BIOLOGY Light

Light main source of food for all living organisms Energy for all life on Earth ultimately comes from photosynthesis. 6CO H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants 2

Photosynthesis Overview Photosynthesis is divided into: light-dependent reactions -capture energy from sunlight -make ATP and reduce NADP + to NADPH carbon fixation reactions -use ATP and NADPH to synthesize organic molecules from CO 2 3

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Light -Photon photon: a particle of light -acts as a discrete bundle of energy -energy content of a photon is inversely proportional to the wavelength of the light photoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons 6

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Pigments Pigments: molecules that absorb visible light Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. 8

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Light-Dependent Reactions Light-dependent reactions occur in 4 stages: 1. primary photoevent – a photon of light is captured by a pigment molecule 2. charge separation – energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3. electron transport – electrons move through carriers to reduce NADP + 4. chemiosmosis – produces ATP Photophosphorylation 11

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Carbon Fixation Reactions Calvin cycle -biochemical pathway that allows for carbon fixation -occurs in the stroma -uses ATP and NADPH as energy sources -incorporates CO 2 into organic molecules 14

Sensory Systems in Plants

Responses to Light Pigments other than those used in photosynthesis can detect light and mediate the plant’s response to it Photomorphogenesis refers to non-directional, light- triggered development Phototropisms are directional growth responses to light Both compensate for plants’ inability to move 16

Responses to Light In Arabidopsis, five forms of phytochromes have been characterized: PHYA to PHYE -Involved in several plant growth responses 1. Seed germination -Inhibited by far-red light and stimulated by red light in many plants 17

Responses to Light 2. Shoot elongation -Etiolation occurs when shoot internodes elongate because red light and active P fr are not available 3. Detection of plant spacing -Crowded plants receive far-red light bounced from neighboring plants -This increases plant height in competition for sunlight 18

Flower Production Four genetically regulated pathways to flowering have been identified 1. The light-dependent pathway 2. The temperature-dependent pathway 3. The gibberellin-dependent pathway 4. The autonomous pathway Plants can rely primarily on one pathway, but all four pathways can be present 19

Light-Dependent Pathway Also termed the photoperiodic pathway -Sensitive to the amount of darkness a plant receives in each 24-hour period -Short-day plants flower when daylight becomes shorter than a critical length -Long-day plants flower when daylight becomes longer -Day-neutral plants flower when mature regardless of day length 20

21 Long-Day Plants Short-Day Plants 24 hours Night Day Early summer Late fall 24 hours Night Day a.a. b. Clover Short length of dark required for Cocklebur Long length of dark required for bloom Night Day Flash of light Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Light-Dependent Pathway In obligate long- or short-day plants there is a sharp distinction between short and long nights, respectively In facultative long- or short-day plants, the photoperiodic requirement is not absolute -Flowering occurs more rapidly or slowly depending on the length of day 22

Light-Dependent Pathway Using light as cue allows plants to flower when environmental conditions are favorable 23 -Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidays

Photoreceptors And Vision 1. A diversity of photoreceptors has evolved among invertebrates 2. Vertebrates have single-lens eyes 3. The light-absorbing pigment rhodopsin triggers a signal-transduction pathway 4. The retina assists the cerebral cortex in processing visual information

Invertebrates photoreceptors Eye cups are among the simplest photoreceptors Detect light intensity and direction — no image formation. The movement of a planarian is integrated with photoreception. Fig. 49.7

Image-forming eyes. Compound eyes of insects and crustaceans. Each eye consists of ommatidia, each with its own light-focusing lens. This type of eye is very good at detecting movement. Fig. 49.8

Single-lens eyes of invertebrates such as jellies, polychaetes, spiders, and mollusks. The eye of an octopus works much like a camera and is similar to the vertebrate eye.

Vertebrates have single-lens eyes Is structurally analogous to the invertebrate single- lens eye. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 49.9

The lens and ciliary body divide the eye into two cavities. The anterior cavity is filled with aqueous humor produced by the ciliary body. Glaucoma results when the duct that drain aqueous humor are blocked. The posterior cavity is filled with vitreous humor. The lens, the aqueous humor, and the vitreous humor all play a role in focusing light onto the retina.

Accommodation is the focusing of light in the retina. In squid, octopuses, and many fish this is accomplished by moving the lens forward and backward.

In mammals accommodation is accomplished by changing the shape of the lens. The lens is flattened for focusing on distant objects. The lens is rounded for focusing on near objects. Fig

Photoreceptors of the retina. About 125 million rod cells. Rod cells are light sensitive but do not distinguish colors. About 6 million cone cells. Not as light sensitive as rods but provide color vision. Most highly concentrated on the fovea – an area of the retina that lacks rods.

Color reception is more complex than the rhodopsin mechanism. There are three subclasses of cone cells each with its own type of photopsin. Color perception is based on the brain’s analysis of the relative responses of each type of cone. In humans, colorblindness is due to a deficiency, or absence, of one or more photopsins. Inherited as an X-linked trait.

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