Biology: Section 6

What is Photosynthesis

Process that converts light energy to chemical energy, which is then stored in sugars or other organic compounds

What are autotrophs/producers?

‘self-feeders’

Organisms that produce their own organic molecules from CO2

What are Photoautotrophs?

A specific type of autotroph that uses light energy to perform photosynthesis. Plants are classic examples of photoautotrophs.

What are Heterotrophs / consumers / decomposers

“other feeders”

Obtain organic molecules from eating other organisms or substances derived from them

Almost all heterotrophs are dependent on what?

photoautotrophs for food – organic molecules – and O2 , a by-product of photosynthesis

What is the primary organ for photosynthesis?

The Leaf

What are stomata?

These are microscopic pores, mainly found on the surface of leaves. Carbon dioxide (CO2) enters the leaf through stomata, and oxygen (O2), a by-product of photosynthesis, exits through them.

How is 1. water (H2O) absorbed and 2. sugar transported within a plant?

1. Water Absorption: Water (H2O) is absorbed by the roots of the plant and then transported to the leaves through specialized vascular tissues called veins.

2. Sugar Export: Once sugars are produced in the leaves, the veins also serve to export these sugars to other parts of the plant where they are needed for energy or storage.

What are chloroplasts and where are they found?

Chloroplasts are the specific organelles within plant cells where photosynthesis occurs.

They are primarily found in the mesophyll tissue, which is the internal tissue of the leaf.

How many chloroplasts are contained in mesophyll cells?

30-40 chloroplasts

Describe the structure of the chloroplasts

A chloroplast is enclosed by a double membrane. Within this membrane, there are distinct compartments:

1. Stroma:

2. Thylakoids:

3. Chlorophyll:

What is the Stroma?

This is the dense fluid filling the space within the chloroplast, surrounding the other internal structures.

What are the Thylakoids and what do they contain?

These are a system of interconnected membrane-bound sacs. In some areas, thylakoids are stacked into structures called grana (singular: granum).

The thylakoid membranes contain chlorophyll.

What is the Chlorophyll?

This is the green pigment responsible for absorbing light energy, which is crucial for initiating photosynthesis.

What is the photosynthesis equation, and what’s being reduced and oxidized?

6CO2​+6H2​O+Light Energy→C6​H12​O6​+6O2​

• CO2 is reduced to sugar

• H2O is oxidized

Is photosynthesis an exergonic or endergonic process? Why?

As electrons move from water to sugar, their potential energy increases. This means that the overall process of photosynthesis requires energy input (from light), making it an endergonic reaction. The electrons are pulled away from a more electronegative atom (oxygen in water) to a less electronegative environment within the sugar molecule.

What are the 2 stages of photosynthesis and where do they occur?

1. The Light Reactions: occurs in the thylakoids

2. The Calvin Cycle: Occurs in the stroma

What is the first step of the light reaction?

Water Splitting: Water (H2O) is split, yielding electrons (e−), hydrogen ions (H+), and oxygen (O2), which is released as a waste product.

2H2O→O2+4H+ + 4e−

What is the second step of the light reaction?

NADPH Production: Two electrons (e−) and a hydrogen ion (H+) are added to an electron carrier molecule, NADP+, reducing it to NADPH. This reduction process uses the absorbed solar energy.

2NADP+ + H+ +4e− +2H+(stroma)→2NADPH

What is the third step of the light reaction?

ATP Generation (Photophosphorylation): Adenosine triphosphate (ATP) is generated through a process called photophosphorylation. This involves adding a phosphate group (Pi) to adenosine diphosphate (ADP). This process is driven by solar energy, which powers chemiosmosis.

ADP+Pi​→ATP

What is the first step of the The Calvin Cycle?

Carbon Fixation: Carbon dioxide (CO2) from the atmosphere is incorporated into existing organic molecules within the stroma.

What is the second step of the The Calvin Cycle?

Simple Sugar Reduction: Using the chemical energy from ATP (which is converted back to ADP) and the reducing power from NADPH (which donates its electrons and is converted back to NADP+), the fixed carbon is reduced to a simple sugar, CH2O.

What happens to the regenerated ADP and NADP+ when regenerated in the Calvin Cycle?

The ADP and NADP+ generated during the Calvin cycle are recycled back to the light reactions to be re-energized.

What is the third step of the The Calvin Cycle?

Sugar (glucose) Production: Sugar is the ultimate product of the Calvin cycle.

What are the main differences between Photosynthesis and Respiration?

They are reverse reactions.

Photosynthesis:

1. Stores energy

2. Required sunlight

3. In chloroplasts

Respiration:

1. Releases energy

2. Does not required sunlight

3. In mitochondria

What’s the difference in the CO2 concentration of plants that undergo Photosynthesis versus Respiration?

In photosynthesis, plants use up (decrease) carbon dioxide — they take in CO₂ from the air to make glucose.

In respiration, plants release (increase) carbon dioxide — they break down glucose and give off CO₂ as a waste product.