ACT Science

Outside Knowledge

In this chapter, I will give an overview of the topics that will NOT be explained in the ACT Science passages that the ACT expects you to know. The ACT assumes you know these topics from school and science class. To gather this info, I dissected dozens of previous ACT Science test sections to find the concepts you have to know.


1. Cell Biology

You need to know certain cell organelles (parts of cells), their functions, and whether they are found in animal or plant cells. The following are some important cell organelles:

  • Lysosomes hold enzymes. Lysosomes digest food or break down the cell when it dies.
  • Mitochondria are organelles that act like a digestive system, which takes in nutrients, breaks them down, and creates energy-rich molecules (ATP) for the cell.
  • The cell nucleus acts as the brain of the cell. It contains the cell’s DNA, or the genetic information, from which proteins are made (see Topic 2, coming up next). It also helps control eating, movement, and reproduction.
  • Chloroplasts only exist in plant cells and assist in the process of photosynthesis, converting light into energy (which only plants do, not animals).
  • The cell membrane holds all of the pieces of the cell and serves as the barrier between the cell and other cells.

Below is a sample question where you need an understanding of these organelles to select the correct answer. The chemical reaction mentioned in the passage was photosynthesis.

Example 1:

In eukaryotic organisms, the chemical reactions associated with the chemical equation shown in the passage typically occur within which of the following structures?

A. Chloroplasts

B. Mitochondria

C. Lysosomes

D. Nuclei

Solution: Knowing that photosynthesis happens in chloroplasts, I can correctly choose answer A.

But nowhere in the passage was it said that chloroplasts are where photosynthesis happens!

This is a fact you had to know before the test.

2. DNA, RNA, Ribosomes, and Protein Synthesis

DNA contains the genetic information needed for making proteins. Protein synthesis involves DNA, RNA, ribosomes, and proteins. DNA acts as the blueprint for protein production.

Messenger RNA (known as mRNA) makes a copy of the sequence of DNA of a specific gene. This process is known as transcription and happens in the nucleus.

Once the mRNA is made, it leaves the nucleus and enters the cytosol of the cell. Ribosomes use mRNA as a guide to make protein of the same amino acid sequence as the original DNA. The process of producing protein from the mRNA is referred to as translation. So, the process of protein synthesis consists of two steps: DNA to mRNA transcription and mRNA to protein translation.

3. Natural Selection

Natural selection is also known as ‘survival of the fittest.’ In a specific environment, traits that allow organisms to reproduce more effectively will become more common, and traits that reduce reproductive success will become less common.

A classic example of this is the change in peppered moth color during the industrial revolution. In England, the burning of coal during the industrial changed tree bark from light brown to dark brown in color. The peppered moth blended in perfectly and was hidden from predators.

However, once clean air acts were passed, the trees quickly returned to a lighter color, making the dark moths easily visible to predators. Meanwhile, lighter colored moths were still hidden from view and survived to lay eggs. Thus, because of natural selection, over the course of years, the moths turned from dark to light in color

4. Genetics

Proteins are chemicals, made up of amino acids that do specific jobs in cells; for example hemoglobin is the protein in your blood that carries oxygen, and is also what gives blood its red color. Proteins are made from a set of instructions contained in the nucleus of cells, called DNA.

DNA is made up of four different bases, shortened to A, T, C and G. They go in pairs, so A is always paired with T, and C is always paired with G. There are two strands in DNA, which fit together like a zipper, because of this pairing. Because of the pairing, the two strands are sort of copies of each other, which prevents mistakes in your genetic code.

DNA are wound into a shape called a chromosome. Humans have 22 pairs of chromosomes. Different regions of chromosomes are called genes.

An allele is a version of a gene. For example, your eye color genes may be the blue allele, or the brown allele. Alleles can be dominant or recessive. For any given gene, you inherit one allele from your mother, and one from your father. This is called your genotype - the set of alleles that you have. Letters are used to denote the different alleles: capital letters for dominant alleles and lower case for recessive.

When the two alleles are different, we call it heterozygous. If the two alleles are the same, we refer to this as homozygous.

On the ACT Scientific Reasoning, you may see a punnett square, which is a tool for working out the genetic traits of the offspring of two parents. Setting up and using a Punnett square is quite simple once you understand how it works.

You begin by drawing a grid of perpendicular lines:

grid of perpendicular lines

Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:

genotype of parents

Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square.

Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.

offspring genotype possibilities

In this example, 100% of the offspring will likely be heterozygous (YG). Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, 100% of the YG offspring will have a yellow phenotype,


1. Molar Mass

The only molar mass concept you need to know is that the mass of a molecule is the sum of the mass of its atoms.

This appears in an ACT question asking about oxygen’s weight versus carbon dioxide’s weight. You need to know that \(O_2) is lighter per molecule than \(CO_2) because \(CO_2) has an extra Carbon atom compared to oxygen.


Which of the following best explains why equal masses of \(O_2) and \(CO_2) at the same temperate and in the same-size vessel had different pressures? The pressure exerted by the \(O_2) was:

A. less, because there were fewer \(O_2) molecules per gram than there were \(CO_2) molecules per gram

B. less, because there were more \(O_2) molecules per gram than there were \(CO_2) molecules per gram

C. greater, because there were more \(O_2) molecules per gram than there were \(CO_2) molecules per gram

D. greater, because there were more \(O_2) molecules per gram than there were \(CO_2) molecules per gram

Solution: In order to answer this question, you need to use this figure from the passage as well as your outside knowledge.

Figure 2 shows that the pressure exerted by \(O_2) was greater, eliminating answer choices A and B. However, nothing in the passage tells you if there are more \(O_2) molecules per gram or \(CO_2) molecules per gram. You now know that \(O_2) is lighter per molecule than \(CO_2) , so the answer is D.

2. Few Common Elements

The ACT Science section expects you to know the basic molecular structure of sugar, fat, protein, and nucleic acids.

  • Sugar: \(C_6H_1_2O_6) is the basic sugar molecule structure
  • Fat: Know that fats are made up of C (Carbon), H (Hydrogen), and O (Oxygen). To differentiate fats from sugar: fats have nearly twice the number of H as C and a very small number of O. Fats are much bigger in size than sugar. For example, and unsaturated fat triglyceride has a chemical formula of \(C_5_5H_9_8O_6) .
  • Amino acids: Proteins are composed of amino acids. All proteins contain C, H, O and N (Nitrogen).
  • Nucleic acids: Two types of nucleic acids that we already discussed are DNA and RNA. Nucleic acids are made up of 3 parts: a 5-carbon sugar, a phosphate group, and a nitrogenous base. Nucleic acids are different from Sugar, Fat, and Proteins because they are made up of P (Phosphorus) and N in addition to C, H, and O.


In the chemical equation shown in the passage, the carbon in \(CO_2) becomes part of which of the following types of molecules?

A. Fat

B. Sugar

C. Protein

D. Nucleic Acid

Solution: In order to answer this question, you need to look at this equation from the passage:

\(6CO_2+12H_2O + energy -> C_6H_1_2O_6 +6O_2+6H_20\)

You then see that the Carbon from the original \(CO_2) becomes a part of \(C_6H_1_2O_6) . However, you need to know that \(C_6H_1_2O_6) is a sugar molecule to get the correct answer B.

Once again, the ACT expects that you know how photosynthesis works, and what the chemical formula for sugar is! You wouldn’t be able to get this information from the passage.

3. Acids or Basis

This is a fundamental concept for ACT scientific reasoning - it comes up time and time again!

Some substances are acidic, some are basic (alkaline) and some are neutral. We can measure the pH of a compound to a compound to find out which it is.

  • If its pH is between 0-7, we classify it as acidic.
  • If it’s 7-14, it’s alkali or basic
  • If it’s 7, it’s neutral

Acid are usually defined as substances that can give away an H+ ion to other substances.

They react with metals and carbonates, taste sour, conduct electricity and turn litmus paper red. Litmus paper is one type of indicator. An indicator is a substance that changes color depending on the acidity/basicity of the solution it’s in.

Bases, or alkalis, are usually defined as substances that can accept an H+ ion from acids. They also conduct electricity, but they taste bitter, and turn litmus paper blue.

Acids and bases react in a neutralization reaction:

acid + base > ionic compound + water


HCL + Na0H > NaCL + \(H_2O)

4. How Charges Interact

Atoms are composed of three types of particles: protons, electrons, and neutrons. Protons are positively charged, electrons are negatively charged, and neutrons have no charge.

How charges interact

Like charges repel each other while opposite charges attract each other. For example, two positive charges will repel each other while a positive and a negative charge will attract.

5. Nuclear chemistry

Atoms that are either very large or have many neutrons, are unstable. This means that they are likely to decay radioactively. There are three types of decay:

  1. Alpha
  2. Beta
  3. Gamma

Gamma decay usually accompanies the other two types. It’s not possible to predict exactly when an atom will decay. However, atoms are so small that any given time a large amount of them are decaying.

The definition of half-life is the amount of time taken for the substance to decay to half its original amount.

6. Diffusion and Osmosis

Diffusion is the process by which particles spread from an area of low concentration to area of high concentration. It happens in liquids and gases, but not in solids.

Osmosis is the process by which water moves to equalize concentration. Water will move from an area of low concentration to an area of high concentration. Yes, this is the opposite of diffusion, because a high concentration of water is normally referred to as a low concentration of solute.

Substances can enter cells by diffusion (sometimes) and water can leave cells by osmosis.

Comparing two solutions, the more concentrated one is said to be hypertonic. The less concentrated one is said to be hypotonic. If they have the same concentration, they are isotonic.

Thus is a really important concept! If your blood is hypotonic with respect to your cells, you are dehydrated, and water is likely to move out of your cells into your blood. This works the other way around too. This is why IV drips in hospitals are saline solution, not just pure water - they need to be isotonic with your blood.


1. Density

Density is the degree of compactness of a substance. To calculate the density of a substance, you use the formula:

Density = mass/volume

You need to know more about density than just the formula. You need to know the main density rule. Denser objects sink, and less dense objects float. Objects only float when they are less dense than the liquid they are placed in.

An easy way to think about this: what happens when you throw a rock into water? It sinks and that is because the rock is denser than water, meaning it weighs more for the same volume.

What about when you throw a styrofoam cup onto water? It floats because styrofoam is less dense than water. For the same volume, styrofoam weighs a lot less than water.

2. Electricity

Electricity is a property of electrons, and one of the fundamental forces in the universe. A flow of electrons (or in fact any charged particle) is an electric current.

Electrons are negatively charged particles, so when something loses electrons it becomes positively charged. An electric current only happens in a complete circuit, but small amounts of charge can be built up when two surfaces rub together and transfer some electrons.

Here are some definitions you should know:

  • Charge is the difference in number of electrons. If a material gains two electrons, its charge will be - 2. If a material loses two electrons, its charge will be +2.
  • Conductors are low resistance materials that allow the flow of electrons easily, for example, metals such as copper.
  • Insulators are high resistance materials that don’t usually allow the flow of electricity, for example, rubber and plastics.
  • If something is grounded, it means it is attached to a large object through which any excess charge dissipates.
  • Electric current is how much charge is flowing past a point in a given time, that is, the rate of charged particles moving.
  • Voltage is most easily thought of as the energy that the current has (but it’s a little more complicated than this).
  • Resistance is a measure of how freely charge can pass through an object. Insulators have high, or virtually infinite resistance.
  • There are two ways of connecting an electric circuit: in series or in parallel. Current, voltage and resistance work differently in each case.

3. Energy and the laws of motion

The first law of thermodynamics states, in simple terms, that energy can’t be created or destroyed; it can only be changed from one form into another.

In order for this concept to make sense, sometimes we have to consider types of potential energy as well. For example, think about compressing a spring - you use energy to compress it, so the spring must contain that stored energy, which we call elastic potential energy.

If you pick up a pencil, you give it gravitational potential energy, and when you drop it, that energy is transformed into kinetic energy plus a little bit of sound and heat energy when it hits the floor. Here are the types of energy:

  • Light energy (which is really a form of electromagnetic energy)
  • Kinetic energy (the energy of a moving object)
  • Sound energy
  • Heat energy (which is really kinetic energy of the atoms and molecules vibrating)
  • Electrical potential energy (two charged object have potential energy, which is proportional to the distance between them)
  • Chemical potential energy (energy stored within chemical bonds)
  • Elastic potential energy
  • Gravitational potential energy

One concept often tested on the ACT is the conversion of kinetic energy to gravitational potential energy.

4. Kinetic theory

Kinetic theory is a set of rules describing how molecules in fluids (liquids and gases) behave. It can explain many everyday observations, such as why balloons deflate, why you can smell a candle throughout the house, and why water boils faster at higher altitudes.

According to kinetic theory, gases are made up of tiny particles that travel in straight lines until they hit each other. When they hit the walls of the container, they create pressure.

The energy of a gas is usually thermal energy that is translated into kinetic energy of the molecules. Because kinetic energy depends on mass, larger particles in the gas travel more slowly. Chemical reactions may happen if the molecules collide with enough energy to react.

There are three main laws that talk about the behavior of gases. If you think enough about them, they are easy to visualize.

  1. Boyle’s law: pressure in a gas is inversely proportional to volume, at constant temperature. (Take a container of gas, and make the container smaller - this will increase the pressure inside the container.)
  2. Gay-Lussac’s law: pressure is proportional to temperature, at constant volume. (Take a container of gas and increase the temperature - the molecules will move faster, causing more collisions, creating more pressure.)
  3. Charles’ law: volume is proportional to temperature, at constant pressure. (Take a container of gas, heat it, and the volume will increase.)

You don’t need to know any of this specifically, but it is a very good idea to have a brief overview of how gases behave.


The crust is between about 5 - 10 km thick, depending on where you are in the world. Below that is the mantle, which is made of a sort of thick fluid, known as magma. The outer core is liquid iron and nickel, which is the reason why the Earth has a magnetic field. Because these metals are in liquid form, the magnetic field of the Earth moves a bit over time. The inner core is solid iron and nickel.

The crust of the Earth is made of a number of tectonic plates. Because these plates are sitting on the mantle, which moves, they move too. This usually happens gradually, but when it happens suddenly, there is an earthquake.

Some of these plates are moving away from each other, creating gaps in the crust of the Earth. Some are moving towards each other, which creates mountain ranges such as the Himalayas.

In places where the crust of the earth is thin, volcanoes are common. Volcanoes happen when magma is compressed, and rises to the surface of the Earth suddenly.

When the Earth’s crust is thin, water that is piped down through the crust can heat up quite quickly. This can cause natural hot springs, and can also be used to create geothermal energy.

There are three kinds of rock - igneous, sedimentary and metamorphic.

There are many kinds of soil, including sand, clay, silt, peat, etc. Soil contains decayed organic matter: the remains of dead animals, insects and plants that are broken down by bacteria that live in the soil. It also contains very small pieces of rock - essentially sand.

The balance of these components, and the composition of the rocks, gives the overall composition of the soil.