William W. answered • 01/26/22
AP Biology teacher with molecular genetics experience
A. At the simplest level, the reactions of photosynthesis take in carbon dioxide from and release oxygen into the atmosphere and the reactions of cellular respiration consume oxygen and release carbon dioxide. This creates a cycle. It's actually a little more involved than it first seems. The oxygen released by plants is not the same oxygen in the carbon dioxide molecules they just took in. The carbon dioxide taken in is incorporated into the sugars produced by photosynthesis. Those sugars get used by the plant (via cell respiration) for food or get eaten by an animal for food. Cell respiration (in either type of organism) releases some of the oxygen in those sugars as carbon dioxide which can be taken in by plants to build more sugars.
B. Plants have holes in their leaves (call stomata). These openings allow oxygen to enter the leaves to be used in photosynthesis. The oxygen reaches the photosynthesizing cells by moving through air spaces within the leaf. The stomata are closed in hot weather to reduce water loss. Insects, interestingly, have a similar system of holes to allow oxygen into their bodies. These are arrayed on the outer surface of the insect. The openings are connected to a series of tubes (tracheae) that branch throughout the interior of the insect bringing the air (and oxygen) in relatively close contact with all of the internal organs for exchange via diffusion. Fish have gills for getting oxygen from their watery environment. Water is passed either actively (mouth and gill covering movement) or passively (the fish swimming) over the gills. The gills have lots of blood vessels running through them that allows for pickup of oxygen by blood cells. Most fish have an interesting arrangement of blood circulation in the gills that allows for near continuous transfer of oxygen across the entire gill surface despite the fact that water at the far end of the gill structure has had some of the oxygen removed. The fish's blood cells carry the oxygen throughout their bodies for exchange with cells.
C. I choose ME as a vertebrate! The contraction of my diaphragm muscle lowers the pressure inside of the air spaces in my lungs. As a result air (containing oxygen) is pushed by atmospheric pressure into my lungs. In the tiny air spaces, oxygen diffuses into the blood. This happens because there is a high concentration of oxygen in the air space and relatively lower concentration in the blood. Diffusion happens to reach equilibrium. The oxygen is held in the red blood cells by hemoglobin molecules. Blood moves back to the left side of my heart from the lungs and is then pumped out to the body, including the muscles. In a muscle, the diffusion process is reversed because the blood has a high concentration of oxygen and the muscle lower. Oxygen moves into the muscle to reach equilibrium.
