Lecture 9 (photosynthesis)

Question 1

Life on earth depends on…

Answer 1

Energy from the sun. The Earth is sustained by taking light energy from the sun and transferring it into chemical energy via photosynthesis.

Question 2

Photosynthesis

Answer 2

The conversion of light energy to chemical energy that is stored in sugars or other organic compounds. This occurs in plants, algae and certain prokaryotes. Photosynthesis forms an energy rich carbohydrate.

Question 3

Under what conditions do the reactions of photosynthesis occur in plants?

Answer 3

The light reactions occur only in the light, and the Calvin cycle occurs both in the light and dark

Question 4

Photosynthesis equation

Answer 4

6CO2 + 6H2O —> C6H12O6 + 6O2

(Plant, light on the arrow)

Question 5

Chloroplast

Answer 5

An organelle found in plants and photosynthetic protists (eukaryotic organism that is not an animal, plant or fungi). Absorbing sunlight for use in photosynthesis. \
Chloroplasts are the site of photosynthesis

Question 6

Chloroplast structure overview

Answer 6

Has a big surface area in order to catch light energy

A chloroplast has 3 membranes (inner, outer and thylakoid)

A chloroplast has 3 compartments ( inter membrane space, stroma, thylakoid space)

Granum = A granum is a coin-shaped stack of thylakoids, which are the membrane-like structures found inside the chloroplasts of plant cells.

Thylakoid = A flattened, membranous sac inside the chloroplast comprising the thylakoid membrane and thylakoid space. Their membranes contain molecular “machinery” used to convert light energy to chemical energy.

Stroma = The dense fluid within the chloroplast containing ribosomes and DNA. Involved in the synthesis of organic molecules from carbon dioxide and water.

Thylakoid space is the space inside of the thylakoid

Question 7

Where do light reactions and carbon fixation occur?

Answer 7

Light reactions take place on the thylakoid membrane. Carbon fixation occurs in the stroma.

Question 8

Light reactions - photosynthetic electron transport chain

Answer 8

Capture light energy and convert it to chemical energy (a photon is a little packet of energy that is absorbed bu a pigment molecule and then that energy is going to be transferred)

Using pigment (molecules that absorb light energy/photons) and protein complexes embedded in the thylakoid membrane

Photosystems are protein complexes that contain the chlorophyll (pigment molecule

Chlorophyll absorbs light energy

Light energy absorbed by chlorophyll produces high energy electrons

High energy electrons travel through the photosynthetic electron transport chain

In these steps, light energy is converted into chemical energy

Question 9

Photosynthetic electron transport chain summary

Answer 9

Order - photosystem II, cytochrome complex, photosystem I, ATP synthase

Photosystem II - Contains chlorophyll, light is absorbed and then the chlorophyll molecule gives off an electron

Cytochrome complex - This doesn’t absorb light, it builds up a proton gradient. As electrons move, protons are pumped inside building up a proton concentration gradient. This proton gradient is used to go through ATP synthase to make energy (ATP)

Photosystem I - Integral membrane protein complex that uses light energy to catalyse the transfer of electrons across the thylakoid membrane. The electrons that are transferred by Photosystem I are used to produce the high energy carrier NADPH.

ATP synthase - Protons move through and this generates ATP

Electrons are replaced by water splitting at photosystem II which makes oxygen as a by product

Question 10

Light reaction summary

Answer 10

In the light reactions, water has been spilt and oxygen is produced, also ATP and NADPH produced

Question 11

The Calvin cycle or carbon fixation summary

Answer 11

ATP and NADPH produced in the light reactions are used to fix CO2 and produce carbohydrate. Plant cells break down glucose from photosynthesis in the mitochondria using cellular respiration

Question 12

The Calvin cycle

Answer 12

The Calvin Cycle fixes CO2 and produces 3-carbon sugars using ATP and NADPH produced in the light reactions of photosynthesis. The 3-carbon sugars can subsequently be converted to glucose and other carbohydrates.

The Calvin cycle is a continuous cycle
1- Fixation : 5 carbon molecule + CO2 —> 2 X 3 carbon molecule

2 - Reduction: 3 carbon molecule converted into a different three carbon molecule (which has more energy than the previous three carbon molecule)

3- Regeneration: converting the 3 carbon molecule back into the five carbon molecule
Three turns of the cycle generates 3 carbon sugar (then glucose and other compounds)

The ATP and NADPH produced in the light reactions are only used in the Calvin cycle (not an output of photosynthesis)

Question 13

Inputs for photosynthesis

Answer 13

light, water and carbon dioxide

Question 14

Outputs for photosynthesis

Answer 14

glucose and oxygen

Question 15

Glucose as an energy supply in plants and animals

Answer 15

Both plants and animals breakdown glucose into cellular respiration to generate ATP

Animals must have an external source (eating etc.) of glucose

Plants generate glucose during photosynthesis and then break this down during reparation

Almost all glucose on earth has directly or indirectly come from photosynthesis

Question 16

ATP synthase

Answer 16

ATP is generated in both reparation and photosynthesis

ATP synthase is responsible for ATP generation in both processes (cellular respiration and photosynthesis)

This requires a proton gradient across a membrane in both the chloroplast and mitochondrion. In both cases this occurs through the movement of electrons through an electron transport chain enabling pumping of protons across a membrane. Once the proton gradient is established, the protons can move from a high to low concentration through ATP synthase and generate ATP

Question 17

ATP synthase in plants vs animals

Answer 17

In both, protons are moving from high to low concentration

In mitochondrion, ATP is made in the matrix and in the chloroplast the ATP is being made in the storm

In mitochondrion, the protons are in the inter-membrane space and in chloroplasts the protons are in the thylakoid space ( the double membrane structures of both allow for the build up of protons)

For both, ATP synthase is in a membrane however different membranes

Question 18

The origin of chloroplasts and mitochondria

Answer 18

Mitochondria and chloroplasts both contain DNA, ribosomes and are able to make some proteins (have a reduced genome now). This is how it is known that they used to be seperate organisms

Both have an outer and inner membrane (chloroplasts have a third membrane system too)

Endosymbiosis theory

Question 19

Photosynthesis summary

Answer 19

Photosynthesis requires carbon dioxide, which diffuses into the lead through small pores and then enters the cells. Inside the cell, carbon dioxide diffuses into the chloroplasts, where photosynthesis takes place. Chloroplasts use energy from light to transform carbon dioxide and water into sugar and oxygen. Zooming into a chloroplasts we see these flattened, membranous sacs called thylakoids. Here, light energy is converted to chemical energy in the first phase of photosynthesis - the light reactions. On the thylakoids membrane are two photosystems, large complexes of proteins and chlorophyll that capture light energy. An electron transport chain connects the two photosystems. Photosystem two absorbs light energy, exciting electrons that enter the electron transport chain. Electrons are replaced by electrons stripped from water, creating oxygen as a byproduct. The energised electrons flow down the electron transport chain, releasing energy that is used to pump hydrogen ions into the thylakoid. In photosystem one, light energy excites electrons, and this time the electrons are captured by an electron carrier molecule called NADPH. The high concentration of hydrogen ions inside the thylakoid membrane powers ATP synthase which produces ATPs. The light reactions in the thylakoid have produced two energy products - ATP and NADPH - these will now power the production of sugar in the Calvin cycle. The Calvin cycle take place outside the thylakoids in the storm, the thick fluid of the chloroplast. At the beginning of the cycle, carbon dioxide molecules combine with molecules called RUBP. The resulting molecules fo through a series of reactions powered by ATP and NADPH from the light reactions. Sugar molecules known as G3Ps are produced. Most of the G3Ps are rearranged back into RUBPs that will begin the Calvin cycle again. But the important product of photosynthesis is the remaining G3P sugar. Some G3Ps are used to build glucose, which can combine into starch or cellulose. Still other g3Ps form sucrose and some of the sugar is broken down by cellular respiration using oxygen in the plants own mitocondiria, generating ATPS that can power other work of the plant.