Biology Exam III- Study Guide

Biology Exam III - Study Guide

Photosynthesis

Photosynthesis is a biological process by which plants utilize solar energy to transform carbon dioxide ($CO2$) and water ($H2O$) into glucose ($C6H{12}O6$) and oxygen gas ($O2$) as a by-product. The equation representing this process is:
ext{Light energy} + 6 ext{CO}2 + 6 ext{H}2 ext{O}
ightarrow C6H{12}O6 + 6 ext{O}2

Components of Photosynthesis
  • Reactants:
    • Water ($H_2O$)
    • Carbon dioxide ($CO_2$)
  • Products:
    • Glucose ($C6H{12}O_6$)
    • Oxygen gas ($O_2$)
Autotrophs and Heterotrophs

  • Autotrophs: Organisms that synthesize their own food, recognized as the primary source of organic compounds for almost all life forms.

    • Photoautotrophs: A subset of autotrophs, these organisms harness light energy to produce food from inorganic sources.

  • Heterotrophs: Organisms that cannot produce their own food and rely on consuming plants, animals, or decomposed organic matter.

Examples of Photoautotrophs
  • Plants
  • Algae
  • Cyanobacteria (photosynthetic bacteria)
Site of Photosynthesis

Photosynthesis predominantly occurs in leaves, particularly within the mesophyll layer, where chloroplasts are heavily concentrated. Chloroplasts contain chlorophyll, a green pigment essential for capturing solar energy.

Structure of Chloroplasts

The chloroplast is a specialized organelle critical for photosynthesis, characterized by an envelope of two membranes:

  • Outer membrane: Acts as a protective layer.
  • Inner membrane: Regulates molecule transport.
  • Intermediate membrane space: Space between the outer and inner membranes.
  • Stroma: A fluid-filled region where the Calvin cycle occurs.
  • Thylakoids: Membranous sacs within the stroma that house the chlorophyll and are critical in capturing light energy to transform it into chemical energy. Thylakoids are arranged in stacks known as grana (plural) or granum (singular).
Chlorophyll

Chlorophyll is embedded in the thylakoid membrane and plays a vital role in capturing light energy during the light reactions of photosynthesis.

Light Reactions of Photosynthesis

Light reactions occur in the thylakoid membranes and involve several key steps:

  1. Photon absorption: Chlorophyll absorbs light energy, which excites electrons.
  2. Water photolysis: Water is split to replenish electrons; this process releases oxygen as a by-product.
  3. Electron Transport Chain (ETC): Excited electrons move through a series of proteins, releasing energy that is used to synthesize ATP.
  4. NADPH production: Electrons are also used to convert NADP$^+$ to NADPH, a high-energy electron carrier.
    • Overall products of light reactions: ATP, NADPH, and $O_2$.
Redox Reactions in Photosynthesis
  • Oxidation: The loss of electrons (in this case, water is oxidized to release $O_2$).
  • Reduction: The gain of electrons (carbon dioxide is reduced to form glucose).
  • These processes are collectively known as redox reactions, integral in deriving energy from the sun to convert $CO_2$ into glucose.

The Calvin Cycle (Light-Independent Reactions)

The Calvin cycle occurs in the stroma and is responsible for synthesizing sugar from $CO_2$ using the energy stored in ATP and NADPH produced during the light reactions.

Stages of the Calvin Cycle
  1. Carbon Fixation: $CO_2$ is fixed to ribulose bisphosphate (RuBP) by the enzyme RuBisCO, forming an unstable sugar that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
  2. Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
  3. Regeneration of RuBP: Some G3P is used to regenerate RuBP, allowing the cycle to continue.
    • For every three molecules of $CO_2$ that enter the cycle, one G3P molecule leaves the cycle to be used for glucose synthesis. Thus, the cycle must run six times to produce enough G3P for one glucose molecule.
Energy Requirements of the Calvin Cycle

To synthesize one G3P molecule, the Calvin Cycle consumes:

  • 9 ATP
  • 6 NADPH

Comparison of C4 and CAM Photosynthesis

  • C4 Photosynthesis: This mechanism separates $CO2$ fixation from the Calvin cycle spatially, with $CO2$ fixation occurring in the mesophyll cells and the Calvin cycle happening in the bundle sheath cells (e.g., corn, sugarcane) to minimize photorespiration.
  • CAM Photosynthesis: In contrast, CAM plants separate these processes temporally, capturing $CO_2$ at night and conducting the Calvin cycle during the day (e.g., cacti, pineapple) to conserve water.

Cellular Respiration

Cellular respiration is an exergonic process wherein glucose is broken down to produce ATP. It encompasses three main stages:

  1. Glycolysis: Occurs in the cytoplasm, converting one glucose molecule into two pyruvate molecules, resulting in a net of 2 ATP and 2 NADH.
  2. Pyruvate Oxidation and Citric Acid Cycle: Occurs in the mitochondrial matrix, fully oxidizing glucose to $CO2$ and generating more NADH and FADH$2$.
  3. Oxidative Phosphorylation: Comprised of the electron transport chain and chemiosmosis, where the majority of ATP is generated.
    • Final electron acceptor is $O2$, forming water ($H2O$).
Total ATP Yield

The complete oxidation of glucose via cellular respiration can yield approximately 30-32 ATP molecules:

  • Glycolysis: 2 ATP
  • Krebs Cycle: 2 ATP
  • Electron Transport Chain: 26-28 ATP
Fermentation under Anaerobic Conditions

When oxygen is absent, cells undergo fermentation to regenerate NAD$^+$, allowing glycolysis to continue. Two types are:

  • Lactic Acid Fermentation: Occurs in muscle cells.
  • Alcohol Fermentation: Occurs in yeast and some microorganisms, producing ethanol and $CO_2$.
    • Major products of fermentation are 2 ATP per glucose compared to around 30-32 ATP during aerobic respiration.