The one difference to remember: Rough ER (RER) has ribosomes stuck on its surface, so it makes proteins. Smooth ER (SER) has no ribosomes (that is why it looks smooth), so it makes lipids and, in animal cells, steroid hormones. Memory hook: "Rough = Ribosomes = pRotein; Smooth = no dots = Lipids."
Rough ER carries ribosomes (dots) on its surface and makes proteins; smooth ER has no ribosomes and makes lipids and steroid hormones.
Your doubts, answered
Why is one ER called rough and the other smooth?
It is only about ribosomes. Rough ER (RER) has ribosomes attached to its outer surface. Under a microscope these ribosomes look like tiny dots, so the membrane looks rough or bumpy. Smooth ER (SER) has NO ribosomes on it, so its surface looks plain and smooth. The name comes from the appearance, and the appearance comes from whether ribosomes are present.
What is the main function of rough ER vs smooth ER?
Rough ER makes proteins, because it carries ribosomes and ribosomes are the protein factories. These proteins are often sent out of the cell (secreted). Smooth ER is the major site for making lipids (fats). In animal cells, SER also makes lipid-like steroid hormones. Simple line: RER = proteins, SER = lipids.
Does smooth ER have ribosomes or not?
No. Smooth ER has NO ribosomes. This is a very common NEET trap. If a question says 'SER has ribosomes attached', that statement is WRONG. Only rough ER (RER) bears ribosomes. This is why SER cannot make proteins for secretion and instead makes lipids.
Which ER makes steroid hormones?
The smooth ER (SER) makes steroid hormones in animal cells. NCERT says: 'In animal cells lipid-like steroidal hormones are synthesised in SER.' So cells that make hormones like testosterone or oestrogen (for example cells of the adrenal gland or testis) have a lot of smooth ER.
Are both RER and SER part of the endomembrane system?
Yes. The endomembrane system includes the endoplasmic reticulum (both RER and SER), Golgi complex, lysosomes and vacuoles. Their jobs are linked together. Note: mitochondria, chloroplasts and peroxisomes are NOT part of the endomembrane system because their functions are not coordinated with these.
Do prokaryotes have RER or SER?
No. Prokaryotic cells (like bacteria) lack the endoplasmic reticulum completely. They do not have any membrane-bound organelles, so they have neither RER nor SER. A NEET statement like 'in prokaryotes only RER is present' is INCORRECT.
⚠️ The NEET trap ✗ Rough ER makes lipids / phospholipid synthesis happens in rough ER. ✓ Lipid and phospholipid synthesis happens in SMOOTH ER. Rough ER handles protein synthesis, signal peptide cleavage, glycosylation and folding. 🧠 If you see 'lipid' or 'phospholipid', point to SMOOTH ER. If you see 'protein' or 'ribosome', point to ROUGH ER. NEET 2018 asked exactly this: phospholipid synthesis does NOT occur in RER.
Real NEET questions
NEET 2018
Which of the following events does not occur in rough endoplasmic reticulum?
A · Cleavage of signal peptide
B · Protein glycosylation
C · Protein folding
D · Phospholipid synthesis ✓
Solution: Rough ER bears ribosomes and handles protein synthesis and its follow-up steps: signal peptide cleavage, glycosylation and folding. Phospholipid (lipid) synthesis is the job of the SMOOTH ER, not the rough ER. So the event that does NOT occur in RER is phospholipid synthesis, option (D).
NEET 2022
Which of the following statements with respect to Endoplasmic Reticulum is incorrect?
A · RER has ribosomes attached to ER
B · SER is devoid of ribosomes
C · In prokaryotes only RER are present ✓
D · SER are the sites for lipid synthesis
Solution: Prokaryotic cells lack membrane-bound organelles, so they have NO endoplasmic reticulum at all, neither RER nor SER. So 'in prokaryotes only RER are present' is the incorrect statement (C). The other three are correct: RER bears ribosomes, SER lacks ribosomes, and SER is the major site of lipid synthesis.
ReNEET 2026
Smooth endoplasmic reticulum ___________.
A · has ribosomes attached to its surface
B · is the major site for the synthesis of lipids ✓
C · is actively involved in protein synthesis
D · is a site for the synthesis of carbohydrates
Solution: Smooth ER lacks ribosomes (that is why it is 'smooth') and is the major site for the synthesis of lipids and steroidal hormones. Protein synthesis happens on the rough ER, and carbohydrate (sugar) synthesis happens in chloroplasts. So the correct answer is (B).
Solved Cell: The Unit of Life NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the single biggest difference between RER and SER?
Ribosomes. RER has ribosomes on its surface and makes proteins. SER has no ribosomes and makes lipids and steroid hormones.
Which ER is more common in protein-secreting cells?
Rough ER. Cells that make and export a lot of protein, such as pancreatic cells or antibody-making plasma cells, have very extensive rough ER because RER is loaded with ribosomes.
Can SER make proteins?
No. SER has no ribosomes, so it cannot make proteins for secretion. It specialises in lipid synthesis and steroid hormone synthesis in animal cells.
Is the endoplasmic reticulum single or double membrane?
The ER is a network of tiny tube-like and sheet-like membranes made of a single membrane. Do not confuse it with double-membrane organelles like mitochondria or the nucleus.
Why does this matter for NEET?
NEET repeatedly asks which ER makes lipids versus proteins, and whether SER has ribosomes. It has appeared in 2018, 2022 and 2026. Getting the RER = protein, SER = lipid rule right is easy marks.
