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6 Jan 2019

Explain the reason for each of the following:

a) If you are trying to see a faint star in the night sky, it is better to look slightly to one side of it rather than straight at it. (think of the cells you are using)

b) Why is it easier to study an object in detail if you look straight at it? (think of the cells)

c) Why does the eye pupil dilate (i.e. widen) in dim light?

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Reid WolffLv2

9 Jan 2019

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Exercise #5: The Nervous system and Perception

Task #1: Color vision

Introduction

In the human retina, there are two kinds of light-sensitive cells: rods and cones. These cells are located on the outside of the retina, away from the vitreous humor and against the choroid layer, and are covered by a later of bipolar and ganglion cells as well as neurons. Light enters the posterior chamber through the lens and must pass through several cell layers before it is absorbed by the rods and cones.

Cones are involved in color vision at daylight intensities. Generator potentials in a cone excite one neuron, giving high-level resolution vision. Cones will not cause neural firing at low light intensities and are, therefore, nonfunctional at night.

Rods provoke black-and-white vision at low light intensities. A bipolar cell associates with several rods, so that generator potentials sum in their effect. This assures neuronal firing at low light intensities and provides a highly sensitive vision but with less resolution.

Receptor convergence. A ganglion cell, which receives input from a number of rods, spread out over the retina, cannot pinpoint the source of a light stimulus as well as a ganglion cell, which is stimulated by a single photoreceptor. In the case of a ganglion cell receiving the added input of several rods, light hitting any one of the rods can cause a neural spike in the ganglion cell, but the brain has no way to know which rod was excited. It is therefore harder to pinpoint the location of light stimuli on the retina when the stimulus is detected by rods (as compared to cones). The result is that our ability to see detail, or our resolving power, is considerably lower under dim light conditions than under bright light conditions.

Dark adaptation. If you go from a lighted area to a very dark room, you will not be able to see. After about ten minutes, however, you begin to be able to see again. This occurs because the rhodopsin pigment, which gets broken down when it absorbs light, is continually re-synthesized in the rods. When there is very little light, very little rhodopsin gets broken down and the amount of rhodopsin gradually builds up, making the rod much more sensitive to dim light.

When light strikes a receptor cell, it triggers a photo-chemical reaction that leads to neuronal firing. If a rod or cone receives high-intensity light, the visual pigments will be temporarily bleached and will not absorb any more light. Within a matter of a few seconds, enzyme systems in the cells will restore the visual pigment to its light-absorbing form. This can be easily demonstrated. If, after viewing a bright light, the eye is quickly shut or turned to a dark wall, the bright image will persist as a positive afterimage because the high generator potential causes the continued firing of neurons. If the eye is cast on a lighter background after a few seconds, a negative afterimage, or a dark image of the object will appear. This is because the still bleached cells are not receptive to the light coming from the light background.

According to the Young-Helmholtz theory of color vision, there are three types of cone cells, which respond respectively to red, green, and blue light.

All other colors are perceived as the brain interprets impulses coming from a mix of these receptors. If an object of one color is viewed for a long period of time at high light intensity, the cones for that color will become bleached. The afterimage of the object will be seen in the complementary color.

The Exercise 6 PowerPoint will demonstrate afterimages to you. Use the slide show option to project the square of red light on your entire screen in a dark room. After staring intensely at the square for 30 seconds or so, without shifting your gaze, click enter to change the slide to a soft white. Continue staring at the screen as the change is made. What do you see? What was the color of the afterimage?

Repeat this experiment using a blue square. Before doing the experiment, hypothesize what color the afterimage will be according to the Young-Helmholtz theory.

Hypothesis to be tested: (2 pts)

What did you see when the experiment was conducted? (2 pts)

Was you hypotheses accepted or rejected? (1 pt)

Explain the colors seen in these afterimages using the Young-Helmholtz theory. (5 pts)

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The Eye Exam It was a sunny day in 2005, and time for Ericâs annual eye exam. Dr. Tim flipped through his charts. âYou know, we really should measure your visual fields. Itâs been at least ten years since you had that done!â âI hate them!â Eric grumbled. âI get disoriented and, anyhow, we already know I have a large blind spot.â He raised his hand, squinting through one eye. âHere. I canât see my finger here.â âI know.â Dr. Tim was sympathetic. âBut werenât you just telling me you havenât seen stars for a while? Itâs important. You had that tumor on your pituitary gland. Your optic nerves cross in that area, right in front of the pituitary, so if anything is happening again weâd probably notice it first by changes in your eyesight. It really is important!â An hour later, Eric was seated in front of a large white screen, staring intently at a light in the middle. âKeep looking at that light, so your eyes donât move. If you see another light anywhere else, push this button.â Eric began the test by focusing on the light he saw in the center. The machine then successively displayed points of light of varying intensity in different places around the entire visual field. When Eric saw the peripheral light, he pushed a button. He did his best to detect the flashes, but realized he was probably missing some. The machine mapped out where he could see and where he could not. Dr. Tim looked at the printout. His face grew serious. âEric, we have a problem. You have limited peripheral vision, and itâs bad enough that I donât think you should be driving! This looks to me like something called retinitis pigmentosa. We should refer you to a specialist.â He pulled out more papers. âHere, look at this. Here are your visual fields. The numbers show how well you can see in that area of your eye, and in the graph on the right the darker the box is the less you can see. Now here are a normal personâs visual fields. Do you see the difference?â

1. How do your eyes work? What is the difference between focal vision and peripheral vision?

2. What is the anatomy around the pituitary gland? Why are the optic nerves likely to be affected by pituitary tumors?

3. Eric has retinitis pigmentosa In this disease pigments lining your eye gradually break down, so you canât sense the light that hits your eye. Why do you need the pigments in order to see the images and light?

This experiment is about Affinity Purification of Green Fluorescent Protein (GFP)

please answer the below questions

1. What is a bacterial colony?

2. Why did you pick one green colony and one white colony from your agar plate(s)? Why do you think you picked one of each color? What could this prove?

3. How are these items helpful in this cloning experiment? a. ultraviolet (UV) light - b. incubator - c. shaking incubator

4. Explain how placing cloned cells in nutrient broth to multiply relates to your overall goal of purifying the fluorescent protein.

5. You have used a bacterium to propagate a gene that produces a green fluorescent protein. Identify the function of these items you need in this procedure. a. Centrifuge â b. Lysozyme â c. Freezer â

6. Can you explain why both liquid cultures fluoresce green?

7. Why did you discard the supernatant in this part of the protein purification procedure?

8. Can you explain why the bacterial cellsâ outer membrane ruptures when the cells are frozen. What happens to an unopened soft drink when it freezes?

9. What was the purpose of rupturing or lysing the bacteria?

10. Briefly describe hydrophobic interaction chromatography and identify its purpose in this lab.

This lab is about Crystal Growth of Hen Egg White Lysozyme so please answer the following question

1. What condition yielded the "best" crystals of lysozyme?

2. What changes did your group try to make, and what effect did they have on the crystals?

3. How many of your conditions yielded crystals? How many did the entire class get? Give the percentage (number of conditions that gave crystals/number of different conditions that were set up) for your attempts as well as for the entire class. What does this tell you about protein crystallization?

4. What methods can you think of to determine if the crystals are lysozyme and not salt (many salt crystals are colorless as well)? (Hint: think of protein properties and ways to assay and look at proteins).

5. Many protein crystals are formed at or near the pI of the protein being studied. Can you rationalize any reason why this observation might be made (in other words, why is the pI so good at helping form protein crystals).