General Biology: Cellular Processes, Signaling, and Energy Metabolism

Calvin Cycle

Carbon Fixation Process

The Calvin cycle is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is responsible for converting atmospheric carbon dioxide into organic molecules.

• Carbon fixation: Incorporation of carbon from the atmosphere into a five-carbon molecule, catalyzed by the enzyme RuBisCO.

• Carbon formation is not stable and intermittently breaks up into two three-carbon molecules.

Reduction

• Electrons are added to the molecules, increasing their energy.

• ATP is used to add phosphate groups; NADPH donates electrons.

• One phosphate group is released during this process.

Regeneration of CO2 Receptor

• Three-carbon molecules (G3P) are recycled to regenerate the five-carbon acceptor (RuBP).

• Some G3P is used to synthesize glucose and other carbohydrates.

• Five three-carbon molecules are recycled into three five-carbon molecules.

Cell Signaling

Types of Cell Communication

Cells communicate to coordinate activities and respond to environmental changes. Signaling can occur over short or long distances.

• Intercellular signaling: Communication between cells.

• Intracellular signaling: Communication within a cell.

• Ligands: Signaling molecules that bind to receptors.

• Paracrine signaling: Signals to nearby cells.

• Synaptic signaling: Involves neurotransmitters between neurons.

• Autocrine signaling: Cells signal to themselves.

• Endocrine signaling: Long-distance signaling via hormones.

Transduction Pathways

• Relay systems pass signals through molecules, often triggering phosphorylation cascades.

• Protein kinases: Enzymes that add phosphate groups to proteins, activating or deactivating them.

Fermentation

Overview

Fermentation is an anaerobic process that allows cells to recycle electron carriers and produce ATP when oxygen is scarce.

• Alcohol fermentation: Glycolysis produces pyruvate, which is converted to ethanol and CO2. NAD+ is regenerated.

• Lactic acid fermentation: Glycolysis produces pyruvate, which is converted to lactic acid. NAD+ is regenerated; lactate can build up until oxygen returns.

Spontaneous Processes

Exergonic and Endergonic Reactions

Biochemical reactions are classified by their energy changes.

• Exergonic reaction: Releases energy; spontaneous.

• Endergonic reaction: Requires energy input; nonspontaneous.

Gibbs Free Energy

• Energy released after a reaction; difference between initial and final states.

Catalysts and Enzymes

• Catalyst: Speeds up reactions without being consumed.

• Enzyme: Biological catalyst, usually a protein.

• Activation energy: Energy required to start a process. Enzymes lower activation energy, increasing reaction rate.

Cell Membrane Structure and Function

Extracellular Matrix Composition

The extracellular matrix (ECM) provides structural support and mediates cell communication.

• Collagen is the primary structural molecule for support and elasticity.

• Proteoglycans contribute to attachment and hydration.

• Integrins connect the ECM to the cytoskeleton.

Cell Junction Types

• Tight junctions: Prevent fluid leakage.

• Desmosomes: Provide mechanical strength.

• Gap junctions: Allow communication between animal cells.

Plant Cell Walls and Plasmodesmata

• Plant cells have rigid cell walls for shape and protection.

• Plasmodesmata are channels for exchange of fluids and small molecules between plant cells.

Protein Secretion and Cellular Export

• Proteins are synthesized in the rough ER, processed in the Golgi apparatus, and exported via exocytosis.

Membrane Structure

• Phospholipid bilayer with hydrophilic heads and hydrophobic tails.

• Fluid mosaic model describes membrane flexibility.

• Cholesterol and unsaturated fatty acids affect fluidity.

Membrane Transport and Signaling

Transport Mechanisms

• Passive transport: Diffusion, osmosis, facilitated diffusion.

• Active transport: Requires energy (e.g., Na+/K+ pump).

• Bulk transport: Endocytosis and exocytosis for large molecules.

Cellular Signaling Mechanisms

• Direct contact (gap junctions), localized signaling (paracrine), and long-distance signaling (endocrine).

• Receptors can be membrane-bound (e.g., GPCRs) or intracellular.

• Signal transduction cascades amplify signals via phosphorylation.

Metabolism and Thermodynamics

Metabolic Pathways

• Anabolic pathways: Build complex molecules.

• Catabolic pathways: Break down molecules.

• Energy transformations follow the laws of thermodynamics.

ATP as Cellular Energy Currency

• ATP stores energy in high-energy phosphate bonds.

• Hydrolysis of ATP releases energy for cellular work.

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Enzymes and Catalysis

Enzyme Function

• Enzymes lower activation energy, increasing reaction rates.

• Enzyme activity is affected by temperature, pH, cofactors, and inhibitors.

Enzyme Kinetics

• Michaelis-Menten kinetics describe enzyme activity.

$V = \frac{V_{max}[S]}{K_m + [S]}$

Cellular Respiration

Overview

• Cellular respiration converts glucose into ATP via glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

• Glycolysis occurs in the cytoplasm; other steps occur in mitochondria.

• Electron transport chain uses NADH and FADH2 to pump protons and generate ATP.

Overall Reaction

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Photosynthesis

Photosynthetic Fundamentals

• Photosynthesis converts solar energy into chemical energy in sugars.

• Occurs in chloroplasts, primarily in leaf mesophyll cells.

• Light reactions capture energy and produce ATP and NADPH.

• Calvin cycle uses ATP and NADPH to fix CO2 into sugars.

Photosynthesis Reaction

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Ecological and Evolutionary Context

Photosynthesis and Food Webs

• Photosynthesis is the primary energy input for most ecosystems.

• Oxygenic photosynthesis transformed Earth's atmosphere.

• Plants produce fruits and nectar for pollination and seed dispersal.

Key Conclusions

• Cellular organization and communication depend on specialized structures and signaling mechanisms.

• Membrane fluidity and transport are dynamically regulated.

• Energy metabolism is driven by electron flow and ATP production.

• Enzymes are essential for metabolic efficiency.

• Cellular respiration and photosynthesis are multi-step processes yielding ATP and organic molecules.

• Photosynthesis sustains life and drives evolutionary innovation.

Important Table: Types of Cell Junctions

Junction Type	Function	Location
Tight Junction	Prevents fluid leakage	Animal cells, epithelial tissue
Desmosome	Provides mechanical strength	Animal cells, tissues under stress
Gap Junction	Allows communication and exchange of ions/molecules	Animal cells, cardiac and smooth muscle
Plasmodesmata	Allows exchange of fluids and small molecules	Plant cells