GED Science Practice Test: Metabolic Processes: Photosynthesis

Plants produce complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in their surroundings, generally using the process of photosynthesis, in which energy from light is converted into chemical energy.

The overall chemical reaction for photosynthesis is:


Photosynthesis takes place in the chloroplastsof eukaryotic cells; similar to the mitochondria, chloroplasts have a two-membrane structure, as shown in the diagram below. However, in chloroplasts, there are also stacked membrane-bound sacs, called thylakoids. The space between the thylakoids and the inner membrane is called the stroma.


There are two main steps of photosynthesis: the light-dependent reactions, and the light-independent reactions. The light-dependent reactions, also called light reactions, are the first stage of photosynthesis, in which plants capture and store energy from sunlight. In this process, light energy is captured by pigments, such as chlorophyll. Light absorbed by chlorophyll excites electrons, resulting in the formation of high-energy ATP and NADPH molecules.  Thus the chlorophyll helps convert the light energy from the sun into chemical energy.

Because ATP and NADPH are not stable forms of chemical energy, the light-independent reactions, or dark reactions, convert the energy in ATP and NADPH into more stable forms of chemical energy, such as glucose. These reactions occur in the stroma of the chloroplast, through a process called the Calvin cycle. These reactions take the light reactions and perform further chemical processes on them. It is important to know that, despite their name, the dark reactions can only occur in the presence of light because these reactions must be able to convert energy from ATP and NADPH to glucose immediately after the ATP and NADPH are formed.  The following diagram summarizes photosynthesis:


Leaf structure and function

The majority of photosynthesis takes place in the leaves of plants, and leaves have specialized structures that are adapted to support efficient photosynthesis.  Leaves must capture as much light energy as possible and also be able to allow carbon dioxide to enter while keeping water from diffusing out. Additionally, the oxygen that is a waste product of photosynthesis must be allowed to escape from the leaf. Pore-like structures called stomata (plural for stoma) are located on the underside of the leaf (thus reducing transpiration, or loss of water from the leaf); these allow CO2 to diffuse into the leaf, and allow oxygen to exit the leaf. Additionally, the cells on the underside of the leaf, called the spongy mesophyll, have a spongy texture with air spaces between cells that enables oxygen and carbon dioxide to diffuse throughout the leaf. In contrast, the cells that comprise the palisade mesophyll on the top side of the leaf are closely packed together and contain many choloroplasts, as these cells play a key role in capturing light energy.



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