Cell Components

Methods Used to Study Cells: Types of Microscopy

Microscopy is essential for visualizing cells and their structures. Different types of microscopes provide varying levels of resolution and magnification.

• Light Microscopy: Uses visible light to observe living or fixed cells; limited resolution.

• Electron Microscopy: Uses electron beams for much higher resolution; includes Transmission Electron Microscopy (TEM) for internal structures and Scanning Electron Microscopy (SEM) for surface details.

• Fluorescence Microscopy: Uses fluorescent dyes to highlight specific cell components.

• Example: TEM can reveal the detailed structure of mitochondria.

Prokaryotes vs. Eukaryotes: Structural Differences

Cells are classified as prokaryotic or eukaryotic based on their internal organization.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is found in the nucleoid region.

• Eukaryotes: Have a nucleus and various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Example: Escherichia coli is a prokaryote; plant and animal cells are eukaryotes.

Plant vs. Animal Cells: Organelles and Functions

Plant and animal cells share many organelles but also have distinct features.

• Plant Cells: Have a cell wall, chloroplasts, and a large central vacuole.

• Animal Cells: Lack cell walls and chloroplasts; contain centrioles.

• Organelles: Nucleus (stores DNA), mitochondria (energy production), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (modifies and ships proteins), lysosomes (digestion), peroxisomes (detoxification).

• Example: Chloroplasts in plant cells enable photosynthesis.

Free vs. Bound Ribosomes: Structure and Function

Ribosomes synthesize proteins and can be found in two forms.

• Free Ribosomes: Float in the cytosol; produce proteins used within the cell.

• Bound Ribosomes: Attached to the endoplasmic reticulum; synthesize proteins for export or for membranes.

• Example: Enzymes for glycolysis are made by free ribosomes.

Microtubule Arrangement in Centrioles, Cilia, and Flagella

Microtubules are structural components involved in cell movement and division.

• Cilia and Flagella: Have a "9+2" arrangement: nine doublets surrounding two central microtubules.

• Centrioles: Have a "9+0" arrangement: nine triplets with no central microtubules.

• Example: Human sperm flagella use the 9+2 microtubule structure for movement.

Extracellular Matrix (ECM) and Cell Junctions

The ECM provides structural support and mediates cell communication. Cell junctions connect cells and regulate passage of substances.

• ECM Components: Collagen, proteoglycans, fibronectin.

• Cell Junctions:

  • Desmosomes: Anchor cells together.

  • Tight Junctions: Prevent leakage between cells.

  • Gap Junctions: Allow communication via ions and small molecules.

• Example: Tight junctions in intestinal epithelium prevent fluid leakage.

The Membrane

Fluid Mosaic Model of the Plasma Membrane

The plasma membrane is described by the Fluid Mosaic Model, which emphasizes its dynamic and heterogeneous nature.

• Phospholipid Bilayer: Provides a flexible barrier.

• Proteins: Embedded or attached; serve as channels, receptors, or enzymes.

• Cholesterol: Modulates fluidity and stability.

• Example: Membrane proteins facilitate glucose transport.

Components of the Cell Membrane and Their Functions

• Phospholipids: Form the basic structure; hydrophilic heads and hydrophobic tails.

• Cholesterol: Maintains membrane fluidity.

• Proteins: Transport, signal, and catalyze reactions.

• Example: Aquaporins are membrane proteins that facilitate water movement.

Saturated vs. Unsaturated Fatty Acids and Membrane Fluidity

The type of fatty acids in phospholipids affects membrane properties.

• Saturated Fatty Acids: Straight chains; pack tightly, making the membrane less fluid.

• Unsaturated Fatty Acids: Have kinks; prevent tight packing, increasing fluidity.

• Example: Cold-adapted organisms have more unsaturated fatty acids in their membranes.

Substances Crossing the Plasma Membrane Without Transport Proteins

• Small, nonpolar molecules: Oxygen (O2), carbon dioxide (CO2).

• Small, uncharged molecules: Water (to some extent).

• Example: Oxygen diffuses freely across the membrane.

Types of Cellular Transport

Cells move substances across membranes via passive, active, and bulk transport.

• Passive Transport: No energy required; includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (ATP); moves substances against concentration gradients.

• Bulk Transport: Endocytosis and exocytosis move large particles or volumes.

• Osmosis: Movement of water across a semipermeable membrane.

• Tonicity: Describes the effect of a solution on cell volume.

  • Isotonic: No net water movement; cell remains unchanged.

  • Hypotonic: Water enters cell; animal cells may burst (lyse).

  • Hypertonic: Water leaves cell; cell shrinks (crenates).

• Example: Red blood cells in a hypotonic solution swell and may burst.

Types of Active Transporters and Sodium-Potassium Pump

• Uniporters: Transport one substance in one direction.

• Symporters: Transport two substances in the same direction.

• Antiporters: Transport two substances in opposite directions.

• Sodium-Potassium Pump: Maintains electrochemical gradients by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed. Equation:

• Example: The sodium-potassium pump is vital for nerve impulse transmission.

Energy and Metabolism

Metabolism: Catabolism and Anabolism

Metabolism encompasses all chemical reactions in cells, divided into catabolic and anabolic pathways.

• Catabolism: Breaks down molecules; releases energy (exergonic).

• Anabolism: Builds molecules; consumes energy (endergonic).

• Example: Cellular respiration is catabolic; protein synthesis is anabolic.

Laws of Thermodynamics

Biological systems obey the laws of thermodynamics.

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases entropy (disorder).

• Example: Heat released during metabolism increases entropy.

Energy and ATP in Metabolism

Energy is the capacity to do work. ATP is the primary energy carrier in cells.

• ATP Function: Transfers energy to drive cellular processes.

• If Cells Cannot Make ATP: Cellular functions cease; cell death occurs.

• Equation:

• Example: Muscle contraction uses ATP.

Spontaneous Processes

Spontaneous processes occur without energy input and can be predicted by free energy changes.

• Spontaneous: If free energy decreases ().

• Equation:

• Example: Diffusion of molecules down a concentration gradient.

Potential vs. Kinetic Energy

Energy exists in two main forms.

• Potential Energy: Stored energy (e.g., chemical bonds).

• Kinetic Energy: Energy of motion (e.g., moving molecules).

• Example: Glucose has potential energy; ATP hydrolysis releases kinetic energy.

Enzyme Activity: Effects of Temperature, pH, and Inhibition

Enzymes are biological catalysts affected by environmental conditions and inhibitors.

• Optimal Conditions: Each enzyme has a specific temperature and pH for maximum activity.

• Allosteric Regulation: Activators or inhibitors bind to sites other than the active site, altering enzyme activity.

• Competitive Inhibition: Inhibitor binds to active site, blocking substrate.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Metabolic Pathways: Series of enzyme-catalyzed reactions; substrates and inhibitors can be identified at each step.

• Example: Feedback inhibition in glycolysis regulates ATP production.

Activation Energy and Reaction Rates

Activation energy is the minimum energy required to start a chemical reaction.

• Effect: High activation energy slows reactions; enzymes lower activation energy, speeding up reactions.

• Equation:

• Example: Amylase lowers activation energy for starch breakdown.