Question 1
A student examining a cell under an electron microscope notices a network of interconnected membranous tubules with no ribosomes attached to their surface. Which function is most likely carried out by this organelle?No clue? Show me the answer
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Method #1Approach 1Step 1: Identify the organelle described
The question describes a network of membranous tubules with no ribosomes on the surface. This is the key distinguishing feature of the smooth endoplasmic reticulum (sER).
Step 2: Recall the function of the smooth ER
The smooth ER is responsible for lipid (phospholipid) synthesis, detoxification of harmful substances (e.g., drugs and alcohol), and calcium ion storage. It does NOT synthesise proteins for secretion — that is the role of the rough ER.
Step 3: Match function to the correct answer
The option stating 'Synthesis of phospholipids and detoxification of drugs' matches the known functions of the smooth ER. This is the correct answer.
Method #2Approach 2Step 1: What is being asked?
We need to identify the function of a membrane-bound tubular organelle without ribosomes on its surface.
Step 2: Eliminate 'Synthesis of proteins destined for secretion'
Protein synthesis for secretion is carried out by the rough ER, which is studded with ribosomes. The structure described has no ribosomes, so this option is eliminated.
Step 3: Eliminate 'Packaging of proteins into vesicles for export'
Packaging and vesicle formation is the role of the Golgi apparatus, which appears as curved, stacked cisternae — not tubules without ribosomes. This option is eliminated.
Step 4: Eliminate 'Hydrolysis of damaged proteins and organelles'
Hydrolytic digestion is performed by lysosomes, which are small spherical vesicles containing digestive enzymes. This does not match the description of a tubular membrane network.
Step 5: Select the correct answer
The remaining option — 'Synthesis of phospholipids and detoxification of drugs' — correctly describes the smooth ER, which is the organelle matching the description provided.
Question 2
In eukaryotic cells, DNA is organised into repeating structural units that allow compaction within the nucleus. Each unit consists of DNA wound around a core of proteins. What is this repeating unit called?No clue? Show me the answer
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Choose a solution method
Method #1Approach 1Step 1: Identify the concept being described
The question describes the basic repeating unit of chromatin in eukaryotes — DNA wrapped around a protein core. This structure allows the very long DNA molecule to be compacted into the nucleus.
Step 2: Recall the structure of chromatin
In eukaryotes, DNA is wound around histone proteins to form a nucleosome — the fundamental unit of chromatin packaging. Multiple nucleosomes connected by linker DNA resemble 'beads on a string' when viewed under an electron microscope.
Step 3: Select the correct answer
The correct term is nucleosome. This is distinct from a ribosome (protein synthesis), a spliceosome (RNA splicing), or a centromere (chromosome attachment during division).
Method #2Approach 2Step 1: What is being asked?
We must name the repeating unit of DNA packaging in eukaryotic chromatin, where DNA winds around a protein core.
Step 2: Eliminate 'Spliceosome'
A spliceosome is a complex involved in pre-mRNA splicing (removing introns). It has nothing to do with DNA packaging or chromatin structure.
Step 3: Eliminate 'Centromere'
A centromere is the region of a chromosome where spindle fibres attach during cell division. It is not a repeating structural unit of DNA packaging.
Step 4: Eliminate 'Ribosome'
A ribosome is the site of protein synthesis (translation). It is not related to the structural organisation of DNA in the nucleus.
Step 5: Select the correct answer
Nucleosome is the correct answer. It is the repeating unit of chromatin, consisting of DNA wound around eight histone proteins.