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 lysosomes and centrioles, 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 drugs, stores calcium ions.

• Lysosome: Contains digestive enzymes for breaking down 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.

Transport Across Membranes

• Direct diffusion: Small, nonpolar molecules (e.g., O2, CO2) can diffuse through the lipid bilayer.

• Impermeable to: Large, polar molecules and ions (e.g., glucose, Na+).

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.

Passive vs. Active Transport

• Passive diffusion: Movement down concentration gradient; no energy required.

• Facilitated diffusion: Passive transport via membrane proteins.

• Active transport: Movement against gradient; requires energy (ATP).

Endocytosis and Exocytosis

• Endocytosis: Cell engulfs material via vesicles (e.g., phagocytosis, pinocytosis).

• Exocytosis: Vesicles fuse with plasma membrane to release contents outside cell.

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 entropy (disorder) of the universe.

Kinetic vs. Potential Energy

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

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

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 molecules, require energy (e.g., photosynthesis).

Entropy

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

Enzyme Function

• 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.

• Human enzymes: optimal temperature ~37°C, pH 7.35–7.45 (except stomach enzymes: pH 1.5–2).

Enzyme Inhibition

• Competitive inhibitors: Bind active site, blocking substrate.

• Non-competitive inhibitors: Bind elsewhere, changing enzyme shape and function.

Cellular Respiration

Purpose and Overview

• Purpose: Convert chemical energy in food to ATP.

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

Stages of Cellular Respiration

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

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

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

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

ATP Production

• Most ATP is produced during oxidative phosphorylation.

• Substrate-level phosphorylation occurs in glycolysis and the citric acid cycle.

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• NAD+ and FAD are reduced to NADH and FADH2 as they accept electrons.

Location of Stages

• Glycolysis: cytosol (anaerobic).

• Pyruvate oxidation, citric acid cycle, oxidative phosphorylation: mitochondria (require oxygen except glycolysis and fermentation).

Fermentation

• Occurs when oxygen is absent.

• Animals: Lactic acid fermentation (pyruvate → lactate).

• Yeast: Alcohol fermentation (pyruvate → ethanol + CO2).

Alternative Fuels

• Other macromolecules (fats, proteins) can enter cellular respiration at various points to produce ATP.

Oxidative Phosphorylation (Essay Focus)

• NADH and FADH2 donate electrons to the ETC, powering proton (H+) pumps.

• H+ ions are pumped into the intermembrane space, creating a gradient.

• ATP synthase uses the flow of H+ back into the matrix (chemiosmosis) to synthesize ATP from ADP and Pi.

Photosynthesis

Relationship to Cellular Respiration

• Photosynthesis and cellular respiration are complementary; products of one are reactants of the other.

Redox in Photosynthesis

• CO2 is reduced to glucose; H2O is oxidized to O2.

Autotrophs vs. Heterotrophs

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

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

Overall Purpose and Equation

• Purpose: Convert light energy to chemical energy (glucose).

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

Light Reactions

• Occur in thylakoid membranes of chloroplasts.

• Produce ATP, NADPH, and O2 (from splitting water).

Chlorophyll and Color

• Chlorophyll is located in the thylakoid membranes.

• Absorbs blue and red light best; reflects green, making leaves appear green.

Calvin Cycle

• Occurs in the stroma of chloroplasts.

• Uses ATP and NADPH to convert CO2 into glucose.

• Does not require light directly ("dark reactions").

Table: Comparison of Cell Types and Organelles

Feature	Prokaryotic Cell	Plant Cell	Animal Cell
Nucleus	No	Yes	Yes
Cell Wall	Yes	Yes (cellulose)	No
Chloroplasts	No	Yes	No
Mitochondria	No	Yes	Yes
Central Vacuole	No	Yes	No
Lysosomes	No	Rare	Yes

Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Osmosis	No	Water: High to Low	Water movement in cells

Example: Application of Tonicity

• If an animal cell is placed in a hypertonic solution, water leaves the cell, causing it to shrink (crenation).

• If a plant cell is placed in a hypotonic solution, water enters, and the cell becomes turgid (normal state for plants).

Example: Recognizing Redox in Respiration

• In the reaction: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$, glucose is oxidized to CO2, and O2 is reduced to H2O.

Example: Enzyme Inhibition

• If an inhibitor binds to the active site and can be outcompeted by increasing substrate, it is competitive.

• If an inhibitor binds elsewhere and cannot be outcompeted, it is non-competitive.

Additional info:

• Some explanations and examples were expanded for clarity and completeness.

• Tables were constructed to summarize and compare key concepts.