Intro to Photosynthesis, Leaf Structure, Nature of Light, and Pigments

Welcome to our continuing AP Biology/ General Biology series here on STEM Talks!

The purpose of this post is to get our readers accustomed to photosynthesis so that when we dive into the in-depth nitty gritty later on, they will be right at home.

We will begin with a short summary of photosynthesis:

Photosynthesis is the basis of the majority of life on Earth as it utilizes Sunlight (which in most places is readily available) and Carbon Dioxide to create chemical energy stored in sugars and other molecules. The system is the basis of plant life as well as all major ecosystems on Earth. Its importance cannot be understated. This process affect the vast majority of life either directly or indirectly.

The equation for photosynthesis is:

Light Energy + 6CO2 + 6H2O = C6H12O6 + 6O2

We will resume our coverage of Photosynthesis at the end of this post as more items need to be known for proper comprehension.


The Leaf: Adaptations for Photosynthesis:


As with everything in the subject of Biology – Structure follows Function.

The Upper Epidermis is the top layers cells in the leaf. These cells secrete a waxy waterproof layer called the Cuticle that protects the leaf from damage.

The middle section of the leaf are called the Mesophyll which contain veins such as Xylem (water transport) and Phloem (sugar transport). The Mesophyll cells also contain Chloroplasts which are the primary organelle for photosynthesis. In each cell there is an average of 30-40 chloroplasts.

The Lower Epidermis is on the bottom of the leaf (underside). Openings called Stomata, which allow CO2 entry and water evaporation, are found in the Lower Epidermis. Each stomata is surrounded by Guard Cells so that when the plant won’t waste energy during the night pumping CO2 in. The water evaporation (transpiration) function is also important because it allows water to be drawn up from the roots against gravity due to various water properties.


Chloroplast Structure and Pigments:


Structure again follows function which here is to carry out photosynthesis. Chloroplasts, like mitochondria, have two membranes aptly named the Outer Membrane and Inner Membrane. Inside the inner membrane there are small stacks of thylakoid sacs called Granum. These Grana are interconnected. The liquid inside the Inner Membrane surrounding all the Grana is called the Stroma (not to be confused with Stomata).

Each Thylakoid is made of a lipid bilayer composed of various proteins which are the basis of Photosyntheisis called the Light Harvesting Antennae. The pigment Chloropyll is found here.


In the diagram above, the shaded green section is called the Porphyrin ring which is reactive to light and is involved in the light reactions of Photosynthesis. The non polar hydrocarbon tail in Chlorophyll helps anchor it into the Thylakoid membrane.

There are multiple types of Chlorophyll – the major types are type a and type b. Both types are bad at absorbing the color green so the color is reflected. This is why plants usually appear green. In temperate regions, the season of autumn may bring color changes to plants and this is due to other pigments being more dominant such as Carotenoids.

Nature of Light:

Light is a part of the Electromagnetic Spectrum. It can either be imagined as a particle (photon) or a wave (measured in nanometers). Visible light is between 400 and 700 nanometers in wavelength. Longer wavelengths mean there is lower energy and therefore a warmer color. The photoelectric effect allows light (as photons) to exit electrons. In Photosynthesis this is called photoexitation. This is very important in the Light Reactions of Photosynthesis.

In the next few posts we will  look further into the actual process of photosynthesis.

-Sai Rachakonda



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