Cellular Metabolism: Glycolysis & Krebs Cycle

Introduction to Cellular Metabolism

• Metabolism refers to all the chemical changes that occur within a cell.
• Anabolism builds up molecules, requiring energy.
• Catabolism breaks down molecules, releasing energy.
• A calorie is a unit of energy found in food.
• ATP is the main energy source for cells.

Glycolysis

• Process involving the breakdown of glucose (C6H12O6) into pyruvic acid or pyruvate.
• ATP or energy is generated through this process.
• This process can occur both in the presence and absence of oxygen.
• The final outcome is 2 pyruvic acid molecules.
• The final outcome is also 2 ATP molecules in anaerobic conditions.
• Additionally, 8 ATP molecules are created when oxygen is present (aerobic).

Krebs Citric Acid Cycle

• Pyruvic acid is converted to acetic acid, which then becomes Acetyl-CoA.
• Acetyl-CoA enters the Krebs cycle within the mitochondria.
• The final outcome of the Krebs Cycle is 6 CO2, 8 NADH2, 2 FADH2, and 2 ATP (GTP).

The Electron Transport System

• A series of reduction/oxidation reactions take place.
• Oxygen (O2) is required.
• Water is produced as a waste product.
• The amount of ATP produced depends on the electron carrier involved.
• The system involves several complexes, including NADH Dehydrogenase (Complex I), Succinate Dehydrogenase (Complex II), Cytochrome bc₁ Complex (Complex III), and Cytochrome c Oxidase (Complex IV).

Summary of ATP Production

• ATP is produced during glycolysis, the citric acid cycle, and electron transport.
• Glycolysis (aerobic) yields 8 ATP.
• The Krebs cycle combined with electron transport produces 28 ATP + 2 GTP, or 30 ATP.
• A single glucose molecule results in 38 ATP molecules.

Fermentation

• Yeast breaks down glucose without oxygen.
• Pyruvic acid is broken down by decarboxylase.
• Carbon dioxide and acetaldehyde are formed.
• Overall 2 ATP, CO2 and ethyl alcohol are the final products.

Anaerobic Production of ATP by Muscles

• Pyruvic acid is converted to lactic acid.
• Lactic acid buildup in muscles can cause fatigue.
• When oxygen becomes available, lactic acid turns back into pyruvic acid.
• 2 ATP molecules are produced per glucose molecule in this process.

Production of ATP from General Food Compounds

• Carbohydrates enter cellular metabolism at the glucose stage and can be stored as liver glycogen or as fat.
• Fats are broken down into fatty acids and glycerol.
• Glycerol enters at the PGA stage of glycolysis.
• Fatty acids enter the Krebs citric acid cycle.
• Proteins are digested into amino acids.
• Amino acids enter the Krebs cycle at various stages, dependent on their chemical structure.

Introduction to Cellular Reproduction

• Cell duplication is significant for this process.
• Mitosis involves the duplication of genetic material.
• Cytokinesis involves the duplication of organelles.
• Meiosis is a reduction division that happens only in the gonads.

The Structure of the DNA Molecule

• Friedrich Miescher discovered DNA in 1869.
• P.A. Levene determined the composition of DNA in the 1920s.
• Rosalind Franklin discovered the helical structure of DNA.
• Watson and Crick determined the three-dimensional structure of DNA.
• DNA arranged in a double helical chain of nucleotides.
• Each nucleotide includes a phosphate group, a five-carbon sugar (deoxyribose), and a nitrogen-containing base.
• Nitrogen containing bases are pyrimidines: thymine and cytosine, and purines: adenine and guanine.
• Pyrimidines pair with purines.
• The chains are held together by hydrogen bonds.
• A gene is a sequence of base pairs that codes for a polypeptide or protein.
• The Human Genome Project identified 3 billion base pairs that code for 30,000 genes.
• DNA duplication involves helicase separating the hydrogen bonds.
• DNA polymerase adds new nucleotides.

Introduction

• All reproduction starts at the cellular level.
• The cell cycle includes interphase, mitosis, and cytokinesis.

Interphase

• Interphase is the time between cell divisions.
• G₁ is the primary growth phase.
• S is when DNA duplication occurs.
• During G₂, centrioles complete duplication, mitochondria replicate, and chromosomes condense and coil.

Mitosis

• Prophase is the initial stage.
• During prophase, chromosomes become visible as chromatids joined by a centromere.
• There are two kinetochores at the centromere.
• Centrioles move to opposite poles.
• The nuclear membrane breaks down.
• Microtubules attach kinetochores to the spindle.
• During Metaphase, chromatids align at the equator of the cell.
• The centromere divides during metaphase.
• Anaphase involves the divided centromere pulling chromatids to opposite poles.
• Cytokinesis starts during anaphase.
• Chromosomes uncoil and decondense during telophase.
• The spindle apparatus breaks down.
• A new nuclear membrane forms.
• Cytokinesis is nearly complete.

Cytokinesis

• In animal cells, a cleavage furrow forms and the cell is pinched into two daughter cells.
• In plant cells, a cell plate forms at the equator and later becomes the new cell wall.

Meiosis

• Meiosis is a reduction division that occurs only in the gonads.
• It reduces the genetic material from diploid to haploid.
• It involves two divisions, which results in four cells.
• During Prophase I of Meiosis, homologous chromosomes pair and cross over.
• During Metaphase I, chromosomes align along the equator.
• During Anaphase I, centromeres are pulled to the poles.
• Telophase I results in one member of each pair being at each pole.
• During Prophase II, involves the spindle forming and the centrioles moving to poles.
• During Metaphase II, chromosomes line up at the equator.
• During Anaphase II, centromeres divide
• Telophase II results in chromatids at each pole, and a new nuclear membrane forms.

Gametogenesis

• Spermatogenesis produces four cells that develop into sperm.
• Oogenesis produces four cells, but only one becomes a functional egg.