Name_________________________
The Cell in Its Environment
The cell membrane is selectively permeable , which means that some substances can pass through it while others cannot. Oxygen, food molecules, and waste products all must pass through the cell membrane. Substances that can move into and out of a cell do so by one of three methods: diffusion, osmosis, or active transport.
Diffusion is the main method by which small molecules move across the cell membrane.
Diffusion is the process by which molecules tend to move from an area of higher concentration to an area of lower concentration. The concentration of a substance is the amount of the substance in a given volume.
Diffusion is caused by molecules moving and colliding. The collisions cause the molecules to push away from one another and spread out. Molecules diffuse through the cell membrane into a cell when there is a higher concentration of the molecules outside the cell than inside the cell.
The diffusion of water molecules through a selectively permeable membrane is called osmosis.
Because cells cannot function properly without adequate water, many cellular processes depend on osmosis.
In osmosis, water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.
The movement of dissolved materials through a cell membrane without using cellular energy is called passive transport . Diffusion and osmosis are both types of passive transport . When a cell needs to take in materials that are in higher concentration inside the cell than outside the cell, the movement of the materials requires energy. Active transport is the movement of materials through a cell membrane using cellular energy (ATP) . The main difference between passive transport and active transport is that active transport requires the cell to use its own energy while passive transport does not.
Cells have several ways of moving materials by active transport. In one method, transport proteins in the cell membrane "pick up" molecules outside the cell and carry them in. Another method of active transport is engulfing, in which the cell membrane wraps around, or engulfs, a particle and forms a vacuole within the cell. This mostly occurs because the molecule entering the cell is too large to pass through the cell membrane without using energy. This is known as endocytosis.
When a vesicle carrying something that needs to leave the cell comes to the cell membrane, it will attach itself to the membrane and then the materials are pushed out of the cell. This is known as exocytosis.
Most cells are very small. Remember that all materials move into and out of cells through the cell membrane. The amount of cell membrane limits the ability of cells to either get substances from the outside or transport waste and other materials outside.
Once a molecule enters a cell, it is carried to its destination by a stream of moving cytoplasm. In a very large cell, streams of cytoplasm must travel farther to carry materials from the cell membrane to all parts of the cell.
This is why growth is limited in a cell by two factors. 1.) One is the total amount of the surface of the cell membrane that is exposed to the environment or surface area and the other is 2.) the total amount of space it occupies or volume. Cell growth is limited by the ratio of a cell’s surface area to its volume. If the cell gets too large, molecules entering the cell won’t be able to travel to other parts of the cell with the materials it needs. As the cell gets larger, its volume increases faster that its surface area if the cell maintains the same shape. The cell stops growing. Instead it divides or reproduces itself to maintain a good surface to volume ratio .

Cell Growth Your body, like that of most animals, is always growing. This growth is the result of two processes: cell growth and cell division .
Cell growth causes a cell to become larger.
Cell division increases the number of cells.
No one knows exactly how many cells make up an adult human body, but scientists estimate the number to be between 10 trillion and 100 trillion cells.
Two factors limit the growth of cells.
1.) Cell volume, or the total amount of space a cell occupies.
2.) Surface area, or the total amount of the surface of the cell membrane that is exposed to the environment .
Cell growth is limited by the ratio of a cell's surface area to its volume.
As a cell grows, its volume increases more quickly than its surface area. Thus, the ratio of the cell's surface area to volume decreases as the cell grows. The following table illustrates how the surface area/volume ratio becomes smaller as a cell grows larger. In this example, the cell is assumed to have the shape of a cube.
Cell Surface Area to Volume Ratio (cube)
Length of Side
1 mm
Surface Area
.
1mm x 1mmx 6s =
6mm 2
Volume
1mmx1mmx1mm=
1 mm 3
Surface Area
Volume Ratio
6:1
3mm 3mm x3mm x 6s=
54 mm 2
3mm x3mm x 3mm =
27 mm 3
2:1
The cell membrane regulates what substances can enter and leave a cell. As a cell grows, its surface area/volume ratio eventually becomes too small to allow substances to enter or leave the cell quickly. At this point, the cell must divide to form two smaller cells. Each smaller cell will have a surface area/volume ratio large enough to allow substances to enter and leave the cell quickly.
What is the correct cell surface area to volume ratio for the cell represented in the table?
Cell Surface Area to Volume Ratio (cube)
Length of Side
2cm
Surface Area
24cm 2
Volume
8cm 3
Surface Area/Volume Ratio
?
A. 8:1
What is the surface area of a cell represented in the table?
Cell Surface Area to Volume Ratio
Surface Area
?
B. 3:1
Volume
1.5mm
3
C. 6:1
4:1
D. 1:3
Surface Area/Volume Ratio
A. 1.5 mm 2 B. 3 mm 2 C. 4mm 2 D. 6mm 2
If the volume of a cell increases, the cell’s ability to move materials from the cell membrane throughout the cell will be more difficult for the cell. Diffusion will not be able to take to move materials in and out of the cell. The cell will not be able to perform the necessary tasks it needs to in order to continue surviving, such as digestion, excretion, circulation and reproduction. The cell will then take the next step and that is to divide, or reproduce itself.

Name _____________________________
The Cell in Its Environment
1.The cell membrane is ____________________ ______________________ which means that some substances can
pass through it while others cannot.
2. List three ways that substances can move into and out of a cell. a. _______________________________________ b. ________________________________________ c. _______________________________________
3.ln diffusion, molecules move from an area of ___________________ concentration to an area of
_______________ concentration.
4.Draw molecules on Part B of the diagram below to show how the molecules are distributed inside and outside the cell after diffusion has occurred.
A. Before diffusion B. After Diffusion
5. In __________________________ , water molecules diffuse through a selectively permeable membrane.
6.Two ways of moving things into and out of cells that do NOT need energy are ___________________ and
_______________________________ _______________________. Moving materials through a cell membrane without using energy ______________ is called _____________transport.
7. How does active transport differ from passive transport?
__________________________________________________________________________________________
8. List three ways that the cell moves things by active transport. a. _______________________________ b. _______________________________ c. _______________________________
9.
True or false? As a cell gets larger, it takes longer for a molecule that as entered the cell to reach the middle of the cell.
1O. What is exocytosis?__________________________________________________________________________
_________________________________________________________________________________________
11. What is endocytosis? __________________________________________________________________________
_______________________________________________________________________________________________________

Name_________________________
The Cell in Its Environment
Understanding Main Ideas
Fill in the blank to identify the process illustrated in each of the following figures.
3.
96.5% water
Water moves out of the cells of a saltwater fish and into the ocean .
1. _____ __________ _
Oxygen moves from the lungs into the bloodstream.
2.__________ _
___
155 units of sodium
Sodium is pumped out of a nerve cell.
3.________________________
Answer the following questions.
4.
Explain how osmosis differs from diffusion.
________________________________________________________________________________
______________________________________________________________________________
5.
Compare and contrast active and passive transport
_______________________________________________________________________________
_______________________________________________________________________________
6.
Identify two methods of active transport.
________________________________________________________________________________
7.
State one reason that cells are small.
________________________________________________________________________________
Building Vocabulary
If the statement is true, write true. If the statement is false, change the underlined word or words to make the statement true.
_________________ 8. Selectively permeable means letting some but not all substances pass through.
_________________ 9. Osmosis is the process by which molecules tend to move from an area of
higher concentration to an area of lower concentration.
_________________ 10 . The process by which water moves across a selectively permeable membrane
is called diffusion.
_________________ 11.Diffusion and osmosis are types of active transport.
_________________ 12.Passive transport requires the cell's own energy.