Many cells rely on chemotaxis -- detecting differences in concentrations of chemicals on one side compared to the other -- in order to determine which way to move. Our neutrophils (white blood cells) use this to find and destroy invading bacteria as a part of our immune system. The video at the link on Blackboard (also below) shows chemotaxis of amoeboid cells. In other words, it shows cells migrating toward a region with a high concentration of a chemical, in this case cAMP. Cells start in a large reservoir on top (outside of the field of view) and migrate toward a large reservoir filled with a high concentration of a chemical that attracts the migrating cells (chemoattractant) which is below the field of view. The point of view is looking down on a horizontal surface so the cells are not "falling" but moving on their own to find the chemoattractant.

the area between the graph and the t axis

scattering of light by tiny particles in air to produce the colors we see on sky, is dependent on the following factors:

(i) wavelength of light

(ii) size of the scattering particle  

(iii) distance traveled by light through the atmosphere.

according to rayleigh's law of scattering, the amount of scattering is inversely proportional to the fourth power of its wavelength. shorter the wavelength, greater is its scattering. hence violet/blue light is scattered more than red.

sun's light is a composite light ( visible light) with wavelengths in the range 400 nm (violet) to 700 nm (red). as sunlight passes through our atmosphere, the molecules of air scatter light according to their wavelengths. during sunrise and sunset, the sun's position is below the horizon. hence light from the sun travels a greater distance in the atmosphere when compared to its position a noon.

as sunlight enters the earth's atmosphere, blue light is scattered away and at the end of its journey in the sky, only red and orange wavelengths remain in the beam.

hence at sunrise and sunset, the sky appears reddish orange.

a 2,000 calorie based diet.