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Photosynthesis (Honors)


How do things grow?

Think about how you grow from a baby to an adult: Every day you take in food – some provides energy, some provides building material. You build new skin, muscles, nerves and bones by stitching together smaller molecules into bigger ones.

But how can plants do this? They don’t eat – so where do they get energy and building blocks from? Photosynthesis!

Photosynthesis is a process used by plants, algae and certain bacteria

Photosynthesis takes energy from sunlight and stores it as chemical energy.

Photosynthesis allows plants to build small organic molecules (sugars), which then gets stitched into more complex molecules (like starches, complex carbs, etc)

What colors of light do plants use?

Plants mostly use red and blue wavelengths of light

When you see a leaf, green is the color that the leaf doesn’t absorb. (Green is reflected.)

How much of each color is reflected, absorbed, or transmitted?

What does this look like, within the leaf itself?

Plant abosrb Red Blue reflects Green spectra

from pinklightaeroponics.net, science-of-futuristic-aeroponic-indoor-gardening.

Do plants use IR (infrared) light?


Plants vs animals

plant vs animal cells

Here’s a 2D representation of a plant cell

Here’s a 3D representation

The photosynthesis equation

How to build glucose (a sugar) – C6H12O6

What’s the simple versions of the photosynthesis equation?

(for the full version scroll down to the Honors Bio section.)

photosynthesis equation simple

This is the SIMPLIFIED photosynthesis equation. It get across the main idea. (From IGCSE 2014 notes, http://igbiologyy.blogspot.com)


How do chloroplasts and mitochondria work together?

Chloroplasts take in CO2, H2O, and light energy. Give off sugar and O2 gas + sugars.

Mitochondria take in sugar, and make ATP + waste (CO2 and H2O)
Chemistry Photosynthesis

How plants produce energy at night?

The Light and Dark Cycles


light-reactions-and-dark-reactions photsynthesis

from eschooltoday.com – photosynthesis

Light reactions make energy molecules

picks up ADP, and P, combines them to make ATP

makes NADP- to combine with H+ forming NADPH.

occur in the chlorophyll of the chloroplasts.

only occur while plant gets light

During daylight all plants cells do cellular respiration, with their mitochondria, like all other organisms.


Dark reactions use ATP and NADPH

better name: light-independent reaction. Because while this cycle does take place during darkness, it actually occurs all the time.

Called the Calvin cycle, occurs in the stroma – fluid filled interior of the chloroplast

Only occurs temporarily, using the ATP made while the plant had been in sunlight. If it stayed dark, eventually plant would run out of ATP, and then the dark cycle would end.

sugars are made from CO2

During nighttime all plants cells do cellular respiration, with their mitochondria, like all other organisms.

Light and Dark cycles Pearson



photosynthesis and respiration color diagram

The H atoms are used to “power up” two types of battery molecules

NADP+                                    +          H           =>       NADPH

Low power form of battery                                          High power form of battery

Energy from the light is also used to add a phosphorous atom (P) to ADP

ADP                                         +          P          =>        ATP

Low power form of battery                                          High power form of battery

  1. Light independent reaction – Uses the battery molecules.
  1. Can occur anytime, even in the dark.
  1. Occurs in the fluid outside the thylakoids – the stroma
  1. Energy from NADPH and ATP is used to break apart CO2, and build C6H12O6 (sugar)

NADPH loses energy and H, turns back into NADP+    (low power form)

ATP loses energy P, turns back into ADP   (low power form)

  1. ADP and NADP+ get recycled during the light cycle (they are rechargeable batteries)

Honors Biology: Details

Where does photosynthesis take place?

Plants have leaves. Each leaf is made up of millions of cells

Each cell has many greenish organelles that look like a stack of pancakes.

Each little “pancake” is called a thylakoid.

A stack of them is called the granum.

Chloroplast structure Thylakoid Granum


What is the full & correct photosynthesis equation?

An experiment conducted by Ruben, Randall and Kamen (1940) proved that O2 evolved during photosynthesis comes from water, not from CO2. Then the conventional equation was modified – as 6 molecules of H2O is not sufficient for the evolution of 6 molecules of O2.

Correct balanced equation of photosynthesis

Ruben, S., Randall, M., Kamen, M., & Hyde, J. L. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society, 63(3), 877-879

From http://www.biologyexams4u.com/2016/11/what-is-correct-balanced-equation-of.html

Why is chlorophyll green?

Why did the chlorophyll pigment evolve to be green? That only absorbs some wavelengths of light. Why not evolve into a black pigment, which would absorb all visible colors of light? The answer has to do with quantum mechanics!

Why did chlorophyll evolve to be green – as opposed to black – which would absorb more energy?


Learning Standards

Massachusetts Curriculum Frameworks: Biology

HS-LS1-5. Use a model to illustrate how photosynthesis uses light energy to transform water and carbon dioxide into oxygen and chemical energy stored in the bonds of sugars and other carbohydrates.

Clarification Statements: Emphasis is on illustrating inputs and outputs of matter and the transfer and transformation of energy in photosynthesis by plants and other photosynthesizing organisms.

State Assessment Boundary: Specific biochemical steps of light reactions or the Calvin Cycle, or chemical structures of molecules are not expected in state assessment.

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