Unit 3 - Ch 9 - Photosynthesis

Question 1

Simplified equation for photosynthesis

Answer 1

6CO2 + 6H2O → C6H12O6 + 6O2

light and chlorophyll power the reaction

CO2 and H2O are the reactants

Question 2

Requirements for photosynthesis

Answer 2

1. CO2
   – from the atmosphere
   - enters the leaf via stomata
   - .04% concentration of CO2 in the atmosphere - diffusion
2. H2O
   – from soil via roots
   - source of e-
3. Chlorophyll
   – pigment that absorbs light energy
4. Light
   - energy source, overall photosynthesis is an endergonic process

Question 3

Biological Importance of Photosynthesis

Answer 3

Trophic Webs
- photosynthesis supports virtually all trophic webs on earth - Ps organisms are producers in the ecosystem

Removes Atmospheric CO2

• Ps is a primary regulator of the carbon cycle
• carbon sequestration - carbon sink

Produces O2

1. Aerobic Respiration - vs. anaerobic
2. Ozone - O3 in the upper atmosphere - protects from UV radiation

Question 4

A leafs flat, thin shape:

Answer 4

– maximizes surface area for light absorption

- leads to water loss from leaf (transpiration)

Question 5

Leaf cuticle

Answer 5

– waxy layer on both leaf surfaces - minimizes transpiration
- vary by species (thicker in desert species)

Question 6

Leaf epidermis

Answer 6

Upper and lower, beneath the cuticle

Question 7

Mesophyll

Answer 7

• middle portion of the leaf (means middle leaf)
  1. Palisade - elongated cells packed with chloroplasts

2. Spongy - more open / gas exchange layer
   - CO2 diffuses into, O2 diffuses out

Question 8

Vascular Tissue

Answer 8

• Leaf veins
  1. Xylem - conducts water from roots to leaf
  2. Phloem - conducts carbohydrate from leaf to roots - girdling

Question 9

Stomata

Answer 9

leaf opening for gas exchange

Question 10

Guard Cells

Answer 10

• two guard cells form a stoma
• osmotically regulated to open and close the stoma
• open = turgor pressure
• closed = not turgid

Question 11

Photochemical Reaction

• Location -
• Energy Input -

Answer 11

also called Light Phase of photosynthesis

Location - occurs within the thylakoid

Energy Input - photons of light

Question 12

Role of Pigments in Photochemical Reaction

Answer 12

1. Chlorophyll (a and b) - absorb primarily red and blue wavelengths of visible light
2. Accessory Pigments - transfer light energy to chlorophyll
   a. Carotenes - orange in color
   b. Xanthophylls - yellow

Question 13

Photosynthetic Units

Answer 13

pigment arrays in the thylakoid membrane - energy is transfered onto a reaction center via a “light funnel”

1. Photosystem I (P700) – maximum absorption at 700 nm
2. Photosystem II (P680) - maximum absorption at 680 nm

Question 14

Major Photochemical Steps (3)

Answer 14

1. Photolysis
2. Photoreduction
3. Photophosphorylation

Question 15

Photolysis

Answer 15

splitting apart water to produce e- for photosystem II

• produces O2
• H+ for production of NADPH

Question 16

Photoreduction

Answer 16

producing NADPH (endergonic)

Question 17

Photophosphorylation (2 types)

Answer 17

Produces ATP from ADP using light energy (endergonic)

a. Noncyclic
- involves both photosystems
- “ z scheme”
- photoreduction occurs (NADPH)
- photollysis occurs (recharge Photosystem II) - one way flow of e-

b. Cyclic
- only involve photosystem I
- no photoreduction or photolysis
- accumulates ATP when CO2 levels are low

Question 18

Thermochemical Reaction

• location -
• energy input -

Answer 18

also called “Dark Phase” and Calvin Cycle

Location
– occurs within the stroma

Energy Input
– ATP and NADPH from photochemical
- light independent

Question 19

Excess PGALs in Thermochemical Reaction

Answer 19

• form glucose-phosphate (6C)
• proteins and lipids are synthesized from carbohydrates in the metabolic pool
• require nitrogen from the environment to form proteins

Question 20

Relationship Between Respiration and Photosynthesis

Answer 20

• Photosynthesis creates organic compounds and O2, products which serve as reactants for cellular respiration
• cellular respiration creates CO2 and H2O, products which serve as reactants for photosynthesis

Question 21

How does the relationship between cellular respiration and photosynthesis relate to tropical deforestation, fossil fuel combustion and global warming?

Answer 21

Combustion of fossil fuels (coal, oil, nat. gas) and decomposition of organic compounds mimic cell respiration
= increase CO2
Deforestation decreases photosynthesis
= CO2 increases in the atmosphere