Cells: The Building Blocks of Life

Cell Theory

• All living organisms are composed of one or more cells.

• The cell is the basic unit of life.

• All cells arise from pre-existing cells.

Types of Cells

• Prokaryotic Cells

  • Found in bacteria and archaea.

  • Lack a nucleus and membrane-bound organelles.

  • DNA is located in the nucleoid region.

  • May have a cell wall, plasma membrane, ribosomes, flagella, or pili.

• Eukaryotic Cells

  • Found in plants, animals, fungi, and protists.

  • Have a nucleus enclosed by a nuclear membrane.

  • Contain membrane-bound organelles for specialized functions.

Key Organelles

• Nucleus

  • Control center; houses DNA; coordinates growth, metabolism, reproduction.

• Ribosomes

  • Sites of protein synthesis; found freely in cytoplasm or on rough ER.

• Endoplasmic Reticulum (ER)

  • Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus

  • Modifies, sorts, and packages proteins and lipids; contains cisternae (flattened sacs).

• Mitochondria

  • Powerhouse of the cell; generates ATP through cellular respiration.

  • Has a double membrane; inner membrane folded into cristae for ATP production.

• Chloroplasts (in plant cells)

  • Sites of photosynthesis; convert light energy into glucose.

  • Double membrane; contains thylakoids (in grana) and stroma.

• Lysosomes

  • Contain digestive enzymes; break down waste materials and cellular debris.

• Peroxisomes

  • Contain enzymes that detoxify harmful substances; break down fatty acids; produce hydrogen peroxide.

• Vacuoles

  • Storage organelles; large central vacuole in plant cells maintains turgor pressure.

• Cytoskeleton

  • Provides structural support; aids in cell division and movement.

  • Microfilaments: Thin, actin filaments for cell shape and movement.

  • Microtubules: Hollow tubes made of tubulin; support cell shape and transport.

  • Intermediate Filaments: Fibrous proteins for mechanical support and cell shape.

Endomembrane System vs. Energy Organelles

• Endomembrane System

  • Includes nuclear envelope, rough and smooth ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

  • Involved in synthesis, modification, and transport of proteins and lipids.

• Energy Organelles

  • Mitochondria: Cellular respiration, ATP production.

  • Chloroplasts: Photosynthesis in plants.

Compartmentalization in Cells

• Refers to membrane-bound organelles creating distinct environments within eukaryotic cells.

• Allows specific biochemical reactions in isolated areas, enhancing efficiency.

  • Example: Lysosomes have an acidic environment; mitochondria have cristae for efficient ATP production.

Differences Between Animal and Plant Cells

• Plant Cells

  • Cell Wall: Provides support and protection.

  • Chloroplasts: For photosynthesis.

  • Large Central Vacuole: Maintains turgor pressure.

  • Plasmodesmata: Channels for cell communication.

• Animal Cells

  • Lysosomes: Involved in waste digestion.

  • Centrioles: Play a role in cell division.

  • Cilia and Flagella: Aid in movement.

Cell Size:

Cell Metabolism and Size

• Metabolism involves chemical reactions for energy, protein synthesis, and waste removal.

• Smaller cells have a higher surface area to volume ratio, enhancing efficiency of material exchange.

Surface Area to Volume Ratio (SA)

• High SA

  ratio benefits material exchange and metabolic efficiency.

• As cell size increases, volume grows faster than surface area, leading to decreased SA

  ratio.

Importance of SA Ratio

• Exchange of Materials: Faster diffusion with high SA

  ratio.

• Metabolic Efficiency: Smaller cells with high SA

  can support higher metabolic rates.

• Temperature Regulation: High SA

  ratio aids in heat loss.

Formulas

• Cuboidal Cells

  • Surface Area (SA) = 6s²

  • Volume (V) = s³

• Spherical Cells

  • Surface Area (SA) = 4πr²

  • Volume (V) = (4/3)πr³

Why Cell Size Matters

• Larger cells face challenges with slower diffusion rates and metabolic inefficiencies.

Strategies to Increase SA Ratio

• Microvilli: Extensions to increase surface area.

• Flattened Shapes: To maximize surface area relative to volume.

• Compartmentalization: Internal membranes and organelles enhance internal surface area.

Plasma Membrane and Membrane Permeability Structure and Function of the Plasma Membrane

Structure and Function of the Plasma Membrane

• Composed of phospholipid bilayer, proteins, and carbohydrates.

• Phospholipids: Hydrophilic heads and hydrophobic tails form bilayer.

• Fluid Mosaic Model: Describes membrane as flexible with a mosaic of proteins in lipid bilayer.

Factors Affecting Membrane Fluidity

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Fatty Acid Composition: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

• Cholesterol: Stabilizes membrane at high temperatures; prevents tight packing at low temperatures.

Membrane Proteins

• Integral Proteins: Span the membrane; act as channels or transporters.

• Peripheral Proteins: Attached to membrane surfaces; involved in signaling and maintaining shape.

Membrane Carbohydrates

• Glycoproteins and Glycolipids: Involved in cell recognition and communication.

Plant Cell Walls

• Structure: Rigid layer of cellulose fibers and matrix.

• Function: Offers support, maintains shape, and prevents bursting in hypotonic environments.

Membrane Transport: Passive and Active Transport

Selective Permeability of the Plasma Membrane

• Allows easy diffusion of small, nonpolar molecules.

• More difficult for larger, polar molecules and ions.

Passive Transport

• Simple Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Facilitated Diffusion: Uses proteins to help molecules cross the membrane.

  • Channel Proteins: Form pores for specific molecules (e.g., aquaporins for water).

  • Carrier Proteins: Bind, change shape, and transport molecules.

Active Transport

• Requires energy (ATP) to move substances against concentration gradients.

• Pumps

  • Sodium-Potassium Pump: Moves sodium out, potassium in; maintains membrane potential.

  • Proton Pump: Moves protons out; crucial for ATP synthesis.

• Cotransport

  • Symport: Two substances move in the same direction.

  • Antiport: Substances move in opposite directions.

  • Example: Sucrose-proton cotransport in plants.

• Exocytosis and Endocytosis

  • Exocytosis: Expels materials using vesicles.

  • Endocytosis: Takes in materials via vesicle formation.

    • Phagocytosis: Engulfs large particles.

    • Pinocytosis: Takes in extracellular fluid.

    • Receptor-Mediated Endocytosis: Specific molecules bind to receptors and are engulfed.

ATP Function in Active Transport

• ATP breaks down to ADP and a phosphate group, releasing energy for transport processes.

Tonicity and Osmoregulation

Tonicity

• Refers to the ability of a solution to cause a cell to gain or lose water.

• Isotonic Solution: Equal solute concentration inside and outside the cell.

• Hypotonic Solution: Lower solute concentration outside the cell; causes cell swelling.

• Hypertonic Solution: Higher solute concentration outside the cell; causes cell shrinking.

Water Potential

• Formula: 𝚿 = 𝚿P + 𝚿s

  • 𝚿P: Pressure potential.

  • 𝚿s: Solute potential.

Solute Potential

• Formula: 𝚿s = -iCRT

  • i: Ionization constant.

  • C: Molar concentration.

  • R: Pressure constant.

  • T: Temperature in Kelvin.

Pressure Potential

• Positive in plant cells due to turgor pressure from the cell wall.