🧬 Life Science: Cells, Body Systems and Ecology

Part 1 of 7 — Life Science on the ASVAB

Topic Overview

Where This Fits on the Test

General Science (GS) is the first subtest of the CAT-ASVAB. It counts toward several service line scores (for example, the Air Force Electronics composite and the Navy and Marine Corps Electronics composites all include GS), so a strong GS score opens technical and medical jobs.

On the CAT that is about 48 seconds per question. Some questions may be unscored tryout items, and you can't tell which ones, so treat every question as if it counts.

What "life science" covers

The official description of GS names life science as one of its three areas, next to earth and space science and physical science. Life science covers four fields:

Key Fact: Life science items are short: one fact, or one cause-and-effect step. You don't need a biology degree. You need a compact set of facts you can recall in a few seconds, and this lesson gives you that set in tables built to memorize.

🧬 Life Science: Cells, Body Systems and Ecology

Part 2 of 7 — Cells and Organelles

The Cell Theory

Every living thing is built from cells. Biology rests on three statements, known together as the cell theory:

1. All living things are made of one or more cells.
2. The cell is the basic unit of life (the smallest thing that is alive).
3. All cells come from existing cells.

Two kinds of cells

Memory hook: pro sounds like "no" (no nucleus); eu sounds like "do" (do have one).

Levels of organization

$\text{cell} \rightarrow \text{tissue} \rightarrow \text{organ} \rightarrow \text{organ system} \rightarrow \text{organism}$

Muscle cells form muscle tissue; tissues form an organ such as the heart; the heart and blood vessels form the circulatory system; all the systems together make the organism.

🧬 Life Science: Cells, Body Systems and Ecology

Part 3 of 7 — Genetics, DNA and Classification

DNA: The Instruction Manual

DNA (deoxyribonucleic acid) carries the instructions for building and running an organism. It is shaped like a twisted ladder, a double helix. The rungs are pairs of four bases, and the pairing never changes:

Memory hook: Apple in the Tree, Car in the Garage. (RNA, DNA's working copy, uses Uracil in place of T.)

From DNA to traits

Sex chromosomes: females are XX and males are XY. Every egg carries an X, so the father's sperm (X or Y) decides the child's sex.

Key Fact: "Dominant" does not mean "most common." It only means the allele shows up when one copy is present. Some dominant conditions are rare.

🧬 Life Science: Cells, Body Systems and Ecology

Part 4 of 7 — Human Body Systems, Nutrition and Health

The Body Systems Chart

Anatomy questions are the single biggest group of life science items. Learn this chart and you can answer most of them.

Organ facts that get tested

• Heart: four chambers (two atria on top receive blood, two ventricles below pump it out). The left ventricle pumps blood to the whole body, so it has the thickest wall.
• Arteries carry blood away from the heart (A for Away); veins return it; capillaries are the tiny vessels where exchange happens.
• Alveoli: tiny air sacs in the lungs where oxygen enters the blood and carbon dioxide leaves.
• Small intestine: where most digestion finishes and most nutrients are absorbed. The large intestine mainly absorbs water.
• Liver: makes bile (stored in the gallbladder) to break up fats, and cleans toxins from the blood.
• Pancreas: makes digestive enzymes and the hormone insulin, which lowers blood sugar.
• Muscle and bone links: tendons join muscle to bone; ligaments join bone to bone.

🧬 Life Science: Cells, Body Systems and Ecology

Part 5 of 7 — Plants, Ecology and Energy Flow

Photosynthesis and Respiration: Mirror Images

Photosynthesis happens in the chloroplasts of plants (and algae). The green pigment chlorophyll captures light energy:

$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}$

$6\,CO_2 + 6\,H_2O \rightarrow C_6H_{12}O_6 + 6\,O_2$

Cellular respiration happens in the mitochondria of plants and animals. It runs the same reaction backward to release the energy stored in sugar:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy}$

Plant parts

Loss of water vapor through the stomata is transpiration. Plant growth toward light is phototropism; roots growing downward with gravity is gravitropism.

Memory hook: xylem carries water ("x marks the water"), phloem carries food.

🧬 Life Science: Cells, Body Systems and Ecology

Part 6 of 7 — Test Strategy and Common Traps

Your Clock on GS

On the CAT-ASVAB, the time and questions remaining show in the lower-right corner of the screen.

How the computer test treats you

• No going back. You can't return to a question once you answer it, so give each one your real best answer before moving on.
• Adaptive. The first question is of average difficulty. A correct answer usually brings a harder question; a wrong one, an easier question. Getting a hard question is a sign you are doing well.
• Tryout questions may be mixed in, and you can't tell which, so treat every question as scored.
• Don't speed-guess at the end. Unfinished questions are scored as if you had guessed at random, and the official guidance says the CAT applies a relatively large penalty when several wrong answers come toward the end of a subtest. If time is short, keep reading and genuinely answering.
• Paper test: answer every question; there is no penalty for guessing.

Key Fact: A life science fact question should take 10–20 seconds if you know it. Save the rest of your 48-second average for questions that need a step of reasoning, like a Punnett square or a food-chain energy question.

🧬 Life Science: Cells, Body Systems and Ecology

Part 7 of 7 — Mixed Mastery Set

How to Use This Set

These questions mix every part of the lesson, in the same short, single-fact or one-step style the CAT-ASVAB uses. Treat it like the real thing:

• Aim for about 48 seconds per question, and most should take far less.
• Commit to an answer before you look at the explanation; on the CAT you can't go back.
• When you miss one, note which part it came from and reread that part's table.

Key Fact: A score of 10 or more out of 12 means you are ready for life science on test day. Below that, the Key Takeaways at the end of each part are your fastest review.