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Schematic representation of a Euglena cell with red eyespot (9) Schematic representation of a Chlamydomonas cell with chloroplast eyespot (4). The eyespot apparatus (or stigma) is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids.
It possesses an eyespot, a small area highly sensitive to light, [41] [42] which triggers the two flagella differently. [43] Those responses are adaptive, on a timescale matched to the rotational period of the cell body, [ 44 ] [ 45 ] [ 46 ] and allow cells to scan the environment and swim toward light. [ 47 ]
The eyespot is located near the cell equator. It is composed of a carotenoid-rich granule layer in the chloroplast which act like a light reflector. [6] The main function of the eyespot is the phototaxis, which consist of the movement (with the flagella) related to a light stimulus. [7]
The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that of amoeboid movement. [20] In low moisture conditions, or when food is scarce, Euglena forms a protective wall around itself and lies dormant as a resting cyst until environmental conditions improve.
It has forward and backwards movement, as well as changes in its direction. Additionally, E. gracilis contains a light-sensitive eyespot, or stigma, which enables it to exhibit phototaxis by moving towards light sources for photosynthesis. The cell also possesses a contractile vacuole responsible for osmoregulation, helping maintain proper ...
Middle: photokinesis involving changes in speed induced by changing light intensity. In patchy light environments, positive photokinesis results in accumulation in low light areas (and vice versa for negative photokinesis). Bottom: true phototaxis results in movement towards or away from a light source, but is not a response to a light gradient.
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Ocelli (the plural of ocellus) are a type of simple eye, or eyespot. They are photoreceptive, but very simple; differing from complex eyes with compound lenses, ocelli cannot morph the sensation of light into a complete image, and are utilized to sense movement and the absence and presence of light.