Overview of the Structure of the Cell

Protoplasm - includes everything in the cell

a). Cytoplasm - everything outside the nucleus

b). Nucleus - control center of the cell

Inside the Cytoplasm:

1). Mitochondrion(a) - the powerhouse of the cell. Provides energy for the cell through a process called cellular respiration. {C6H12O6 + 6O2 6CO2 + 6H2O + ENERGY (ATP)}

- bean-shaped, has its own membrane and its own DNA

2). Ribosomes - are where proteins are made (enzymes too)

- use building blocks called amino acids

- smallest organelle in the human body

3). Endoplasmic Reticulum (ER)

- a series of canals carrying messages throughout the cytoplasm.

- there are two types:

a). Rough ER - this is the site where specialized proteins are produced. (Eg. Pancrease - balancing sugar levels)

b). Smooth ER - this where fats and lipids are made to produce things like steroids.

4). Golgi Apparatus (complex) - packs proteins for transport out of the cell

- looks like a stack of pancakes

5). Lysosome - known in the text as "suicide sacs" because when they enter an area the foreign object is digested as well as the lysosome.

- the white blood cells carry lots of lysosomes

6). Microtubules - are hollow pipe-like structures used for transporting materials around and are link to cell movement mechanisms like cilia and flagella.

7). Microfilaments - are solid thread-like structures used for support and movement

8). Vacuoles - large sacs that are used to store minerals, proteins ans sugars

- "water sacs"

- very large in plants

9). Cell Membrane - a selectively-permeable phospholipid bilayer membrane with proteins embedded throughout for the purposes of structure, transport and cell communication.

Specific to Plants:

10). Plastids - are bean-shaped organelles the store and produce food and pigments

There are three types:

Chloroplasts - contain chlorophyll which is used in a process known as Photosynthesis.

Chromoplasts - contain yellow and orange pigments

Amyloplast - colorless and contain starch (eg. Potatoes)

11). Cell wall - protects the cell and provides structural support

- mainly made up of cellulose

- a pectin-like substance between the primary and secondary cell wall.

- pectin is used to make jello and jam solids

Parts of the Nucleus:

12). Nuclear envelope (membrane) - surrounds the nucleus and contains the nuclear material.

13). Nuclear Pore - passageway between nucleus and cytoplasm

14). Nucleus - the control center of the cell.

15). Nucleolus - unknown, has been linked to the synthesis of ribosomes

16). Chromatin - genetic material of the cell.

The Fluid Mosaic Model: Plasma Membrane

• lipids and proteins are the main ingredients of membranes, although other chemical compounds are present.
• lipids are a group or family of compounds such as fats, phospholipids, and steroids, that are insoluble in water.
• proteins are a 3-D biological polymer constructed from a set of 20 different monomers called amino acids.
• the phospholipid bilayer has 2 layers of phospholipids with the hydrophobic tails pointing towards one another and the hydrophilic heads face the water.
• the proteins are spread throughout the membrane and are not on the surface of the membrane but are in fact embedded in it.
• it has also been found that the membrane is not fixed in place but actually the membrane is moving as if it were a fluid, hence the word fluid in the name.

Why call it fluid? Experiments with human and mice cells fused together have shown that proteins specific to humans and those specific to mice eventually spread around the entire surface of the new fused cell. It was also noted that certain proteins do not move because they are attached to the cytoskeleton, a network of microfilaments and microtubules that maintain the shape of the cell. The plasma membrane remains fluid as the temperature decreases, until finally, at some critical temperature, it solidifies, much like bacon grease when it cools. The plasma membrane is as fluid as salad oil.

Why is it mosaic?

• a membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer.
• the plasma membrane and the membrane of the various organelles each have their unique collection of proteins.
• more than 50 kinds of proteins have been found in the plasma membrane of red blood cells, for example, and more have not yet been detected.

Some functions of the membrane proteins

• Transport proteins: i) a protein may act as a tunnel through the membrane. ii) some proteins use ATP to pump substances across the membrane

• Enzymes: a protein built into the membrane that can be used to control reaction rates of substances that it comes in contact.
• Proteins as receptor sites: a protein with a specific shape that allows the connection of a messenger molecule known as a hormone. Hormones binding to this site cause specific chemical reactions to occur in the cell.
• Cell Adhesion: membrane proteins connect with similar proteins of adjacent cells.
• Attachment to the cytoskeleton: microfilaments of other elements of the cell structure may be bonded to membrane proteins to maintain the shape of the cell or to lock down certain proteins to a certain area.

Other important molecules of the plasma membrane

• glycoproteins - proteins that carry special sugars on the surface that act as identifiers. These sugar molecules differ between individuals of the same species. In humans they help to distinguish blood types. It will also help your immune system to identify foreign invaders. This also helps to explain why transplant organs are often rejected by recipients.
• glycolipids - are sugars that are in place of the phospholipids. These may include things like cholesterol. May also aid in identification or recognition.
  Passive and Active Transport

• the cell membrane acts as a selectively-permeable barrier
• the cell must be able to take in food and eliminate wastes
• the movement of materials across a cell membrane without cell energy expenditure is called Passive Transport
• transport proteins in the cell membrane can allow certain materials to diffuse across the membrane.
• There are 3 types of passive transport:

i) diffusion

ii) osmosis

iii) facilitated diffusion

Diffusion

• molecules move about randomly and collide (called Brownian motion)
• diffusion is the movement of molecules from an area of high concentration to an area of low concentration (e.g. Perfume)
• molecular collisions cause diffusion

Osmosis

• Osmosis is the diffusion of water through a selectively-permeable membrane. (Such as cheese cloth of a coffee filter)
• solute - molecules that dissolve in water
• solvent - water or the dissolver
• Isotonic solutions - are solutions in which the concentration of solute molecules outside the cell are equal to the concentration inside the cell. (Water in the cell does not move)
• Hypotonic solutions - are solutions in which the concentration of the solute molecules outside the cell is lower than inside the cell. (Water moves into the cell)
• Hypertonic solutions - are solutions in which the concentration of the solute molecules outside the cell is higher than inside the cell. (Water leaves the cell)

Facilitated Diffusion

• transport proteins in the cell membrane are similar in function to enzymes. When they come in contact with specific molecules or ions they activate and bind with the molecule. To unbind with the molecule that protein must expel the molecule into the cell.

Active Transport

• This involves the use of cell energy to move materials across the cell membrane against the concentration gradient.
• The movement of materials from an area of low concentration to an area of higher concentration is called active transport.
• Endocytosis - is the process by which particles too large to pass through the cell membrane are transported into the cell. There are two types: Pinocytosis and Phagosytosis.
• certain ions and amino acids are moved into the cell by active transport. This means that the cell is using transport proteins and at the same time using up metabolic energy to obtain these particles of matter.

Surface Area to Volume Ratio

Procedure

1. Prepare a table with the column headings Cube, Volume, Surface Area

2. Calculate the volume and surface area of each of the following cubes:
a) micro 1 cm x 1 cm x 1 cm
b) macro 10 cm x 10 cm x 10 cm
c) multimicro each side made of 10 micro cubes

Questions

1. When the cube dimensions increase by a factor of 10 (from 1 cm to 10 cm), what is the factor of increase for the surface area? The volume?

2. For the two cubes of 10 cm x 10 cm x 10 cm, the size and volume have a factor of one. Calculate the factor for the difference in surface area between these two cubes.

3. For which two cubes does the ratio between surface area and volume stay the same when the size increases?

4. Imagine three cells with the dimensions given.

Cell	Length (cm)	Width (cm)	Height (cm)
A	1	1	1
B	2	0.5	1
C	10	0.1	1

Calculate the surface area to volume ration for each. What do you notice?

Organelles Review

1. What are the three points in the Cell Theory?

2. List some difference between prokaryotic and eukaryotic cells.

3. What is the endosymbiont theory?

4. What is some of the evidence for the endosymbiont theory?

5. Why is surface area:volume ratio important for cells?

6. What is the difference between cytoplasm and cytosol?

7. Organelle means "little organ." How are organelles similar to organs?

8. All large organisms have cells with organelles. What advantage do organelles give the cell?

9. Describe the current model of the cell membrane.

10. The cell requires instructions in order to function correctly; where are these instructions found?

11. Describe the characteristic of mitochondria that makes them well suited to their role in the cell.

12. How are the functions of mitochondria and chloroplasts similar?

13. By comparing a bee's body mass to its wing span, it has been calculated that a bee should not be able to fly. Cell biologists have since found that the muscles which control the wings of the bee have a huge number of mitochondria. Explain why this discovery may help explain why bees are able to fly.

14. What is the function of the ER?

15. Describe the function of the Golgi apparatus.

16. What is the function of the ribosome?

17. Cells of the stomach lining have large numbers of ribosomes and Golgi. Explain.

18. What are the functions of lysosomes?

19. A Biology 11 student has observed an amoeba and made a drawing. You are marking the drawing and notice that he has drawn in a chloroplast. Do you take marks off or not? Explain.

20. Name some types of organisms that have cell walls and state what the walls are made of.

21. How does a cell membrane differ from a cell wall?

22. The ER is called "rough" if it has what organelle attached to it?

23. List the organelles that are only found in plants. List those found only in animals.

Membrane Structure & Function

1.         General

            a.         The cell membrane forms a barrier between the cell and the external environment

            b.         A membrane performs several jobs for the cell:

i.         The cell must be able to retain the molecules it requires.

                        ii.        The cell must be able to exclude unwanted molecules.

                        iii.       Cells recognize one another by molecules attached to their membranes.

            c.         Membranes are selectively permeable barriers. This means that some molecules are able to pass through while others cannot.

i.         Substances able to pass through a membrane

                                    (1)       Nonpolar molecules like fats lipids.

                                    (2)       Small polar molecules like water.

                        ii.        Substances that are unable to pass through a membrane

                                    (1)       Ions and charged molecules like salts dissolved in water.

                                    (2)       small polar molecules like glucose.

                                    (3)       Macromolecules

2.         Membrane structure

            a.         Cell membranes are made of two layers of phospholipids sandwiched together.

            b.         Remember from the organelles section, that the membrane is a fluid mosaic. Components of the membrane can move sideways like buoys floating in water.

            c.         Phospholipids have a hydrophilic (or polar) head and two long, hydrophobic (or nonpolar) tails. This means the heads like to be near (or interact with) water while the tails prefer to be away from (or not interact with) water. Molecules like this are called amphipathic.

            d.         In general, nonpolar molecules do not interact with polar molecules. This can be seen when oil (nonpolar) is mixed with water (polar). Polar molecules interact with other polar molecules and ions while nonpolar molecules interact with other nonpolar molecules. E.g., table salt (ionic) dissolves in water (polar).

            e.         The easiest way to satisfy both heads and tails is for the phospholipids to from two layers with the tails between them and the heads facing water on both sides. Phospholipids do this spontaneously. The nature of phospholipids also means that any small tear in a membrane will repair itself spontaneously so that the tails avoid contact with water. It also means that two membranes can fuse together very easily.

3.         Membrane function

            a.         The primary function is to control the passage of materials into and out of the cell.

            b.         Membranes contain other molecules as well

i.         Proteins in the membrane do a variety of jobs like:

                                    (1)       channels for moving materials into and out of the cell

                                    (2)       markers used for cell recognition

                                    (3)       receptors for hormones

                                    (4)       pumps for transporting materials across the membrane

                        ii.        Glycoproteins are proteins with sugars attached. They function in cell recognition, allowing cells to identify other cells. This is particularly important in the immune system where cells need to be able to identify and destroy foreign invaders such as bacteria or viruses.

                        iii.       Proteins associated with the cell membranes of animal cells may bind to proteins of adjacent cells.

Membrane Transport

1.         The cell membrane prevents equal motion of water and dissolved substances from inside to outside the cell. Many substances cannot pass through biological membranes. The transport of materials across a cell membrane falls into two general categories.

2.         Passive transport - does not require energy use by the cell.

            a.         Diffusion

i.         All molecules move continuously by random simple diffusion. This movement is spontaneous and does not require energy. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.

                        ii.        Heat energy causes molecules to move randomly. This motion is called Brownian motion.

                        iii.       If the concentration of molecules in 2 areas is different diffusion will cause molecules to move from the area with the higher concentration to the area with the lower concentration.

                        iv.       The greater the concentration difference, the more rapid the net diffusion.

                        v.         Diffusion evens out the concentrations so they are equal everywhere, i.e., equilibrium - when there is uniform concentration.

                        vi.       The movement is due to collisions between particles and a few factors affect the rate

                                    (1)       Temperature - rate of diffusion increases as temperature increases.

                                    (2)       Pressure - rate of diffusion increases as pressure increases.

                                    (3)       Concentration - rate of diffusion increases as concentration increases.

                                    (4)       Size - rate of diffusion decreases as molecule size increases.

            b.         Osmosis

i.         Remember that, in a solution, the substance present in the greatest amount is called the solvent. In biology the solvent is almost always water.

                        ii.        Solutes are the substances dissolved in the solvent.

                        iii.       When water diffuses across a selectively permeable membrane, it is called osmosis. It occurs when a solute cannot pass through a membrane but the solvent (water) can.

                        iv.       Because it is a special kind of diffusion, osmosis occurs spontaneously and requires no energy.

                        v.         Water moves from an area where there is more water to an area where there is less water. In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.

                        vi.       The amount of water in a solution is indirectly proportional to the amount of solute in the solution. Think of a dilute solution (call it A) as having a high water concentration (because it has few solutes) and a concentrated solution (call it B) as having a lower water concentration (because it has lots of solutes). If these two solutions were separated by a selectively permeable membrane, water would flow from high water to low water concentration. I.e., from A to B.

                        vii.      3 types of solutions

                                    (1)       Hypotonic - a solution which is more dilute (i.e., less solutes) than the cytosol. The cell gains water and swells. This is a problem for many freshwater organisms.

                                    (2)       Hypertonic - a solution which is more concentrated (i.e., more solutes) than the cytosol. The cells loses water and shrinks. This is a problem for many marine organisms.

                                    (3)       Isotonic - a solution which has the same concentration of solutes as the cytosol. The cell neither gains nor loses water and remain unchanged.

                                    (4)       Osmotic swelling dilutes the cytosol and can eventually cause the cell to burst (lyse). The opposite problem, the cell shrinking, would occur in a hypertonic solution. Cells have different ways of dealing with these differences in concentration

                                                (a)       Cell walls of plant, fungal and bacterial cells are rigid and prevent swelling. The walls are strong enough to allow a fairly high pressure gradient. In plants this is called turgor pressure. Plant cells placed in a hypertonic solution will undergo plasmolysis, a condition where the plasma membrane pulls away from the cell wall as the cell shrinks. The cell wall is rigid and does not shrink.

                                                (b)       Some simple, single-celled organisms have contractile vacuoles which store excess water and then squirt it out.

                                                (c)       Most cells pump ions out of the cell. This increases the solute concentration outside the cell and water follows by osmosis.

                                                (d)       In complex organisms such as humans, the blood is isotonic to cytosol so that cells do not have to face these problems.

            c.         Facilitated Diffusion

i.         Some important molecules, like glucose, are helped across the cell membrane so that they move into a cell faster.

                        ii.        Special protein channels help move these substances across the membrane.

                        iii.       Each protein channel is specific for the molecule it is transporting.

                        iv.       This process is spontaneous and does not require energy.

3.         Active transport

            a.         Active transport

i.         Often, a cell requires substances that are at a lower concentration outside the cell than inside the cell. These substances will not move by diffusion.

                        ii.        Special proteins in the membrane use energy to transport these substances into the cell. Note the similarity to facilitated diffusion.

                        iii.       Enables a cell to concentrate materials inside itself that are a a low concentration in the environment.

                        iv.       In humans, active transport can account for 30% of your resting energy use.

            b.         Endocytosis (phagocytosis and pinocytosis)

i.         These processes are used for macromolecules that are too big to pass through the cell membrane normally.

                        ii.        The cell membrane bends inward, forming a vesicle containing extracellular fluid and other substances dissolved in it.

                        iii.       Can bring in large molecules such as proteins which would not diffuse across the cell membrane.

                        iv.       The vesicle is then fused with a lysosome to be digested macromolecules.

                        v.         Phagocytosis is bringing particles into the cell while pinocytosis is bringing in fluid.

                        vi.       Exocytosis is the opposite - materials are excreted from a cell but the mechanism is the same.

4.         Comparison of simple diffusion, facilitated diffusion and active transport.

Property	Simple Diffusion	Facilitated Transport	Active Transport
Requires special membrane proteins	No	Yes	Yes
Highly selective	No	Yes	Yes
Transport saturates	No	Yes	Yes
Can be inhibited	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP energy	No	No	Yes

Diffusion and Osmosis Through Membranes

Cell membranes are selectively-permeable by nature and as such, exert some control over substances passing through them into and out of cells. In the presence of a concentration gradient, substances will diffuse from an area of high concentration to one of low concentration across a selectively permeable membrane. Water, too, will diffuse through membranes in a process called osmosis. The net movement of water is from an area of high [H₂O] (i.e., low [solute]) to one of low [H₂O] (i.e., high [solute]).

Starch molecules react with iodine to form a dark blue compound. Clinitest tablets are used to detect the presence of glucose. If glucose is present, the mixture will turn a series of shades of green. The appearance of a yellow or brown compound means the glucose is rather concentrated (check the package for concentration ranges).

A. Osmosis

Procedure

1. Obtain a 10 cm piece of 2.5-cm dialysis tubing that has been soaking in water. Twist one end of the tubing and fold it over, then tie off that end with string to form a bag. To open the other end of the bag, rub the end between your thumb and forefinger until the sides separate.

2. Pour molasses solution into the tube until it is about 5 cm from the top.

3. Twist the top of the tube and tie securely. Be sure to leave a gap of about 2 cm between the top of the liquid and the thread.

4. Rinse the outside of the tube with water to remove any molasses which may have contaminated it.

5. Place about 300 mL of water in a 500 mL beaker and place the tube inside. Label the beaker 'A.'

6. Let stand overnight and record observations in the morning.

B. Diffusion

Procedure

1. Prepare another tube as in procedure A but pour starch solution in the tube.

2. Tie and rinse the tube.

3. Place the tube in a beaker of water and label it 'B.'

4. Add iodine solution to the water slowly until the water is a faint yellow color.

5. Let stand 20 min and record observations.

6. Prepare a similar setup except put glucose solution in the tube and do not add iodine solution. Be sure to rinse the tube before putting it in the beaker of water. Label this beaker 'C.' Let stand 20 min.

7. Pour about 5 mL of water from beaker 'C' into a test tube and add half of a Clinitest tablet. Do not shake the test tube. Record the color about 15 s after the tablet stops bubbling.

8. Let beaker 'B' stand overnight and then record any changes.

Thought-provoking Questions (at least, that's the idea)

1. Explain your observations.

2. Is the membrane 100% selective? Explain.

3. Suppose that a tube filled with a 5% molasses solution was placed in a beaker filled with a 10% molasses solution. What would happen? Why?

4. Describe and account for any changes that occurred in beaker 'A.'

5. Describe and account for any changes that occurred in beaker 'B.'

6. Describe and account for any changes that occurred in beaker 'C.'

7. What happened to the tube in beaker 'B' after it was allowed to stand overnight? What does this suggest?

8. For each of the following questions, explain your answers.
a) Which molecules pass through the membrane with ease?
b) What molecules pass through the membrane slowly?
c) What molecules were unable to pass through the membrane?

Diffusion and Osmosis Review

1. Discuss three factors that affect diffusion rates.

2. Describe the difference between active and passive transport.

3. What is a selectively permeable membrane?

4. Explain the process of osmosis.

5. What factor determines the direction in which molecules will move during osmosis?

6. The point at which water molecules enter and leave a cell at the same rate is called .

7. Define isotonic, hypotonic, and hypertonic.

8. What happens to the size of a cell placed in a hypotonic solution?

9. You are fertilizing the lawn one day and spill some fertilizer in one spot. What do expect to observe over the next few days? Explain.

10. How does facilitated diffusion differ from normal diffusion and osmosis? Why is it a benefit to cells?

11. What is meant by the terms turgor pressure and plasmolysis?

12. Why is turgor pressure not used in reference to animal cells?

13. How do the effects of osmosis differ in plant and animal cells?

14. The movement of molecules from a higher concentration to a lower concentration is
a) active transport (b) transpiration (c)absorption (d) diffusion

15. When cells are placed in isotonic solutions,
a) the cell contents shrink (b) the cell remains in a state of homeostasis (c) the cell contents swell (d) the concentration gradient increases

16. Water moves mainly in which direction through the pores of a cell placed in a hypertonic solution?

17. Osmosis is the diffusion of _____.

18. Which of the following conditions describe a hypotonic solution that a cell is placed in?
a) a higher solute concentration outside the cell (b) a lower solute concentration outside the cell (c) a equal solute concentration both inside and outside the cell (d) none of the above

19. Movement of molecules across the plasma membrane requiring the aid of cellular energy is
a) passive transport (b) active transport (c) osmosis (d) diffusion

20. The movement of molecules from a region of low concentration to a region of high concentration is
a) endocytosis (b) osmosis (c) diffusion (d) active transport

21. The red blood cell which is about 80% water will do the following when placed in distilled water
a) shrink (b) swell (c) stay the same (d) plasmolyze

22. A plant cell placed in a salt solution will undergo
a) plasmolysis (b) diffusion (c) turgor (d) deplasmolysis

23. A ___ barrier allows only certain particles to pass through.

24. Which of the following require the expenditure of energy by the cell?
a) osmosis (b) exocytosis (c) diffusion (d) phagocytosis (e) active transport (f) endocytosis (g) facilitated diffusion

25. Suppose that the concentration of carbon dioxide in the fluid outside a cell became higher than that on the inside. Predict what would happen.

26. A marathon runner collapses after running on a hot day. Although the runner consumed adequate water along the route, blood testing showed that many of his red blood cells had burst. Why was this the case? (hint: on hot days, runners drink fluids that contain sugar and salt.)

27. When gardeners bring in fresh vegetables from the garden, they sometimes soak them in saltwater before rinsing them and soaking them in freshwater. Why would they do this?