Quiz 9 — The Cell | Look Again

The Guide

The smallest thing that is alive

Cut a leaf, a liver, or a loaf of bread mold into smaller and smaller pieces and you eventually reach something that cannot be divided again without killing it. That is a cell. Everything alive is either one cell or a great many.

Cell theory, in three lines

Every living thing is made of one or more cells.
The cell is the smallest unit that carries out life.
Cells come from cells that already exist.

The third line took the longest to establish and is the one that mattered most. For centuries people assumed that living things could form on their own from non-living material. Settling that question is what the passage in this quiz is about.

Two kinds of cell

Every cell has an outer membrane, a jelly inside called cytoplasm, and DNA. The difference is where the DNA sits.

In a prokaryotic cell the DNA floats loose in the cytoplasm. There are no separate compartments inside. Bacteria are the example, and they are small, simple, and by far the most numerous cells on Earth.

In a eukaryotic cell the DNA is packed inside a nucleus, and the cell contains a set of enclosed compartments, each doing a particular job. Plants, animals, fungi, and the single-celled organism from Quiz 8 are all built this way.

The parts, and what they do

Cell membrane: The outer boundary of every cell. It decides what gets in and what gets out. Quiz 10 is entirely about this one.
Cytoplasm: The jelly filling the cell. Everything else sits in it, and most of the cell's chemistry happens there.
Nucleus: Holds the DNA and directs the cell's work. Only in eukaryotic cells.
Mitochondrion: Releases energy from sugar so the rest of the cell can use it. Plural: mitochondria. Quiz 12 comes back to this.
Ribosome: Builds proteins. Tiny, and there are thousands of them. Found in both kinds of cell.
Cell wall: A stiff outer layer, outside the membrane, in plant cells and bacteria. It holds the cell's shape. Animal cells have none.
Chloroplast: Uses sunlight to make sugar. Green, and the reason plants are. Plant cells only.
Large vacuole: A big sac of water that presses outward and keeps the plant firm. When it empties, the plant wilts. Plant cells have one large one; animal cells may have several small ones.

The pair worth separating now. Cell wall and cell membrane are not two names for the same thing. The membrane is thin, flexible, alive, and present in every cell. The wall is stiff, sits outside the membrane, and is found in plants, fungi, and most bacteria, and in some other one-celled living things, but never in animal cells. A plant cell has both. An animal cell has just the membrane. Mixing these two costs points every year.

On the factory comparison. You will hear the cell described as a factory, with the nucleus as the manager and the mitochondria as the power plant. It helps at first and then it misleads, because a factory is designed and run from the top, and a cell is not. Nothing gives the orders. The parts work because of chemistry, not instruction. Use the comparison to remember the names, then drop it.

Worked Examples

Two questions, worked through

Example 1. A cell is examined and found to have a membrane, cytoplasm, ribosomes, and DNA lying loose with no nucleus around it. What kind of cell is it?

Work down the list rather than guessing from the total. Membrane, cytoplasm, ribosomes, and DNA are in every cell, so those four tell you nothing about which kind. The one useful detail is the last: the DNA is loose.

A prokaryotic cell — a bacterium. Most of what you are told about a cell is shared by all cells. Look for the one feature that only some cells have.

Example 2. Yeast is a fungus. A yeast cell has a nucleus, mitochondria, and a cell wall, but no chloroplasts. Is it a plant cell?

The nucleus and mitochondria make it eukaryotic. The cell wall is tempting, since plants have walls — but so do bacteria and fungi, so a wall on its own does not make something a plant. The missing chloroplasts settle it: without them the cell cannot make its own food from sunlight.

Not a plant cell. One shared feature is not evidence; a feature that only one group has is. This is the same reasoning as Quiz 5, applied to a cell instead of a study.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 5

1.Which structure is drawn in both the animal cell and the plant cell?

ACell wall
BChloroplast
CLarge vacuole
DNucleus

2.Which structure does the plant cell have that the animal cell does not?

AChloroplast
BCytoplasm
CNucleus
DRibosome

3.What is the job of the nucleus?

AIt breaks down worn-out parts of the cell.
BIt controls what enters and leaves the cell.
CIt holds the DNA and directs the cell's work.
DIt releases energy from sugar for the cell.

4.How does a cell wall differ from a cell membrane?

AThe cell wall lies inside the cell membrane, not outside it.
BThe cell wall is stiff and the membrane is flexible.
CThe cell wall and the membrane do the same job.
DAnimal cells have a cell wall but no membrane.

5.A student finds an unfamiliar cell with a cell wall, chloroplasts, and one large vacuole. What can she conclude?

AIt is a bacterial cell, since bacteria have walls.
BIt is an animal cell that has been damaged.
CIt cannot be identified from these three parts.
DIt is a plant cell, since those three go together.

Table — questions 6 to 8

	Prokaryotic cell (a bacterium)	Eukaryotic cell (a plant or animal)
Cell membrane	Yes	Yes
Cytoplasm	Yes	Yes
DNA	Yes, loose in the cytoplasm	Yes, inside a nucleus
Nucleus	No	Yes
Enclosed compartments	No	Yes
Typical width	1 to 5 micrometers	10 to 100 micrometers

6.According to the table, what does a bacterial cell lack?

ACell membrane
BCytoplasm
CDNA
DNucleus

7.A cell is found with its DNA loose in the cytoplasm and no enclosed compartments. Which is it?

AEukaryotic cell
BPlant cell
CProkaryotic cell
DVirus

8.According to the table, what do the two kinds of cell have in common?

ABoth have a membrane, cytoplasm, and DNA.
BBoth keep their DNA inside a nucleus.
CBoth carry out photosynthesis for energy.
DBoth are surrounded by a rigid cell wall.

A word the passage needs. In an experiment, the control group is the part that is left alone, so there is something to compare the changed part against.

Passage — questions 9 and 10

For centuries most people believed that living things could form on their own out of non-living material — that maggots arose from meat and mold from damp cloth.

In the 1660s Francesco Redi filled several jars with meat. He sealed some with fine cloth that air could pass through but flies could not, and left the others open. Maggots appeared in the open jars. In the covered jars none appeared, though the meat rotted just the same.

Two hundred years later Louis Pasteur boiled broth in flasks shaped so that air could reach the liquid but airborne dust could not. Nothing grew. When he tipped a flask so dust reached the broth, growth appeared within days.

9.Which line of cell theory do these two experiments support?

AAll living things are made of one or more cells.
BCells come from cells that already exist.
CThe cell is the basic unit of life's structure.
DCells are too small to be seen without a lens.

10.In Redi's experiment, which jars served as the control group?

AThe covered jars, since covering them was the new treatment.
BBoth sets of jars together, since both were observed.
CThe uncovered jars, since they were left as they were.
DNeither set, since no control is possible outdoors.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. D — nucleus. It is drawn in both cells and it is the only one of the four that is. The other three carry gold outlines, and the note at the bottom says what gold means. The gold convention is doing real work here. Once you know it, three of the four options rule themselves out without your having to remember any biology.

2. A — chloroplast. Gold outline, drawn only in the plant cell. Chloroplasts capture sunlight, which is why plants can feed themselves and animals cannot. B, C, and D are in both cells on the page. A part that everything has cannot be what makes one thing different from another.

3. C — holds the DNA and directs the cell's work. The nucleus is the container for the DNA, and the DNA carries the instructions the cell runs on. B is the cell membrane's job and D is the mitochondrion's. Both are correct sentences about the cell, attached to the wrong part. Wrong answers on this topic are rarely false statements — they are true statements filed under the wrong name.

4. B — the wall is stiff, the membrane is flexible. Look at the plant cell in the diagram: the gold wall is the outer rectangle and the dark membrane is the line just inside it. Two separate layers, drawn separately on purpose. A reverses them. In the diagram the wall is plainly outside, and the order matters — the wall is built by the cell from the inside out, so the membrane has to come first.

5. D — a plant cell. Any one of those three parts on its own would leave room for doubt. All three together do not. A is the good objection, and it is the one worked example 2 was about: bacteria do have cell walls, so a wall by itself proves nothing. What settles it is the chloroplasts and the vacuole arriving alongside it. This is the Quiz 5 habit — one shared feature is weak, several together are strong.

6. D — nucleus. Read across the nucleus row: no for the bacterium, yes for the eukaryote. It is the only row where the bacterium says no to something the other cell has, apart from the compartments. C is the classic error. Bacteria certainly have DNA — the table says so. What they do not have is a nucleus to keep it in. No nucleus is not the same as no DNA.

7. C — a prokaryotic cell. Loose DNA and no compartments are the two rows that define the left-hand column. D would be right only if this were not a cell at all. A virus, for example, has no membrane and no cytoplasm of its own. This one has a membrane, cytoplasm and DNA, so whatever it is, it is a cell.

8. A — membrane, cytoplasm, and DNA. Three rows read yes and yes straight across. Those three are what makes something a cell at all. B is true of only one column and D of neither as stated. Reading a comparison table means reading across the rows, not down one column and guessing at the other.

9. B — cells come from cells that already exist. Both experiments tested whether living things appear where none were before. When nothing could get in, nothing appeared. When something could, it did. A and C are also lines of cell theory and also true, which is what makes them work as distractors. Neither experiment was about what living things are made of or how small the unit is. Match the evidence to the claim it actually tests.

10. C — the uncovered jars. The control group is the one left alone, so there is something to compare against. Redi changed one thing — the cloth — and left the rest as they were. A has it backward, and the giveaway is inside the option itself: it calls the covered jars the treatment and then calls them the control. This is the same reasoning about controls that Quiz 2 teaches, used on a biology experiment. The reasoning does not change when the subject does.

Your Score

What the number means

8 to 10	Solid. Check topic 9 on your map and go on to Quiz 10, on the membrane.
6 or 7	Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewer	Worth another pass. Draw both cells from memory and label them — badly drawn is fine. The naming sticks through the hand faster than through the eye.

Misses on 1, 2, or 6 mean the diagram or the table was read too fast. Misses on 3 or 4 are the naming, and drawing the cells by hand fixes that better than rereading. A miss on 5, 9, or 10 is the useful kind: each of those asks you to reason rather than recall, and item 10 is a Part I question that happens to be set in a biology laboratory.

The rooms that walk the same ground

Two of the Life Science rooms cover what this quiz covers, by hand. Two Kinds of Cells puts a bacterium beside an animal cell at true scale, lays out the three domains of life, and sets a plant cell beside an animal one. Made to Order takes you inside one cell to learn its parts by name and by job, and then has you make a protein and ship it out, the way the guide above describes it. The rooms teach the naming; this quiz asks the test’s kind of question about it.

The plate for this quiz

The gallery image is a page from Robert Hooke's Micrographia, 1665 — a slice of cork seen through a microscope made to his own design, drawn as a field of small empty boxes. He called them cells, from the Latin word for a small room, and the name stuck to every living thing on Earth.

What he was looking at were the dead walls of cells whose contents were long gone. He named the container, not the life inside it, and the word we still use is a description of an empty room. Public domain by age, and available at high resolution.