Plant Biology and Photosynthesis

Question 5

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

Answer 5

chloroplasts and vacuoles

Chloroplasts aid in photosynthesis and are actually never present in animal cells. Vacuoles are large structures used for the 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 6

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

Answer 6

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 7

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

Answer 7

Reactants:

• Carbon dioxide
• Water

Products:

• Glucose
• Oxygen

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

Technically, light is also a reactant, but light is not a molecule.

Question 8

What is the net reaction of photosynthesis?

Answer 8

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 9

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

Answer 9

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 11

Are plant cells eukaryotic or prokaryotic?

Answer 11

Eukaryotic, like animal cells.

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

Question 13

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

Answer 13

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 15

Define:

grana

Answer 15

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 16

Define:

thylakoids

Answer 16

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

Thylakoids are found in stacks known as grana.

Question 17

Name the two types of vascular tissue in plants.

Answer 17

• xylem
• 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 18

Define:

stomata

Answer 18

These 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 desiccation (drying out).

Question 19

Name the two broad reaction types that occur during photosynthesis.

Answer 19

1. light-dependent reactions
2. 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 20

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

Answer 20

• Chlorophyll a
• chlorophyll b
• carotenoids

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

Question 21

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

Answer 21

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 22

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

Answer 22

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 23

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

Answer 23

• Chlorophyll (both chlorophyll a and b) is green
• 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 24

Define:

photophosphorylation

Answer 24

The formation of ATP using energy gained from light.

ATP is synthesized from ADP and phosphate.

Question 25

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

Answer 25

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 26

Name the two types of photophosphorylation.

Answer 26

1. cyclic
2. noncyclic

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 27

Briefly explain the steps involved in noncyclic photophosphorylation.

Answer 27

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 out of the lumen through ATP synthase, generating ATP.
5. The electrons end at photosystem I, where they are excited again, initiating a second electron transport chain that produces NADPH.

Question 28

Briefly explain the steps involved in cyclic photophosphorylation.

Answer 28

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 29

Define:

carbon fixation

Answer 29

The conversion of CO₂ into other organic compounds, mainly carbohydrates.

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

Question 30

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 30

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 31

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

Answer 31

6

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 32

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

Answer 32

• 18 ATP
• 12 NADPH

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

Question 33

Which enzyme catalyzes the first step of the Calvin cycle?

Answer 33

Rubisco

ribulose bisphosphate carboxylase

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

Question 34

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

Answer 34

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, an enzyme-containing fluid, is found in this inner region.

Question 35

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 35

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 36

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

Answer 36

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.

Question 37

Define:

chemiosmosis

Answer 37

The movement of ions down their concentration gradient across a selectively permeable membrane.

Chemiosmosis is often 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.