Biology Light Light main source of food for all living organisms - PowerPoint Presentation

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Biology

Light Slide2

Light main source of food for all living organisms

Energy for all life on Earth ultimately comes from photosynthesis.

6CO

2 + 12H2O C6H12O6 + 6H2O + 6O2 Oxygenic photosynthesis is carried out by:cyanobacteria, 7 groups of algae, all land plants

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Photosynthesis Overview

Photosynthesis is divided into:

light-dependent reactions

-capture energy from sunlight -make ATP and reduce NADP+ to NADPHcarbon fixation reactions -use ATP and NADPH to synthesize organic molecules from CO23Slide4

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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 lightphotoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons6Slide7

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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.8Slide9

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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 molecule2. charge separation – energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule3. electron transport – electrons move through carriers to reduce NADP+4. chemiosmosis – produces ATP Photophosphorylation

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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 CO2 into organic molecules14Slide15

Sensory Systems in PlantsSlide16

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 developmentPhototropisms are directional growth responses to lightBoth compensate for plants’ inability to move16Slide17

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 plants17Slide18

Responses to Light

2. Shoot elongation

-Etiolation occurs when shoot internodes elongate because red light and active Pfr 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 sunlight18Slide19

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 pathwayPlants can rely primarily on one pathway, but all four pathways can be present19Slide20

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 length20Slide21

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Long-Day Plants

Short-Day Plants

24

hours

Night

Day

Early summer

Late fall

24

hours

Night

Day

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.Slide22

Light-Dependent Pathway

In

obligate long-

or short-day plants there is a sharp distinction between short and long nights, respectivelyIn facultative long- or short-day plants, the photoperiodic requirement is not absolute -Flowering occurs more rapidly or slowly depending on the length of day 22Slide23

Light-Dependent Pathway

Using light as cue allows plants to flower when environmental conditions are favorable

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-Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidaysSlide24

Photoreceptors

And Vision

1. A diversity of photoreceptors has evolved among invertebrates

2. Vertebrates have single-lens eyes3. The light-absorbing pigment rhodopsin triggers a signal-transduction pathway4. The retina assists the cerebral cortex in processing visual informationSlide25

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. 49.7Slide26

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. 49.8Slide27

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.Slide28

Vertebrates

have single-lens eyes

Is structurally analogous to the invertebrate single-lens eye.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 49.9Slide29

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.Slide30

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.Slide31

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. 49.10Slide32

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.Slide33

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.Slide34

Fig. 49.15