Cell Structure, Membranes, and Energy Conversion: General Biology Study Notes

Microscopy and Cell Study

Types of Microscopy

Microscopes are essential tools for visualizing cells and their internal structures, which are typically too small to be seen with the naked eye.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images. Example: Viewing living cells and some organelles.

• Electron Microscopes (EM): Use beams of electrons for much higher resolution than light microscopes. Types:

  • Scanning Electron Microscope (SEM): Provides detailed 3D images of cell surfaces.

  • Transmission Electron Microscope (TEM): Reveals internal structures by passing electrons through thin sections of specimens.

• Key Parameters:

  • Magnification: Ratio of image size to actual size.

  • Resolution: Clarity of the image; minimum distance between distinguishable points.

  • Contrast: Differences in brightness between parts of the sample.

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components for detailed study.

• Process: Cells are broken apart, and organelles are separated by centrifugation.

• Purpose: Allows researchers to study the function of individual organelles.

Cell Types and Basic Features

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic Cells:

  • No membrane-bound organelles.

  • DNA is located in a region called the nucleoid.

  • Cytoplasm is bound by the plasma membrane.

  • Domains: Bacteria and Archaea.

• Eukaryotic Cells:

  • DNA is enclosed within a double-membrane nucleus.

  • Contains membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Organisms in this group include Protists, Fungi, Animals, and Plants.

• Common Features of All Cells:

  • Plasma membrane

  • Cytosol (semifluid substance)

  • Chromosomes (carry genes)

  • Ribosomes (protein synthesis)

Surface Area to Volume Ratio

The size and efficiency of cells are limited by the surface area to volume ratio.

• As a cell grows, its volume increases faster than its surface area, limiting the rate of material exchange.

• Small cells have a higher surface area to volume ratio, facilitating efficient transport.

Internal Organization of Eukaryotic Cells

Genetic Information Storage and Transmission

• Nucleus: Contains chromosomes (DNA packaged as chromatin) and the nucleolus (site of ribosomal RNA synthesis).

• Nuclear Envelope: Double membrane enclosing the nucleus, with nuclear pores for transport.

• Nuclear Lamina: Protein network maintaining nuclear shape and structure.

Ribosomes: Protein Factories

• Composed of ribosomal RNA and protein.

• Sites of protein synthesis:

  • Free ribosomes: In cytosol; produce proteins for use within the cell.

  • Bound ribosomes: Attached to endoplasmic reticulum or nuclear envelope; produce proteins for export or membranes.

The Endomembrane System

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins and glycoproteins, distributes transport vesicles, and is a membrane factory.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs and poisons, stores calcium.

• Golgi Apparatus: Consists of flattened membranous sacs called cisternae; modifies, sorts, and packages products from the ER into transport vesicles.

• Lysosomes: Membranous sacs containing hydrolytic enzymes for digesting macromolecules.

  • Phagocytosis: Engulfing food particles and digesting them.

  • Autophagy: Recycling the cell's own organelles and macromolecules.

• Vacuoles: Large vesicles derived from the ER and Golgi apparatus.

  • Food vacuoles: Formed by phagocytosis.

  • Contractile vacuoles: Pump excess water out of cells.

  • Central vacuoles (plants): Store ions and contribute to cell growth.

Energy Conversion Organelles

Mitochondria and Chloroplasts

These organelles convert energy into forms usable by the cell.

• Mitochondria: Sites of cellular respiration, generating ATP from organic molecules.

• Chloroplasts: Found in plants and algae; sites of photosynthesis, converting solar energy to chemical energy.

• Peroxisomes: Oxidative organelles that detoxify substances and break down fatty acids.

Endosymbiotic Theory

This theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• States that these organelles originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Supporting evidence: Double membranes, circular DNA, ribosomes similar to prokaryotes, and independent replication.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that provides structural support, maintains cell shape, and enables movement.

Fiber Type	Main Protein	Functions
Microtubules	Tubulin	Cell shape, chromosome movement, organelle transport, cilia/flagella movement
Microfilaments	Actin	Cell shape, muscle contraction, cytoplasmic streaming, cell motility
Intermediate Filaments	Keratin, others	Cell shape, anchorage of nucleus/organelles, nuclear lamina

Cilia and Flagella

• Structures for cell movement; composed of microtubules arranged in a "9+2" pattern.

• Flagella: Usually one or few per cell; longer, whip-like motion.

• Cilia: Numerous per cell; shorter, coordinated beating.

• Movement driven by the motor protein dynein.

Extracellular Components and Cell Junctions

Plant Cell Walls

• Provide structural support, protection, and regulate water intake.

• Layers:

  • Primary cell wall: Thin and flexible.

  • Middle lamella: Rich in pectin; glues adjacent cells together.

  • Secondary cell wall: Added in some cells for extra strength.

Extracellular Matrix (ECM) in Animal Cells

• Composed of glycoproteins (e.g., collagen, proteoglycans, fibronectin).

• Provides structural support, regulates cell behavior, and facilitates communication via integrins.

Cell Junctions

• Connect neighboring cells and coordinate cellular activities.

• Plasmodesmata (plants): Channels allowing transport of water, ions, and small molecules between cells.

• Tight junctions (animals): Seal cells together, preventing leakage of extracellular fluid.

• Desmosomes (animals): Anchor cells together into strong sheets.

• Gap junctions (animals): Provide cytoplasmic channels for communication between cells.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Organelles	Absent	Present (membrane-bound)
Size	Smaller (1-10 μm)	Larger (10-100 μm)
Domains	Bacteria, Archaea	Protists, Fungi, Animals, Plants

Example: Phagocytosis is a process by which a cell engulfs particles to form an internal compartment known as a food vacuole. Lysosomes then fuse with the vacuole to digest the contents.

Membrane Structure and Function

Overview of Cellular Membranes

Cellular membranes are essential structures that separate the interior of the cell from its external environment. They are primarily composed of lipids and proteins, forming a dynamic and selectively permeable barrier.

• Fluid Mosaic Model: Describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Phospholipid Bilayer: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a bilayer.

• Proteins: Embedded within or attached to the membrane, responsible for most membrane functions.

• Membrane Fluidity: Maintained by weak hydrophobic interactions; lipids and some proteins can move laterally, rarely flip-flop.

Phospholipid Bilayer Structure

• Amphipathic Nature: Phospholipids have hydrophilic heads facing outward toward water and hydrophobic tails facing inward.

• Unsaturated vs. Saturated Fatty Acids: Membranes with unsaturated fatty acids are more fluid; saturated fatty acids make membranes more viscous.

• Temperature Effects: Lower temperatures can cause membranes to solidify; fluidity depends on lipid composition.

Membrane Proteins

• Proteins embedded in the membrane perform a variety of functions, including transport, signaling, and structural support.

• Integral Proteins: Penetrate the hydrophobic core; some span the membrane (transmembrane proteins).

• Peripheral Proteins: Bound to the surface of the membrane.

• Functions of Membrane Proteins:

  • Transport: Move substances across the membrane.

  • Enzymatic Activity: Catalyze reactions at the membrane surface.

  • Signal Transduction: Relay signals from outside to inside the cell.

  • Cell Recognition: Allow cells to identify each other.

  • Intercellular Joining: Connect adjacent cells.

  • Attachment: Anchor the membrane to the cytoskeleton or extracellular matrix.

Membrane Permeability

Selective Permeability

The plasma membrane is selectively permeable, allowing certain molecules to pass while restricting others.

• Hydrophobic Molecules: (e.g., hydrocarbons, O2, CO2) pass easily through the lipid bilayer.

• Hydrophilic Molecules: (e.g., sugars, water, ions) usually require transport proteins.

• Transport Proteins: Facilitate the movement of specific molecules across the membrane.

Types of Transport Across Membranes

Passive Transport

• Simple Diffusion: Movement of particles from high to low concentration.

• Facilitated Diffusion: Transport proteins (channel and carrier) help hydrophilic substances cross the membrane.

Active Transport

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

  • Sodium-Potassium Pump: Maintains high K+ and low Na+ inside animal cells.

Bulk Transport

• Exocytosis: Vesicles fuse with the membrane, releasing contents outside the cell.

• Endocytosis: Membrane engulfs materials, bringing them into the cell.

  • Phagocytosis: Engulfs large particles.

  • Pinocytosis: Engulfs extracellular fluid.

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Osmosis and Tonicity

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, influenced by solute concentration.

• Osmosis: Water moves from regions of lower solute concentration to higher solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

  • Isotonic: No net movement; cell remains the same.

  • Hypotonic: Water enters the cell; cell swells.

  • Hypertonic: Water leaves the cell; cell shrivels.

Electrochemical Gradients and Membrane Potential

Membrane Potential

Membrane potential is the voltage across a membrane, created by differences in ion distribution. It drives the movement of charged substances (ions) across the membrane.

• Electrogenic Pumps: Transport proteins that generate voltage across membranes (e.g., sodium-potassium pump, proton pump).

Transport Mechanisms Table

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Bulk Transport (Exocytosis/Endocytosis)	Yes (ATP)	Varies	Secretion of proteins, uptake of macromolecules

Key Equations

• Diffusion: Movement down concentration gradient; no specific equation, but described by Fick's Law:

$J = -D \frac{dC}{dx}$

• Osmotic Pressure Equation:

$\Pi = iMRT$

• Membrane Potential: Where V is membrane potential.

Examples and Applications

• Medical Application: Oral rehydration therapy uses Na+/glucose cotransport to treat dehydration from diarrhea.

• Cell Recognition: HIV resistance in individuals lacking CCR5 co-receptor.

Fluid Mosaic Model of Membrane Structure

Dynamic Structure

The fluid mosaic model describes the membrane as a dynamic structure with proteins and other molecules embedded in or attached to a fluid lipid bilayer.

• Fluidity: Lipids and proteins can move laterally within the layer, allowing for flexibility and self-healing.

• Mosaic: The membrane is a patchwork of different proteins and lipids, each with specific functions.

Membrane Proteins: Types and Functions

• Integral (transmembrane) proteins: Span the membrane; involved in transport, signal transduction, and cell adhesion.

• Peripheral proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

• Functions: Transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Membrane Carbohydrates and Cell Recognition

• Carbohydrates on the cell surface play a crucial role in cell-cell recognition and communication.

• Often attached to proteins (glycoproteins) or lipids (glycolipids).

• Serve as identification tags recognized by other cells (e.g., immune response).

Metabolism and Energy Conversion

Metabolic Pathways

Metabolism is the sum of all chemical reactions within an organism, transforming matter and energy to sustain life.

• Catabolic Pathways: Break down complex molecules into simpler compounds, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy (e.g., synthesis of proteins from amino acids).

• Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.

Free Energy and Spontaneity

• Free Energy Change ($\Delta G$): Determines if a reaction occurs spontaneously. Reactions with a negative $\Delta G$ are spontaneous.

• Exergonic Reactions: Release free energy; $\Delta G$ is negative; these reactions are spontaneous (e.g., cellular respiration).

• Endergonic Reactions: Absorb free energy; $\Delta G$ is positive; these reactions are nonspontaneous and require energy input (e.g., synthesis of glucose).

$\Delta G = \Delta H - T\Delta S$

• Where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy (total energy), $T$ is temperature in Kelvin, and $\Delta S$ is the change in entropy.

ATP: The Energy Currency of the Cell

• ATP (adenosine triphosphate) stores energy in high-energy phosphate bonds, which can be released through hydrolysis.

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• The energy released is used to drive endergonic (energy-consuming) reactions.

• ATP is regenerated from ADP and inorganic phosphate ($P_i$) using energy from catabolic reactions.

• This cycle is continuous, with each cell recycling thousands to millions of ATP molecules per second.

Energy Coupling

• Cells use ATP hydrolysis to couple exergonic and endergonic reactions, making otherwise nonspontaneous processes possible.

• Example: ATP powers muscle contraction, active transport, and biosynthesis.

Enzymes and Metabolic Regulation

Enzyme Structure and Function

• Enzymes are proteins that act as catalysts, speeding up reactions by lowering activation energy without being consumed.

• Active Site: The region on the enzyme where the substrate binds and the reaction occurs.

• Induced Fit: The enzyme changes shape slightly to fit the substrate more closely, enhancing catalysis.

• Enzymes are highly specific for their substrates due to the unique shape of their active sites.

• Enzyme activity can be affected by temperature, pH, and the presence of inhibitors or activators.

Enzyme Regulation

• Allosteric Regulation: Enzyme activity is regulated by molecules binding to sites other than the active site, causing conformational changes.

• Feedback Inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

• Competitive Inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive Inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing activity.

Enzyme Localization

• Enzymes are often compartmentalized within specific organelles or regions of the cell to increase efficiency and regulation.

Summary Table: Key Concepts in Energy and Metabolism

Concept	Definition	Explanation	Example
First Law of Thermodynamics	Energy cannot be created or destroyed, only transformed	Sunlight to chemical energy in plants	Photosynthesis
Second Law of Thermodynamics	Every energy transfer increases entropy	Heat loss during energy transfer	Cellular respiration
Metabolism	Sum of all chemical reactions in an organism	Cellular respiration, photosynthesis	ATP production
ATP	Main energy currency of the cell	Muscle contraction, active transport	ATP hydrolysis
Enzyme	Biological catalyst that speeds up reactions	Lowering activation energy	Sucrase breaking down sucrose
Catabolic Pathway	Breaks down molecules, releases energy	Glycolysis	Cellular respiration
Anabolic Pathway	Builds molecules, consumes energy	Protein synthesis	Photosynthesis

Cellular Respiration

Definition and Major Steps

Cellular respiration is the process of breaking down organic molecules (e.g., glucose) to produce ATP, the cell's energy currency.

• Major Steps: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, and Electron Transport Chain.

• Products: ATP, carbon dioxide (CO2), and water (H2O).

• Electron Carriers: NAD+ and FAD are reduced to NADH and FADH2 during respiration.

Overall Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Fermentation

Fermentation is an anaerobic process that allows cells to produce ATP without oxygen.

• Types: Alcohol fermentation (produces ethanol and CO2), lactic acid fermentation (produces lactate).

• Key Molecules: NAD+ is regenerated to allow glycolysis to continue.

• Comparison: Fermentation yields less ATP than aerobic respiration.

• Example: Muscle cells perform lactic acid fermentation during intense exercise.

Stages of Cellular Respiration

Stage	Location	Main Products
Glycolysis	Cytosol	2 Pyruvate, 2 ATP, 2 NADH
Pyruvate Oxidation & Citric Acid Cycle	Mitochondria	CO2, NADH, FADH2, ATP
Oxidative Phosphorylation	Inner mitochondrial membrane	ATP, H2O

Glycolysis

• Occurs in the cytoplasm; converts glucose to pyruvate, producing ATP and NADH.

• ATP is formed by substrate-level phosphorylation.

• Phases:

  • Energy Investment Phase: Consumes 2 ATP to phosphorylate substrates.

  • Energy Payoff Phase: Produces 4 ATP and 2 NADH; net gain is 2 ATP.

Citric Acid Cycle (Krebs Cycle)

• Occurs in the mitochondria; completes the oxidation of organic molecules.

• Pyruvate is converted to acetyl CoA, releasing CO2 and producing NADH.

• Each turn of the cycle adds two carbons from acetyl CoA, releases 2 CO2, and produces 3 NADH, 1 FADH2, and 1 ATP.

• Intermediates formed during the cycle are used in other metabolic pathways.

Oxidative Phosphorylation

• Final stage of cellular respiration, generating most of the cell's ATP.

• NADH and FADH2 donate electrons to the electron transport chain, creating a proton gradient across the inner mitochondrial membrane.

• ATP synthase uses the H+ gradient to synthesize ATP from ADP and inorganic phosphate.

• Oxygen acts as the final electron acceptor, forming water.

Fermentation: Anaerobic Harvesting of Energy

• Allows cells to produce ATP in the absence of oxygen, using glycolysis followed by alternative pathways to recycle NAD+.

• Lactic acid fermentation: Pyruvate is reduced to lactate, regenerating NAD+.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+.

• Fermentation is less efficient than aerobic respiration, producing only 2 ATP per glucose.

Comparison Table: Aerobic vs. Anaerobic Respiration

Process	Oxygen Required?	ATP Yield (per glucose)
Aerobic Respiration	Yes	~32 ATP
Fermentation	No	2 ATP

Entry Points of Macromolecules into Cellular Respiration

Macromolecule	Entry Point
Carbohydrates	Glycolysis
Fats	Glycerol (glycolysis), Fatty acids (acetyl CoA)
Proteins	Various intermediates after deamination

Regulation and Biosynthesis

• Intermediates from cellular respiration are used for biosynthesis of other organic molecules.

• Metabolic pathways are regulated by feedback inhibition and allosteric regulation.

• Excess calories can be stored as fat, even on a low-fat diet due to conversion of carbohydrates and proteins into fatty acids.

Special Function of Brown Fat

• Brown fat contains mitochondria that can generate heat by allowing protons to flow freely across the inner mitochondrial membrane, dissipating the proton gradient without producing ATP.

• Brown fat is more active in individuals exposed to cold and can contribute to calorie burn.

Photosynthesis and Cellular Respiration: Energy for Life

Photosynthesis

• Photosynthesis is the conversion of light energy to chemical energy in the form of glucose.

• Equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Occurs in chloroplasts of plant cells.

• Stages: Light reactions (produce ATP and NADPH) and Calvin cycle (synthesize glucose).

Comparison: Cellular Respiration vs. Photosynthesis

Feature	Cellular Respiration	Photosynthesis
Energy Source	Chemical (glucose)	Light
Location	Mitochondria	Chloroplasts
Electron Carriers	NADH, FADH2	NADPH
Products	ATP, CO2, H2O	Glucose, O2
Organisms	All eukaryotes	Plants, algae, some bacteria

Pigments and Photosynthetic Cells

• Pigments such as chlorophyll absorb light energy for photosynthesis.

• Other pigments (carotenoids, phycobilins) expand the range of light absorption.

• Photosynthetic cells are found in leaves and other green tissues.

• Comparison: Plant cells have chloroplasts for photosynthesis; animal cells do not.

Additional info: Some content inferred from standard biology curriculum and Campbell Biology, 11th edition. Key terms and processes expanded for clarity and completeness.