Problem 5 - Biology

Question

What property prevents the ligands of cell-surface receptors from entering the cell? a. The molecules bind to the extracellular domain. b. The molecules are hydrophilic and cannot penetrate the hydrophobic interior of the plasma membrane. c. The molecules are attached to transport proteins that deliver them through the bloodstream to target cells. d. The ligands are able to penetrate the membrane and directly influence gene expression upon receptor binding.

Step-by-Step Solution

Answer

Option 'b'.

Step 1: Understand the question

Identify what the question is asking: Which property prevents the ligands of cell-surface receptors from entering the cell?

Step 2: Evaluate option 'a'

Option 'a' states that the molecules bind to the extracellular domain. This doesn't explicitly explain why they cannot enter the cell.

Step 3: Evaluate option 'b'

Option 'b' states that the molecules are hydrophilic and cannot penetrate the hydrophobic interior of the plasma membrane. Hydrophilic molecules have difficulty passing through the hydrophobic lipid bilayer of the plasma membrane.

Step 4: Evaluate option 'c'

Option 'c' states that the molecules are attached to transport proteins that deliver them through the bloodstream to target cells. This information is about transport to the cell but doesn't address why they can't enter the cell.

Step 5: Evaluate option 'd'

Option 'd' states that the ligands are able to penetrate the membrane and directly influence gene expression. This is incorrect because it implies the ligands do enter the cell.

Step 6: Conclusion

Option 'b' is the most accurate because it directly addresses the hydrophilic nature of the molecules preventing them from penetrating the hydrophobic interior of the plasma membrane.

Key Concepts

Hydrophilic ligands

Hydrophilic ligands are molecules that love water. They can easily dissolve in water because they form hydrogen bonds with water molecules.
However, this hydrophilic nature also means they have trouble passing through cell membranes.
Cell membranes are made up of a lipid bilayer that is hydrophobic at its core, which repels water and substances that dissolve in water. So when hydrophilic ligands approach the cell membrane, they cannot easily pass through it.

• Hydrophilic ligands include many hormones and neurotransmitters.
• They bind to cell-surface receptors situated on the membrane’s outer layer.

This binding triggers a sequence of events inside the cell, transmitting the signal without the ligand actually entering the cell itself.

Plasma membrane structure

The plasma membrane acts as a shield for the cell, maintaining its internal environment. This membrane is composed of a double layer of lipids, called a lipid bilayer.
In this bilayer, the lipids have hydrophilic (water-attracting) heads facing outwards, and hydrophobic (water-repelling) tails facing inwards. This arrangement forms a barrier that protects the cell from its surroundings.

• Membrane proteins are embedded within the lipid bilayer, which allows selective passage of substances.
• Receptors on the cell surface are a type of membrane protein.

Because of its hydrophobic interior, the plasma membrane is impermeable to most water-soluble molecules, including hydrophilic ligands.
Only specific channels or carriers can facilitate the entry and exit of these molecules.

Cell signaling mechanisms

Cell signaling is how cells communicate with each other. When a hydrophilic ligand binds to a cell-surface receptor, it starts a signaling cascade inside the cell.
This signaling cascade often involves a series of events that amplify the signal, allowing the cell to respond appropriately.

• Receptors can be of different types, such as G-protein-coupled receptors (GPCRs) and tyrosine kinase receptors.
• These receptors change shape upon ligand binding, which allows them to interact with other proteins inside the cell.

As these intracellular proteins are activated, they carry the signal further inside the cell, leading to a specific response, such as changes in gene expression or cellular activity.

Hydrophobic barriers

The hydrophobic barrier of the plasma membrane plays a crucial role in cell signaling. While it effectively protects the cell, it also restricts the entry of hydrophilic substances.

• This barrier ensures that the internal environment of the cell remains stable.
• Only certain molecules, like gases or small lipid-soluble molecules, can pass through this barrier without assistance.

Hydrophilic ligands, thus, need to bind to receptors on the membrane surface to transmit their signal.
These hydrophobic barriers contribute to the selective permeability of the cell membrane, ensuring that only the right substances enter or exit the cell.