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The Cell

Lab 3 Cell Structure & Func on

Lab 3: Cell Structure & Func on

Introduc on

A cell is the fundamental unit of life. All living organisms originate from a single cell. Some remain as a single cell, while others become mul cellular (like you!). Though most cells are di cult to see with the naked eye, using the microscope, cytologists have iden ed many of their features. These range from the characteris cs of the outer membranes, to internal structures such as the nucleus and mito chondria and have become the founda on for what is now known as “cell theory”.

Cell theory states:

All cells are generated from previous cells All cells pass on their gene c informa on All living things are made of cell(s) Energy metabolism occurs inside cells The chemical make up of cells is similar

Although all organisms are made up of cells, not all cells are iden cal. Prokaryotes and eukaryotes are two structurally di erent types of cells.

Prokaryotes are the most primi ve and basic organisms, and span the taxonomic classes of bacteria and archaea. They lack a membrane bound nucleus and membrane bound organelles (specialized structures). The term prokaryote comes from the La n words “pro” (before) and “karyote” (nucleus).

Eukaryote are much more complex organisms with two characteris cs that set them apart from prokaryotes: a de ned nucleus and membrane bound organelles. The term “eukaryote” comes from the La n words “eu” (true) and “karyote” (nucleus). Pro sts, fungi, plant and ani mal cells are all eukaryo c cell(s).

Concepts to explore:

What is a cell? Prokaryotes Eukaryotes

Cell structure Func on of cell structures Di usion Rare of Di usion

Cytologists are scien sts who study cells. The study of the cell is known as cytology.

Lab 3: Cell Structure & Func on

Prokaryotes Eukaryotes

Bacteria and archaea (both prim i ve cells) are the only prokary otes

Are very small (.1μm to 2μm)

Reproduce asexually. This means sexual reproduc on is absent, and there is li le gene c varia on between genera ons

Have simple cellular components

Are capable of living almost any where and o en thrive in harsh condi ons

Are unicellular

2 Billion years younger than pro karyo c cells

Great biological diversity

All mul cellular organisms are eukaryotes

Signi cantly larger than most prokaryo c cells

More complex shapes and inter nal structure than prokaryotes

Some are capable of capturing light energy (chloroplasts in plant cells and cones and rods of the eye)

Figure 1: A prokaryo c cell showing some of the major structures

Nucleoid (nucleus-like) Region

Lab 3: Cell Structure & Func on

Both prokaryotes and eukaryotes have a plasma membrane (also known as the cell membrane) that separates the cellular content from the external environment. This structure is o en referred to as a phospholipid bi layer, as it is composed of two layers of lipids with proteins oa ng between these lay

Figure 2: Major structures of eukaryotes; Top: an animal cell; bo om le : a plant cell; Bo om right: a para mecium.

Plant Cell

Lab 3: Cell Structure & Func on

ers. The proteins in the structure are responsible for carrying out the majority of the func ons speci c to the membrane and impart a selec vity to certain materials that can pass through the membrane. Many cells within the prokaryo c and eukaryo c families have cell walls outside the cell membrane hat help to protect them and provide support (note: animal cells and protozoa do not have cell walls). Un like the cell membrane, this barrier is not selec ve and does not allow materials to pass through easily. Prokaryo c cells have a thick, rigid cell wall composed of amino acids and sugars (pep doglycan), but the cell wall composi on within eukaryotes varies (e.g., fungi cell walls include a polysaccharide called chi n while plants exhibit cell walls with the polysaccharide cellulose).

In all cells, the plasma membrane encases the cytoplasm (also called cytosol), which is a semiliquid, gel like substance that is the founda on of the cell. Within the cytoplasm of eukaryo c cells, a number of membrane bound organelles exist to provide speci c func ons within the cell. Prokaryotes do not have these specialized bodies to compartmentalize the intercellular func ons and are therefore everything is free oa ng within the cell. As you examine the structures of prokaryotes and eukaryotes in Figures 1 and 2, you will note these di erences.

Some Organelles Found in Eukaryotes:

Nucleus: Houses the gene c content (DNA) of the cell.

Nuclear Envelope: An outer membrane that surrounds the nucleus.

Nuclear Pores: Holes in the nuclear envelope that permit communica on between the internal nuclear environment and the cytoplasm.

Nucleolus: (plural: nucleolus) A part of the nucleus that is made of RNA, Protein and Chroma n and manufactures RNA and ribosomes.

Ribosomes: Ribosomes are large molecules found in all living cells. Ribosomes are responsible for catalyzing protein forma on during transla on. A strand of mRNA docks onto a ribosome molecule and the correct amino acids are then recruited to the ribosome to create a protein.

Mitochondrion: (plural: mitochondria) The “power plant” of the cell. They are a membrane bound organelle (inner and outer membrane) with their own circular DNA, and make ATP (energy) for the rest of the cell.

Endoplasmic Re culum (ER): A series of membranes extending throughout the cytoplasm that can be peppered with ribosomes (rough ER) or not (smooth ER) and is the site of protein syn thesis within a cell.

Golgi Apparatus: (also called the Golgi Body) A series of a ened sack like bodies that process es the cell’s proteins and lipids before they are released to their nal des na on.

Peroxisomes: Contain enzymes that help the cell destroy toxins.

Lysosomes: A sack of enzymes found within the cell that aid in the diges on of food into usa ble products for the cell.

Cytoskeleton: The “skeleton” found in all eukaryo c cells that provides shape to the cell while also enabling it to move. It consists of three parts:

1. Micro laments: Small strands that help the cell resist tension. Think of it as a piece of wire.

Lab 3: Cell Structure & Func on

2.Intermediate laments: Anchors the organelles in the cell and provide addi onal stability.

3.Microtubules: Small hollow tubes that help the cell maintain its shape, move things around within the cell and form other key structures.

Centriole: Barrel shaped structures that help make cilia and agella. They also play a key role in cell division.

Cilia: Small “hairs” on the outside of the cell. They help the cell move and are sensory recep tors.

Flagella: The structure of eukaryo c agella is far more complex than prokaryo c agella as the consist of mul ple laments. They provide mobility by rota ng back and forth, they help transport uids and serve as sensory receptors.

Chloroplast: Think of them as the plant version of mitochondria. The main di erence is that they take light energy and convert it to mechanical energy.

Vacuole:Membrane bound “sacs” that provide storage and provide transporta on within the cell (excre on, secre on).

Vesicle: Plays a similar role to vacuoles, but are smaller.

Structure Prokaryo c Cell Eukaryo c Cell

Nucleus No Yes

Plasma Membrane Yes Yes

Cell Wall Yes Yes (in most cells)

Cytoplasm Yes Yes

Flagella and Pili Occasionally Flagella Occasionally

Pili No Cilia No Occasionally

Glycocalyx Occasionally Occasionally

Cytoskeleton No Yes

Endoplasmic Re culum No Yes

Mitochondria No Yes

Golgi Apparatus No Yes

Chloroplast No In plants and many pro sts

Ribosome Yes Yes

Lysosome No Yes

Peroxisome No Yes

Vacuole and Vesicle No Yes (in most cells)

Prokaryo c vs. Eukaryo c Cells

Lab 3: Cell Structure & Func on

Although prokaryo c cells do not have a nucleus, they do have DNA. The DNA is a closed loop and ex ists freely in an unorganized manner within the cytoplasm, in an area known as the nucleoid region. They can also have a slime coa ng, called the glycocalyx, which is used to protect the cell and enable it to a ach to surfaces (such as teeth and lungs). Prokaryotes also have ribosomes to facilitate the pro duc on of proteins. All cells, prokaryotes and eukaryotes, must have a means to regulate nutrients and wastes, and also require a supply of energy to exist. Metabolic ac vi es such as photosynthesis and respira on can be carried out by both cell types. In eukaryotes, photosynthe c ac vity ini ates in the chloroplasts, while in prokaryotes it occurs in the thylakoid.

Regardless of the cell type or structure, di usion of molecules is almost always a factor. Molecules are constantly in mo on due to the kine c energy pre sent in every atom. This energy results in the net movement of molecules from areas of high concen tra on to areas of low concentra on, or di usion (Figure 3). If uninhibited, this movement will con n ue un l equilibrium is reached and the molecules are uniformly distributed.

The rate of di usion depends on the medium used, size of the molecule, and polarity of molecule. Be cause the medium will not change in a biological sys tem, the di usion rate is usually dictated by molecu lar characteris cs. Small, non polar molecules ex hibit a higher rate of di usion than large, charged ones.

The direc on of di usion depends on concentra on gradients, heat and pressure. The concentra on gradient is the change of molecular density over a given area. Temperature and pressure typically re main constant in biological systems, making the concentra on gradient the best indicator of direc on ality. In general, molecules will move towards areas of lower concentra ons.

Figure 3 Di usion through a semi permeable membrane

(lipid bilayer)

Lab 3: Cell Structure & Func on

Experiment 1: Iden fying Cell Structures

View the slide pictures and images below, paying a en on to detail, and note the di erent characteris cs of prokaryotes and eukaryotes. On each picture, label the parts indicated if they are visible. If you

can not see them, draw and label them where they would be located.

Bacteria: Nucleoid region, cell wall, plasma membrane, ribosomes, agella

Pro st: Macronucleus, micronucleus, plasma membrane, cytoplasm, contrac le vacuole

Figure 3

Figure 4

Lab 3: Cell Structure & Func on

Figure 6

Figure 5

Plant Cell: Nucleus, cell wall, plasma membrane, cytoplasm, chloroplast, mitochondria, vacuoles

Animal Cell: Nucleus, nucleolus, plasma membrane, cytoplasm, mitochondria, golgi apparatus, rough ER, ribosome

Lab 3: Cell Structure & Func on

Ques ons

1. For each structure iden ed, do you think its loca on a ects its ability to func on? Why or why not? (Hint: those buried deep in the cell probably do di erent things than those closer to the cell membrane)

2. Draw a labeled diagram of a small sec on of the plasma membrane and brie y describe its structure and func on.

3. Describe the di erences between animal and plant cells.

4. Which of the following structures are present in both prokaryo c and eukaryo c cells? Plas ma membrane, Golgi apparatus, DNA, lysosomes and peroxisomes, cytoplasm

5. Where is gene c material found in plant cells?

6. Mitochondria contain their own DNA (circular) and have a double membrane. What explana on for this observa on can you come up with?

(Hint 1: Where else do we see circular DNA?) (Hint 2: What do you know about the rela ve age of eukaryo c cells?)

7. How is the structure of the plant’s cellulose based cell wall related to its func on?

8. Defects in structures of the cell can lead to many diseases. Pick one structure of a eukaryo c cell and develop a hypothesis as to what you think the implica ons would be if that structure did not func on properly.

Lab 3: Cell Structure & Func on

9. Using books, ar cles, the internet, etc. conduct research to determine if your hypothesis was correct.

Experiment 2: Direc on and Concentra on Gradients

In this experiment, we will inves gate the e ect of solute concentra on on osmosis. A semi permeable membrane (dialysis tubing) and sucrose will create an osmo c environment similar to that of a cell. Using di erent concentra ons of sucrose (which is unable to cross the membrane) will allow us to ex amine the net movement of water across the membrane.

Materials

30% Sucrose solu on

4 15 cm Pieces dialysis tubing**

3 250 mL Beakers

8 Rubber bands

Concepts to explore:

Water*

Watch*

*You must provide

**Cut to exact length

Note:

Dialysis tubing can be rinsed and used again if you make a mistake.

Dialysis tubing must be soaked in water before you will be able to open it up to create the dialysis “bag”. Follow the direc ons for the experiment, beginning with soaking the tubing in a beaker of water. Then, place the dialysis tubing between your thumb and fore nger and rub the two digits together in a shearing manner. This should open up the "tube" so you can ll it with the di erent solu ons.

Lab 3: Cell Structure & Func on

Procedure

1. Submerge the four pieces of dialysis tubing into a 250 mL beaker lled with 100 mL of water for at least 10 minutes.

2. A er 10 minutes, remove one piece of tubing from the beaker. On one end (not the whole tube), gently twirl the tubing into a long, thin cylindrical piece that is able to t into the hole of the yellow bead.

3. Insert the long cylindrical end of the tube into the center hole in the yellow bead. Once it is through, pull the cylindrical end un l there is about 1.5 to 2 cm of tubing extending beyond the bead

4. Take the extra tubing you just pulled through the bead and fold it back over the bead, towards the remaining, non folded tube. Place a rubber band above the bead and around the extra tubing as to be sure no solu on can leak out of the tube (see Figure 4).

To test that no solu on can leak out, add a few drops of water and look for water leakage. Make sure you pour the water out before con nuing to the next step.

5. Repeat steps 2 4 with the three remaining dialysis tubes, using each of the three remaining bead colors (Figure 5).

6. Table 1 provides a dis nc on as to what bead belongs to which tube. Using a 10 mL graduated cylinder, measure and ll the appropriate dialysis bag with the designated concentra on of su crose solu on (3%, 15% or 30%) by adding the volumes of sucrose and water listed in Table 1.

Figure 4: Fold the bag un l you have a piece narrow enough to be threaded

through the bead.

Figure 5: Beads help to secure the ends of the dialysis bags and iden fy each one.

Lab 3: Cell Structure & Func on

7. Rinse the outside of the bags with water to remove any remaining sucrose.

8. Pour 150 mL of the stock sucrose solu on (30%) into the 250 mL beaker (beaker #1). Using the graduated cylinder, measure 20 mL of the stock sucrose solu on and 180 mL of water to cre ate a 3% sucrose solu on and place it into the 250 mL beaker (beaker #2).

9. Place bags #1 3 (red, blue, yellow) into beaker 2 and bag #4 (green) into beaker 1 (Figure 6).

10. In Table 2, predict whether water will ow in or out of each dialysis bag.

11. Allow the bags to sit for one hour. While wai ng, dump out the water in the 250 mL beaker that was used to soak the dialysis tubing in step 1. We will use this in the last part of the ex periment.

12. A er allowing the bags to sit for one hour, remove them from the beakers.

Bead Color Bag Number Stock Sucrose Solu on Water

Yellow Bag #1: 30% sucrose 10 mL 0 mL

Red Bag #2: 15% sucrose 5 mL 5 mL

Blue Bag #3: 3% sucrose 1 mL 9 mL

Green Bag #4: 3% sucrose 1 mL 9 mL

Table 1: How to Make a Serial Dilu on of Sucrose

Figure 6: The dialysis bags are lled with varying con centra ons of sucrose solu on and placed in one of two beakers.

Lab 3: Cell Structure & Func on

13. Carefully open the bags, no ng that o en mes the tops may need to be cut as they tend to dry out. Measure the solu on volumes of each dialysis bag using the empty 250 mL beaker. Record your data in Table 2.

Ques ons

1. For each of the bags, iden fy whether the solu on inside was hypertonic, hypotonic or isotonic in comparison to the beaker solu on it was placed in.

2. Which bag increased the most in volume? Why?

3. What does this tell you about the rela ve tonicity between the contents of the bag and the solu on in the beaker?

4. What would happen if bag 1 is placed in a beaker of dis lled water?

Ini al Volume Sucrose % Predic on: Will water move in or out? Final Volume

Bag#1 10 mL

Bag #2 10 mL

Bag #3 10 mL

Bag #4 10 mL

Table 2: Water Movement