Cellular Processes: Membranes, Energy, Enzymes, Respiration, and Photosynthesis

Cell Membrane

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed mainly of phospholipids, embedded proteins, and other molecules. It regulates the movement of substances in and out of the cell and maintains cellular integrity.

• Bilayer Formation: Phospholipids spontaneously form bilayers due to their hydrophilic heads and hydrophobic tails.

• Membrane Fluidity:

  • Temperature and cholesterol affect fluidity.

  • Unsaturated fats increase fluidity; saturated fats decrease it.

• Membrane Proteins:

  • Transporters, enzymes, surface receptors, and identity markers are key types.

• Fluid Mosaic Model: Describes the membrane as a fluid, dynamic structure with proteins embedded in or associated with the lipid bilayer.

• Selective Permeability:

  • Small, hydrophobic molecules pass easily; large, hydrophilic molecules require transport proteins.

Molecule Transport

Transport across the plasma membrane occurs via passive or active mechanisms, depending on the nature of the molecules and the concentration gradients.

• Passive Transport:

  • Simple Diffusion: Movement of molecules from high to low concentration without energy input.

  • Facilitated Diffusion: Movement via transport proteins or carrier proteins, still down the concentration gradient.

• Active Transport: Requires energy (usually ATP) to move molecules against their concentration gradient.

• Endocytosis/Exocytosis: Bulk transport mechanisms for large molecules or particles.

Energy and Thermodynamics

Energy in Cells

Cells require energy to perform work, which is provided in various forms and governed by the laws of thermodynamics.

• Forms of Energy:

  • Potential energy (stored in bonds)

  • Kinetic energy (energy of motion)

• Oxidation and Reduction:

  • Oxidation is loss of electrons; reduction is gain of electrons.

  • NAD+ is an oxidizing agent; NADH is a reducing agent.

• Two Laws of Thermodynamics:

  • Energy is neither created nor destroyed.

  • Entropy (disorder) in the universe is always increasing.

• Gibbs Free Energy ($\Delta G$):

  • Equation: $\Delta G = \Delta H - T\Delta S$

  • Negative $\Delta G$ (exergonic) = spontaneous; positive $\Delta G$ (endergonic) = non-spontaneous.

• Activation Energy: Energy required to initiate a reaction; enzymes lower activation energy.

• ATP: The energy currency of the cell; hydrolysis of ATP releases energy for cellular processes.

Enzymes and Metabolism

Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy. They are highly specific for their substrates.

• Catalysts: Increase reaction rates without being consumed.

• Substrate and Active Site: Substrates bind to the enzyme's active site, forming an enzyme-substrate complex.

• Enzyme Function:

  • Kinase – adds phosphate from ATP

  • Phosphatase – removes phosphate

  • Dehydrogenase – transfers hydrogen

• Cofactors and Coenzymes:

  • Cofactors: Metal ions aiding electron transfer

  • Coenzymes: Organic molecules (e.g., NAD+, FAD)

• Enzyme Regulation: Pathways are regulated by feedback inhibition.

Metabolism

Metabolism encompasses all chemical reactions in an organism, including catabolism (breakdown) and anabolism (synthesis).

• Catabolic Pathways: Break down molecules to release energy.

• Anabolic Pathways: Build complex molecules from simpler ones.

• Biochemical Pathways: Series of reactions where the product of one enzyme is the substrate for the next.

Cellular Respiration: Glycolysis

Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose by oxidizing it, producing ATP.

• Autotrophs: Use sunlight to make energy (plants, green algae).

• Heterotrophs: Obtain energy by consuming nutrients.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm, where glucose is split into pyruvate.

• Inputs: 1 Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

• Key Enzymes:

  • Hexokinase

  • Phosphofructokinase (PFK) – major regulatory enzyme

  • Pyruvate kinase

• Net ATP Yield: 2 ATP per glucose

Cellular Respiration: Pyruvate Oxidation and Krebs Cycle

Pyruvate Oxidation

Pyruvate oxidation links glycolysis to the Krebs cycle, converting pyruvate to Acetyl CoA in the presence of oxygen.

• Occurs in: Mitochondrial matrix

• Inputs: 2 Pyruvate, 2 NAD+, 2 Coenzyme A (CoA)

• Outputs: 2 Acetyl CoA, 2 NADH, 2 CO2

• No ATP is synthesized during pyruvate oxidation.

Cellular Respiration: Electron Transport Chain (ETC)

Electron Transport Chain

The ETC is located in the inner mitochondrial membrane and is responsible for the majority of ATP production during cellular respiration.

• Electron Carriers: NADH and FADH2 donate electrons to the chain.

• Proton Gradient: Electrons move through complexes, pumping protons across the membrane, creating a gradient.

• ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and Pi.

• Oxygen: Final electron acceptor, forming water.

• Inputs: NADH, FADH2, O2

• Outputs: 28 ATP, H2O

• Overall ATP Yield: Aerobic respiration produces about 32-34 ATP per glucose.

Regulation of Respiration

Respiration is regulated by energy needs and key enzymes such as phosphofructokinase (PFK).

• High ATP inhibits PFK; low ATP and AMP activate it.

Photosynthesis and the Calvin Cycle

Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

• Occurs in: Chloroplasts

• Light-dependent Reactions: Capture light energy to produce ATP and NADPH.

• Light-independent Reactions (Calvin Cycle): Use ATP and NADPH to fix CO2 into glucose.

• Chlorophyll: Main pigment absorbing light; chlorophyll a and b absorb different wavelengths.

• Electron Transport: Electrons move through photosystems to reduce NADP+ to NADPH.

The Calvin Cycle

The Calvin Cycle is the set of light-independent reactions that fix carbon dioxide into organic molecules.

• Occurs in: Stroma of chloroplasts

• Overall Equation: $6CO_2 + 12NADPH + 10H_2O + 18ATP \rightarrow 2C_3 + 12NADP^+ + 18ADP$

• Phases:

  • Carbon fixation: CO2 combines with RuBP to form 3-PGA.

  • Reduction: ATP and NADPH reduce 3-PGA to G3P.

  • Regeneration: RuBP is regenerated for the cycle to continue.

Photorespiration and Plant Adaptations

Photorespiration

Photorespiration occurs when Rubisco uses O2 instead of CO2, leading to reduced photosynthetic efficiency.

• Increased temperatures increase photorespiration.

• C3 plants use only the Calvin cycle.

• C4 plants and CAM plants have adaptations to minimize photorespiration by spatial or temporal separation of CO2 fixation.

Plant Type	CO2 Fixation Method	Photorespiration Minimization	Examples
C3	Calvin Cycle only	None	Wheat, rice
C4	Calvin Cycle + PEP carboxylase	Spatial separation (bundle sheath cells)	Corn, sugarcane
CAM	Calvin Cycle + PEP carboxylase	Temporal separation (night/day)	Cacti, pineapple

Additional info: The above notes expand on brief points and fill in missing context for clarity and completeness, including definitions, equations, and examples relevant to a college-level biology course covering cellular processes.