Chapter 8: Photosynthesis

• Organisms- classified into 2 groups, according to their method of obtaining food– 1. Autotrophs- can make their own

food from CO2 and an energy source such as SUNLIGHT (ex. Plants, algae, some bacteria)

• Foods made by autotrophs are mainly carbohydrates(ex. Glucose, a 6 carbon sugar)

• Autotrophs are known as PRODUCERS– Why? Because they can PRODUCE their

own food as well as provide food for other organisms

Heterotrophs• Organisms that

cannot make their own food (includes animals, fungi and many unicellular organisms)– Are known as

CONSUMERS• Why? Because

they need to consume other organisms to obtain food

ATP Molecule

An ATP molecule releases chemical energy whenever a bond holding a phosphate group is broken. Energy released can be used by a cell to do work.

Adenosine triphosphate (ATP)Adenine

Sugar

3 phosphate groups

USAGE of ATP

• 1. Active Transport

• 2. Powers movement within the cell

• ATP vs. Glucose?

8-2 Photosynthesis: An Overview

• Jan Van Helmont: trees gain most of their mass from water

• Joseph Priestley: finds out that the plant releases oxygen and keeps candle burning in his experiment

• Jan Ingenhousz: concludes that plants nee sunlight to produce oxygen

• Autotrophs that perform photosynthesis contain chemicals called pigments

• Pigment- a molecule that absorbs certain wavelengths of light and reflects others– The reflected wavelengths determine what

color you perceive an object to be– EXAMPLE: an apple

If the pigments of the apple absorb all wavelengths of light EXCEPT RED, the red wavelength is reflected to your eye---therefore you see a RED apple.

Autotrophs contain certain pigments essential for photosynthesis

Chlorophyll (a pigment)

• Absorbs violet, blue and red light- the wavelengths that provide energy for photosynthesis

• DOES NOT absorb green light, which is why most plants look green– Chlorophyll A- is the primary pigment for

photosynthesis• Chlorophyll is located in specialized organelles called

chloroplasts

8-3 Chloroplast• 1. Thylakoid- contain

light collecting photosystems with chlorophyll inside

• 2. Grana- stacks of thylakoids, disc shaped sacs of membrane

• 3. Stroma- gel-like material that lies between grana in the chloroplast

Photosynthesis-Conversion of sunlight to usable energy

Occurs in 2 steps (stages)

1. Light- Dependent reactions

2. The Calvin Cycle

Light- Dependent Reactions

• 1. light is absorbed in the grana of chloroplast– REACTANTS: WATER and SUNLIGHT– PRODUCTS: ATP, NADPH and then O2

ATP and NADPH are used to power the CALVIN CYCLE

Calvin Cycle• Requires input of

CO2 to produce sugars

• Uses ATP and NADPH from the light dependent reactions to produce the sugars

The Light-Dependent Reactions: Generating ATP

and NADPH– Thylakoids contain clusters of chlorophyll

and proteins known as photosystems.– Photosystems absorb sunlight and

generate high-energy electrons that are then passed to a series of electron carriers embedded in the thylakoid membrane.

Photosystem II– Light energy is absorbed by electrons in the pigments within photosystem II, increasing the electrons’ energy level.

– The high-energy electrons are passed to the electron transport chain, a series of electron carriers that shuttle high-energy electrons during ATP-generating reactions.

Photosystem I– Because some energy has been used to pump H+ ions across the thylakoid membrane, electrons do not contain as much energy as they used to when they reach photosystem I.

– Pigments in photosystem I use energy from light to reenergize the electrons.

Photosystem I

– At the end of a short second electron transport chain, NADP+ molecules in the stroma pick up the high-energy electrons and H+ ions at the outer surface of the thylakoid membrane to become NADPH.

Hydrogen Ion Movement and ATP Formation

– Powered by the gradient, H+ ions pass through ATP synthase and force it to rotate.

– As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP.

Factors Affecting Photosynthesis

– What factors affect photosynthesis?

– Among the most important factors that affect photosynthesis are

– temperature, light intensity, and the availability of water.

Temperature, Light, and Water

– The reactions of photosynthesis are made possible by enzymes that function best between 0°C and 35°C.

– Temperatures above or below this range may affect those enzymes, slowing down the rate of photosynthesis or stopping it entirely.

Temperature, Light, and Water

– High light intensity increases the rate of photosynthesis.

– After the light intensity reaches a certain level, however, the plant reaches its maximum rate of photosynthesis, as is seen in the graph.

Temperature, Light, and Water

– Because water is one of the raw materials in photosynthesis, a shortage of water can slow or even stop photosynthesis.

– Water loss can also damage plant tissues.

– Plants that live in dry conditions often have waxy coatings on their leaves to reduce water loss. They may also have biochemical adaptations that make photosynthesis more efficient under dry conditions.

Photosynthesis Under Extreme Conditions

– In order to conserve water, most plants under bright, hot conditions close the small openings in their leaves that normally admit carbon dioxide.

– This causes carbon dioxide within the leaves to fall to very low levels, slowing down or even stopping photosynthesis.

– C4 and CAM plants have biochemical adaptations that minimize water loss while still allowing photosynthesis to take place in intense sunlight.

C4 Photosynthesis

– C4 plants have a specialized chemical pathway that allows them to capture even very low levels of carbon dioxide and pass it to the Calvin cycle.

– The name “C4 plant” comes from the fact that the first compound formed in this pathway contains 4 carbon atoms.

– The C4 pathway requires extra energy in the form of ATP to function.

– C4 organisms include crop plants like corn, sugar cane, and sorghum.