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Photosynthesis Intro

What are we learning? Photosynthesis – the way that plants (*) get energy, and create new molecular building blocks.

(*) Also green algae, and some bacteria and protista.

Why are we learning this? Think about how you grow from a baby to an adult: You take in food – for energy, or building material. You build new skin, muscles, nerves and bones by stitching together smaller molecules into bigger ones. Which leads us to our guiding question – Plants don’t eat – so where do they get energy and building material from?

Target vocabulary: autotroph, cell membrane (lipid bilayer), cell walls, chlorophyll, chloroplast, glucose, heterotroph, mitochondria, photosynthesis, starch, synthesize.

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)

How are plants different from animals?

Autotrophs (aka producers) – Capture solar energy to produce their own food. Examples: Plants, green algae; some bacteria; some protists.

Heterotrophs (aka consumers) – An organism that needs to ingest another organism for food. Most life is heterotrophic, e.g. mammals, reptiles, birds, insects. All fungi (including mushrooms) and many bacteria.

* Plant cells have a cell membrane (a lipid bilayer ) and a cell wall (cellulose)

Animal cells only have a cell membrane.

* Plant cells have one huge vacuole (organelle for storage)

Animal cells a few tiny vacuoles (they aren’t even visible in this diagram)

* Plant cells have an organelle called a chloroplast.

Using energy from light, this is where photosynthesis occurs.

plant and animal cell ribosomes

plant cell vs animal cell


Here we see where plant and animal cells are similar/different:

Animal cell versus plant cell

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.

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

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

photosynthesis vs cellular respiration

from the Amoeba sisters


photosynthesis equation simple

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


The Fruit Loops Lab

Modeling Photosynthesis: The Fruit-Loop Lab


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

Give off ATP, and waste (CO2 and H2O)
Chemistry Photosynthesis

Synthesizing starches


photosynthesis and respiration color diagram

How does O2 and CO2 build trees?

In photosynthesis plants take in O2 and CO2 molecules. We saw above how they are combined to create sugars.

Well, what you saw above was only the beginning. Turns out that with enzymes, plants can modify these sugars and create other types of sugars.

And then with yet other enzymes, plants can break those sugars apart and connect them together to create other organic molecules.

Then these molecules can be stitched (“bonded”) to form chains, like lignin and cellulose.

Then these chains can wrap around each other to form more complex structures, which create wood.

Lignin–carbohydrate complex (LCC) in wood cell wall Cellulose Tree

From Direct evidence for α ether linkage between lignin and carbohydrates in wood cell walls, Hiroshi Nishimura et al, Scientific Reports volume 8, Article number: 6538 (2018)


MCAS example problem

Feb 2016 MCAS.  Earth’s atmosphere is 20.9% molecular oxygen (O2). Which process provides most of the oxygen in the atmosphere?

A. cellular respiration .  B. osmosis .  C. photosynthesis .   D. transpiration

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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