OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which organelle is responsible for protein synthesis in both animal and plant cells?

1. Nucleus

2. Cytoplasm

3. Ribosomes

4. Mitochondria

The correct answer is: Ribosomes

The ribosomes are the organelles that are primarily responsible for protein synthesis in both animal and plant cells. They play a crucial role in translating messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins. Ribosomes can be found free-floating in the cytoplasm or attached to the endoplasmic reticulum, contributing to the production of proteins that are either secreted from the cell or incorporated into cellular membranes. While the nucleus is essential for storing genetic information and directing protein synthesis via transcription of DNA to mRNA, it does not carry out the actual process of synthesizing proteins. The cytoplasm provides a medium where ribosomes can operate, but it is the ribosomes themselves that perform the synthesis. Mitochondria are involved in energy production through respiration, but they do not synthesize proteins in the way ribosomes do. Understanding this distinction highlights the specific function of ribosomes as the site of protein synthesis in all living cells.