Cell Structure and Function

 

 

Cell Structure and Function

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Cell Shape and Movement

You might recall that all living things are made up of one or more cells. As illustrated in Figure 1, cells come in many shapes and sizes. The size and shape of a cell relates to its job or function. For example, a human red blood cell cannot be seen without a microscope. Its small size and disk shape enable it to pass easily through the smallest blood vessels. The shape of a nerve cell enables it to send signals over long distances. Some plant cells are hollow and make up tubelike structures that carry materials throughout a plant.

 

The structures that make up a cell also have unique functions. Think about how the players on a football team perform different tasks to move the ball down the field. In a similar way, a cell is made of different structures that perform different functions that keep a cell alive. You will read about some of these structures in this lesson.

Figure 1 The shape of a cell relates to the function it performs.

 

Cell Membrane

Although different types of cells perform different functions, all cells have some structures in common. As shown in Figure 2 and Figure 3, every cell is surrounded by a protective covering called a membrane. The cell membrane is a flexible covering that protects the inside of a cell from the environment outside a cell. Cell membranes are mostly made of two different macromolecules—proteins and a type of lipid called phospholipids. Think again about a football team. The defensive line tries to stop the other team from moving forward with the football. In a similar way, a cell membrane protects the cell from the outside environment.

1. Reading Check What are cell membranes made of?

Cell membranes are made of proteins and lipids.

Figure 2 The cell wall maintains the shape of a plant cell.

 

Figure 3 The cytoskeleton maintains the shape of an animal cell.

Visual Check Compare this animal cell to the plant cell in Figure 2.

The shape of the plant cell is maintained by the rigid cell wall, whereas the animal cell’s shape is maintained by an internal framework provided by the cytoskeleton. A plant cell contains chloroplasts and a large central vacuole. An animal cell has lysosomes.

 

Cell Wall

Every cell has a cell membrane, but some cells are also surrounded by a structure called the cell wall. Plant cells such as the one in Figure 2, fungal cells, bacteria, and some types of protists have cell walls. A cell wall is a stiff structure outside the cell membrane. A cell wall protects a cell from attack by viruses and other harmful organisms. In some plant cells and fungal cells, a cell wall helps maintain the cell’s shape and gives structural support.

 

Cell Appendages

Arms, legs, claws, and antennae are all types of appendages. Cells can have appendages too. Cell appendages are often used for movement. Flagella (fluh JEH luh; singular, flagellum) are long, tail-like appendages that whip back and forth and move a cell. A cell can also have cilia (SIH lee uh; singular, cilium) like the ones shown in Figure 4. Cilia are short, hairlike structures. They can move a cell or move molecules away from a cell. A microscopic organism called a paramecium (pa ruh MEE shee um) moves around its watery environment using its cilia. The cilia in your windpipe move harmful substances away from your lungs.

SPL/Photo Researchers

Figure 4 Lung cells have cilia that help move fluids and foreign materials.

 

Cytoplasm and the Cytoskeleton

You might recall that water is the main ingredient in a cell. Most of this water is in the cytoplasm, a fluid inside a cell that contains salts and other molecules. The cytoplasm also contains a cell’s cytoskeleton. The cytoskeleton is a network of threadlike proteins that are joined together. The proteins form a framework inside a cell. This framework gives a cell its shape and helps it move. Cilia and flagella are made from the same proteins that make up the cytoskeleton.

 

1 NOTE

ACADEMIC VOCABULARY

12.D.i
12.D.viii

Explain that every cell part has a special job, such as the protective function of the cell membrane. Ask students to read the Academic Vocabulary definition.

Ask: Why is the function of the cell membrane important to all the other parts of the cell?

The function of the cell membrane is to protect the inside of the cell, just as the function of an eggshell is to protect the egg inside it.

ACADEMIC VOCABULARY

function
(noun) the purpose for which something is used

 

1 NOTE

WORD ORIGIN

12.D.iv
12.D.x

Have students read the word origins for cytoplasm and discuss the word origins in relation to the word definition. Discuss how knowing word origins can help in understanding new vocabulary words.

Ask: What makes up the cytoplasm of a cell?

Water, salts, and other molecules make up the cytoplasm of a cell.

WORD ORIGIN

cytoplasm
from Greek kytos, means “hollow vessel”; and plasma, means “something molded”

 

4 NOTES

Cell Types

Prokaryotic and Eukaryotic Cells

Differentiated Instruction

Differentiated Instruction

Cell Types

Some students may have difficulty sounding out the words prokaryotic and eukaryotic. Write these words on the board, divide them into syllables, and pronounce them for students. After students read the first paragraph, use the scaffolded questions below to connect their experience in the MiniLab to the lesson.

Prokaryotic and Eukaryotic Cells

Students may have difficulty conceptualizing cell structures. Build on the football analogy used earlier to suggest how different cell structures each play a role in the function of the cell. Have them turn back to Figures 2 and 3 as needed for your discussions. Compare and contrast the features of eukaryotic cells with prokaryotic cells.

Guiding Questions

In what way have microscopes helped scientists learn more about cells?

Possible answer: Microscopes help scientists see tiny things, like cells, that cannot be seen with the unaided eye.

Key Concept Check How are prokaryotic cells and eukaryotic cells similar, and how are they different?

Both cell types have genetic material. In prokaryotic cells, the genetic material floats freely in cytoplasm. In eukaryotic cells, the genetic material is inside the nucleus. Eukaryotic cells are usually larger and have more parts than prokaryotic cells.

Differentiated Instruction

Same and Different? Have student pairs create a flipchart that shows how prokaryotic and eukaryotic cells are the same on one side of the chart and how they are different on the flip side.

Differentiated Instruction

The Word Game! Provide student pairs with a set of ten important terms from this section, such as prokaryotic cell.

1. Each student creates word cards with the term on one side and a drawing representing the term on the other side.

2. One set of cards should be placed on a table with the terms showing. The other set of cards should be placed with the drawings showing.

3. Each student takes a turn selecting a term card and choosing a matching picture card. The term is read and the chosen card turned over. If the two cards match, the student keeps the cards. If they do not match, the cards are returned.

4. Students take turns until all the cards are matched.

Cell Types

Recall that the use of microscopes enabled scientists to discover cells. With more advanced microscopes, scientists discovered that all cells can be grouped into two types— prokaryotic (proh ka ree AH tihk) cells and eukaryotic (yew ker ee AH tihk) cells.

 

Prokaryotic Cells

The genetic material in a prokaryotic cell is not surrounded by a membrane, as shown in Figure 5. This is the most important feature of a prokaryotic cell. Prokaryotic cells also do not have many of the other cell parts that you will read about later in this lesson. Most prokaryotic cells are unicellular organisms and are called prokaryotes.

Figure 5 In prokaryotic cells, the genetic material floats freely in the cytoplasm.

 

Eukaryotic Cells

Plants, animals, fungi, and protists are all made of eukaryotic cells, such as the ones shown in Figure 2 and Figure 3, and are called eukaryotes. With few exceptions, each eukaryotic cell has genetic material that is surrounded by a membrane. Every eukaryotic cell also has other structures, called organelles, which have specialized functions. Most organelles are surrounded by membranes.

Eukaryotic cells are usually larger than prokaryotic cells. About ten prokaryotic cells would fit inside one eukaryotic cell.

1. Key Concept Check How are prokaryotic cells and eukaryotic cells similar, and how are they
    different?

Both cells types have genetic material. In prokaryotic cells, the genetic material floats freely in cytoplasm. In eukaryotic cells, the genetic material is inside the nucleus. Eukaryotic cells are usually larger and have more parts than prokaryotic cells.

 

1 NOTE

How do eukaryotic and prokaryotic cells compare ?

3.B.i
2.A.v

Prep: 10 min Class: 25 min

Purpose
To model a eukaryotic cell and then compare it with a prokaryotic cell.

Materials
scissors, glue or glue sticks, craft supplies (for example: craft foam, yarn, pipe cleaners, pom-poms, markers/crayons, construction paper, cardboard, egg cartons, modeling clay, aluminum foil, paper towel tubes)

Guide the Investigation
To save time and limit student movement, place craft supplies on each table for students to share. If this activity is done as an introduction to organelles, direct students to appropriate figures later in this lesson to see pictures of the organelles. Additional visuals of eukaryotic cells and/or organelles for students to observe would be helpful.

Cell Organelles

 

As you have just read, organelles are eukaryotic cell structures with specific functions. Organelles enable cells to carry out different functions at the same time. For example, cells can obtain energy from food, store information, make macromolecules, and get rid of waste materials all at the same time because different organelles perform the different tasks.

 

The Nucleus

The largest organelle inside most eukaryotic cells is the nucleus, shown in Figure 6. The nucleus is the part of a eukaryotic cell that directs cell activities and contains genetic information stored in DNA. DNA is organized into structures called chromosomes. The number of chromosomes in a nucleus is different for different species of organisms. For example, kangaroo cells contain six pairs of chromosomes. Most human cells contain 23 pairs of chromosomes.

 

In addition to chromosomes, the nucleus contains proteins and an organelle called the nucleolus (new KLEE uh lus). The nucleolus is often seen as a large dark spot in the nucleus of a cell. The nucleolus makes ribosomes, organelles that are involved in the production of proteins. You will read about ribosomes later in this lesson.

 

Surrounding the nucleus are two membranes that form a structure called the nuclear envelope. The nuclear envelope contains many pores. Certain molecules, such as ribosomes and RNA, move into and out of the nucleus through these pores.

1. Reading Check What is the nuclear envelope?

A nuclear envelope is a structure formed by two membranes that surrounds the nucleus of a cell.

(r) Dr. Donald Fawcett/Visuals Unlimited/Getty Images

Figure 6 The nucleus directs cell activity and is surrounded by a membrane.

 

Manufacturing Molecules

You might recall that proteins are important molecules in cells. Proteins are made on small structures called ribosomes. Unlike other cell organelles, a ribosome is not surrounded by a membrane. Ribosomes are in a cell’s cytoplasm. They also can be attached to a weblike organelle called the endoplasmic reticulum (en duh PLAZ mihk • rih TIHK yuh lum), or ER. As shown in Figure 7, the ER spreads from the nucleus throughout most of the cytoplasm. ER with ribosomes on its surface is called rough ER. Rough ER is the site of protein production. ER without ribosomes is called smooth ER. It makes lipids such as cholesterol. Smooth ER is important because it helps remove harmful substances from a cell.

2. Reading Check Contrast smooth ER and rough ER.

Smooth ER makes lipids (cholesterol), removes harmful substances from the cell, and does not have ribosomes. Rough ER has ribosomes and is the site of protein production.

(l) Dr. Donald Fawcett/Visuals Unlimited/Getty Images, (r) Dennis Kunkel / Phototake

Figure 7 The endoplasmic reticulum is made of many folded membranes. Mitochondria provide a cell with usable energy.

 

Processing Energy

All living things require energy in order to survive. Cells process some energy in specialized organelles. Most eukaryotic cells contain hundreds of organelles called mitochondria (mi tuh KAHN dree uh; singular, mitochondrion), shown in Figure 7. Some cells in a human heart can contain a thousand mitochondria.

 

Like the nucleus, a mitochondrion is surrounded by two membranes. Energy is released during chemical reactions that occur in the mitochondria. This energy is stored in high-energy molecules called ATP—adenosine triphosphate (uh DEH nuh seen • tri FAHS fayt). ATP is the fuel for cellular processes such as growth, cell division, and material transport.

 

Plant cells and some protists, such as algae, also contain organelles called chloroplasts (KLOR uh plasts), shown in Figure 8. Chloroplasts are membrane-bound organelles that use light energy and make food—a sugar called glucose—from water and carbon dioxide in a process known as photosynthesis (foh toh SIHN thuh sus). The sugar contains stored chemical energy that can be released when a cell needs it.

3. Reading Check Which types of cells contain chloroplasts?

Plant cells and some protists, such as algae, contain chloroplasts.

(l) Dr. R. Howard Berg/Visuals Unlimited/Getty Images, (r) Dennis Kunkel / Phototake

Figure 8 Plant cells have chloroplasts that use light energy and make food. The Golgi apparatus packages materials into vesicles.

Processing, Transporting, and Storing Molecules

Near the ER is an organelle that looks like a stack of pancakes. This is the Golgi (GAWL jee) apparatus, shown in Figure 8. It prepares proteins for their specific jobs or functions. Then it packages the proteins into tiny, membrane-bound, ball-like structures called vesicles. Vesicles are organelles that transport substances from one area of a cell to another area of a cell. Some vesicles in an animal cell are called lysosomes. Lysosomes contain substances that help break down and recycle cellular components.

 

Some cells also have saclike structures called vacuoles (VA kyuh wohlz). Vacuoles are organelles that store food, water, and waste material. A typical plant cell usually has one large vacuole that stores water and other substances. Some animal cells have many small vacuoles.

 

4. Key Concept Check What is the function of the Golgi apparatus?

It prepares proteins for their specific functions and packages them into vesicles for transport.

 

1 NOTE

SCIENCE USE V. COMMON USE

Ask: How is the function of the nuclear envelope similar to that of an envelope you use for letters?

The nuclear envelope allows for gathering and protecting molecules, much like a paper envelope gathers information—a letter—and protects it until it is removed.

SCIENCE USE V. COMMON USE

envelope
Science Use an outer covering

Common Use a flat paper container for a letter

 

Lesson Review

Visual Summary

A cell is protected by a flexible covering called the cell membrane.

Cells can be grouped into two types—prokaryotic cells and eukaryotic cells.

In a chloroplast, light energy is used for making sugars in a process called photosynthesis.

Dr. R. Howard Berg/Visuals Unlimited/Getty Images

 

Lesson Assessment

 

Use Vocabulary

1. Distinguish between the cell wall and the cell membrane.

The cell wall is a rigid structure that provides support and protection in plants and some bacteria, whereas the cell membrane is a flexible covering that is found around all cells.

2. Use the terms mitochondria and chloroplasts in a sentence.

Possible answer: Mitochondria and chloroplasts are used to process energy.

3. Define organelle in your own words.

An organelle is a structure in a cell that is surrounded by a membrane and used for a specialized task.

 

Understand Key Concepts

4. The nucleus of a eukaryotic cell contains genetic information stored in DNA. DNA is organized
     into what structures?

     A. chromosomes

     B. lysosomes

     C. mitochondria

     D. ribosomes

5. Which organelle is used to store water?

     A. chloroplast

     B. lysosome

     C. nucleus

     D. vacuole

6. Explain the role of the cytoskeleton.

The cytoskeleton provides structural support to the cell.

7. Draw a prokaryotic cell and label its parts.

Cell drawing should lack organelles and show DNA without a membrane around it.

8. Compare the roles of the endoplasmic reticulum and the Golgi apparatus.

The endoplasmic reticulum makes proteins and lipids, whereas the Golgi apparatus prepares proteins for their specific functions and packages them into vesicles for transport.

9. The arrow below is pointing to which cell part?

     A. chloroplast

     B. mitochondrion

     C. cell membrane

     D. cell wall

10. Which best describes vacuoles?

     A. lipids

     B. proteins

     C. contained in mitochondria

     D. storage compartments

11. Which cell shown below can send signals over long distances?

A.

B.

C.

D.

 

 

2. The figure below shows a cell. What is the arrow pointing to?

     A. chloroplast

     B. cytoplasm

     C. mitochondrion

     D. nucleus

 

Interpret Graphics

13. Explain how the structure of the cells below relates to their function.

These tubelike cells are hollow and transport materials in plants.

14. Compare Copy the table below and fill it in to compare the structures of a plant cell to the 
     structures of an animal cell.

 

Critical Thinking

15. List the function of each of the following cell parts: cell membrane, cell wall, chloroplast, 
     cytoplasm, cytoskeleton, mitochondrion, and nucleus.

cell membrane: controls the materials going in and out of a cell
cell wall: maintains the shape of some plant cells
chloroplast: traps energy from the Sun
cytoplasm: holds the structures of a cell in a watery mix
cytoskeleton: maintains the shape of an animal cell
mitochondrion: processes energy for cellular processes
nucleus: controls the activities of a cell

16. Name the kinds of organisms that have cells with cell walls. Name the kinds of organisms that 
     have cells without cell walls. Briefly describe the benefits of cell walls for organisms.

Plants, fungi, bacteria, and some protists have cells with cell walls. Animals and some protists do not have cells with cell walls. Cell walls provide support for the cells. Cell walls also provide some protection for the cells from viruses and other organisms that can cause diseases and injury.

17. Draw simple diagrams of an animal cell and a plant cell. Label the nucleus, the cytoplasm, the 
     mitochondria, the cell membrane, the chloroplasts, the cell wall, and the central vacuole in the 
     appropriate cells. Briefly describe the main differences between the two cells.

The diagrams should show the correct locations and relative sizes of the nucleus, cytoplasm, mitochondria, cell membrane, chloroplasts, cell wall, and central vacuole. The main differences between the two cells are that the plant cell has a rigid cell wall that gives a fixed shape to the cell, chloroplasts that perform photosynthesis, and a central vacuole that holds water and other substances.

18. Analyze Why are most organelles surrounded by membranes?

to perform functions without interference from other organelles

19. Compare the features of eukaryotic and prokaryotic cells.

A prokaryotic cell lacks a nucleus and most other organelles, while a eukaryotic cell has a nucleus and many other organelles.

20. Compare prokaryotes and eukaryotes by copying and filling in the table below.

 

21. The photo below shows a protozoan. What structures enable it to get food into its mouth?

Dr. Dennis Kunkel/Visuals Unlimited/Getty Images

The protozoan has cilia around its mouth, which catch food and move it toward the mouth.

 

Writing in Science

22. Write a five-sentence paragraph relating the cytoskeleton to the walls of a building. Be sure to 
     include a topic sentence and a concluding sentence in your paragraph.

 

Student answers will vary. The cytoskeleton provides support for cells just like the walls in a building. The proteins that make up the cytoskeleton are joined together, similar to walls. The cytoskeleton provides a framework for the cell, just as walls provide a framework inside a building.

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Levels of Organization of all living things!

 

Cells > Tissue > Organ > Organ Systems > Organisms

 

Cells make up tissue
Tissues makes up organs
Organs make Organ Systems
Organs Systems make
Organisms