Phototransduction: How we see photons

how often are we aware of what's going on in individual cells in our bodies we are made of trillions of cells yet notice very few for example neutrophils a type of white blood cell circulate throughout our bodies protecting us from infection every hour of our life billions of these cells are born and die yet we don't know this in contrast we are exquisitely aware of some cells such as nerve cells in our eyes and other sensory organs at the back of the eye is the retina containing light sensing cells called photoreceptors as discussed in a previous video our dark adapted rods can detect individual photons that activate a light sensing molecule within them this means our visual system has astonishing sensitivity we can detect the events affecting not just a single cell but individual molecules how we go from activating a single molecule to affecting the whole rod so that it can inform the brain is through a process of immense and rapid amplification called phototransduction phototransduction is the process by which light is converted into electrical signals in photoreceptors there are two types of photoreceptors in this video we'll be focusing on the rods a similar process occurs in cones but they are less sensitive to individual photons requiring much brighter light the light sensitive region of the rod is here where inside there are numerous disc shaped structures stacked atop each other these discs are membranes that contain millions of light-sensing molecules called rhodopsin what happens when light activates rhodopsin rhodopsin has two parts a small molecule called retinol attached to a protein called opsin when light is absorbed it results in a rearrangement of a double bond in the middle to change it from this to this this tiny rearrangement of retinal pushes on parts of the rhodopsin activating it the challenge is this this activated rhodopsin must produce a signal large enough to affect the whole cell in addition to rhodopsin there are two other important proteins in the disk membranes transducin and phosphodiesterase after a photon activates a rhodopsin the rhodopsin will activate transducin proteins it encounters the activated transducin in turn will bind to and activate phosphodiesterase effectively transducin is an intermediary between ferodopsin and phosphodiesterase having this chain of activation allows for amplification here's how recall from chemistry that molecules in solution are in constant motion rhodopsin water molecules and all other molecules in the disk membrane are constantly colliding with each other these collisions bring rhodopsin into contact with many transducin activating them each activated transducer in turn will bind to and activate phosphodiesterase in these visualizations time is slowed down and the violent motions of the molecules are reduced and simplified so we can see what's happening this is the first step of the amplification one activated rhodopsin leads to many activated phosphodiesterase proteins in a few hundred milliseconds in the dark rods contain many small molecules called cyclic gmp cyclic gmp is also in constant random motion and when it encounters activated phosphodiesterase it is converted into another chemical for visual simplicity i'm showing the cyclic gmp disappearing when they encounter activated phosphodiesterase as cyclic gmp is the important molecule for phototransduction this is the second step of amplification each phosphodiesterase will destroy several cyclic gmp molecules thus a single activated rhodopsin will result in the loss of thousands of cyclic gmp molecules resulting in a decrease in the concentration of cyclic gmp rods have a constant circulating electric current in the dark at one end of the cell positive charge in the form of ions like sodium are constantly pumped out of the cell by pumping these positive charges out of the cell the cell becomes negatively charged which will show in blue like this the cell membrane is an electric insulator because ions cannot freely cross it to re-enter the cell but there are proteins called ion channels that are like tunnels allowing charge to enter the cell at the other end when these ion channels are open sodium ions can move through these to re-enter the cell why do they enter the cell recall that opposite charges attract the cell is negatively charged attracting the positive ions how does this relate to cyclic gmp cyclic gmp can bind to these channels the channels are open when cyclic gmp is bound and charge can enter the cell the channels are closed when cycle gmp is not bound to the channel blocking the entry of positive charge into the cell when cyclic gmp is plentiful it will bind to and unbind from the channel many times per second because it is only weakly attached to the channel after a photon activates rhodopsin thousands of cyclic gmp molecules are destroyed by phosphodiesterase depleting the cyclic gmp in that part of the cell this causes the nearby channels to close reducing the amount of positive charge entering the pumps at the other end of the rod keep pushing positive charge out of the cell as a result the cell becomes more negatively charged this is the last step of phototransduction the loss of thousands of cyclic gmp molecules leads to closing of a small fraction of the channels this makes the electric potential across the cell membrane more negative by about a millivolt this electric signal though small is enough to affect the cell and be passed on to other nerve cells and all of this occurs in less than a second any one of the millions of rhodopsin molecules in the rod can activate this cascade brighter lights made of more photons can activate multiple rhodopsins at the same time more activated rhodopsins will lead to more destruction of cyclic gmp and a greater electric signal from closing more channels after about a second the rhodopsin becomes inactivated along with transducin and phosphodiesterase other proteins return cyclic gmp levels to normal and the channels reopen returning the rod to a more positive membrane potential the amplification in the phototransduction cascade is immense realize that the whole process begins with a tiny change to a single molecule no atoms are added or removed and even the number of bonds in the molecule remains the same phototransduction relies on random molecular encounters between molecules sometimes the rhodopsin will activate more transducins and sometimes fewer but because rhodopsin activates many molecules one photon can be distinguished from two phototransduction is one example of a more general phenomenon all of life's chemical reactions rely on similar random collisions to what you see here quantitative thinking is essential for understanding the chemical reactions fundamental to biology i made these animations to let you see that biological processes are physical processes to which we can apply our physical and mathematical intuition also i invite you to be curious notice that rhodopsin and phosphodiesterase are confined to a plane diffusing in two dimensions instead of three how does this affect the collision rates another question increasing temperature will increase the speed of molecules how might phototransduction be different in cold-blooded animals compared to us what other questions do you have put your thoughts or questions in the comments section if there's interest i can make a video discussing these as there's some surprising biophysics and math there if you're interested in more see my previous video exploring whether the signal to noise in rods lets us see individual photons

Channel: Kerry Kim

Views: 12,846

Rating: 4.9846153 out of 5

Keywords:

Id: NjrFe7JHY1o

Length: 8min 7sec (487 seconds)

Published: Fri Aug 20 2021

Reddit Comments

[u/blender-rules-bot]

Hi, friendly bot here!

I noticed "Feedback" in your post. Are you asking for feedback on your work? If so, you can use the "Need Feedback" flair. If you are asking for help on a specific task, you can add the "Help!" flair.

This will make it easier for others to notice you're asking for tips!

---

About | Feedback