Which organelles contribute to synthesizing protein hormones and packaging them into secretory vesicles?

Which organelles contribute to synthesizing protein hormones and packaging them into secretory vesicles?

The ribosomes on the rough endoplasmic reticulum, and Golgi complex contribute to the synthesis of protein hormones and packaging them into secretory vesicles.

 

What happens on the mitochondria cristae and in the mitochondria matrix?

 

What happens on the cristae and in the matrix of mitochondria?

 

Enzymes are located on the cristae and in the matrix of the mitochondria that catalyze the chemical reactions that are a part of the aerobic phase of cellular respiration that supplies most of the cell's ATP.

 

Define an organelle

An organelle is a specialized intracellular structure with a characteristic shape and present to perform specific functions in cellular growth, maintenance, and reproduction.

 

Which organelles are surrounded by a membrane and which are not?

The centrosome, cilia, flagella, ribosomes, and proteasomes are organelles not surrounded by a membrane. The rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, mitochondria and nucleus are organelles that are surrounded by a membrane.

 

What are some of the chemicals present in cytosol?

What is the function of cytosol?

What are some of the chemicals present in cytosol?

The chemicals present in cytosol include water, various ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, waste products, and possibly some aggregates of stored organic molecules.

What is the function of cytosol?

The function of cytosol is to serve as location for many chemical reactions required for cell existence.

 List the three main parts of a cell and explain their functions

The three main/basic parts of the cell are:

1. Cell Membrane (Plasma Membrane)
2. Cytoplasm
3. Nucleus

Explanation:

Each cell is surrounded by a lipid-rich Cell membrane (also called the Plasma Membrane) that forms a boundary between the cell and its environment.

The membrane encloses the Cytoplasm, which includes all cell contents (except the Nucleus, in cells that have one). Cytosol is the fluid of Cytoplasm.

Nucleus is the central part of an atom, and the process of milk production and secretion begins here; the organelle that contains DNA in eukaryotic cells.

List the three main parts of a cell and explain their functions

 

1. Plasma membrane - a selective barrier that surrounds the cell, separating the outside of the cell from the internal components of the cell and establishes and maintains an appropriate environment for normal cellular activities.
2.Cytoplasm - cellular contents between the plasma membrane and nucleus consisting of the cytosol that suspends particles and dissolves solutes and the various cellular organelles responsible for conducting specific cellular functions.
3. Nucleus - large cellular organelle that houses most of the cells' DNA and functions as the primary control center for heredity and cell structure and function.

 

 

Study Guide-The Cellular Level

STUDY GUIDE-THE CELLULAR LEVEL

1. INTRODUCTION
2. A cell is the basic, living, structural, and functional unit of the body.
3. Cell biology or cytology is the study of cell structure and function.

 

1. PARTS OF A CELL
2. A generalized view of the cell is a composite of many different cells in the body as seen in No single cell includes all of the features seen in the generalized cell.
3. The cell can be divided into three principal parts for ease of study.
4. Plasma (cell) membrane
5. Cytoplasm
6. Cytosol
7. Organelles (except for the nucleus)
8. Nucleus
9. chromosomes
10. genes

 

1. THE PLASMA MEMBRANE
2. The plasma membrane is a flexible, sturdy barrier that surrounds and contains the cytoplasm of the cell.
3. The fluid mosaic model describes its structure.
4. The membrane consists of proteins in a sea of lipids.
5. The Lipid Bilayer
6. The lipid bilayer is the basic framework of the plasma membrane and is made up of three types of lipid molecules: phospholipids, cholesterol, and glycolipids.
7. The bilayer arrangement occurs because the lipids are amphipathic molecules. They have both polar (charged) and nonpolar (uncharged) parts with the polar “head” of the phospholipid pointing out and the nonpolar “tail” pointing toward the center of the membrane.
8. cholesterol molecules are weakly amphipathic
9. Arrangement of Membrane Proteins
10. The membrane proteins are divided into integral and peripheral proteins
11. Integral proteins extend into or across (transmembrane) the entire lipid bilayer among the fatty acid tails of the phospholipid molecules.
12. Peripheral proteins are found at the inner or outer surface of the membrane and can be stripped away from the membrane without disturbing membrane integrity.
13. Integral membrane proteins are amphipathic.
14. Those that stretch across the entire bilayer and project on both sides of the membrane are termed transmembrane proteins.
15. Many integral proteins are glycoproteins.
16. Glycocalyx: formed by the carbohydrate portions of

                  glycolipids and glycoproteins

3. The combined glycoproteins and glycolipids form the glycocalyx which helps cells recognize one another, adhere to one another, and be protected from digestion by enzymes in the extracellular fluid.
4. Functions of Membrane Proteins
5. Membrane proteins vary in different cells and functions as ion channels, carriers (transporters), receptors, enzymes, linkers, and cell-identity markers.
6. The different proteins help to determine many of the functions of the plasma membrane.
7. Membrane Fluidity
8. Membranes are fluid structures, rather like cooking oil, because most of the membrane lipids and many of the membrane proteins easily move in the bilayer.
9. Membrane lipids and proteins are mobile in their own half of the bilayer.
10. Cholesterol serves to stabilize the membrane and reduce membrane fluidity.
11. Membrane Permeability
12. Plasma membranes are selectively permeable, meaning that some things can pass through and others cannot.
13. The lipid bilayer portion of the membrane is permeable to small, nonpolar, uncharged molecules but impermeable to ions and charged or polar molecules. Although thought to be slightly permeable to water, the observed permeability to water is most likely the result of aquaporin channels or proteins imbedded within the plasma membrane that are selective for water molecules.
14. Transmembrane proteins that act as channels or transporters increase the permeability of the membrane to molecules that cannot cross the lipid bilayer.
15. Macromolecules are unable to pass through the plasma membrane except by vesicular transport.
16. Gradients Across the Plasma Membrane
17. A concentration gradient is the difference in the concentration of a chemical between one side of the plasma membrane and the other.
18. Oxygen and sodium ions are more concentrated outside the cell membrane with carbon dioxide and potassium ions more concentrated inside the cell membrane.
19. The inner surface of the membrane is more negatively charged and the outer surface is more positively charged. This sets up an electrical gradient, also called the membrane potential.
20. Maintaining the concentration and electrical gradients are important to the life of the cell.
21. The combined concentration and electrical gradients are called the electrochemical gradient.

 

1. TRANSPORT ACROSS THE PLASMA MEMBRANE
2. Processes to move substances across the cell membrane are essential to the life of the cell.
3. Some substances cross the lipid bilayer while others cross through ion channels.
4. Transport processes that move substances across the cell membrane are either active or passive.
5. Passive processes include simple diffusion, facilitated diffusion, and osmosis, and are driven by concentration gradients
6. Active processes include active transport and vesicular transport, and these require cellular energy.
7. Passive processes
8. The principle of diffusion
   1. Diffusion is the random mixing of particles that occurs in a solution as a result of the kinetic energy of the particles.
   2. Diffusion rate across plasma membranes is influenced by several factors: steepness of the concentration gradient, temperature, mass of the diffusing substance, surface area, and diffusion distance.
9. Simple Diffusion
10. Nonpolar, hydrophobic molecules such as respiratory gases, some lipids, small alcohols, and ammonia can diffuse across the lipid bilayer without the help of transport proteins
11. It is important for gas exchange, absorption of some nutrients, and excretion of some wastes.
12. Facilitated Diffusion

Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can cross the plasma membrane by a passive process called facilitated diffusion. In this process, an integral membrane protein assists a specific substance across the membrane. The integral membrane protein can be either a membrane channel or a carrier.

1. Channel mediated facilitated diffusion: a solute moves down its concentration gradient across the lipid bilayer through a membrane channel.
   • Most membrane channels are ion channels
   • Some membrane channels are gated
2. Carrier mediated facilitated diffusion: a solute binds to a specific transporter on one side of the membrane and is released on the other side after the transporter undergoes a conformational change.
3. Substances that move across the plasma membrane by carrier mediated facilitated diffusion include glucose, fructose, galactose, and some vitamins
4. Osmosis
5. Osmosis is the net movement of a solvent through a selectively permeable membrane, or in living systems, the movement of water (the solute) from an area of higher concentration to an area of lower concentration across the membrane.

1. Water molecules penetrate the membrane by diffusion through the lipid bilayer or through aquaporins, transmembrane proteins that function as water channels.
2. Water moves from an area of lower solute concentration to an area of higher solute concentration. Movement of water can generate hydrostatic pressure.
3. Osmosis occurs only when the membrane is permeable to water but not to certain solutes.
4. Tonicity of a solution relates to how the solution influences the shape of body cells
   1. In an isotonic solution, red blood cells maintain their normal shape.
   2. In a hypotonic solution, red blood cells undergo hemolysis.
   3. In a hypertonic solution, red blood cells undergo cremation.
   4. There are important medical uses of isotonic, hypotonic, and hypertonic solutions.
5. Clinical Connection: Medical Uses of Isotonic, Hypertonic, and Hypotonic Solutions
6. Active Processes

Active Transport: energy is required for the carrier proteins to move solutes across the membrane against the concentration gradient.

1. Primary Active Transport

1. In primary active transport, energy derived from ATP changes the shape of a transporter protein, which pumps a substance across a plasma membrane against its concentration gradient.
2. The most prevalent primary active transport mechanism is the sodium ion/potassium ion pump.
3. Clinical Connection: Digitalis slows the sodium ion-calcium ion antiporters, allowing more calcium to stay inside heart muscle cells, which increases the force of their contraction and thus strengthens the heartbeat.
   2. Secondary Active Transport
4. In secondary active transport, the energy stored in the form of a sodium or hydrogen ion concentration gradient is used to drive other substances against their own concentration gradients.
5. Plasma membranes contain several antiporters and symporters powered by the sodium ion gradient.
   3. Transport in Vesicles
6. Endocytosis

• In endocytosis, materials move into a cell in a vesicle formed from the plasma membrane.
• Receptor-mediated endocytosis is the selective uptake of large molecules and particles by cells.

1. The steps of receptor-mediated endocytosis include binding, vesicle formation, uncoating, fusion and endosome formation, recycling of receptors, degradation in lysosomes, and transcytosis.
2. Viruses can take advantage of this mechanism to enter cells.
3. Clinical connection: Viruses and Receptor-Mediated Endocytosis

• Phagocytosis is the ingestion of solid particles.

1. a) Only a few body cells, termed phagocytes, are able to carry out phagocytosis
2. macrophages and neutrophils

Clinical Connection: Phagocytosis and Microbes

• Pinocytosis is the ingestion of extracellular fluid. Also called bulk phase endocytosis

1. Exocytosis
   • In exocytosis membrane-enclosed structures called secretory vesicles that form inside the cell fuse with the plasma membrane and release their contents into the extracellular fluid
   • Transcytosis is a transport process that includes both endocytosis and exocytosis.

 

1. CYTOPLASM
2. Cytosol, the intracellular fluid, is the semifluid portion of cytoplasm that contains inclusions and dissolved solutes.
3. Cytosol is composed mostly of water, plus proteins, carbohydrates, lipids, and inorganic substances.
4. The chemicals in cytosol are either in solution or in a colloidal (suspended) form.
5. Functionally, cytosol is the medium in which many metabolic reactions occur.
6. The cytoskeleton is a network of protein filaments that extends cytosol
7. The cytoskeleton is a network of several kinds of protein filaments that extend throughout the cytoplasm and provides a structural framework for the cell.
8. It consists of microfilaments, intermediate filaments, and microtubules.

• Most microfilaments are composed of actin and function in movement and mechanical support.
• Intermediate filaments are composed of several different proteins and function in support and to help anchor organelles such as the nucleus.
• Microtubules are composed of a protein called tubulin and help determine cell shape and function in the intracellular transport of organelles and the migration of chromosome during cell division.

1. Organelles: Organelles are specialized structures that have characteristic shapes and perform specific functions in cellular growth, maintenance, and reproduction.
2. Centrosomes are dense areas of cytoplasm containing the centrioles, which are paired cylinders arranged at right angles to one another, and serve as centers for organizing microtubules in interphase cells and the mitotic spindle during cell division.
3. Clinical Connection: Cilia and Smoking
4. Cilia and Flagella
5. Cilia are numerous, short, hairlike projections extending from the surface of a cell and functioning to move materials across the surface of the cell.
6. Flagella are similar to cilia but are much longer; usually moving an entire cell. The only example of a flagellum in the human body is the sperm cell tail.
7. Ribosomes
8. Ribosomes are tiny spheres consisting of ribosomal RNA and several ribosomal proteins; they occur free (singly or in clusters) or together with endoplasmic reticulum.
9. Functionally, ribosomes are the sites of protein synthesis.
10. Endoplasmic Reticulum
11. The endoplasmic reticulum (ER) is a network of membranes that form flattened sacs or tubules called cisterns.
12. Rough ER is continuous with the nuclear membrane and has its outer surface studded with ribosomes.
13. Smooth ER extends from the rough ER to form a network of membrane tubules but does not contain ribosomes on its membrane surface.
14. The ER transports substances, stores newly synthesized molecules, synthesizes and packages molecules, detoxifies chemicals, and releases calcium ions involved in muscle contraction.
15. Clinical Connection: The role of the smooth ER in chemical detoxification has a role in drug tolerance.

 

 

5. Golgi Complex
6. The Golgi complex consists of four to six stacked, flattened membranous sacs (cisterns) referred to as cis, medial, and trans.
7. The the Golgi complex’s principle function is to process, sort, and deliver proteins and lipids to the plasma membrane, lysosomes, and secretory vesicles.
8. Lysosomes
9. Lysosomes are membrane-enclosed vesicles that form in the Golgi complex and contain powerful digestive enzymes.
10. Lysosomes function in intracellular digestion, digestion of worn-out organelles (autophagy), digestion of cellular contents (autolysis) during embryological development, and extracellular digestion.
11. Clinical connection: Tay-Sachs disease is an example of a disorder caused by faulty lysosomes.
12. Perioxosomes
13. Peroxisomes are similar in structure to lysosomes, but are smaller.
14. They contain enzymes (e.g., catalase) that use molecular oxygen to oxidize various organic substances.
15. Proteosomes
16. Proteosomes are structures that destroy unneeded, damaged, or faulty proteins.
17. They contain proteases which cut proteins into small peptides.
18. Clinical Connection: Tay-Sachs Disease: proteosomes are thought to be a factor in several diseases.
19. Mitochondria
20. The mitochondrion is bound by a double membrane. The outer membrane is smooth with the inner membrane arranged in folds called cristae.
21. Mitochondria are the site of ATP production in the cell by the catabolism of nutrient molecules.
22. Plays an important role in apoptosis
23. Mitochondria self-replicate using their own DNA.
24. Mitochondrial DNA (genes) are usually inherited only from the mother.

 

VI. NUCLEUS

The nucleus is usually the most prominent feature of a cell.

1. Most body cells have a single nucleus; some (red blood cells) have none, whereas others (skeletal muscle fibers) have several.
2. The parts of the nucleus include the nuclear envelope which is perforated by channels called nuclear pores, nucleoli, and genetic material (DNA)
   1. nucleoli: function in producing ribosomes. Each nucleolus is simply a cluster of protein, DNA, and RNA; it is not enclosed by a membrane
3. Within the nucleus are the cell’s hereditary units, called genes, which are arranged in single file along chromosomes.
4. Each chromosome is a long molecule of DNA that is coiled together with several proteins.
5. Human somatic cells have 46 chromosomes arranged in 23 pairs.
6. The various levels of DNA packing are represented by nucleosomes, chromatin fibers, loops, chromatids, and chromosomes.
7. The main parts of a cell and their functions.
8. Clinical Connection: Genomics, the study of the genome and its relationship to body function, has the potential for increasing our understanding of normal and abnormal conditions.

 

1. PROTEIN SYNTHESIS
2. Much of the cellular machinery is devoted to synthesizing large numbers of diverse proteins.
3. The proteins determine the physical and chemical characteristics of cells.
4. The instructions for protein synthesis is found in the DNA in the nucleus.
5. Protein synthesis involves transcription and translation.
6. Transcription
7. Transcription is the process by which genetic information encoded in DNA is copied onto a strand of RNA called messenger RNA (mRNA), which directs protein synthesis.
8. Besides serving as the template for the synthesis of mRNA, DNA also synthesizes two other kinds of RNA, ribosomal RNA (rRNA), and transfer RNA (tRNA).
9. tRNA brings in additional amino acids, utilizing binding affinity with its anitcodon region, which interacts with a corresponding region or codon of the strand.
10. Transcription of DNA is catalyzed by RNA polymerase.
    • RNA polymerase uses a region of the mRNA called the promoter to start synthesis of a new strand
    • Transcription of the DNA strand ends at another special nucleotide sequence called a terminator
11. Not all parts of a gene actually code for parts of a protein. For instance, regions within a gene called introns do not code for parts of proteins. Introns are located between regions called exons, which do code for segments of a protein.
12. Translation
13. Translation is the process of reading the mRNA nucleotide sequence to determine the amino acid sequence of the protein.
14. The sequence of translation is as follows.
15. Messenger RNA associated with ribosomes, which consist of tRNA and proteins.
16. Specific amino acids attach to molecules of tRNA. Another portion of the tRNA has a triplet of nitrogenous bases called an anticodon, a codon is a segment of three bases of mRNA.
17. Transfer RNA delivers a specific amino acid to the codon; the ribosome moves along an mRNA strand as amino acids are joined to form a growing polypeptide.
18. Clinical Connection: As a result of recombinant DNA techniques, genetic engineering has arisen; strains of recombinant bacteria produce important therapeutic substances such as human growth hormones, insulin, and vaccines against several viruses.

 

1. CELL DIVISION
2. Cell division is the process by which cells reproduce themselves. It consists of nuclear division (mitosis and meiosis) and cytoplasmic division (cytokinesis).
3. Cell division that results in an increase in body cells is called somatic cell division and involves a nuclear division called mitosis, plus cytokinesis.
4. Cell division that results in the production of sperm and eggs is called reproductive cell division and consists of a nuclear division called meiosis plus cytokinesis.
5. The Cell Cycle in Somatic Cells
6. The cell cycle is an orderly sequence of events by which a cell duplicates its contents and divides in two. It consists of interphase and the mitotic phase.
7. Interphase
8. During interphase the cell carries on every life process except division. Interphase consists of three phases: G₁, S and G₂.

• In the G₁ phase, the cell is metabolically active, duplicating its organelles and cytosolic components except for DNA.
  1. Cells that remain in G1 for a very long time, perhaps destined never to divide again, are said to be in the G0 phase
• In the S phase, chromosomes are replicated.
• In the G₂ phase, cell growth continues and the cell completes its preparation for cell division.

1. A cell in interphase shows a distinct nucleus and the absence of chromosomes.
2. Mitotic Phase
3. The mitotic phase consists of mitosis (or nuclear division) and cytokinesis (or cytoplasmic division).
4. Nuclear division: mitosis

• Mitosis is the distribution of two sets of chromosomes, one set into each of two separate nuclei.
• Stages of mitosis are prophase, metaphase, anaphase, and telophase.

1. During prophase, the chromatin condenses and shortens into chromosomes.
2. During metaphase, the centromeres line up at the exact center of the mitotic spindle, a region called the metaphase plate or equatorial plane region.
3. Anaphase is characterized by the splitting and separation of centromeres and the movement of the two sister chromatids of each pair toward opposite poles of the cell.
4. Telophase begins as soon as chromatid movement stops; the identical sets of chromosomes at opposite poles of the cell uncoil and revert to their threadlike chromatin form, microtubules disappear or change form, a new nuclear envelope forms, new nucleoli appear, and the new mitotic spindle eventually breaks up.
5. Cytoplasmic Division: Cytokinesis

• Cytokinesis is the division of a parent cell’s cytoplasm and organelles. The process begins in late anaphase or early telophase with the formation of a cleavage furrow.
• When cytokinesis is complete, interphase begins.

1. Clinical Connection: Inhibiting the formation of the mitotic spindle has a role in the treatment of cancer.
2. Control of Cell Destiny
3. The three possible destinies of a cell are to remain alive and functioning without dividing, to grow and divide, or to die.
4. Enzymes called cyclin-dependent protein kinase can regulate DNA replication. Turning these on and off is a function of proteins called cyclins.
5. Cell death, a process called apoptosis, is triggered either from outside the cell or from inside the cell due to a “cell-suicide” gene.
6. Necrosis is a pathological cell death due to injury.
7. Clinical Connection: Tumor-suppressor genes can produce proteins that normally inhibit cell division resulting in the uncontrollable cell growth known as cancer.
8. Reproductive cell division
   1. The replication of DNA in Meiosis is similar to Mitosis
   2. Meiosis involves two stages
9. Meiosis I

• The two pairs of sister chromatids pair off to form a tetrad
• During the formed tetrad parts of the sister chromatids of the homologous chromoses is traded, a process called crossing over.
• As a result of crossing over, the resulting sister chromatids are not genetically identical, allowing genetic recombination
• The net result is a haploid cell with only one of the pair of homologous chromosomes, but with paired sister chromatids.

 

 

1. Meiosis II

• The paired sister chromatids making up each homologous chromosome are separated
• The net result of Meiosis II is a haploid cell with one chromatid
• Net result of meiosis is the production of four haploid cells that are genetically different
• Compare mitosis and meiosis

 

IX CELLULAR DIVERSITY

1. Not all cells look alike, nor do they perform identical functional roles in the body.
2. The shapes of cells vary considerably.

 

X CELLS AND AGING

1. Aging is a normal process accompanied by a progressive alteration of the body’s homeostatic adaptive responses; the specialized branch of medicine that deals with the medical problems and care of elderly persons is called geriatrics.
2. The physiological signs of aging are gradual deterioration in function and capacity to respond to environmental stresses.
3. These signs are related to a net decrease in the number of cells in the body and to the remaining cells’ dysfunction.
4. The extracellular components of tissues (e.g., collagen fibers and elastin) also change with age.
5. Clinical Connection: Free Radicals. There are many theories of aging, including genetically programmed cessation of cell division, glucose addition to proteins, free radical reactions, and excessive immune responses, but none successfully answers all experimental objections.
6. Clinical Connection: Progeria and Werner Syndrome are disorders of aging.

 

 

1. DISORDERS: HOMEOSTATIC IMBALANCES
2. Cancer is a group of diseases characterized by uncontrolled cell proliferation.
3. Cells that divide without control develop into a tumor or neoplasm.
4. A cancerous neoplasm is called a malignant tumor or malignancy. It has the ability to undergo metastasis, the spread of cancerous cells to other parts of the body. A benign tumor is a noncancerous growth.
5. Types of Cancer
6. Carcinomas arise from epithelial cells.
7. Melanomas are cancerous growths of melanocytes.
8. Sarcomas arise from muscle cells or connective tissues.
9. Leukemia is a cancer of blood-forming organs.
10. Lymphoma is a cancer of lymphatic tissue.
11. Growth and Spread of Cancer
12. Cancer cells divide rapidly and continuously.
13. They trigger angiogenesis, the growths of new networks of blood vessels.
14. Cancer cells can leave their site of origin and travel to other tissues or organs, a process called metastasis.
15. Causes of Cancer
16. Environmental agents can cause cancer growth. A chemical agent, or radiation that produces cancer, is termed a carcinogen and induces mutations in DNA.
17. Viruses can cause cancer.
18. Cancer-causing genes are known as oncogenes
19. The normal counterparts of oncogenes are called proto-oncogenes; these are found in every cell and carry out normal cellular functions until a malignant change occurs via a mutation.
20. Some cancers may also be caused by genes called anti-oncogenes or tumor-suppressing genes. These genes may produce proteins that normally oppose the action of an oncogene or inhibit cell division.
21. Carcinogenesis is a multistep process involving mutation of oncogenes and anti-oncogenes; as many as 10 distinct mutations may have to accumulate in a cell before it becomes cancerous.
22. Treatment of Cancer
23. Treatment of cancer is difficult because it is not a single disease and because all the cells in a tumor do not behave in the same way.
24. Many cancers are removed surgically.
25. Cancer that is widely distributed throughout the body or exists in organs with essential functions, such as the brain, which might be greatly harmed by surgery, may be treated with chemotherapy and radiation therapy instead.
26. Another potential treatment for cancer that is currently under development is virotherapy, the use of viruses to kill cancer cells.
27. Researchers are also investigating the role of metastasis regulatory genes that control the ability of cancer cells to undergo metastasis. Scientists hope to develop therapeutic drugs that can manipulate these genes and, therefore, block metastasis of cancer cells.

In theory, any chemical reaction can proceed in either direction under the right conditions. 

 

Reactants may synthesize into a product that is later decomposed. 

2.3 Chemical Reactions

Learning Objectives

By the end of this section, you will be able to:

• Distinguish between kinetic and potential energy, and between exergonic and endergonic chemical reactions
• Identify four forms of energy important in human functioning
• Describe the three basic types of chemical reactions
• Identify several factors influencing the rate of chemical reactions

One characteristic of a living organism is metabolism, which is the sum total of all of the chemical reactions that go on to maintain that organism’s health and life. The bonding processes you have learned thus far are anabolic chemical reactions; that is, they form larger molecules from smaller molecules or atoms. But recall that metabolism can proceed in another direction: in catabolic chemical reactions, bonds between components of larger molecules break, releasing smaller molecules or atoms. Both types of reaction involve exchanges not only of matter, but of energy.

The Role of Energy in Chemical Reactions

Chemical reactions require a sufficient amount of energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ones formed. In general, kinetic energy is the form of energy powering any type of matter in motion. Imagine you are building a brick wall. The energy it takes to lift and place one brick atop another is kinetic energy—the energy matter possesses because of its motion. Once the wall is in place, it stores potential energy. Potential energy is the energy of position, or the energy matter possesses because of the positioning or structure of its components. If the brick wall collapses, the stored potential energy is released as kinetic energy as the bricks fall.

In the human body, potential energy is stored in the bonds between atoms and molecules. Chemical energy is the form of potential energy in which energy is stored in chemical bonds. When those bonds are formed, chemical energy is invested, and when they break, chemical energy is released. Notice that chemical energy, like all energy, is neither created nor destroyed; rather, it is converted from one form to another. When you eat an energy bar before heading out the door for a hike, the honey, nuts, and other foods the bar contains are broken down and rearranged by your body into molecules that your muscle cells convert to kinetic energy.

Chemical reactions that release more energy than they absorb are characterized as exergonic. The catabolism of the foods in your energy bar is an example. Some of the chemical energy stored in the bar is absorbed into molecules your body uses for fuel, but some of it is released—for example, as heat. In contrast, chemical reactions that absorb more energy than they release are endergonic. These reactions require energy input, and the resulting molecule stores not only the chemical energy in the original components, but also the energy that fueled the reaction. Because energy is neither created nor destroyed, where does the energy needed for endergonic reactions come from? In many cases, it comes from exergonic reactions.

Forms of Energy Important in Human Functioning

You have already learned that chemical energy is absorbed, stored, and released by chemical bonds. In addition to chemical energy, mechanical, radiant, and electrical energy are important in human functioning.

• Mechanical energy, which is stored in physical systems such as machines, engines, or the human body, directly powers the movement of matter. When you lift a brick into place on a wall, your muscles provide the mechanical energy that moves the brick.
• Radiant energy is energy emitted and transmitted as waves rather than matter. These waves vary in length from long radio waves and microwaves to short gamma waves emitted from decaying atomic nuclei. The full spectrum of radiant energy is referred to as the electromagnetic spectrum. The body uses the ultraviolet energy of sunlight to convert a compound in skin cells to vitamin D, which is essential to human functioning. The human eye evolved to see the wavelengths that comprise the colors of the rainbow, from red to violet, so that range in the spectrum is called “visible light.”
• Electrical energy, supplied by electrolytes in cells and body fluids, contributes to the voltage changes that help transmit impulses in nerve and muscle cells.

Characteristics of Chemical Reactions

All chemical reactions begin with a reactant, the general term for the one or more substances that enter into the reaction. Sodium and chloride ions, for example, are the reactants in the production of table salt. The one or more substances produced by a chemical reaction are called the product.

In chemical reactions, the components of the reactants—the elements involved and the number of atoms of each—are all present in the product(s). Similarly, there is nothing present in the products that are not present in the reactants. This is because chemical reactions are governed by the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

Just as you can express mathematical calculations in equations such as 2 + 7 = 9, you can use chemical equations to show how reactants become products. As in math, chemical equations proceed from left to right, but instead of an equal sign, they employ an arrow or arrows indicating the direction in which the chemical reaction proceeds. For example, the chemical reaction in which one atom of nitrogen and three atoms of hydrogen produce ammonia would be written as N + 3H→NH3. Correspondingly, the breakdown of ammonia into its components would be written as NH3→N + 3H.

Notice that, in the first example, a nitrogen (N) atom and three hydrogen (H) atoms bond to form a compound. This anabolic reaction requires energy, which is then stored within the compound’s bonds. Such reactions are referred to as synthesis reactions. A synthesis reaction is a chemical reaction that results in the synthesis (joining) of components that were formerly separate (Figure 2.12a). Again, nitrogen and hydrogen are reactants in a synthesis reaction that yields ammonia as the product. The general equation for a synthesis reaction is A + B→AB.

Figure 2.12 The Three Fundamental Chemical Reactions The atoms and molecules involved in the three fundamental chemical reactions can be imagined as words.

In the second example, ammonia is catabolized into its smaller components, and the potential energy that had been stored in its bonds is released. Such reactions are referred to as decomposition reactions. A decomposition reaction is a chemical reaction that breaks down or “de-composes” something larger into its constituent parts (see Figure 2.12b). The general equation for a decomposition reaction is: AB→A+B.

An exchange reaction is a chemical reaction in which both synthesis and decomposition occur, chemical bonds are both formed and broken, and chemical energy is absorbed, stored, and released (see Figure 2.12c). The simplest form of an exchange reaction might be: A+BC→AB+C. Notice that, to produce these products, B and C had to break apart in a decomposition reaction, whereas A and B had to bond in a synthesis reaction. A more complex exchange reaction might be:AB+CD→AC+BD. Another example might be: AB+CD→AD+BC.

In theory, any chemical reaction can proceed in either direction under the right conditions. Reactants may synthesize into a product that is later decomposed. Reversibility is also a quality of exchange reactions. For instance, A+BC→AB+C could then reverse to AB+C→A+BC. This reversibility of a chemical reaction is indicated with a double arrow: A+BC⇄AB+C. Still, in the human body, many chemical reactions do proceed in a predictable direction, either one way or the other. You can think of this more predictable path as the path of least resistance because, typically, the alternate direction requires more energy.

Factors Influencing the Rate of Chemical Reactions

If you pour vinegar into baking soda, the reaction is instantaneous; the concoction will bubble and fizz. But many chemical reactions take time. A variety of factors influence the rate of chemical reactions. This section, however, will consider only the most important in human functioning.

Properties of the Reactants

If chemical reactions are to occur quickly, the atoms in the reactants have to have easy access to one another. Thus, the greater the surface area of the reactants, the more readily they will interact. When you pop a cube of cheese into your mouth, you chew it before you swallow it. Among other things, chewing increases the surface area of the food so that digestive chemicals can more easily get at it. As a general rule, gases tend to react faster than liquids or solids, again because it takes energy to separate particles of a substance, and gases by definition already have space between their particles. Similarly, the larger the molecule, the greater the number of total bonds, so reactions involving smaller molecules, with fewer total bonds, would be expected to proceed faster.

In addition, recall that some elements are more reactive than others. Reactions that involve highly reactive elements like hydrogen proceed more quickly than reactions that involve less reactive elements. Reactions involving stable elements like helium are not likely to happen at all.

Temperature

Nearly all chemical reactions occur at a faster rate at higher temperatures. Recall that kinetic energy is the energy of matter in motion. The kinetic energy of subatomic particles increases in response to increases in thermal energy. The higher the temperature, the faster the particles move, and the more likely they are to come in contact and react.

Concentration and Pressure

If just a few people are dancing at a club, they are unlikely to step on each other’s toes. But as more and more people get up to dance—especially if the music is fast—collisions are likely to occur. It is the same with chemical reactions: the more particles present within a given space, the more likely those particles are to bump into one another. This means that chemists can speed up chemical reactions not only by increasing the concentration of particles—the number of particles in the space—but also by decreasing the volume of the space, which would correspondingly increase the pressure. If there were 100 dancers in that club, and the manager abruptly moved the party to a room half the size, the concentration of the dancers would double in the new space, and the likelihood of collisions would increase accordingly.

Enzymes and Other Catalysts

For two chemicals in nature to react with each other they first have to come into contact, and this occurs through random collisions. Because heat helps increase the kinetic energy of atoms, ions, and molecules, it promotes their collision. But in the body, extremely high heat—such as a very high fever—can damage body cells and be life-threatening. On the other hand, normal body temperature is not high enough to promote the chemical reactions that sustain life. That is where catalysts come in.

In chemistry, a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change. You can think of a catalyst as a chemical change agent. They help increase the rate and force at which atoms, ions, and molecules collide, thereby increasing the probability that their valence shell electrons will interact.

The most important catalysts in the human body are enzymes. An enzyme is a catalyst composed of protein or ribonucleic acid (RNA), both of which will be discussed later in this chapter. Like all catalysts, enzymes work by lowering the level of energy that needs to be invested in a chemical reaction. A chemical reaction’s activation energy is the “threshold” level of energy needed to break the bonds in the reactants. Once those bonds are broken, new arrangements can form. Without an enzyme to act as a catalyst, a much larger investment of energy is needed to ignite a chemical reaction (Figure 2.13).

Figure 2.13 Enzymes Enzymes decrease the activation energy required for a given chemical reaction to occur. (a) Without an enzyme, the energy input needed for a reaction to begin is high. (b) With the help of an enzyme, less energy is needed for a reaction to begin.

Enzymes are critical to the body’s healthy functioning. They assist, for example, with the breakdown of food and its conversion to energy. In fact, most of the chemical reactions in the body are facilitated by enzymes.

 

 

2.2 Chemical Bonds - Anatomy and Physiology | OpenStax

11–14 minutes

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Learning Objectives

By the end of this section, you will be able to:

• Explain the relationship between molecules and compounds
• Distinguish between ions, cations, and anions
• Identify the key difference between ionic and covalent bonds
• Distinguish between nonpolar and polar covalent bonds
• Explain how water molecules link via hydrogen bonds

Atoms separated by a great distance cannot link; rather, they must come close enough for the electrons in their valence shells to interact. But do atoms ever actually touch one another? Most physicists would say no, because the negatively charged electrons in their valence shells repel one another. No force within the human body—or anywhere in the natural world—is strong enough to overcome this electrical repulsion. So when you read about atoms linking together or colliding, bear in mind that the atoms are not merging in a physical sense.

Instead, atoms link by forming a chemical bond. A bond is a weak or strong electrical attraction that holds atoms in the same vicinity. The new grouping is typically more stable—less likely to react again—than its component atoms were when they were separate. A more or less stable grouping of two or more atoms held together by chemical bonds is called a molecule. The bonded atoms may be of the same element, as in the case of H₂, which is called molecular hydrogen or hydrogen gas. When a molecule is made up of two or more atoms of different elements, it is called a chemical compound. Thus, a unit of water, or H₂O, is a compound, as is a single molecule of the gas methane, or CH₄.

Three types of chemical bonds are important in human physiology, because they hold together substances that are used by the body for critical aspects of homeostasis, signaling, and energy production, to name just a few important processes. These are ionic bonds, covalent bonds, and hydrogen bonds.

Ions and Ionic Bonds

Recall that an atom typically has the same number of positively charged protons and negatively charged electrons. As long as this situation remains, the atom is electrically neutral. But when an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons, the atom will then become positively or negatively charged. This happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either by gaining electrons to fill a shell that is more than half-full, or by giving away electrons to empty a shell that is less than half-full, thereby leaving the next smaller electron shell as the new, full, valence shell. An atom that has an electrical charge—whether positive or negative—is an ion.

Interactive Link

Visit this website to learn about electrical energy and the attraction/repulsion of charges. What happens to the charged electroscope when a conductor is moved between its plastic sheets, and why?

Potassium (K), for instance, is an important element in all body cells. Its atomic number is 19. It has just one electron in its valence shell. This characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron. (It is easier for potassium to donate one electron than to gain seven electrons.) The loss will cause the positive charge of potassium’s protons to be more influential than the negative charge of potassium’s electrons. In other words, the resulting potassium ion will be slightly positive. A potassium ion is written K⁺, indicating that it has lost a single electron. A positively charged ion is known as a cation.

Now consider fluorine (F), a component of bones and teeth. Its atomic number is nine, and it has seven electrons in its valence shell. Thus, it is highly likely to bond with other atoms in such a way that fluorine accepts one electron (it is easier for fluorine to gain one electron than to donate seven electrons). When it does, its electrons will outnumber its protons by one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride, and is written as F^–. A negatively charged ion is known as an anion.

Atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. A cation that has donated two electrons has a net charge of +2. Using magnesium (Mg) as an example, this can be written Mg⁺⁺ or Mg²⁺. An anion that has accepted two electrons has a net charge of –2. The ionic form of selenium (Se), for example, is typically written Se^2–.

The opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity forming an ionic bond. An ionic bond is an ongoing, close association between ions of opposite charge. The table salt you sprinkle on your food owes its existence to ionic bonding. As shown in Figure 2.8, sodium commonly donates an electron to chlorine, becoming the cation Na⁺. When chlorine accepts the electron, it becomes the chloride anion, Cl^–. With their opposing charges, these two ions strongly attract each other.

Figure 2.8 Ionic Bonding (a) Sodium readily donates the solitary electron in its valence shell to chlorine, which needs only one electron to have a full valence shell. (b) The opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of a bond of attraction called an ionic bond. (c) The attraction of many sodium and chloride ions results in the formation of large groupings called crystals.

Water is an essential component of life because it is able to break the ionic bonds in salts to free the ions. In fact, in biological fluids, most individual atoms exist as ions. These dissolved ions produce electrical charges within the body. The behavior of these ions produces the tracings of heart and brain function observed as waves on an electrocardiogram (EKG or ECG) or an electroencephalogram (EEG). The electrical activity that derives from the interactions of the charged ions is why they are also called electrolytes.

Covalent Bonds

Unlike ionic bonds formed by the attraction between a cation’s positive charge and an anion’s negative charge, molecules formed by a covalent bond share electrons in a mutually stabilizing relationship. Like next-door neighbors whose kids hang out first at one home and then at the other, the atoms do not lose or gain electrons permanently. Instead, the electrons move back and forth between the elements. Because of the close sharing of pairs of electrons (one electron from each of two atoms), covalent bonds are stronger than ionic bonds.

Nonpolar Covalent Bonds

Figure 2.9 shows several common types of covalent bonds. Notice that the two covalently bonded atoms typically share just one or two electron pairs, though larger sharings are possible. The important concept to take from this is that in covalent bonds, electrons in the two atoms' overlapping atomic orbitals are shared to fill the valence shells of both atoms, ultimately stabilizing both of the atoms involved. In a single covalent bond, a single electron pair is shared between two atoms, while in a double covalent bond, two pairs of electrons are shared between two atoms. There even are triple covalent bonds, where three electron pairs are shared between two atoms.

Figure 2.9 Covalent Bonding

You can see that the covalent bonds shown in Figure 2.9 are balanced. The sharing of the negative electrons is relatively equal, as is the electrical pull of the positive protons in the nucleus of the atoms involved. This is why covalently bonded molecules that are electrically balanced in this way are described as nonpolar; that is, no region of the molecule is either more positive or more negative than any other.

Polar Covalent Bonds

Groups of legislators with completely opposite views on a particular issue are often described as “polarized” by news writers. In chemistry, a polar molecule is a molecule that contains regions that have opposite electrical charges. Polar molecules occur when atoms share electrons unequally, in polar covalent bonds.

The most familiar example of a polar molecule is water (Figure 2.10). The molecule has three parts: one atom of oxygen, the nucleus of which contains eight protons, and two hydrogen atoms, whose nuclei each contain only one proton. Because every proton exerts an identical positive charge, a nucleus that contains eight protons exerts a charge eight times greater than a nucleus that contains one proton. This means that the negatively charged electrons present in the water molecule are more strongly attracted to the oxygen nucleus than to the hydrogen nuclei. Each hydrogen atom’s single negative electron therefore migrates toward the oxygen atom, making the oxygen end of their bond slightly more negative than the hydrogen end of their bond.

Figure 2.10 Polar Covalent Bonds in a Water Molecule

What is true for the bonds is true for the water molecule as a whole; that is, the oxygen region has a slightly negative charge and the regions of the hydrogen atoms have a slightly positive charge. These charges are often referred to as “partial charges” because the strength of the charge is less than one full electron, as would occur in an ionic bond. As shown in Figure 2.10, regions of weak polarity are indicated with the Greek letter delta (δ) and a plus (+) or minus (–) sign.

Even though a single water molecule is unimaginably tiny, it has mass, and the opposing electrical charges on the molecule pull that mass in such a way that it creates a shape somewhat like a triangular tent (see Figure 2.10b). This dipole, with the positive charges at one end formed by the hydrogen atoms at the “bottom” of the tent and the negative charge at the opposite end (the oxygen atom at the “top” of the tent) makes the charged regions highly likely to interact with charged regions of other polar molecules. For human physiology, the resulting bond is one of the most important formed by water—the hydrogen bond.

Hydrogen Bonds

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. In other words, hydrogen bonds always include hydrogen that is already part of a polar molecule.

The most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens before your eyes whenever two raindrops merge into a larger bead, or a creek spills into a river. Hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules (Figure 2.11).

Figure 2.11 Hydrogen Bonds between Water Molecules Notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms. Hydrogen bonds are relatively weak, and therefore are indicated with a dotted (rather than a solid) line.

Water molecules also strongly attract other types of charged molecules as well as ions. This explains why “table salt,” for example, actually is a molecule called a “salt” in chemistry, which consists of equal numbers of positively-charged sodium (Na⁺) and negatively-charged chloride (Cl^–), dissolves so readily in water, in this case forming dipole-ion bonds between the water and the electrically-charged ions (electrolytes). Water molecules also repel molecules with nonpolar covalent bonds, like fats, lipids, and oils. You can demonstrate this with a simple kitchen experiment: pour a teaspoon of vegetable oil, a compound formed by nonpolar covalent bonds, into a glass of water. Instead of instantly dissolving in the water, the oil forms a distinct bead because the polar water molecules repel the nonpolar oil.