Meditaliano IMAT Preparation

Lesson 10: Photosynthesis & The Cell Cycle

Introduction: Energy Conversion & Proliferation

In this lesson, we explore two fundamental processes essential for life: Photosynthesis (energy capture) and the Cell Cycle (reproduction and growth). Understanding these processes is key to understanding energy flow in ecosystems and the basis of biological continuity.

Part 1: Photosynthesis

1.1 Overview: The Process and Its Importance

Photosynthesis is a vital anabolic process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is the reverse of cellular respiration in terms of overall reactants and products.

Overall Equation (Endergonic)

$$6CO_2 + 6H_2O + \text{Light Energy} \longrightarrow C_6H_{12}O_6 + 6O_2$$

In eukaryotes, this process occurs in the Chloroplasts. These organelles contain:

  • Thylakoids: Flattened sac-like membranes where the light-dependent reactions occur. Stacks are called grana.
  • Stroma: The dense fluid surrounding the thylakoids where the Calvin cycle (light-independent reactions) occurs.

Image 1: Ultrastructure and Functional Compartment of Chloroplast

Ultrastructure and Functional Compartment of Chloroplast
A high-resolution look at the chloroplast's internal architecture, highlighting the thylakoid network and the stroma.

Visual Analysis: Chloroplast Architecture

This visual tracks the structural hierarchy required for the two stages of photosynthesis (LO 10.1).

  • Thylakoid Network: Visualizes the stacked grana and the interconnected lamellae, providing massive surface area for light absorption.
  • Stroma: The aqueous fluid surrounding the thylakoids, where the enzymes of the Calvin cycle reside.
  • Double Membrane: Shows the outer and inner envelopes, reflecting the organelle's endosymbiotic origins.
Overview of Photosynthesis in a Chloroplast
Process Overview: The Light Reactions in the thylakoid membranes split water and release $O_2$, producing ATP and NADPH. The Calvin Cycle in the stroma uses these energy carriers to fix $CO_2$ into sugar. Source: Wikimedia / Anton Nel
光合成の全体的なプロセス
Schematic of photosynthesis

Diagram: Structure of a Chloroplast

Outer Membrane Stroma Granum (stack) Thylakoid

1.2 Light-Dependent Reactions (Thylakoids)

These reactions capture solar energy and convert it into chemical energy (ATP and NADPH). Oxygen is a byproduct.

Key Event: Photolysis
Water is split to provide electrons to Photosystem II. $$H_2O \longrightarrow 2H^+ + 2e^- + \frac{1}{2}O_2$$
  • Photosystem II (PS II): Absorbs light (680nm). Excites electrons which enter the Electron Transport Chain (ETC).
  • Electron Transport Chain: Pumps protons ($H^+$) from stroma into the thylakoid space, creating a gradient.
  • Photosystem I (PS I): Re-energizes electrons with light (700nm), reducing $NADP^+$ to NADPH.
  • ATP Synthase: Uses the proton gradient (Chemiosmosis) to generate ATP via photophosphorylation.
Inputs & Outputs of Light Reactions
Inputs Outputs
Light Energy ATP (to Calvin Cycle)
Water ($H_2O$) NADPH (to Calvin Cycle)
ADP + $P_i$ Oxygen ($O_2$) - Released
$NADP^+$

Diagram: The Light-Dependent Reactions (Z-Scheme)

The overall z schema and chloroplast structure

Visual Analysis: The Z-Scheme & Photochemistry

This visual maps the energetic "uphill" flow of electrons through the thylakoid membrane (LO 10.2).

  • Photosystem Integration: Shows the coordinated action of PSII and PSI in capturing light and exciting electrons.
  • Proton Pumping: Illustrates how the ETC uses electron energy to build the H+ gradient required for ATP synthesis.
  • Terminal Products: Tracks the final destination of electrons in NADPH and the generation of ATP via chemiosmosis.
Stroma (Low H+) Thylakoid Lumen (High H+) PS II Cyt b6f PS I ATP Syn H₂O → ½O₂ + 2H⁺ + 2e⁻ NADP⁺ → NADPH H⁺ H⁺ ADP → ATP

1.3 Light-Independent Reactions (Calvin Cycle)

Occurring in the stroma, this cycle uses the chemical energy from the light reactions to fix atmospheric $CO_2$ into organic sugar molecules.

  1. Carbon Fixation: The enzyme RuBisCO attaches $CO_2$ to RuBP (5-carbon sugar), creating unstable intermediates that split into 3-PGA.
  2. Reduction: ATP and NADPH are used to convert 3-PGA into G3P (Glyceraldehyde-3-phosphate), a high-energy sugar building block.
  3. Regeneration: Most G3P is used to regenerate RuBP (requiring more ATP) so the cycle can continue.
カルビンサイクルの詳細図
カルビンサイクルの詳細図: 炭素固定、還元、再生の各段階と、ATPおよびNADPHの消費を示す。 Source: Science Facts
Inputs & Outputs (per 1 G3P / 3 turns)
Inputs Outputs
3 $CO_2$ 1 G3P (Sugar precursor)
9 ATP 9 ADP + 8 $P_i$
6 NADPH 6 $NADP^+$

Diagram: The Calvin Cycle

The three major stages of calvin cycle

Visual Analysis: The Calvin Cycle Hub

This visual details the "dark" reactions where inorganic carbon is transformed into life-sustaining sugar (LO 10.3).

  • Carbon Fixation: Highlights the role of RuBisCO in merging CO2 with RuBP.
  • Reduction & Sugar Synthesis: Shows where ATP and NADPH are consumed to produce G3P.
  • RuBP Regeneration: Tracks the ATP-driven recycling phase that allows the cycle to turn continuously.
3 CO₂ Enter RuBisCO 1. Fixation 2. Reduction Uses ATP & NADPH 3. Regeneration Uses ATP 1 G3P (Sugar) Exits

1.4 Factors Affecting Photosynthesis

The rate of photosynthesis is limited by the factor in the shortest supply (Law of Limiting Factors).

  • Light Intensity: Rate increases with light intensity until it plateaus (saturation point) as enzymes operate at max speed.
  • Carbon Dioxide Concentration: Rate increases with $CO_2$ until saturation.
  • Temperature: Rate increases to an optimum temperature. Above this, enzymes (like RuBisCO) denature, causing a rapid drop in rate.
Graphs of Limiting Factors on Photosynthesis
Graphs showing how Light Intensity, Temperature, and $CO_2$ concentration affect the rate of photosynthesis. Note the bell curve for temperature. Source: IB BioNinja

Part 2: The Cell Cycle

The Cell Cycle is the ordered series of events involving cell growth and division to produce two new daughter cells. It consists of two major phases: Interphase and Mitotic (M) Phase.

細胞周期のフェーズ
細胞周期のフェーズ: インターフェーズ（G1, S, G2）とM期（有糸分裂と細胞質分裂）を示す図。 Source: Science Facts

Molecular Surveillance: Cell Cycle Checkpoint Control

functional phases of cell cycle with checkpoint regulation

Figure 2.1: Critical checkpoints (G1/S, G2/M, and M) ensuring genomic integrity during the cell cycle.

Diagram: The Cell Cycle Phases

functional phases of cell cycle with checkpoint regulation

Visual Analysis: Cell Cycle & Regulation

This visual tracks the cell's lifespan and the molecular surveillance system that prevents errors (LO 10.4).

  • Phase Integration: Clearly shows the sequence of G1, S, G2, and M phases.
  • Checkpoint Control: Highlights the G1/S, G2/M, and M checkpoints where Cdks and cyclins regulate progress.
  • G0 Rest State: Visualizes the exit point for non-dividing or quiescent cells.
M G₁ S (DNA Syn) G₂ INTERPHASE (G1 + S + G2)

2.1 Interphase: Preparation for Division

Interphase accounts for 90% of the cycle. It is divided into:

  • G₁ Phase (First Gap): Cell grows, produces organelles, and synthesizes proteins. If the cell does not receive a "go" signal, it may enter G₀ (resting state).
  • S Phase (Synthesis): DNA replication occurs. Chromosomes are duplicated to form Sister Chromatids joined at the centromere.
  • G₂ Phase (Second Gap): Final growth and preparation. Cytoskeletal elements needed for mitosis are assembled.

2.2 Part 3: Mitosis (M Phase)

The M Phase includes Mitosis (nuclear division) and Cytokinesis (cytoplasmic division).

Stages of Mitosis

  1. Prophase: Chromatin condenses into visible chromosomes. Spindle forms.
  2. Metaphase: Chromosomes align at the metaphase plate.
  3. Anaphase: Sister chromatids separate and move to opposite poles.
  4. Telophase: New nuclei form; chromosomes de-condense.

Cytokinesis

Animal Cells: Formation of a Cleavage Furrow (contractile ring of microfilaments).

Plant Cells: Formation of a Cell Plate (from Golgi vesicles) which becomes the new cell wall.

Diagram: Stages of Mitosis

step by step mitosis and cytokinesis in animal cell

Visual Analysis: Mitotic Progression

This visual tracks the physical partition of the nucleus and cytoplasm into two identical daughter cells (LO 11.2).

  • Chromosomal Landmarks: Clearly identifies Prophase condensation, Metaphase alignment, and Anaphase separation.
  • Spindle Dynamics: Shows the active pulling of sister chromatids to opposite poles.
  • Cytokinesis: Visualizes the cleavage furrow mechanism in animal cells, ensuring the physical separation of the daughter nuclei.
Prophase Metaphase Anaphase Telophase

2.3 Part 4: Checkpoints & Regulation

The cell cycle is driven by a chemical control system that triggers events. Checkpoints act as stop/go signals.

  • G₁ Checkpoint (Restriction Point): The most critical. Checks for cell size, nutrients, and DNA damage. If denied, the cell exits to G₀.
  • G₂ Checkpoint: Ensures DNA replication is complete and undamaged before Mitosis.
  • M Checkpoint (Spindle Checkpoint): Ensures all chromosomes are attached to spindle fibers during Metaphase.
Molecular Control:
Cyclins: Proteins whose concentration fluctuates cyclically.
Cdks (Cyclin-dependent kinases): Enzymes active only when attached to Cyclins.
MPF (Maturation-Promoting Factor): A specific Cyclin-Cdk complex that triggers the cell's passage into the M phase.

Comprehensive Practice Quiz