Cracking the AP Biology Exam
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:
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.
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 proteins, pass through it unaided. 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 cyptoplasm while their hydrophobic regions interact with the tails of the membrane phospholipids. Some integral proteins do not extend all the way through the membrane (transmembrane proteins). This arrangement of phospholipids and proteins is known as the fluid-mosaic model.
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 proteins of the extracellular matrix or 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 chains. They are found only on the outer surface of the plasma membrane. Cholesterol molecules are also found in the phospholipid bilayer because they help stabilize membrane fluidity in animal cells.
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.
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 RNA and two subunits of proteins. They 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 “studded” with ribosomes is called the rough ER (RER). Proteins made on the rough ER are the ones “earmarked” to be exported out of the cell. 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.
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.
Mitochondria: The Powerhouses of the Cell
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. As we’ll see later, most of the production of ATP is done on the cristae.
Since mitochondria are the cell’s powerhouses, you’re most likely to find more of them in cells that require a lot of energy. Muscle cells, for example, are rich in mitochondria.
Throughout the cell are small, membrane-bound structures called lysosomes. These tiny sacs carry digestive enzymes, which they use to break down old, worn-out organelles, debris, or large ingested particles. The lysosomes make up the cell’s cleanup crew, helping to keep the cytoplasm clear of unwanted flotsam.
The centrioles are small, paired, cylindrical structures that are found within microtubule organizing centers (MTOCs). Centrioles are most active during cellular division. When a cell is ready to divide, the centrioles produce microtubules, which pull the replicated chromosomes apart and move them to opposite ends of the cell. Although centrioles are common in animal cells, they are not found in plant cells.
In Latin, the term vacuole means “empty cavity.” But vacuoles are far from empty. They are fluid-filled sacs that store water, food, wastes, salts, or pigments.
Peroxisomes are organelles that detoxify various substances, producing hydrogen peroxide as a byproduct. They also contain enzymes that break down hydrogen peroxide (H2O2) into oxygen and water. In animals, they are common in the liver and kidney cells.
Have you ever wondered what actually holds the cell together and enables it to keep its shape? The shape of a cell is determined by a network of fibers called the cytoskeleton. The most important fibers you’ll need to know are microtubules and microfilaments.
Microtubules, which are made up of the protein tubulin, participate in cellular division and movement. These small fibers are an integral part of three structures: centrioles, cilia, and flagella. We’ve already mentioned that centrioles help chromosomes separate during cell division.Cilia and flagella are threadlike structures best known for their locomotive properties in single-celled organisms. The beating motion of cilia and flagella structures propels these organisms through their watery environments.
The two classic examples of organisms with these structures are the Euglena, which gets about using its whiplike flagellum, and the Paramecium, which is covered in cilia. The rhythmic beating of the Paramecium’s cilia enables it to motor about in waterways, ponds, and microscope slides in your biology lab. You’ve probably already checked these out in lab, but here’s what they look like:
Though we usually associate such structures with microscopic organisms, they aren’t the only ones with cilia and flagella. As you probably know, these structures are also found in certain human cells. For example, the cells lining your respiratory tract possess cilia that sweep constantly back and forth (beating up to 20 times per second), helping to keep dust and unwanted debris from descending into your lungs. And every sperm cell has a flagellum, which enables it to swim through the female reproductive organs to fertilize the waiting ovum.
Microfilaments, like microtubules, are important for movement. These thin, rodlike structures are composed of the protein actin, they are involved in cell mobility, and play a central role in muscle contraction.