Cell Structure and Function

Main Cell Organelles and Their Functions

Cells contain specialized structures called organelles that perform distinct functions necessary for life. Understanding these organelles is fundamental to cell biology.

• Nucleus: Contains genetic material (DNA); controls cellular activities.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Found in plant cells; site of photosynthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes; break down waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Vacuoles: Storage organelles; large central vacuole in plants maintains turgor pressure.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments); provides structural support, cell movement, and transport.

• Extracellular Matrix (ECM): Network outside animal cells; provides structural support and signaling.

• Cell Junctions: Connections between cells (tight junctions, desmosomes, gap junctions).

• Channels: Protein structures in membranes that allow passage of ions and molecules.

Example: The mitochondria are often called the "powerhouse" of the cell because they generate ATP through cellular respiration.

Differences Between Eukaryotes and Prokaryotes

Cells are classified as eukaryotic or prokaryotic based on their structure.

• Eukaryotes: Have a nucleus and membrane-bound organelles (animals, plants, fungi, protists).

• Prokaryotes: Lack a nucleus and membrane-bound organelles (bacteria, archaea).

Organelles in Different Cell Types:

• Animal Cells: No cell wall, have lysosomes, centrioles.

• Plant Cells: Cell wall, chloroplasts, large central vacuole.

• Fungi: Cell wall (chitin), no chloroplasts.

• Protists: Diverse; may have features of both plant and animal cells.

• Prokaryotes: Cell wall (peptidoglycan in bacteria), no nucleus, no membrane-bound organelles.

Membrane Structure and Function

Properties of the Cell Membrane

The cell membrane is a selectively permeable barrier composed mainly of a phospholipid bilayer with embedded proteins.

• Composition: Phospholipids, proteins, cholesterol, carbohydrates.

• Permeability: Allows selective passage of substances; small, nonpolar molecules pass easily, while ions and large polar molecules require transport proteins.

• Function: Protects cell, regulates transport, facilitates communication.

Factors Influencing Membrane Properties: Temperature, lipid composition (saturated vs. unsaturated fatty acids), presence of cholesterol, and types of proteins.

Tonicity and Osmosis

Tonicity describes the relative concentration of solutes in the environment compared to the cell.

• Hypotonic: Lower solute concentration outside; water enters cell, causing swelling.

• Hypertonic: Higher solute concentration outside; water leaves cell, causing shrinkage.

• Isotonic: Equal solute concentration; no net water movement.

Example: Red blood cells in a hypotonic solution swell and may burst (hemolysis).

Transport Across the Cell Membrane

Cells use various mechanisms to move substances across membranes.

• Diffusion: Passive movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive transport via protein channels or carriers.

• Active Transport: Requires energy (ATP) to move substances against concentration gradient.

• Aquaporins: Specialized proteins that facilitate rapid water transport.

• Exocytosis: Export of materials via vesicles fusing with membrane.

• Endocytosis: Import of materials by engulfing them in vesicles.

Example: Sodium-potassium pump uses ATP to maintain ion gradients in animal cells.

Metabolism and Energy

Reduction and Oxidation

Redox reactions involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Catabolic vs. Anabolic Reactions

Metabolic pathways are classified as catabolic (breakdown) or anabolic (synthesis).

• Catabolic: Break down molecules, release energy (e.g., cellular respiration).

• Anabolic: Build molecules, require energy (e.g., photosynthesis).

Spontaneity of Reactions: Determined by change in free energy ().

• Exergonic: Release energy; ; spontaneous.

• Endergonic: Require energy; ; non-spontaneous.

Example: ATP hydrolysis is exergonic and powers cellular processes.

Enzymes and Their Properties

Enzymes are biological catalysts that speed up reactions by lowering the activation energy barrier.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: Region of enzyme where substrate binds.

• Inhibitors: Molecules that decrease enzyme activity (competitive, noncompetitive).

• Activation Energy: Energy required to start a reaction.

ATP Synthase: Enzyme that synthesizes ATP from ADP and inorganic phosphate during cellular respiration and photosynthesis.

Cellular Respiration and Photosynthesis

Major Steps and Locations

Both processes involve multiple steps, each occurring in specific cellular locations.

• Cellular Respiration:

  • Glycolysis: Cytoplasm; breaks down glucose into pyruvate.

  • Pyruvate Oxidation: Mitochondrial matrix; converts pyruvate to acetyl-CoA.

  • Krebs Cycle (Citric Acid Cycle): Mitochondrial matrix; completes glucose oxidation.

  • Electron Transport Chain: Inner mitochondrial membrane; produces most ATP.

• Photosynthesis:

  • Light Reactions: Thylakoid membranes; convert light energy to chemical energy.

  • Calvin Cycle: Stroma; synthesizes glucose from CO2.

Reactants and Products:

Electron Flow: In respiration, electrons flow from glucose to O2; in photosynthesis, electrons flow from water to NADP+.

Energy Storage: Energy is stored in ATP and NADH during respiration; in glucose during photosynthesis.

Main Photosynthetic Pigment and Light Absorption

Chlorophyll is the main pigment in plants, absorbing blue and red wavelengths most efficiently.

Krebs and Calvin Cycles

Both cycles regenerate key molecules, making them cyclical and essential for their respective processes.

• Krebs Cycle: Regenerates oxaloacetate; produces NADH, FADH2, ATP.

• Calvin Cycle: Regenerates RuBP; produces G3P (precursor to glucose).

Importance: Regeneration allows continuous operation and efficient energy conversion.

Photosynthesis vs. Cellular Respiration

Both processes involve electron transport, ATP synthesis, and cycles, but differ in direction and purpose.

• Photosynthesis: Converts light energy to chemical energy; stores energy.

• Cellular Respiration: Releases energy from chemical bonds; produces ATP.

Common Processes: Electron transport chains, ATP synthesis, cycles.

Photorespiration

Photorespiration occurs when Rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

Oxygen's Role

• Cellular Respiration: Final electron acceptor; forms water.

• Photosynthesis: Produced as a byproduct of water splitting.

Fermentation

Fermentation is an anaerobic process occurring in the cytoplasm, allowing ATP production without oxygen.

• Lactic Acid Fermentation: Produces lactic acid.

• Alcohol Fermentation: Produces ethanol and CO2.

C3 vs. C4 Plants

C3 and C4 plants differ in their photosynthetic pathways.

Example: Corn is a C4 plant; wheat is a C3 plant.

Logical Application of Concepts

Understanding these concepts allows students to analyze real-life biological cases, interpret figures, and apply data to solve problems.

Additional info: Logical thinking and application are emphasized in modern biology education, encouraging students to go beyond memorization and understand underlying mechanisms.