Plants and Photosynthesis

Question 1

Name two organelles that are present in all plant cells, but not in all animal cells.

Answer 1

Chloroplasts and vacuoles

Chloroplasts aid in photosynthesis and are actually never present in animal cells. Vacuoles are large structures used for storage of waste, nutrients, or water. Some animal cells do have vacuoles, but they are much smaller than the vacuoles found in plant cells.

Question 2

Are plant cells eukaryotic or prokaryotic?

Answer 2

Plant cells are eukaryotic, like animal cells.

Even fungal cells are eukaryotic; only bacteria and Archaea are prokaryotic.

Question 3

What structure, which human cells lack, surrounds the outer face of the plasma membrane in plant cells?

Answer 3

A cell wall

Plants have cell walls made of cellulose (a carbohydrate), fungi have cell walls composed of chitin, and bacteria have cell walls that consist of peptidoglycan. Humans do not have cell walls; our cells are surrounded by the plasma membrane alone.

Question 4

In general, plants are able to gain energy from sunlight and use it to synthesize organic compounds. Organisms with this ability are known as:

Answer 4

Photoautotrophs

Specifically, “photo-“ refers to the ability of plants to acquire their energy from photons (light). “Autotroph” refers to producers, or organisms able to make their own complex organic compounds.

Question 5

In the net reaction of photosynthesis, which molecules are reactants and which molecules are products?

Answer 5

Carbon dioxide and water are reactants, meaning that they are consumed in the process. Technically, light is also a reactant, but light is not a molecule.

Glucose and oxygen are products. The purpose of photosynthesis is to make glucose (a carbohydrate food source) to fuel the plant’s cells.

Question 6

What is the net reaction of photosynthesis?

Answer 6

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

Be sure to remember that C₆H₁₂O₆ is the chemical formula for glucose, the carbohydrate (sugar) produced by photosynthetic plants.

Question 7

Name the waxy covering that protects plant leaves from water loss.

Answer 7

The waxy covering of a leaf is called the cuticle.

This structure is what you see when you look at the surface of a leaf. Without the cuticle, plants would lose excess water via evaporation.

Question 8

What fluid fills the inner compartment of a chloroplast and serves as the location of the Calvin cycle?

Answer 8

The stroma

The stroma is an enzyme-containing fluid encased by the inner membrane of the chloroplast. Here, the grana (stacks of thylakoids) can be found. The stroma is also the location of the Calvin cycle, or light-independent reactions of photosynthesis.

Question 9

The compartment of the chloroplast that holds the stroma is analogous to which compartment of a mitochondrion?

Answer 9

The mitochondrial matrix

Like chloroplasts, mitochondria are encased in both an outer and an inner membrane. The region between the membranes is termed the intermembrane space, and the region enclosed by the inner membrane is termed the matrix. In chloroplasts, the stroma, or enzyme-containing fluid, is found in this inner region.

Question 10

Define:

grana

Answer 10

In chloroplasts, grana are stacks of disc-shaped thylakoids found in the stroma, or fluid in the inner compartment.

The singular of “grana” is “granum.”

Question 11

Define:

thylakoids

Answer 11

In chloroplasts, thylakoids are disc-shaped structures that contain chlorophyll, the pigment involved in photosynthesis.

Thylakoids are found in stacks known as grana.

Question 12

Name the two types of vascular tissue in plants.

Answer 12

Xylem and phloem

These tissues are structured as bundles that can transport water or nutrients. Xylem transports water (and some minerals) from the roots to the rest of the plant, including the leaves. Phloem transports nutrients (sugars, in particular) throughout the plant.

Question 13

Define:

stomata

Answer 13

Stomata are openings that allow gas exchange through the cuticle and epidermis of the plant.

Stomata are closed by guard cells when not needed. This reduces water loss, which would otherwise occur via dessication (drying out).

Question 14

Name the two broad reaction types that occur during photosynthesis.

Answer 14

Light-dependent and light-independent reactions

Light-dependent, or “light,” reactions directly involve light (photons). Light-independent, or “dark,” reactions do not. The light-independent reactions are also known as the Calvin cycle.

Question 15

Define:

photophosphorylation

Answer 15

Photophosphorylation is the formation of ATP using energy gained from light.

ATP is synthesized from ADP and phosphate.

Question 16

Name the two major products of the light reactions of photosynthesis.

Answer 16

ATP and NADPH

ATP (or adenosine triphosphate) is the main energy storage molecule in cells. NADPH is an electron carrier that is used in the later steps of photosynthesis (the dark reactions, or Calvin cycle).

Question 17

In which structures within plant cells do the light reactions of photosynthesis take place?

Answer 17

The light reactions take place in the thylakoids.

These reactions require the excitation of electrons within pigments, including chlorophyll. Remember, the thylakoids are the disc-shaped structures in which chlorophyll is found!

Question 18

Name three pigments that participate in the light-dependent reactions of photosynthesis.

Answer 18

Chlorophyll a, chlorophyll b, and carotenoids

These pigments are colored molecules that can absorb energy from photons (light). This energy can be used to excite electrons.

Question 19

With regard to color, how does chlorophyll differ from the carotenoid pigments?

Answer 19

Chlorophyll (both chlorophyll a and b) is green, while carotenoids are yellow, orange, or red.

These differences in color stem from the wavelengths of light that are reflected (as opposed to absorbed) by these pigments.

Question 20

What is the key structural difference between photosystem I and photosystem II?

Answer 20

The two photosystems contain different pairs of chlorophyll a molecules, causing them to have different wavelengths at which they absorb maximum light.

Photosystem I contains P₇₀₀, which has its maximum absorption at 700 nanometers. Photosystem II contains P₆₈₀, which has its maximum absorption at 680 nanometers.

Question 21

Name the two types of photophosphorylation.

Answer 21

Cyclic and noncyclic photophosphorylation

Cyclic photophosphorylation involves photosystem I and generates only ATP (not NADPH). Noncyclic photophosphorylation involves both photosystems I and II and generates both ATP and NADPH.

Question 22

Briefly explain the steps involved in noncyclic photophosphorylation.

Answer 22

1. Electrons within the P₆₈₀ reaction center of photosystem II are excited.
2. These electrons are passed to a molecule termed the primary electron acceptor.
3. The electrons move along an electron transport chain from one protein to another. During this process, protons are pumped into the thylakoid lumen, creating a proton gradient.
4. Protons move back out of the lumen through ATP synthase, creating ATP.
5. The electrons end at photosystem I, where they are excited again, initiating a second electron transport chain that produces NADPH.

Question 23

Briefly explain the steps involved in cyclic photophosphorylation.

Answer 23

1. Electrons within the P₇₀₀ reaction center of photosystem I are excited.
2. The electrons move along an electron transport chain from one protein to another. During this process, protons are pumped into the thylakoid lumen, creating a proton gradient.
3. Protons move out of the lumen through ATP synthase, creating ATP.
4. The electrons eventually return to photosystem I, where they can take part in photophosphorylation again.

Question 24

Define:

chemiosmosis

Answer 24

Chemiosmosis is the movement of ions down their concentration gradient across a selectively permeable membrane.

On the AP Biology exam, chemiosmosis is tested in relation to cellular respiration. Specifically, it refers to the movement of protons down their concentration gradient and through ATP synthase, a process that allows for the creation of ATP.

Question 25

Define:

carbon fixation

Answer 25

Carbon fixation is the conversion of CO₂ to other organic compounds, namely carbohydrates.

In plants, carbon fixation is accomplished via the Calvin cycle, a set of light-independent (“dark”) reactions.

Question 26

A plant is kept in a completely dark room for an extended period of time. At the end of this interval, is the Calvin cycle likely taking place in plant cells?

Answer 26

No, the Calvin cycle is not likely to be occurring.

While the Calvin cycle (also termed the “dark” reactions of photosynthesis) does not directly require light, its reactants include ATP and NADPH. In plants, both of these molecules can only be generated in the light-dependent reactions, which will not be taking place here due to the dark surroundings.

Question 27

In the Calvin cycle, how many carbon dioxide molecules are required to create one glucose molecule?

Answer 27

6 carbon dioxide molecules are required.

An easy way to remember this is to memorize the chemical formula of glucose, which is C₆H₁₂O₆. Since this formula includes six carbon atoms and carbon dioxide only includes one, we must need six CO₂ molecules to form one glucose.

Question 28

In the Calvin cycle, how many ATP and NADPH molecules are required to create one glucose molecule?

Answer 28

18 ATP and 12 NADPH molecules are required.

The other reactants involved in the Calvin cycle are carbon dioxide (CO₂) and H⁺.

Question 29

Which enzyme catalyzes the first step of the Calvin cycle?

Answer 29

Rubisco (ribulose bisphosphate carboxylase)

This reaction involves the combination of carbon dioxide with ribulose bisphosphate (RuBP).

Question 30

The first step of the Calvin cycle is also known as C₃ carbon fixation. In this name, to what does “C₃” refer?

Answer 30

C₃ refers to the fact that this first step produces a 3-carbon molecule as its first stable product.

Specifically, this molecule is 3-phosphoglycerate.