Biology and Life Science: Practice Questions & Explanations

The cell is the basic structural and functional unit of all living organisms. Cell theory (one of biology's foundational principles): (1) All living things are composed of one or more cells; (2) The cell is the basic unit of life; (3) All cells come from pre-existing cells. Levels of biological organization (smallest to largest): atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biome → biosphere. Cell types: (1) Prokaryotic — bacteria and archaea; no membrane-bound nucleus; smaller, simpler; circular DNA in nucleoid; (2) Eukaryotic — plants, animals, fungi, protists; membrane-bound nucleus containing DNA in chromosomes; membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts in plants). Major cell structures: cell membrane (boundary, selectively permeable), cytoplasm (interior), nucleus (control center, DNA), mitochondria (powerhouse, ATP production), ribosomes (protein synthesis). Plant cells additionally have: cell wall (rigid, cellulose), chloroplasts (photosynthesis), large central vacuole.

Photosynthesis occurs in plants, algae, and some bacteria, converting light energy into chemical energy stored in glucose. Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Plants take in CO₂ through stomata in leaves; absorb water through roots; capture light using chlorophyll in chloroplasts (green pigment); produce glucose (food) and release oxygen as byproduct. Two main stages: (1) Light-dependent reactions — occur in thylakoid membranes; produce ATP and NADPH; split water releasing O₂; (2) Light-independent reactions (Calvin cycle) — occur in stroma; use ATP and NADPH to fix CO₂ into glucose. Photosynthesis is the foundation of nearly all food chains; provides oxygen for aerobic respiration. Cellular respiration is essentially the reverse: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Plants do BOTH photosynthesis (during light) and respiration (all the time). Net effect during day: photosynthesis dominates, plants produce O₂ and consume CO₂. At night: only respiration, plants consume O₂ and produce CO₂.

Arteries carry blood AWAY from the heart (think: A for Away, A for Artery). Veins carry blood TOWARD the heart. Capillaries are tiny vessels between arteries and veins where exchange of gases, nutrients, and waste occurs. Arterial vs venous characteristics: (1) Arteries — thick muscular walls (withstand high pressure), elastic, carry oxygenated blood (except pulmonary arteries which carry deoxygenated blood from heart to lungs), no valves; (2) Veins — thinner walls, lower pressure, carry deoxygenated blood (except pulmonary veins carrying oxygenated blood from lungs to heart), have valves preventing backflow; (3) Capillaries — single-cell thick walls allow diffusion of gases, nutrients, waste between blood and tissues. Circulation paths: Systemic circulation — heart pumps oxygenated blood from left ventricle through aorta to body, returns deoxygenated through superior/inferior vena cava to right atrium. Pulmonary circulation — right ventricle pumps deoxygenated blood through pulmonary arteries to lungs, returns oxygenated through pulmonary veins to left atrium. Heart chambers: right atrium, right ventricle (pulmonary), left atrium, left ventricle (systemic).

DNA (deoxyribonucleic acid) is the genetic material in all known living organisms. Structure: (1) Double helix — two strands wound around each other; (2) Nucleotides — building blocks; each contains a sugar (deoxyribose), phosphate group, and one of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); (3) Base pairing — A always pairs with T (2 hydrogen bonds); C always pairs with G (3 hydrogen bonds); this complementary pairing allows accurate replication; (4) Sugar-phosphate backbone — sides of the 'ladder'; bases are the 'rungs'. DNA structure discovered by Watson, Crick, Franklin, Wilkins (1953). DNA → genes → chromosomes: a gene is a DNA sequence coding for a protein; genes are organized into chromosomes; humans have 46 chromosomes (23 pairs). The Central Dogma: DNA → RNA → protein. (1) Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus; (2) Translation: mRNA is read by ribosomes; transfer RNAs (tRNAs) bring amino acids; amino acids link to form proteins. Codons: three-letter mRNA sequences each coding for one amino acid (e.g., AUG = methionine/start). DNA replication: occurs before cell division; semi-conservative (each new molecule has one old and one new strand); enzymes include helicase (unwinds), DNA polymerase (adds nucleotides). Mutations: changes in DNA sequence (point mutations, insertions, deletions); can be harmful, neutral, or beneficial.

Mitosis is cell division producing two genetically identical diploid daughter cells. Phases: (1) Prophase — chromosomes condense, nuclear envelope breaks down, spindle fibers form; (2) Metaphase — chromosomes line up at cell equator; (3) Anaphase — sister chromatids separate, pulled to opposite poles; (4) Telophase — nuclear envelopes reform around each set; (5) Cytokinesis — cytoplasm divides, two cells separate. Mnemonic: PMAT (Prophase, Metaphase, Anaphase, Telophase). Uses of mitosis: growth (single fertilized egg → trillions of cells); tissue repair (healing wounds, replacing dead cells); asexual reproduction (in some organisms). Versus MEIOSIS: produces four genetically different haploid gametes (sperm, eggs); occurs in reproductive cells only; involves two divisions and genetic recombination through crossing over. Meiosis introduces genetic variation; mitosis preserves it. Cell cycle: G1 (growth) → S (DNA replication) → G2 (preparation) → M (mitosis). G0 — non-dividing state for cells like neurons. Cell cycle regulation involves checkpoints; failure can lead to cancer (uncontrolled cell division). Tumor suppressor genes (p53) and oncogenes regulate the cycle; mutations disrupt control.

Mitochondria produce ATP (cellular energy) through aerobic respiration — the powerhouse of the cell.

O negative is the universal donor for red blood cells — it lacks A, B, and Rh antigens, making it compatible with all blood types.

DNA (Deoxyribonucleic Acid) is the molecule that contains the GENETIC INSTRUCTIONS for every living organism. Located primarily in the NUCLEUS of cells. STRUCTURE: Double helix — two strands wound around each other; each strand made of nucleotides with four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C). BASE PAIRING: A pairs with T; G pairs with C. FUNCTION: Stores instructions for making proteins (via transcription → RNA → translation → protein); copied during cell division so each daughter cell receives a complete copy; mutations in DNA can cause cancer or heritable diseases. GENOME: Complete DNA sequence of an organism — human genome has ~3 billion base pairs.

The MITOCHONDRIA are the 'powerhouse of the cell' — they produce energy in the form of ATP through cellular respiration. ASVAB General Science tests basic biology. Other organelles: NUCLEUS (contains DNA, controls the cell); RIBOSOMES (make proteins); CELL MEMBRANE (controls what enters/exits); CHLOROPLASTS (in plant cells, do photosynthesis). The mitochondria-as-powerhouse fact is one of the most commonly tested cell biology points.

WHITE BLOOD CELLS (leukocytes) are primarily responsible for FIGHTING INFECTION as part of the immune system. ASVAB General Science covers the circulatory/immune system. Blood components: RED BLOOD CELLS (carry oxygen via hemoglobin); WHITE BLOOD CELLS (fight infection/immunity); PLATELETS (help blood clot); PLASMA (the liquid that carries cells and nutrients). Knowing the function of each blood component — especially white blood cells fighting infection — is commonly tested.

PHOTOSYNTHESIS is the process by which plants (using chlorophyll in chloroplasts) convert SUNLIGHT, WATER, and CARBON DIOXIDE into GLUCOSE (food/energy) and release OXYGEN. ASVAB General Science covers this key biological process. Equation concept: CO2 + H2O + light energy → glucose + O2. Photosynthesis is the basis of most food chains and produces the oxygen we breathe. CELLULAR RESPIRATION is roughly the reverse (using glucose and oxygen to release energy). Photosynthesis is one of the most commonly tested biology topics.

Humans INHALE OXYGEN (needed for cellular respiration to produce energy) and EXHALE CARBON DIOXIDE (a waste product of respiration). ASVAB General Science covers the respiratory system. In the lungs, oxygen from inhaled air passes into the blood, and carbon dioxide passes out of the blood to be exhaled (gas exchange in the alveoli). This is essentially the opposite of plant photosynthesis (which uses CO2 and releases O2). The oxygen-in, carbon-dioxide-out cycle is fundamental, commonly tested biology.

The mitochondria are often called the 'powerhouse of the cell' because they produce most of the cell's energy in the form of ATP through cellular respiration. The nucleus contains the cell's DNA and controls its activities; ribosomes build proteins; and the cell membrane controls what enters and leaves the cell. Knowing the main organelles and their functions is core General Science content. Mitochondria convert nutrients and oxygen into usable energy, which is why cells with high energy needs (like muscle cells) contain many of them.

Photosynthesis is the process by which plants (and some other organisms) use sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen. It occurs mainly in the chloroplasts, which contain the green pigment chlorophyll that captures light energy. Respiration is almost the reverse process — using oxygen to release energy from glucose. Photosynthesis is fundamental to life because it produces oxygen and forms the base of most food chains. The basic equation is: carbon dioxide + water + light energy → glucose + oxygen.

DNA (deoxyribonucleic acid) carries the genetic instructions that determine an organism's traits and guide the building of proteins. It is found in the nucleus of cells and is organized into structures called chromosomes. DNA's famous double-helix shape stores information in sequences of four bases. Proteins are built according to DNA's instructions but do not store the genetic code themselves; glucose is an energy sugar; and water is essential to life but carries no genetic code. DNA as the molecule of heredity is fundamental biology content.