AP Biology Unit 2: Cell Structure and Function Notes Pt. 1

Collegeboard Video 2.2

1) Choice B- It was mostly incorporated into proteins that manage metabolic reactions. 

The number is largest in mitochondria since its main function is to produce energy from macromolecules. This energy is then used to help the entire cell function by regulating and managing metabolic reactions

Collegeboard 2.3 video 1

Incorrect answer - Choice A

Choice A is incorrect since the graph shows that the surface area to volume ratio vs the length of sides of cube shaped cells are constant. This is incorrect since in the exercise, it is shown that surface area is calculated by SA = s^2 and V = s^3. When going through the activity with the beets and bleach, the data was taken and the conclusion was that the surface area to volume ratio decreases as the length of sides of cube shaped cells increases. Therefore, the data would represent this statement by having a negative slope, which is choice D. 

Correct answer - Choice D

Collegeboard 2.3 video 2 

Incorrect answer - Choice C

Choice C is incorrect because the values of the ratio are flipped. When the answer is calculated, 300/350 would be 0.86 and 2500/400 would be 0.63. This means the ratio decreased, so choice C is incorrect. It also states that more stomata are needed at higher CO2 concentrations and this is incorrect. The data reflects that fewer stomata are needed at higher CO2 concentrations. Therefore, the correct answer in this case would be choice A. 

Correct answer - Choice A

Collegeboard Practice FRQ

Read each question carefully. Write your response in the space provided for each part of each question. Answers must be written out in paragraph form. Outlines, bulleted lists, or diagrams alone are not acceptable and will not be scored.

The stems and fruits of pineapple plants contain a group of protein-digesting enzymes collectively called bromelain and often used as an antibrowning agent for fruits and vegetables. Fruits and vegetables brown when they are bruised during transport or sliced and exposed to air. This browning is controlled by enzymatic pathways that produce brown pigments. The browning of fruits and vegetables reduces the nutritional value of the food, so antibrowning agents such as bromelain are used.

(a) Identify the type of monomer of which this enzyme is composed.

The protein digesting enzyme is composed of amino acids and the enzymes that cause browning are composed of amino acids too. This is because proteins are composed of amino acids.

(b) Bromelain works by breaking the enzymes that cause browning into smaller molecules. Explain how the reaction that breaks up the enzymes occurs.

The reaction that breaks up the enzymes is hydrolysis since the bromelain breaks down the amino acid by breaking the peptide bonds between amino acids.

(c) The pH of a solution determines the charge of certain R groups. The pH of pineapple fruit ranges from 3.5 to 5.2. Predict the effect on the activity of bromelain if it is used in a product with a pH of 11.

Bromelain is efficient in the acidic environment, so when placed in a basic environment, its efficiency decreases. The enzyme becomes denatured since the R group is impacted. Optimal range is 3.5 to 5.2. 

(d) Provide reasoning to justify your prediction.

The activity of bromelain would decrease since the enzyme is not in its most efficient environment. The pH of 11 is basic when compared to the pH of a pineapple, ranging from 3.5 to 5.2, which is acidic. Since the new environment is basic, the enzyme is less efficient in breaking down the enzymes that cause browning. 

Directions: Answers must be in essay form. Outline form is not acceptable. Labeled diagrams may be used to supplement discussion, but in no case will a diagram alone suffice. It is important that you read each question completely before you begin.

Water is important for all living organisms. The functions of water are directly related to its physical properties.

Describe how the properties of water contribute to TWO of the following.

• transpiration

• thermoregulation in endotherms

• plasma membrane structure

• Water plays a major role in transpiration since it is adhesive. Transpiration is the process by which water molecules move through a plant. This is only possible due to water molecules being adhesive meaning that it sticks to other surfaces. These other surfaces that the water molecules stick to are the xylem tubes which are the structure inside the plant, that acts like a tube to transport water throughout the plant, all the way to the leaf. This is mainly due to the hydrogen bonding between water molecules.

• Water also contributes to the thermoregulation in endotherms by having a high heat capacity. The high heat capacity allows for no instantaneous temperature changes to occur. This allows for an optimal environment because it is harder to change the temperature. This is caused by the hydrogen bonding between water molecules making it harder to break the bonds and causing temperature changes due to the high heat of evaporation.

• Water is a polar molecule that is a contributing factor for the structure of the plasma membrane since it causes the phospholipids to create a bilayer to allow the hydrophobic tails to be protected by the hydrophilic heads. When cells are placed in an environment, the phospholipids spontaneously react to create a barrier between the aqueous cytosol and the external environment being aqueous, which means it contains water. 

Cytoskeletal Proteins and Motor proteins Introduction

Cells are highly structured in much the same way as our own bodies. They have a network of filaments known as the cytoskeleton (literally, “cell skeleton”), which not only supports the plasma membrane and gives the cell an overall shape, but also aids in the correct positioning of organelles, provides tracks for the transport of vesicles, and (in many cell types) allows the cell to move. In eukaryotes, there are three types of protein fibers in the cytoskeleton: microfilaments, intermediate filaments, and microtubules. 

Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They have a diameter of about 7 nm and are made up of many linked monomers of a protein called actin, combined in a structure that resembles a double helix. Because they are made of actin monomers, microfilaments are also known as actin filaments. Actin filaments have directionality, meaning that they have two structurally different ends.

Actin filaments have a number of important roles in the cell. For one, they serve as tracks for the movement of a motor protein called myosin, which can also form filaments. Because of its relationship to myosin, actin is involved in many cellular events requiring motion.

For instance, in animal cell division, a ring made of actin and myosin pinches the cell apart to generate two new daughter cells. Actin and myosin are also plentiful in muscle cells, where they form organized structures of overlapping filaments called sarcomeres. When the actin and myosin filaments of a sarcomere slide past each other in concert, your muscles contract.

Actin filaments may also serve as highways inside the cell for the transport of cargoes, including protein-containing vesicles and even organelles. These cargoes are carried by individual myosin motors, which "walk" along actin filament bundles.

Actin filaments can assemble and disassemble quickly, and this property allows them to play an important role in cell motility (movement), such as the crawling of a white blood cell in your immune system.

Finally, actin filaments play key structural roles in the cell. In most animal cells, a network of actin filaments is found in the region of cytoplasm at the very edge of the cell. This network, which is linked to the plasma membrane by special connector proteins, gives the cell shape and structure.

Intermediate filaments

Intermediate filaments are a type of cytoskeletal element made of multiple strands of fibrous proteins wound together. As their name suggests, intermediate filaments have an average diameter of 8 to 10 nm, in between that of microfilaments and microtubules.

Intermediate filaments come in a number of different varieties, each one made up of a different type of protein. One protein that forms intermediate filaments is keratin, a fibrous protein found in hair, nails, and skin. For instance, you may have seen shampoo ads that claim to smooth the keratin in your hair!

Unlike actin filaments, which can grow and disassemble quickly, intermediate filaments are more permanent and play an essentially structural role in the cell. They are specialized to bear tension, and their jobs include maintaining the shape of the cell and anchoring the nucleus and other organelles in place.

Microtubules

Despite the “micro” in their name, microtubules are the largest of the three types of cytoskeletal fibers, with a diameter of about 25 nm. A microtubule is made up of tubulin proteins arranged to form a hollow, straw-like tube, and each tubulin protein consists of two subunits, α-tubulin and β-tubulin.

Microtubules, like actin filaments, are dynamic structures: they can grow and shrink quickly by the addition or removal of tubulin proteins. Also similar to actin filaments, microtubules have directionality, meaning that they have two ends that are structurally different from one another. In a cell, microtubules play an important structural role, helping the cell resist compression forces.

In addition to providing structural support, microtubules play a variety of more specialized roles in a cell. For instance, they provide tracks for motor proteins called kinesins and dyneins, which transport vesicles and other cargoes around the interior of the cell. During cell division, microtubules assemble into a structure called the spindle, which pulls the chromosomes apart.

Flagella, cilia, and centrosomes

Microtubules are also key components of three more specialized eukaryotic cell structures: flagella, cilia and centrosomes. You may remember that our friends the prokaryotes also have structures have flagella, which they use to move. Don't get confused—the eukaryotic flagella we're about to discuss have pretty much the same role, but a very different structure.

Flagella (singular, flagellum) are long, hair-like structures that extend from the cell surface and are used to move an entire cell, such as a sperm. If a cell has any flagella, it usually has one or just a few. Motile cilia (singular, cilium) are similar, but are shorter and usually appear in large numbers on the cell surface. When cells with motile cilia form tissues, the beating helps move materials across the surface of the tissue. For example, the cilia of cells in your upper respiratory system help move dust and particles out towards your nostrils.

Despite their difference in length and number, flagella and motile cilia share a common structural pattern. In most flagella and motile cilia, there are 9 pairs of microtubules arranged in a circle, along with an additional two microtubules in the center of the ring. This arrangement is called a 9 + 2 array. You can see the 9 + 2 array in the electron microscopy image at left, which shows two flagella in cross-section.

The centrosome is duplicated before a cell divides, and the paired centrosomes seem to play a role in organizing the microtubules that separate chromosomes during cell division. However, the exact function of the centrioles in this process still isn’t clear. Cells with their centrosome removed can still divide, and plant cells, which lack centrosomes, divide just fine.

Questions 

1) What did you understand about microtubules, microfilaments and intermediate filaments? Give an example for each of them making a specific structure in the cell.

~ Microtubules are the thickest of the cytoskeletal fibers, with a diameter of 25 nm. It is made up of actin monomers and combines into a structure that resembles a double helix. It is made up of alpha and beta tubulin dimers which form hollow tubes and often form 9+2 arrangement of microtubule doublets. They provide tracks for motor proteins called kinesins and dyneins, which transport vesicles and other cargoes around the interior of the cell. 

~ Microtubules are the intermediate of the cytoskeletal fibers, with a diameter of 8 to 10 nm. It is made up of fibrous proteins like keratin, and are wound into thicker cables. An example of this making a specific structure is how keratin makes hair, nails, and even skin. 

~ Microfilaments are the thinnest of the cytoskeletal fibers, with a diameter of nm. They play a large role in structural support. There is a network of actin filaments found near the edge of the cell in the cytoplasm which is linked to the plasma membrane by special connector proteins. 

2) Explain about the structure of flagellum and cilium.

~ In most flagella and motile cilia, there are 9 pairs of microtubules arranged in a circle, along with an additional two microtubules in the center of the ring. This arrangement is called a 9 + 2 array. There are two central singlet tubules, and nine outer doublet microtubules, near the plasma membrane. 

3) What are the roles of motor proteins and what type of motor protein did you learn in this text?

~ Kinesins and dyneins are motor proteins, which transport vesicles and other cargoes around the interior of the cell. This is done using microtubules which are made up of actin monomers. 

Endosymbiotic Theory

It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

Evidence for this is based on the following:

1. Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divided by binary fission, and the mitochondria of some protists have Fts homologs at their division plane.

2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.

3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.

4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.

Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.

1. Briefly describe what is meant by the endosymbiotic theory.

- The endosymbiotic theory suggests that there was a host cell that was originally a prokaryotic cell. This cell then had parts of the cell membrane break off to create a protective barrier around the nucleic material, forming a nucleus. Then, a proteobacterium entered the cell but was not digested. It becomes the mitochondria due to its endosymbiont nature. Then a cyanobacterium entered the cell but was not digested either. It becomes chloroplast. 

2. Give three points of evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.
- Three points that are evidence are the nature of the nucleus, the proteobacterium becoming the mitochondria, and the cyanobacterium becoming chloroplast.