old objects, for example the remains of extinct animals such as mammoths. The amount of the carbon isotope carbon-14 in an object is examined to determine its age.

Things to do

  1. Fill in the missing words and numbers to describe the similarities and differences between isotopes of the same element.                                                                                 a) Isotopes are atoms with the same            number but different                   number. b) They have the same number of protons and electrons but different numbers of                      .
  2. Name the scientist who discovered the neutron.
  3. Why are isotopes the same element?
  4. How do we refer to isotopes?
  5. Name the three isotopes of carbon.
  6. Why is the isotope carbon-14 useful?
  7. Using your knowledge about isotopes, fill in the gaps in the table a, b and c.
Isotope name Atomic number Mass number
 Tin-116  50 a
b c 118

Remember that the atomic number is the same for isotopes of the same element.

Active reading exercise: The Atom

A bit of history

In 1805 the English Chemist John Dalton published his atomic theory that said:

  • Everything is made up from tiny particles called atoms
  • Atoms are tiny hard spheres (= balls) that cannot be broken down into smaller parts
  • Atoms in one element are all identical

This helped to explain many properties of materials. However, later experiments showed that atoms contained even smaller particles. In 1897 another British scientist, JJ Thomson, discovered the electron. The nucleus which makes up the middle of an atom was discovered by Ernest Rutherford in 1913.

The Structure of the atom

Today we know that atoms are made from three subatomic particles: proton, neutron and electron.

Protons and neutrons are found in the centre of the atom which is called the nucleus. Both have a mass of 1. Protons have a positive (+) charge and neutrons are neutral (= no charge).

Electrons have a negative (-) charge and have almost no mass at all. They are found on the electron shells on the outside of the atom, circling the nucleus.

What you need to remember

  • Atoms are made from protons, neutrons and electrons called subatomic particles
  • Protons: found in nucleus, positive (+) charge and a mass of 1
  • Neutrons: found in nucleus, neutral (no charge) and a mass of 1
  • Electrons: found on electron shells, negative (-) charge and almost no mass

 

Things to do

  1. Name the scientist who first introduced atomic theory.
  2. Name the scientists who discovered the electron and the nucleus.
  3. State the names of the three subatomic particles as well as their masses and charges.
  4. State where in the atom protons and neutrons are found.
  5. State where in the atom electrons are found.
  6. Copy and label the image of the atom. Words: electron, proton, neutron, shellLithium atom
  7. Describe in your own words what an atom looks like. Include information about the charges and masses of the subatomic particles.