Diffusion and Human body

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Diffusion and Human body

Read closely the article on Diffusion and Human body, highlight or underline any important information relevant to the topic, and answer the questions at the end. Cite the evidence from your reading to justify your answers. (CCSS.ELA-LITERACY.RST.6-8.1), (CCSS.ELA-LITERACY.RST.6-8.2)

  1. The Cell Membrane

The cell is the basic functional unit of all living things. If one is to understand what is happening within an organism, it is necessary to study what is happening to the individual cells within the organism .The cell membrane, sometimes referred to as the plasma membrane, is a phospholipid bilayer that creates a definite boundary between the inside of the cell and the outside of the cell. The structure of cell membrane allows it to easily regulate the materials that enter and exit the cell. Because the cell membrane allows certain materials to pass through it, and keeps other materials out of it, it is referred to as a semi-permeable or selectively permeable membrane. Much like the security guard at a government center, movement into and out of the cell is highly regulated.

  1. Diffusion

Water, carbon dioxide, and oxygen are a few substances that are able to pass through the cell membrane without any help from the proteins embedded in the membrane. These materials can simply pass through the membrane without the use of energy and they will do so in an attempt to reach an equal solute concentration inside and outside of the cell. This process is called diffusion, when substances move from a region of high concentration to a region of low concentration. The "goal" of diffusion is to reach a state of equilibrium. This does not mean that movement of molecules stops once equilibrium is reached. Equilibrium is a state of balance, where for every molecule of a substance that moves into a cell, another one moves out of the cell. To visualize diffusion, imagine two rooms that are connected by one door. If the door connecting the two rooms is closed and everyone is crammed into only one of the rooms, as soon as the door opens, people will begin to walk into the other room to spread out. Most likely, they will not all move into the newly opened room. Instead, they will spread out until every area of the two rooms has roughly the same density (or concentration) of people. People will still be able to walk around and mingle, but they will likely readjust so that every part of the room is equally comfortable at all times. Diffusion is an easy concept to model. Simply place a drop of food coloring into a glass of water (being careful to make sure that the fluid in the glass is completely still). The food coloring will slowly spread out in the water until it is evenly mixed throughout.

  1. Diffusion In the Human Body

Within the body, diffusion is vital to the functioning of the cardiovascular system and other body systems as well. When oxygen is inhaled into the lungs, it ends up at high concentration in the alveoli, the round sacs at the end of the bronchioles. Because blood that is flowing through your lungs is deoxygenated, there is a higher concentration of oxygen inside the alveoli than in the blood. In an attempt to reach equilibrium, the oxygen simply diffuses into the blood across the cells at the alveolar/capillary boundary. Similarly, the blood circulating through the capillaries in your lungs contains high amounts of carbon dioxide that has been picked up as a waste product from cellular respiration. The concentration of carbon dioxide in the blood is much higher than it is in the alveoli, and the carbon dioxide moves into the alveolar gas through simple diffusion; it is then exhaled out of the body 7. In regards to cells, simple diffusion is only possible if the material is able to permeate the membrane.
  1. Examples of Diffusion

Below are examples of diffusion in the body across concentration gradients.


Particles that move




Digested food particles

Gut cavity

Blood in capillary of villus in small intestine



Alveolar air space

Blood circulating around the lungs


Carbon Dioxide

Blood circulating around the lungs

Alveolar air space

Remember: Particles continue to move from a high to a low concentration until all the particles are evenly and randomly distributed.

  1. Osmosis

Osmosis is simply a special type of diffusion. It occurs when water molecules pass through a partially permeable membrane. Some membranes in plant and animal cells allow certain particles to pass through them but not others. They are partially permeable membranes. During osmosis, more water molecules pass from the pure water into the dilute solution than pass back the other way. This is because there is a higher concentration of water molecules in the pure water than in the solution. This results in more water molecules diffusing across the concentration gradient from the water to the solution. Eventually, the level on the more concentrated side of the membrane will rise, while that on the less concentrated side falls.

  1. Osmosis is the overall movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane. This is still like diffusion, as the water is moving from a higher concentration of water to a lower concentration of water. When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero, and the system is in equilibrium. If red blood cells are placed in pure water, water enters them by osmosis and the red blood cells swell up and burst.

Name: _____________________ Date: ______________

Period: ______________________
1. What is this article about?


2. What is the author communicating about in the first paragraph? Why is this information important?


3. What does the word semipermeable mean? How is this word related to the cell?


4. In paragraph 2, the author uses an example to visualize diffusion. Does this example help you to understand what diffusion is? How?


5.What is the relationship of diffusion and human body? Explain with the help of citing textual evidence from the article.



6. How is osmosis similar and different from diffusion? Use textual evidence to support your answer.

7. Explain with the help of textual evidence how diffusion takes place in the alveoli and capillaries?


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