Chapter 5 Cells MICROSCOPES AND THE STUDY OF CELLS Cells are studied using different types of microscopes. Light microscopes, (for example, the compound microscopes commonly found in labs) are used to study stained or living cells. They can magnify the size of an organism up to 1,000 times. Electron microscopes are used to study detailed structures of a cell that cannot be easily seen or observed by light microscopy. They are capable of resolving structures as small as a few nanometers in length, such as individual virus particles or the pores on the surface of the nucleus. A prokaryotic cell, which is a lot smaller than a eukaryotic cell, is relatively simple. Bacteria, for example, are prokaryotic cells. The inside of the cell is filled with a substance called cytoplasm. The genetic material in a prokaryote is one continuous, circular DNA molecule that is found free in the cell in an area called the nucleoid (this is not the same as a nucleus!). Most prokaryotes have a cell wall composed of peptidoglycans that surrounds a lipid layer called the plasma membrane. Prokaryotes also have ribosomes (though smaller than those found in eukaryotic cells), and they may also have one or more flagella, which are long projections used for motility (movement). Eukaryotic cells are more complex than prokaryotic cells. Fungi, protists, plants, and animals are eukaryotes. Eukaryotic cells are more complex than prokaryotic cells. Fungi, protists, plants, and animals are eukaryotes. Eukaryotic cells are organized into many smaller structures called organelles. Some of these organelles are the same structures seen in prokaryotic cells, but many are uniquely eukaryotic. A good way to remember the difference is that prokaryotes do not have any membrane-bound organelles. Their only membrane is the plasma membrane. Organelles A eukaryotic cell is like a microscopic factory. It’s filled with organelles, each of which has its own special tasks. Let’s take a tour of a eukaryotic cell and focus on the structure and function of each organelle. Here’s a picture of a typical animal cell and its principal organelles. Plasma Membrane The cell has an outer envelope known as the plasma membrane. Although the plasma membrane appears to be a simple, thin layer surrounding the cell, it’s actually a complex, double-layered structure made up of phospholipids and proteins. The hydrophobic fatty acid tails face inward and the hydrophilic phosphate heads face outward. It is called a phospholipid bilayer since two lipid layers are forming a hydrophobic sandwich. The plasma membrane is important because it regulates the movement of substances into and out of the cell. The membrane itself is semipermeable, meaning that only certain substances, namely small hydrophobic molecules (such as O2 and CO2), pass through it unaided. Anything large and/or hydrophilic can only pass through the membrane via special tunnels (discussed more later in this chapter). Many proteins are associated with the cell membrane. Some of these proteins are loosely associated with the lipid bilayer (peripheral proteins). They are located on the inner or outer surface of the membrane. Others are firmly bound to the plasma membrane (integral proteins). These proteins are amphipathic, which means that their hydrophilic regions extend out of the cell or into the cytoplasm, while their hydrophobic regions interact with the tails of the membrane phospholipids. Some integral proteins extend all the way through the membrane (transmembrane proteins). This arrangement of phospholipids and proteins is known as the fluid-mosaic model. This means that each layer of phospholipids is flexible, and it is a mosaic because it is peppered with different proteins and carbohydrate chains. Remember, anything hydrophilic should not go through the hydrophobic interior. This means that the phospholipids on one side should never flip-flop to the other side of the membrane (because that would require their polar heads to pass through the hydrophobic area). Why should the plasma membrane need so many different proteins? It’s because of the number of activities that take place in or on the membrane. Generally, plasma membrane proteins fall into several broad functional groups. Some membrane proteins form junctions between adjacent cells (adhesion proteins). Others serve as docking sites for arrivals at the cell, such as hormones (receptor proteins). Some proteins form pumps that use ATP to actively transport solutes across the membrane (transport proteins). Others form channels that selectively allow the passage of certain ions or molecules (channel proteins). Finally, some proteins, such as glycoproteins, are exposed on the extracellular surface and play a role in cell recognition and adhesion (recognition and adhesion proteins). Attached to the surface of some proteins are carbohydrate side chain. They are found only on the outer surface of the plasma membrane. As mentioned above, cholesterol molecules are also found in the phospholipid bilayer because they help stabilize membrane fluidity in animal cells. The Nucleus The nucleus, which is usually the largest organelle, is the control center of the cell. The nucleus not only directs what goes on in the cell, it is also responsible for the cell’s ability to reproduce. It’s the home of the hereditary information— DNA—which is organized into large structures called chromosomes. The most visible structure within the nucleus is the nucleolus, which is where rRNA is made and ribosomes are assembled. Ribosomes The ribosomes are the sites of protein synthesis. Their job is to manufacture all the proteins required by the cell or secreted by the cell. Ribosomes are round structures composed of two subunits. The structure is composed of RNA and proteins. Ribosomes can be either free floating in the cell or attached to another structure called the endoplasmic reticulum (ER). Endoplasmic Reticulum (ER) The endoplasmic reticulum (ER) is a continuous channel that extends into many regions of the cytoplasm. The region of the ER that is attached to the nucleus and “studded” with ribosomes is called the rough ER (RER). Proteins generated in the rough ER are trafficked to or across the plasma membrane, or they are used to build Golgi bodies, lysosomes, or the ER. The region of the ER that lacks ribosomes is called the smooth ER (SER). The smooth ER makes lipids, hormones, and steroids and breaks down toxic chemicals. Golgi Bodies The Golgi bodies, which look like stacks of flattened sacs, also participate in the processing of proteins. Once the ribosomes on the rough ER have completed synthesizing proteins, the Golgi bodies modify, process, and sort the products. They’re the packaging and distribution centers for materials destined to be sent out of the cell. They package the final products in little sacs called vesicles, which carry the products to the plasma membrane. Golgi bodies are also involved in the production of lysosomes. Mitochondria Another important organelle is the mitochondrion. The mitochondria are often referred to as the “powerhouses” of the cell. They’re power stations responsible for converting the energy from organic molecules into useful energy for the cell. The energy molecule in the cell is adenosine triphosphate (ATP). The mitochondrion is usually an easy organelle to recognize because it has a unique oblong shape and a characteristic double membrane consisting of an inner portion and an outer portion. The inner mitochondrial membrane forms folds known as cristae and separates the innermost area called the matrix from the intermembrane space. The outer membrane separates the intermembrane space from the cytoplasm. As we’ll see later, most of the production of ATP is done on the cristae. More Mitochondria = More Energy Since mitochondria are the cell’s powerhouses, you’re likely to find more of them in cells that
