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Passive Transport The movement of materials across the cell membrane without using cellular energy is called passive transport.

Diffusion is the process by which particles move from an area of high concentration to an area of lower concentration.

Facilitated diffusion is the process by which molecules that cannot directly diffuse across the membrane pass through special protein channels.

Osmosis is the facilitated diffusion of water through a selectively permeable membrane.

• 
  Aquaporins are water channel proteins that allow water to pass through cell membranes.
  
• 
  Two adjacent solutions are isotonic if they have the same concentrations of solute.
  
• 
  Hypertonic solutions have a higher concentration of solute compared to another solution.
  
• 
  Hypotonic solutions have a lower concentration of solute compared to another solution.
  

Osmotic pressure is the force caused by the net movement of water by osmosis.

Active Transport The movement of materials against a concentration difference is called active transport. Active transport requires energy.

Transport proteins that act like pumps use energy to move small molecules and ions across cell membranes.

The bulk transport of large molecules and clumps of materials into and out of cells occurs by movements of the cell membrane, which require energy.

Passive Transport

For Questions 1–4, write the letter of the correct answer on the line at the left.

1. Which of the following must be true for diffusion to occur?

A. Molecules or particles must have different sizes.

B. Special protein channels must always be available.

C. There must be areas of different concentrations.

D. Energy must be available.

2. Which term refers to the condition that exists when no net change in concentration results from diffusion?

A. concentration C. osmosis

B. equilibrium D. randomness

3. Air has a higher concentration of oxygen molecules than does the cytoplasm of your lung cells. Where in your lungs will there be a net increase of oxygen?

A. in the air breathed in C. outside of the lung cells

B. in the air breathed out D. inside of the lung cells

4. Which of the following statements tells how facilitated diffusion differs from simple diffusion?

A. Particles move through cell membranes without the use of energy by cells.

B. Particles tend to move from high concentration to lower concentration.

C. Particles move within channel proteins that pass through cell membranes.

D. Particles tend to move more slowly than they would be expected to move.

For Questions 5–7, match the situation with the result. Write the letter of the correct answer on the line at the left.

THINK VISUALLY
8. In the table below, draw how each type of cell will look after being placed in a hypertonic solution.

Appearance of Cells in a Hypertonic Solution
Animal Cells	Plant Cells

Active Transport

9. 
   What is the function of active transport in moving small molecules and ions across cell membranes? Give an example.
   

9. 
   How does ATP enable transport proteins to move ions across a cell membrane?
   

9. 
   What are the proteins used in active transport called?
   
10. 
    Complete the table to summarize the types of bulk transport.
    

Types of Bulk Transport
Type	Description
Endocytosis
Phagocytosis
Exocytosis

Apply the Big idea

9. 
   Most sports drinks are isotonic in relation to human body fluids. Explain why athletes should drink solutions that are isotonic to body fluids when they exercise rather than ones that are hypotonic to body fluids (contain a greater proportion of water in comparison to the fluids in and around human body cells).
   

7.4 Homeostasis and Cells

Lesson Objectives

Explain how unicellular organisms maintain homeostasis.

Explain how multicellular organisms maintain homeostasis.

Lesson Summary

The Cell as an Organism Sometimes a single cell is an organism. Single-celled organisms must be able to carry out all the functions necessary for life.

Unicellular organisms maintain homeostasis, relatively constant internal conditions, by growing, responding to the environment, transforming energy, and reproducing.

Unicellular organisms include both prokaryotes and eukaryotes.

Unicellular organisms play many important roles in their environments.

Multicellular Life Cells of multicellular organisms are interdependent and specialized.

The cells of multicellular organisms become specialized for particular tasks and communicate with one another to maintain homeostasis.

Specialized cells in multicellular organisms are organized into groups.

• 
  A tissue is a group of similar cells that performs a particular function.
  
• 
  An organ is a group of tissues working together to perform an essential task.
  
• 
  An organ system is a group of organs that work together to perform a specific function.
  

The cells of multicellular organisms communicate with one another by means of chemical signals that are passed from one cell to another.

• 
  Certain cells form connections, or cellular junctions, to neighboring cells. Some of these junctions hold cells together firmly.
  
• 
  Other cells allow small molecules carrying chemical signals to pass directly from one cell to the next.
  
• 
  To respond to a chemical signal, a cell must have a receptor to which the signaling molecule can bind.
  

The Cell as an Organism

For Questions 1–5, complete each statement by writing the correct word or words.

1. 
   The term refers to the relatively constant internal physical and chemical state of a living cell.
   
2. 
   Unicellular prokaryotes, called , are adapted to living in a remarkable number of different places.
   
3. 
   Some unicellular eukaryotes, called , contain chloroplasts.
   
4. 
   Yeasts are unicellular , which are eukaryotes.
   
5. 
   Other unicellular eukaryotes include and algae.
   

6. 
   How do single-celled organisms maintain homeostasis?
   

6. 
   Why is maintaining homeostasis particularly important to single-celled organisms?
   

Multicellular Life

8. 
   How are the cells of a multicellular organism like a baseball team?
   

8. 
   How does a multicellular organism maintain homeostasis?
   

8. 
   Complete the table by describing the functions of the specialized cells.
   

Examples of Specialized Cells
Type of Cell	Name of Specialized Cell Part	Function of Specialized Cell Part
cells that line the upper air passages in humans	cilia
pine pollen grains	wings

8. 
   The Venn diagram below consists of four concentric circles. Complete the diagram to show the relationships among four levels of organization of life. Use the terms cells, organ, organ system, and tissue.
   

8. 
   Starting with the outermost circle of the diagram, explain how each level is related to the next level within each circle.
   

8. 
   What is the name of the areas that hold adjacent cells together and enable them to communicate?
   

Apply the Big idea

8. 
   On the Venn diagram above, where would you add a circle that represents the organism level of life? Where would you add a circle that represents another organ of the same organ system?
   

CIDEB Biology Academy First Semester Aug-Dec 2011 Portfolio 3 page