Question

Which of the following statements is not correct? a. Some metabolic pathways are cyclic. b. Glucose can diffuse directly through a lipid bilayer. c. All coenzymes are cofactors. d. Osmosis is an example of diffusion.

Short answer

Statement (b) is not correct; glucose cannot diffuse directly through a lipid bilayer without help.

Step-by-step solution

Understanding Statement (a)

Statement (a) says "Some metabolic pathways are cyclic." This statement is correct because there are indeed metabolic pathways that are cyclic, such as the Krebs cycle.

Analyzing Statement (b)

Statement (b) claims "Glucose can diffuse directly through a lipid bilayer." This statement is not correct because glucose is a large and polar molecule, and it cannot diffuse directly through a lipid bilayer without the aid of a transport protein.

Reviewing Statement (c)

Statement (c) states "All coenzymes are cofactors." This is correct because coenzymes, which are organic molecules, are a specific type of cofactor involved in enzymatic reactions.

Evaluating Statement (d)

Statement (d) is "Osmosis is an example of diffusion." This statement is correct as osmosis is indeed a type of diffusion where water molecules move across a semipermeable membrane.

Key concepts

Diffusion

Diffusion is a fundamental process in biological systems where molecules move from an area of high concentration to an area of low concentration. This process is driven by the kinetic energy of molecules, resulting in a net movement towards equilibrium.
Diffusion is crucial in processes such as the exchange of gases in the lungs and the dispersal of nutrients and waste products in cells.
It occurs without the use of energy, making it a form of passive transport.

• Molecules move until they are evenly distributed, achieving equilibrium.
• Diffusion can involve gases, liquids, and solutes.
• Factors affecting diffusion include temperature, concentration gradient, and the size of the molecules.

Understanding diffusion helps explain how substances like oxygen and carbon dioxide are moved in biological systems.

Coenzymes

Coenzymes are organic molecules that play a critical role in metabolic pathways by assisting enzymes in catalyzing reactions. They are not consumed in the reaction but temporarily bind to the enzymes, enhancing their activity.
Coenzymes are often derived from vitamins, making them essential nutrients that we need to obtain from our diet.

• Examples include NAD (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide), both crucial in energy production.
• Without coenzymes, many reactions and metabolic pathways would not proceed efficiently.
• They ensure proper enzymatic function and also assist in the transfer of chemical groups like hydrogen or electrons during reactions.

These molecules are central to our body's ability to produce energy and process nutrients.

Lipid Bilayer Transport

The lipid bilayer is a structural foundation of cell membranes, composed mainly of phospholipids, which form a double layer with hydrophobic tails facing inward and hydrophilic heads facing outward.
This membrane functions as a selective barrier, determining what substances can enter or leave the cell.
Transport across the lipid bilayer can occur in several ways:

• Simple Diffusion: Small, nonpolar molecules (like oxygen and carbon dioxide) can pass freely through the membrane.
• Facilitated Diffusion: Larger or polar molecules (such as glucose) require protein channels or carriers to move across.
• Active Transport: Energy-dependent process that moves molecules against the concentration gradient using transport proteins.

The lipid bilayer's selective permeability is vital for maintaining homeostasis within cells.

Osmosis

Osmosis is a specific type of diffusion that involves the movement of water molecules through a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. It is crucial for maintaining cell turgor and regulating fluid balance in biological systems.
In biology, osmosis helps cells manage water intake, ensuring that they do not shrink or swell excessively.

• Water moves to equalize solute concentrations on both sides of the membrane.
• The process can affect cell volume and pressure.
• Osmosis is a passive process, requiring no energy expenditure by the cell.

Understanding osmosis is fundamental in explaining fluid movement in tissues and the kidneys' role in concentrating urine.