Comprehensive Study Guide: Cell Structure, Membrane Function, Metabolism, Cellular Respiration, and Photosynthesis

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells lack a membrane-bound nucleus and organelles; their DNA is located in the nucleoid region.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles.

• Both cell types have a plasma membrane, cytoplasm, ribosomes, and genetic material.

• Key differences: Eukaryotes are generally larger, have complex internal structures, and can be multicellular; prokaryotes are usually unicellular.

Surface Area to Volume Ratio

• The surface area to volume ratio ($\text{SA:V} = \frac{\text{surface area}}{\text{volume}}$) limits cell size.

• As cells grow, volume increases faster than surface area, reducing efficiency of material exchange.

• Smaller cells have a higher SA:V ratio, allowing more efficient transport of molecules.

Microscopy

• Light microscopes can view living cells and structures down to about 200 nm (e.g., most cells, nuclei, chloroplasts).

• Electron microscopes (TEM and SEM) provide higher resolution (down to ~2 nm), allowing visualization of organelles, viruses, and macromolecules.

• Choose the microscope based on the size of the structure to be observed.

Prokaryotic Cell Structures

• Nucleoid region: Area where DNA is concentrated, not membrane-bound.

• Flagella: Tail-like structures for movement.

• Cell wall: Provides shape and protection; composed of peptidoglycan in bacteria.

Plant vs. Animal Cells

• Plant cells have a cell wall, chloroplasts, and a large central vacuole; animal cells do not.

• Animal cells have centrioles and lysosomes, which are rare in plant cells.

Organelle Functions

• Nucleus: Stores genetic material (DNA); site of transcription.

• Ribosomes: Sites of protein synthesis; found free in cytosol or bound to rough ER.

• Chloroplast: Site of photosynthesis in plants and algae.

• Mitochondria: Site of cellular respiration and ATP production.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membranes.

• Smooth ER: Synthesizes lipids, detoxifies toxins, stores calcium ions.

• Lysosome: Contains digestive enzymes for breakdown of macromolecules.

• Flagella: Motility structures in some animal cells and prokaryotes.

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) for support, shape, and movement.

Endomembrane System

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

• Proteins/lipids are synthesized in the ER, modified in the Golgi, and transported in vesicles to their destinations or out of the cell (exocytosis).

Membrane Structure and Function

Phospholipid Bilayer

• Composed of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward, forming a semi-permeable membrane.

Membrane Transport

• Simple diffusion: Small, nonpolar molecules (e.g., O2, CO2) pass directly through the bilayer.

• Facilitated diffusion: Polar or charged molecules move via protein channels or carriers; no energy required.

• Active transport: Movement against concentration gradient using energy (ATP); e.g., Na+/K+ pump.

• Endocytosis: Uptake of materials via vesicle formation.

• Exocytosis: Release of materials via vesicle fusion with the plasma membrane.

Osmosis and Tonicity

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

• Tonicity terms:

  • Hypertonic: Higher solute concentration outside; water leaves cell (cell shrinks).

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

  • Hypotonic: Lower solute concentration outside; water enters cell (cell swells).

• Plant cells prefer hypotonic environments (turgid); animal cells prefer isotonic.

Endosymbiosis Theory

• Explains origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes similar to bacteria.

Metabolism and Enzymes

Thermodynamics in Biology

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

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

Kinetic vs. Potential Energy

• Kinetic energy: Energy of motion (e.g., heat, movement).

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

Exergonic vs. Endergonic Reactions

• Exergonic: Release energy; spontaneous; $\Delta G < 0$.

• Endergonic: Require energy input; non-spontaneous; $\Delta G > 0$.

Catabolic vs. Anabolic Pathways

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

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

Entropy

• Entropy (S): Measure of disorder; increases in spontaneous processes.

Enzymes

• Enzymes: Biological catalysts that lower activation energy ($E_a$) of reactions.

• Active site: Region where substrate binds and reaction occurs.

• Substrate: Reactant acted upon by enzyme.

• Enzyme activity is affected by temperature and pH; optimal conditions vary by enzyme.

• Inhibitors:

  • Competitive: Bind active site, block substrate.

  • Non-competitive: Bind elsewhere, change enzyme shape.

Cellular Respiration

Purpose and Overview

• Converts chemical energy in food (glucose) into ATP.

• Overall equation: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

Stages of Cellular Respiration

• Glycolysis: Occurs in cytosol; glucose → 2 pyruvate, 2 ATP, 2 NADH.

• Pyruvate oxidation: Mitochondrial matrix; pyruvate → acetyl-CoA, NADH, CO2.

• Krebs (Citric Acid) Cycle: Mitochondrial matrix; acetyl-CoA → CO2, NADH, FADH2, ATP.

• Oxidative phosphorylation: Inner mitochondrial membrane; electron transport chain (ETC) and chemiosmosis produce most ATP.

ATP Production

• Most ATP is produced during oxidative phosphorylation via the ETC and ATP synthase.

• NADH and FADH2 donate electrons to the ETC, powering H+ pumping and ATP synthesis.

Fermentation

• Occurs when oxygen is absent; glycolysis continues, but pyruvate is converted to lactate (animals) or ethanol (yeast).

• Produces less ATP than aerobic respiration.

Other Food Molecules

• Fats, proteins, and carbohydrates can enter cellular respiration at various points to produce ATP.

Photosynthesis

Relationship to Cellular Respiration

• Photosynthesis stores energy in glucose; cellular respiration releases it.

• Reactants and products are essentially reversed between the two processes.

Autotrophs vs. Heterotrophs

• Autotrophs: Organisms that produce their own food (e.g., plants, algae).

• Heterotrophs: Obtain food by consuming other organisms (e.g., animals, fungi).

Photosynthesis Overview

• Overall equation: $\mathrm{6CO_2 + 6H_2O + light \ energy \rightarrow C_6H_{12}O_6 + 6O_2}$

• Occurs in chloroplasts; light reactions and Calvin cycle.

Light Reactions

• Occur in thylakoid membranes; convert light energy to chemical energy (ATP, NADPH), release O2.

• Chlorophyll is located in thylakoid membranes; absorbs blue and red light best, reflects green (why leaves appear green).

Calvin Cycle

• Occurs in stroma; uses ATP and NADPH to convert CO2 into glucose.

• Main products: G3P (a sugar), which can be used to form glucose and other carbohydrates.

Table: Comparison of Diffusion Types

Type	Energy Required?	Direction (Gradient)	Transport Protein Needed?	Example
Simple Diffusion	No	High to Low	No	O2, CO2
Facilitated Diffusion	No	High to Low	Yes	Glucose, ions
Active Transport	Yes (ATP)	Low to High	Yes	Na+/K+ pump

Table: Cellular Respiration Stages

Stage	Location	O2 Required?	Main Products	ATP Produced
Glycolysis	Cytosol	No	2 Pyruvate, 2 NADH	2
Pyruvate Oxidation	Mitochondrial Matrix	Yes	2 Acetyl-CoA, 2 NADH, 2 CO2	0
Krebs Cycle	Mitochondrial Matrix	Yes	6 NADH, 2 FADH2, 4 CO2	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	Yes	H2O, NAD+, FAD, ATP	~26-28

Example: Oxidative Phosphorylation (Essay Topic)

• NADH and FADH2 donate electrons to the electron transport chain (ETC).

• As electrons move through the ETC, energy is used to pump H+ ions from the mitochondrial matrix to the intermembrane space, creating a proton gradient.

• H+ ions flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi (chemiosmosis).

• O2 is the final electron acceptor, forming H2O.

Equation for ATP synthesis:

$\mathrm{ADP + P_i + H^+_{gradient} \xrightarrow{ATP\ synthase} ATP}$

Additional info: The process is highly efficient and produces the majority of ATP in aerobic respiration.