Lecture 2.6 - Photosynthesis

Flashcards covering key concepts related to photosynthesis and biological systems.

Photosynthesis

The process by which chloroplasts in plants capture sunlight energy and convert it into chemical energy in the form of sugars.

Autotrophs

Self-feeder, produce organic molecules from CO2 and other inorganic molecules

Heterotrophs

Other feeding, cannot make their own food and live on compounds produced by other organisms

Chloroplast

Organelles in eukaryotic plant cells that carry out photosynthesis, containing their own DNA and ribosomes.

RuBisCO

An enzyme that catalyzes the fixation of carbon dioxide in the Calvin Cycle, and is considered one of the most important enzymes on Earth.

Calvin Cycle

The second stage of photosynthesis where ATP and NADPH are used to fix CO2 and synthesize simple sugars.

Photorespiration

A process in which RuBisCO adds O2 to the Calvin Cycle instead of CO2, leading to a waste of carbon and energy.

C3 plants

Plants that use RuBisCO for carbon fixation into a 3-carbon sugar, commonly affected by photorespiration under low CO2 conditions.

C4 plants

Plants that fix CO2 into a 4-carbon sugar in a separate layer of cells, allowing efficient photosynthesis in hot and arid environments.

CAM plants

Plants that fix CO2 at night and close stomata during the day to conserve water, incorporating CO2 as organic acids.

Sunlight provides…

The energy needed to feed almost the entire living world

Two main nutrition modes

Autotrophs and heterotrophs

Photoautotrohps

Use sunlight as energy source

Which of the following statements is a correct distinction

between autotrophs and heterotrophs?

1. Autotrophs, but not heterotrophs, can nourish themselves beginning

with CO2 and other nutrients that are inorganic.

2. Only heterotrophs require chemical compounds from the environment.

3. Only heterotrophs have mitochondria.

4. Cellular respiration is unique to heterotrophs

Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic

Endosymbiotic theory

Chloroplasts were photosynthetic prokaryotes that lived inside ancestral eukaryotic plant cell

How many membranes does chloroplasts have?

Two, own DNA and ribosomes, divided by binary fission

What is inside the chloroplast?

Stacks of thylakoid sacs, which contain chlorophyll

Chlorophyll

The pigment that absorbs sunlight energy and makes photosynthesis possible

Name

The chemistry of photosynthesis

How is O2 formed

From H2O

Stages of photosynthesis

Light reactions, calvin cycle

Light reactions

Use light to synthesise ATP and harvest electron in NADPH

Calvin cycle

Uses ATP + NADPH to fix CO2 and synthesise simple sugars

Properties of light

• Has both wave-like and particle-like properties

  • A particle of light is called a photon and each photon has a fixed amount of energy

Photosystem

Composed of several pigment molecules surrounding a reaction-center complex

The reaction-center complex is composed of

Special chlorophyll molecules and primary electron acceptor

Special chlorophyll molecules

Transfer energy as e- rather than photon

Primary electron acceptor

Captures that electron (redox reaction)

Name

Photosystems 1 and 2

Properties of photosystems 2

• Has chlorophyll a that is best at absorbing at 680nm in the reaction center (“P680”)

• Harvests electrons from water (H2O → ½O2 + 2H+ + 2e-) to replenish P680

• Passes excited e- to an electron transfer chain (ETC), which also powers an ATP synthase via chemiosmosis

Properties of photosystems 1

• Has chlorophyll a that is best at absorbing at 700nm in the reaction center (“P700”)

• Accepts electrons from ETC to replenish P700

• Passes excited e- to another ETC, which reduces NADP+

Plants need more ________ than ________

ATP, NADPH

Photosystems 1 can produce…

ATP instead of NADPH

When light strikes chlorophyll molecules of photosystems 2, they lose electrons, which are ultimately replaced by…

Electrons released by splitting water

Calvin Cycle uses…

• 3x CO2 as carbon source

• 6x NADPH as a source of high-energy electrons

  • 9x ATP as energy source to power the anabolic (endergonic) reaction

To make a 3-carbon sugar…

Glyceraldehyde 3-phosphate which is then used as starting material for organic molecules

What does RuBisCO stand for

Ribulose bisphosphate carboxylase oxygenase

RuBisCO

Catalyses carbon fixation, and thus arguably the most important enzyme on Earth

Photosynthesis is a redox reaction. H2O is _________ during the light reactions and CO2 is ________ during the Calvin cycle

Oxidized, reduced

Is RuBisCO perfectly ditinguish between CO2 and O2

RuBISCO does not perfectly distinguish between CO2 and O2

Early atmosphere CO2 and O2 percentage

20-30% CO2 and no O2

CO2 and O2 percentage today

0.04% CO2 and 21% O2

Photorespiration

Adding O2 to the Calvin Cycle instead of CO2, which wastes the cycle and instead of a 3-carbon sugar releases 2 molecules of CO2

What does plant cells need to conduct photosynthesis?

CO2 in and O2 out

Where goes gas exchange occur in plants?

Pores on leaf surface

Stomata

Small openings on the leaf surface that regulate gas exchange by allowing CO2 to enter and O2 to exit.

Main avenue of transpiration in plants

Stomata

What happens to plants on hot days?

Plants close their stomata to reduce water loss but that also means CO2 can no longer get in (or O2 out)

What does most plants rely on?

RuBisCO to fix CO2 into a 3-carbon sugar (called C3 plants)

What happens when internal CO2 levels drop?

RuBisCO starts using O2 instead and wastage by photorespiration increases

C4 plants

Have additional layer of cells where CO2 is first fixed on a 4-carbon sugar which is then spoon fed to RuBisCO in bundle sheath cells

C4 name

Spatial Separation

CAM plants name

Temporal separation

CAM plants

Others sock up on CO2 during the night, and then close their stomata during the day, slowing releasing the stored CO2 to feed RuBisCO

CAM plants CO2

Incorporated in variety of acids

Name

C4