Biology 107 The Cell I. Introduction A. As we learned in the first chapter, the cell is the structural and functional unit of life. In this chapter we are going to look at some of the general features of the cell. B. Cells vary greatly in size, length shape and function. Some cells such as the red blood cell are very small. Other cells such as an ovum (female egg) are very large. Some cells are round (a ovum) where others are elongated (muscle cells) and some have very strange shapes such as the neuron or nerve cell. (See the diagrams below.) You do not have to recognize these cells at this point in time – just realize there are differences in shape and function between types of cells. Red Blood Cell: Muscle Cells: Nerve Cells: Cells also vary greatly in function. Red blood cells or erythrocytes carry oxygen and carbon dioxide. Muscle cells contract and thus allow for movement. Neurons allow for the electrochemical conduction or the conduction of nerve impulses. Although these sizes, shapes, and functions vary greatly all cells have certain structural and functional features in common. That is what we are going to discuss now - the structural and functional features common to all cells. C. We will now look at some of the basic features of a generalized animal cell. Remember, humans are animals, therefore we are interested in an animal cell. Keep in mind that there are other types of cells, for example plant cells. The features of animal cells are not necessarily the same as for other types of cells. There are three basic parts to the cell: the plasma membrane or cell membrane, the nucleus, and the cytoplasm. (See figure 2.13 on page 48 as well.) 1. The plasma membrane or cell membrane is a boundary between the intracellular fluid (ICF) and the extracellular fluid (ECF). 2. The nucleus is frequently the large structure centrally located within the cell. It contains the cells genetic material, deoxyribonucleic acid or DNA. DNA controls all cellular activity by coding for protein synthesis (the production of proteins), and because of the function of coding for proteins, the nucleus if frequently referred to as the “control center of the cell”. Recall the importance of proteins. Many functions of proteins were identified in the Introductory Material. 3. Surrounding the nucleus and enclosed by the plasma membrane is the cytoplasm. The cytoplasm consists of two components: the organelles and the cytosol. a. Organelles are “tiny organs". Just as the human body has organs with specific functions, the cell has specific structures that have very distinct functions. These structures are called organelles. There are many specific organelles within the cell, however we will only be discussing two of the organelles at this time. i. Vesicles: Vesicles a sacs with a membrane surrounding them that are used to transport materials within the cell as well as into or out of the cell. Materials that are released by a cell, such as hormones or neurotransmitters are released from a vesicle. Many things that are taken in to the cell are taken in via a vesicle II. b. Mitochondrion (Mitochondria – pleural). Mitochondria are small organelles of great importance in the cell. Mitochondria carry out chemical reactions that break down nutrients to release ATP. Recall ATP is the bodies’ usable source of energy. Thus mitochondria are frequently referred to as the powerhouse of the cell. Cytosol is the fluid within the cytoplasm that suspends the organelles. Do not confuse the cytosol and the cytoplasm D. Extracellular Fluid. The extracellular fluid, as previously mentioned is the fluid surrounding the outside (extra) of the cell. II. The Plasma (Cell) Membrane. Functions: The plasma membrane has several functions. First, the plasma membrane separates the cellular contents from the extracellular fluid. Second, the plasma membrane provides for chemical receptors. (Remember receptors from the Introductory Material.) Receptors are the sites to which various chemical messengers bind to in order to alter cellular activity. Here is a diagram of a cell secreting a messenger (such as a hormone) and two other cells with receptors. The messenger will attach to the receptor and alter the activity of the cell. NOTE: The chemical messenger is stored within a vesicle within the cell before it is released. Third, the plasma membrane is the location of the identification markers. (Again remember the Introductory Material). Identification markers will allow for the differentiation of different cell types. For example identification markers differentiate a kidney cell from a bone cell. Identification markers also allow for self vs. non-self; that is your bodies individual cells have ID markers that identify it as belonging to your body. When structures enter the body that don’t have a “self ID’ marker, the body recognizes it as foreign and attacks it. So for example a virus will enter into the body. It has different identification markers than your cells; thus the body recognizes the virus as foreign and attacks it. Finally, the plasma membrane regulates the passage of materials into and out of the cells. Certain substances need to enter into the cell (nutrients such as glucose (blood sugar), oxygen, etc.). Other substances need to leave the cell (CO2, waste products, etc.) Some substances should not leave the cell (organelles) while others shouldn’t enter (other cells, various ions). The plasma membrane selects what is permeable (can cross the cell membrane) and what is impermeable (can’t cross the cell membrane). Because some materials are permeable to the plasma membrane and others are not, the plasma membrane is said to be selectively permeable. Structure of the plasma membrane. (See figure 2.14 on page 49.) The plasma membrane is said to be a bi-layer lipid membrane. That is, a two layered membrane comprised mostly of lipids. The types of lipids that make of the membrane are phospholipids (the primary component of the plasma membrane) and cholesterol. In addition to the lipids, the membrane contains proteins and carbohydrates. Movement across the Plasma Membrane. As stated earlier some substances are permeable and others are impermeable. Of those substances that are permeable there are two general processes by which substances can cross the plasma membrane. They are Active Processes and Passive Processes. Passive processes usually involve movement “down a concentration gradient”, that is the substance moves from an area of high concentration to an area of low concentration. The cell does not need to expend any cellular energy in the form of ATP for this process to occur. In Active processes, the substance is moving ‘up a concentration gradient”, that is from an area of low concentration to an area of high concentration. There are many different types of active and passive processes. The book mentions some of them. I will not be discussing them here. As they become relevant I will briefly mention them.
