Photosynthetic Organisms

Chloroplasts

Now let's talk about chloroplasts very carefully.

A chloroplast is a special structure inside the cells of plants (and some algae) where photosynthesis happens.
You can think of a chloroplast as a tiny factory whose entire job is to capture sunlight and turn it into sugar.

Each chloroplast is surrounded by a double membrane.
A membrane is like a thin skin or barrier. A double membrane means there are two layers, one inside the other.
The reason it has two layers is for extra protection and control over what substances move in and out — kind of like having a front door and a back door at your house that you can lock separately.

Inside the chloroplast, there are smaller structures called thylakoids (which we’ll talk about soon) and a liquid-like area called the stroma.

In short, the chloroplast is the place where sunlight gets captured and turned into chemical energy.

Analogy:
Imagine a smoothie shop. The shop has walls (the double membrane) and inside the workers (the thylakoids and stroma) are busy blending up smoothies (sugar) using sunlight as the blender's power source.

Light Reactions (Photochemical Reactions)

The light reactions happen inside the thylakoids, which are tiny flat disk-like structures stacked inside the chloroplast.

Here’s what happens during the light reactions:

• Sunlight, made of particles called photons, hits the chlorophyll molecules in the thylakoid membranes.

• The energy from the photons excites electrons in the chlorophyll.
  "Excites" here means that the electrons absorb energy and move to a higher energy level — like when you get super hyped up after drinking an energy drink.

Now, these excited electrons don’t just stay put. They start moving along something called an electron transport chain, which is a series of proteins that pass the electrons along like a bucket brigade.

While the electrons are moving:

• Water molecules (H₂O) are split into hydrogen, oxygen, and electrons.

• The oxygen (O₂) is released as a waste product — it’s the oxygen we breathe!

• ATP and NADPH are made. These are high-energy molecules that will be used in the Calvin Cycle later.

Reactants (what you need at the start): -light

• Water (H₂O)

• NADP⁺ (an empty carrier molecule, kind of like an Uber without passengers)

• ADP + Pi (a partly used battery and a loose piece called a phosphate)

Products (what you get at the end):-light

• Oxygen (O₂) — the leftover gas from splitting water.

• NADPH — the Uber now full of energy passengers (electrons).

• ATP — a fully charged battery ready to power the Calvin Cycle.

purpose

The main purpose of the light reactions is to capture sunlight energy and turn it into chemical energy stored inside ATP and NADPH.

The Calvin Cycle (Biochemical Reactions)

The Calvin Cycle takes place in the stroma.
The stroma is the watery, jelly-like fluid that fills up the inside of the chloroplast, all around the thylakoids.

Imagine the stroma as the workshop floor inside a factory.
You already made batteries (ATP) and delivered passengers (NADPH) in the light reactions —
Now the workers in the stroma are going to use that energy to build sugars.

What happens during the Calvin Cycle?

The Calvin Cycle does NOT use light directly.
Instead, it uses:

• CO₂ (carbon dioxide gas from the air)

• ATP (energy battery made earlier)

• NADPH (high-energy electron shuttle made earlier)

Here’s how it works:

1. Carbon dioxide molecules enter the chloroplast through little pores called stomata.

2. The carbon dioxide molecules are captured and linked together in a series of chemical steps.

3. Using the energy from ATP and the electrons from NADPH, the plant builds sugar molecules — mostly glucose (C₆H₁₂O₆).

4. After the energy is used up:

   • ATP becomes ADP + Pi (it’s now an empty battery).

   • NADPH becomes NADP⁺ (it dropped off its electrons and is now empty). These can go back to the light reactions to get recharged!

Reactants (stuff you need to start): -calvin

• Carbon dioxide (CO₂)

• ATP

• NADPH

Products (stuff you make at the end):

• Sugars (like glucose)

• NADP⁺ (empty shuttle)

• ADP + Pi (empty battery + spare phosphate)

🔥 KEY REMEMBER TIP:

• Light reactions = thylakoid.

• Calvin Cycle = stroma.

• Light makes ATP/NADPH.

• Calvin Cycle makes glucose.

Mitochondria vs Chloroplast Comparison

Chloroplasts are found in plants and some algae.
They capture sunlight and make food (sugar) during photosynthesis.

Mitochondria are found in almost all eukaryotic cells (plants, animals, fungi).
They take food (sugar) and make usable energy (ATP) during cellular respiration.

• Chloroplasts make sugar → using sunlight + CO₂ + H₂O.

• Mitochondria use sugar → to make ATP (energy your body/cell can use immediately).

In other words:

🌞 Sunlight → Chloroplast → Makes Sugar
🍬 Sugar → Mitochondria → Makes ATP

They are connected like a cycle.

What is chlorophyll?

Chlorophyll is a molecule inside chloroplasts that gives plants their green color and absorbs light energy for photosynthesis to start.

It’s a pigment — meaning it’s a special molecule that soaks up some colors of light and reflects others.

When chlorophyll absorbs sunlight, its electrons get excited (gain energy).
These excited electrons are what power the reactions of photosynthesis.

Deeper structure:

Chlorophyll isn’t just random green goo — it has a very specific and fancy structure:

It has a porphyrin ring.

• This is a big ring-shaped molecule that can absorb light.

• At the very center of the ring is a magnesium atom (Mg).

• The ring has delocalized electrons.
  Delocalized means the electrons can move freely around the ring — they aren't stuck in one place.

These moving electrons are perfect for absorbing the energy from sunlight.

Why is chlorophyll green

Why is it green?
Chlorophyll absorbs red and blue/violet light from the sun, but reflects green light, so that’s the color we see.

Visible Spectrum

Definition: The visible spectrum is the range of light wavelengths that human eyes can see.
It ranges from about 400 nanometers (violet light) to about 700 nanometers (red light).

Important for photosynthesis:
Plants mostly absorb light in the blue-violet and red parts of the visible spectrum.
They reflect green light — that’s why plants look green!

Other Accessory Pigments

Breaking it down:

• Carotenoids produce yellow to orange colors (like carrots 🍊).

• Xanthophylls make yellow colors (more in leaves).

• Anthocyanins make red colors (stored in vacuoles inside plant cells).

Role:
These pigments help absorb extra light that chlorophyll might miss.
They also protect the plant by soaking up extra light that could damage cells

Photosynthetically Active Radiation (PAR)

Photosynthetically Active Radiation (PAR)

wavelengths of light between 400 nm - 700 nm

supports photosynthesis

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Definition: PAR is the range of light wavelengths (400 to 700 nanometers) that plants can actually use for photosynthesis.

Plants only absorb light in this range. Anything outside (like UV or infrared) is not useful for photosynthesis.

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Real-life analogy:
Imagine you have a radio that only picks up FM stations between 88 and 108 MHz.
PAR is like that — plants only "hear" light between 400 and 700 nm.

Leaves

Definition: Leaves are the main parts of plants where photosynthesis happens.
They are flat and wide so they can catch as much sunlight as possible.

More surface area = more light absorption = more food making.

Stomata

Definition: Stomata are tiny openings or pores on the underside of leaves.
They open and close based on water movement (osmosis) and potassium ions (K⁺).

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Jobs of Stomata:

1. Let CO₂ in → plants need carbon dioxide for the Calvin Cycle.

2. Let O₂ out → oxygen is a waste product of photosynthesis.

3. Allow water vapor out → this helps cool the plant (like sweating).

Evaporative Cooling

When water evaporates out of stomata, it takes heat with it — cooling down the plant, just like when you sweat to cool off.