Cell and Developmental Biology

Polar covalent bonds are formed when the electrons in the bond are not shared equally between the two nuclei. Which one of these molecules contains polar bonds?

A. Water (H2O) B. Methane (CH4) C. Molecular oxygen (O2) D. Propane (C3H8) Answer: A

There are 90 naturally occurring elements on the earth, __________of which compose 96% of the mass of living organisms.

A. 1 B. 4 C. 7 D. 6 Answer: B

The Ras protein is a GTPase that functions in many growth factor-signaling pathways. In its active form, with GTP bound, it transmits a downstream signal that leads to cell proliferation; in its inactive form, with GDP bound, the signal is not transmitted. Mutations in the gene for Ras are found in many cancers. Of the choices below, which alteration of Ras activity is most likely to result in Ras being "turned on" and contribute to the uncontrolled growth of cancer cells?

A. A change that prevents Ras from being transcribed. B. A change that increases the affinity of Ras for GDP. C. A change that decreases the affinity of Ras for GTP. D. A change that decreases the rate of hydrolysis of GTP by Ras. Answer: D

Motor proteins use the energy in ATP to transport organelles, rearrange elements of the cytoskeleton during cell migration, and move chromosomes during cell division. Which of the following mechanisms is sufficient to ensure the unidirectional movement of a motor protein along its substrate?

A. A conformational change is coupled to the release of a phosphate (Pi). B. The substrate on which the motor moves has a conformational polarity. C. A conformational change is coupled to the binding of ADP. D. A conformational change is coupled to ATP hydrolysis. Answer: D

Squid and sea slugs (Aplysia) may seem like odd choices as animal models for neuroscience. However, these creatures have provided important insights into the fundamental mechanisms of neuronal function. Why are these marine invertebrates good models for neuroscientists?

A. All answers provided here are correct. B. Neurons from both vertebrates and invertebrates operate using similar fundamental principles. C. They have large, identifiable neurons. D. These creatures have relatively simple behaviors. Answer: A

The fact that all cells share a similar basic chemistry and store genetic information in the form of DNA supports which of the three tenets of cell theory?

A. All cells come from pre-existing cells. B. All living organisms are constructed from one or more cells. C. The cell is the basic structural and functional unit of life as we know it. Answer: A

Which of the following statements about allostery is TRUE?

A. Allosteric regulators are often products of other chemical reactions in the same biochemical pathway. B. Allosteric regulation is always used for negative regulation of enzyme activity. C. Enzymes are the only types of proteins that are subject to allosteric regulation. D. Binding of allosteric molecules usually locks an enzyme in its current conformation, such that the enzyme cannot adopt a different conformation. Answer: A

Suppose that you used cryo-EM to determine the structure of your favorite protein. What visual model would best display the surface features of your protein's tertiary structure?

A. Backbone B. Ribbon C. Wire D. Space-filling Answer: D

Which of the following statements about disulfide bonds is TRUE?

A. Disulfide bonds are formed by the cross-linking of methionine residues. B. Disulfide bonds are formed mainly in proteins that are retained within the cytosol. C. Disulfide bonds stabilize but do not change a protein's final conformation. D. Disulfide bonds are more common for intracellular proteins, compared to extracellular proteins. Answer: C

Which of the following monomer building blocks is necessary to assemble selectively permeable boundaries around and inside cells?

A. Fatty acids B. Sugars C. Nucleotides D. Amino acids Answer: A

β sheets can participate in the formation of amyloid fibers, which are insoluble protein aggregates. What drives the formation of amyloid fibers?

A. Formation of biofilms by infectious bacteria. B. β-sheet stabilization of abnormally folded proteins. C. Extension of β sheets into much longer β strands. D. Denaturation of proteins containing β sheets. Answer: B

Which of the following atoms comprise 60% of the atoms in the human body?

A. Hydrogen B. Carbon C. Nitrogen D. Oxygen Answer: A

Both DNA and RNA are synthesized by covalently linking a nucleoside triphosphate to the previous nucleotide, constantly adding to a growing chain. In the case of DNA, the new strand becomes part of a stable helix. The two strands are complementary in sequence and antiparallel in directionality. What is the principal force that holds these two strands together?

A. Hydrogen bonds B. Ionic interactions C. van der Waals interactions D. Covalent bonds Answer: A

Cyclic AMP (cAMP) is a small molecule (derived from ATP) that associates with its binding site with a high degree of specificity. Which types of noncovalent interactions are the most important for providing the "hand in a glove" binding of cAMP?

A. Hydrogen bonds. B. Hydrophobic interactions C. Electrostatic interactions. D. van der Waals interactions. Answer: A

Lysozyme is an enzyme that specifically recognizes bacterial polysaccharides, which renders it an effective antibacterial agent. Into what classification of enzymes does lysozyme fall?

A. Hydrolase B. Isomerase C. Nuclease D. Protease Answer: A

Scientists learned that cell death is a normal and even important part of life by studying the development of the nematode worm C. elegans. What was the most important feature of C. elegans for the study of programmed cell death?

A. Its genome has been sequenced. B. Seventy percent of C. elegans genes have homologs in humans. C. The developmental pathway of each cell in the adult worm was known. D. The nematode is smaller and simpler than the fruit fly. Answer: C

Lysozyme lowers the activation energy for the hydrolysis of a glycosidic bond in its polysaccharide substrate. Which of the following statements about the lysozyme-catalyzed reaction is FALSE?

A. Lysozyme speeds up the rate at which water molecules collide with the substrate. B. Lysozyme polarizes the charge of specific atoms to favor a reaction. C. Lysozyme binds irreversibly to the substrate so that it cannot dissociate. D. Lysozyme strains the conformation of the substrate to favor the hydrolysis reaction. Answer: C

Oligosaccharides are short sugar polymers that can become covalently linked to proteins and lipids through condensation reactions. These modified proteins and lipids are called glycoproteins and glycolipids, respectively. Within a protein, which of the amino acids (shown in the figure below) is the most probable target for this type of modification? Hint: find the side chain of each amino acid and determine which is most likely to participate in a condensation reaction.

A. Methionine B. Glycine C. Phenylalanine D. Serine Answer: D

The elemental composition of the human body is similar to that of the earth's crust.

False

Noncovalent interactions mediate specific binding of a ligand to a protein. Which of the following noncovalent interactions are NOT involved in binding between a protein and its ligand?

A. None of the above. All of these are examples of noncovalent interactions that mediate protein-ligand interactions. B. Hydrogen bonds. C. van der Waals interactions. D. Hydrophobic forces. E. Electrostatic interactions. Answer: A

Proteins bind selectively to small-molecule targets called ligands. The selection of one ligand out of a mixture of possible ligands depends on the number of weak, noncovalent interactions in the protein's ligand-binding site. Where is the binding site typically located in the protein structure?

A. On the surface of the protein. B. Buried in the interior of the protein. C. Between two adjacent amino acids. D. Inside a cavity on the protein surface. Answer: D

Which of the following statements is TRUE?

A. Peptide bonds are the only covalent bonds that can link together two amino acids in proteins. B. There is free rotation around all covalent bonds in the polypeptide backbone. C. Acidic and basic amino acids tend to be found on the exterior of properly folded proteins. D. The sequence of the atoms in the polypeptide backbone varies between different proteins. Answer: C

The biosynthetic pathway for the two amino acids E and H is shown schematically in Figure 4-33. You are able to show that E inhibits enzyme V, and H inhibits enzyme X. Which biosynthetic product is most likely the inhibitor of enzyme T?

A. Product B B. Product C C. Product E D. Product H Answer: B

Living systems are incredibly diverse in size, shape, environment, and behavior. It is estimated that there are between 10 million and 100 million different species. Despite this wide variety of organisms, it remains difficult to define what it means to say something is alive. Which of the following can be described as the smallest living unit?

A. Protein B. DNA C. Cell D. Mycoplasma Answer: Cell

Some proteins have α helices, some have β sheets, and still others have a combination of both. What makes it possible for proteins to have these common structural elements?

A. Specific amino acid sequences. B. Side-chain interactions. C. The hydrophobic-core interactions. D. Hydrogen bonds along the protein backbone. Answer: D

Studies conducted with a lysozyme mutant that contains an Asp→Asn change at position 52 and a Glu→Gln change at position 35 exhibited almost a complete loss in enzymatic activity. What is the most likely explanation for the decrease in enzyme activity in the mutant?

A. The binding pocket is too hydrophobic for water molecules to enter. B. The mutant lysozyme no longer has a substrate-binding pocket. C. The mutant protein lacks negative charges in the active site that are required for catalysis. D. The mutant lysozyme has increased affinity for the polysaccharide substrate. Answer: C

To study how proteins fold, scientists must be able to purify the protein of interest, use solvents to denature the folded protein, and observe the process of refolding at successive time points. What is the effect of the solvents used in the denaturation process?

A. The solvents break all covalent interactions. B. The solvents disrupt all noncovalent interactions C. The solvents break some of the covalent bonds and some of the noncovalent interactions. D. The solvents create a new, stable protein conformation. Answer: B

Consider the definitions below and select the one that best fits the term "protein domain."

A. The tertiary structure of a substrate-binding pocket. B. Two alpha helices and a beta sheet. C. A complex of more than one polypeptide chain. D. A segment of a polypeptide that folds independently into a stable structure. Answer: D

The correct folding of proteins is necessary to maintain healthy cells and tissues. The presence of unfolded proteins are associated with some neurodegenerative disorders as Alzheimer's disease, Huntington's disease, and Creutzfeldt-Jakob disease (the specific faulty protein is different for each disease). What happens to these disease-causing, unfolded proteins?

A. They are degraded. B. They bind a different target protein. C. They form structured filaments. D. They form protein aggregates. Answer: D

Which of the following statements about molecular chaperones is FALSE?

A. They can interact with unfolded polypeptides in a way that changes the final fold of the protein. B. They assist polypeptide folding by helping the folding process follow the most energetically favorable pathway. C. They can isolate proteins from other components of the cells until folding is complete. D. They help streamline the protein-folding process by making it a more efficient and reliable process inside the cell. Anser: A

The three-dimensional coordinates of atoms within a folded protein are determined experimentally. After researchers obtain a protein's structural details, they can use different techniques to highlight particular aspects of the structure. What visual model best displays a protein's secondary structures (α helices and β sheets)?

A. Wire B. Backbone C. Ribbon D. Space-filling Answer: C

Caenorhabditis elegans is a nematode worm that produces more than 1000 cells during its development. However, the adult worm has only 959 somatic cells. The process by which 131 cells are specifically targeted for destruction is called

A. autophagy. B. cell ablation. C. necrosis. D. programmed cell death Answer: D

Energy required by the cell is generated in the form of ATP. The hydrolysis of ATP is used to power many cellular processes. A byproduct of ATP hydrolysis is ADP. As ATP is used up, the relative amount of ADP molecules also increases. When present at high levels, ADP can bind to glycolytic enzymes, which will lead to the production of more ATP. The best way to describe this mechanism of regulation is

A. feedback inhibition. B. oxidative phosphorylation. C. allosteric activation. D. substrate-level phosphorylation. Answer: C

In some cases, small molecules are integral to the function of enzymes, and are dubbed "coenzymes." Which of the following is a coenzyme for the enzyme carboxypeptidase?

A. heme B. biotin C. retinal D. zinc Answer: D

A. thaliana, or Arabidopsis, is a common weed. Biologists have selected it over hundreds of thousands of other flowering plant species to serve as an experimental model organism. Which of the follow is NOT a reason that Arabidopsis was chosen as a model plant?

A. it can reproduce in 8-10 weeks. B. unlike many plants, Arabidopsis has a diploid genome. C. it produces thousands of offspring per plant. D. Arabidopsis is a valuable cash crop species. Answer: D

The primary structure of a protein is the...

A. lowest energy conformation. B. average size of the amino acid side chains. C. amino acid composition. D. amino acid sequence. Answer: D

The phosphorylation of a protein is typically associated with a change in its activity, the assembly of a protein complex, or the triggering of a downstream signaling cascade. The addition of ubiquitin, a small polypeptide, is another type of covalent modification that can affect the protein function. Ubiquitylation often results in

A. membrane association. B. protein degradation. C. protein secretion. D. nuclear translocation. Answer: B

You want to measure the size of a typical cell in the leaf of a peanut plant. What unit of length should you use to report this cell's dimensions?

A. millimeters B. centimeters C. micrometers D. nanometers Answer: C

Select the answer that BEST completes the following statement: Chemical reactions in living systems occur in an __________environment, within a narrow range of temperatures.

A. organic B. extracellular C. terrestrial D. aqueous Answer: D

The Table below indicates the number and arrangement of electrons in the first four atomic electron shells for selected elements. Based on the information in the chart and what you know about atomic structure and biology, which elements are least likely to be involved in biological chemistry?

A. phosphorus; chlorine B. sodium; potassium C. magnesium; calcium D. helium; neon Answer: D

Complete the sentence with the best option provided below. The secondary structures of a protein are the

A. regular, repeated folds present in a lowest energy conformation. B. temporary, unstable protein folding conformations. C. interactions between polar amino acid side chains. D. chemical modifications of amino acid side chains. Answer: A

Which of the following tripeptides are most likely to be in the inner core of a cytosolic protein?

A. serine-threonine-tyrosine B. alanine-glycine-leucine C. proline-serine-alanine D. lysine-arginine-aspartic acid Answer: B

The amino acids glutamine and glutamic acid are shown in the figure below. They differ only in the structure of part of their side chains (circled). At pH7, what type of interactions are possible for glutamic acid but not for glutamine?

A. van der Waals interactions B. hydrogen bonds C. ionic bonds D. covalent bonds Answer: C

Match the general type of biochemical reaction catalyzed with the class of enzyme.

ATPase =Hydrolyzes ATP. Polymerase =Catalyzes the synthesis of polymers such as RNA and DNA. Ligase =Joins two ends of DNA together. Protein kinase =Adds phosphate groups to proteins. Isomerase =Rearranges bonds within a single molecule. Nuclease =Hydrolyzes bonds between nucleotides. Protease =Hydrolyzes peptide bonds. Phosphatase =Removes a phosphate group from a protein.

Mice and zebrafish are both very popular vertebrate model organisms. However, mice have some advantages over zebrafish for modeling human health and disease. For which of the following studies would zebrafish be a poor choice? (choose ALL correct answers)

Asthma Breast cancer

Electrostatic interactions are an important type of noncovalent bond that helps proteins adopt their most stable, lowest energy conformation. It is possible for electrostatic bonds to form between atoms found in which of the following parts of a polypeptide? (choose ALL correct answers)

Charged amino acid side chains The free amino and carboxyl termini of a protein.

Match each term related to the structure of nucleic acids with one of the descriptions provided.

Base= Nitrogen-containing aromatic ring. Glycosidic bond= Linkage between the sugar and the base. Nucleoside= Sugar linked to a base. Nucleotide= Sugar linked to a base and one or more phosphates. Phosphoanhydride bond= Linkages between phosphate groups. Phosphoester bond= The linkage between the 5′ sugar hydroxyl and a phosphate group. Phosphodiester bond= The linkage between two nucleotides. Ribose= Five-carbon sugar found in RNA Deoxyribose= Five-carbon sugar found in DNA

Choose the answers that best fit the following statement: ________ is an essential component of biological membranes. Although it is much smaller than the typical phospholipids and glycolipids in the membrane, it is a/an ___________ molecule, having both hydrophilic and hydrophobic regions.

Cholesterol amphipathic

__________ are long molecules of DNA held within the ________ of eukaryotic cells.

Chromosomes, nucleus

For each process (A - E), identify the model system best suited to study that process from the options in each dropdown. (A) Programmed cell death (B) Chloroplast function (C) Immunology (D) Neuron development (E) Cell movements during early vertebrate development

Cultured human fibroblasts Arabidopsis Mouse Drosophila Zebrafish

Which species was the key model organism for the advancement of molecular biology (understanding DNA replication, decoding the DNA to make proteins, etc.)?

D. melanogaster C. elegans S. pombe E. coli Answer: E coli

Biologists cannot possibly study all living species. Instead, they try to understand cell behavior by studying a select subset of species. Which of the following characteristics are useful in an organism chosen for use as a model in laboratory studies? (choose ALL correct answers)

Ethical considerations. Ability to grow under controlled conditions. Amenability to genetic manipulation. A rapid rate of reproduction.

Multicellular organisms are composed of cells that all share the same genome, but can look and function very differently from one another. This is possible because genetically identical cells can express different genes.

Express

Protein phosphorylation is another way to alter the conformation of an enzyme and serves exclusively as a mechanism to increase enzyme activity.

False

The cytosol contains very few proteins and has a water-like consistency.

False

Indicate whether the molecules below are inorganic or organic.

Glucose=Organic Ethanol=Organic Sodium chloride=Inorganic Water=Inorganic Cholesterol=Organic Adenosine triphosphate=Organic Phosphate=Inorganic Calcium=Inorganic Glycine=Organic Triacylglycerol=Organic Oxygen=Inorganic Phospholipid=Organic

Sugars form part of many organic molecules in the cell. In which of the following macromolecules contain one or more sugar molecules as part of their structure? (choose ALL correct answers)

Glycolipids DNA

1. Statins inhibit __________ to block intracellular cholesterol synthesis. 2. Methotrexate inhibits _________ , which subsequently leads to blocked DNA replication. Because they proliferate at a high rate, cancer cells are _________ sensitive to inhibition of chromosome replication. 3. Gleevec inhibits the BCR-ABL oncoprotein, a kinase that is only produced in certain types of leukemia cells. It blocks the activity of this protein by binding tightly in the __________ site of the enzyme.

HMG CoA reductase dihydrofolate reductase more substrate-binding

Indicate whether the common molecular groups below are hydrophilic (water-loving) or hydrophobic (water-fearing) in character.

Methyl group (-CH3)= Hydrophobic Hydroxyl group (-OH)= Hydrophilic Carboxyl group (-COOH)= Hydrophilic Phosphate group (-PO3_2-)= Hydrophilic Amino group (-NH2)= Hydrophilic

"Life" is easy to recognize but difficult to define. According to one popular biology text, living things: 1. Are highly organized compared to natural inanimate objects. 2. Display homeostasis, maintaining a relatively constant internal environment (e.g. temperature, pH). 3. Reproduce themselves. 4. Grow and develop from simple beginnings. 5. Take energy and matter from the environment and transform it. 6. Respond to stimuli. 7. Show adaptation to their environment. Score a peanut, a phone, and the SARS-CoV-2 virus with respect to these characteristics.

Peanut: Yes, Yes, Yes, Yes, Yes, Yes, Yes Phone: Yes, No, No, No, Yes, Yes, No SARS-COV-2: Yes, No, No, No, No, No, No

Consider this polypeptide that is five amino acids long: Phe-Ala-Arg-Met-Ile _________ is the N-terminal amino acid and _________ is at the C-terminus.

Phenylalanine (Phe) Isoleucine (Ile)

"Nurse and Hartwell first treated _______ cells with a chemical mutagen. The mutated population of cells was then grown and observed. Cells that demonstrated defects in cell-cycle regulation (characterized by _________ -than-normal cells) were isolated, including mutants for a gene they called cdc2. The scientists were able to "rescue" the mutant by supplying a normal copy of the cdc2gene from _________ , a distantly-related yeast species. After this big result, the scientists went on to show that the homologous gene from humans could also rescue the mutant phenotype. These results demonstrated that there are common principles underlying cell-cycle regulation across a large range of __________ organisms."

S. pombe larger S. cerevisiae eukaryotic

Match the small organic building block with the large organic molecule that they form.

Sugars= Polysaccharides Fatty acids= Phospholipids Amino acids= Proteins Nucleotides= Nucleic acids

Van der Waals attractions are weakly attractive forces that influence how polypeptides fold into their stable, lowest energy conformation. Van der Waals attractions occur between all atoms (polar or nonpolar) that are in close proximity to one another.

True

Mitochondria are membrane-bound organelles found in eukaryotic cells. Mitochondria contain their own DNA, reproduce by dividing, produce energy, and are thought to derive from aerobic bacteria that were engulfed by the ancestor of today's eukaryotic cells. However, mitochondria are not considered to be alive. Based on your understanding of what qualifies something to be "alive", provide at least one reason why mitochondria are not living cells.

They are incapable of surviving and replicating on their own.

Binding of an allosteric regulator to an enzyme's regulatory site can cause a conformational change in the protein that may either increase or decrease its catalytic activity, depending on the specific enzyme.

True

Condensation reactions occur in the synthesis of all the macromolecules found in cells.

True

Hydrophobic forces play an important role in the formation of both lipid droplets and phospholipid bilayers.

True

Researchers learned a lot about human cancers by studying cell cycle genes in yeast.

True

Two or three __________ can sometimes wrap around each other to form coiled-coils. The stable wrapping of one helix around another is typically driven by ________ interactions. Coiled-coils are typically found in proteins that form an elongated structural framework, such as ________ .

alpha helices hydrophobic collagen

Proteins with similar sequences of ______ are ______ to perform similar functions, even if they are from distantly related organisms

amino acids more likely

In addition to its usefulness in brewing and baking, Brewer's yeast is an experimental organism used to study eukaryotic cells. However, it does have some limitations. Select all the processes below that CANNOT be studied in yeast (choose ALL correct answers).

cell motility photosynthesis cell differentiation

Even though a squid giant axon cell and a Mycoplasma bacterium are very different in size and shape, they are similar in that they share a basic fundamental _______. For example, they both use the genetic information stored in ________to _______ RNA, which is often ______ into protein.

chemistry, DNA, transcribe, translated

Polypeptides are synthesized from amino acid building blocks through a _________ reaction between the _______ of the growing polypeptide chain and the _________ of the next amino acid to be added.

condensation C-terminal carboxyl group amino group

Hydrolysis reactions ________ a molecule of water and commonly occur during the ________ of large organic molecules in the cell.

consume breakdown

The primary structure of a polypeptide is held together by _______ bonds. The α helices and β sheets form because of ______ bonds and are examples of a protein's ________ structure. The three-dimensional conformation of a properly folded protein is its ________ structure. A protein such as hemoglobin, which is composed of more than one _________ , provides a good example of a protein's __________ structure.

covalent noncovalent secondary tertiary subunit quaternary

Unsaturated fatty acids contain carbon-carbon _________ and their presence in phospholipids _________ the fluidity of cell membranes

double bonds increases

Although covalent bonds are 10-100 times stronger than noncovalent interactions, many dynamic biological processes depend upon the number and type of noncovalent interactions between molecules. Which two noncovalent interactions below will contribute most to the strong and specific binding of two molecules, such as a pair of proteins?

electrostatic attractions hydrogen bonds

Proteins destined for secretion from an animal cell must first go through two membrane-bound compartments called the _________ and __________ .

endoplasmic reticulum Golgi apparatus

GTP-binding proteins typically have GTPase activity, and the hydrolysis of GTP transforms them to the _______ conformation.

inactive

Which combination of answers best completes the following statement: When atoms are held together by ________ , they are typically referred to as __________.

ionic interactions salts

Feedback inhibition is defined as a mechanism of down-regulating enzyme activity by the accumulation of a product ______ in the pathway.

late

Any substance that will bind to a protein is known as a/an ________. Enzymes bind their ______ at the _______. The enzyme hexokinase is so specific that it reacts with only one of the two _______ of glucose. Enzymes catalyze a chemical reaction by lowering the _________, because they provide conditions favorable for the formation of a/an _________ intermediate called the __________. Once the reaction is completed, the enzyme releases the _________ of the reaction.

ligand substrates active site isomers activation energy high energy transition state products

The cytoskeleton in animal cells is comprised of three major types of protein filaments: actin filaments, intermediate filaments, and ________________.

microtubules

Biochemical subcompartments form because of a large number of _________ between macromolecules. These intracellular condensates ________ important biochemical reactions by concentrating macromolecules within a specific region of the cell.

noncovalent interactions facilitate

Macromolecules in the cell can often interact transiently as a result of ___________ . These ____________ interactions also produce stable, highly specific interactions between molecules.

noncovalent interactions weak

Deoxyribonucleic acid is classified as an _________ molecule because it contains __________ .

organic carbon

Proteins are _______ built from amino acids, which each have an amino group and a _______ group attached to the central _________. There are 20 possible _______ that differ in structure and are generally referred to as "R." In solutions of neutral pH, amino acids are _________, carrying both a positive and a negative charge. When a protein is made, amino acids are linked together through _________ , which are formed by condensation reactions between the carboxyl end of the last amino acid and the __________ end of the next amino acid to be added to the growing chain.

polypeptides carboxyl alpha-carbon side chains ionized peptide bonds amino

A covalent bond between two atoms is formed as a result of the _________ . An ionic bond between two atoms is formed as a result of the ___________.

sharing of electrons transfer of electrons from one atom to the other

Mitochondria oxidize ______ to produce ________.

sugar, ATP

Chloroplasts use the energy of ________ to produce ________ in a process called __________ .

sunlight sugar photosynthesis

Carbon, hydrogen, oxygen, and nitrogen are the most abundant elements in living things. These elements all have ______ outer electron shells, making them ________ likely to react with one another to form molecules.

unfilled more