The expression of the lac structural genes is when lactose is absent from the culture medium and is when lactose is added, because lactose binds to the lac and inactivates it.

The expression of the lac structural genes is LOW when lactose is absent from the culture medium and is HIGH when lactose is added, because lactose binds to the lac REPRESSOR and inactivates it.

Explanation:

Lac operon is expressed only when lactose sugar is available to the bacteria. When lactose is not present, the repressor protein binds to the operator sequence and does not allow RNA polymerase to transcribe the genes. Therefore, the expression of the lac operon is prevented in the absence of lactose. The presence of lactose allows its expression. Under these conditions, allolactose binds to the repressor protein rendering it not able to bind to the operator sequence.

True

Explanation:

During resting potential, the inside of the membrane is more negative compared to the outside. The activation gates of Na+ channels are closed and the membrane is impermeable for sodium ions. When a stimulus is strong enough to fire an action potential, change in the shape of the channel proteins opens the activation gates.  

Now, the gated Na+ channels are opened making the membrane more permeable for sodium ions.  The membrane is permeable for sodium ions for a brief period only when both activation and inactivation gates are open. This allows the entry of sodium ions inside the axon and makes is more positive.