Cell Structure and Membrane Function

Phospholipid Bilayer Structure

The phospholipid bilayer forms the fundamental structure of cell membranes, providing a semi-permeable barrier between the cell and its environment.

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer arrangement: Hydrophobic tails face inward, hydrophilic heads face outward.

• Provides fluidity and flexibility to the membrane.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of components that move fluidly within the lipid bilayer.

• Fluid: Lipids and proteins can move laterally within the layer.

• Mosaic: Diverse proteins, cholesterol, and carbohydrates are embedded in the bilayer.

Membrane Transport Mechanisms

Cells exchange substances with their environment through various transport mechanisms.

• Simple diffusion: Movement of small, nonpolar molecules down their concentration gradient without energy or proteins.

• Facilitated diffusion: Movement of molecules down their gradient via membrane proteins (channels or carriers); no energy required.

• Active transport: Movement of molecules against their gradient using energy (usually ATP) and transport proteins.

Osmosis and Solutions

Isotonic, Hypotonic, and Hypertonic Solutions

Osmosis is the movement of water across a semi-permeable membrane in response to solute concentration differences.

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, which may swell or burst (lysis in animal cells, turgid in plant cells).

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing shrinkage (crenation in animal cells, plasmolysis in plant cells).

Osmotic lysis occurs when a cell bursts due to excessive water intake in a hypotonic environment.

Plasmolysis is the shrinking of the cytoplasm away from the cell wall in plant cells in a hypertonic environment.

Enzymes and Metabolism

Endergonic and Exergonic Reactions

Chemical reactions in cells can either require energy or release energy.

• Endergonic reactions: Absorb energy; products have more energy than reactants (e.g., photosynthesis).

• Exergonic reactions: Release energy; products have less energy than reactants (e.g., cellular respiration).

Anabolism and Catabolism

Metabolism consists of two main types of reactions:

• Anabolism: Building complex molecules from simpler ones; usually endergonic.

• Catabolism: Breaking down complex molecules into simpler ones; usually exergonic.

Enzyme Activity and Inhibition

Enzymes are biological catalysts that speed up chemical reactions.

• Enzyme activity: Influenced by temperature, pH, substrate concentration, and inhibitors.

• Competitive inhibition: Inhibitor competes with substrate for the active site.

• Non-competitive inhibition: Inhibitor binds elsewhere, changing enzyme shape and reducing activity.

• Allosteric sites are regulatory sites on enzymes where molecules can bind to modulate activity.

Cellular Respiration

Redox Reactions

Cellular respiration involves a series of oxidation-reduction (redox) reactions.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• In the reaction , glucose is oxidized and oxygen is reduced.

Aerobic Respiration Steps

Aerobic respiration is the process by which cells convert glucose and oxygen into ATP, carbon dioxide, and water.

• Major steps: Glycolysis, Pyruvate oxidation, Citric acid cycle (Krebs cycle), Electron transport chain, and Oxidative phosphorylation.

• Electron carriers: NADH, FADH2 transport electrons to the electron transport chain.

• ATP is produced mainly during oxidative phosphorylation.

• Oxygen acts as the final electron acceptor in the electron transport chain.

• Total ATP yield per glucose: ~30-32 ATP (most from oxidative phosphorylation).

Fermentation

Fermentation is an anaerobic process that allows ATP production without oxygen.

• Occurs when oxygen is absent; pyruvate is converted to lactate (in animals) or ethanol and CO2 (in yeast).

• Produces much less ATP than aerobic respiration (2 ATP per glucose).

• Final electron acceptor: organic molecules (not oxygen).

Photosynthesis

Light and Dark Reactions

Photosynthesis converts light energy into chemical energy in plants, algae, and some bacteria.

• Light reactions: Occur in the thylakoid membranes; use light to produce ATP and NADPH, and release O2.

• Dark reactions (Calvin cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

• Chlorophyll absorbs light energy, exciting electrons.

• Electron flow: Water → Photosystem II → Electron transport chain → Photosystem I → NADP+ → NADPH.

• Rubisco catalyzes carbon fixation in the Calvin cycle.

Photosynthesis Steps and Locations

Key events and their cellular locations:

• Oxygen production: Thylakoid membrane (light reactions).

• ATP production: Stroma (Calvin cycle in plants), cytoplasm (animal cells).

• Electron transport: Thylakoid membrane (plants), mitochondria (animals).

Order of Photosynthesis Events

The main steps in photosynthesis, in order:

1. CO2 fixation

2. ATP production

3. Glucose production

4. Chlorophyll excitation and electron transfer

5. Water splitting to produce oxygen

6. NADPH production

Key Terms Table

Additional info: Some explanations and order of events were inferred from standard biology curriculum to ensure completeness and clarity.