Biology Lecture on Cell Biology and Metabolism

Cellular Respiration and Organelles

Mitochondria and Oxygen

• ATP Production: The mitochondria is the primary site for ATP (adenosine triphosphate) production, though it can be made elsewhere, it yields the most ATP.

• Aerobic Respiration: This process, also known as the respiratory chain, is highly dependent on oxygen.

• Oxygen's Role: Oxygen acts as the final electron acceptor in the electron transport chain, which is crucial for this metabolic process.

• Anaerobic Respiration: When oxygen is insufficient (e.g., during intense exercise like walking up steps or prolonged talking causing breathlessness):

  • Muscles switch to anaerobic respiration.

  • Results in the production of lactic acid, leading to muscle fatigue and tiredness.

  • The body's response is to gasp for breath to increase oxygen supply to tissues.

• Blood Doping: Athletes sometimes use methods like blood doping to increase oxygen in their blood and tissues, which reduces fatigue and soreness, allowing for sustained performance.

Muscle Types: Dark vs. White Meat

• Dark Meat (e.g., chicken legs/thighs, duck):

  • Contains a high concentration of mitochondria, indicating high aerobic activity.

  • Rich in myoglobin, a protein that carries oxygen to tissues.

  • Associated with muscles used for sustained activity (e.g., chickens that primarily walk/run, ducks that fly, swim, and run).

• White Meat (e.g., chicken breast):

  • Generally has fewer mitochondria and less myoglobin.

  • Associated with muscles used for short bursts of activity (e.g., chicken wings for short flights, though wings can be debatable).

Peroxisomes

• Detoxification: Primarily responsible for detoxifying harmful substances within the cell, such as medicines and poisons.

• Metabolic Breakdown: Important for the breakdown of fatty acids and amino acids, acting like a cellular garbage disposal unit.

Plant vs. Animal Cells (Eukaryotic Cells)

• Similarities: Both are eukaryotic cells, meaning they possess a nucleus.

• Key Differences in Plant Cells:

  • Cell Wall: A rigid outer layer primarily made of cellulose (a structural sugar). It provides rigidity and structural support to the plant, contributing to turgor pressure. Cellulose is considered dietary fiber for humans, as we cannot break it down for energy.

  • Large Central Vacuole: This large organelle is primarily responsible for storing water, often in the form of sap. It plays a significant role in maintaining turgor pressure, which keeps plants rigid and upright. A lack of water causes plants to wilt.

  • Chloroplasts: Organelles unique to plants and other photosynthetic organisms. Animal cells do not have chloroplasts.

Osmosis and Tonicity

• Osmosis Defined: The movement of water across a semipermeable membrane, driven by concentration differences, to dilute the more concentrated side.

  • Water generally moves from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration).

  • This process is natural and does not require energy.

• Tonicity (Tautness of the Cell):

  • Isotonic Solution: The concentration of solutes outside the cell is the same as inside the cell. Water moves freely in and out, but there is no net change in cell shape (e.g., human blood plasma is ideally isotonic to red blood cells).

  • Hypertonic Solution: The solution outside the cell is more concentrated than inside the cell. Water rushes out of the cell to dilute the external solution, causing the cell to shrivel (e.g., a plant in saltwater will wilt and die). In humans, high blood sugar (diabetes) creates a hypertonic environment for cells, causing water to leave cells and enter the blood, leading to symptoms like frequent urination, excessive thirst, and potential edema.

  • Hypotonic Solution: The solution outside the cell is less concentrated than inside the cell. Water rushes into the cell to dilute the internal environment, causing the cell to swell and potentially burst (e.g., a plant watered with distilled water, which lacks solutes, might experience this).

• Practical Implications:

  • Drinking saltwater is deadly because it creates a hypertonic environment, drawing water out of body cells.

  • Drinking excessive amounts of distilled water is also not recommended for humans as it lacks essential minerals and can disrupt the body's osmotic balance.

  • Distilled water is used in machines (like CPAPs or bottle warmers) to prevent the buildup of mineral deposits (e.g., calcium) found in tap water.

Endosymbiosis Theory

• Concept: This theory proposes that existing cells engulfed other cells, leading to a symbiotic relationship where both cells benefited and became a single, more complex organism.

• Explanation for Mitochondria: The theory explains the origin of mitochondria (and by extension, chloroplasts in plant cells).

  • An ancestral cell (which needed nutrition but couldn't produce its own energy) engulfed a prokaryote (bacterium) capable of producing energy.

  • This led to a symbiotic relationship, forming the early eukaryotic cells with energy-producing organelles.

• Evidence: Mitochondria possess their own distinct DNA (mitochondrial DNA).

  • Mitochondrial DNA is inherited exclusively from the mother.

  • It remains virtually unchanged through generations, supporting the idea of a separate evolutionary origin.

Other Key Organelles

• Lysosomes:

  • Function as the cell's recycling centers.

  • Break down old cells, old organelles, and biomolecules (proteins, lipids, carbohydrates) into smaller components for reuse or excretion.

• Endoplasmic Reticulum (ER):

  • A network of membranes that connect with the nucleus.

  • Rough Endoplasmic Reticulum (RER):

    • Studded with ribosomes, which synthesize proteins.

    • Involved in modifying newly synthesized proteins.

  • Smooth Endoplasmic Reticulum (SER):

    • Lacks ribosomes, giving it a 'smooth' appearance.

    • Involved in the synthesis of lipids, carbohydrates, and steroids.

    • Crucially, it serves as a major storage site for calcium ions (e.g., sarcoplasmic reticulum in muscle cells).

• Golgi Body (or Golgi Apparatus):

  • Functions like a cellular sorting and packaging center.

  • Modifies, sorts, and packages proteins and lipids into vesicles for transport to their final destinations (either within the cell or outside).

  • Employs 'chemical tags' (often lipids or sugars) to direct proteins to the correct location, similar to sorting packages by ZIP code.

• Centrosomes and Microtubules:

  • Centrosomes: Are the main microtubule-organizing centers in animal cells.

  • Microtubules: Are tiny projections that play a vital role in:

    • Cell movement.

    • Chromosomal segregation during cell division (mitosis and meiosis), where they form spindle fibers that pull DNA to opposite poles.

    • General intracellular transport, acting like cellular highways.

Chloroplasts and Photosynthesis

• Location: Found only in plants and other organisms capable of photosynthesis (e.g., some bacteria, certain fungi).

• Structure: Possess a double membrane, similar to mitochondria.

• Primary Function: Convert light energy into chemical energy in the form of sugar.

• Photosynthesis Process:

  • Takes in light energy, water (H2O), and carbon dioxide (CO2).

  • Produces sugar (chemical energy) and oxygen (O_2) as byproducts.

  • It's a unique process that converts inorganic substances (CO2, H2O) into organic compounds (sugar), performed by autotrophs.

• Symbiotic Relationship with Animals:

  • Plants produce O_2 that animals inhale and sugar that animals break down for energy.

  • Animals exhale CO_2 that plants inhale.

  • This forms a crucial symbiotic cycle, where plants are self-sustaining, requiring only water and sunlight to produce their own food and energy. Plant cells also possess mitochondria to break down the sugars they produce for their own cellular energy, just like animal cells.