Respiration and Gaseous Exchange

Metabolic pathway

a series of interconnected chemical reactions that occur in a specific sequence within cells, leading to the transformation of a substrate molecule into a final product. Each step in this pathway is catalysed by a specific enzyme, ensuring efficiency and regulation of the process.

Key characteristics of metabolic characteristics

1. Enzyme specificity: Each reaction is catalysed by a specific enzyme, which reduces the activation energy required for the reaction.

2. Regulation: Enzymes in the pathway are often regulated to ensure homeostasis, e.g., through feedback inhibition.

3. Sequential Steps: The product of one reaction becomes the substrate for the next, creating a streamlined process.

4. Energy coupling: Catabolic pathways often generate ATP, while anabolic pathways consume it

Metabolites

Substances involved in the physical and chemical processes. Many of which Are synthesised in cells, while others are imported from outside the cell such as food, water, CO₂ and O₂

Anabolic reactions

Built-up of large molecules from smaller ones, using energy, require energy (ATP)

Catabolic reactions

break down of large molecules to smaller ones releasing energy, supply energy (ATP)

Exergonic reactions

release free energy by the process of respiration

Endergonic reactions

requires energy for synthesis

Basal Metabolic Rate (BMR)

The energy needed to maintain body condition and involuntary processes such as sleep. Measured in terms of heat produced per unit time: kJm^-2h^-1. Energy consumed to sustain vital functions such as heartbeat, breathing, nervous activity etc. Different tissues have different BMR

Factors affecting BMR

• Age: The BMR is greatest during first few years of life.

• Sex: Females have been found to have 2 to 12% less BMR than males.

• Climate: higher in colder regions (poles).

• Body Temperature: a rise in body temperature of 1°F, the BMR increases by 7%.

• Hormones: Thyroxine and Adrenaline raise BMR.

• Sleep: decrease BMR by 10%.

• Pregnancy: BMR raised during the 3^rd trimester.

• Role of Physical activity: regular exercise raises BMR.

Role of enzymes in Control of Metabolic Pathways

Enzymes are biological catalysts that control the rate of each reaction in a metabolic pathway by:

1. Lowering Activation Energy: Enzymes stabilize the transition state of a reaction, allowing it to proceed more quickly.

2. Regulation of Flow: Key enzymes act as "gatekeepers," determining the flux of metabolites through the pathway.

3. Feedback Inhibition: The end product of a pathway often inhibits an earlier enzyme to prevent overproduction. This is a form of negative feedback.

Activation energy

The minimum amount of energy needed to reach the transition state

Location of phosphofructokinase (PFK)

Glycolysis pathway

Function of PFK

Catalyses the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate using ATP

Regulation of PFK

• Inhibited by ATP- When energy levels are high, excess ATP binds to allosteric sites on PFK, reducing its activity.

• Activated by AMP- Low energy levels (high AMP) signal the need for more ATP, enhanicnf PFK activity

Significance of PFK

It is a rate-limiting enzyme, controlling the pace of glycolysis and ensuring energy production matches cellular demands.

Significance of ATP in Metabolism

ATP (adenosine triphosphate) is the primary energy currency of the cell, playing a central role in metabolism. It provides an immediate source of energy for various biological processes, including:

1. Active Transport: Energy is used to move molecules against concentration gradients (e.g., Na⁺/K⁺ pump).

2. Mechanical Work: Powers processes like muscle contraction and flagellar movement.

3. Biosynthesis: Supplies energy for the synthesis of macromolecules like proteins, nucleic acids, and lipids.

4. Anabolism- molecule build-up such as protein synthesis

5. Secretion- enzymes and hormones exported from cells

6. Cell division- formation of organelles

Structure of ATP

ATP consists of adenine, ribose, and three phosphate groups. The bonds between phosphate groups (phosphoanhydride bonds) are high-energy bonds.

Hydrolysis of ATP

ATP is hydrolysed to ADP (adenosine diphosphate) and inorganic phosphate (P_i), releasing ~30.5 kJ/mol of energy

Regeneration of ATP

ATP is continuously regenerated via cellular respiration, particularly in the mitochondria.

Respirometer

is a device used to measure the rate of respiration by quantifying the consumption of oxygen or production of carbon dioxide in living organisms or tissues.

Components of respirometer

1. Sealed Chamber: Contains the organism or tissue.

2. Manometer: Measures pressure changes due to gas exchange.

3. Soda Lime or KOH: Absorbs CO₂ to isolate O₂ consumption.

4. Water Bath: Maintains a constant temperature for accurate measurements.

Principle of respirometer

During aerobic respiration, organisms consume O₂ and produce CO₂. By absorbing CO₂, any volume changes in the chamber are solely due to O₂ uptake, which correlates with the respiration rate

Aerobic respiration

Requires O₂ where sugars are oxidised to CO₂ + H₂O

Anaerobic respiration

does not require O₂ where sugars are converted to alcohol instead.

Why do organisms need energy?

To maintain and repair body structure and for activities such as nutrition, excretion, movement, reproduction etc.

Glycolysis Overview

Glycolysis is the first stage of cellular respiration, where one molecule of glucose (a six-carbon monosaccharide) is converted into two molecules of pyruvate (a three-carbon compound). This process occurs in the cytoplasm of cells and involves ten enzyme-catalysed steps. It is anaerobic. The process can be summarized as follows:

1. Energy Investment Phase (Steps 1–5): Two ATP molecules are consumed to phosphorylate and split glucose into two molecules of glyceraldehyde-3-phosphate (G3P).

2. Energy Payoff Phase (Steps 6–10): G3P is oxidized and phosphorylated, producing ATP, NADH, and pyruvate

Key stages in Glycolysis

1. phosphorylation of Hexose molecules

2. breakdown to Glyceraldehyde-3-Phosphate (G3P)

3. Oxidation to 3-Phosphoglycerete (3PG)

4. Pyruvate formation

Phosphorylation of Hexose Molecules

• The first step involves phosphorylation of glucose to glucose-6-phosphate (G6P) by hexokinase (enzyme not required for recall). This reaction consumes one ATP molecule.

• G6P is then rearranged into fructose-6-phosphate (F6P), which undergoes a second phosphorylation.

• Phosphofructokinase (PFK) catalyses the phosphorylation of F6P to fructose-1,6-bisphosphate (F1,6BP), using another ATP molecule.

• This phosphorylation step is critical for committing glucose to glycolysis and is tightly regulated by PFK as an allosteric enzyme:

  • Inhibition by ATP: When ATP levels are high, ATP binds to an allosteric site on PFK, decreasing its activity.

  • Activation by AMP: Low ATP levels increase AMP concentration, which binds to PFK, enhancing its activity

Breakdown of Glyceraldehyde-3-Phosphate (G3P)

• F1,6BP is cleaved into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).

• Only G3P proceeds in the glycolysis pathway, but DHAP is rapidly converted to G3P by an isomerase enzyme.

• Thus, from one glucose molecule, two G3P molecules enter the subsequent steps.

Oxidation to 3-Phosphoglycerate (3PG)

• G3P is oxidized to 1,3-bisphosphoglycerate (1,3-BPG) in a reaction that reduces NAD+ to NADH + H. This reaction is crucial for capturing energy in the form of reduced coenzyme (NADH), which will later be used in oxidative phosphorylation.

• 1,3-BPG donates a high-energy phosphate group to ADP, forming ATP via substrate-level phosphorylation, and is converted into 3-phosphoglycerate (3PG).

• Substrate-level phosphorylation: ATP is directly synthesized from ADP using a high-energy intermediate (1,3-BPG in this case), without the need for an electron transport chain

Pyruvate Formation (brief)

After further reactions (not specified here), 3PG is converted into phosphoenolpyruvate (PEP), which donates its phosphate group to ADP, forming another ATP molecule via substrate-level phosphorylation.

Pyruvate

The final product of glycolysis

Energy Yield of Glycolysis

1. Energy Consumed: 2 ATP molecules are used in the energy investment phase (steps catalysed by hexokinase and PFK).

2. Energy Produced:

   • 4 ATP molecules are produced in the energy payoff phase (2 ATP per G3P molecule).

   • 2 NADH molecules are produced (1 NADH per G3P molecule).

3. Net Gain: Net ATP= 4 produces – 2 used= 2ATP
   Additionally, 2 NADH and 2 pyruvate molecules are produced per glucose.

Significance of Glycolysis

1. Energy Harvest: Glycolysis provides ATP even in the absence of oxygen, supporting anaerobic conditions.

2. Intermediates: The pyruvate and NADH generated serve as key inputs for subsequent stages of respiration (e.g., the Krebs cycle and oxidative phosphorylation under aerobic conditions, or fermentation under anaerobic conditions).

3. Regulation by PFK: PFK ensures that glycolysis occurs at a rate appropriate to the cell's energy needs, preventing wastage of glucose and ATP.

Aerobic respiation equetion

Glucose + oxygen carbon dioxide + water + ENERGY

C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂O + ENERGY

Feature as ATP as an energy currency

• It is universal to all living organisms

• It can move easily between cytosol and organelles by diffusion.

• It can take part in the many reactions of metabolism.

• It can deliver energy in relatively small amounts, sufficient to drive individual reactions.

• It is involved in energy-requiring reactions and in the energy-releasing steps of respiration.

• It is the source of energy for most biological processes

Redox reaction

one substance transfers electrons to another substances

reduction reaction

gain of one or more electrons by an atom, ion or molecule

Oxidation reaction

loss of one or more electrons

Phosphorylation

Addition of a phosphate group

Decarboxylation

Removal of a carbon atom

Aerobic respiration

involves the complete oxidation of pyruvate, derived from glycolysis, to generate energy in the form of ATP. The process occurs in two key stages:

1. Oxidation of Pyruvate to Acetyl Coenzyme A (Acetyl-CoA).

2. The Krebs Cycle (Citric Acid Cycle).

Both processes take place in the mitochondrial matrix and contribute to the production of CO₂, reduced coenzymes (NADH + H + NADH + H⁺, FADH₂), and a small amount of ATP.

Location of oxidation of pyruvate to Acetyl Coenzyme A

Mitochondrial matrix

Process of Pyruvate to Acetyl Coenzyme A

• Each pyruvate molecule (3 carbons) is oxidized in a reaction catalyzed by the pyruvate dehydrogenase complex (PDC).

• During this reaction:

  • One carbon atom is released as carbon dioxide (CO₂).

  • Two electrons and one proton (H⁺) are transferred to NAD⁺, reducing it to NADH + H⁺.

  • The remaining 2-carbon fragment is combined with coenzyme A (CoA) to form acetyl coenzyme A (acetyl-CoA).

Summary of Oxidation of Pyruvate to Acetyl Coenzyme A

Pyruvate + NAD⁺ +CoA Acetyl-CoA+ NADH+ H⁺+ CO₂

• For one glucose molecule (2 pyruvate molecules from glycolysis), this reaction produces:

  • 2 CO₂,

  • 2 NADH

  • 2 acetyl-CoA.

Location of The Krebs Cycle

Mitochondrial matrix

Overview of the Krebs Cycle

Acetyl-CoA (2-carbon) enters the Krebs cycle, where it is completely oxidized to CO₂. In the process, electrons are transferred to NAD⁺ and FAD, forming reduced coenzymes (NADH + H+ NADH + H⁺ and FADH₂), and ATP is produced via substrate-level phosphorylation

Key Steps of the Krebs Cycle (brief)

1. Condensation with Oxaloacetate

2. Decarboxylation and Reduction

3. ATP production

4. Regeneration of Oxaloacetate

Condensation with Oxaloacetate

Acetyl-CoA (2C) combines with oxaloacetate (4C) to form a 6-carbon compound (citrate). This is catalysed by citrate synthase

Significance of Oxaloacetate

It is regenerated at the end of the cycle to ensure the cycle can continue.

Decarboxylation and Reductions

• Two carbon atoms are removed as CO₂, one at a time. This is the source of the carbon dioxide exhaled by organisms.

• At the same time, three NAD⁺ molecules are reduced to NADH + H⁺, and one FAD molecule is reduced to FADH₂ per cycle.

ATP Production

One molecule of ATP (or GTP, depending on the cell type) is produced via substrate-level phosphorylation in the cycle

Regeneration of Oxaloaxetate

Through a series of intermediate steps, oxaloacetate (4C) is regenerated, enabling the cycle to continue

Summary of the Kreb Cycle

For each acetyl-CoA molecule (per turn of the Krebs cycle):

• 2 CO₂ are released.

• 3 NADH+ H⁺ are produced.

• 1 FADH₂ is produced.

• 1 ATP (or GTP) is synthesized.

Since one glucose molecule generates 2 acetyl-CoA molecules, the Krebs cycle runs twice per glucose, doubling the outputs.

Outputs from the Krebs Cycle (Per Glucose Molecules)

• Carbon Dioxide: 4 molecules (2x2).

• Reduced Coenzymes:

  • 6 NADH + H⁺ (2x3).

  • 2 FADH₂(2x1).

• ATP: 2 molecules (2x1).

Important of Reduced Coenzymes

• NADH+ H⁺ and FADH₂ are critical for the electron transport chain (ETC), where their stored energy is used to drive the production of ATP via oxidative phosphorylation.

• Each NADH contributes approximately 2.5 ATP, and each FADH₂ contributes approximately 1.5 ATP during the ETC.

Overall Role in Cellular Respiration

• The oxidation of pyruvate and the Krebs cycle represent key stages in the complete oxidation of glucose.

• These processes generate reduced coenzymes that are essential for the majority of ATP production in the final stage of respiration.

• They also release carbon dioxide as a waste product, which is expelled from the body during respiration.

The Role of the Electron Transport Chain (ETC) in generating ATP

The electron transport chain (ETC) is the final stage of cellular respiration, located in the inner mitochondrial membrane. It uses electrons from reduced coenzymes (NADH+ H+ NADH + H⁺ and FADH₂) to create a proton gradient across the membrane, driving ATP synthesis through oxidative phosphorylation. This process includes chemiosmosis, where protons flow back into the mitochondrial matrix via ATP synthase, producing ATP.

Steps of the ETC and ATP Generation

1. electron transfer

2. proton gradient

3. Chemiosmosis and ATP Synthesis

4. Role of Oxygen

Electron Transfer

1. Electrons from NADH+ H⁺ and FADH₂ are transferred to the ETC.

2. The ETC consists of protein complexes and electron carriers embedded in the inner membrane, organized as follows:

   • Complex I: NADH dehydrogenase.

   • Complex II: Succinate dehydrogenase (directly linked to the Krebs cycle).

   • Ubiquinone (Coenzyme Q): A mobile electron carrier that shuttles electrons between complexes.

   • Complex III: Cytochrome bc1 complex.

   • Cytochrome c: A mobile protein that transfers electrons to Complex IV.

   • Complex IV: Cytochrome c oxidase.

3. Electrons lose energy as they pass through these complexes, which is used to pump protons (H⁺) across the inner membrane into the intermembrane space.

Proton Gradient (Proton Motive Force)

1. The movement of H⁺ into the intermembrane space creates an electrochemical gradient (high H⁺ concentration in the intermembrane space and low in the matrix).

2. This gradient stores potential energy, termed the proton motive force

Chemiosmosis and ATP Synthesis

Protons flow back into the matrix through ATP synthase, a protein complex embedded in the inner membrane.

Oxidative phosphorylation

The flow of protons provides energy to ATP synthase to phosphorylate ADP, forming ATP

Role of Oxygen

1. At the end of the ETC, electrons combine with H⁺ and molecular oxygen (O₂) to form water: 4e^- +4H⁺ +O₂→2H₂O

2. Oxygen is the final electron acceptor, and without it, the ETC would stop, halting ATP production.

ATP Yield from Oxidative Phosphosylation

• Each NADH contributes approximately 2.5 ATP.

• Each FADH₂ contributes approximately 1.5 ATP.

• Together, the ETC produces ~28 ATP per glucose molecule.

Structure of a Typical mitochondrion

The mitochondrion is a double-membraned organelle and the site of aerobic respiration. Its structure supports its function in the Krebs cycle and the electron transport chain.

1. outer membrane

2. inner membrane

3. intermembrane space

4. matrix

Outer Membrane of Mitochondria

• Smooth and permeable to small molecules and ions.

• Contains porins (protein channels) that allow diffusion of molecules like pyruvate into the intermembrane space.

Inner Membrane of Mitochondria

1. Highly folded into cristae, increasing surface area for the ETC and ATP synthase.

2. Contains:

   • Protein complexes of the ETC.

   • ATP synthase.

   • Transport proteins (e.g., for ADP/ATP exchange).

Intermembrane Space in Mitochondria

1. Space between the outer and inner membranes.

2. High concentration of protons (H⁺) during ETC, forming the proton gradient for chemiosmosis.

Matrix in Mitochondria

1. The central space enclosed by the inner membrane.

2. Contains:

   • Enzymes of the Krebs cycle.

   • Enzymes for the oxidation of pyruvate.

   • Mitochondrial DNA and ribosomes.

Site of the Krebs Cycle

1. The Krebs cycle occurs in the mitochondrial matrix.

2. Produces reduced coenzymes (NADH+ H+ NADH + H⁺, FADH₂) and CO₂ as waste.

Site of the ETC

1. The inner mitochondrial membrane hosts the ETC.

2. This membrane's organization supports the sequential transfer of electrons, proton pumping, and ATP synthesis.

Glycolysis Overview

Anaerobic respiration occurs when oxygen is unavailable or insufficient to act as the final electron acceptor in the electron transport chain (ETC). In these situations, the pyruvate produced during glycolysis does not undergo complete oxidation in the Krebs cycle. Instead, it is metabolized in pathways that regenerate NAD⁺ allowing glycolysis to continue and produce ATP. These pathways lead to the formation of lactic acid in animals such as in muscle cells or ethanol and CO₂ in yeast.

Situations Requiring Anaerobic Respiration

1. Muscle cells- During intense exercise, oxygen delivery to muscle cells may not meet demand. To maintan energy production, cells switch to anaerobic respiration, converting pyruvate to lactic acid

2. Microorganisms- Organisms like yeast live in anaerobic or oxygen-limited environments. They convert pyruvate to ethanol and CO₂ to regenerate NAD⁺ for glycolysis.

Pathways in Anaerobic Respiration

1. Formation of Lactic Acid in Animals

2. Formation of Ethanol and CO₂ in yeast

Formation of Lactic Acid in Animals

• Process:

  • Pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase.

  • NADH+ H+ NADH + H⁺ is oxidized to NAD⁺, allowing glycolysis to continue.

• Equation: Pyruvate + NADH + H⁺ Lactic Acid + NAD⁺

• Outcome:

  • Lactic acid accumulates in muscles, causing fatigue and a drop in pH (acidosis).

  • This process is reversible; lactic acid is later transported to the liver and converted back to glucose (via the Cori cycle).

Formation of Ethanol and CO₂ in yeast

• Process (Alcoholic Fermentation):

  1. Pyruvate is first decarboxylated to acetaldehyde and CO₂ by pyruvate decarboxylase.

  2. Acetaldehyde is reduced to ethanol by alcohol dehydrogenase, with NADH + H⁺ oxidised to NAD⁺

• Equation: Pyruvate Acetaldehyde + CO₂
  Acetaldehyde + NADH + H⁺ Ethanol + NAD⁺

• Outcome:

  • This process is used in brewing and baking, producing ethanol and CO₂.

Oxygen Dept

Oxygen debt refers to the extra oxygen required after exercise to metabolize accumulated lactic acid and restore the body to its resting state.

Causes of oxygen dept

• During intense exercise, when oxygen supply is insufficient, anaerobic respiration leads to lactic acid accumulation.

• Post-exercise, oxygen is needed to:

  • Convert lactic acid back to pyruvate or glucose in the liver (Cori cycle).

  • Replenish ATP and creatine phosphate stores in muscles.

  • Restore oxygen levels in myoglobin.

Process of oxygen dept

Recovery involves increased breathing and heart rate to deliver oxygen to cells, enabling the breakdown or recycling of lactic acid

The metabolic pool

refers to the interconnected pathways of metabolism, where molecules from carbohydrates, fats, and proteins serve as substrates that can be interconverted to meet cellular energy demands or biosynthetic requirements. This flexibility ensures that cells can adapt to varying energy sources and physiological conditions.

Fat Respiration

Fats are stored as triglycerides in adipose tissue. During energy demand, triglycerides are hydrolysed into glycerol and fatty acids, which enter different metabolic pathways:

1. Glycerol Metabolism:

2. Beta-Oxidation:

Glycerol Metabolism

• Glycerol is converted to glyceraldehyde-3-phosphate (G3P), an intermediate in glycolysis.

• G3P can either:

  • Enter glycolysis for ATP production.

  • Be used in gluconeogenesis to form glucose.

Beta- Oxidation

• Fatty acids undergo beta-oxidation in the mitochondrial matrix.

• In this process:

  • Fatty acids are broken down into acetyl-CoA molecules.

  • NADH and FADH₂ are also produced and feed electrons into the electron transport chain.

Fat Synthesis

When energy is abundant, excess carbohydrates and proteins can be converted to fats for storage. This occurs primarily in the liver and adipose tissue.

Fat Synthesis Process

• Acetyl-CoA from glycolysis is converted to fatty acids via a multi-step enzymatic process.

• Fatty acids are esterified with glycerol to form triglycerides.

Fat Synthesis Significiance

This pathway provides long-term energy storage

Gluconeogenesis

is the synthesis of glucose from non-carbohydrate precursors, ensuring a continuous supply of glucose, especially during fasting or low carbohydrate intake. It occurs primarily in the liver (and to a lesser extent, in the kidneys).

Key Precursors

1. Lactate:

   • From anaerobic glycolysis in muscles.

   • Converted back to pyruvate via the Cori cycle.

2. Glycerol:

   • From triglyceride breakdown.

   • Converted to dihydroxyacetone phosphate (DHAP), an intermediate in gluconeogenesis.

3. Amino Acids (Glucogenic Amino Acids):

   • From protein breakdown.

   • Converted to intermediates of the Krebs cycle (e.g., oxaloacetate) or pyruvate.

Significance of gluconeogenesis

• Maintains blood glucose levels during fasting or starvation.

• Provides glucose for tissues that rely exclusively on it, such as the brain and red blood cells.

Amino Acid Metabolism

Proteins are not stored in the body but can be broken down into amino acids, which are used for energy production, gluconeogenesis, or biosynthesis.

1. Deamination

2. Fates of Carbon Skeletons

   1. Glucogenic Amino Acids

   2. Ketogenic Amino Acids

   3. Both Glucogenic and Ketogenic- Some amino acids, like isoleucine and phenylalanine, can act as both glucogenic and ketogenic substrates

3. Protein Synthesis

Deamination

The amino group is removed from amino acids, producing ammonia and a carbon skeleton. Ammonia is converted to urea in the urea cycle and excreted

Glucogenic Amino Acids

Carbon skeletons are converted to glucose via gluconeogenesis. Examples: Alanine, serine

Ketogenic Amino Acids

Carbon skeletons are converted to acetyl-CoA or acetoacetate, which can enter the Krebs cycle or be used in ketone body formation. Examples: Leucine, lysine

Protein Synthesis

Amino acids are used to synthesize new proteins or nitrogenous compounds like nucleotides

Integration of the Metabolic Pool

1. Carbohydrates:

   • Glucose can be oxidized for energy (glycolysis and respiration) or stored as glycogen.

   • Excess glucose can be converted to fats via lipogenesis.

2. Fats:

   • Fatty acids can be oxidized for ATP or stored as triglycerides.

   • Glycerol can feed into gluconeogenesis or glycolysis.

3. Proteins:

   • Amino acids can be used for gluconeogenesis, energy production, or biosynthesis.

Energy Yields and Pathway Significance

• Carbohydrates are the primary quick energy source, producing ~36-38 ATP per glucose in aerobic respiration.

• Fats provide the highest energy yield (~9 kcal/g compared to ~4 kcal/g for carbohydrates), but require more oxygen for oxidation.

• Proteins are not a preferred energy source and are used mainly during prolonged fasting or starvation

Gaseous exchnage

the interchange of O₂ and CO₂ between an organism and its environment.

The importance of gas exchange to multicellular organisms

Energy metabolism requires O₂ and produces CO₂. There are three phases of gas exchange:

• Breathing

• Transport of gases by the circulatory system

• Servicing of cells within the body tissues

Frick’s law of diffusion

illustrates the relationship between environmental variables that influence rate of diffusion. For gas exchange it may be written as:
Q=DA ((P₁-P₂)/L)

Where:
Q = is the rate at which a gas such as O2 diffuses between two locations
D = diffusion coefficient; depends on diffusing substance, the medium and temperature A = cross-sectional area through which the gas is diffusing
P1 and P2 are the partial pressures of the gas at the two locations.
L = the path length, or distance between the two locations.