LAB EXAM 2 PRACTICE QUESTIONS Labs 8-10

1. Compare and contrast the exponential and logistic models.

• Exponential: J-shaped curve, assumes unlimited resources (dN/dt = rN), growth rate remains constant.

• Logistic: S-shaped curve, accounts for carrying capacity (K) where resources limit growth (dN/dt = rN[(K-N)/K]); growth slows as N nears K.

2. Distinguish between density-dependent and density-independent factors. Provide examples for each.

• Density-Dependent: Effects change with population density (e.g., competition, predation, disease).

• Density-Independent: Affect population size regardless of density (e.g., natural disasters like floods, fires, extreme weather).

3. Considering the entire human population, what type of growth model best fits our population?

Exponential growth (rapid, J-shaped historical trajectory).

4. Can this trend continue? Why or why not?

• No. Earth has a finite carrying capacity; limited resources and environmental resistance will eventually cap growth.

5. Make a sketch of a pyramid for a population that is: a) growing; b) shrinking; c) staying the same in population size.

• Growing: Wide base (many young/pre-reproductive individuals), tapering top.

• Shrinking: Narrow base, wider middle/top (fewer young individuals).

• Stable: Proportion of young and middle-aged individuals is relatively even, tapering at the top.

6. What non-biological factors might influence population growth of humans?

• Technology, healthcare/medicine, public policy, economy, education, and cultural norms.

1. What are phytoplankton and why are they important?

Microscopic photosynthetic organisms (algae/cyanobacteria) drifting in water. They form the base of aquatic food webs and produce a major share of Earth's oxygen.

2. What specific pigment do all of these organisms have in common? Why is this

particular pigment so important?

Chlorophyll a. Essential for capturing light energy to drive photosynthesis.

3. What adaptations allow phytoplankton to remain in the photic zone?

Small size, oil droplets, flagella, spines, or buoyant shapes that slow sinking to stay in sunlit waters.

4. Consider the following image:

a. Identify the type of phytoplankton.

b. Name one (1) photosynthetic pigment this

organism possesses.

• a. Cyanobacteria (photosynthetic bacteria / blue-green algae).

• b. Chlorophyll a (or phycocyanin).

5. Consider the following image:

a. Identify the type of phytoplankton.

b. What structure allows for gas exchange in these

organisms?

a. Diatoms

b. Striae

1. What is the function of chlorophyll a in diatoms or other autotrophs?

Absorbs light energy (photons) to power photosynthesis (converting CO2 and water into glucose).

2. Why measure chlorophyll a concentrations? What information does this provide?

Serves as a direct proxy for phytoplankton biomass and aquatic primary productivity.

3. Why was it important to keep the samples in the dark and cold?

Prevents chlorophyll degradation and halts active photosynthesis/respiration before extraction.

4. Why did we have to freeze the glass fiber filters before we could extract the chlorophyll?

Disrupts cell membranes (lysis), making it easier for solvents to extract the pigments.

5. Why did we use acetone as a solvent to extract chlorophyll? Why not water?

Chlorophyll is hydrophobic (fat-soluble), so organic solvents like acetone dissolve it effectively; water is polar and cannot.

6. Why was it important to centrifuge the samples?

• Pellets cell debris and filter fragments so the liquid supernatant is clear for spectrophotometry.

7. Chlorophyll a shows absorption peaks at 664 nm and 432 nm, and the peak at 432 nm is greater than the peak at 664 nm. If 432 nm shows a higher peak, why did we measure chlorophyll absorption at 664 nm?

664 nm minimizes interference from other overlapping accessory pigments (like chlorophyll b or carotenoids) that absorb heavily near 432 nm.

8. To calculate the concentration of chlorophyll a we also measured the absorption at 630 nm and 647 nm. Why?

Corrects for background turbidity and other chlorophyll types present in the sample.

What occurs at the other wavelengths?

Light is either transmitted, scattered, or absorbed by non-target pigments.