Photosynthesis Flashcards
(140 cards)
1
Q
What is photosynthesis?
A
The process by which green plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.
2
Q
What is the equation for photosynthesis?
A
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
3
Q
Where do light-dependent reactions occur?
A
Thylakoid membranes of chloroplasts
4
Q
What are the inputs of light-dependent reactions?
A
Light, H₂O, ADP, NADP⁺
5
Q
What are the outputs of light-dependent reactions?
A
ATP, NADPH, O₂
6
Q
What is photolysis of water?
A
2H₂O → 4H⁺ + 4e⁻ + O₂
7
Q
What is the role of Photosystem II in light-dependent reactions?
A
Absorbs light, excites electrons, initiates electron transport chain
8
Q
How is ATP produced in light-dependent reactions?
A
Via chemiosmosis and photophosphorylation
9
Q
What is the process of reducing NADP⁺ to NADPH?
A
Conducted by Photosystem I
10
Q
Where do light-independent reactions occur?
A
Stroma of chloroplast
11
Q
What are the inputs of the Calvin Cycle?
A
CO₂, ATP, NADPH
12
Q
What are the outputs of the Calvin Cycle?
A
Glucose (C₆H₁₂O₆), ADP, NADP⁺
13
Q
What is the first step of the Calvin Cycle?
A
Carbon Fixation - CO₂ combines with RuBP
14
Q
Which enzyme catalyzes carbon fixation?
A
RuBisCO
15
Q
What happens during the reduction phase of the Calvin Cycle?
A
ATP and NADPH are used to reduce 3-PGA to G3P
16
Q
What occurs during the regeneration phase of the Calvin Cycle?
A
Some G3P is used to remake RuBP; the rest makes glucose
17
Q
How many turns of the Calvin Cycle are needed to form 1 glucose molecule?
A
6 turns
18
Q
What is a key characteristic of C₃ plants?
A
Use only the Calvin Cycle; first stable product = 3-PGA
19
Q
What is a key characteristic of C₄ plants?
A
First product = 4-carbon oxaloacetate; CO₂ fixed in mesophyll cells
20
Q
What adaptation do CAM plants have for CO₂ fixation?
A
Open stomata at night to fix CO₂, stored as malate
21
Q
What is the mnemonic for the phases of the Calvin Cycle?
A
Fixation → Reduction → Regeneration = FRR
22
Q
True or False: Photorespiration is more common in C₃ plants.
A
True
23
Q
What is the role of water in light-dependent reactions?
A
Source of electrons and protons; releases O₂
24
Q
What molecule carries electrons from light-dependent reactions to the Calvin cycle?
A
NADPH
25
What structure in the chloroplast contains the photosystems?
Thylakoid membrane
26
What are the products of the light-dependent reactions of photosynthesis?
ATP, NADPH, and O₂
## Footnote
The light-dependent reactions use light energy to split water and produce ATP and NADPH, releasing O₂ as a byproduct.
27
Where in the chloroplast do the light-dependent reactions occur?
Thylakoid membranes
## Footnote
The thylakoid membranes contain chlorophyll and other pigments organized into photosystems, as well as the electron transport chain and ATP synthase.
28
Which of the following events does not occur during the light-dependent reactions?
Carbon fixation
## Footnote
Carbon fixation occurs in the Calvin cycle (light-independent reactions) and not in the light-dependent reactions.
29
Photosystem II (PSII) is primarily responsible for:
Splitting water and releasing O₂
## Footnote
PSII contains the water-splitting complex that releases O₂ and protons.
30
The oxygen released in photosynthesis comes from the splitting of which molecule?
Water (H₂O)
## Footnote
The O₂ produced during photosynthesis is derived from water during the light reactions.
31
Photophosphorylation refers to:
The conversion of ADP to ATP using light energy in chloroplasts
## Footnote
Photophosphorylation is the process of generating ATP from ADP and inorganic phosphate using the energy of light.
32
In non-cyclic photophosphorylation (the 'Z-scheme'), electrons ejected from the reaction center of PSII ultimately:
Are used to reduce NADP⁺ to NADPH
## Footnote
Electrons travel through the electron transport chain to PSI and are ultimately used to reduce NADP⁺.
33
Which statement correctly distinguishes cyclic from non-cyclic photophosphorylation?
Cyclic photophosphorylation produces ATP but not NADPH or O₂
## Footnote
In cyclic photophosphorylation, electrons from PSI are cycled back instead of reducing NADP⁺.
34
In the light reactions of photosynthesis, the final electron acceptor is:
NADP⁺
## Footnote
NADP⁺ accepts electrons at the end of the PSI electron transport chain, becoming reduced to NADPH.
35
The proton (H⁺) gradient created during the light reactions is used to:
Synthesize ATP from ADP and Pi
## Footnote
The proton gradient drives ATP synthase to convert ADP + Pi into ATP.
36
What is the role of water in the light reactions of photosynthesis?
It is split to provide replacement electrons and H⁺ ions, releasing O₂
## Footnote
Water provides electrons to PSII’s reaction center and contributes to the proton gradient.
37
'P700' is the special chlorophyll a molecule found in the reaction center of:
Photosystem I
## Footnote
P700 absorbs light optimally at 700 nm, while Photosystem II’s reaction center is P680.
38
Which component of the photosynthetic electron transport chain actively transports protons (H⁺) across the thylakoid membrane?
Cytochrome b₆f complex
## Footnote
The cytochrome b₆f complex pumps protons into the thylakoid lumen as electrons pass through it.
39
During the light reactions, the pH of the thylakoid lumen ______, while the pH of the stroma ______:
Decreases (becomes more acidic); increases (becomes more basic)
## Footnote
Protons accumulate in the thylakoid lumen, lowering its pH.
40
If a toxin (e.g. the herbicide DCMU) blocks the flow of electrons from PSII to plastoquinone (PQ), what is the immediate effect on photosynthesis?
Oxygen production ceases, and no ATP or NADPH are produced via non-cyclic electron flow
## Footnote
Blocking electron transfer disrupts PSII's ability to split water.
41
If Photosystem II is not operational, how can a chloroplast continue to produce ATP?
Through cyclic electron flow around Photosystem I
## Footnote
Photosystem I can still work in a cyclic mode to produce ATP.
42
Which sequence correctly traces the path of electrons in non-cyclic (linear) electron flow during the light reactions?
PSII (P680) → Plastoquinone (PQ) → Cytochrome b₆f → Plastocyanin (PC) → PSI (P700) → Ferredoxin (Fd) → NADP⁺
## Footnote
This sequence involves both photosystems and produces ATP and NADPH.
43
The term 'Z-scheme' in photosynthesis refers to:
The zigzag shape of the energy diagram as electrons are excited in PSII and PSI
## Footnote
The 'Z-scheme' illustrates changes in energy levels of electrons during the light reactions.
44
Which of the following does NOT occur on or within the thylakoid membranes?
Enzyme-catalyzed carbon fixation reactions
## Footnote
Carbon fixation occurs in the stroma, not in the thylakoid membranes.
45
When a chlorophyll molecule in the light-harvesting complex absorbs a photon of light, what is the immediate result?
An electron in the chlorophyll is boosted to a higher energy level (excited state)
## Footnote
This excitation energy is transferred to drive photochemical events.
46
Where do the light-independent reactions (Calvin cycle) take place within the chloroplast?
Stroma
## Footnote
The stroma contains the enzymes that fix carbon dioxide into carbohydrates.
47
In C₃ photosynthesis, what is the first stable product formed after CO₂ fixation?
3-phosphoglycerate (3-PGA)
## Footnote
This is formed when CO₂ is fixed by rubisco onto RuBP.
48
Which enzyme catalyzes the fixation of CO₂ in the Calvin cycle (C₃ pathway)?
Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
## Footnote
Rubisco is the most abundant enzyme in chloroplasts and reflects its role in carbon fixation.
49
The Calvin cycle requires all of the following except:
O₂
## Footnote
The Calvin cycle does not use O₂; high O₂ can inhibit the cycle.
50
The Calvin cycle produces a three-carbon sugar that can be used to form glucose and other carbohydrates. This direct product of the Calvin cycle is:
Glyceraldehyde-3-phosphate (G3P, also called PGAL)
## Footnote
G3P can be further processed to form glucose and other carbohydrates.
51
What are the inputs of the Calvin cycle?
CO₂, ATP, NADPH
## Footnote
The cycle fixes CO₂ into sugars using energy from ATP and reducing power from NADPH produced in light reactions.
52
What is the direct product of the Calvin cycle?
Glyceraldehyde-3-phosphate (G3P)
## Footnote
G3P is a 3-carbon sugar phosphate that can be converted into glucose and other carbohydrates.
53
For the net synthesis of one molecule of glucose, how many ATP and NADPH are consumed in the Calvin cycle?
18 ATP and 12 NADPH
## Footnote
This is because fixing six CO₂ requires six turns of the cycle, using 3 ATP and 2 NADPH per CO₂.
54
What role does NADPH play in the Calvin cycle?
Provides high-energy electrons to convert 3-PGA into G3P
## Footnote
NADPH donates electrons during the reduction phase, while ATP provides energy.
55
How many turns of the Calvin cycle are required to produce one net molecule of G3P?
3 turns
## Footnote
Each turn fixes one CO₂, yielding two G3Ps; five of six G3Ps are recycled to regenerate RuBP.
56
What happens to the majority of G3P produced in the Calvin cycle?
It is used to regenerate RuBP
## Footnote
This regeneration step is crucial for the cycle to continue functioning.
57
Photorespiration becomes more likely when:
Stomata are closed on a hot, dry day
## Footnote
This leads to low internal CO₂ and high O₂, causing rubisco to bind O₂ instead of CO₂.
58
What is the result of photorespiration?
Uptake of O₂ and release of CO₂, with a net loss of carbon
## Footnote
It is a wasteful process that reduces photosynthesis efficiency.
59
Which statement about rubisco is FALSE?
Rubisco is a small, fast enzyme with an exceptionally high turnover rate
## Footnote
Rubisco is actually a large, slow enzyme, which is why it is abundant.
60
Why can’t the Calvin cycle proceed for long in the dark?
It requires ATP and NADPH produced by light reactions
## Footnote
The cycle needs a constant supply of these molecules, which deplete in the dark.
61
Which compound is regenerated during the Calvin cycle?
Ribulose-1,5-bisphosphate (RuBP)
## Footnote
RuBP is essential for the continuous fixation of CO₂ in the cycle.
62
How many CO₂ molecules must be fixed to form one molecule of glucose?
6
## Footnote
Each CO₂ contributes one carbon to glucose, necessitating six CO₂ for one glucose molecule.
63
Which structure is NOT found within a chloroplast?
Cristae
## Footnote
Cristae are found in mitochondria, while chloroplasts contain stroma, thylakoids, and grana.
64
Where are chlorophyll molecules located in a chloroplast?
Thylakoid membranes
## Footnote
These membranes contain chlorophyll for capturing light energy.
65
What is a granum?
A stack of thylakoid discs within a chloroplast
## Footnote
Grana increase the surface area for light capture.
66
What structure contains chlorophyll and the electron transport chain for light reactions?
Thylakoid membrane
## Footnote
This membrane is where light-dependent reactions occur.
67
Chloroplasts are thought to have evolved from:
Photosynthetic prokaryotes engulfed by a eukaryotic cell
## Footnote
This theory is supported by chloroplasts having their own DNA and ribosomes.
68
Where is ATP synthase located in chloroplasts?
Thylakoid membrane
## Footnote
ATP synthase generates ATP as protons flow back into the stroma.
69
During light reactions, protons accumulate in which compartment?
Inside the thylakoid (thylakoid lumen)
## Footnote
A proton gradient is established to drive ATP production.
70
Which fraction contains the enzymes for carbon fixation?
The stromal fraction (fluid)
## Footnote
These enzymes, including rubisco, are soluble in the stroma.
71
What is evidence supporting the endosymbiotic origin of chloroplasts?
Chloroplasts contain circular DNA and bacteria-like ribosomes
## Footnote
These features suggest they originated from engulfed cyanobacteria.
72
How many membrane layers must a molecule cross from the cytosol to the thylakoid lumen?
3
## Footnote
A molecule crosses the outer membrane, inner membrane, and thylakoid membrane.
73
Which pigment is essential for the light reactions?
Chlorophyll a
## Footnote
It is the primary pigment in the reaction center of photosystems.
74
Which pigment is found in the reaction centers of photosystems and is essential for the light reactions to occur?
A. Chlorophyll a
## Footnote
Chlorophyll a is the primary pigment in the reaction center of both photosystem I and II, directly involved in converting light energy to chemical energy.
75
Chlorophyll appears green because:
B. It reflects green light and absorbs primarily red and blue wavelengths.
## Footnote
Chlorophyll absorbs light most strongly in the red and blue parts of the spectrum and poorly in the green range.
76
Which portion of the visible spectrum is least effective at driving photosynthesis in green plants?
C. Green light (~550 nm)
## Footnote
Green light is poorly absorbed by chlorophyll, contributing little to the photosynthetic process.
77
Accessory pigments like chlorophyll b and carotenoids help the plant by:
B. Expanding the range of light wavelengths that can be absorbed and used for photosynthesis.
## Footnote
They capture wavelengths that chlorophyll a absorbs less effectively and transfer that energy to chlorophyll a.
78
What is a primary function of carotenoid pigments in photosynthesis?
B. To absorb light energy in wavelengths that chlorophyll misses and to protect the chloroplast from excess light damage.
## Footnote
Carotenoids broaden light absorption and act as antioxidants.
79
Which of the following best describes an action spectrum of photosynthesis?
B. A graph showing the rate of photosynthesis (e.g., O₂ production or CO₂ uptake) at different wavelengths of light.
## Footnote
The action spectrum indicates the effectiveness of different light wavelengths in driving photosynthesis.
80
The absorption spectrum of chlorophyll a alone differs from the action spectrum of photosynthesis because:
B. The action spectrum is broader and shows high photosynthetic activity at some wavelengths that chlorophyll a does not strongly absorb, due to the contribution of accessory pigments.
## Footnote
Accessory pigments enhance the light absorption capacity, leading to a broader action spectrum.
81
Engelmann’s classic experiment (1883) with algae and aerobic bacteria demonstrated that:
B. Blue and red light support the highest rates of photosynthesis, as evidenced by bacteria clustering where oxygen production was greatest.
## Footnote
The experiment confirmed that blue and red light are most effective for photosynthesis.
82
Which pigment, abundant in red algae, allows them to absorb the available light and gives them their red color?
C. Phycoerythrin
## Footnote
Phycoerythrin absorbs blue and green light efficiently, allowing red algae to photosynthesize in deeper water.
83
If a leaf appears yellow, what does that tell you about the pigments and light absorption in that leaf?
B. The leaf is reflecting yellow light and likely contains carotenoid pigments (such as xanthophylls) that absorb other colors.
## Footnote
A yellow leaf indicates high presence of carotenoids or degraded chlorophyll.
84
Which of the following is a characteristic of C₃ plants (as opposed to C₄ or CAM plants)?
B. The first stable product of CO₂ fixation is a 3-carbon molecule (3-phosphoglycerate).
## Footnote
C₃ plants directly fix CO₂ via rubisco, yielding 3-PGA as the first product.
85
In C₄ photosynthesis, the initial fixation of CO₂ and the Calvin cycle occur in different locations. CO₂ is first fixed in the ____ cells, and the Calvin cycle then takes place in the ____ cells.
C. mesophyll; bundle sheath
## Footnote
C₄ plants separate these processes spatially to enhance efficiency.
86
“Kranz anatomy” refers to:
A. The wreath-like arrangement of bundle sheath cells around leaf veins, containing chloroplasts – a characteristic of C₄ plant leaves.
## Footnote
This structure minimizes photorespiration by isolating rubisco in high CO₂ environments.
87
Which enzyme allows C₄ plants to fix CO₂ efficiently even when CO₂ is low inside the leaf?
B. PEP carboxylase
## Footnote
PEP carboxylase has a higher affinity for CO₂ than rubisco and does not use O₂ as a substrate.
88
Which of these plants is an example of a CAM plant (crassulacean acid metabolism)?
B. Pineapple
## Footnote
CAM plants fix CO₂ at night and release it for the Calvin cycle during the day.
89
A key difference between CAM and C₄ plants is:
A. CAM plants separate initial CO₂ fixation and the Calvin cycle in time (night vs. day), while C₄ plants separate these processes in different cells.
## Footnote
Both strategies enhance CO₂ utilization efficiency.
90
Compared to C₃ plants, C₄ plants:
B. Are generally more water-efficient and can photosynthesize efficiently at higher temperatures and light intensities.
## Footnote
C₄ plants minimize photorespiration and maintain high internal CO₂ levels.
91
What is the first stable product of carbon fixation in C₄ plants?
B. Oxaloacetate (a 4-carbon compound)
## Footnote
Oxaloacetate is formed during the initial fixation of CO₂ in mesophyll cells.
92
Which statement is true regarding photorespiration in different types of plants?
B. Both C₄ and CAM plants have adaptations that greatly reduce photorespiration compared to C₃ plants.
## Footnote
These adaptations enhance photosynthetic efficiency under various conditions.
93
How do CAM plants minimize water loss while still acquiring CO₂ for photosynthesis?
By opening their stomata only at night when it’s cooler and more humid.
## Footnote
This adaptation allows for CO₂ uptake without excessive water loss during the day.
94
What do CAM plants do at night to minimize water loss?
They open their stomata at night to take in CO₂ when evaporation is lower.
95
How do CAM plants store CO₂ for use during the day?
CO₂ is fixed into malic acid and stored.
96
Why do C₄ plants have an advantage over C₃ plants in hot, sunny, and dry conditions?
C₄ plants avoid most photorespiration and can keep stomata partially closed.
97
What is the extra ATP cost for each CO₂ fixed in the C₄ cycle compared to the C₃ cycle?
2 additional ATP per CO₂ fixed.
98
What anatomical feature is characteristic of C₄ plants?
A ring of large bundle sheath cells around the veins, packed with chloroplasts (Kranz anatomy).
99
What happens to malate produced in the mesophyll of a C₄ plant?
It is transported to the bundle sheath cells, where it is decarboxylated to release CO₂.
100
Under which condition would a typical C₃ plant likely outperform a C₄ plant?
Cool temperatures with moderate light and ample water.
101
What is the primary factor limiting a plant's rate of photosynthesis in darkness?
Light (no light available).
102
What change would generally increase the rate of photosynthesis?
Increasing light intensity from very low to a higher level.
103
What is the light compensation point of a plant?
The light intensity at which the rate of photosynthesis equals the rate of respiration.
104
As light intensity increases, how does the rate of photosynthesis in a green plant generally respond?
Increase initially in a roughly linear fashion, then level off at high light intensities.
105
What factor often becomes limiting for photosynthesis in C₃ plants on a bright, clear day?
Carbon dioxide concentration.
106
How does temperature affect the rate of photosynthesis in a typical C₃ plant?
The rate increases with temperature up to an optimum range, then declines if the temperature rises further.
107
Why does the rate of photosynthesis decrease at very high temperatures in C₃ plants?
Enzymes become less efficient or start to denature, and stomata may close.
108
What effect does increasing CO₂ concentration around a C₃ plant usually have?
Increase the rate of photosynthesis, up to a saturation point.
109
What environmental conditions favor the occurrence of photorespiration in plants?
High O₂ concentration and low CO₂ concentration.
110
How does severe water stress usually affect the rate of photosynthesis?
It causes stomata to close, reducing CO₂ uptake and slowing photosynthesis.
111
On a bright, sunny day, what factor often limits the photosynthetic rate of a C₃ plant?
Availability of CO₂.
112
What is photoinhibition in the context of photosynthesis?
Damage to the photosynthetic apparatus by excess light, leading to a decline in efficiency.
113
What is photoinhibition in the context of photosynthesis?
Damage to the photosynthetic apparatus (like PSII) by excess light, leading to a decline in photosynthetic efficiency at high light intensities
## Footnote
Photoinhibition occurs when excess light damages the D1 protein of PSII and generates reactive oxygen species, impairing photosystems.
114
According to Blackman’s law of limiting factors, what is the most effective way to increase the photosynthetic rate at low light intensity?
Increase the light intensity (since light is the limiting factor at low light)
## Footnote
Blackman’s law states that the rate of a physiological process is limited by the factor in shortest supply.
115
What effect does a very high oxygen concentration have on photosynthesis in C₃ plants?
It actually lowers net photosynthesis by enhancing photorespiration
## Footnote
High O₂ increases the rate at which rubisco acts as an oxygenase, releasing more CO₂ and wasting energy.
116
Why can enriching the air with CO₂ enhance plant growth?
Higher CO₂ allows plants to photosynthesize faster (more CO₂ for the Calvin cycle), leading to higher growth, as long as light and other factors are adequate
## Footnote
This is known as the CO₂ fertilization effect.
117
Approximately what percentage of the sunlight energy hitting a plant is converted into chemical energy through photosynthesis under typical field conditions?
~1–2%
## Footnote
This low efficiency is due to energy lost as heat or used in respiration.
118
Which of the following is NOT a reason why photosynthesis fails to convert 100% of light energy to chemical energy?
Plants can capture and use every photon of light with perfect efficiency
## Footnote
This statement is false; plants cannot achieve perfect efficiency.
119
What is the benefit of having many veins in a leaf?
It ensures ample water delivery to photosynthetic cells and rapid export of sugars, preventing any slowdown from water stress or sugar accumulation
## Footnote
High vein density supports efficient hydration and sugar transport.
120
How do thin, broad, and darker green leaves help shade-tolerant plants in low-light conditions?
Thinner, broader leaves with more chlorophyll capture diffuse light more effectively, maximizing light absorption in shady conditions
## Footnote
These adaptations allow efficient photosynthesis under low-light conditions.
121
What is a benefit of having smaller, thicker leaves in high-light environments?
Smaller, thicker leaves help avoid overheating and excess water loss in intense sunlight, while still housing a lot of photosynthetic machinery
## Footnote
These adaptations prevent damage and enhance efficiency in bright conditions.
122
Why are most leaves broad and flat?
To maximize surface area for light absorption and gas exchange
## Footnote
This shape facilitates the capture of sunlight and CO₂ uptake.
123
How does a reflective leaf surface help desert and high-light plants?
It reflects a portion of the intense sunlight, preventing overheating and photoinhibition of the leaf
## Footnote
This adaptation protects the photosynthetic apparatus from excessive light.
124
What is the role of sun-screen pigments in photosynthetic tissues?
They protect the chloroplast by absorbing and dissipating excess light that could otherwise harm the photosystems
## Footnote
These pigments help prevent photoinhibition and oxidative stress.
125
Why have many plants in arid environments evolved C₄ or CAM photosynthetic pathways?
To increase water-use efficiency by reducing the need to keep stomata open during the hottest, driest times, while still fixing carbon effectively
## Footnote
These adaptations help minimize water loss while allowing photosynthesis.
126
What does quantum yield refer to in photosynthesis?
The number of photons required to fix one CO₂ molecule or release one O₂ molecule
## Footnote
It measures how efficiently light photons are used to drive photosynthesis.
127
Why might a plant allocate more nitrogen to its leaves when growing in high-light conditions?
To increase the amount of chlorophyll and photosynthetic enzymes (like rubisco) in the leaves, boosting photosynthetic capacity
## Footnote
Higher nitrogen content supports greater photosynthetic machinery.
128
How do aquatic plants or algae adapt to low-light conditions underwater?
They produce additional pigments (like phycobilins in red algae or chlorophyll c in some algae) that absorb blue-green light more effectively
## Footnote
Specialized pigments enable photosynthesis in deeper or murkier water.
129
How does solar tracking (heliotropism) benefit photosynthesis?
It allows leaves to maximize light capture when light is limiting and minimize excess light when it’s too intense
## Footnote
This behavior optimizes light absorption and can enhance overall photosynthesis.
130
Which of the following is NOT an adaptation plants use to balance maximizing photosynthesis with minimizing water loss?
Waxy, reflective leaf surfaces and smaller leaves to reduce water loss in high light
## Footnote
This statement is true; it is an adaptation to minimize water loss.
131
What adjustment do plants make to reduce overheating and photoinhibition?
Tilt leaves to reduce direct exposure to excessive light
## Footnote
This adjustment optimizes light absorption and enhances overall photosynthesis while protecting the plant from light extremes.
132
Which of the following is NOT an adaptation plants use to balance maximizing photosynthesis with minimizing water loss?
D. Keeping stomata open at all times, even when soil moisture is low
## Footnote
Keeping stomata open constantly would lead to excessive water loss and likely death in dry conditions.
133
What are the genuine adaptations plants use to conserve water?
* Waxy, reflective leaf surfaces
* Smaller leaves to reduce water loss
* CAM photosynthesis, taking in CO₂ at night
* Shedding leaves during extreme drought
## Footnote
These adaptations help plants manage water loss while maintaining photosynthesis.
134
What is 'midday depression' in photosynthesis?
A drop in the rate of photosynthesis around midday despite maximal light
## Footnote
This phenomenon occurs in many plants due to environmental stressors.
135
What is a common cause of midday decline in photosynthetic rate?
Excess heat and light lead to partial stomatal closure and photoinhibition
## Footnote
This results in less CO₂ entering the plant and potential damage to photosystems.
136
True or False: High light intensity always directly inhibits photosynthesis.
False
## Footnote
While high light can contribute to photoinhibition, it is not the sole cause of reduced photosynthesis.
137
What happens to stomata during midday to conserve water?
They may partially close
## Footnote
This limits CO₂ supply for photosynthesis and can reduce the rate of photosynthesis.
138
Fill in the blank: Plants may take in CO₂ at night through _______ photosynthesis.
CAM
## Footnote
This adaptation allows plants to minimize water loss during the hotter daytime.
139
What can happen to photosystems during conditions of high light and heat?
They may be temporarily damaged
## Footnote
This can lead to reduced efficiency in photosynthesis.
140
What is one strategy drought-deciduous plants use to reduce water loss?
Shedding leaves during severe dryness
## Footnote
This strategy sacrifices photosynthesis temporarily to conserve water.
