Context: The cell membrane is like the bag holding the Jell-O. It encloses the cytoplasm of the cell. It forms a barrier between the cytoplasm and the environment outside the cell. The function of the cell membrane is to protect and support the cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other substances inside or outside the cell. The structure of the cell membrane explains how it can control what enters and leaves the cell. The membrane is composed mainly of two layers of phospholipids. Figure 3.8 shows how the phospholipids are arranged in the cell membrane. Each phospholipid molecule has a head and two tails. The heads are water loving (hydrophilic), and the tails are water fearing (hydrophobic). The water-loving heads are on the outer surfaces of the cell membrane. They point toward the watery cytoplasm within the cell or the watery fluid that surrounds the cell. The water-fearing tails are in the middle of the cell membrane. Hydrophobic molecules like to be near other hydrophobic molecules. They fear being near hydrophilic molecules. The opposite is true of hydrophilic molecules. They like to be near other hydrophilic molecules. They fear being near hydrophobic molecules. These likes and fears explain why some molecules can pass through the cell membrane while others cannot. Hydrophobic molecules can pass through the cell membrane. Thats because they like the hydrophobic interior of the membrane and fear the hydrophilic exterior of the membrane. Hydrophilic molecules cant pass through the cell membrane. Thats because they like the hydrophilic exterior of the membrane and fear the hydrophobic interior of the membrane. You can see how this works in the video at this link: . MEDIA Click image to the left or use the URL below. URL: Cytoplasm is everything inside the cell membrane (except the nucleus if there is one). It includes the watery, gel-like cytosol. It also includes other structures. The water in the cytoplasm makes up about two-thirds of the cells weight. It gives the cell many of its properties. Why does a cell have cytoplasm? Cytoplasm has several important functions. These include: suspending cell organelles. pushing against the cell membrane to help the cell keep its shape. providing a site for many of the biochemical reactions of the cell. Crisscrossing the cytoplasm is a structure called the cytoskeleton. It consists of thread-like filaments and tubules. The cytoskeleton is like a cellular skeleton. It helps the cell keep its shape. It also holds cell organelles in place within the cytoplasm. Figure 3.9 shows several cells. In the figure, the filaments of their cytoskeletons are colored green. The tubules are colored red. The blue dots are the cell nuclei. Eukaryotic cells contain a nucleus and several other types of organelles. These structures carry out many vital cell functions. The nucleus is the largest organelle in a eukaryotic cell. It contains most of the cells DNA. DNA, in turn, contains the genetic code. This code tells the cell which proteins to make and when to make them. You can see a diagram of a cell nucleus in Figure 3.10. Besides DNA, the nucleus contains a structure called a nucleolus. Its function is to form ribosomes. The membrane enclosing the nucleus is called the nuclear envelope. The envelope has tiny holes, or pores, in it. The pores allow substances to move into and out of the nucleus. The mitochondrion (mitochondria, plural) is an organelle that makes energy available to the cell. Its like the power plant of a cell. It uses energy in glucose to make smaller molecules called ATP (adenosine triphosphate). ATP packages energy in smaller amounts that cells can use. Think about buying a bottle of water from a vending machine. The machine takes only quarters, and you have only dollar bills. The dollar bills wont work in the vending machine. Glucose is like a dollar bill. It contains too much energy for cells to use. ATP is like a quarter. It contains just the right amount of energy for use by cells. A ribosome is a small 
 Question: ______threadlike filaments and tubules that crisscross the cytoplasm
Output: cytoskeleton