2.4 Cell Transport and Homeostasis Study Guide

2.4 Cell Transport and Homeostasis

(Bio.2.4)

Explore this Phenomenon

Your intestinal cells must take in glucose and other nutrients from the food you eat, and then transport the glucose into blood cells.

1. How do you think the glucose molecules get from the food you eat into the cells? Create a model to communicate your idea.
2. How could we investigate the movement of matter, like glucose, in and out of plant or animal cells? Describe the steps we could take to investigate the movement of matter across cell membranes.

Bio.2.4 Transporting Materials

Plan and carry out an investigation to determine how cells maintain stability within a range of changing conditions by the transport of materials across the cell membrane. Emphasize that large and small particles can pass through the cell membrane to maintain homeostasis. (LS1.A)

Cells can maintain stability, even when conditions around the cell are changing, by transporting materials in or out of the cell. As you read this chapter, pay attention to how transport of materials across the membrane helps a cell maintain homeostasis.

Homeostasis

What happens if stability is disrupted?

If any of the stones were removed, the whole arch would collapse. The same is true for the human body. All the systems work together to maintain stability or homeostasis. If one system is disrupted, the other systems can also be affected.

Homeostasis refers to the balance, or equilibrium, within the cell or a body. It is an organism's ability to keep a constant internal environment. Keeping a stable internal environment requires constant adjustments as conditions change inside and outside the cell. Abiotic factors like temperature and pH must be kept at just the right levels to support life processes. Homeostasis is a dynamic equilibrium rather than an unchanging state. For example, if your body temperature decreases, you start to shiver. This generates heat, causing your temperature to rise. If your body temperature increases too much, you begin to sweat. This lowers your body temperature. Even though your temperature is changing, it never gets too high or too low.

Individual cells must also maintain homeostasis. This includes keeping the right amount of salt, water, sugars, and other molecules inside the cell. As the surrounding conditions change, cells must not let the amount of these substances increase or decrease too much.

Diffusion

The membrane surrounding the cell helps maintain the correct amount of substances inside the cell. The following figure shows the parts of a cell membrane.

Cell transport is the movement of substances across the cell membrane either into or out of the cell. Small molecules like oxygen, carbon dioxide, and water can pass through the cell  membrane freely.

If a molecule is big, it won't make it through the cell membrane on its own. Larger molecules need the assistance of a protein.

These proteins act like a doorway that larger molecules can go through. The protein channel in the diagram is one example of a protein that enables larger molecules to move into or out of the cell.

Because not all molecules can move through the cell membrane on their own, we say that the cell membrane is selectively permeable, or semipermeable. This gives the cell some control over which molecules move into or out of the cell. In this way, cell membranes help maintain a state of homeostasis within the cell (and tissues, organs, and organ systems) so that an organism can stay alive and healthy.

A selectively permeable, or semipermeable, membrane allows certain molecules through, but not others. Can you tell which molecules can move through the center membrane and which would need a protein doorway?

Which direction?

What determines if molecules will move from the outside of the cell to the inside, or from the inside of the cell to the outside?

Molecules tend to move from an area with a high concentration to an area with a low concentration. If there is a higher concentration of carbon dioxide molecules outside the cell than inside, more molecules of carbon dioxide will move from the outside of the cell to the inside of the cell.

Image caption: The alveolus of the lungs contain a lower concentration of CO 2 and a higher concentration of O 2 than the blood. The CO 2 and O 2 diffuse across cell membranes from a high concentration to a low concentration. What are the relative concentrations in the lungs and blood that cause the CO 2 to diffuse into the lung and the O 2 to diffuse out of the lung?

The movement of molecules from a high to a low concentration is called diffusion, and it requires no energy. If the molecules must use a protein doorway to diffuse through the cell membrane, it is called facilitated diffusion. Facilitated diffusion does not require energy.

Diffusion can help cells maintain homeostasis. For example, as a cell uses oxygen molecules, the concentration of oxygen inside the cells decreases. When the concentration of oxygen inside the cell becomes lower than the concentration of oxygen outside the cell, oxygen molecules will move into the cell through diffusion. This helps the cell maintain homeostasis, because as oxygen molecules inside the cell are used, they will be replaced by oxygen molecules diffusing into the cell.

The picture above shows a red blood cell in three different environments. Describe the conditions in each environment that would cause water to move as shown in the picture.

Osmosis- the movement of water across the cell membrane from an area of high water potential ( more water/ less solute) to an area of low water potential ( less water/more solute).

Active Transport

There may be times when a cell needs to move molecules from a low concentration to a high concentration to maintain homeostasis. In this case, molecules will diffuse in the opposite direction that the cell needs them to. How does a cell maintain homeostasis in this situation?

Active transport is a process that moves molecules from a low concentration to a high concentration. This process requires a protein to move the molecules, and it also requires energy. Active transport is important in helping cells maintain the right concentration of molecules, even when the concentration of molecules outside the cell is changing.

Image caption: The proteins in the plasma membrane (cell membrane) are moving the potassium and sodium from one side to another. When ATP is broken into ADP + Phosphate, energy is made available for the proteins to transport particles across the membrane. Are the potassium and sodium moving from a high to a low concentration, or a low to a high concentration?

Exostosis- when a vesicle forms around a substance INSIDE a cell, moves to the cell membrane, fuses with it and releases its cargo outside the cell.

Endocytosis- when a patch of the cell membrane encloses a substance OUTSIDE the cell, it sinks into the cell, pulling it inside. 

Putting It Together

Your intestinal cells must take in glucose and other nutrients from the food you eat, and then transport the glucose into blood cells.

1. Review and revise your model showing how glucose molecules get from the food you eat into your intestinal cells.
2. The conditions in your intestines are always changing. Develop a model or explanation to show how your intestinal cells are able to maintain homeostasis, even when the concentrations of molecules in the intestines are always changing.
3. What other phenomena can you use your understanding of homeostasis and cell transport to explain?