Photosynthesis and Cellular Energy

Comprehensive practice flashcards covering cellular energy, ATP structure and function, chloroplast anatomy, pigments, and the step-by-step mechanisms of the light-dependent and light-independent reactions of photosynthesis.

What does the abbreviation $ATP$ stand for, and why is it essential to living cells?

$ATP$ stands for adenosine triphosphate. It is considered the most important energy-related compound for living organisms, providing the immediate chemical energy required to power cellular activities and maintain homeostasis.

What are the three chemical components that compose an $ATP$ molecule?

An $ATP$ molecule is composed of: 1) adenine (a nitrogenous base), 2) ribose (a 5-carbon sugar), and 3) three phosphate groups.

How do cells store and release chemical energy using $ATP$ and $ADP$?

Energy is released when the chemical bond between the second and third phosphate groups is broken, converting $ATP$ into $ADP$. Energy is stored when a third phosphate group is bonded to $ADP$, restoring it to $ATP$.

In this visual analogy comparing cellular energy carriers to a battery, what do the partially charged battery and the fully charged battery represent?

The partially charged battery represents $ADP$ (which has two phosphate groups and less stored energy), while the fully charged battery represents $ATP$ (which has three phosphate groups and high chemical potential energy to do cellular work).

Why do cells store only small amounts of $ATP$ at any given time?

$ATP$ is not well-suited for long-term energy storage. Instead, cells maintain only enough $ATP$ for a few seconds of activity and regenerate it rapidly and easily from $ADP$ using the energy stored in food molecules such as glucose.

What is the primary difference between autotrophs and heterotrophs?

Autotrophs are organisms that make their own food by capturing light or chemical energy (such as plants via photosynthesis), whereas heterotrophs must consume other living things to obtain food energy.

What is the ultimate source of energy for most living organisms, and how does photosynthesis make it usable?

The sun is the ultimate energy source. Photosynthesis converts sunlight energy into chemical bond energy stored in glucose ($C_6H_{12}O_6$), which organisms then consume and break down to synthesize $ATP$.

What is the primary photosynthetic pigment in plants, and what are its two chemical forms?

Chlorophyll is the primary pigment, and it exists in two forms: Chlorophyll a and Chlorophyll b.

Based on this absorption spectrum, why do plant leaves containing chlorophyll appear green?

Chlorophyll absorbs light exceptionally well in the blue-violet and red regions of the visible spectrum, but it does not absorb green light well; green light is reflected off the leaf surface.

What are the primary internal structures of the chloroplast shown in this diagram?

The chloroplast consists of: thylakoids (flattened, pancake-shaped sac membranes containing pigments), grana (stacks of thylakoids), and stroma (the fluid or liquid portion surrounding the thylakoids inside the inner membrane).

How does sunlight create 'high-energy electrons' during photosynthesis?

Light energy from the sun is absorbed by pigments, which excites atomic electrons and moves them farther away from the nucleus of the atom to higher energy levels.

What is the role of $NADP^+$ in photosynthesis, and what reaction converts it to $NADPH$?

$NADP^+$ functions as an electron carrier that accepts a pair of high-energy electrons along with a hydrogen ion ($H^+$) to transport energy from the thylakoid membranes to the stroma: $NADP^+ + 2e^- + H^+ \rightarrow NADPH$.

What is the overall balanced chemical equation for photosynthesis?

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ (six molecules of carbon dioxide plus six molecules of water, powered by light energy, yield one molecule of glucose plus six molecules of oxygen).

What are the location, inputs, and outputs of the light-dependent reactions shown here?

They take place in the thylakoids. They use light energy and water ($H_2O$) along with $ADP$ and $NADP^+$ to produce oxygen ($O_2$) as a byproduct, along with energy-rich $ATP$ and $NADPH$.

What are the location, inputs, and outputs of the light-independent reactions shown here?

They take place in the stroma. They take in carbon dioxide ($CO_2$) and use the energy carriers $ATP$ and $NADPH$ from the light reactions to produce high-energy sugars (glucose), returning $ADP$ and $NADP^+$ back to the thylakoids.

What takes place during Photosystem II at the beginning of the light-dependent reactions?

Light energy is absorbed by pigments, exciting electrons to a higher energy level. Water molecules ($H_2O$) are split to replace these lost electrons, releasing $2e^-$, $2H^+$ ions into the thylakoid space, and oxygen gas ($O_2$) into the atmosphere.

What is the function of the electron transport chain (ETC) located between Photosystem II and Photosystem I?

The ETC is a system of proteins in the thylakoid membrane that passes high-energy electrons toward Photosystem I. As electrons move along the chain, their energy is used to pump hydrogen ions ($H^+$) from the stroma into the thylakoid space (lumen).

What occurs in Photosystem I during the light-dependent reactions?

Pigments absorb light energy to re-energize electrons arriving from the ETC. At the end of Photosystem I, a second short electron transport chain transfers these electrons and $H^+$ to $NADP^+$ in the stroma to form $NADPH$.

According to this diagram of the thylakoid membrane, how does $ATP$ synthase synthesize $ATP$?

A high concentration of $H^+$ inside the thylakoid space creates a positive charge gradient. $H^+$ ions pass out into the stroma exclusively through $ATP$ synthase, causing the protein to rotate and provide the mechanical energy needed to bond a phosphate group to $ADP$, generating $ATP$.

What happens in the first step of the Calvin Cycle (carbon fixation)?

Six molecules of carbon dioxide ($CO_2$) entering from the atmosphere are combined with six 5-carbon molecules (RuBP), catalyzed by the enzyme Rubisco, forming unstable 6-carbon compounds that immediately split into twelve 3-carbon compounds.

In the sugar-production stage of the Calvin Cycle, how many 3-carbon molecules are converted into glucose versus recycled?

Two of the twelve 3-carbon molecules are removed from the cycle to synthesize one 6-carbon glucose molecule (and other organic compounds), while the remaining ten 3-carbon molecules are recycled using $ATP$ to recreate the six 5-carbon starting molecules.

What three environmental factors significantly affect the rate of photosynthesis?

1) Temperature: Photosynthetic enzymes function optimally between $0\text{--}35\text{ }^{\circ}\text{C}$; extreme temperatures slow or denature them. 2) Light intensity: Higher intensity increases the rate up to a maximum plateau. 3) Water availability: Water is a necessary reactant; scarcity slows or stops the process.

How do $C_4$ plants adapt their photosynthetic pathway, and what is an example of a $C_4$ plant?

$C_4$ plants have a specialized chemical pathway that first fixes carbon into a 4-carbon compound. This allows the Calvin Cycle to keep running under conditions of intense light, high temperature, and very low $CO_2$ levels. An example is corn.

How does Crassulaceae Acid Metabolism (CAM) enable plants like cacti and pineapples to survive in dry climates?

CAM plants open their stomata only at night to take in and store carbon dioxide as organic acids, preventing daytime water loss. During the daylight, the stomata close tightly, and the stored carbon is released to run the Calvin Cycle using light-generated energy.