Cell Biology: Membrane Structure and Function

Fluid Mosaic Model

• The cell membrane is a flexible layer of phospholipids with proteins embedded.
• Phospholipids are amphipathic with a hydrophilic head and hydrophobic tails.
• The phospholipid bilayer has heads facing outward and tails facing inward.

Membrane Fluidity

• Temperature affects membrane fluidity.
• Higher temperatures lead to increased fluidity (more movement).
• Unsaturated fatty acids increase fluidity due to kinks preventing tight packing.
• Saturated fatty acids decrease fluidity as they pack tightly.

Cholesterol's Role

• Cholesterol helps maintain membrane fluidity by preventing it from becoming too rigid or too fluid.

Membrane Protein Functions

• Transport proteins help move substances in and out of the cell.
• Receptor proteins receive signals from outside the cell.
• Enzymes catalyze reactions at the membrane.

Movement Across Membranes

• Small nonpolar molecules (O2, CO2) can easily cross the membrane.
• Large or charged molecules (glucose, ions) require transport proteins.

Types of Passive Transport

• Simple diffusion: Movement of small molecules directly through the membrane.
• Facilitated diffusion: Movement of larger molecules through protein channels.

Diffusion vs. Osmosis

• Diffusion involves the movement of molecules from high to low concentration.
• Osmosis is the movement of water from low solute concentration to high solute concentration.

Tonicity Terms

• Hypertonic solutions have a higher solute concentration outside the cell, causing water to move out (cell shrinks).
• Hypotonic solutions have a lower solute concentration outside the cell, causing water to move in (cell swells).

Active Transport vs. Passive Transport

• Active transport moves substances against their concentration gradient, requiring energy (ATP).
• Passive transport moves substances down their concentration gradient without energy.

Co-Transport

• Co-transport involves the simultaneous transport of two substances across a membrane.
• One substance moves with its concentration gradient, providing energy for the other substance to move against its gradient.

Bulk Transport

• Endocytosis: The cell takes in large materials by engulfing them (e.g., phagocytosis for solids).
• Exocytosis: The cell expels materials using vesicles that fuse with the membrane.

Catabolic vs. Anabolic Reactions

• Catabolic reactions break down larger molecules into smaller ones, releasing energy (e.g., cellular respiration).
• Anabolic reactions build larger molecules from smaller ones, requiring energy (e.g., protein synthesis).

Types of Energy

• Potential energy is stored energy (e.g., energy in chemical bonds).
• Chemical energy is potential energy stored in molecular bonds (e.g., glucose).
• Kinetic energy is the energy of motion (e.g., moving molecules).
• Thermal energy is a form of kinetic energy related to molecular motion.
• Heat is energy transferred due to temperature differences and is not usable for work.

Entropy

• Entropy measures disorder or randomness in a system.
• In a closed system, entropy increases as energy is dispersed (e.g., ice melting).
• Living organisms maintain order by using energy to counteract increasing entropy.

Free Energy

• Free energy is the energy available to do work in a system.
• Reactions with products having lower free energy than reactants are likely to proceed spontaneously.
• Exergonic reactions release free energy (e.g., cellular respiration).
• Endergonic reactions require free energy (e.g., photosynthesis).

Spontaneous Reactions

• Spontaneous reactions occur without external input and increase the entropy of the universe.
• A reaction can be spontaneous even if it is slow.

Reaction Coupling

• Reaction coupling links an exergonic reaction with an endergonic reaction.
• Energy released from the exergonic reaction (e.g., ATP hydrolysis) is used to power the endergonic reaction.

Role of ATP

• ATP (adenosine triphosphate) is the primary energy carrier in cells.
• It stores energy in its high-energy phosphate bonds and releases energy when hydrolyzed to ADP.

Enzymes and Chemical Reactions

• Enzymes speed up reactions by lowering activation energy.
• Substrates bind to the active site of the enzyme, allowing the enzyme to be reused.

Reaction Rates and Energy Levels

• Lowering the energy level of the transition state increases the reaction rate.
• Adding or removing enzymes changes reaction rates.
• Competitive inhibitors block active sites, slowing reactions.
• Non-competitive inhibitors change enzyme shape, also slowing reactions.

Activation Energy and Regulation

• Enzymes lower activation energy, making reactions easier to start and enabling regulation of metabolic pathways.

Relationship Between Cellular Respiration and Photosynthesis

• Photosynthesis captures light energy and converts it into chemical energy (glucose).
• Cellular respiration breaks down glucose to release energy for cellular processes.
• Cellular respiration (glucose + oxygen --> carbon dioxide + water + ATP) is essentially the reverse of photosynthesis.

Energy Change Diagram

• Glucose and oxygen are reactants, and carbon dioxide, water, and ATP are products.
• The energy change during cellular respiration is a release of energy.

Redox Reactions

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• The oxidizing agent gains electrons and is reduced.
• The reducing agent loses electrons and is oxidized.

Redox Reactions in Cellular Processes

• Combustion is similar to cellular respiration, with the oxidation of fuel (glucose) producing carbon dioxide and water.
• Cellular respiration involves redox reactions where glucose is oxidized and oxygen is reduced.

Function of NAD

• NAD (nicotinamide adenine dinucleotide) is an electron carrier.
• It becomes NADH when it gains electrons during metabolic reactions, which is crucial for energy production in cellular respiration.

Major Steps of Cellular Respiration

• Glycolysis:

  • Location: Cytoplasm
  • Reactants: Glucose
  • Products: 2 pyruvate, 2 ATP, 2 NADH

• Pyruvate Oxidation:

  • Location: Mitochondrial matrix
  • Reactants: 2 pyruvate
  • Products: 2 Acetyl-CoA, 2 NADH, 2 CO2

• Citric Acid Cycle (Krebs Cycle):

  • Location: Mitochondrial matrix
  • Reactants: 2 Acetyl-CoA
  • Products: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• Oxidative Phosphorylation:

  • Location: Inner mitochondrial membrane
  • Reactants: NADH, FADH2, O2
  • Products: ATP, H2O

ATP Generation

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP to form ATP (occurs in glycolysis and the citric acid cycle).
• Oxidative phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain, utilizing the proton gradient.

Effects of Oxygen Supply

• Anaerobic conditions: If oxygen is scarce, cells switch to anaerobic respiration (e.g., fermentation).
• Fermentation results in the buildup of lactic acid or ethanol instead of CO2 and water.

Response to Oxygen Demand

• Anaerobic conditions: When oxygen is scarce, ATP production is less efficient leading to fatigue during intense exercise.
• Cells may switch to anaerobic pathways (e.g., lactic acid fermentation) to produce ATP.

Impact of Circulatory System Changes

• Reduced blood flow can lead to decreased oxygen delivery, impairing aerobic respiration, and causing reliance on anaerobic pathways, reducing ATP yield.

Flow of Matter and Energy

• Autotrophs (like plants) produce their own food through photosynthesis, converting light energy into chemical energy (glucose).
• Heterotrophs (like animals) obtain energy by consuming autotrophs or other heterotrophs.
• Energy flow: Energy from the sun is captured by autotrophs, producing glucose which is consumed by heterotrophs, transferring energy and matter through food chains.

Structure of Leaves and Chloroplasts

• Leaves have broad, flat surfaces to maximize light absorption.
• Chloroplasts contain chlorophyll and have a double membrane structure with thylakoids (light reactions) and stroma (Calvin cycle).
• Chloroplasts are arranged in leaves to optimize light capture and gas exchange.

Net Reaction for Photosynthesis

• Chemical equation: Carbon dioxide + water + light energy --> glucose + oxygen
• Carbon in glucose originates from carbon dioxide, while oxygen comes from water.

Stages of Photosynthesis

• Light Reactions: Convert solar energy into chemical energy (ATP and NADPH).
  • Location: Thylakoid membranes
• Calvin Cycle: Uses ATP and NADPH to convert CO2 into glucose.
  • Location: Stroma
• Light reactions produce energy carriers needed to power the Calvin cycle.

Light Absorption and Energy Harvesting

• Different pigments (e.g., chlorophyll) absorb different wavelengths of light, mainly blue and red light.
• Absorption spectra show the amount of light absorbed at different wavelengths, indicating which pigments are present and how they affect photosynthesis.

Chlorophyll Excitation

• Chlorophyll absorbs light energy, exciting electrons to a higher energy state, which can be used in photosynthesis.
• When excited electrons fall back to their original state, energy is released, often as heat or light.

Photosystems

• Located in the thylakoid membranes.
• Capture and transfer light energy to drive the electron transport chain.

Energy and Matter Flow

• Electron Transport Chain: Energized electrons from photosystems are passed along a series of proteins, releasing energy used to pump protons into the thylakoid lumen, creating a proton gradient.
• ATP and NADPH Production: ATP synthase uses the proton gradient to synthesize ATP, while electrons ultimately reduce NADP+ to NADPH.

Water Splitting Reaction

• During light reactions, water is split into oxygen, protons, and electrons.
• Oxygen is released as a byproduct, and electrons replenish those lost by chlorophyll.

Calvin Cycle

• Three phases:
  • Carbon Fixation: CO2 is added to ribulose bisphosphate (RuBP).
  • Reduction: ATP and NADPH convert 3-PGA to G3P (sugar).
  • Regeneration: RuBP is regenerated to continue the cycle.
• ATP and NADPH are required for the energy and reducing power to convert CO2 into glucose.

Comparison of Photosynthesis and Cellular Respiration

• Both use electron carriers (NADPH in photosynthesis, NADH in respiration) to transport electrons.
• Both create gradients (protons in photosynthesis and respiration) to drive ATP synthesis via ATP synthase.

Citric Acid Cycle vs. Calvin Cycle

• The citric acid cycle (catabolic) produces ATP, NADH, and FADH2 from the breakdown of glucose.
• The Calvin cycle (anabolic) uses ATP and NADPH to synthesize glucose.

Predicting Changes in Closed Systems

• Plant-only system: CO2 levels decrease, O2 levels increase.
• Animal-only system: CO2 levels increase, O2 levels decrease.
• Mixed system: Levels depend on the ratio of plants to animals, achieving a balance.

Explore the intricacies of the fluid mosaic model and how membrane components contribute to cell functions. This quiz covers topics like phospholipid bilayers, membrane fluidity, the role of cholesterol, and various types of membrane proteins. Test your understanding of how substances move across cell membranes.