If we feel hungry, we usually find some snacks in kitchen. But when plants are "hungry," what do they do? Although we don't think of plants as creatures that can cook for themselves, in fact, they make "food" on their own through a process call photosynthesis.
Autotrophs and heterotrophs
There’re two modes that organisms obtain energy and construct organic compounds. Autotrophs use inorganic substances to synthesize organic matter, so they are producers in biosphere. The sunlight is necessary for plants, some bacteria and algae obtain to produce saccharide, and that’s where the name photoautotroph comes from.
Heterotrophs can’t directly use inorganic substances to generate organic substances. They must intake organic substances from other living things. Imagine an apple tree. When the apples are ripe, they store a lot of saccharide. A hungry squirrel decides to climb the tree to eat some apples. Actually, it indirectly gets energy from sunlight. Some heterotrophic pathways are more subtle. Some heterotrophs consume the remains of other organisms and organic waste, such as feces and fallen leaves. They are called decomposers. Most fungi and prokaryotes obtain nutrients in this way.
Photosynthesis occurs in chloroplasts
Chloroplasts are organelles in plant cells responsible for photosynthesis. They’re found in green stems, unripe fruits, and all green parts of plants, but most abundant in leaves. About 500,000 chloroplasts are present in one square millimeter of leaf fragment. They’re mainly distributed in mesophyll cells on the leaf. A typical mesophyll cell has about 30-40 chloroplasts.
Chloroplasts live within eukaryotes like symbiotic prokaryotes; originally, they probably were prokaryotes. They have a rich membrane system to run photosynthesis. The outermost double membrane separates them from the cytoplasm, and encloses a space called stroma where enzymes and various active substances participate in triose synthesis. The thylakoids in the stroma form the third complex membrane system. These small sacs are rich in chlorophyll that captures solar photons, and the complete set of protein complexes (electron transport chains) that convert light energy into chemical energy. The pigments chlorophyll gives plants their green color. Thylakoids sometimes stack into columnar structures to form a larger surface area for more efficient light absorption.
The chemical equation of photosynthesis: how it occurs
Photoautotrophs directly use natural resources—especially solar energy—to produce nutrients. But how do plants do this? Three essential elements are needed: carbon dioxide, water, and sunlight. The chemical equation is simplified as: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂. Photosynthesis consumes 12 water molecules and produces 6 water molecules, so the simplified equation on the right doesn't include water.
Unlike respiration that absorbs oxygen, plants absorb carbon dioxide through tiny openings in their leaves, stems, and roots. Water infiltrates into cells from root hairs. Then, water is transported to large leaves through vessels. Transpiration and root pressure (root cells actively transport water) help water overcome gravity. Some aquatic plants can directly absorb water from their environment through their stems and leaves because their epidermis lacks the collenchyma that prevents water evaporation in terrestrial plants.
The most critical factor is the sun. Sunlight triggers a complex photochemical reaction that converts water and carbon dioxide into glucose and oxygen. Glucose quickly transforms into fructose and sucrose to participate in nutrient transport. They are also synthesized into starch and stored in various parts, especially roots and fruits. Oxygen and carbon dioxide share a common channel. They are released into the air to become essential elements for aerobic respiration in organisms. Plants not only self-sustain but also selflessly support other life on Earth.
Although it looks like inverted cellular respiration, photosynthesis is actually more complex. Of course, it’s not as simple as the equation listed above. It’s an endothermic and entropy-increasing reaction. It can be divided into two major parts: light and dark reactions (also known as the Calvin cycle).
In light reaction of photosynthesis, all the biochemical reactions depend on light energy. It occurs on thylakoid membrane. Electrons from chlorophyll are excited twice by photons. After the first excitation, high-energy electrons release energy along the electron transport chain on thylakoid membrane. This process is similar to electron transfer on the mitochondrial respiratory chain, so it also forms an electrochemical proton gradient and drives ATP synthase to synthesize ATP in the stroma. High-energy electrons and protons are received by NADP⁺ to form NADPH in the second excitation. Electrons provided by chlorophyll are replaced by electrons from water ultimately. This causes water molecules to decompose into oxygen as a by-product. Energy from sunlight is temporarily stored in ATP and NADPH.
The second stage is dark reaction that doesn't depend on light directly. Because it was discovered by Calvin, it's also called Calvin cycle. The energy for fixing and reducing carbon dioxide comes from high-energy compounds ATP and NADPH. Triose containing 3 carbons is the product of this reaction. They are synthesized into glucose in chloroplast stroma.