Which of the following is not true of a chemical synapse?

Options

A. Signals can move in either direction across the synapse
B. Neurotransmitter molecules are removed from the synaptic cleft after they bind with receptor molecules
C. A minimum number of neurotransmitter-receptor complexes must form before the impulse is propagated in the postsynaptic neuron
D. Neurotransmitter secreted is not the same for all synapses

Explanation

To determine which statement is not true of a chemical synapse, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Chemical Synapses**:
- A chemical synapse is a junction between two neurons, where the axon terminal of one neuron (presynaptic neuron) meets the dendrite of another neuron (postsynaptic neuron). They are separated by a small gap known as the synaptic cleft.

2. **Analyzing the Options**:
- **Option 1**: "Signals can move in either direction across the synapse."
- This statement is **not true**. In a chemical synapse, signals are transmitted in one direction only, from the presynaptic neuron to the postsynaptic neuron. Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.

- **Option 2**: "Neurotransmitter molecules are removed from the synaptic cleft after they bind with receptor molecules."
- This statement is **true**. After neurotransmitters bind to their receptors, they are typically removed from the synaptic cleft to terminate the signal and prevent continuous stimulation of the postsynaptic neuron.

- **Option 3**: "A minimum number of neurotransmitter-receptor complexes must form before the impulse is propagated in the postsynaptic neuron."
- This statement is **true**. For an action potential to be initiated in the postsynaptic neuron, a certain threshold of neurotransmitter-receptor binding must occur.

- **Option 4**: "Neurotransmitter secreted is not the same for all synapses."
- This statement is **true**. Different synapses can release different types of neurotransmitters, which are specific to the types of receptors present on the postsynaptic neuron.

3. **Conclusion**:
- The statement that is **not true** of a chemical synapse is: "Signals can move in either direction across the synapse." This is incorrect because the transmission of signals in chemical synapses is unidirectional.

### Final Answer:
The statement that is not true of a chemical synapse is: **"Signals can move in either direction across the synapse."**

---