Organization of life: cells, tissues, organisms

Where the answer comes from: the passage says all plant and animal cells share "a cell membrane, cytoplasm, a nucleus and mitochondria," so those structures cannot tell the cells apart. The diagram shows that only Cell Y has a cell wall (the thick outer border) and chloroplasts (the green ovals). Cell X has neither.

Step by step:

1. List what both cells have: cell membrane, cytoplasm, nucleus, mitochondria. These are useless as clues.
2. List what only Cell Y has: cell wall, chloroplasts, one large central vacuole.
3. Cell walls and chloroplasts are found in plant cells, not animal cells. So Cell Y is the leaf cell, and Cell X is the cheek cell.

Answer: Cell Y came from the leaf, because it has a cell wall and chloroplasts.

Tip: when a question asks for evidence, the evidence must actually distinguish the choices. A true fact that applies to both cells (both have a nucleus) is not evidence.

Where the answer comes from: in the table, Sample 1 is the only sample with "No" in the Nucleus row (and also "No" for mitochondria). It is also the smallest, about 2 micrometers wide.

Step by step:

1. Bacteria are prokaryotes: their DNA floats in the cytoplasm instead of sitting inside a nucleus, and they have no membrane-covered organelles such as mitochondria.
2. Scan the Nucleus row: Samples 2, 3 and 4 have one; Sample 1 does not.
3. Samples 2, 3 and 4 are therefore eukaryotes (fungus, plant and animal).

Answer: Sample 1, because it has no nucleus.

Check: bacteria are usually only a few micrometers long, and Sample 1 is the smallest cell (2 micrometers), which fits.

Tip: good evidence must belong to only one choice. A cell wall or a missing chloroplast is shared by several samples, so it cannot single one out.

Where the answer comes from: the stem describes "many similar cells that work together" to do one job (releasing digestive juices). That is the definition of a tissue.

Step by step:

1. Recall the levels: organelle → cell → tissue → organ → organ system → organism.
2. A tissue is a group of similar cells that perform the same function.
3. The whole stomach is an organ because it is built from several tissues (lining, muscle, nerve and connective tissue).
4. The stomach, intestines, liver and other organs together form the digestive system, an organ system.

Answer: a tissue.

Check: the layer is made of similar cells doing one job, so it cannot be an organ, which needs several tissue types.

Tip: ask "How many kinds of cells are involved?" One kind working together = tissue; several tissues = organ; several organs = organ system.

Where the answer comes from: the passage defines osmosis: "water moves from the side with fewer dissolved particles (more water) to the side with more dissolved particles (less water)." It also says "Salt does not easily cross the membrane of a potato cell." The table shows the core in distilled water went from 5.00 g to 5.60 g (+12%).

Step by step:

1. Distilled water has no dissolved particles at all.
2. Potato cells contain dissolved sugars, salts and other substances, so the inside of the cells has more dissolved particles.
3. Water therefore moves into the cells by osmosis, and the core gets heavier.
4. Nothing in distilled water could enter except water, so the extra mass must be water.

Answer: Water moved into the potato cells, where there were more dissolved particles than in the distilled water.

Check: in the 6% salt cup the outside has more dissolved particles, and the core lost mass (−16%), the opposite direction, which fits the same rule.

Tip: in osmosis, follow the water, not the salt: water moves toward the side that is "saltier" (more dissolved particles).

Where the answer comes from: the graph shows 180 of the 200 cells in interphase. The passage says "the fraction of cells found in a stage estimates the fraction of the whole cell cycle that the stage takes."

Step by step:

1. Fraction in interphase: \dfrac{180}{200} = 0.90 = 90\%.
2. So the cells spend about 90% of the cell cycle in interphase, growing and copying DNA, and only about 10% actually dividing.
3. Check the other choices against the graph:
   • Metaphase (4 cells) is not the longest stage of mitosis; prophase (12 cells) is.
   • Anaphase and telophase have 2 cells each, which is half of metaphase's 4, not twice.
   • Nothing in the data tells us whether cells are alive or dead; counting stages says nothing about that.

Answer: The cells spend about 90% of the cell cycle in interphase.

Tip: on data questions, compute a quick fraction or percent and match it to a choice. Reject any choice that brings in an idea the data never measured.

Where the answer comes from: the passage says "a fruit fly body cell has 8 chromosomes, so for a fruit fly 2n = 8." The diagram shows that the four cells at the end of meiosis II each have n chromosomes, half the parent's number.

Step by step:

1. Write what you know: 2n = 8.
2. Solve for n: n = 8 \div 2 = 4.
3. The diagram labels every cell after meiosis I and after meiosis II as n. Meiosis II separates the copies of each chromosome, so it does not halve the number again.

Answer: 4 chromosomes.

Why not 2? It is tempting to halve once in meiosis I and again in meiosis II. But the DNA was copied before meiosis began, so the second division only splits the copies. The diagram confirms this: both rows after meiosis I are labeled n.

Tip: read the labels on a process diagram carefully. They often answer the question if you translate symbols (2n, n) into numbers.

Where the answer comes from: the table pairs each job with a mitochondria percentage. The heart muscle cell, which "contracts nonstop… for life," has the highest value (30%). The liver cell, which "carries out hundreds of chemical reactions," is next (20%). Cells with lighter energy demands (fat storage, 2%) have far less, and the red blood cell has none.

Step by step:

1. Rank the cells by mitochondria: heart 30 > liver 20 > skeletal muscle 5 > fat 2 > red blood cell 0.
2. Match to jobs: the top two do constant, energy-hungry work; the bottom cells store energy or simply carry oxygen.
3. Conclusion: cells that do continuous, energy-demanding work tend to devote more of their volume to mitochondria.
4. Check the others: the table gives no cell sizes; skeletal muscle (5) has less than heart muscle (30); and the passage never says fat cells lack sugar.

Answer: Cells that do continuous, energy-demanding work tend to have more of their volume made of mitochondria.

Tip: "best supported" means the conclusion must follow from the data shown. Watch for choices that sneak in data the table does not contain (like cell size).

Where the answer comes from: the passage ends with the three parts of the cell theory: "all living things are made of one or more cells; the cell is the basic unit of structure and function in living things; and all cells come from pre-existing cells."

Step by step:

1. Part 1 says every living thing is made of one or more cells. So the organism has at least one cell, but it could be single-celled.
2. Part 3 says every cell comes from a cell that existed before it. So its cells came from earlier cells.
3. The theory says nothing about a nucleus: bacteria are living cells without one.
4. Cells forming from minerals in vent water would be spontaneous generation, which Pasteur's experiment argued against.

Answer: It is made of at least one cell, and its cells came from earlier cells.

Check: this statement uses only parts of the theory, so it must be true for any organism, known or new.

Tip: "must be true" questions reward the choice that follows directly from the rule. Choices that add extra details (a nucleus, many cells) go beyond what the rule promises.

Where the answer comes from: the table shows, out of 500 cells each, the number in each stage. Dividing cells (prophase + metaphase + anaphase + telophase):

• Normal skin: 8 + 3 + 2 + 2 = 15
• Skin tumor: 50 + 20 + 10 + 10 = 90

Step by step:

1. Because both samples have 500 cells, the counts can be compared directly: 90 dividing tumor cells vs. 15 dividing normal cells.
2. So a much greater share of tumor cells were dividing, which fits the passage's statement that cancer cells divide "without the controls that normally stop them."
3. Check the other choices: 410 tumor cells were still in interphase, so they do not skip it; 15 normal cells were dividing, so normal skin cells do divide; and chromosome numbers were never counted.

Answer: A greater share of the tumor cells than of the normal skin cells were dividing.

Check: as percents, 15/500 = 3% and 90/500 = 18%; 18% is six times 3%.

Tip: the words never, skip and all are red flags. One counter-example in the table (15 dividing normal cells) is enough to reject them.

Where the answer comes from: the table shows that as side length grows from 1 to 3 cm, the surface area-to-volume ratio falls from 6 : 1 to 3 : 1 to 2 : 1, and the volume turned pink falls from 100% to 88% to 70%. The passage says "the base had soaked in about 0.5 cm from each face" in every cube.

Step by step:

1. The base moved the same distance (0.5 cm) into every cube, so the speed of diffusion did not change.
2. In a bigger cube, more of the volume lies farther than 0.5 cm from any face, so a smaller fraction is reached.
3. The bigger the cube, the smaller its surface area compared with its volume, and the smaller the share that the base reached.

Answer: As cube size increased, the surface area-to-volume ratio decreased and a smaller percent of the volume was reached by the base.

Check: the largest cube (3 cm) has the biggest surface area (54 cm²) but the lowest percent pink (70%), so "more surface area" alone does not mean better supply; the ratio is what matters.

Tip: when a table has several columns, the best conclusion usually links the variable that was changed (size) to the result that was measured (percent pink).

Where the answer comes from: the table shows a gain at 1% salt (+3%) and a loss at 2% salt (−4%).

Step by step:

1. A core keeps the same mass when water moves in and out equally, which happens when the solution has about the same concentration of dissolved particles as the potato cells.
2. The change in mass is positive up to 1% and negative from 2% on. Somewhere between 1% and 2% it must pass through zero.
3. The gain at 1% (+3) and the loss at 2% (−4) are similar in size, so the zero point is near the middle, roughly 1.4%. Any value between 1% and 2% fits.

Answer: between 1% and 2% salt.

Check: at 0% and 1% water moved into the cores (gain); at 2%, 4% and 6% water moved out (loss). Only the gap from 1% to 2% contains the switch.

Tip: to find where a change equals zero, look for the place where the sign changes from + to −. The answer lies between those two rows.

Where the answer comes from: the graph shows 4 cells in metaphase out of 200 total. The passage says the fraction of cells in a stage "estimates the fraction of the whole cell cycle that the stage takes."

Step by step:

1. Fraction in metaphase: \dfrac{4}{200} = 0.02.
2. Time in metaphase: 0.02 \times 24\text{ h} = 0.48\text{ h}.
3. Convert to minutes: 0.48 \times 60 = 28.8\text{ min} \approx 29\text{ min}.

Answer: about 29 minutes.

Why the traps are tempting:

• 4 minutes treats the count of cells as a time.
• 48 minutes reads 0.48 hours as "48 minutes." An hour has 60 minutes, not 100.
• 4.8 hours uses only the 20 dividing cells as the whole: \tfrac{4}{20} \times 24 = 4.8. But the cycle includes interphase, so the whole is all 200 cells.

Tip: in multi-step problems, write the units at every step. Converting decimal hours to minutes (multiply by 60) is a classic GED trap.

Where the answer comes from: the diagram shows that meiosis produces cells with n, half the parent's chromosomes, while mitosis keeps 2n. The passage says that "at fertilization, a sperm and an egg join to form a single cell called a zygote."

Step by step:

1. Human body cells: 2n = 46, so n = 23.
2. Meiosis gives each sperm and each egg 23 chromosomes.
3. Fertilization: 23 + 23 = 46. The zygote has the same number as each parent's body cells.
4. If gametes were made by mitosis, each would have 46, and the zygote would have 46 + 46 = 92. The number would double every generation.

Answer: Meiosis gives each gamete 23 chromosomes, so a sperm and an egg together restore the normal 46 in the zygote.

Why the others fail: choice B gets the numbers wrong (meiosis halves, it does not keep 46). Choice C is false: mitosis happens throughout the body, and the passage never says otherwise. Choice D contradicts the diagram, which says meiosis makes "genetically different" cells.

Tip: for chromosome questions, write the numbers at each step (body cell → gamete → zygote). The math reveals the answer.

Where the answer comes from: the table shows that the cores did not all start at the same mass: some started at 5.00 g and others at 4.00 g.

Step by step:

1. A heavier core has more cells, so it can gain or lose more grams of water even if each cell changes by the same amount.
2. The percent change divides the change by the starting mass, which removes the effect of size: \text{percent change} = \dfrac{\text{final} - \text{start}}{\text{start}} \times 100.
3. Example: the 1% core gained 4.12 - 4.00 = 0.12 g, which is \dfrac{0.12}{4.00} \times 100 = 3\%. A 5.00 g core changing by 3% would gain 0.15 g. In grams these look different, but in percent they are the same.

Answer: The cores started with different masses, and percent change allows a fair comparison between cores of different sizes.

Check: if all cores had started at exactly 5.00 g, grams and percent would rank the cores in the same order, so the starting masses are the reason percent is needed here.

Tip: use percent (or rate per unit) whenever the starting amounts differ. It turns unequal starting points into a fair comparison.

Where the answer comes from: the 1859–1862 entry says Pasteur's boiled broth "stayed free of microbes as long as dust from the air could not reach it, but microbes grew once the neck was broken off."

Step by step:

1. The idea being tested: can living cells (microbes) appear on their own from nonliving broth, or must they come from other cells?
2. Pasteur changed one condition: whether dust (which carries microbes) could reach the broth.
3. Result: no dust, no microbes; once dust could enter, microbes grew. So the microbes came from existing microbes, not from the broth itself.
4. Virchow also stated that cells come from cells, but the timeline describes his entry as a statement, not an experiment. Hooke, Leeuwenhoek and Schwann made observations that support other parts of the theory.

Answer: Pasteur's experiments with broth in bent-neck flasks.

Check: Pasteur's is the only entry with a condition that was changed and a result that was compared, which is what makes it an experiment.

Tip: match the evidence to the exact claim. "All animals are made of cells" is true, but it supports a different part of the theory.

Where the answer comes from: the table gives the dog 78 chromosomes (2n). The passage says meiosis makes gametes with "half that number (n)" and mitosis "keeps the chromosome number the same."

Step by step:

1. Mitosis: one skin cell → 2 cells, each with the same number: 78.
2. Meiosis: one cell → 4 cells (after meiosis I and II), each with half the number: 78 \div 2 = 39.
3. Mitosis copies are identical to the parent; meiosis products are genetically different from each other.

Answer: Mitosis produces 2 cells with 78 chromosomes each; meiosis produces 4 cells with 39 chromosomes each.

Check: a sperm (39) plus an egg (39) gives a zygote with 78, the normal dog number, and the zygote then grows by mitosis, keeping 78 in every body cell ✓

Tip: remember two numbers for each process: mitosis = 2 cells, 2n; meiosis = 4 cells, n.

Where the answer comes from: paragraph 2 traces exported proteins through the cell: ribosomes on the "rough endoplasmic reticulum (ER) build the protein," then "the Golgi apparatus modifies the protein and packages it into small sacs called vesicles," and "Mitochondria supply the ATP for all of this work."

Step by step:

1. The pancreas cell's job is to make and export large amounts of enzyme protein.
2. The organelles in that pathway are the rough ER (with ribosomes) and the Golgi apparatus.
3. Structure matches function: a cell that exports much protein should have much more rough ER and Golgi than a cell that exports little.
4. Reject the others: protein export uses ATP, so fewer mitochondria makes no sense; animal cells such as these have no cell wall or chloroplasts.

Answer: much more rough ER and a larger Golgi apparatus.

Check: the diagram itself shows the rough ER and Golgi taking up a large part of the pancreas cell, which fits its job.

Tip: on structure-function questions, list the cell's main job, then name the organelle that does that job. More of the job → more of that organelle.

Where the answer comes from: the passage says the student "cut five cylinders (cores)" and placed each one in a different salt solution. So there was only one core for each concentration.

Step by step:

1. With one core per concentration, a single odd core (a damaged one, or one from a part of the potato with more sugar) could make that result wrong, and she would not know.
2. Testing several cores at each concentration and averaging their percent changes reduces the effect of any one unusual core.
3. The other choices do not repeat the test: changing the time or the salt in only some cups adds new variables, and skipping the blotting adds error from surface water.

Answer: Test several cores at each salt concentration and average the results.

Check: after the change, the independent variable (salt concentration) and the dependent variable (percent change in mass) stay the same; only the number of trials goes up.

Tip: on the GED, "more reliable" almost always means repeated trials or a larger sample, with everything else kept the same.

Where the answer comes from: three pieces of the stimulus must be combined.

• Paragraph 1: cellular respiration in mitochondria "uses oxygen."
• The table: a red blood cell's job is to carry "oxygen from the lungs to other cells."
• Final paragraph: a mature red blood cell gets "the small amount of energy it needs from a process that breaks down sugar without using oxygen."

Step by step:

1. If a red blood cell had mitochondria, they would use some of the oxygen the cell is supposed to deliver.
2. Without mitochondria, the red blood cell makes its energy without oxygen, so the oxygen it carries reaches other cells.
3. This is a structure–function match: the missing organelle helps the cell do its job.

Answer: Without mitochondria, the cells do not use up the oxygen they are delivering to other cells.

Why the distractors fail:

• B contradicts the table: red blood cells have 0% mitochondria, so they cannot out-produce heart cells, which have 30%.
• C contradicts the passage, which says a red blood cell needs "the small amount of energy it needs."
• D contradicts the table: liver and muscle cells, which do not store fat, have many mitochondria.

Tip: hard GED inference items often require joining two separate pieces of the stimulus. Underline the key verb in each piece ("uses oxygen," "carries oxygen") and look for the link.

Where the answer comes from: the table shows the starting tumor counts: metaphase 20, anaphase 10, telophase 10. The question says cells "cannot move on to anaphase" but cells "already past metaphase finish dividing normally."

Step by step:

1. Cells keep arriving at metaphase from prophase, but none can leave it. The metaphase count goes up.
2. Cells already in anaphase and telophase finish dividing, and no new cells replace them. Those counts go down.
3. The drug acts only on dividing cells, so the overall pattern is a pile-up at metaphase and an empty space after it.

Answer: More cells would be in metaphase, and fewer cells would be in anaphase and telophase.

Check: this is how such drugs are used in real labs: blocking metaphase makes chromosomes easy to count, because many cells are held at that stage.

Tip: for "what would happen if" questions about a process with steps, find the blocked step: things build up before or at the block and run out after it.

Where the answer comes from: the passage says that in a cell "materials such as oxygen and nutrients enter through the surface (the cell membrane), but every part of the cell's volume needs them." The table shows that bigger cubes have a smaller surface area-to-volume ratio and that a smaller share of their volume is reached (100% → 88% → 70%).

Step by step:

1. Treat each cube as a model cell and the base as oxygen or nutrients.
2. In a large cell, the surface is small compared with the volume, so materials cannot reach the center quickly enough.
3. Many small cells each keep a high surface area-to-volume ratio, so every part of every cell gets supplied.
4. An elephant grows large by adding more cells, not by making each cell enormous.

Answer: Small cells have a large surface area compared with their volume, so materials can reach all parts of each cell quickly.

Check: an elephant's cells are about the same size as a mouse's cells; the elephant simply has many more of them, which fits this explanation.

Tip: when a passage uses a model (agar cubes), apply the model's result to the real thing it stands for (cells).