8. A molecule of hemoglobin __ 1. is shaped like a biconcave disk packed almost entirely with iron 2. contains four glycoprotein units studded with oxygen 3. consists of four globin proteins, each bound to a molecule of heme 4. can carry up to 120 molecules of oxygen

## Step 1<br />Hemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the rest of the body. It is not shaped like a biconcave disk packed almost entirely with iron, so option 1 is incorrect.<br /><br />## Step 2<br />Hemoglobin does not contain four glycoprotein units studded with oxygen. Instead, it consists of four globin proteins, each bound to a molecule of heme. This makes option 2 incorrect.<br /><br />## Step 3<br />Hemoglobin consists of four globin proteins, each bound to a molecule of heme. This is the correct description of the structure of hemoglobin, making option 3 correct.<br /><br />## Step 4<br />Hemoglobin can carry up to four molecules of oxygen, not 120. This makes option 4 incorrect.