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.

• This structure allows selective passage of substances.

Fluid Mosaic Model

The fluid mosaic model describes the dynamic nature of the cell membrane.

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

• Mosaic: The membrane is composed of various proteins, lipids, and carbohydrates.

Membrane Transport Mechanisms

Cells use several mechanisms to transport substances across membranes.

• 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 against the concentration gradient, requiring 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. The effect of different solutions on cells:

• 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, causing swelling (animal cells may burst, plant cells become turgid).

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

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

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

Enzymes and Metabolism

Endergonic and Exergonic Reactions

Chemical reactions in cells can be classified by energy changes:

• Endergonic reactions: Require energy input (e.g., synthesis of macromolecules).

• Exergonic reactions: Release energy (e.g., breakdown of glucose).

Anabolism and Catabolism

• 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 reactions by lowering activation energy.

• 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 locations other than the active site where molecules can bind and regulate enzyme activity.

The concentration of inhibitors affects the degree of inhibition in both models.

Cellular Respiration

Redox Reactions

Cellular respiration involves 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 produce ATP using oxygen.

• 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.

• Up to 32-38 ATP molecules are produced per glucose molecule.

Fermentation

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

• Final electron acceptor: Organic molecules (e.g., pyruvate).

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

• Mechanism: Regenerates NAD+ for glycolysis.

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; produce ATP and NADPH, release O2.

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

Chlorophyll and Pigments

• Chlorophyll: Main pigment absorbing light for photosynthesis.

• Absorption of light excites electrons, initiating electron transport.

• Other pigments (carotenoids, etc.) assist in capturing light energy.

Electron Flow in Photosynthesis

Electron flow starts with water splitting, producing O2 and transferring electrons through photosystems II and I.

• Final electron acceptor: NADP+, forming NADPH.

• ATP and NADPH are used in the Calvin cycle.

Calvin Cycle

• Primary function: Fixation of CO2 into organic molecules.

• Key enzyme: Rubisco catalyzes carbon fixation.

Order of Photosynthetic Events

• CO2 fixation

• ATP production

• Glucose production

• Chlorophyll excitation and electron transfer

• Water splitting to produce oxygen

• NADPH production

Cellular Locations of Photosynthetic Events

Additional info:

• Some questions reference specific steps and molecules (e.g., NADH, FADH2, ATP) in cellular respiration and photosynthesis. These are key electron carriers and energy molecules.

• Rubisco is the most abundant enzyme on Earth and is crucial for carbon fixation in the Calvin cycle.

• ATP synthase is the enzyme responsible for synthesizing ATP during oxidative phosphorylation.