Chapter 4: Photosynthesis and Cellular Respiration Flashcards
Outline the light dependent stage of photosynthesis
- Occurs in thylakoids and grana
- Inputs → Water (H2O), NADP+ and ADP + Pi
- Outputs → Oxygen (O2), NADPH and ATP
- Energy is provided by light
- Chlorophyll absorbs light energy
- Sunlight energy excites electrons, providing energy to convert ADP to ATP
- Sunlight energy is used to split water
- Hydrogen binds to NADP+ to form NADPH
- Oxygen is a waste product that will diffuse out through the plasma membrane
NOTE: The light dependent stage is the same for all three plant types; C3, C4 and CAM.
Outline the light independent stage of photosynthesis
- Occurs in the stroma
- Inputs → ATP, NADPH and CO2
- Outputs → ADP + Pi, NADP+ and glucose
- Energy is provided by ATP molecules
- Rubisco, along with energy from ATP and NADPH, is used to convert carbon dioxide to glucose
- ADP + Pi and NADP+ is returned to the grana to be reused
NOTE: The synthesis of glucose is an anabolic reaction as ATP is being used.
Explain Rubisco
-
Enzyme in C3 plants that bring carbon dioxide from the air into the Calvin cycle where glucose is made
- Earth’s most abundant enzyme
- Rubisco works most efficiently in
- High levels of carbon dioxide
- Low oxygen levels
- Moderate temperatures
Explain the issues faced by CAM and C4 plants (desert and tropical plants)
- These plants are generally in hot and dry areas where water is scarce
- Limited water results in closed stomata leading to an accumulation of oxygen, hence, an increased rate of photorespiration as rubisco will bind to oxygen rather than CO2
Explain photorespiration
- Photorespiration occurs when plants take up oxygen instead of carbon dioxide resulting in a decreased rate of photosynthesis
- Photorespiration occurs as temperatures increase or as conditions dry out
- This is because stomata remain closed (to prevent water loss) resulting in the accumulation of oxygen as carbon dioxide cannot enter
Explain how C4 plants have adapted to avoid photorespiration
- Plants in warm and tropical environments (e.g. corn and sugar cane)
- Initial CO2 fixation and the calvin cycle are separated into different cells
- PEP carboxylase fixes carbon dioxide (converts it into a similar molecule, malic acid) in leaf mesophyll cells
-
Glucose production via the Calvin cycle occurs in bundle sheath cells
- Malic acid breaks down in bundle sheath cells releasing CO2
- CO2 concentration increases allowing Rubisco to bind to it
Explain how the structure of C4 plants facilitate the adaptations they have to minimise photorespiration
- Bundle-sheath cells are surrounded by mesophyll cells for the efficient transfer of malic acid
- Stomata are adjacent to mesophyll cells for fast initial carbon fixation
- Less air pockets to store gases exchanged in photosynthesis
Explain how CAM plants have adapted to avoid photorespiration
- Plants in hot and arid environments (e.g. cacti and orchids)
- Initial CO2 fixation occurs at night and the calvin cycle occurs during the day
- Night: CAM plants open their stomata allowing CO2 to enter
- PEP carboxylase fixes CO2 into malic acid (or other organic acids)
- Malic acid is stored in vacuoles in mesophyll cells
- DAY: CAM plants close their stomata to prevent water loss
- Malic acid is released from storage and is broken down to release CO2
- CO2 concentration increases allowing Rubisco to bind to carbon dioxide
NOTE: Both stages take place in leaf mesophyll cells.
Explain how CAM plants photosynthesise when water is available and their stomata are open
- They revert back to the C3 process of photosynthesis
- Carbon fixation is not split up
- The light dependent and light independent stage both occur during the day
Explain why PEP carboxylase is able to avoid photorespiration
- PEP carboxylase only binds to carbon dioxide at its active site (photorespiration cannot occur)
- Not capable of binding oxygen
Explain the importance of maximising photosynthetic efficiency
- Maximising photosynthetic efficiency increases glucose production and allows for the conservation of energy
Explain how light availability can affect the rate of photosynthesis
- The rate of photosynthesis increases as light availability increases until a maximum point is reached
Explain how water availability can affect the rate of photosynthesis
- When water is scarce, stomata close resulting in a decreased rate of photosynthesis
- This is because closed stomata prevent the uptake of carbon dioxide needed for the calvin cycle
Explain how carbon dioxide concentration can affect the rate of photosynthesis
- The rate of photosynthesis increases as carbon dioxide concentration increases until a limiting factor is reached
- E.g. Rubisco or PEP carboxylase may already be working at their maximum rate
Explain why temperature affects the rate of photosynthesis
- The rate of photosynthesis increases as temperature increases until a maximum point is reached
- This is because molecules are moving faster and are more likely to collide
NOTE: Temperature also influences whether stomata are open or closed. High temperatures = closed stomata = decreased rate of photosynthesis.
Outline the three interconnected stages of aerobic cellular respiration
- Glycolysis → one glucose molecule is split into two pyruvate molecules
- Does not require oxygen
- Occurs in the cytosol
- Krebs cycle → further breakdown of pyruvate resulting in the formation of NADH and FADH2coenzymes
- Does not require oxygen
- Occurs in the mitochondrial matrix
- Electron transport chain → energy from electrons (via loaded coenzymes) used to make ATP
- Requires oxygen
- Occurs in the cristae (inner mitochondrial membrane)
State the inputs and outputs of glycolysis
- Inputs → Glucose, NAD+ (2) and ADP + Pi (2)
- Outputs → Pyruvate (2), NADH (2) and ATP (2)
Explain pyruvate oxidation
- Before pyruvate enters the Krebs cycle, pyruvate oxidation occurs
- Pyruvate moves into the mitochondria and is modified into acetyle coenzyme a (acetyl-CoA)
State the inputs and outputs of the Krebs cycle
- Inputs → Pyruvate, NAD+, FAD and ADP + Pi
- Outputs → CO2, NADH, FADH2 and ATP
State the inputs and outputs of the electron transport chain
- Inputs → O2, NADH, FADH2 and ADP + Pi
- Outputs → H2O, NAD+, FAD and ATP
State how much ATP is made in each stage of cellular respiration (+ net yield)
- Glycolysis → 2
- Krebs cycle → 2
- Electron transport chain → 26-28
- Net → 30-32
Explain why ATP is produced and not just glucose
- Glucose has greater chemical energy content than ATP, however, the direct use of glucose leads to the production of excessive heat and waste
- ATP is a more useable form of energy as its energy can be released in a single step (energy isinstantlyavailable for use by cells)
- Release of energy from glucose involves a multistep pathway that is about 100 times slower than that for ATP energy release
State the cellular location and ATP output of fermentation
- Anaerobic fermentation occurs in the cytoplasm (usually of human skeletal muscle cells) when oxygen supply from aerobic cellular respiration cannot keep up with the demand for ATP
- Produces a net of 2 ATP molecules
State the equation for lactic acid fermentation in animals
Glucose → Lactic acid + 2ATP
State the inputs (reactants) and outputs (products) of lactic acid fermentation in animals
- Inputs → Glucose, NAD+ and ADP + Pi
- Outputs → Lactic acid, NAD+ and ATP
State the equation for ethanol fermentation in yeasts
Glucose → Ethanol + CO2 + 2ATP
State the inputs (reactants) and outputs (products) of fermentation in yeasts
- Inputs → Glucose, NAD+ and ADP + Pi
- Outputs → Ethanol, CO2, NAD+ and ATP
State how glucose and oxygen availability affects the rate of cellular respiration using the term limiting factor
- As glucose and oxygen availability increases, the rate of cellular respiration increases
- This rate eventually levels off from limiting factors
Explain the difference between aerobic and anaerobic cellular respiration
- Anaerobic operates without oxygen
- Anaerobic takes place totally within the cytosol of cells
- Anaerobic produces less ATP per glucose molecule
- Anaerobic does not involve ETC
- Anaerobic produces ATP quicker (about 100 times faster)
- Anaerobic takes up more glucose to get the same amount of energy (as aerobic, therefore more wasteful)
NOTE: Anaerobic cellular respiration occurs at a faster rate enabling more ATP to be produced per unit of time.
Describe two ways CRISPR-Cas9 could improve crop yield
- Target certain genes that impact crop yield by either inserting advantageous genes or removing disadvantageous genes
- This improves photosynthetic efficiency and speed allowing for higher crop yield
Describe one potential way CRISPR-Cas9 could be used to increase the efficiency of photosynthesis
- Genes that code for Rubisco can be edited to make it less likely to bind with oxygen (reduce photorespiration)
- Delete genes that increase the binding of Rubisco and oxygen
- Insert genes that would promote Rubisco to bind with carbon dioxide
- This reduces the likelihood of photorespiration and therefore improve photosynthetic efficiency
List the benefits of using CRISPR-Cas9 over other methods to increase crop yield
- More precise than traditional methods
- Able to edit multiple target genes simultaneously
- Can knock out certain genes
- Can insert certain genes
Define biofuel
- Any source of energy made up of from biomass (organic material)
List 3 substances that can be used as biomass
- Wood, ethanol and waste
Describe how biomass can be converted to biofuel using fermentation
- Microorganisms (e.g. bacteria and yeast) metabolize plant sugars and produce ethanol
List advantages and disadvantages biofuel has over other fuel types
- Advantages
- It is a renewable energy source
- Causes less pollution
- Many ways to apply it and use for energy
- Disadvantages
- Extraction and storage of biomass can be expensive
- Large areas are required for the different processes involved in harnessing energy from biomass
Chloroplasts
- Chloroplasts are present in all plants but not all plant cells (e.g. roots and onion epidermal cells)
- Most abundant in shaded regions, allowing plants to maximise limited light availability
Explain the function of photosynthesis
- Converts light energy into chemical energy stored as glucose
- Glucose provides energy to produce ATP, which the cell can use for energy and to catalyse reactions
C3 plants
- C3 plants comprise about 85% of terrestrial plants worldwide
- Cool to temperate conditions
- Carbon fixation occurs only once
State the optimal wavelengths of light for photsynthesis
- Plants with chlorpohyll best absorbs violet, blue and red light
- They absorbs less light of green wavelength
