Cell as a Factory, Carbohydrates, Lipids, and ATP

Cell as a Factory

Cells function like factories, taking in materials, breaking them down, and utilizing food molecules for survival. This concept applies from individual cells to entire organisms.

Important Molecules and Energy Generation

Understanding the crucial molecules and how cells generate energy from them is key. The primary focus is on how cells utilize these molecules to produce energy.

Carbohydrates

• General Formula: Carbohydrates, particularly glucose, follow a basic hydrocarbon formula, making them excellent sources of stored energy.

• Transport: They are easily transported, especially in multicellular organisms like humans, via the bloodstream and within cells.

• Carbon Skeletons: These skeletons, the remaining structure after part of the molecule is removed, serve as substrates for other reactions, converting them into different molecule types.

Types of Carbohydrates

• Monosaccharides: Simple sugars like glucose.

• Disaccharides: Two simple sugars linked by covalent bonds.

• Oligosaccharides: Intermediate-sized sugars, containing roughly 3 to 20 monosaccharides.

• Polysaccharides: Large sugars with hundreds to thousands of monosaccharides linked together. They are significant for storing carbohydrate energy within human cells.

Glucose

• Nearly all cells can utilize glucose as an energy source.

• Glucose exists in two forms, typically as a ring structure (hexagon-like).

• It has alpha and beta forms that can interconvert.

Importance of Glucose

• Each bond in a glucose molecule contains electrons as covalent links.

• Extracting these electrons is a crucial process for energy generation.

Other Monosaccharides

• Hexoses: Such as glucose are primarily used as fuel sources.

• Pentoses: Found predominantly in DNA nucleotides such as deoxyribose and ribose, which are five-carbon sugars.

• Three-Carbon Sugars: Glyceraldehyde.

• Six-Carbon Sugars: Include galactose and fructose, which are main dietary carbohydrates obtained from the digestion of complex carbohydrates.

Disaccharides Formation

• Monosaccharides join to form disaccharides via an alpha linkage, particularly at carbon number one (alpha carbon) on glucose.

• Sucrose: A combination of 50% glucose and 50% fructose.

• The bond that holds them together involving an oxygen is a Glycosidic linkage, indicating a covalent bond between the two monomers.

• Maltose: Consists of two structural isomers of D-glucose.

Oligosaccharides Function

• Composed of 3-20 monomers, these are often linked with proteins or membranes rather than being used for energy production. ABO blood types are built around carbohydrates attached to proteins on the red blood cells.

Polysaccharides Structure and Function

• Starch (in plants): Used to store glucose as a highly branched carbohydrate structure.

• Glycogen (in humans): Stored mainly in muscle tissue and the liver, it is a highly branched structure allowing for more glucose to be packed into a given space.

• Cellulose (in plants): Useless for energy production in humans; it forms plant cell walls and provides structural stability.

Lipids

• Lipids are like hydrocarbons. They are great sources of fuel. They pack a substantial amount of energy into a small space.

• They do not strictly form polymers. However, when multiple lipids interact, they function similarly to a polymer.

Functions

• Fats and Oils: Oils are primarily used by plants for energy storage. Fats are used by humans to store energy.

• Structural Roles:

  • Phospholipids form cell membranes.

  • Carotenoids and chlorophylls (in plants) are lipid-based.

  • Hormones (in humans) can be lipid-based.

Structure of Fats and Oils

• Most fats and oils are triglycerides, consisting of three fatty acid chains linked to a glycerol molecule (an alcohol-based molecule).

• Fatty acid chains store energy.

• Excess energy is stored as fatty acids built from acetyl CoA and attached to glycerol.

• Fatty acids can be broken down two at a time to yield energy.

Types of Fatty Acids

• Saturated Fatty Acids: Linear, without double bonds. Animal fats tend to be saturated and solid at room temperature.

• Unsaturated Fatty Acids: Contain double bonds, leading to a twisted structure. This is divided into:

  • Monounsaturated: One double bond.

  • Polyunsaturated: More than one double bond.

Amphipathic Nature of Fatty Acids

• Contains carboxyl group (Hydrophilic) at the head and chains of carbon and hydrogen (Hydrophobic).

• Phospholipids

  • Phosphate Group: Hydrophilic head.

  • Fatty Acid chains: Hydrophobic tails.

Membrane Formation

• Due to their amphipathic nature, phospholipids can spontaneously form membranes.

• In these formations, hydrophobic tails are sandwiched between hydrophilic head groups.

• Membranes act as physical barriers within cells and between the cell and its external environment.

Cells as Factories

Cells operate akin to factories by importing materials, conducting processes, and expelling waste while producing other substances. ATP is crucial for capturing and storing free energy.

ATP (Adenosine Triphosphate)

• ATP captures free energy released from molecule breakdown.

• It facilitates the transfer of chemical groups.

• ATP is formed by adding a phosphate group onto adenosine diphosphate (ADP).

• Breaking down ATP into ADP releases energy by transferring a phosphate group to another molecule.

• ATP can phosphorylate proteins, thus altering their function, and transfer phosphates to other chemicals within the cell.

• ATP is a nucleotide, it hydrolyzes into ADP by cleaving the phosphate, releasing significant free energy.

• Has a negative delta G value, meaning it releases free energy (exergonic).

Biochemical Reaction

In bioluminescence, the enzyme luciferase converts luciferin into oxyluciferin using oxygen and ATP, which products a lot of light.

ATP Formation

Requires input of free energy.

ATP Role

Links exergonic and endergonic processes. Catabolic processes release energy to produce ATP, which is then used in active transport, cell movement, and anabolic processes.

Glucose Metabolism

Glucose is the primary dietary carbohydrate for humans, and many organisms metabolize. The goal:

• Glucose oxidation to release chemical energy.

• Oxidative phosphorylation to form ATP.

• Energy extraction from glucose without oxygen.

• Regulation of these processes.

Glucose as Fuel

Glucose acts as a fuel and store of potential energy. Metabolic processes involve chemical transformations catalyzed by enzymes.

Regulation and Control

These processes are similar across different life forms, and in eukaryotes, compartmentalization via organelles adds another layer of control.

Burning Glucose

Comparable to burning, producing carbon dioxide, water, and a large amount of heat. However, cells break the molecular structure of glucose into small packets of energy.

• Complete combustion of glucose releases a massive amount of energy but this is not how biology captures the energy.

• Cells trap free energy in ATP rather than releasing it all at once as heat.

• \Delta G is the change in free energy of a reaction.

• If glucose is completely combusted, it has a very negative \Delta G value, indicating a large release of energy.

Metabolic Pathways

• Glycolysis: Converts glucose into pyruvate.

• Cellular Respiration (Aerobic): Converts pyruvate into water and carbon dioxide when oxygen is present.

• Fermentation (Anaerobic): Converts pyruvate into ethanol (in yeast) or lactic acid/lactate (in muscles) when oxygen is absent.

Diagram of Metabolic Processes

• Sunlight captured via photosynthesis leads to glucose production which is broken down through glycolysis. It splits glucose (6-carbon) into pyruvate (3-carbon).

• Without oxygen, glycolysis yields only 2 ATPs per glucose molecule.

• With oxygen, pyruvate can be completely oxidized to produce 32 ATPs.

Citric Acid Cycle and Electron Transport

If oxygen is present, uses pyruvate to produce ATP but if not, fermentation of pyruvate yields less ATP.

Redox Reactions

• Oxidation (loss of electrons) and reduction (gain of electrons) occur together.

• Transferring electrons in the process is referred to as redox reactions.

• Oxidation involves the movement of hydrogen atoms. Gaining hydrogen makes a covalent bond, and breaking a covalent bond releases it.

OIL RIG is an easy way to remember: Oxidation Is Loss, Reduction Is Gain.

Capturing Electrons

The electrons can strip out of molecules allowing the cell to make ATP.

Oxidizing and Reducing

• If burning glucose, then it is the reducing agent while oxygen is the oxidizing agent.

• Energy is transferred in redox reactions from the reducing agent to the reduced product.

• Deconstructing molecules goes from high free energy (more complexity and electrons in bonds) to lower free energy (more oxidized state).

Coenzyme NAD

• Serves as an electron carrier and captures electrons.

• It has oxidized and reduced forms, the only difference being the presence or absence of a hydrogen atom.

Function

NAD captures hydrogen from food molecules to form NADH which then passes that hydrogen/electron onto another molecule, these energetic reactions enable metabolic reactions.

The reason humans breathe is to allow oxygen to accept electrons because the electrons in NADH will be passed to oxygen to form water (H2O), thus the oxidizing agent here is molecular oxygen.

NAD has a complex structure with only two hydrogens versus one when it's reduced, but it is within that bond where the electrons are stored/captured, and cleaving that bond releases energy. The capture of electrons leads to formation of water, thus the oxidizing agent is that molecular oxygen.