Final answer:
The demand curve for a purely competitive firm is perfectly elastic, allowing it to sell any quantity at the market price, while a pure monopolist faces a downsloping demand curve, meaning it can sell more only by lowering the price.
Step-by-step explanation:
The demand curve facing a firm depends on the market structure it operates in. For a purely competitive firm, the demand curve is perfectly elastic, signifying that the firm can sell any quantity it desires at the prevailing market price without influencing the price.
This is because each firm is a price taker due to the product being homogeneous and the presence of many sellers in the market. On the other hand, a pure monopolist faces a downsloping demand curve, which is the market demand curve and indicates that the monopolist can sell more only by reducing the price.