▶️ Answer/Explanation
(a)
- As temperature increases, the rate of enzyme-catalyzed reactions increases.
- This is due to increased molecular motion, which results in more frequent collisions between enzyme and substrate.
- However, high temperatures can denature enzymes.
- Denaturation changes the shape of the active site, preventing substrate binding.
- Enzymes function best at an optimum temperature (specific to each enzyme).
- At very low temperatures, enzyme activity slows due to reduced collisions.
(b)
- The thyroid gland secretes thyroxin.
- Thyroxin increases the metabolic rate.
- A higher metabolic rate leads to more heat production.
- This helps maintain or raise body temperature.
- Thyroxin can also stimulate shivering and brown adipose tissue to generate heat.
- More thyroxin is released when body temperature is too low (negative feedback mechanism).
(c)
- Humans release carbon dioxide (CO₂) into the atmosphere.
- This primarily occurs through the burning of fossil fuels (coal, oil, natural gas).
- Forest fires and deforestation contribute by releasing CO₂ and reducing carbon uptake by plants.
- Humans also release methane (CH₄), notably from livestock (e.g. cattle digestion).
- Both CO₂ and CH₄ are greenhouse gases.
- They allow shortwave solar radiation to pass into Earth’s atmosphere.
- Earth’s surface emits longwave (infrared) radiation after warming.
- Greenhouse gases absorb or reflect this longwave radiation, trapping heat.
This results in a greenhouse effect, leading to a rise in global temperatures.
Markscheme
(a)
a. Speed of reaction/catalysis increases as temperature rises;
b. Faster molecular motion so more collisions between substrate and active site;
c. Denaturation at higher temperatures;
d. (Denaturation causes) shape/conformation/structure of enzyme/active site altered/damaged;
e. An enzyme works fastest at its optimum temperature;
f. Inactivation at lower temperatures (due to very few collisions);
g. Sketch graph to model the effect of temperature on enzyme activity;
(b)
a. Secretes thyroxin;
b. Thyroxin causes the metabolic rate to rise;
c. Heat released by metabolism;
d. Thyroxin increases generation of body heat;
e. Thyroxin stimulates shivering/stimulates brown adipose tissue (to release heat);
f. More thyroxin secreted if body temperature too low/converse;
(c)
a. Release of carbon dioxide;
b. Combustion of fossil fuels produces carbon dioxide;
c. Forest fires (caused by humans) produce carbon dioxide;
d. Deforestation reduces carbon dioxide uptake by photosynthesis;
e. Release of methane;
f. From cattle/sheep/ruminant digestive systems / other verified source of anthropogenic methane;
g. Greenhouse effect / carbon dioxide/methane is a greenhouse gas;
h. Carbon dioxide/methane allow short wave radiation in sunlight to pass through the atmosphere;
i. Longer wave/infra-red radiation emitted by the warmed Earth’s surface;
j. Carbon dioxide/methane absorbs/reflects back longer wave/infra-red radiation;
▶️ Answer/Explanation
a.
- Carbon dioxide is a greenhouse gas.
- Methane, nitrogen oxides, and water vapor are also greenhouse gases.
- Sunlight (shortwave radiation) passes through the atmosphere and reaches the Earth’s surface.
- The Earth’s surface emits longwave (infrared) radiation.
- Greenhouse gases absorb or reflect back this longwave radiation.
- This trapped heat causes the warming of the Earth’s surface.
- Greenhouse gases allow shortwave radiation to pass through but absorb longwave radiation.
b.
- Light is used in photosynthesis, particularly in light-dependent reactions.
- Chlorophyll absorbs red and blue wavelengths most efficiently.
- Green light is mostly reflected, not absorbed.
- The absorption spectrum of chlorophyll shows peaks in the red and blue regions.
- The action spectrum shows that photosynthesis is most efficient under red and blue light.
- Accessory pigments absorb additional wavelengths and expand the range of usable light.
- Violet has the shortest wavelength; red has the longest.
- Red and far-red light help measure the duration of light and dark periods (important in flowering).
c.
- Transport occurs in the phloem.
- Organic compounds move through sieve tube elements.
- Movement happens by mass flow.
- Compounds move from sources (e.g., leaves) to sinks (e.g., roots, fruits).
- Sugars are actively loaded into the phloem at the source.
- This causes a high solute concentration in the phloem.
- Water enters by osmosis, increasing pressure.
- A pressure gradient is created, driving flow from source to sink.
Transport can occur in both directions depending on source and sink locations.
Markscheme
a.
a. Carbon dioxide is a greenhouse gas
b. Methane/nitrogen oxide/water vapor is a greenhouse gas
c. Sunlight/light/(solar) radiation passes through the atmosphere (to reach the Earth’s surface)
d. CO2 in atmosphere/greenhouse gases absorb/trap/reflect back some radiation/heat (emitted by the Earth’s surface)
e. CO2 in atmosphere/greenhouse gases allow short wave radiation to pass (through atmosphere) but absorb long wave/infrared
f. Solar radiation/sunlight is (mostly) short wave
g. Radiation/heat emitted by the Earth is long wave/infrared
Allow answers presented in a clearly annotated diagram.
b.
a. Light used in photosynthesis/light-dependent reactions/photolysis/photosystems/photophosphorylation/excitation of electrons/switch to flowering
b. Chlorophyll absorbs red AND blue light (more)
c. Chlorophyll/leaf/plant reflects/does not absorb/does not use green light
d. Absorption spectrum of chlorophyll has peaks in the red and blue/sketch graph to show this
e. Action spectrum shows which wavelengths plants use in photosynthesis/sketch graph of action spectrum showing peaks in the blue and red
f. Accessory/other (named) photosynthetic pigments absorb different wavelengths/colors
g. Violet is the shortest wavelength and red the longest
h. Red light and far red/infrared absorbed to measure length of light/dark periods
c.
a. Transported in/translocated in/loaded into phloem
b. In sieve tubes
c. By mass flow
d. From sources to sinks
e. From leaves/other example of source to roots/other example of sink
f. Loading (of sugars/organic compounds) by active transport
g. Cause high concentration of solutes (in phloem/sieve tubes)
h. Water uptake (in phloem/sieve tubes) by osmosis/water diffuses into phloem
i. Rise in (hydrostatic) pressure at source (in phloem)
j. Creates a (hydrostatic) pressure gradient/higher pressure in source than sink
k. Flow can be in either direction/bidirectional
▶️ Answer/Explanation
a.
- Starch is a polysaccharide made up of glucose units.
- It consists of two components:
- Amylose – linear/helical structure.
- Amylopectin – branched structure.
- Starch is used by plants for energy storage.
- It is insoluble and does not cause osmotic problems in cells.
b.
- Light energy is absorbed by chlorophyll, mainly red and blue wavelengths.
- Light-dependent reactions convert light energy into chemical energy (ATP and NADPH).
- Photolysis of water produces oxygen, protons, and electrons.
- Carbon dioxide is fixed in the Calvin cycle.
- Carbohydrates (e.g., glucose) are synthesized from fixed carbon using ATP and NADPH.
c.
- Carbon dioxide (CO₂) sources:
- Cellular respiration by organisms.
- Combustion of fossil fuels.
- Decomposition of organic matter.
- Methane (CH₄) sources:
- Anaerobic respiration in wetlands, rice paddies.
- Digestive systems of ruminants (e.g., cattle).
- Landfills and waste treatment.
- Carbon sinks:
- Photosynthesis removes CO₂ from the atmosphere.
- CO₂ absorbed by oceans.
- Fossilization stores carbon long-term.
- Climate change consequences:
- CO₂ and CH₄ are greenhouse gases that trap heat.
- Increased global temperatures (global warming).
- Ocean acidification due to excess CO₂.
- Disruption of weather patterns and ecosystems.
Markscheme
a. Structure and function of starch:
• Starch is a polysaccharide composed of glucose molecules.
• Contains amylose (linear/helical) and amylopectin (branched).
• Function: Energy storage in plants; does not draw water.
b. Production of carbohydrates in photosynthesis:
• Light absorbed by chlorophyll (red/blue light).
• Light energy → chemical energy (ATP/NADPH).
• Photolysis of water → O2 + H+.
• CO2 fixation into carbohydrates (Calvin cycle).
c. Carbon cycle and climate change:
• CO2 sources: respiration, combustion, decomposition.
• CH4 sources: anaerobic respiration (e.g., wetlands, cattle).
• CO2 sinks: photosynthesis, ocean absorption, fossilization.
• Greenhouse effect: CO2/CH4 trap heat → global warming.
• Consequences: Climate disruption, ocean acidification, ecosystem shifts.
▶️ Answer/Explanation
a.
- X: Incoming solar radiation with shorter wavelengths, such as visible or ultraviolet light
- Y: Outgoing radiation from the Earth, typically with longer wavelengths like infrared
b.
- Gases in the atmosphere trap heat by absorbing infrared energy
- These gases then radiate some of this energy back toward the Earth’s surface
- This process contributes to warming the lower atmosphere
c.
- The birds’ nesting grounds could be lost due to rising ocean levels
- More frequent storms may damage their breeding areas
- Warmer ocean temperatures could reduce fish stocks, limiting their food supply
Markscheme
a.
X: short/ultraviolet/UV/visible/EMR/electromagnetic radiation
Y: long/infrared/IR
b.
• Greenhouse gases (CO2, methane, etc.) present at Z
• These gases absorb long-wavelength infrared radiation
• Radiation is re-emitted/reradiated, warming the Earth
c.
• Rising sea levels may flood nesting islands
• Extreme weather may destroy breeding habitats
• Warming oceans may disrupt food chains/fish populations