LET Elementary Biological Science Reviewer 2026

Answer: B. All cells come from pre-existing cells.

Step 1: Recall the three statements of Cell Theory: (1) All living things are made of one or more cells. (2) The cell is the basic unit of structure and function. (3) All cells come from pre-existing cells. Step 2: The third statement was contributed by Rudolf Virchow. Step 3: Option A is wrong because only plant cells and some other organisms have chloroplasts. Option C is wrong because animal cells lack a cell wall. Option D is wrong because most body cells use mitosis, not meiosis. Step 4: Therefore, the correct answer is that all cells come from pre-existing cells.

Answer: C. Central vacuole; it stores water and keeps the plant cell firm (turgid).

Step 1: Plant cells have a large central vacuole that can occupy up to 90% of the cell's interior. Step 2: Its main role is to store water, nutrients, and waste products, and to maintain turgor pressure that keeps the plant firm. Step 3: Lysosomes are small and mainly found in animal cells for waste digestion. Step 4: Mitochondria are oval-shaped structures involved in energy production, not the large fluid-filled structure described. Step 5: The Golgi apparatus is a stack of flat membranes, not a large fluid-filled sac.

Answer: B. Prokaryotic cells lack a membrane-bound nucleus; eukaryotic cells have one.

Step 1: The key structural difference between prokaryotes and eukaryotes is the presence or absence of a true, membrane-bound nucleus. Step 2: Prokaryotes (like bacteria) have their DNA floating freely in the cytoplasm — they have no nuclear envelope. Step 3: Eukaryotes (animals, plants, fungi, protists) have their DNA enclosed in a membrane-bound nucleus. Step 4: Mitochondria are found in eukaryotic cells, not prokaryotic cells. Step 5: Prokaryotic cells are actually smaller than eukaryotic cells, not larger.

Answer: B. Water will move out of the cell, causing it to shrink (crenate).

Step 1: The solution described has a higher solute (salt) concentration outside the cell — this is called a hypertonic solution. Step 2: Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. Step 3: Since the salt concentration is higher outside, water moves out of the cell to equalize concentrations. Step 4: As water leaves, the red blood cell shrinks — this process is called crenation. Step 5: Option A describes what happens in a hypotonic (low solute) solution, and Option C is incorrect because osmosis moves water, not solutes, across the membrane.

Answer: B. 46 chromosomes

Step 1: Mitosis is a type of cell division used for growth and repair of body (somatic) cells. Step 2: The purpose of mitosis is to produce two daughter cells that are genetically identical to the parent cell. Step 3: Before mitosis begins, DNA is replicated so the cell has a full copy to give to each daughter cell. Step 4: After mitosis and cytokinesis, each daughter cell ends up with the same number of chromosomes as the original parent cell — in this case, 46 (diploid, 2n). Step 5: Meiosis (not mitosis) reduces the chromosome number to 23 (haploid) to form gametes.

Answer: B. Four haploid (n) daughter cells that are genetically varied.

Step 1: Meiosis involves two rounds of cell division (Meiosis I and Meiosis II), resulting in four daughter cells. Step 2: Each daughter cell is haploid (n), meaning it has half the chromosome number of the original cell — 23 chromosomes in humans instead of 46. Step 3: Genetic variation arises during meiosis through a process called crossing over during Prophase I, where homologous chromosomes exchange segments of DNA. Step 4: This genetic variation is essential because it gives natural selection something to act upon in evolution. Step 5: Option A describes mitosis; Options C and D describe incorrect combinations of cell number, ploidy, and genetic outcome.

Answer: C. Adenine pairs with Thymine; Guanine pairs with Cytosine.

Step 1: DNA is a double-stranded molecule shaped like a twisted ladder (double helix). Step 2: The 'rungs' of the ladder are pairs of nitrogen bases held together by hydrogen bonds. Step 3: The base pairing rule in DNA is strict: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). A memory aid: 'AT' sounds like 'AT' as in 'at the market'; GC sounds like 'GC' as in 'good chemistry.' Step 4: Option D is incorrect because Uracil replaces Thymine only in RNA, not in DNA. Step 5: Correct base pairing allows DNA to replicate accurately and pass genetic information faithfully to daughter cells.

Answer: B. 3 tall : 1 short

Step 1: Set up the Punnett square for Tt x Tt. Step 2: The four possible offspring genotypes are: TT, Tt, Tt, tt. Step 3: Genotypic ratio = 1 TT : 2 Tt : 1 tt. Step 4: Since T (tall) is dominant over t (short), both TT and Tt plants will appear tall. Only tt plants will appear short. Step 5: This gives a phenotypic ratio of 3 tall : 1 short — a classic Mendelian result. Option A (1:1) is the result of a test cross (Tt x tt). Option D describes incomplete dominance, not simple dominance.

Answer: B. Heterozygous genotype; tall phenotype.

Step 1: Genotype refers to the allele combination a plant carries — Tt has two different alleles (one dominant T, one recessive t). Step 2: An organism with two different alleles is called heterozygous. Step 3: Phenotype refers to the physical appearance — since T (tall) is dominant, it masks the effect of t (short). Step 4: Therefore, the Tt plant looks tall even though it carries the recessive allele. Step 5: Homozygous means having two identical alleles (TT or tt). A tt plant would show the short phenotype because both alleles are recessive.

Answer: B. Individuals with heritable traits better suited to the environment survive and reproduce more.

Step 1: Darwin's theory of natural selection rests on four observations: overproduction (more offspring are born than can survive), variation (individuals differ), heritability (traits are passed on), and differential survival (better-suited individuals survive and reproduce more). Step 2: Option A describes Lamarck's incorrect theory of inheritance of acquired characteristics — for example, giraffes stretching their necks and passing longer necks to offspring. Step 3: Darwin's key point is that the variation must be heritable (based on genes), not acquired during a lifetime. Step 4: Option C is wrong because variation within a population is the foundation of natural selection. Step 5: Evolution does not always trend toward complexity — it trends toward fitness in a given environment.

Answer: A. Cell → Tissue → Organ → Organ System → Organism

Step 1: The cell is the basic unit of life – the smallest living unit. Step 2: Similar cells group together to form a tissue (e.g., muscle tissue). Step 3: Different tissues working together form an organ (e.g., the heart). Step 4: Related organs cooperate as an organ system (e.g., the circulatory system). Step 5: All organ systems together make up the complete organism. The other options disrupt this correct order, which is a standard LET exam sequence.

Answer: B. Respiratory system

Step 1: Gas exchange means swapping oxygen (O₂) for carbon dioxide (CO₂). Step 2: The lungs, trachea, and diaphragm make up the respiratory system. Step 3: When you breathe in, oxygen enters the lungs and moves into the blood; when you breathe out, CO₂ leaves the blood and exits through the lungs. Step 4: The circulatory system then delivers oxygen to the rest of the body – it transports, it does not exchange gases. Step 5: The excretory system removes liquid waste, and the digestive system processes food – neither performs gas exchange.