Advanced Biology: Leaf Anatomy, Photosynthesis Review, and Exhaustive Guide to Cellular Respiration

Internal Structures of a Leaf:
- Cuticle: A protective, waxy outer layer found on both the upper and lower surfaces of the leaf to prevent water loss.
- Upper Epidermis: The outermost layer of cells on the top side of the leaf.
- Palisade Mesophyll Cell: Column-shaped cells located under the upper epidermis; high concentration of chloroplasts for photosynthesis.
- Spongy Mesophyll Cells: Loosely packed cells below the palisade layer, allowing for gas exchange.
- Bundle Sheath Cell: Cells surrounding the vascular tissue (vein).
- Vascular Tissue (Vein): Contains the transport systems of the plant.
  - Xylem: Transports water and minerals upward from the roots to the rest of the tree.
  - Phloem: Transports sugars (products of photosynthesis) downward from the leaves to the rest of the tree.
  - Orientation: Within the vascular bundle, xylem is typically located on the inside, while phloem is on the outside.
- Lower Epidermis: The outermost layer of cells on the bottom side of the leaf.
- Stoma (plural: Stomata): Pores in the lower epidermis that allow for gas exchange ( $CO_2$ in, $O_2$ and water vapor out).
- Guard Cells: Specialized cells that flank the stoma and regulate its opening and closing.

Autotroph: An organism that produces its own food (complex organic compounds like carbohydrates, fats, and proteins) using light or chemical energy.
Heterotroph: An organism that cannot produce its own food and must derive its energy from the consumption of other organisms or organic substances.

Key Questions and Concepts:
- 1. Reactant used in light reactions: Water ( $H_2O$ ).
- 2. Product obtained directly from water: Oxygen ( $O_2$ ).
- 3. Other substance obtained from water during light reactions: Hydrogen ( $H$ ).
- 4. Substance used in dark reactions (Calvin Cycle): Carbon dioxide ( $CO_2$ ).
- 5. Resulting product of carbon dioxide conversion: Carbohydrate ( $C_6H_{12}O_6$ ).
- 6. Requirements to complete carbohydrate formation: $NADPH$ and $ATP$ (provided by the light reactions).
- 7. Energy source for the basic equation: Light energy.

The Gasoline Analogy:
- A can of gasoline stores energy. This energy remains stored until the gasoline is used.
- Simply having the gasoline near a car is not enough to make the car run.
- The gasoline must be burned (broken down) to release the energy required to power the vehicle.
Biological Application:
- Carbohydrates are like the gasoline in the can; they store energy.
- Carbohydrates must be broken down through a series of processes to release energy for the cell.
- This process of breaking down food molecules to release energy is called Cellular Respiration.

The Cellular Respiration Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$
Redox Reactions (Leo says GeR):
- Oxidation: The loss of electrons ( $e^-$ ).
- Reduction: The gaining of electrons ( $e^-$ ).
Three Main Steps of Cellular Respiration:
1. Glycolysis: Site of substrate-level phosphorylation.
2. Kreb’s Cycle (Citric Acid Cycle).
3. Electron Transport Chain (ETC): Site of oxidative phosphorylation.

Outer Membrane: The smooth outermost boundary of the mitochondrion.
Intermembrane Space: The narrow region between the inner and outer membranes.
Inner Membrane: Highly folded into structures called Cristae to increase surface area for energy production.
Matrix: The internal space enclosed by the inner membrane; contains enzymes for the Kreb's cycle.

Location: Occurs in the cytoplasm (cytosol) of the cell.
Process Detail:
- Initial Investment: $2$ molecules of $ATP$ are used to activate Glucose.
- Glucose is converted into Fructose diphosphate.
- Fructose diphosphate splits into two molecules of $PGAL$ (Phosphoglyceraldehyde).
- Each $PGAL$ is converted into Pyruvic acid (Pyruvate).
- During this conversion, $NAD^+$ is reduced to $NADH$ and $ADP$ is phosphorylated to $ATP$ .
End Products (Net Yield):
- $2$ Net $ATP$ molecules (4 produced, 2 used).
- $2$ $NADH$ molecules.
- $2$ Pyruvate molecules.
The Bridge Step:
- Before entering the Kreb's Cycle, Pyruvic acid ( $3$ carbons) is converted into Acetyl CoA ( $2$ carbons).
- One carbon is released as Carbon Dioxide ( $CO_2$ ).

Location: Occurs in the mitochondrial matrix.
Chemical Step-by-Step:
1. Acetyl CoA ( $2C$ ) combines with Oxaloacetic acid (OAA) ( $4C$ ) to form Citric acid ( $6C$ ).
2. Citric acid is converted to Isocitric acid ( $6C$ ).
3. Isocitric acid is converted to a-ketoglutarate ( $5C$ ), releasing $CO_2$ and reducing $NAD^+$ to $NADH$ .
4. a-ketoglutarate is converted to Succinyl CoA ( $4C$ ), releasing $CO_2$ and reducing $NAD^+$ to $NADH$ .
5. Succinyl CoA becomes Succinic acid ( $4C$ ), generating $ATP$ (via $GTP$ ).
6. Succinic acid becomes Fumaric acid ( $4C$ ), reducing $FAD$ to $FADH_2$ .
7. Fumaric acid becomes Malic acid ( $4C$ ).
8. Malic acid (Malate) is converted back into Oxaloacetic acid (OAA), reducing $NAD^+$ to $NADH$ , completing the cycle.
Total Products of the Kreb's Cycle (per Glucose molecule):
- $FADH_2$
- $NADH$
- Carbon Dioxide ( $CO_2$ )
- $2$ $ATP$

Location: Occurs in the inner mitochondrial membrane.
Mechanism:
- A series of protein complexes (Complex I, II, III, and IV) embedded in the membrane.
- $NADH$ and $FADH_2$ deposit electrons into the chain.
- As electrons move through the complexes, energy is released to pump Hydrogen ions ( $H^+$ ) from the matrix into the intermembrane space.
- Oxygen ( $O_{2}$ ) acts as the final electron acceptor, combining with electrons and $H^+$ ions to form Water ( $H_2O$ ).
Chemiosmosis:
- An energy-coupling mechanism.
- Uses energy stored in the form of an $H^+$ gradient across a membrane to drive cellular work.
- $H^+$ ions flow back into the matrix through ATP Synthase, a protein complex that acts as a turbine to generate $ATP$ from $ADP + P_i$ .
Products of the Electron Transport System:
- Water ( $H_2O$ ).
- 32–34 ATP depending on the cell type:
  - Prokaryotic Cell: Yields approximately $34$ $ATP$ .
  - Eukaryotic Cell: Yields approximately $32$ $ATP$ .

Aerobic Respiration:
- Occurs when sufficient amounts of Oxygen ( $O_2$ ) are present.
- Produces a significantly higher amount of $ATP$ .
Anaerobic Respiration (Fermentation):
- Occurs when there are insufficient amounts of Oxygen ( $O_2$ ).
- Produces significantly less $ATP$ .
- End Products of Fermentation:
  - In Animals: Lactic acid (causing muscle fatigue).
  - In Plants/Yeast: Ethyl alcohol.