Electron transport, Chemiosmosis and the Photosystems

Last time, i talked about photosynthesis, and pigments. I'm just admitting it now, I forgot to mention/explain one thing, so I am explaining it here instead. Pigments are compounds that absorb light, and in the thylakoid membrane there are the chlorophylls a and b. However, there are more compounds, including carotenoids. Carotenoids don't absorb yellow, orange or brown. In fall, many plants lose their chlorophylls, but keep their carotenoids, making the leaves take on many different colors. Why is this important? Well, the chlorophylls and carotenoids are grouped together in clusters within the thylakoid membrane. Each cluster is referred to as a photo system. there are types of Photosystem, Photosystem 1 and Photosystem 2. Although similar in terms of the pigments each contains, they have very different roles in the light reaction process. The light reactions begin with absorbing light. By absorbing light, the pigment molecules aquire some of the energy carried by light waves. Within each photosystem, the aquired energy is quickly passed to other pigment molecules until it reaches a specific pair if chlorophyll a molecules. Now, I can explain this in five steps. Step 1: The light energy forces electrons to have a higher energy level in the two chlorophyll a molecules in photosystem 2. These electrons are said to be 'excited' Step 2: The excited now have enough energy to leave the chlorophyll a molecules. Since they have lost electrons, the chlorophyll a molecules have now undergone an oxidation reaction. Step 3: The electrons are now passed along a series of molecules called an electron transport chain. However, along the way, they lose most of the energy gained from sunlight. This energy is used to move protons into the thykaloid. Step 4:As light excites the electrons in photosystem 2, it does the same in photosystem 1. It specifically excites the electrons in the chlorophyll a molecules. The electrons move away from that pair, and continue being passed along until they reach a primary electron acceptor. The electrons from photosystem 2 replace the ones lost in photosystem 1 Step 5: Electrons from photosystem 1 travel along an electron transport chain, and at the end of the chain, they combine with NADP+ and H+ to make NADPH. Finally, we are at chemiosmosis. This is derived from greek, chemia meaning alchemy and osmosis meaning pushing. Why bring this up? Well, alchemy was meant to be able to turn lead to gold, which is a little unrelated, or is it? The driving chemical energy of photosynthesis is ATP. ATP is produced by glucose. Comparison; "lead" glucose is turned into the "Gold" or ATP. Chemiosmosis relies on the concentration gradient of protons across the thylakoid membrane. I cant remember if I included this or not last time, but when water is broken down inside the thylakoid, some protons are produced. This makes up for some of the protons used, but not all of them. The others are brought in from the stroma. The energy required to due so is gained from the excited electrons from photosystem 2. See? It all ties into itself. Oh, wait there's more to chemiosmosis. The protons brought in from the stroma are used to build a higher and higher concentration gradient. Now, the concentration gradient of protons represents potential energy. ATP harnesses this potential energy into chemical energy. NADPH uses some of the protons too, and together, ATP and NADPH provide energy for the second set of reactions in photosynthesis. So, this is all a very detailed explanation of the first set of photosynthesis. It is called the light cycle too sometimes, because it harnesses energy from light. The second cycle is called the dark cycle, or, more commonly, the Calvin cycle. This will be explained further in the next post. Sorry for so many posts on photosynthesis, but the process is very long and very complicated. Also, it is probably important for the survival of every species of the planet, but let's not get ahead of ourselves

Photosynthesis

Organisms can be classified based on how they obtain energy. There are autotrophs, organisms that manufacture their own energy, and heterotrophs, organisms that can't make their own energy. Animals, like humans, can't obtain their energy on their own, so they eat autotrpohs or heterotrophs that eat autotrophs. Confusing? Yeah, a little. Think of it like this, we eat steak, which comes from cows. But cows don't eat other cows, they eat grass. Grass is an autotroph, thus the food chain cycle is shown. But, how does a plant like grass obtain its energy? Plants get their energy through a process called photosynthesis. Photosynthesis involves a complex series of chemical reactions, even using a by product of a previous reaction. A plant releases oxygen as a byproduct in an early reaction, but need it in a later one. Back on topic, photosynthesis uses light as its fuel. A plant absorbs sunlight through chloroplasts. The light reactions take place in the thylakoids. To clear things up, the chloroplasts have a membrane surrounding three things. These are the thylakoids, grana and stroma. Thylakoids stack to form a grana, and is surrounded by a solution called the stroma. Moving on, plants are often picky with the light they absorb. As you probably know, the visible light spectrum includes red, orange, yellow, green, blue, indigo and violet. But, why are plants green most of the time? A part of the chloroplast is the chlorophyll. There are several types , but the two important ones are chlorophyll a and chlorophyll b. These are pigments, basically a compound that absorbs light. Chlorophyll a absorbs red to yellow light in the color spectrum, while chlorophyll b absorbs blue to violet light. This is why plants typically appear green. This is only the first stage in the entire process, but I'll get to electron transfer, chemiosmosis, and the different photo systems in my next post.

The Discovery of Cells

Cells are covered in many of the textbooks given to students in middle school and high school. But, who was it that discovered cells? The first observation of cells is attributed to an English scientist named Robert Hooke. He used a microscope to examine a thin slice of cork in 1665. He noticed that the cells appeared in "little boxes". You and me know that plant cells are the "little boxes" Hooke observed, but Robert was the first person to see them. He looked at carrots, ferns, and the stems of elder trees, and all of them displayed a similar formation of cells. Pattern? He was looking only at plant cells. The first person to observe living cells was a Dutch microscope maker named Anton van Leeuwenhoek. After 150 years, scientists began to organize the observations begun by Hooke and van Leeuwenhoek. They formed the cell theory. The theory has three parts: 1. All living things are composed of one or more cells. 2. Cells are the basic units of structure and function in an organism and 3. Cells only come from the reproduction of existing cells. The evidence to support this theory was provided by a trio of German scientists. In 1838, the botanist Matthias Schleiden concluded that all plants are made of cells. A year later, zoologist Theodor Schwann made the same conclusion about animals. Finally, in 1855, a physician named Rudolf Virchow reasoned that cells only come from existing ones. Over the years, modern scientists have gathered a lot of additional information that strongly supports the cell theory.