Unit 3 Cellular Energetics

Photosynthesis and Cellular Respiration

Photosynthesis

Photosynthesis

The process by which autotrophs convert light energy from the sun into chemical energy in the form of organic compounds such as carbohydrates.

Organisms are classified by what?

The way they get energy

Autotrophs

use energy from the sun or chemicals to make organic compounds 

 Heterotrophs

animals that must get energy from food 

Light reactions

Light energy (absorbed from the sun) is converted into chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH. 
Products: ATP and NADPH

Light independent reactions

Organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH.
Products: Glucose and Oxygen 

Photosynthesis chemical equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ 
6 carbon molecules + 6 water molecules = glucose + 6 oxygen molecules

Where does the absorption of light occur

Chloroplast

The inner membrane in chloroplast contains

Flattened sacs called thylakoids. 

Thylakoids stack to form

Grana

Surrounding the grana

Stroma

Although light from the sun appears white, it is actually made of various colors.

Visible spectrum

A prism

can separate colors and range from red to violet.

pigment

a compound that absorbs specific wavelengths of light, leaving only the others to be reflected back. 

Where in the chloroplast are pigments found?

membranes of thylakoids

What are the most common chlorophylls?

Chlorophyll A and Chlorophyll B.
Chlorophyll A absorbs more red light, chlorophyll B absorbs more blue light. Neither absorbs much green light (RBR)

What other pigment is found in plants?

The carotenoids that reflect orange, yellow, and brown. 

Why are plants green in spring and orange in the fall.

Since chlorophylls are more abundant, generally, they mask the other pigments. In many plants, the chlorophylls break down in the fall (because of lower temperatures and less sunlight), revealing the carotenoids. 

What are photosystems?

Hundreds of pigment molecules clustered within protein complexes, including chlorophylls and other pigments.

Energy absorbed by the pigments

are passed from pigment to pigment until it reaches a pair of chlorophyll A molecules. 

Step 1 (light reactions)

Light energy forces electrons to enter a higher energy level in the two chlorophyll A molecules of photosystem II. These electrons are said to be “excited.” They can leave the chlorophyll A molecules. 

Step 2 (light reactions)

A molecule in the thylakoid membrane called the primary electron acceptor takes the electrons from the chlorophyll A molecules. 

Step 3 (light reactions)

The primary electron acceptor gives electrons to another molecule in the thylakoid membrane, which gives them to another, and another, and so on. These molecules are called an electron transport chain. Each time electrons are passed, they lose some energy, which is used to move protons (H+) into the thylakoid.

Step 4 (light reactions)

The electrons finally go to replace the electrons that are leaving another group of pigments in photosystem I. These electrons also move from molecule to molecule in another electron transport chain (ETC).

Step 5 (light reactions)

The ETC brings electrons to the side of the thylakoid closest to the stroma, combining with NADP+ and a proton to make NADPH. 

Both photosystems would stop if

electrons were not replaced. 

oxygen-evolving complex (OEC)

An enzyme beside photosystem II that splits water molecules into electrons, protons & oxygen helps to replace the electrons that left photosystem II. 

The protons from OEC

get pushed into the thylakoid, releasing oxygen from the plant. 

The purpose of the light reactions

to push protons from the stroma to the inside of the thylakoid. This happens each time electrons pass from one molecule to the next. 

All the protons in the Thylakoid

Become highly concentrated and want to leave. 

ATP synthase

is located in the membrane and lets protons escape and uses the energy from them to put together ATP and NADPH molecules. 

The Calvin Cycle

the second set of reactions in photosynthesis that uses energy from ATP and NADPH during light reactions to produce organic molecules in the form of sugars. 

In the Calvin Cycle

a series of enzyme-assisted reactions produce a 3-carbon sugar. CO2 from the atmosphere is bonded or “fixed” into organic compounds in this process. This process is called carbon fixation. 

The Calvin Cycle occurs

in the stroma of the chloroplasts. 

RuBP

a 5-carbon molecule

Step 1 (Calvin Cycle)

CO2 diffuses into the stroma. An enzyme combines each CO2 with RuBP. The resulting 6-carbon molecule is unstable and immediately splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA). 

Step 2 (Calvin Cycle)

Each 3-PGA is converted into another 3-carbon molecule called glyceraldehyde 3-phosphate (G3P) in a two-part process: 1st, each 3-PGA receives a phosphate from an ATP. This compound receives a proton (H+) from NADPH and releases the phosphate group, producing G3P. 

Step 3 (Calvin Cycle)

 one of the G3P molecules leaves the Calvin cycle and is used to make carbohydrates. 

Step 4 (Calvin Cycle)

The other G3P molecule is converted back into RuBP by adding a phosphate from an ATP. This RuBP can be used again in the Calvin cycle.

Stomata

Pores on the underside of a leaf.

Although the Calvin cycle is the most common way plants fix carbon plants that live in hot/dry climates can

Open and close their stomata. This helps them keep water in but reduces the amount of CO2 and O2 that enters and leaves the plant. Alternative pathways help them fix this problem.

The C4 Pathway

C4 plants partially close their stomata during hot parts of the day. This prevents water from leaving and keeps most CO2 from getting in. C4 plants have special enzymes that can still fix small amounts of CO2 into 4-carbon compounds. These compounds can be sent to other parts of the plant where they are turned into CO2 to enter the Calvin cycle. 

The CAM pathway

CAM = crassulacean acid pathway and is named after the types of plants that do it (cactuses, pineapples, and certain others). These plants close their stomata during the day and open them at night. CO2 at night is fixed into several different compounds converted back to CO2 during the day to fuel the Calvin cycle. 

Some products of the Calvin cycle

are used in the light reactions and vice versa.

The other products of the Calvin cycle are used to

build molecules like amino acids, lipids, and carbs. 

Light Intensity

will cause the rate of photosynthesis to increase until all available electrons are excited. Then, the rate will stay at its max even if more light is available. 

CO2 Levels

increasing CO2 will also cause photosynthesis to increase until the rate of photosynthesis levels off. 

Temperature

Photosynthesis will increase as temperature increases, but if it gets too hot, enzymes stop working, and stomata close, preventing CO2 from entering.