OCR GCSE Biology Practice Exam 2025 – Comprehensive All-in-One Guide to Exam Success

Transmitter substances, also known as neurotransmitters, are stored in specialized structures called vesicles within neurons. These vesicles are small, membrane-bound sacs that compartmentalize the neurotransmitters, preventing them from diffusing and becoming inactive before they are released. When a neuron is stimulated to send a signal (an action potential), these vesicles move to the presynaptic membrane and release their contents into the synaptic cleft, allowing the neurotransmitters to bind to receptors on the postsynaptic neuron. This precise storage mechanism is critical for the efficient and controlled transmission of nerve impulses, ensuring that signals are effectively communicated between neurons. The other options do not accurately describe the way neurotransmitters are stored; for instance, fluid sacs and cell membranes do not provide the required environment for the storage and release process, and protein chains represent a completely different biological structure.