How a cell is built, fed, and copied
TEAS 7 outline: Cell structure, function and organization
Also covers HESI A2 Biology (The cell)
- Only eukaryotic cells have a nucleus and membrane-bound organelles.
- Mitochondria make ATP; ribosomes build proteins.
- Plant cells add a cell wall, chloroplasts, and a central vacuole.
- Water moves toward more solute; active transport uses energy, usually ATP.
- Mitosis makes two identical cells; meiosis makes four different haploid cells.
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Between divisions, the cell is in interphase. Its chromosomes are long, thin threads inside the nucleus. This simplified cell has four chromosomes; a human body cell has 46.
Key facts
- All cells have a plasma membrane, cytoplasm, DNA, and ribosomes; only eukaryotic cells have a nucleus and membrane-bound organelles.
- Mitochondria make ATP and ribosomes build proteins; the rough ER makes proteins for export, the Golgi apparatus modifies them, and vesicles carry them.
- Plant cells have a cellulose cell wall, chloroplasts, and a large central vacuole; animal cells have a centrosome and lysosomes, which most plant cells lack.
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- In osmosis, water moves toward the side with more dissolved solute: a red blood cell swells and can burst in a hypotonic solution and shrinks in a hypertonic one.
- Active transport uses energy, usually ATP, to move substances against their concentration gradient; the sodium-potassium pump moves 3 Na+ out for every 2 K+ in.
- Mitosis produces two genetically identical cells; meiosis produces four genetically different haploid cells, so a human egg or sperm carries 23 chromosomes.
- Homologous chromosomes pair up and cross over in prophase I and separate in meiosis I; sister chromatids separate in mitosis and in meiosis II.
Quick check1 question · 2 more at the end
Which event takes place during meiosis I but not during mitosis?
Correct answer: C.
In prophase I of meiosis, each pair of homologous chromosomes (one from each parent) pairs up tightly, and nonsister chromatids swap segments by crossing over; the pairs then separate in meiosis I. Mitosis never pairs homologs, so it produces identical cells, while crossing over helps make every gamete genetically different. Lining up at the middle, chromatids separating, and nuclear envelopes reforming all happen in mitosis too.
A Sister chromatids separate in mitosis and in meiosis II; in meiosis I they stay joined while the homologous pairs separate.
B Chromosomes line up on the metaphase plate in mitosis as well as in meiosis, so this is not unique to meiosis I.
C Correct: pairing of homologous chromosomes and crossing over happen only in prophase I of meiosis.
D Nuclear envelopes re-form in telophase of mitosis too, so this does not distinguish meiosis I.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Prokaryotic cells (bacteria and archaea) have no nucleus or membrane-bound organelles; eukaryotic cells do. The nucleus stores DNA, ribosomes build proteins, and mitochondria make ATP. Plant cells add a cell wall, chloroplasts, and a central vacuole. Diffusion and osmosis need no energy, but active transport uses ATP. Mitosis makes two identical cells; meiosis makes four different haploid cells.
1 What is the difference between prokaryotic and eukaryotic cells?Every cell, no matter how simple, has four things: a plasma membrane around it, cytoplasm inside, DNA as its instructions, and ribosomes to build proteins. 1 min
Every cell, no matter how simple, has four things: a plasma membrane around it, cytoplasm inside, DNA as its instructions, and ribosomes to build proteins.
Prokaryotic cells (bacteria and archaea) stop there. They have no nucleus and no membrane-bound organelles. Their DNA sits in a central region called the nucleoid, and most bacteria have a cell wall. They are also tiny, about 0.1 to 5.0 micrometers across.
Eukaryotic cells (animals, plants, fungi, protists) are roughly 10 to 100 micrometers across. They keep their DNA inside a nucleus and divide their work among organelles, small compartments wrapped in membranes, each with its own job.
2 What does each organelle do?A protein that a cell exports follows an assembly line. 1 min
| Organelle | Main job |
|---|---|
| Nucleus | Stores DNA; its double envelope has pores that control what goes in and out |
| Nucleolus (inside the nucleus) | Assembles ribosome subunits from rRNA and proteins |
| Ribosomes | Build proteins; float free in the cytoplasm or sit on the rough ER |
| Rough ER | Studded with ribosomes; makes membrane proteins and proteins for export |
| Smooth ER | Lacks the ribosomes of rough ER; main site of lipid synthesis |
| Golgi apparatus | Stack of flat membranes that modifies proteins and lipids made in the ER |
| Mitochondria | Make ATP, the cell's main energy molecule; have their own DNA |
| Lysosomes | Digest large molecules, worn-out organelles, and pathogens |
| Vesicles | Small membrane sacs that carry cargo between organelles |
A protein that a cell exports follows an assembly line. Ribosomes on the rough ER build it, and a vesicle carries it to the Golgi. The Golgi modifies it, and another vesicle fuses with the plasma membrane to release it outside the cell.
3 How are plant and animal cells different?Both are eukaryotic, and both have mitochondria. 1 min
Both are eukaryotic, and both have mitochondria. Plant cells add three things: a cell wall made mainly of cellulose, chloroplasts for photosynthesis, and a large central vacuole that holds water. Animal cells have a centrosome (with a pair of centrioles) and lysosomes, which most plant cells lack.
4 How do substances cross the cell membrane?The plasma membrane is a phospholipid bilayer with proteins embedded in it. 1 min
The plasma membrane is a phospholipid bilayer with proteins embedded in it. Passive transport across it uses no cell energy. Substances simply move down their concentration gradient, from high to low.
- Simple diffusion: small uncharged molecules such as oxygen and carbon dioxide slip straight through the membrane.
- Facilitated diffusion: ions and polar molecules still move from high to low, but need a channel or carrier protein.
- Osmosis: the diffusion of water across a membrane. Water moves toward the side with more dissolved solute (and therefore less free water).
The solution around a cell decides which way water goes:
- Hypotonic (less solute outside): water rushes in. A red blood cell swells and can burst. A plant cell becomes firm (turgid) because its wall stops it from bursting.
- Hypertonic (more solute outside): water leaves. A red blood cell shrinks. In a plant cell, the membrane pulls away from the rigid wall (plasmolysis), so the plant wilts.
- Isotonic: there is no net change.
Active transport moves substances against their gradient, from low to high, so it costs energy, usually ATP. The classic example is the sodium-potassium pump, which pushes 3 sodium ions out of the cell for every 2 potassium ions it brings in. If a cell runs out of ATP, pumps like this stop, but diffusion and osmosis keep going.
5 How are mitosis and meiosis different?Before either kind of division, the cell copies its DNA during the S phase of interphase, so each chromosome becomes two identical sister chromatids. 1 min
Before either kind of division, the cell copies its DNA during the S phase of interphase, so each chromosome becomes two identical sister chromatids.
Mitosis turns one cell into two identical copies. Its phases are:
- Prophase: chromosomes condense and the nuclear envelope breaks down. Some books split off a prometaphase when spindle fibers attach.
- Metaphase: chromosomes line up at the middle.
- Anaphase: sister chromatids pull apart to opposite poles.
- Telophase: new nuclear envelopes form.
Cytokinesis then splits the cytoplasm: a pinching cleavage furrow in animal cells, a new cell plate in plant cells.
Meiosis makes gametes (eggs and sperm). It has two divisions after one round of copying:
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells produced | 2 | 4 |
| Chromosome sets | Same as parent (diploid) | Half (haploid) |
| Genetically | Identical | All different |
| Unique events | None | Homologs pair and cross over (prophase I) |
In meiosis I, homologous chromosomes (one from mom, one from dad) pair up, swap segments by crossing over, and then separate. In meiosis II, sister chromatids separate, much like mitosis. Humans have 23 chromosome pairs, so each egg or sperm ends up with 23 chromosomes.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Prokaryotes lack a nucleus, not DNA or ribosomes; their DNA sits in a region called the nucleoid.
Watch out: In osmosis water moves toward the higher solute concentration, not away from it.
Watch out: A hypertonic solution makes a cell shrink, not swell; the prefix describes the solution outside, which has more solute.
Watch out: Plant cells have mitochondria too; chloroplasts are an addition, not a replacement.
Watch out: Meiosis I separates homologous pairs, not sister chromatids; sister chromatids split in meiosis II.
How to remember it
Memory hook: "Water follows salt." Whichever side has more dissolved solute pulls the water toward it. And "meiosis makes me": it makes the gametes that make a new person.
Self-check:
- Name the four things every cell has, then the one thing only eukaryotes have.
- A red blood cell is dropped into pure water. Say which way water moves and what happens to the cell.
- Explain which division separates homologous pairs and which separates sister chromatids.