Which one is the correct sequence of events that takes place during phototransduction when light falls onto the retina?
Structural changes in rhodopsin → activation of transducin → decrease in intracellular cGMP → closure of Na. channels → decreased release of glutamate
Phototransduction Sequence Explained
Phototransduction is the process by which light is converted into electrical signals in the retina of the eye. This process occurs in the photoreceptor cells (rods and cones). In the dark, these cells are depolarized and release neurotransmitters. When light strikes, they hyperpolarize, and neurotransmitter release decreases. The question asks for the correct sequence of events that leads to this hyperpolarization when light is present.
Understanding Phototransduction Steps
Let's break down the sequence of events initiated by light:
- Structural Changes in Rhodopsin: The process begins when a photon of light is absorbed by the photopigment, such as rhodopsin in rod cells. This absorption causes a conformational change in the rhodopsin molecule, specifically in the retinal component (from 11-cis-retinal to all-trans-retinal), activating the rhodopsin.
- Activation of Transducin: The activated rhodopsin molecule interacts with and activates a G-protein called transducin. Transducin then dissociates into its subunits, one of which activates the enzyme phosphodiesterase (PDE).
- Decrease in Intracellular cGMP: The activated PDE hydrolyzes cyclic guanosine monophosphate (cGMP) into 5'-GMP. This enzymatic activity reduces the concentration of cGMP within the photoreceptor cell cytoplasm.
- Closure of Na$^{+}$ Channels: In the dark, high levels of cGMP bind to and keep open cGMP-gated cation channels (primarily permeable to Na$^{+}$ and Ca$^{2+}$) in the photoreceptor plasma membrane. When cGMP levels decrease due to PDE activity, these channels close. This reduces the influx of Na$^{+}$ ions into the cell.
- Decreased Release of Glutamate: The closure of Na$^{+}$ channels leads to hyperpolarization of the photoreceptor cell membrane (becomes more negative inside) because fewer positive ions are entering the cell. Hyperpolarization of the cell membrane potential reduces the opening of voltage-gated calcium channels at the synaptic terminal. This reduction in calcium influx leads to a decrease in the fusion of synaptic vesicles containing the neurotransmitter glutamate with the presynaptic membrane, resulting in less glutamate being released into the synapse with the bipolar cells.
This step-by-step process accurately describes the events of phototransduction in the retina, leading from light absorption to a change in neurotransmitter release.
Analyzing the Provided Sequence
The sequence given in the correct option is:
Structural changes in rhodopsin $\rightarrow$ activation of transducin $\rightarrow$ decrease in intracellular cGMP $\rightarrow$ closure of Na$^{+}$ channels $\rightarrow$ decreased release of glutamate.
Comparing this sequence with the biological process of phototransduction, we see that it perfectly matches the order of events initiated by light striking the photoreceptor.
| Step | Event | Explanation |
|---|---|---|
| 1 | Light Absorption | Photon hits rhodopsin. |
| 2 | Rhodopsin Activation | Structural change in rhodopsin. |
| 3 | Transducin Activation | Activated rhodopsin activates transducin. |
| 4 | PDE Activation | Activated transducin activates phosphodiesterase (PDE). |
| 5 | cGMP Hydrolysis | PDE breaks down cGMP. |
| 6 | cGMP Decrease | Intracellular cGMP concentration falls. |
| 7 | Channel Closure | cGMP-gated Na$^{+}$ channels close. |
| 8 | Hyperpolarization | Cell membrane hyperpolarizes due to reduced Na$^{+}$ influx. |
| 9 | Glutamate Release Decrease | Reduced calcium influx at synapse leads to less glutamate release. |
The sequence provided in the correct option aligns with steps 2, 3, 6, 7, and 9 from the detailed breakdown above, representing the key downstream events following initial light absorption.
Important Questions from Cell signaling
In the table below Column A lists ligands and Column B lists classes of receptors
Column A Column B A Serotonin i Binding activates a G-protein which activates or inhibits an enzyme that generates a specific second messenger and opens ion channel B Interferons ii Binding causes receptor monomers to dimerize. Dimeric receptor then interacts with and activates one or more cytosolic tyrosine protein kinases C Glycine iii Binding changes the conformation of the receptor so that specific ions flow through it D Insulin iv Binding leads to activation of intrinsic tyrosine kinase activity Which one is the correct match?
Receptor for which one of the following proteins spans the plasma membrane of target cells but DOES NOT contain intrinsic protein kinase activity?
Which one of the following amino acids is characteristic of and is conserved in response regulator of plant two-component systems?
Which one of the following signaling molecules is NOT a protein or a peptide?
Receptor tyrosine kinases (RTKs) form a large and important class of cell surface receptors whose ligands are soluble or membrane bound protein or peptide hormones. Which of the following statement is INCORRECT?
