Life Science: Cells, Body Systems and Ecology

🧬 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.

Your Secret Weapon: Word Roots

Biology vocabulary is built from a small set of Greek and Latin pieces. Knowing them lets you decode a word you have never seen, which is often enough to pick the answer.

The four big lists you will build

• The organelles and their jobs (Part 2)
• The levels of classification: domain, kingdom, phylum, class, order, family, genus, species (Part 3)
• The body systems and their main organs (Part 4)
• The feeding levels of an ecosystem: producers, consumers, decomposers (Part 5)

Key Fact: If a question asks about the "dermis," you already know it is about skin. If it asks what an "herbivore" eats, the root answers it. Roots turn many "I never learned that" questions into easy points.

Warm-up. 🎯 Three quick questions to see what you already know. Missing some is fine: each one is taught later in the lesson.

Decode the word. 🔽 Use the root table above. You have never needed to memorize these words; the roots give them away.

Key Takeaways — Part 1

• GS is the first CAT-ASVAB subtest: 15 scored questions in 12 minutes, about 48 seconds each
• Life science covers botany, zoology, anatomy and physiology, and ecology
• Most items test one fact or one cause-and-effect step, so memorized tables pay off
• Word roots (bio-, cardi-, hem-, derm-, neur-, photo-, -cide) let you decode unfamiliar terms
• Four lists carry most of the points: organelles, classification levels, body systems, feeding levels

Up Next: Part 2 — cells and the organelles inside them.

🧬 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.

The Organelle Job Chart

Think of a cell as a small factory. Each organelle has one job you should be able to name in a second.

Plant cells vs animal cells

Key Fact: Plants have both chloroplasts and mitochondria. Chloroplasts make sugar; mitochondria then burn that sugar for energy, just as in animal cells. "Plants have chloroplasts instead of mitochondria" is a classic wrong answer.

Moving Materials and Making New Cells

Diffusion and osmosis

• Diffusion: particles spread from where they are more concentrated to where they are less concentrated. No energy is needed. (A drop of food coloring spreading through water.)
• Osmosis: the diffusion of water across a membrane. A wilted stalk of celery placed in fresh water firms up as water moves into its cells.
• Active transport: the cell spends energy to move material from low concentration to high, "uphill."

Mitosis vs meiosis

Human body cells have 46 chromosomes (23 pairs). Cells divide in two different ways:

When a sperm (23) joins an egg (23) at fertilization, the new cell has 46 again.

Worked example

Question: A cell from a sample has a nucleus, a cell wall and chloroplasts. Where did it come from?

Reasoning: A nucleus rules out bacteria. A cell wall plus chloroplasts means a plant. (Fungi have cell walls but no chloroplasts; animals have neither.)

Label the cell. 🔽 Name each numbered structure in Figure 1.

Organelle jobs. 🎯

Count the chromosomes. ✍️ A dog's body cells have $78$ chromosomes.

1) How many chromosomes are in each cell made when a dog skin cell divides by mitosis?

2) How many chromosomes are in each dog egg cell made by meiosis?

3) How many new cells does one round of meiosis produce from one starting cell?

Key Takeaways — Part 2

• Cell theory: all living things are made of cells; the cell is the basic unit of life; cells come from cells
• Prokaryotes (bacteria) have no nucleus; eukaryotes (plants, animals, fungi, protists) do
• Organelle jobs: nucleus = control and DNA, mitochondria = energy, ribosomes = proteins, membrane = gatekeeper, chloroplasts = photosynthesis
• Only plant cells have a cell wall, chloroplasts and a large central vacuole; plants also have mitochondria
• Osmosis = water diffusing across a membrane; diffusion goes from high to low concentration
• Mitosis → 2 identical cells (46 in humans); meiosis → 4 sex cells with half (23)

Up Next: Part 3 — DNA, genetics and how living things are classified.

🧬 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.

Punnett Squares

A Punnett square predicts the chances for offspring. Put one parent's alleles across the top, the other's down the side, and fill each box with one letter from each.

Worked example: two heterozygous parents

In pea plants, tall (T) is dominant over short (t). Cross Tt × Tt:

• Genotypes: 1 TT : 2 Tt : 1 tt
• Phenotypes: 3 tall : 1 short
• Chance of a short plant: $\frac{1}{4} = 25\%$

Worked example: heterozygous × homozygous recessive

Cross Tt × tt:

Two boxes are Tt (tall) and two are tt (short): a 50% chance of each.

The three crosses to know cold

Classifying Living Things

Scientists sort organisms into nested groups, from the broadest to the most specific:

Going down the list, each group holds fewer kinds of organisms that are more closely related. A species is a group that can breed and produce fertile offspring.

Scientific names use two words, the genus then the species: Homo sapiens (humans), Canis lupus (gray wolf). This is called binomial nomenclature.

The kingdoms

Many textbooks use six kingdoms: animals, plants, fungi, protists, and two kingdoms of bacteria (older books combine the bacteria into one, giving five). The fact tested most: mushrooms, molds and yeast are fungi, not plants. Fungi cannot make their own food; they absorb it, often as decomposers.

Animals with backbones (vertebrates)

Animals without backbones are invertebrates: insects (6 legs, 3 body sections), spiders and other arachnids (8 legs, 2 body sections), worms, jellyfish, snails.

DNA and heredity. 🎯

Genetics vocabulary. 🔽

Run the crosses. ✍️ In guinea pigs, black fur (B) is dominant over white fur (b). Enter each answer as a percent (number only).

1) Cross Bb × bb. What percent of offspring are expected to have white fur?

2) Cross BB × bb. What percent of offspring are expected to have black fur?

3) Cross Bb × Bb. What percent of offspring are expected to be homozygous (BB or bb)?

Key Takeaways — Part 3

• DNA is a double helix; bases pair A–T and C–G (RNA uses U instead of T)
• Gene = section of DNA; allele = version of a gene; humans have 46 chromosomes
• Dominant shows with one copy; recessive needs two; genotype = letters, phenotype = what you see
• Punnett results: Aa × Aa → 3 : 1 (25% recessive); Aa × aa → 1 : 1 (50%)
• Females XX, males XY; the father's sperm decides the sex
• Dear King Philip Came Over For Good Soup: domain → species; scientific name = genus + species
• Mushrooms are fungi; whales, bats and dolphins are mammals; spiders (8 legs) are not insects (6 legs)

Up Next: Part 4 — the systems of the human body, nutrition and disease.

🧬 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.

Blood, Digestion and Breathing in Order

The parts of blood

The path of food

$\text{mouth} \rightarrow \text{esophagus} \rightarrow \text{stomach} \rightarrow \text{small intestine} \rightarrow \text{large intestine}$

Digestion starts in the mouth (chewing, and saliva begins breaking down starch). The stomach uses acid to break down protein.

The path of air

$\text{nose or mouth} \rightarrow \text{trachea} \rightarrow \text{bronchi} \rightarrow \text{lungs} \rightarrow \text{alveoli}$

The diaphragm, a dome-shaped muscle under the lungs, contracts to pull air in.

Worked example

Question: A soldier's blood test shows a low platelet count. Which problem is most likely?

Reasoning: Platelets clot the blood. Too few platelets means cuts bleed longer and bruises form easily. (Low red cells would cause tiredness from too little oxygen; low white cells would mean more infections.)

Nutrition and Disease

The six nutrients

Fiber (from plants) is a carbohydrate you can't digest; it keeps digestion moving.

Deficiency diseases

What causes disease

A vaccine contains a weakened, killed or partial form of a germ, so the immune system learns to make antibodies before a real infection arrives.

Key Fact: Antibiotics kill bacteria and do nothing against viruses. That makes "antibiotics cure the flu" one of the most common wrong answers on any science test.

Body systems. 🎯

Match the nutrient fact. 🔽

Count the Calories. ✍️ A snack bar contains $10$ g of fat, $20$ g of carbohydrate and $5$ g of protein.

1) How many Calories come from the fat?

2) How many Calories come from the carbohydrate and protein together?

3) How many Calories are in the whole bar?

Key Takeaways — Part 4

• Know each system's main organs and job: circulatory, respiratory, digestive, nervous, skeletal, muscular, endocrine, excretory, immune, integumentary, reproductive
• Arteries go away from the heart; alveoli exchange gases; kidneys make urine; skin is the largest organ; adults have 206 bones
• Small intestine absorbs most nutrients; large intestine absorbs water; pancreas makes insulin
• Blood: red cells carry O₂ (hemoglobin, iron), white cells fight infection, platelets clot
• Fat 9, carbohydrate 4, protein 4 Calories per gram
• Vitamin C → scurvy, vitamin D → rickets, iron → anemia
• Antibiotics kill bacteria, not viruses; vaccines train the immune system to make antibodies

Up Next: Part 5 — plants, ecosystems and how energy flows through a food chain.

🧬 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.

Ecology: Who Eats Whom

Feeding roles

A food chain is one path of energy (grass → rabbit → fox). A food web is all the connected chains in an ecosystem.

The 10% rule

Only about 10% of the energy at one level is passed to the next. The rest is used for living (movement, body heat) or lost as heat. That is why food chains are short and why there are far fewer hawks than rabbits. Drawn as levels, this makes an energy pyramid: wide at the producers, narrow at the top.

Relationships between species (symbiosis)

Predation (a lion eating a zebra) and competition (two species after the same food) are the other key interactions.

Levels of ecology and the big cycles

$\text{organism} \rightarrow \text{population} \rightarrow \text{community} \rightarrow \text{ecosystem} \rightarrow \text{biome} \rightarrow \text{biosphere}$

• A population is one species in one area; a community is all the species there; an ecosystem adds the nonliving parts (water, soil, air).
• Carbon cycle: photosynthesis removes CO₂ from the air; respiration, decay and burning fuel put it back.
• Nitrogen cycle: air is mostly nitrogen, but plants can't use it directly. Bacteria (many in the root nodules of beans and peas) "fix" nitrogen into a usable form.

Read the pyramid. ✍️ Use the 10% rule with Figure 2.

1) About how many kcal of energy reach the grasshoppers (A)?

2) About how many kcal reach the snakes (B)?

3) What percent of the grass's energy reaches the snakes?

Plants and ecosystems. 🎯

Name the relationship. 🔽

Key Takeaways — Part 5

• Photosynthesis (chloroplasts): CO₂ + water + light → glucose + O₂
• Respiration (mitochondria, in plants and animals): glucose + O₂ → CO₂ + water + energy
• Xylem carries water up; phloem carries food; stomata are leaf pores; transpiration is water loss from leaves
• Producers → primary → secondary → tertiary consumers; decomposers (bacteria, fungi) recycle nutrients
• About 10% of energy passes to each higher level, so pyramids narrow fast
• Mutualism (both helped), commensalism (one helped, one unaffected), parasitism (one harmed)
• Population (one species) → community (all species) → ecosystem (plus nonliving parts); bacteria fix nitrogen

Up Next: Part 6 — test strategy and the traps that wrong answers are built from.

🧬 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.

The Traps Wrong Answers Are Built From

Three Moves for Every Question

1. Find the key word. "Away from the heart" → artery. "Makes its own food" → producer. "No nucleus" → bacteria. "Shows with one copy" → dominant.
2. Decode with roots. Unknown term? Break it apart: hemo- blood, derm- skin, pulmon- lung, photo- light, -cide kill.
3. Watch the direction. Many wrong answers flip the right one: in or out, toward or away, gives off or takes in. Decide the direction first, then look at the choices.

Trap check. 🎯 Each question is built around one of the traps above.

Decode the term. 🔽 You may not have met these words before. The roots give them away.

Key Takeaways — Part 6

• 15 questions in 12 minutes on the CAT: about 48 seconds each; fact questions should take 10–20 seconds
• No going back on the CAT, and don't speed-guess at the end: a run of wrong answers late in a subtest is penalized heavily
• Know the classic traps: plants have both organelles, antibiotics ≠ viruses, the pulmonary artery carries oxygen-poor blood, spiders aren't insects, mushrooms are fungi, the father determines sex
• Three moves: find the key word, decode with roots, check the direction

Up Next: Part 7 — a mixed mastery set across the whole lesson.

🧬 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.

Mastery set. 🎯 Ten mixed questions.

Two quick numbers. ✍️

1) How many chambers does the human heart have?

2) A serving of turkey has $15$ g of protein and no fat or carbohydrate. How many Calories does the protein supply?

Key Takeaways — Part 7

• Cells: ribosomes build proteins; diffusion spreads particles from high to low concentration
• Genetics: two dominant-trait parents with a recessive-trait child are both heterozygous; classification groups are nested
• Body: tendons join muscle to bone; the spinal cord links brain and body; the heart has 4 chambers
• Nutrition: vitamin D → rickets; protein and carbohydrate 4 Calories per gram, fat 9
• Ecology: a population is one species; legume bacteria fix nitrogen; decomposers recycle nutrients

Next step: Take the exit quiz for this topic. A missed question points you back to the part that teaches it.