The cell is one of the most basic units of life. There are millions of different types of cells. There are cells that are organisms onto themselves, such as microscopic amoeba and bacteria cells. And there are cells that only function when part of a larger organism, such as the cells that make up your body. The cell is the smallest unit of life in our bodies. In the body, there are brain cells, skin cells, liver cells, stomach cells, and the list goes on. All of these cells have unique functions and features. And all have some recognizable similarities. All cells have a 'skin', called the plasma membrane, protecting it from the outside environment. The cell membrane regulates the movement of water, nutrients and wastes into and out of the cell. Inside of the cell membrane are the working parts of the cell. At the center of the cell is the cell nucleus. The cell nucleus contains the cell's DNA, the genetic code that coordinates protein synthesis. In addition to the nucleus, there are many organelles inside of the cell - small structures that help carry out the day-to-day operations of the cell. One important cellular organelle is the ribosome. Ribosomes participate in protein synthesis. The transcription phase of protein synthesis takes places in the cell nucleus. After this step is complete, the mRNA leaves the nucleus and travels to the cell's ribosomes, where translation occurs. Another important cellular organelle is the mitochondrion. Mitochondria (many mitochondrion) are often referred to as the power plants of the cell because many of the reactions that produce energy take place in mitochondria. Also important in the life of a cell are the Lysosome. Lysosomes are organelles that contain enzymes that aid in the digestion of nutrient molecules and other materials. There are many different types of cells. One major difference in cells occurs between plant cells and animal cells. While both plant and animal cells contain the structures inside the cell membrane the plant cell has a cell wall for support. Discussed above, plant cells have some additional specialized structures. Many animals have skeletons to give their body structure and support. Plants do not have a skeleton for support and yet plants don't just flop over in a big spongy mess. This is because of a unique cellular structure called the cell wall. The cell wall is a rigid structure outside of the cell membrane composed mainly of the polysaccharide cellulose. As pictured at left, the cell wall gives the plant cell a defined shape that helps support individual parts of plants. In addition to the cell wall, plant cells contain an organelle called the chloroplast. The chloroplasts allow plants to harvest energy from sunlight. Specialized pigments in the chloroplast (including the common green pigment chlorophyll) absorb sunlight and use this energy to complete the chemical reaction: 6 CO2 + 6 H2O + energy (from sunlight) => C6H12O6 + 6 O2In this way, plant cells manufacture glucose and other carbohydrates that they can store for later use. Organelles are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells.
Schleiden and Shwann proposed the Cell Theory in 1838. They stated that all living things are made of cells; Cells are the basic units of life; and Cells come only from other cells Protocells is one theory of the origins of cells states that the first life on earth consisted of several types of tiny protocells, cell-like organisms. These organisms were able to survive and reproduce in a very limited environment because of their simplicity. Over time, some of these protocells came together and shared their specialization in a symbiotic relationship. These colonies of protocells eventually became the cells we know today. There are two types of cells. They are:...
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