A Level Biology Practice Exam

What is plasma primarily composed of?

1. White blood cells and platelets

2. Red blood cells and platelet fragments

3. A yellow fluid containing many dissolved substances

4. Only oxygen and carbon dioxide

The correct answer is: A yellow fluid containing many dissolved substances

Plasma is the liquid component of blood and is primarily composed of a yellowish fluid that contains a variety of dissolved substances. This includes water, which makes up about 90% of plasma, along with proteins (such as albumins, globulins, and fibrinogen), electrolytes, nutrients, hormones, gases, and waste products. The protein content plays crucial roles in maintaining osmotic pressure, transporting substances, and facilitating immune responses. The other choices do not accurately describe the primary composition of plasma. While white blood cells and platelets are essential components of blood, they are not found in plasma; they are cellular components that are suspended in it. Red blood cells are also cellular components and are responsible for transporting oxygen, but they are not part of plasma composition since they are not dissolved in the fluid. Additionally, only including oxygen and carbon dioxide in the description of plasma is too limited since plasma carries a much broader range of substances. Thus, the selection emphasizing the diverse dissolved constituents accurately reflects the primary composition of plasma.