Cell biology: practice questions with explanations

1What a cell is

The cell is the smallest unit of life. The cell theory says that all living things are made of cells, and that every cell comes from an existing cell.

There are two main types:

• Prokaryotic cells, such as bacteria, are small and simple. Their DNA lies loose in the cell; they have no nucleus and no other membrane-bound organelles.
• Eukaryotic cells, such as those of animals, plants and fungi, are larger. Their DNA is kept in a nucleus, and they have many organelles: small compartments with their own job.

Plant cells have three things animal cells lack: a cell wall of cellulose, chloroplasts for photosynthesis, and a large central vacuole.

Think of a eukaryotic cell as a factory. The nucleus is the office with the plans, the ribosomes are the machines, the mitochondria are the power plant, and the membrane is the gate.

2Organelles and the membrane

• 1 Cell membrane: a phospholipid bilayer with proteins. It controls what enters and leaves.
• 2 Nucleus: holds the DNA. The nucleolus inside it makes ribosomes.
• 3 Rough ER: a membrane network covered with ribosomes, which make proteins. The smooth ER, without ribosomes, makes lipids.
• 4 Golgi apparatus: modifies, sorts and packages proteins into vesicles.
• 5 Mitochondrion: makes most of the cell’s ATP, its energy currency, by aerobic respiration.
• 6 Lysosome: breaks down worn-out parts and engulfed material with enzymes.

Substances cross the membrane in different ways:

• Diffusion: particles move from high to low concentration, without energy. Small non-polar molecules such as O₂ and CO₂ cross this way.
• Osmosis: water moves through the membrane towards the side with more dissolved particles.
• Facilitated diffusion: diffusion through a channel or carrier protein, still without energy. Glucose enters most cells this way.
• Active transport: a pump uses ATP to move substances against their gradient, such as the sodium–potassium pump.

3DNA, proteins and cell division

DNA is a double helix of two strands of nucleotides. Each nucleotide has a sugar (deoxyribose), a phosphate and one of four bases. The bases pair: A with T, and C with G.

Before a cell divides, it copies its DNA. Replication is semi-conservative: each new double helix has one old strand and one new strand.

A gene is turned into a protein in two steps:

1. Transcription in the nucleus: RNA polymerase copies the gene into mRNA. RNA has uracil (U) instead of thymine.
2. Translation at a ribosome: every three bases of mRNA, a codon, code for one amino acid. tRNA brings the amino acids, and the chain folds into a protein.

Cells divide in two ways:

• Mitosis: one cell gives two genetically identical cells with the full set of chromosomes (46 in humans). Used for growth and repair.
• Meiosis: one diploid cell gives four genetically different haploid cells with half the chromosomes (23). Used to make eggs and sperm. Crossing over and independent assortment create the variation.

The phases of mitosis are prophase, metaphase, anaphase (the chromatids are pulled apart) and telophase, followed by cytokinesis.

4The method step by step

Most questions test one of four themes: structure, transport, the flow of information (DNA → RNA → protein), or division.

1. Find the theme of the question and the key word: an organelle, a type of transport, an enzyme, a phase.
2. Recall the function that belongs to it, in one sentence.
3. For a process, follow the steps in order: where it starts, which structures it passes, where it ends.
4. For a case, ask what changes: which structure or step is broken, and what follows from that.
5. Eliminate options that mix up similar terms: mitosis and meiosis, transcription and translation, diffusion and active transport.

5A worked example

Follow every step. Try to predict the next one before you open it.

6Common mistakes

• Mixing up mitosis and meiosis. Mitosis: 2 identical diploid cells. Meiosis: 4 different haploid cells.
• Mixing up transcription (DNA → mRNA, in the nucleus) and translation (mRNA → protein, at the ribosome).
• Thinking prokaryotes have no DNA. They have DNA, but no nucleus.
• Thinking osmosis moves solute. Osmosis is the movement of water.
• Calling facilitated diffusion active. It uses a protein but no energy, and goes down the gradient.
• Pairing A with U in DNA. In DNA, A pairs with T; U appears only in RNA.

7How exam questions look

• Easy: key terms and facts. The function of an organelle, base pairing, what diffusion or mitosis is, plant versus animal cells.
• Medium: applying the concepts. Osmosis in a given solution, the phases of mitosis and meiosis, codons and mutations, glycolysis and respiration.
• Hard: cases and analysis. Classic experiments (Meselson–Stahl, Hershey–Chase), diseases explained at the cell level, cell signalling, the cell cycle and cancer.

In a case question, name the structure or process that fails first. The correct option is the direct consequence of that failure.