Biology Notes - Unit 1: Cell - Lesson 1 PDF

BIOLOGY NOTES Unit 1: CELL Lesson 1 Size of the Cell to diffuse to the center and the wastes...

BIOLOGY NOTES Unit 1: CELL Lesson 1 Size of the Cell to diffuse to the center and the wastes to be eliminated. As the size of a cell - Cells are generally small. Although they increases, its volume also increases at a are found at the lower level in the greater rate than its surface area, hierarchy of the biological organization, which then decreases its surface area life already exists in them. Most cells are to volume ratio. On the other hand, if far smaller than 1 mm, and some are even the size of the cell is smaller, its as small as 1 µm (as shown in Fig. 1.1.1). volume decreases at a slower rate than Subcellular structures and macromolecules its surface area, which then increases that are smaller than a micrometer are its surface area to volume ratio. The measured in terms of nanometers. Because surface area of bigger cells becomes of this, the cell can only be viewed under inadequate for the exchange of the microscope to magnify its size in the materials that their volume requires. field of view. Surface area to volume ratio is FIG 1.1.1. important that it favors a smaller cell size in terms of the efficiency of the 1.1.1 movement of molecules. Having a large surface area to volume ratio is important to the functioning of the cells as most processes require molecules obtained from external sources and involve the production of wastes. FIG 1.1.2. - The diversity of cell sizes ranges from the smallest bacterial cell to the largest avian egg. Note that there are some cells that can be seen with a human's naked eye. Surface Area to Volume Ratio in Cells The cell itself is a system. The exchange of nutrients and metabolic wastes happens through its surface. A cell needs a surface area large enough relative to its volume to The small size of cells provides them with a allow adequate nutrients to enter and relatively higher ratio of surface area to sufficient waste to be eliminated. Small cells volume. A living entity may maintain the are likely to have more available surface area same total volume, but having small cells for the movement of these molecules. Bigger cells, by contrast, have a larger volume will allow it to have a greater surface area relative to their surface area that it gets available for the movement of molecules. difficult for nutrients  BIOLOGY NOTES Unit 1: CELL Lesson 1 Shown in Fig. 1.1.2 is a comparison of the surface area to volume ratio of a small box (one-unit dimension) and a big box (five-unit dimension). The larger box is shown to have a smaller surface area to volume ratio compared with the smaller one. Despite its relatively larger volume, the bigger box would be deemed inefficient in terms of the movement of molecules because of the consequent decrease in the Regulation of the Internal Environment surface area. By contrast, the formation of smaller unit boxes - An organism’s ability to keep a constant internal from the same large box will state is called homeostasis. Homeostasis involves constant adjustments as the internal and external maintain volume but significantly conditions of the cell continuously change. increase the total surface area. It Maintenance of these conditions, usually at a normal should also be emphasized that an or optimal level, is important because most cells of an organism can still increase its total organism require a specific set of conditions to volume (as shown during growth and function normally. If conditions go beyond a particular development) while maintaining a optimal range, some cells would cease to function properly. smaller cell size along the process. - As an example in humans, cells will only function General Functions of the Cell normally at a constant internal temperature of 37 °C. During extremely cold weather, some cells, - The cell is a basic feature in any particularly the fibers of skeletal muscles, may be living organism, from the unicellular stimulated to contract involuntarily which results in shivering. This bacteria, protists, and yeast, to more mechanism allows the body to generate heat (or complex multicellular forms such as thermogenesis) during cold weather to plants and animals. It is the smallest allow bodily chemical reactions to normally take place. unit that exhibits different attributes On the other hand, perspiration of life. In multicellular organisms, cells involves water evaporation through the skin to cool are more specialized—they are down the body temperature during hot seasons. The sweat glands in the skin release water committed to performing particular that covers the body and instantly serve functions (such as in Fig. 1.1.3) that as the cooling system to remove excess heat in the contribute to the overall maintenance body. Homeostasis is exhibited in the of the interacting systems in these attempt of the body to use its cells to return the organisms. temperature to a normal range.  BIOLOGY NOTES Unit 1: CELL Lesson 1 Acquisition and Utilization of Energy - Cells acquire energy from the nutrients in food that organisms consume. This chemical energy is stored in the bonds present in food molecules, and it will be converted by the cells into more usable forms. Energy is needed by cells to drive most of the chemical reactions and other functions in the organism’s body. For example, energy is constantly needed for the heart muscles to continuously pump blood throughout our bodies. Energy is also needed Protection and Support in other bodily functions such as the breakdown of macromolecules during digestion, the contraction of - Cells protect and support their internal skeletal muscles to initiate motion, and for the cells environment through their cellular membranes. The of the nervous system to conduct information. Cells, chemicals outside the cells could affect or influence too, invest energy to release more energy from the normal cellular processes. Cells may form linings of food molecules they metabolize. organs to serve as the first line of defense from the external environment. In addition, some Responsiveness to Their Environment specialized cells, particularly immune cells in complex multicellular animals, also impart protection - The cell’s environment changes constantly and against pathogens and other foreign bodies that rapidly. To survive, cells also respond to various may enter the general circulation. signals that indicate any form of change in their environment. These changes may include the shift in the activities of enzymatic molecules, chemicals that pass through the cell membrane, and signals to various membrane- transport processes. Responsiveness is related to homeostasis. A cell must first be able to determine the changes that have taken place before deciding the necessary responses that will ultimately result in the maintenance of normal internal conditions. One classical example is the pigmented cells in the skin of humans. Whenever these cells are exposed to ultraviolet radiation from the sun, they synthesize and release more pigment to impart protection to the underlying cells especially UV History of the Development of Cell Theory radiation that can damage DNA. - Historically, the cell theory was proposed to disprove the spontaneous generation theory. It is now universally accepted even though it is still a theory. But how does cell theory develop from a mere idea to how it is being widely discussed and accepted today? BIOLOGY NOTES Unit 1: CELL Lesson 1 BIOLOGY NOTES Unit 1: CELL Lesson 1  BIOLOGY NOTES Unit 1: CELL Lesson 1.2 Components of the Cell: Major Parts of the Cell One of the principles of cell theory states that How do the major cellular structures help in the all living organisms are composed of at least one different cellular processes? cell. Cells are the fundamental units of structure and function in all living organisms. A The Plasma Membrane lthough microscopic in size, cells are composed of structures that are complex in function. The Structure of the Plasma Membrane From the warmup activity above, the words you unlocked consist of some of the important Every cell has its own plasma membrane or cell structures in cells, a general diagram of which membrane. Plasma membranes mostly consist of is shown in Fig. 1.2.1. Cells are divided into two phospholipids, which is a form of lipid molecule. Each major regions—the cell membrane and the phospholipid of the cell membrane is composed of protoplast. The protoplast, likewise, is divided two distinct regions. The first part is the “head” into two compartments—the cytoplasm and the with a negatively charged phosphate group with nucleus. Nucleus serves as the polar covalent bonds. The second part is made up of control center of the cell, whereas the the two nonpolar fatty acid “tails”. These cytoplasm provides a vessel where the phospholipids group together to form a two-layered organelles can be suspended, alongside the barrier called a phospholipid bilayer. In some ways, cytoskeleton and the semi-fluid cytosol. the structure of the plasma membrane can be Organelles are structures in cells that perform compared to a sandwich. The layer that comprises specialized functions, which will be further the heads resembles the bread, and the layer that elaborated in the discussion next chapter. This comprises the tails resembles the cheese, meat, and lesson will focus on how the cell membrane, tomatoes. cytoplasm, cytoskeleton, and cell wall contributes to normal cell functioning.  BIOLOGY NOTES Unit 1: CELL Lesson 1.2 As you can see in Fig. 1.2.2, the hydrophilic (water- The Roles of the Lipid Bilayer loving) head region of the phospholipid bilayer faces outward. This orientation exposes it to the - The plasma membrane serves as the primary wall aqueous solutions on the outside and inside of the or barrier of the cell from its external cell. By contrast, the hydrophobic (water-fearing) environment. As previously mentioned, it is tails are oriented inward and are together shielded semipermeable in nature. It means it only allows from water. The middle portion of the phospholipid certain molecules to pass through. The molecules bilayer makes the plasma membrane selectively that are free to move across the membrane permeable. This means that not all substances can generally require no energy to pass through, i.e., pass through this membrane. Lipids and small, they can move passively. Generally, nonpolar or nonpolar molecules such as oxygen and carbon hydrophobic molecules can readily diffuse through dioxide can freely pass into and out of the cell. the middle portion of the phospholipid bilayer at However, the chemical nature of the tails block no energy cost. Polar or hydrophilic molecules, ions and other charged polar molecules like however, require energy expenditure for them to glucose. be transported because they are chemically incompatible with the center of the phospholipid A side from having a phospholipid bilayer, the cell bilayer. Small, uncharged molecules such as carbon membrane is also composed of proteins and other dioxide, oxygen, glycerol, and alcohol can move molecules that are free to drift laterally within freely across the membrane. Charged ions, glucose, the bilayer. The phospholipid bilayer is often fructose, amino acids, and proteins cannot readily described as a fluid mosaic model. According to cross the membrane. Water, though polar in this model, proteins, carbohydrates, and nature, can pass through the membrane because of cholesterol molecules are embedded in the its small molecular size. membrane so that they look like a mosaic. This characteristic is emphasized because of the The Roles of Membrane Proteins fluidity that the lipid bilayer imparts. This fluidity is due to the natural viscosity of the membrane, - Most molecules that cannot readily pass through which allows the protein molecules to freely float the lipid bilayer do so with the aid of the proteins and drift in the membrane. This fluidity results embedded in the membrane. Some of these serve from the presence of cholesterol molecules and as channel proteins (as shown in Fig. 1.2.3.) that the kinks (slightly bent portions) in the fatty acid allow transport of certain molecules through the tails of the phospholipids. membrane. It forms a channel or passageway that simply allows certain substances to move from one The aforementioned membrane proteins also aid in side to the other. regulating the molecules that pass through the bilayer. Some of them form channels or tunnels that shield ions and polar molecules as they pass through the hydrophobic center of the membrane. Phospholipids and proteins that have carbohydrate chains attached are called glycolipids and glycoproteins, respectively.  BIOLOGY NOTES Unit 1: CELL Lesson 1.2 chemically incompatible with the center of the phospholipid bilayer. Small, uncharged molecules such as carbon dioxide, oxygen, glycerol, and alcohol can move freely across the membrane. Charged ions, glucose, fructose, amino acids, and proteins cannot readily cross the membrane. Water, though polar in nature, can pass through the membrane because of its small molecular size. The Roles of Membrane Proteins Most molecules that cannot readily pass Microtubules through the lipid bilayer do so with the aid of the proteins embedded in the membrane. Some of these - Microtubules are straight, long, and hollow cylinders serve as channel proteins (as shown in Fig. 1.2.3.) that that consist of globular proteins called tubulin. There allow transport of certain molecules through the are two types of tubulin, namely, the alpha-tubulin and membrane. It forms a channel or passageway that beta-tubulin (as shown in Fig. 1.2.5). In terms of simply allows certain substances to move from one side assembly, alpha and beta-tubulin proteins come to the other. Another function of membrane proteins together by forming dimers. These dimers are in the plasma membrane is that they also serve as cell arranged in rows. Microtubules have 13 rows of tubulin recognition proteins, like glycoproteins. These proteins dimers, surrounding an empty central core. Generally, are helpful in recognizing the presence of pathogens so these cytoskeletal elements grow out and are that certain immune responses can be elicited. organized by microtubule-organizing centers or MTOCs. The MTOCs in animal cells are in the form of Lastly, membrane proteins may also serve as receptor centrosomes. proteins. These proteins have a shape that allows only specific molecules to bind to them. When a molecule - The microtubules form a dynamic scaffolding for binds to a receptor protein, the protein changes its many cellular processes that quickly shape which brings about particular cellular responses. assemble when needed and disassembled when This is very much evident in the coordination of the unneeded. They radiate from the centrosome to body’s organs. For example, maintain the shape of the cell and to serve as tracks the liver stores glucose after it is signaled to do so by where organelles can move. Microtubules are also the hormone insulin. helpful during cell division. They assemble into structures called spindle fibers that separate the The Cytoskeleton duplicated chromosomes then disassemble afterward. Cells are constantly exposed to v arious physical forces. All eukaryotic cells have a system of interconnected protein filaments from their nucleus to the plasma membrane. This, collectively, is known as the cytoskeleton (as shown in Fig. 1.2.4), which extends throughout the cytoplasm (and outside the cytoplasm in some cells). This network of protein fibers serves as a structural framework that has diverse functions. The cytoskeletal elements of cells include the microtubules, microfilaments, and intermediate filaments.  BIOLOGY NOTES Unit 1: CELL Lesson 1.2 Microfilaments Microfilaments are long and flexible fibers composed of the protein actin which is the thinnest component of the cytoskeleton (shown in Fig. 1.2.4). They occur in a bundle of meshlike networks. Each microfilament contains two chains of globular actin monomers twisted into a helical manner. Microfilaments provide structural support as a dense, complex web, to which they are anchored by special proteins. They can also facilitate cell movement. They can move the cell and its organelles by interacting with motor molecules such as that in muscle proteins. Motor molecules require energy to pull the microfilaments and trigger movement. Intermediate Filaments Intermediate filaments, as the name implies, have support and movement. Furthermore, it maintains characteristics that lie between that of cell shape and it allows the cell and its organelles microfilaments and microtubules. Intermediate to move. The cytoskeleton is also known for being filaments are made of a variety of proteins. They form dynamic, i.e., it can rearrange its protein a ropelike assembly of fibrous polypeptides. components whenever there is a need to respond to the changes in the internal and external Different types of intermediate filaments support environments. An example of this is that the cells and tissues making them the most stable element cytoskeletal elements of muscle cells or fibers can of the cytoskeleton. Some intermediate filaments change orientation to allow the contraction and support the nuclear envelope by anchoring the nucleus, relaxation of entire muscle tissue. while others support the plasma membrane. They also impart great strength to skin cells because one of The Cytoplasm their components include the structural protein keratin. The entire region of the cell between the nucleus and the plasma membrane is the cytoplasm (shown The General Role of the Cytoskeleton in Fig. 1.2.7.). In eukaryotic cells, it is in the cytoplasm that all the various subcellular If compared to a factory building, the cytoskeleton structures are suspended. The plasma membrane serves as the foundation and beams that support the encloses it together with all other cellular whole structure of the factory. A view of the contents. The cytoplasm is composed of the cytoskeleton of a cell by using fluorescent dye is shown cytosol, a semifluid solution that consists of water in Fig.1.2.6.. The cytoskeleton also supports the and inorganic and organic molecules. Organic structure of the cell itself and provides anchorage and molecules include a variety of enzymes that are reinforcement for many organelles. It also serves as involved in increasing the rate of metabolic both the skeleton and the “muscles” of the cell, which reactions. provides  BIOLOGY NOTES Unit 1: CELL Lesson 1.2 The Role of the Cytoplasm The Role of the Cell Wall If the factory building has a floor where all raw The cell wall is one of the features that materials, machines, other equipment, and distinguish plant cells from animal cells. As employees work and manufacture products, the mentioned, it is also present in prokaryotic cell has its cytoplasm to serve as a vessel for cells. It is a rigid structure that provides various functions to take place. The role of the protection and structural support to cells. A cytoplasm is simply to put all organellesand cell wall is permeable to water which is why it is other cellular contents in place. It is where all easier for dissolved substances to cross it to other molecules inside the cell are suspended, the plasma membrane. Although permeable to most of which are substrates for enzymatic water, the cell wall maintains cell shape, and it reactions to take place. keeps cells from absorbing too much water so that they will not burst especially in plant The Cell Wall cells. The Structure of the Cell Wall The cell wall is a rigid structure that surrounds most prokaryotic cells, including archaea and bacteria. The cell wall of most species of bacteria contains peptidoglycan, a complex of polysaccharides that cannot be found in the cell wall of archaea. Furthermore, cell walls are also present in the cells of fungi and plants in addition to their plasma membrane. They also differ in composition because cell walls of plants consist of cellulose fibers (as shown in Fig. 1.2.8), while that of fungi consists of chitin.