Meditaliano IMAT Prep

Part 1: Photosynthesis - Capturing Light Energy

Photosynthesis is the process used by plants, algae, and some bacteria to convert light energy into chemical energy in the form of glucose. It occurs in the chloroplasts and is summarized by the equation: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$. It consists of two main stages.

1.1 Light-Dependent Reactions

These reactions occur in the thylakoid membranes. They capture light energy and use it to produce ATP and NADPH.

• Photosystem II (PS II): Light energy is absorbed by pigment molecules (like chlorophyll) and funneled to a reaction-center complex, exciting electrons. These electrons are passed to an electron transport chain. To replace the lost electrons, a water molecule is split, releasing O₂, protons (H⁺), and electrons.
• Electron Transport Chain: As electrons move down the chain between PS II and PS I, their energy is used to pump H⁺ ions from the stroma into the thylakoid space, creating a proton gradient (or proton-motive force).
• Photosystem I (PS I): Electrons are re-energized by more light and are finally transferred to NADP⁺, reducing it to NADPH.
• ATP Synthase: The H⁺ ions flow back down their concentration gradient into the stroma through ATP synthase, driving the synthesis of ATP from ADP and Pi (photophosphorylation). This process is an example of chemiosmosis.

1.2 The Calvin Cycle (Light-Independent Reactions)

These reactions occur in the stroma. They use the ATP and NADPH from the light reactions to convert CO₂ into sugar (G3P).

1. Carbon Fixation: The enzyme RuBisCO attaches one CO₂ molecule to a five-carbon sugar called Ribulose-1,5-bisphosphate (RuBP). This six-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: Each 3-PGA molecule receives a phosphate group from ATP and is then reduced by NADPH, forming Glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
3. Regeneration: For every six G3P molecules produced, one exits the cycle as a net product (to be used for glucose synthesis). The other five are rearranged, using more ATP, to regenerate the three molecules of RuBP needed to continue the cycle.

Part 2: The Cell Cycle - The Life of a Cell

The cell cycle is the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells. It consists of two main periods: Interphase and the Mitotic (M) phase.

2.1 Interphase: Growth and DNA Replication

Interphase accounts for about 90% of the cell cycle and is divided into three subphases:

• G₁ Phase (First Gap): The cell grows, synthesizes proteins, and produces new organelles, carrying out its normal metabolic functions.
• S Phase (Synthesis): The cell replicates its DNA. Each chromosome, initially a single chromatid, becomes two identical sister chromatids, joined at the centromere.
• G₂ Phase (Second Gap): The cell continues to grow and prepares for division, synthesizing proteins and structures (like microtubules) needed for mitosis.

2.2 M Phase: Mitosis and Cytokinesis

The M Phase is where the cell divides. It includes:

• Mitosis: The nucleus divides, distributing the duplicated chromosomes into two new nuclei. Mitosis is a continuum but can be described in four stages: prophase, metaphase, anaphase, and telophase.
• Cytokinesis: The cytoplasm divides, forming two separate daughter cells. In animal cells, this occurs via a cleavage furrow; in plant cells, a cell plate forms.

2.3 Cell Cycle Checkpoints and Regulation

The cell cycle is tightly regulated by a molecular control system involving checkpoints. The major checkpoints are:

• G₁ Checkpoint: The most important checkpoint. It checks for cell size, nutrients, growth factors, and DNA damage. If a cell receives a go-ahead signal, it will usually complete the S, G₂, and M phases. If not, it may enter the G₀ phase.
• G₂ Checkpoint: Ensures that DNA replication is complete and not damaged before the cell enters mitosis.
• M Checkpoint: Occurs during metaphase and ensures that all sister chromatids are properly attached to the mitotic spindle before anaphase begins.

Progression is controlled by cyclins and cyclin-dependent kinases (Cdks). The cyclin-Cdk complex MPF (Maturation-Promoting Factor) is crucial for triggering the G₂ checkpoint and initiating mitosis. Loss of cell cycle control can lead to cancer.

Interactive Practice Quiz

Test your understanding of photosynthesis and the cell cycle. Choose the best answer for each question (A-E) and then submit to see your results.