Chapter 3 - Cell Processes and Energy

Section 1 - Chemical Compounds in Cells


  An element is any substance that cannot be broken down into simpler substances. The smallest unit of an element is called an atom. The elements found in living things include carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. When two or more elements combine chemically, they form a compound. The smallest unit of many compounds is called a molecule.

  Water is a compound made up of hydrogen and oxygen. Most chemical reactions within cells could not take place without water. Water also helps cells keep their size and shape and keeps the temperature of cells from changing rapidly.

  Many of the compounds found in living things contain the element carbon. Most compounds that contain carbon are called organic compounds. Carbohydrates, proteins, lipids, and nucleic acids are important groups of organic compounds in living things. Compounds that do not contain the element carbon are called inorganic compounds.   A carbohydrate is an energy-rich organic compound made of the elements carbon, hydrogen, and oxygen. Sugars and starches are examples of carbohydrates. Carbohydrates are important components of some cell parts, including cell walls and cell membranes.

  Fats, oils, and waxes are all lipids. Lipids are energy-rich organic compounds made of carbon, hydrogen, and oxygen. Lipids contain more energy than carbohydrates. Cells store energy in lipids for later use.

  Proteins are large organic molecules made of carbon, hydrogen, oxygen, nitrogen, and, in some cases, sulfur. Protein molecules are made up of smaller molecules called amino acids. Proteins make up much of the structure of cells. An enzyme is a type of protein that speeds up a chemical reaction in a living thing. Without enzymes, many chemical reactions that are necessary for life would either take too long or not occur at all.

  Nucleic acids are very long organic molecules made of carbon, oxygen, hydrogen, nitrogen, and phosphorus. Nucleic acids contain the instructions that cells need to carry out all the functions of life. There are two kinds of nucleic acids: DNA and RNA. Deoxyribonucleic acid, or DNA, is the genetic material that carries information about an organism that is passed from parent to offspring and directs all of the cell's functions. Ribonucleic acid, or RNA, plays an important role in the production of proteins. RNA is found in the cytoplasm as well as in the nucleus.




Section 2 - The Cell in Its Environment


  The cell membrane is selectively permeable, which means that some substances can pass through it while others cannot. Oxygen, food molecules, and waste products all must pass through the cell membrane. Substances that can move into and out of a cell do so by one of three methods: diffusion, osmosis, or active transport.

  Diffusion is the main method by which small molecules move across the cell membrane. Diffusion is the process by which molecules tend to move from an area of higher concentration to an area of lower concentration. The concentration of a substance is the amount of the substance in a given volume. Diffusion is caused by molecules moving and colliding. The collisions cause the molecules to push away from one another and spread out. Molecules diffuse through the cell membrane into a cell when there is a higher concentration of the molecules outside the cell than inside the cell.

  The diffusion of water molecules through a selectively permeable membrane is called osmosis. Because cells cannot function properly without adequate water, many cellular processes depend on osmosis. In osmosis, water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.

  The movement of dissolved materials through a cell membrane without using cellular energy is called passive transport. Diffusion and osmosis are both types of passive transport. When a cell needs to take in materials that are in higher concentration inside the cell than outside the cell, the movement of the materials requires energy. Active transport is the movement of materials through a cell membrane using cellular energy. The main difference between passive transport and active transport is that active transport requires the cell to use its own energy while passive transport does not. Cells have several ways of moving materials by active transport. In one method, transport proteins in the cell membrane "pick up” molecules outside the cell and carry them in. Another method of active transport is engulfing, in which the cell membrane wraps around, or engulfs, a particle and forms a vacuole within the cell.

  Most cells are very small. One reason is related to the fact that all materials move into and out of cells through the cell membrane. Once a molecule enters a cell, it is carried to its destination by a stream of moving cytoplasm. In a very large cell, streams of cytoplasm must travel farther to carry materials from the cell membrane to all parts of the cell.




Section 3 - Photosynthesis


  The sun is the source of energy for most living things. All cells need energy to carry out their functions. The process by which a cell captures the energy in sunlight and uses it to make food is called photosynthesis.

  Nearly all living things obtain energy either directly or indirectly from the energy of sunlight captured during photosynthesis. Plants, such as grass, use energy from the sun to make their own food through the process of photosynthesis. An organism that makes its own food is called an autotroph. An organism that cannot make its own food is called a heterotroph. Many heterotrophs obtain food by eating other organisms.

  Photosynthesis is a complex process. During photosynthesis, plants and some other organisms use energy from the sun to convert carbon dioxide and water into oxygen and sugars. Photosynthesis takes place in two stages: (1) capturing the sun's energy and (2) producing sugars. In plants, this energy-capturing process occurs mostly in the leaves. The chloroplasts in plant cells give plants their green color. The green color comes from pigments, colored chemical compounds that absorb light. The main photosynthetic pigment in chloroplasts is chlorophyll. Chlorophyll captures light energy and uses it to power the second stage of photosynthesis to produce sugars. The cell needs two raw materials for this stage: water (H2O) and carbon dioxide (CO2). Plant roots absorb water from the soil, and the water then moves up to the leaves. Carbon dioxide enters the plant through small openings on the undersides of the leaves called stomata. Once in the leaves, the water and carbon dioxide move into the chloroplasts.

  Inside the chloroplasts, the water and carbon dioxide undergo a complex series of chemical reactions and produce two important products of photosynthesis: sugar and oxygen. Plant cells use sugar for food and to make other compounds, such as cellulose. Plant cells also store sugar for later use. Oxygen exits the leaf through the stomata. Almost all of the oxygen in Earth's atmosphere was produced by living things through photosynthesis. The events of photosynthesis can be summed up by the following chemical equation:





Section 4 - Respiration


  Cells store and use energy in a way that is similar to the way you deposit and withdraw money from a savings account. When you eat a meal, you add to your body's energy savings account. When your cells need energy, they make a withdrawal by breaking down the carbohydrates in food to release energy.

  The process by which cells obtain energy from glucose (a type of sugar) is called respiration. During respiration, cells break down simple food molecules such as sugar and release the energy they contain. Because living things need a continuous supply of energy, the cells of all living things carry out respiration continuously. The term respiration also is used to mean breathing, that is, moving air in and out of your lungs. To avoid confusion, the respiration process that takes place inside cells sometimes is called cellular respiration. The two kinds of respiration are related. Breathing brings oxygen into your lungs, and oxygen is necessary for cellular respiration to occur in most cells.

  Like photosynthesis, respiration is a two-stage process. The first stage takes place in the cytoplasm of the organism's cells. There, glucose molecules are broken down into smaller molecules. Oxygen is not involved in this stage of respiration, and only a small amount of energy is released. The second stage of respiration takes place in the mitochondria. There, the small molecules are broken down into even smaller molecules. These chemical reactions require oxygen, and a great deal of energy is released. Two other products of respiration are carbon dioxide and water.

  Photosynthesis and respiration can be thought of as opposite processes. Together, these two processes form a cycle that keeps the levels of oxygen and carbon dioxide fairly constant in the atmosphere.

  Some cells obtain their energy through fermentation, an energy- releasing process that does not require oxygen. Fermentation provides energy for cells without using oxygen. One type of fermentation occurs in yeast and some other single-celled organisms. This process is sometimes called alcoholic fermentation because alcohol is one of the products made when these organisms break down sugars. Another type of fermentation takes place at times in your body when your muscles run out of oxygen-for example, when you've run as fast as you could for as long as you could. One product of this type of fermentation is an acid known as lactic acid. When lactic acid builds up, your muscles feel weak and sore.




Section 5 - Cell Division


  The regular sequence of growth and division that cells undergo is known as the cell cycle. The cell cycle is divided into three main stages.

  The first stage of the cell cycle is called interphase.During interphase, the cell grows, makes a copy of its DNA, and prepares to divide into two cells. During the first part of interphase, the cell grows to full size and produces all the structures it needs. In the next part of interphase, the cell makes an exact copy of the DNA in its nucleus in a process called replication. At the end of DNA replication, the cell contains two identical sets of DNA.

  Once interphase is complete, the second stage of the cell cycle begins. Mitosis is the stage during which the cell's nucleus divides into two new nuclei. During mitosis, one copy of the DNA is distributed into each of the two daughter cells. Scientists divide mitosis into four parts, or phases: prophase, metaphase, anaphase, and telophase. During prophase, the threadlike chromatin in the cell's nucleus condenses to form double-rod structures called chromosomes. Each identical rod in a chromosome is called a chromatid. The two chromatids are held together by a structure called a centromere. As the cell progresses through metaphase, anaphase, and telophase, the chromatids separate from each other and move to opposite ends of the cell. Then two nuclei form around the chromatids at the two ends of the cell.

  After mitosis, the final stage of the cell cycle, called cytokinesis, completes the process of cell division. During cytokinesis, the cytoplasm divides, distributing the organelles into each of the two new cells. Each daughter cell has the same number of chromosomes as the original parent cell. At the end of cytokinesis, each cell enters interphase, and the cycle begins again. The length of each stage and cell cycle varies, depending on the type of cell.

  DNA replication ensures that each daughter cell will have all of the genetic information it needs to carry out its activities. The two sides of the DNA ladder are made up of alternating sugar and phosphate molecules. Each rung of the DNA ladder is made up of a pair of molecules called nitrogen bases. There are four kinds of nitrogen bases: adenine, thymine, guanine, and cytosine. Adenine only pairs with thymine, and guanine only pairs with cytosine. DNA replication begins when the two sides of the DNA molecule unwind and separate. Next, nitrogen bases that are floating in the nucleus pair up with the bases on each half of the DNA molecule. Because of the way in which the nitrogen bases pair with one another, the order of the bases in each new DNA molecule exactly matches the order in the original DNA molecule. Once the new bases are attached, two new DNA molecules are formed.



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