The Scientific Method

The Scientific Method

• The method that scientists use to make new discoveries about natural phenomena

• Involves several steps:1. Making observations2. Forming and testing hypotheses3. Experimentation4. Analyzing results5. Making conclusions

Step 1. Making Observations

• Data is any recorded observations.• Qualitative data describes phenomena.– Example: Color and formation of product in

chemical reaction• Quantitative data gives numerical

measurements.– Example: Height of plant, today’s temperature

Question

• Identify the following as qualitative or quantitative data.

1. The barometric pressure is 760mmHg.2. A solution of sodium chloride conducts

electricity.3. A plant grown in sunlight reaches 23.6cm in a

week.

Step 2. Forming and Testing Hypotheses

• A question can be formed from an observation.

• A tentative answer to the question is known as hypothesis.

Example

• Observation: A plant grown in soil A produces orange flowers, while the same plant grown in soil B produces white flowers.

• Possible question: Does the soil have any effect on the color of the flowers?

• Possible hypothesis: The iron content of soil has an effect on the color of the flowers.

Question

• The following observation was made: A plant flowers from May 15-Jun 15. What is a possible question?

A. Could the amount of sunlight have an effect on the flowering?

B. Could the soil have an effect on the flowering?C. Could the location of the plant in the garden

have an effect on the flowering?

Question

• What could be a possible hypothesis?A. Less sunlight causes plants not to flower.B. The phosphorus content of the soil causes

plants not to flower.C. The crowdiness of a garden causes plants not

to flower.

Step 3. Experimentation

• The hypothesis is proven true or false by selecting methods that prove or disprove the hypothesis.

• The different techniques will be taught during the various sessions.

Step 4. Analyzing Results

• Data may be given in tabular or graphical form.

• Qualitative data is usually reported in tabular form, whereas quantitative data is usually reported in either tabular or graphical form.

Presenting data in tabular form

• The following data was given for growth of plant: Day 1, 1cm; Day 2, 2cm; Day 3, 3cm; Day 4, 4cm.

Day Length of plant (cm)

1 12 23 34 4

Presenting data in graphical form

Day 1 Day 2 Day 3 Day 40

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Day 1 Day 2 Day 3 Day 40

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Step 5. Making Conclusions

• A conclusion is a statement that can be derived from the data.

• Do not exaggerate the conclusion.

Basic Chemistry in Biology

Matter

• Matter is anything that takes up space and has mass.

• Elements cannot be broken down into other substances by chemical reactions.

• Compounds are composed of two or more elements combined in a fixed ratio.

• Essential elements are needed by an organism’s body to be healthy.

• Trace elements are required only in minute quantities.

The Atomic Theory of Matter

• An atom is the smallest unit of matter that retains the properties of an element. It contains the following subatomic particles:– Neutrons have no charge.– Electrons have a negative charge.– Protons have a positive charge.– Protons and neutrons are found in the nucleus.• The dalton (Da) or atomic mass unit (amu) is the

unit used for subatomic particles.

The Atomic Theory of Matter• The atomic number is the number of protons

of an element and is unique to the element.• The mass number is the sum of the number of

protons and the number of neutrons and is an approximation of the atomic mass.

• Some elements have different forms with more neutrons, which are known as isotopes.

• The atomic mass is the mass reported in the periodic table based on the availabilities of the different isotopes .

The Atomic Theory of Matter

• The number of electrons is similar to the number of protons in neutral elements.

• The number of electrons depends on the charge of the element if it is an ion.

• A positive ion is called a cation and has less electrons, and a negative ion is called an anion and has more electrons.

Question

• Na+ has mass number 23 and 11 protons. Identify the number of neutrons and electrons.

• Solution: Since mass number=number of neutrons + number of protons, number of neutrons= 23-11=12.

• Since the ion has charge +1, number of electrons is 1 less than the number of protons, resulting in 10 electrons.

Technique: Radioactive Isotopes

• A radioactive isotope has its nucleus decaying spontaneously, resulting in formation of particles and energy.

• Several radioactive isotopes may be used, such as 3H, 13C, 14C, 32P, 35S, 59Fe, 125I, and 131I.

• Step 1. One ingredient that cells use to make certain molecules is labeled with one of the radioactive isotopes.

Technique: Radioactive Isotopes• Step 2: The ingredients are added to the cells in

culture and incubated in various parameters (pH, temperature, etc.)

• Step 3: Once the cells have synthesized the molecules of interest, the molecules are isolated from the cells.

• Step 4. Scintillation fluid is added to the molecules. As the radioactive isotope decays, the radiation emitted excites the chemicals in the fluid, giving off light.

• Step 5. Light is then measured by a scintillation counter.

Question

pH 1 pH 3 pH 5 pH 70

0.5

1

1.5

2

2.5

3

3.5

4

4.5 • Suppose that the following is a graph of DNA samples labeled with 3H and measured with a scintillation counter. At what pH is the cell living optimally?

Energy

• Energy is the capacity to cause change.• Potential energy is energy that matter

possesses due to location or structure.• Electron shells are the possible locations of an

electron, forming concentric circles around the nucleus, each with a characteristic average distance and energy level.

• Electrons farther from the nucleus have higher potential energies.

Electron Distribution• Valence electrons are the electrons found in the

outermost electron shell, and the electron shell containing them is the valence shell.

• Elements with a complete set of valence electrons in the valence shell are chemically unreactive.

• The first electron shell contains only two electrons, while succeeding electron shells contain eight electrons.

• However, since electrons are constantly moving around, the electron shells were soon replaced by orbitals.

Chemical Bonds• Orbitals are three-dimensional regions where an

electron is found 90% of the time and may only contain two electrons at any given time.

• Atoms are usually held together by chemical bonds to complete their valence shell.

• Covalent bonds share electrons with the atoms involved, while ionic bonds transfer electrons from one atom to the other.

• Ionic compounds are called salts.• A single bond represents two electrons being shared.

Atoms may also have double bonds or triple bonds.

Electronegativity• Electronegativity is the strength of attraction of

a shared covalent bond to a certain atom.• Nonpolar covalent bonds occur when the two

atoms bonded have similar electronegativities, while polar covalent bonds occur when the two atoms bonded have different electronegativities.

• Ionic bonds occur when the two atoms bonded have very high difference in electronegativities.

• Electronegativity increases from left to right in the periodic table and from bottom to top.

Question

• The valence electron of K is 1, and the valence electron of Cl is 7.

A. What compound is formed by the two elements?B. Is the compound covalent or ionic?• In the molecule O2,

A. Is the covalent bond polar or nonpolar?B. How many bonds do they share?

Weak Bonds

• Weak bonds are not covalent chemical bonds and may occur only temporarily between molecules that are in close proximity.

• Hydrogen bonds occur between hydrogen and oxygen, sulfur, or nitrogen.

• Van der Waals interactions occur between two nonpolar molecules due to occasional dipole moments.

• Molecular shape often determines function in biology, as this allows for specificity of reactions.

Question

• What weak bonds can occur between the following molecules, if any?

A. H2S, NH3

B. O2, H2

C. CO2, SO2

Chemical Reactions• Chemical reactions involve the breaking and

formation of new chemical bonds.• The starting materials are called reactants,

while the new molecules formed are called products.

• Most chemical reactions are reversible to a certain extent.

• Chemical equilibrium is achieved when the forward and reverse reactions balance each other.

Properties of Water• Weak bonding between similar molecules results in a

phenomenon called cohesion.• Weak bonding between different molecules results in

a phenomenon called adhesion.• Cohesion between water molecules and adhesion

between water molecules and the plant cells are responsible for water transport against gravity in plants.

• Another phenomenon due to weak bonding is surface tension, which is a measure of how difficult it is to stretch or break the surface of a liquid.