The Working Cell: Energy from Sunlight

Chapter 8

What process is this?

Photosynthesis uses light energy to make food

• Photosynthesis is the opposite of cellular respiration

• Photosynthesis is the process in which plants and other producers convert the energy of sunlight into energy stored in organic molecules

• The cellular organelle in which photosynthesis takes place…

Chloroplast

• Chloroplasts contain chlorophyll, a chemical compound that give the chloroplast a green color

• In most plants leaves contain the most chloroplasts and are the major site of photosynthesis

• Leaves have tiny pores, called stomata, throughout their surface which allow CO2 in and O2 out

• Chloroplasts have an inner and outer membrane• Inside the inner membrane is a thick fluid called

stroma• Suspended in the stroma are many disk-shaped

sacs called thylakoids• A stack of thylakoids is called a grana

Photosynthesis occurs in two stages:1. The light reactions = convert the energy in

sunlight to chemical energy

2. The Calvin cycle = makes sugar from the atoms in carbon dioxide, hydrogen ions, and high-energy electrons

Overview of Photosynthesis

• Electromagnetic energy= a form of energy that exhibits wavelike behavior as it travels through space– Visible light, microwaves, X rays

•Wavelength= the shortest distance between equivalent points on a continuous wave

• Electromagnetic spectrum = range of types of electromagnetic energy

• The most important segment for life is a narrow band between 380 to 760nm…visible light

• ROY G BIV• A particle of electromagnetic radiation is called a

photon

• Pigments= chemical compounds that determine a substance’s color

• When light shines on a material that contains pigments, three things can happen:

1. Absorption

2. Transmission (goes through the material)

3. Reflection (bounces back)

• Why do leaves look green?

• Within the thylakoid membrane, chlorophyll and other molecules are arranged in clusters called photosystems– think solar panel!

• Each photosystem contains a few hundred pigment molecules, including chlorophyll a,chlorophyll b, and carotenoids– Chlorophyll a, the dominant pigment,

absorbs best in the red and blue wavelengths, and least in the green

• Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.– Chlorophyll b, with a slightly different

structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

– Carotenoids are pigments that range in color from pale yellow to deep red

• they are involved in the color changes of leaves in the fall

• they funnel the energy from other wavelengths to chlorophyll a.

Carotenoids

Photosynthesis

• Overview:Chlorophyll absorbs light

↓

Energy transferred to electrons

↓

Energy level of electrons increased (unstable)

↓

Carbohydrates, oxygen

Light Reactions

• The light reactions involve two photosystems, photosystem I and photosystem II, connected by an electron transport chain

• Photons of light, as well as water molecules, come into the thylakoid and strikes photosystem I

• The oxygen is “sliced” off of the water molecule and is released through the stomata into the atmosphere– Called photolysis

• This leaves hydrogen ions and electrons• The electrons are funneled to the electron

transport chain

• The electrons move from carrier to carrier within the electron chain

• Each time an electron moves to the next carrier energy is produced

• When the electrons exit the electron chain they move to photosystem II

• Photosystem II is called the “NADPH producing system”

• What is NADPH?

Nicotinamide Adenine Dinucleotide Phosphate It’s a carrier molecule!!

• What does it carry?Electrons!!!

• The electrons are carried by NADPH to the Calvin cycle

• Now, back to the hydrogen ions…• The ETC produces energy by moving the

electrons from carrier to carrier• This energy is used to pump hydrogen

ions across the membrane from an area of low concentration to area of high concentration

• Potential energy builds up and the hydrogen ions rush through the ATP synthase which produces what?

ATP

• The ATP is then used in the Calvin Cycle

The Calvin cycle makes sugar from carbon dioxide

• The light reactions produce oxygen, a very important product for life to exist

• But life needs more…organic molecules!

The Calvin cycle

• Carbon enters as CO2

• ATP and NADPH, from the light reactions, provide energy and electrons that are used to make the sugar

• The sugar that is produced is called G3P, a smaller molecule than glucose (three carbons instead of 6 carbons)

• G3P exits the cycle• G3P is the raw material that plants use to make

glucose and other organic molecules

Summary of Photosynthesis• Overall equation for photosynthesis

Sunlight + 6 CO2 + 6 H2O C6H1206 + 6 O2

• Light reactions – take place in thylakoid membrane– use light and water– convert light energy to the ATP and NADPH– product is oxygen

• Calvin cycle– takes place in the stroma – Uses ATP and NADPH to convert carbon dioxide to

sugar

Photosynthesis has a global impact

• On a global scale, photosynthesis is the most important process to the welfare of life on Earth

• Photosynthetic organisms make about 160 billion metric tons of organic material per year

• Earth’s atmosphere

• Carbon dioxide traps heat from the sun and prevents it from escaping back out into outer space

• The greenhouse effect, keeps the world climate warm enough for life to exist

• In recent years the amount of CO2 in the atmosphere has been rising