Photosynthesis
What organelle is responsible for photosynthesis in plant cells?
Answer: The chloroplast.
Why? Chloroplasts contain chlorophyll, which absorbs sunlight. They use light energy to help plants make sugars from carbon dioxide and water.
What are the three main subatomic particles in an atom?
Answer: Protons, neutrons, and electrons.
Why? Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. The electrons, especially those in the outer energy level, are important for forming chemical bonds.
What type of bond forms between neighboring water molecules?
Why? The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another water molecule.
Important: Covalent bonds hold the atoms together within a water molecule. Hydrogen bonds attract different water molecules to each other.
What are the four major types of biological macromolecules?
Answer: Carbohydrates, lipids, proteins, and nucleic acids.
Why? Each has an important job in living things. Carbohydrates provide energy, lipids store energy and form membranes, proteins carry out many cellular functions, and nucleic acids store genetic information.
What is the main job of an enzyme?
Answer: To speed up chemical reactions by lowering activation energy.
Why? Activation energy is the energy barrier that must be overcome for a reaction to occur. Enzymes make that barrier easier to overcome, allowing reactions to happen faster.
What is the main difference between photosynthesis and chemosynthesis?
Photosynthesis uses sunlight for energy, while chemosynthesis gets energy from chemical reactions.
Why? Both processes allow organisms to make their own organic molecules, but they use different energy sources. That's why certain organisms can survive deep in the ocean without sunlight.
What is the difference between an element and a compound?
Answer: An element contains only one type of atom. A compound contains two or more different elements chemically combined.
Why? Oxygen (O₂) contains only oxygen atoms, so it represents an element. Water (H₂O) contains hydrogen and oxygen atoms bonded together, so it is a compound.
What's the difference between cohesion and adhesion?
Answer: Cohesion is when water sticks to water. Adhesion is when water sticks to another surface.
Why? Hydrogen bonding helps water molecules attract each other and interact with other polar surfaces.
Memory trick: Cohesion = together. Adhesion = attaches to something else.
Why can carbon form so many different biological molecules?
Answer: Carbon has four valence electrons and can form four covalent bonds.
Why? Carbon can bond with itself and other elements to make long chains, branches, and rings. This allows living things to make many different types of molecules.
Remember: Four available bonds = lots of possible structures.
What is the difference between a monomer and a polymer?
Answer: A monomer is a small building block. A polymer is a larger molecule made of many repeating building blocks.
Why? Think of monomers as individual beads and a polymer as a necklace made by connecting the beads.
Example: Amino acids join together to form polypeptides.
Where do the light-dependent reactions and Calvin cycle occur in the chloroplast?
Answer: Light-dependent reactions happen in the thylakoid membranes. The Calvin cycle happens in the stroma.
Why? The thylakoid membranes contain the proteins that capture light and help produce ATP and NADPH. The stroma contains enzymes that use ATP and NADPH to help build sugars from CO₂.
Carbon-12 and carbon-14 are isotopes. How are they different?
Answer: They have the same number of protons but different numbers of neutrons.
Why? Both have six protons, which makes them carbon. Carbon-12 has six neutrons, while carbon-14 has eight neutrons. The different neutron counts explain their different masses.
Why does ice float on liquid water?
Answer: Ice is less dense than liquid water.
Why? When water freezes, hydrogen bonds hold the molecules in an open crystal structure. This spreads them farther apart, making ice less dense, so it floats.
Remember: Same amount of water, more space when frozen = lower density.
What are proteins made of, and what makes the 20 common amino acids different?
Answer: Proteins are made of amino acids. Amino acids differ in their R groups, also called side chains.
Why? Different R groups have different chemical properties. The order of amino acids and their interactions determine how a protein folds and functions.
How are the energy levels of reactants and products different in exothermic and endothermic reactions?
Answer: Exothermic reactions release energy, so the products have lower energy than the reactants. Endothermic reactions absorb energy, so the products have higher energy.
Why? Exothermic reactions transfer energy to the surroundings. Endothermic reactions take in energy from the surroundings.
Memory trick: Exo = exits. Endo = enters.
Why do hydrogen ions (H⁺) build up inside the thylakoid lumen?
Answer: The electron transport chain moves H⁺ into the lumen, and splitting water adds even more H⁺.
Why? This creates an area with lots of hydrogen ions inside the lumen. When H⁺ flows back through ATP synthase, that movement provides the energy needed to make ATP.
Think of it like: Water building up behind a dam. When it flows through a turbine, energy can be captured.
Oxygen attracts electrons more strongly than hydrogen. How does this make water polar?
Answer: Oxygen pulls the shared electrons closer, making the oxygen side slightly negative and the hydrogen side slightly positive.
Why? The electrons are not shared equally. Because water also has a bent shape, these partial charges make the whole water molecule polar.
Remember: Unequal electron sharing + bent shape = polar water molecule.
A solution changes from pH 7 to pH 4. How does its hydrogen ion concentration change?
Answer: It becomes 1,000 times greater.
Why? Each decrease of one pH unit means 10 times more hydrogen ions.
10 x 10 x 10 = 1,000
The difference between pH 7 and pH 4 is three units.
Remember: Lower pH = higher H⁺ concentration.
Why do lipids store more energy per gram than carbohydrates?
Answer: Lipids contain many carbon-hydrogen bonds and are generally more chemically reduced than carbohydrates.
Why? When the body oxidizes lipids, more energy can be released. This is why fats provide about 9 calories per gram compared with about 4 calories per gram from carbohydrates.
Remember: Lipids are useful for long-term energy storage.
An enzyme works best at 37°C but barely works at 80°C. Why?
Answer: The high temperature may cause the enzyme to denature, changing its shape.
Why? Enzymes need the correct three-dimensional structure to work. If the active site's shape changes, the substrate may no longer bind properly, causing enzyme activity to decrease.
Remember: Enzyme shape affects enzyme function.
A chemical makes the thylakoid membrane freely permeable to hydrogen ions. What happens to ATP production?
Answer: ATP production decreases dramatically.
Why? The hydrogen ions can leak across the membrane instead of flowing through ATP synthase. Without a strong H⁺ gradient, ATP synthase loses the energy source it needs to produce ATP.
Remember: No proton gradient = very little ATP from chemiosmosis.
How can radioactive isotopes be useful to scientists, and why can they also be dangerous?
Answer: They can help scientists determine the age of certain materials, but their radiation can damage cells and DNA.
Why? Radioactive isotopes decay at predictable rates. Scientists use this information for dating materials and in medical applications. However, exposure to too much ionizing radiation can damage living tissue.
Water moves up narrow tubes inside plant stems. What two properties of water help this happen?
Answer: Cohesion and adhesion.
Why? Adhesion helps water stick to the tube walls, while cohesion helps water molecules stay connected. Together, they contribute to capillary action.
Important connection: Transpiration pull is also a major force moving water upward through tall plants.
Fatty Acid A has only single carbon-carbon bonds. Fatty Acid B has three carbon-carbon double bonds. Identify both and explain how their structures affect their properties.
Answer: Fatty Acid A is saturated. Fatty Acid B is polyunsaturated.
Why? Saturated fatty acids have no carbon-carbon double bonds and generally pack tightly. Polyunsaturated fatty acids have multiple double bonds. Cis double bonds introduce bends that make it harder for the chains to pack closely together, often lowering their melting points.
Memory trick: Saturated = no C=C bonds. Polyunsaturated = multiple C=C bonds.
A reaction needs 80 kJ/mol of activation energy without an enzyme and 30 kJ/mol with an enzyme. How does the enzyme affect the reaction?
Answer: The enzyme lowers the activation energy by 50 kJ/mol, allowing the reaction to happen faster.
80-30=50kJ/mol
Why? More reacting molecules can overcome a lower energy barrier. However, the enzyme does not change the overall energy difference between reactants and products.