Photosynthesis

- Chapter 8

8-1 Energy and Life

• Energy is the ability to do work

• Living things depend on energy to maintain homeostasis

• Without the ability to obtain and use energy, life would cease to exist

• Where does this energy come from?

Chemical Energy and ATP

• Energy comes in many forms

Exs. – light, heat and electricity

• ATP and ADP  – Cell activities are powered by chemical

fuels– One of the principal chemical compounds

that living things use to store and release energy is ATP (adenosine triphosphate)

• An ATP molecule consists of a nitrogen-containing compound called adenine, a 5-carbon sugar called ribose, and three phosphate groups

pg. 202

Fig. 8-2 ATP is used by all types of cells as their basic energy source. For example, the energy needed by the cells of a soccer player comes from ATP.

• ADP (adenosine diphosphate) has a structure that is similar to ATP

• There is one important difference: ADP has two phosphate groups instead of three

• This is the key to the way in which cells store energy

• A cell can store small amounts of energy by adding a phosphate group to ADP molecules, producing ATP molecules

• ATP is like a fully charged battery, ready to power the machinery of the cell.

pg. 203

Releasing Energy From ATP

• Energy stored in ATP is released when ATP is converted into ADP and a phosphate group

• Cells can add and subtract a third phosphate group giving it a way of storing and releasing energy as needed.

iText Video – ATP Formation (pg. 203)

• Most cells have only a small amount of ATP, enough to last for only a few seconds of activity

• ATP is a great molecule for transferring energy.

• It is not good for long term energy storage

The ATP Cycle (ATP Formation)

The ATP Cycle

Using Biochemical Energy

• ATP carries just enough energy to power a variety of cellular activities– Exs.:

• Active Transport (Protein Pumps)• Movement (cilia, flagella, muscles)• Light (Fireflies)

• The characteristics of ATP make it an exceptionally useful molecule that is used by all types of cells as their basic energy source

Autotrophs and Heterotrophs

• Originally, nearly all energy in food comes from the Sun.

• The energy that living things need comes from food

• Autotrophs are organisms such as plants, which make their own food

• Heterotrophs obtain energy from the foods they consume – Ex.: Impalas, leopards, & mushrooms

Fig. 8-1 Autotrophs vs. Heterotrophs   Autotrophs use light energy from the sun to produce food. These impalas get their energy by eating grass. A leopard, in contrast, gets its energy by eating impalas and other animals. Impalas and leopards are both heterotrophs.

pg. 201

8–2 Photosynthesis: An Overview 

• Photosynthesis is a process in which plants use the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy carbohydrates (sugars and starches) that can be used as food.

An Overview of Photosynthesis

An Overview of Photosynthesis

• The overall equation for photosynthesis can be shown as follows:

6CO2 + 6H2O sunlight C6H12O6 + 6O2

chlorophyll

carbon dioxide + water sugar + oxygen

Investigating Photosynthesis

• The experiments of many scientists have contributed to the modern understanding of the process of photosynthesis

Van Helmont’s Experiment (1600s) • Experiment to find out if plants grew by taking

material out of the soil• Van Helmont concluded that most of the

mass the plant gained had come from water, because that was the only thing that he had added to the pot.

• Priestley’s Experiment  (100+ yrs later)– Took a candle, placed a glass jar over it,

and watched as the flame gradually died out

– Placed a live sprig of mint under the jar – Discovered that plants release oxygen

• Jan Ingenhousz – Showed that the effect observed by Priestley

occurred only when the plant was exposed to light

– Light is necessary for plants to produce oxygen

• The experiments performed by van Helmont, Priestley, Ingenhousz, and other scientists reveal that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen

Chlorophyll and Chloroplasts

• Energy from the Sun travels to Earth as light.

• Our eyes perceive this as “white light” which is actually mixture of different wavelengths.

• We see these wavelengths as:

Red, Orange, Yellow, Green, Blue, Indigo and Violet.

• In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll, a molecule in chloroplasts

• Plants gather the sun’s energy with light-absorbing molecules called pigments

• The plants’ principal pigment is chlorophyll

• There are two main types of chlorophyll: – chlorophyll a – chlorophyll b

• Chlorophyll absorbs the blue and red regions of the visible spectrum

• Chlorophyll does not absorb light well in the green region of the spectrum

• This is why plants are green

• Plants also contain red and orange pigments that absorb light in other regions of the spectrum– Ex.: carotene

Fig. 8-5 Photosynthesis requires light and chlorophyll, which absorbs light energy. In the graph, notice how chlorophyll a absorbs light in the violet and red regions of the visible spectrum, while chlorophyll b absorbs light in the

blue and red regions of the visible spectrum.

pg. 207

Chloroplasts

• iText Video, Pg. 208

• Inside a Chloroplast– Photosynthesis takes place inside

chloroplasts – Chloroplasts contain saclike

photosynthetic membranes called thylakoids

• Thylakoids:– Are arranged in stacks known as grana

(singular: granum)

– The fluid portion of the chloroplast outside the thylakoids is known as the stroma.

– Contain clusters of chlorophyll and other pigments and photosystems (proteins that are able to capture the energy of sunlight.

High-Energy Electrons

• Sunlight excites electrons in chlorophyll and the electrons gain a great deal of energy

• These high-energy electrons require a special carrier

• One of these carrier molecules is a compound known as NADP+

• The conversion of NADP+ into NADPH is one way in which some of the energy of sunlight can be trapped in chemical form.

Fig. 8-4 Photosynthesis is a series of reactions that uses energy from the sun to convert water and carbon dioxide into sugars and oxygen. Photosynthesis takes place in a plant organelle called the chloroplast.

pg. 206

• The photosynthesis reaction has two stages: – The Light-Dependent reactions

• take place within the thylakoid membranes

– The Light-Independent reactions (Calvin Cycle)

• The Calvin Cycle takes place in the stroma, the region outside the thylakoid membranes.

The Process of Photosynthesis

Light-Dependent Reactions

Light-Independent Reactions

pg. 209

iText Video - Pg. 209

Light-Dependent Reactions(ATP and NADPH)

• Require light

• Why plants need light to grow

• Use energy from light to produce oxygen gas, ATP and NADPH

iText Video – Light-Dependent Reactions (Part 2)

Pg. 211

8–3 The Process of Photosynthesis 

Light-Independent Reactions(Sugars)

• The light-independent (dark) reactions use ATP and NADPH from the light-dependent reactions to produce high-energy sugars

• These reactions are also called the Calvin Cycle

• The Calvin Cycle does not require light

iText Video – Calvin Cycle Pg. 212

pg. 212

Factors Affecting Photosynthesis

• Many factors affect the rate at which photosynthesis occurs

• Temperature:– Temperatures above or below 0°C and

35°C may slow down the rate of photosynthesis

– At very low temperatures, photosynthesis may stop entirely

– Plants at these temperatures can carry out photosynthesis only on sunny days

• Intensity of Light – Affects the rate at which photosynthesis

occurs.– Increasing light intensity increases the

rate of photosynthesis– At a certain level, the plant reaches its

maximum rate of photosynthesis– This level varies from plant to plant.

• Water:– A shortage of water can slow or even

stop photosynthesis – Plants that live in dry conditions (desert

plants and conifers) have a waxy coating on their leaves that reduces water loss

Photosynthesis Under Extreme Conditions

• Plants have small openings (stomata) in their leaves that admit CO2 for photosynthesis

• To prevent the plant from drying out, these openings must close to conserve water.

• This may slow down or stop the process of photosynthesis

C4 and CAM Photosynthesis

• Some plants have adapted to extremely bright, hot conditions

• There are two major groups of these specialized plants:– C4 plants– CAM plants

• These processes minimize water loss while still allowing photosynthesis in intense sunlight

• C4 plants have an added step during the carbon-fixing stage to preserve moisture in hotter climates.

C4 plants capture CO2 so that plants can keep working under intense light and high temperatures.

• C4 Plants:– Corn– Soy Beans– Sugar Cane – Crabgrass

• CAM plants take in CO2 only at night, trapping carbon in the leaves.

• During the day, when the openings in the leaves are tightly closed, the CO2 is released allowing photosynthesis to take place.

• CAM Plants:– Pineapple– Cactus