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The Only Actual Science You Need to Know for ACT Science


While you may have been told you need zero science knowledge to answer any ACT Science questions, this is NOT true. There are typically around four questions per test that you cannot answer without previous background knowledge. That's the difference between a 31 and a 36 on the Science section!

In this article, I will give an overview of the 18 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. As far as we can tell, we're the only ones who have taken the time to research this.

If you've gotten bad grades in science classes in school, this article will give you a refresher on the only concepts you need to know.

Reminder: If you haven't already, check out our Ultimate Study Guide for ACT Science. It contains dozens of ACT Science guides going into every question type tested, strategies to tackle the questions, and how you should be organizing your prep to raise your score.

In order to best answer this question of what science you actually need to know for the ACT Science section, I am going to start with what topics the ACT says the Science section covers. According to the ACT makers, "The content of the Science Test includes biology, chemistry, physics, and the Earth/space sciences (for example, geology, astronomy, and meteorology)."


body_ACTSciencetheonlyscienceoverwhelmed.jpgSo much information.


Do You Need to Be a Science Expert to Succeed on ACT Science?

No, and even the ACT makers admit that "Advanced knowledge in these subjects is not required, but background knowledge acquired in general, introductory science courses is needed to answer some of the questions. The test emphasizes scientific reasoning skills over recall of scientific content, skill in mathematics, or reading ability."

In other words, it's more important that you understand the principles behind scientific thought and logical reasoning than the science itself!


Why Don't You Need Expert Knowledge?

In the ACT Science passages, they give you the majority of the information you need to answer the questions. They explain most of the terms within the passage. See for yourself in this example passage, from a free ACT online practice test:




You can see in this passage they give you definitions for most of the terms: refracted, seismograph, focus, shadowzone, the types of seismic waves, the difference between p and s waves. Because of that, you won't have to know tons about geology to answer this question.


How Much Background Knowledge Do You Need to Answer Most ACT Science Questions?

Very little. Most of the questions can be answered using your knowledge of reading graphs and charts. Check out our article on the types of ACT Science Passages for more information about this strategy. For the questions that you can't answer with the visuals, you can usually figure them out by reading the passage.

However, there will be times you can't find the answer in the passage—and that's what we're discussing in this article.


What Information Will Not Be Given in the Passage?

As I mentioned earlier, there are typically around four questions per test that you cannot answer without previous knowledge. In my study of old ACT Science sections, I have found a total of 18 topics spanning biology, chemistry, physics, and math (yes, math) that you need to know. Here is the overview. I'll go into more depth on each topic below.

  • Biology
    • Cell Biology
    • DNA, RNA, and Ribosomes
    • Natural Selection
    • Greenhouse Gases
    • Photosynthesis and Respiration
    • Taxonomic Rank
    • Genetics
  • Chemistry
    • Basic Molecule Structure
    • Freezing/Boiling Point of Water in Celsius
    • pH Scale
    • Molar Mass Concepts
    • How Charges Interact
    • Phase Changes
  • Physics
    • Gravity
    • Density Formula
    • Density Rules
    • Kinetic vs Potential Energy
  • Math
    • Basic Math Skills

The basic overviews that I present below should be all you need to know for the test, but I provide links to more in-depth explanations if you would like to do more reading.

In our first topic, Cell Biology, you'll also see the first example of a question that you wouldn't be able to answer without prior knowledge, even after reading the passage.


Knowledge Subject 1: Biology

Topic 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.


body_actscienceanimalcell.jpgAnimal Cell Structure


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.




Knowing that photosynthesis happens in chloroplasts, I can correctly choose answer F.

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.

While there are many other organelles, the ones listed above and ribosomes (which I will cover in the next topic) should be the only organelles you need to know for the test. Also, you should not need to know any other information about these organelles. If you would like to do more reading on cell biology, click here.


Topic 2: DNA, RNA, Ribosomes, and Protein Synthesis

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


The Process of Protein Synthesis


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.


Topic 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! Click here for more information on evolution by natural selection.


body_actsciencenaturalselection.jpgThe Strong Survive, the Weak Hang On


Topic 4: Greenhouse Gases

Greenhouse gases are gases that trap heat in the atmosphere. Sunlight can pass through them as it goes towards the earth, but greenhouse gases prevent the heat sunlight produces at Earth's surface from leaving the atmosphere. This mean the heat stays close to Earth, increasing the planet's temperature. Human activity has caused the amount of greenhouse gases in the atmosphere to increase in recent decades, and they are a major contributor to climate change.

There are six main greenhouse gases:

  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Nitrous oxide (N2O)
  • Ozone (O3)
  • Water Vapor (H2O)
  • Fluorinated gases (also known as chlorofluorocarbons or CFCs)

Look at this sample question:


The correct answer is is J because methane (CH4), as a greenhouse gas, absorbs heat not when it enters the earth from space, but when it comes up from the earth. This causes warming.


Topic 5: Photosynthesis and Respiration

Photosynthesis is the process where plants capture sunlight to make food for themselves. During photosynthesis, the plant takes in carbon dioxide, water, and energy, and it produces glucose (which it consumes to live) and oxygen (which humans and other animals breathe). You don't need to memorize the formula for photosynthesis, but you may see it on the ACT, and it's useful to understand it ahead of time. This is what it looks like:

6 CO2 + 6 H2O + light energy→ C6H12O6 + 6 O2

Respiration is basically the inverse of photosynthesis. Respiration is when cells break down molecules into a type of energy they can use. While only plants go through photosynthesis, both plants and animals go through respiration. In respiration, glucose and oxygen are converted into carbon dioxide, water, and ATP (a chemical that provides energy to cells). Here's the formula:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP


Topic 6: Taxonomic Rank

Taxonomy is the science of naming and classifying all organisms. It allows scientists to see how closely different organisms are related and what characteristics they share. Taxonomic rank is the seven levels organisms are classified in. Know the seven taxonomic levels which are (from broadest to most specific):



If two species share one level of taxonomic rank, they also share all of the broader taxonomic ranks above it. So, if two species are in the same family, they are also in the same order, class, phylum, and kingdom.


Topic 7: Genetics

The genetics information you need to know for the ACT centers around which traits are passed on to offspring. The key genetic term to know is "allele". Alleles are pairs of genes responsible for particular traits. Allele pairs can be made up of two dominant genes, two recessive genes, or a dominant and recessive gene. Dominant genes are usually expressed as capital letters and recessive genes are expressed as lower-case letters. So an allele pair shown as "Tt" would have one dominant and one recessive allele.

Learn more about genetics here.


Knowledge Subject 2: Chemistry

Topic 8: Basic Molecule Structure

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

C6H12O6 is the basic sugar molecule structure (for more on sugar molecules, click here).

There are many different kids of fats: saturated, unsaturated, trans (you may have seen this on nutrition labels). The test does not expect you to know each structure. You only need to know that fats are made up of C (Carbon), H (Hydrogen), and O (Oxygen), and 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 (more information on fat molecules here). For example, an unsaturated fat triglyceride has a chemical formula of C55H98O6.

Proteins are composed of amino acids (as I mentioned in the above section on protein synthesis, proteins are made based on the original DNA sequence). There are many different protein structures, but all proteins contain C, H, O and N (Nitrogen). Click for more information on proteins and amino acids.

Nucleic acids are biomolecules. Two types of nucleic acids that we already discussed are DNA and RNA. Nucleic acids are made up of three 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. For more information on nucleic acids, click here.

Here is a sample question from the ACT:




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




You then see that the Carbon from the original CO2 becomes a part of C6H12O6. However, you need to know that C6H12O6 is a sugar molecule to get the correct answer G.

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.


Topic 9: Freezing/Boiling Point of Water in Celsius

Water freezes at 0 degrees Celsius and boils at 100 degrees Celsius. That is all you need to know. Memorize those numbers. For more information on freezing and boiling points and temperature scales, click here.


Topic 10: pH Scale

A pH scale is a measure of how acidic or basic a substance is. While the pH scale formally measures the activity of hydrogen ions in a substance or solution, it is typically approximated as the concentration of hydrogen ions.

All you need to know is that a pH of below 7 is acidic, above 7 is basic, and at 7 is considered neutral.

Fun fact: the beverage Coke has a pH of 2.50 while drinking water typically has a pH of 7.00, and hand soap has a pH of around 10. Coke's very acidic!


Topic 11: Molar Mass Concepts

Remember the periodic table? Don't worry—you don't need to memorize molar weights of elements.

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 O2 is lighter per molecule than CO2 because CO2 has an extra Carbon atom compared to oxygen. Check out this ACT Science question:




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 O2 was greater, eliminating answer choices F and G. However, nothing in the passage tells you if there are more O2 molecules per gram or CO2 molecules per gram. You now know that O2 is lighter per molecule than CO2, so ;the answer is J. For more information about O2 versus CO2, click here.


Topic 12: 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.

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.

Learn more about atomic particles here.


Topic 13: Phase Changes

I already mentioned the freezing and boiling point of water in Celsius, but you also need to know the order of phase changes. Below freezing point, a material will be in solid form, just above freezing point a material will be in liquid form, above boiling point, liquid becomes gas (is vaporized).

One natural way to think about this is in terms of water. When it's really cold, it turns to ice (solid). When it warms up, it turns to liquid. Then, when you boil it, it turns to steam (gas). Gases are generally less dense than liquids, and liquids are generally less dense than solids.

For more information on phase changes, click here.


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Knowledge Subject 3: Physics

Topic 14: Gravity

You need to know that gravity is a downward force that acts on objects, and other forces (such as a spring or pulley) can counteract gravity. This will come up a lot in passages that show experiments using springs or pulleys. For more on gravity, click here.


Topic 15: Density Formula

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

Density = mass/volume

For more information on density, mass, and volume, click here.


Topic 16: Density Rules

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's 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.

For more on the main density rule, click here.


Topic 17: Kinetic vs Potential Energy

Energy is the ability to do work. Kinetic energy is energy that results from an object's motion. Some examples are airplanes flying, skiers going downhill, and a car driving along a road. If an object isn't moving, it has no kinetic energy.

Potential energy is energy that results from an object's position or arrangement. Examples include a stretched rubber band, someone sitting at the top of a slide, and a charged battery.

When a stationary object begins to move, its potential energy is converted to kinetic energy.


Knowledge Subject 4: Math

Topic 18: Basic Math Skills

You are not allowed to use your calculator on the science section. Yet, there are problems that require math skills like the one below.



In order to solve this problem, you have to use the given information from the passage that Algol C is a 1.7 solar-mass MS Star. Then, you have to multiply 1.7 by the mass of the sun (solar-mass) given to you in the question, 2.0 x 10^30. You need to be able to do basic multiplication.

1.7 x 2.0 x 10^30 = 3.4 X 10^30, so the answer is C.

In order to refresh your basic math skills, I suggest during your practice to attempt some of the easier ACT Math Section questions (the first 15 questions) without using your calculator.


And that's it! By knowing these concepts, you'll be able to answer any basic science question the ACT throws at you.

If any of these concepts are unfamiliar to you, review them and brush off the cobwebs—you won't need to know the details beyond what's in this article, but it won't hurt to get more attuned to the science topics.


What's Next?

Now that you know the basic science for ACT science, it's time to tackle the rest of the science section. Check out our article about the 3 Types of ACT Science Passages. Learn the big secret of ACT Science and the best way to read ACT Science passages.

Reminder: If you haven't already, check out our Ultimate Study Guide For ACT Science. It contains dozens of ACT Science guides going into every question type tested, strategies to tackle the questions, and how you should be organizing your prep to raise your score.



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Dora Seigel
About the Author

As an SAT/ACT tutor, Dora has guided many students to test prep success. She loves watching students succeed and is committed to helping you get there. Dora received a full-tuition merit based scholarship to University of Southern California. She graduated magna cum laude and scored in the 99th percentile on the ACT. She is also passionate about acting, writing, and photography.

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