Biology (A)

Cell Transport and Homeostasis

Lesson Summary

• Molecules and ions cross the plasma membrane either by passive transport or active the transport.
• Passive transport is the movement of molecules across the cell membrane without an input of energy from the cell.
• Diffusion is the movement of molecules or ions from an area of high concentration to an area of lower concentration. The molecules keep moving down the concentration gradient until equilibrium is reached.
• Osmosis is the diffusion of water molecules across a semipermeable membrane and down a concentration gradient. They can move into or out of a cell, depending on the concentration of the solute.
• Active transport moves molecules across a cell membrane from an area of lower concentration to an area of higher concentration. Active transport requires the use of energy.
• The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane.
• The sodium-potassium pump is an example of a cell membrane pump. It moves three sodium ions out of the cell and two potassium ions into the cell. The sodium-potassium pump uses ATP.
• Endocytosis and exocytosis are active transport mechanisms in which large molecules enter and leave the cell inside vesicles.
• In endocytosis, a substance or particle from outside the cell is engulfed by the cell membrane. The membrane folds over the substance and it becomes completely enclosed by the membrane. There are two main kinds of endocytosis: pinocytosis and phagocytosis.

Self-Check Questions

-1. Identify the two ways that particles cross the plasma membrane.
0. How does osmosis differ from diffusion?
1. Outline how the sodium-potassium pump works.
2. Are vesicles involved in passive transport? Explain.
3. What is the difference between endocytosis and exocytosis?
4. Why is pinocytosis (cellular drinking) a form of endocytosis?
5. Identify which type of feedback mechanism is most common in homeostasis, and give an example of that type.
6. Imagine you have discovered a new cell that has not been seen before. How would you go about identifying it based on its structure alone?
7. Homeostasis can be thought of as a dynamic equilibrium rather than an unchanging state. Do you agree with this statement? Explain your answer.

8. 

9. This image shows plant cells. The central vacuole of each cell has shrunk and is smaller than normal. What is the likely solute concentration of the cells' environment which has caused this change?