Pete NatsewayMukti Ryan- A&P 241

The Cellular Level of Organization- Chapter 3 ReviewCells are the basic units of all living things in this world. They can be divided into to types: Prokaryotic and Eukaryotic. Prokaryotic cells are defined by their lack of a nucleus and their simplicity. Most of them tend to be unicellular bacteria and Archaea. But our focus will be on the second type, the eukaryotic cell. This type of cell is defined by its presence of a nucleus, and its complex subcellular structures, or organelles.When talking about these eukaryotic cells, we can see a common structure present throughout. They are all composed of three main structures: the plasma membrane (or plasmalemma), the cytoplasm, and the nucleus. The plasma membrane is the cells outer boundary that keeps its internal environment separate from the outside. A key characteristic of this membrane is its selective permeability, meaning it allows the passage of some material but not others. The general rule of thumb is that small, neutrally charged, lipid-soluble substances can pass freely, while large, water-soluble molecules (proteins) cannot. These freely passing substances move across the membrane by a process called passive transport. They can move without any input of energy, or ATP. They move down their concentration gradient, from higher concentration to lower. Some examples include the diffusion of solutes (high to low concentration), the osmosis of water (low concentration of solute to high), and facilitated diffusion (either channel-mediated or carrier-mediated). Active transport processes, on the other hand, do require the breakdown of ATP to move solutes across the membrane against their concentration gradient (low to high). An active transport process found in all cells is the sodium-potassium pump. This type of movement is possible due to integral proteins found throughout the plasma membrane.Transmembrane proteins span the entire phospholipid bi layer, while peripheral proteins attach either to the inner or outer surfaces of the membrane. These proteins can be categorized into ion channels, transporters (selectively moving substances through the membrane), receptors (for cell recognition), and enzymes (catalyzing chemical reactions). Some of the transporter proteins are classified as antiporters, meaning they carry two substances across the membrane in opposite directions, while symporters carry two substances in the same direction. Sometimes, substances can move in and out of the cell by cytosis. These substances can enter the cell by forming a vesicle, which is a spherical sac formed by budding off from the membrane, a process known as endocytosis. The three types of endocytosis are receptor-mediated, phagocytosis, and bulk-phase cytosis (pinocytosis). The opposite movement, known as exocytosis, is when the vesicles fuse with the plasma membrane, releasing their contents into the extracellular fluid. A combination of these two processes is transcytosis. Moving further into the cell, we have the cytoplasm.The cytoplasm is a gelatin-like substance, containing structural fibers and organelles. The cytosol is the intracellular fluid that surrounds the organelles, and it is the site of many chemical reactions. These reactions usually release energy, and also provide the building blocks for cell maintenance, structure, function, and growth. The structural fibers, together known as the cytoskeleton, are the microtubules (made of tubulin), microfilaments (made of actin), and intermediate filaments (made of both proteins). The organelles inside the cell are highly specialized structures.The endoplasmic reticulum is a network of membranes that are shaped like flattened tubes or sacs. The ER is divided into two sections. The rough ER is connected to the nuclear envelope and is a series of flattened sacs. The surface of the rough ER is studded with ribosomes (sites of protein synthesis). The rough ER produces various proteins. The second part, the smooth ER, is a network of tubules, and synthesizes fatty acids and steroids. It also serves to detoxify certain drugs.The Golgi complex consists of around three to twenty flattened, membranous sacs (cisternae). This organelle modifies, sorts, and packages proteins for transport to different destinations. Various vesicles send these proteins, including lysosomes (containing powerful digestive enzymes) and smaller peroxisomes (containing detoxifying enzymes). Peroxisomes are abundant in the liver, where they detoxify substances such as alcohol.The powerhouses of the cells, known as mitochondria, generate ATP used for all of the active processes in the body. They have inner and outer membranes similar in structure to the plasma membrane. The inner membrane contains a series of folds called cristae. The mitochondria also have a large central fluid-filled cavity called the matrix. This organelle contains its own DNA (inherited from your mother), and self-replicates during times of increased cellular demand or before cell division. Lastly, our cells contain the nucleus, which is a spherical or oval shaped structure. It is usually the most prominent feature of the cell, and is characterized by its nuclear envelope (double membrane), nuclear pores (controlling movements of substances between nucleus and cytoplasm, and nucleolus (spherical body that produces ribosomes). The nucleus also contains chromosomes, which are protein molecules combined with long molecules of DNA.