Cell Structure and Function · 085_089_MIS_G6_S_C02_RE_L3_XXXXX89 89085_089_MIS_G6_S_C02_RE_L3_XXXXX89 89 8/14/17 5:29:18 PM8/14/17 5:29:18 PM. 90 Cell Structure and Function Moving

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Key Concepts

• How do materials enter

and leave cells?

• How does cell size affect

the transport of materials?

Passive TransportThe membranes of cells and organelles perform different

functions. They form boundaries between cells. They also control the movement of substances into and out of cells.

Cell membranes are semipermeable. This means that only certain materials can enter or leave a cell. Substances can pass through a cell membrane by one of several different processes. The type of process depends on the physical and chemical properties of the substance that is passing through the membrane.

Small molecules, such as oxygen and carbon dioxide, pass through a cell’s membrane by a process called passive transport. Passive transport is the movement of substances through a cell membrane without using the cell’s energy. Passive transport depends on the amount of a substance on each side of the membrane. If there are more oxygen molecules outside a cell than there are inside a cell, oxygen molecules will move into the cell by passive transport. Oxygen molecules will move into a cell until the amount of oxygen outside the cell equals the amount of oxygen inside the cell. There are different types of passive transport.

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before

column if you agree with the statement or a D if you disagree. After

you’ve read this lesson, reread the statements to see if you have

changed your mind.

Before Statement After

5. Diffusion and osmosis are the same process.

6. Cells with large surface areas can transport

more than cells with smaller surface areas.

Asking Questions Before

you read the lesson,

preview all the headings.

Make a chart and write a What or How question for

each heading. As you read,

write the answers to your

questions.

Make a two-tab book to

organize information about

the different types of

passive and active

transport.

Cell Structure and Function

Moving Cellular Material

LESSON 3

CHAPTER 2

Reading Essentials Cell Structure and Function 85

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Diffusion When the concentration, or amount per volume, of a substance

is unequal on each side of a membrane, molecules will move from the side with a higher concentration of the substance to the side with the lower concentration. Diffusion is the movement of substances from an area of higher concentration to an area of lower concentration.

ACADEMIC

VOCABULARYconcentration(noun) the amount of a given substance in a certain area.

Visual Check

1. Predict What would

the water in the beaker on

the right look like if the

membrane did not let

anything through?

Reading Check

2. Compare osmosis and

diffusion.

Diffusion will continue until the concentration on each side of the cell membrane is equal. The figure above shows how dye passed through the membrane into the clear water until there were equal concentrations of water and dye on both sides of the membrane.

Osmosis—The Diffusion of WaterDiffusion is the movement of any small molecules from areas

of higher concentrations to areas of lower concentrations. Osmosis is the diffusion of water molecules only through a membrane. Water molecules pass through a semipermeable membrane from an area of high concentration to an area of low concentration. For example, plant cells lose water because of osmosis. The concentration of water in the air around a plant is less than the concentration of water in the cells of the plant. Water will leave plant cells and diffuse into the air. If the plant is not watered to replace the water lost by its cells, the plant will wilt and might die.

Facilitated Diffusion

Some molecules are too large or are chemically unable to move through a membrane by diffusion. Facilitated diffusion is the movement of molecules through a cell membrane using special proteins called transport proteins. Facilitated diffusion does not use the cell’s energy to move the molecules. The transport proteins do the work. There are two types of transport proteins.

Carrier Proteins Carrier proteins are transport proteins. They carry large molecules, such as the sugar molecule glucose, through the cell membrane.

86 Cell Structure and Function Reading Essentials


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Channel Proteins Channel proteins are also transport proteins. They form pores through the cell membrane. Ions, such as sodium and potassium, pass through the cell membrane by channel proteins. Transport proteins are shown below.

Visual Check

4. Identify Circle the

type of transport protein

that carries large molecules

through the cell membrane.

Reading Check

3. Explain how materials

move through the cell

membrane in facilitated

diffusion.

Reading Check

5. Summarize how a cell

uses active transport.

Active TransportSometimes a cell uses energy when a substance passes through

its membrane. Active transport is the movement of substances through a cell membrane only by using the cell’s energy.

Substances moving by active transport move from areas of lower concentration to areas of higher concentration. Active transport is important for cells and organelles. Cells can take in nutrients from the environment through carrier proteins by using active transport. Some molecules and waste materials leave cells by active transport.

Endocytosis and Exocytosis

Some substances are too large to enter a cell membrane by diffusion or by using a transport protein. There are other ways that substances can enter a cell.

Endocytosis The process during which a cell takes in a substance by surrounding it with the cell membrane is called endocytosis (en duh si TOH sus). Some cells take in bacteria and viruses using endocytosis.



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Exocytosis Some substances are too large to leave a cell by diffusion or by using a transport protein. They can leave using exocytosis (ek soh si TOH sus). Exocytosis is the process during which a cell’s vesicles release their contents outside the cell. Proteins and other substances are removed from a cell through exocytosis. Both endocytosis and exocytosis are shown below.

Cell Size and TransportFor a cell to successfully transport materials, the size of the cell

membrane must be large compared to the space inside of the cell. This means that the surface area of the cell must be larger than the volume of the cell. When a cell grows, both its surface area and its volume increase. However, the volume of a cell increases faster than its surface area. If a cell becomes too large, it might not survive. Its surface area will be too small to move enough nutrients into the cell and remove waste materials from the cell.

Key Concept

Check

6. Explain how materials

enter and leave cells.

Visual Check

8 . Identify the structure

needed for exocytosis.

A ratio is a comparison of

two numbers, such as

surface area and volume. If

a cell were cube shaped,

you would calculate surface

area by multiplying its

length (ℓ) by its width (w) by

the number of sides (6).

Surface area: ℓ × w × 6

You would calculate the

volume of the cell by

multiplying its length (ℓ) by

its width (w) by its height (h).

Volume: ℓ × w × h

To find the surface-area-to-

volume ratio of the cell,

divide its surface area by its

volume.

Surface area

_________ Volume

7. Use Ratios What is the

surface-area-to-volume ratio

of a cube-shaped cell

whose sides are 6 mm long?

Math Skills

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Mini Glossary

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that comparespassive and active transport.

2. Fill in the table below to compare active and passive transport.

Energy needed? Structures Involved Examples

Active transport yes/no

Passive

transport

yes/no

active transport: the movement of substances through a cell membrane only by using the cell’s energy

diffusion: the movement of substances from an area of higher concentration to an area of lower concentration

endocytosis (en duh si TOH sus): the process during which a cell takes in a substance by surrounding it with the cell membrane

exocytosis (ek soh si TOH sus): the process during which a cell’s vesicles release their contents outside the cell

facilitated diffusion: when molecules pass through a cell membrane using special proteins called transport proteins

osmosis: the diffusion of water molecules only

passive transport: the movement of substances through a cell membrane without using the cell’s energy

Reread the statements at the beginning of the

lesson. Fill in the After column with an A if you

agree with the statement or a D if you dis-

agree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.



90 Cell Structure and Function

Moving Cellular Material

Predict three things that will be discussed in Lesson 3. Read the headings, and look at the photos and illustrations. Write your predictions in your Science Journal.

Lesson 3C

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List 2 functions of membranes.

1.

2.

Organize information about passive transport.

Passive Transport

Definition: Depends on: Example:

Assess information about diffusion. Read the statements below. If the statement is true, write true on the line. I f it is false, rewrite the underlined portion of the statement so that it is true.

Diffusion is the movement of substances from an area of lower concentration to an area of higher concentration.

Diffusion continues until the concentration of a substance is higher inside a cell than outside a cell.

Passive Transport

I found this on page .


Diffusion


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Cell Structure and Function 91

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Lesson 3 | Moving Cellular Material (continued)

Osmosis—The Diffusion

of Water



Complete the sentence about osmosis.

Osmosis is a type of that involves movement

of only through the cell membrane.

Explain the process of facilitated diffusion.

Facilitated Diffusion

A molecule is too

large to pass through

a cell membrane.

Cell uses two types of

proteins.

proteins

pass large molecules

through membrane by

pass large molecules

through membrane by

proteins

Organize information about active transport.

Definition:

the movement

of substances

through a

only by

can move

substances

from areas of

concentration to

concentration

Active Transport

used to bring in

and take

out

Active Transport



92 Cell Structure and Function

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Lesson 3 | Moving Cellular Material (continued)

Compare and contrast facilitated diffusion and active transport by writing yes or no in each empty box of the chart.

DescriptionFacilitated

Diffusion

Active

Transport

Uses carrier proteins

Transports materials across cell

membrane

Requires cellular energy

Able to move materials from an

area with lower concentration to

an area with higher concentration



Cell Size and T ransport


Cells are very small. Yet, as living things, they have the ability to grow. What keeps cells from growing to much larger sizes than they do?

Identify each process as either endocytosis or exocytosis.

Process Description

Materials entering cell

Materials being expelled from cell

Explain how cell size and transport are related. Underline the term that correctly completes each sentence.

As a cell grows, both its volume and surface area (increase/ decrease). Volume increases (faster/slower) than surface area.Eventually, the cell’s membrane would be (too large/too small) to move enough materials into and out of the cell.


Cell Structure and Function · 085_089_MIS_G6_S_C02_RE_L3_XXXXX89 89085_089_MIS_G6_S_C02_RE_L3_XXXXX89 89 8/14/17 5:29:18 PM8/14/17 5:29:18 PM. 90 Cell Structure and Function Moving

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