I. Cell

Key focus of this chapter: biological macromolecules

This chapter reviews biological macromolecules, cell structures and signaling, membrane transport, enzymes, photosynthesis, and cellular respiration.

A. Biological Macromolecules

Most biological molecules are composed primarily of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S), collectively known as CHNOPS.

Nucleic acids: C, H, O, N, and P
Proteins: primarily C, H, O, and N; some proteins also contain S

1. Carbohydrates (Sugars)

Composed primarily of carbon, hydrogen, and oxygen, often in an approximate 1:2:1 ratio
Functions: energy source, energy storage, structural support, and cell recognition

Carbohydrates (sugar)	Classification	Description
Monosaccharide	Glucose	• A six-carbon aldose sugar; the primary energy source for most cells
	Ribose	• A five-carbon sugar found in RNA
	Deoxyribose	• A five-carbon sugar found in DNA
	Galactose	• A component of lactose
	Fructose	• A six-carbon ketose sugar found in many fruits; a component of sucrose
Disaccharide	Sucrose	• Glucose + fructose • α(1→2)β glycosidic linkage
	Maltose	• Glucose + glucose • α(1→4) glycosidic linkage
	Lactose	• Glucose + galactose • β(1→4) glycosidic linkage
Polysaccharide	Starch	• A glucose-storage polysaccharide in plants • Composed of amylose and amylopectin • Contains α(1→4) linkages and α(1→6) branch points
	Glycogen	• A highly branched glucose-storage polysaccharide in animals and fungi • Contains α(1→4) linkages and α(1→6) branch points • More highly branched than starch
	Cellulose	• The primary structural polysaccharide of plant cell walls • Composed of β-glucose monomers joined by β(1→4) glycosidic linkages • Parallel cellulose chains are linked by hydrogen bonds to form microfibrils
	Chitin	• A structural polysaccharide composed of N-acetylglucosamine (NAG) monomers • A major component of fungal cell walls and arthropod exoskeletons

2. Proteins

a. Amino acid

Amino acids are the monomers that make up proteins.
A typical amino acid has an α-carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).
The 20 common amino acids differ in their R groups.

b. Polypeptide

Amino acids are linked by peptide bonds.
Proteins function in catalysis, hormonal signaling, storage, regulation, defense, transport, structural support, and movement.
Heat or extreme pH can disrupt a protein’s secondary, tertiary, or quaternary structure and reduce or eliminate its function.

Structures	Features
Primary (1°) structure	• Unique linear amino acid sequence; peptide bonds.
Secondary (2°) structure	• α-helices and β-pleated sheets; stabilized by hydrogen bonds between backbone atoms.
Tertiary (3°) structure	• Overall 3D shape of a single polypeptide; stabilized by hydrogen bonds, ionic interactions, disulfide bonds, van der Waals forces, and hydrophobic interactions among R groups.
Quaternary (4°) structure	• Association of two or more polypeptide subunits into one functional protein; Example: hemoglobin.

3. Lipid

a. Fat

Unsaturated fatty acids contain one or more carbon–carbon double bonds. Cis double bonds introduce kinks that reduce packing and increase membrane fluidity.

– e.g., many vegetable oils

Saturated fatty acids contain no carbon–carbon double bonds; their straight chains pack tightly and tend to decrease membrane fluidity.

– e.g., butter

Fats provide approximately 9 kcal/g.

** Carbohydrates and proteins provide approximately 4 kcal/g.

– A triacylglycerol consists of glycerol esterified to three fatty acids.

b. Phospholipid

Amphipathic phospholipids have hydrophilic heads and hydrophobic tails and form the bilayer of cell membranes.

c. Steroids

Four fused carbon rings.
Cholesterol buffers membrane fluidity and reduces permeability to small water-soluble molecules.
Found in most animal cell membranes.
e.g., cholesterol

4. Nucleic acids

DNA stores hereditary information; RNA participates in gene expression and can have regulatory and catalytic roles.
Nucleic acids are polymers of nucleotides. Each nucleotide contains a five-carbon sugar, a nitrogenous base, and one or more phosphate groups.
Phosphodiester bonds join the 3′ hydroxyl of one nucleotide’s sugar to the 5′ phosphate of the next, forming the sugar–phosphate backbone.
In double-stranded DNA, complementary bases on the two strands pair through hydrogen bonds.
The two DNA strands are antiparallel: one runs 5′→3′ and the other 3′→5′.

Classification	Description
Nucleoside	Five-carbon sugar + nitrogenous base
Nucleotide	Five-carbon sugar + nitrogenous base + phosphate group
Nucleic acids	Polymer of nucleotides linked by phosphodiester bonds

	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Structure	Usually double-stranded	Usually single-stranded
Bonds	Hydrogen bonds (between strands) and phosphodiester bonds	Phosphodiester bonds in the backbone; hydrogen bonds can form between complementary bases in folded RNA or RNA duplexes

B. Shapes and functions of the cells

1. Cell theory

The cell is the smallest unit capable of performing the functions of life.
All living organisms consist of one or more cells.
All cells arise from preexisting cells.

2. The difference between prokaryotic and eukaryotic cells

Prokaryotic cell	Eukaryotic cell
• Cell division by binary fission • Can exchange genetic material by conjugation • Nucleoid (lacks a nuclear envelope)	• Mitotic and meiotic division • Generally larger than prokaryotic cells • Nucleus enclosed by a nuclear envelope • Membrane-bound organelles

3. Structure of prokaryotic and eukaryotic cells

Structures	Prokaryote	Eukaryote
Structures	Prokaryote	Plant	Animal
Cell wall	Present in most bacteria (peptidoglycan) Archaeal cell walls lack peptidoglycan	Present (cellulose)	None
Cytoplasm	Present	Present	Present
Ribosome	Present	Present	Present
Cytoskeleton	Present	Present	Present
Nucleoid	Present	None	None
Nucleus	None	Present	Present
Mitochondria	None	Present	Present
ER, Golgi	None	Present	Present
Chloroplast	None	Present	None
Vacuole	None	Present (large central vacuole)	Small vacuoles or vesicles may be present
Centriole	None	None	Present
Lysosome	None	None	Present

a. The functions of organelles

Organelles	Functions and features
Flagella	• Cell locomotion
Nucleus	• DNA replication and transcription • Stores genetic information
Ribosomes	• Site of protein synthesis. • In eukaryotes, cytosolic ribosomal subunits are assembled in the nucleolus. • Functional ribosomes are found in the cytoplasm, mitochondria, and chloroplasts. • Free in the cytoplasm or bound to the rough endoplasmic reticulum (ER). • Composed of rRNA and proteins.
Rough ER	• Studded with ribosomes. • Synthesizes secretory proteins, membrane proteins, and membrane components.
Smooth ER	• Lacks ribosomes • Lipid synthesis (including steroid hormones) • Calcium storage • Carbohydrate metabolism • Detoxification
Golgi Apparatus	• Receives proteins and lipids from the ER. • Modifies, sorts, and packages proteins and lipids. • Ships vesicles from the trans face to the plasma membrane or other destinations.
Lysosomes	• Membrane-bound organelles containing hydrolytic enzymes. • Develop from endosomes supplied with hydrolytic enzymes from the Golgi apparatus. • Primarily found in animal cells.
Peroxisomes	• Contain oxidative enzymes that produce and break down H₂O₂. • Oxidize fatty acids and detoxify harmful compounds. • Form through ER-derived precursors and through growth and division of existing peroxisomes.
Vacuole	• Stores nutrients and waste products. • Isolates harmful substances. • Maintains internal pH and turgor pressure. • Surrounded by the tonoplast membrane.
Mitochondria	• Site of aerobic cellular respiration and ATP production. • Contains its own ribosomes and DNA. • Double membrane, cristae, and matrix. • Originated through endosymbiosis.
Chloroplasts	• Site of photosynthesis. • Contains its own ribosomes and DNA. • Double membrane, stroma, thylakoids, and chlorophyll. • Originated through endosymbiosis.
Cytosol	• Main site of many metabolic reactions, including glycolysis.
Centriole	• A component of animal-cell centrosomes, which organize microtubules • Helps form the mitotic spindle during cell division • Found primarily in animal cells

b. Cytoskeletons

Cytoskeletons	Functions and features
Microtubules	• Hollow tubes composed of tubulin • Form the mitotic spindle during cell division • Motile eukaryotic cilia and flagella typically have a 9+2 axoneme; primary cilia usually have a 9+0 arrangement • Major component of centrosomes and centrioles • Form cilia and flagella (with dynein for movement) • Serve as tracks for intracellular transport by kinesin and dynein
Microfilaments (Actin filament)	• Cleavage furrow in cytokinesis • Cell crawling (amoeboid movement) • Support the cell cortex and microvilli • Interact with myosin during muscle contraction
Intermediate filaments	• Tensile strength • Nuclear lamina • Keratin provides mechanical strength to epithelial cells.

c. Organelles classified by number of membranes

Membranes	Organelles
No membrane	Centriole, Ribosome
Single membrane	Peroxisome, Lysosome, ER, Golgi apparatus, Vacuole
Double membrane	Chloroplast, Mitochondria, Nucleus

d. Extracellular matrix

• Collagen is the most abundant structural protein in animals and forms a triple-helix structure.

e. Endomembrane system

** Simplified pathway for DAT review. Some lipids may follow alternative intracellular transport pathways.

f. Cell growth

If a cell’s linear dimensions increase tenfold without changing shape, its surface area increases 10²-fold.
Its volume increases 10³-fold, so its surface-area-to-volume ratio decreases tenfold.

C. Cell communication

Cells communicate with one another to coordinate growth, development, and responses to environmental signals.

1. Signal

a. Direct cell-to-cell communication

Cell types	Classification	Features
Animal cell	Tight junction	Tightly associated membranes between two cells
	Gap junction	Channels that directly connect the cytoplasm of adjacent cells
	Desmosomes	Anchoring junctions held together by adhesion proteins
Plant cell	Plasmodesmata	Perforated channels in plant cell wall

b. Local signal

Paracrine signal

– Signaling molecules diffuse through the extracellular fluid to nearby target cells.

– Local distance

– Examples: growth factors, clotting factors, histamine, nitric oxide

Synaptic signal

– Release of neurotransmitters into the synaptic cleft.

c. Long distance signal

Endocrine signal

– Hormones are released into the bloodstream and travel to distant target cells.

– Long distance

– Examples: estradiol, testosterone, epinephrine (adrenaline), insulin

2. Reception

a. Cell surface reception

Cell-surface receptors bind signaling molecules at the plasma membrane.

Ligand-gated ion channels

– Neurotransmitter binding opens or closes these channels, changing ion movement across the membrane.

G protein-coupled receptors

– Activate G proteins, which relay signals to intracellular effector proteins and enzymes.

Receptor tyrosine kinases

– Ligand binding activates receptor tyrosine kinases, which use ATP to phosphorylate themselves and intracellular proteins.

b. Intracellular reception

Intracellular receptors bind hydrophobic signaling molecules, such as steroid hormones, inside the cell.

3. Transduction

a. Phosphorylation cascade

A series of protein kinase activations in which each kinase phosphorylates and activates the next kinase.

Phosphorylation

– Protein kinase: An enzyme that transfers phosphate groups from ATP to target proteins.

– Protein phosphatase: An enzyme that removes phosphate groups from proteins.

b. G protein signaling pathway

In the cAMP pathway, a first messenger activates a G_s-coupled receptor. The receptor promotes GDP–GTP exchange on the G protein’s α subunit, which then activates adenylyl cyclase.
Adenylyl cyclase converts ATP to cAMP.
cAMP acts as a second messenger and activates protein kinase A (PKA).

c. Calcium and IP₃ signaling pathway

In this pathway, a first messenger activates a G_q-coupled receptor. Gα_q exchanges GDP for GTP and activates phospholipase C.
Phospholipase C (PLC) hydrolyzes PIP₂ into two second messengers: IP₃ and DAG.
IP₃ binds to IP₃-gated Ca²⁺ channels in the endoplasmic reticulum, causing Ca²⁺ to be released into the cytosol.
Ca²⁺ acts as a second messenger and activates various target proteins.
DAG, together with Ca²⁺, activates protein kinase C (PKC).

4. Response

The cellular response may involve changes in gene expression or cytoplasmic activity.

D. Membrane

1. Function

Separates the inside of the cell from the outside environment.
Selectively transports molecules into and out of the cell.
Allows cell growth and movement.
Enables communication with other cells.

2. Components of structure

Phospholipids – Form a bilayer; each phospholipid has a hydrophilic head and hydrophobic tails.
Proteins – Integral proteins (including transmembrane proteins) and peripheral proteins.
Cholesterol – Embedded in animal cell membranes, where it helps regulate membrane fluidity.
Glycolipids – Function in cell recognition and cell-to-cell communication.
Fluid Mosaic Model – The membrane is a dynamic structure composed of phospholipids, proteins, and cholesterol.

3. Passive transport

Net movement of substances down their concentration gradients, or electrochemical gradients for ions.

Does not require a cellular energy input such as ATP hydrolysis.
Simple diffusion occurs directly through the phospholipid bilayer.
Facilitated diffusion requires transport proteins but does not require ATP.
Water moves by osmosis through aquaporins.

a. Diffusion

Net movement of molecules from an area of higher concentration to an area of lower concentration due to random molecular motion.
Examples: movement of O₂ and CO₂ across cell membranes, movement of dissolved solutes in a solution.

b. Osmosis

Diffusion of water across a selectively permeable membrane from an area of high water potential to an area of low water potential.
e.g., Water moves through aquaporin channels in cell membranes.
If a cell is placed in:

– Hypotonic solution: Animal cells swell and may lyse; plant cells become turgid

* Water enters the cell by osmosis.

* For example, a red blood cell swells up and bursts when it is placed in distilled water.

– isotonic solution: Animal cells → Normal, Plant cells → Flaccid

– hypertonic solution: Animal cells → Shrunken (crenated), Plant cells → Plasmolyzed

* Water leaves the cell by osmosis.

4. Active transport

Active transport moves substances against their concentration or electrochemical gradients using cellular energy, directly or indirectly.

Requires energy.
Examples: Sodium–potassium pumps in axonal membranes, electrogenic pump, cotransport (symport and antiport), proton pump.

5. Bulk transport

: movement of large molecules across the membrane.

Classification		Features
Endocytosis	Pinocytosis	Uptake of extracellular fluid into membrane-bound vesicles
	Phagocytosis	Engulfment of large particles into membrane-bound vesicles
	Receptor-mediated endocytosis	Specific ligands bind to receptors on the plasma membrane and are internalized in vesicles
Exocytosis		Release of vesicle contents when the vesicle membrane fuses with the plasma membrane

E. Energy and life

1. Metabolism

Metabolism is the sum of chemical reactions that sustain life.

Catabolic pathways – break down complex molecules into simpler ones and release energy.
Anabolic pathways – use energy to build complex molecules from smaller units.

2. Laws of energy

a. First law of thermodynamics

Principle of conservation of energy – energy can be transferred and transformed, but not created or destroyed.
e.g., Chemical energy from food can be converted into the kinetic energy of movement.

b. Second law of thermodynamics

Every spontaneous process increases the total entropy of the system and its surroundings.
e.g., Organisms release heat during metabolism.

3. Homeostasis

Homeostasis is the maintenance of relatively stable internal conditions despite changes in the external environment.

Negative feedback.

– A response that counteracts a change in a regulated variable and moves it toward its set point.

– Most homeostatic control mechanisms in animals use negative feedback.

Positive feedback – a response that amplifies the initial change, as in uterine contractions during childbirth.

F. Enzymes

An enzyme is a biological catalyst, usually a protein, that speeds up a chemical reaction by lowering its activation energy without being consumed.

Substrate binding can cause a conformational change in the enzyme (induced fit).
Enzymes do not change K_eq, the equilibrium concentrations of reactants and products, or the reaction’s ΔG and ΔS.
Enzyme activity is affected by temperature, pH, enzyme concentration, and substrate concentration.
Each enzyme has an optimal temperature and pH.
Enzymes can be reused after catalyzing a reaction.

1. Reaction

Exergonic reaction: releases free energy (ΔG < 0).
Endergonic reaction: requires an input of free energy (ΔG > 0).

2. Enzyme structure

Allosteric site

– A regulatory site distinct from the active site.

– Binds regulatory molecules, such as allosteric activators and inhibitors.

Active site – the site where a substrate binds and catalysis occurs.

3. Enzyme regulation

a. Allosteric Enzyme

: Regulatory molecules bind to an allosteric site and increase or decrease enzyme activity.

Activation – an activator binds to an allosteric site and increases enzyme activity.
Inhibition – an inhibitor binds to an allosteric site and decreases enzyme activity.
Cooperativity.

– In positive cooperativity, substrate binding to one subunit increases the other subunits’ affinity for the substrate.

– This makes the enzyme’s response to changes in substrate concentration more sensitive.

b. Enzyme inhibition

Non-competitive inhibition

– In pure noncompetitive inhibition, an inhibitor binds equally well to the free enzyme and the enzyme–substrate complex at a site distinct from the active site. V_max decreases, K_m is unchanged, and adding more substrate does not overcome inhibition.

Competitive inhibition

– An inhibitor competes with the substrate for binding to the active site. In classical competitive inhibition, the apparent K_m increases and V_max is unchanged; sufficiently increasing substrate concentration can overcome inhibition.

Feedback inhibition

– The end product of a metabolic pathway inhibits an enzyme acting early in that pathway.

G. Photosynthesis

1. Redox reaction

a. Oxidation

Loss of electrons; often accompanied by loss of hydrogen in biological reactions.
Oxidation may also involve gain of oxygen.

b. Reduction

Gain of electrons; often accompanied by gain of hydrogen in biological reactions.
Reduction may also involve loss of oxygen.

** OILRIG – Oxidation Is Losing electrons and Reduction Is Gaining electrons.

2. Chloroplast

: A double-membrane organelle in plants and algae that converts light energy into chemical energy during photosynthesis.

a. Structure of chloroplast

b. Chloroplast structures, from largest to smallest

c. Photosynthesis

Photosynthesis converts light energy into chemical energy, producing ATP and NADPH that are used to synthesize carbohydrates.

Light + 6H₂O + 6CO₂ → C₆H₁₂O₆ + 6O₂
Occurs in some bacteria, algae, and plants.

i. Light dependent (light reaction)

Takes place in the thylakoid membrane.
Photosynthetic pigments are light-absorbing molecules.

Chlorophyll has a magnesium-containing ring and a hydrophobic hydrocarbon tail.

– chlorophyll a: primary pigment that appears blue-green.

– chlorophyll b: accessory pigment that appears yellow-green.

– carotenoid: accessory pigment that appears yellow, orange, or red and provides photoprotection (protection of chlorophyll by dissipating excessive light energy).

At photosystem II, water oxidation supplies electrons, releases protons into the thylakoid lumen, and produces O₂.

2H₂O (split into) → 4e⁻ + 4H⁺ + O₂

Photosystem II (P680) supplies electrons from water to the electron transport chain, releasing O₂ and helping establish the proton gradient used by ATP synthase.
Photosystem I (P700) re-energizes electrons; NADP⁺ reductase transfers them to NADP⁺, producing NADPH.
Electron transport through the cytochrome b₆f complex contributes to proton translocation into the thylakoid lumen, helping establish the proton gradient.
H⁺ moves down its electrochemical gradient through ATP synthase, driving ATP synthesis.

ii. Light-independent reactions (Calvin cycle)

Takes place in the chloroplast stroma; it does not use light directly but relies on ATP and NADPH from the light reactions.
Uses ATP and NADPH to synthesize carbohydrate in the Calvin cycle.
Calvin cycle

RuBP (ribulose-1,5-bisphosphate) + CO₂ (by Rubisco enzyme)

→ Two 3-phosphoglycerate (PGA) per turn of Calvin cycle

Glyceraldehyde 3-phosphate (G3P, also called PGAL) is the net carbohydrate product of the Calvin cycle and is used to synthesize glucose and other organic molecules.

d. Photosynthesis in C3, C4, CAM plants

C3 plants

– The first stable product of CO₂ fixation by Rubisco is the three-carbon compound 3-phosphoglycerate. Photorespiration increases when Rubisco binds O₂ instead of CO₂.

– e.g., common crops (rice, wheat, soybeans).

C4 plants

– adapted to hot, dry conditions.

– spatial separation: initial CO₂ fixation by PEP carboxylase produces four-carbon compounds in mesophyll cells. These compounds deliver CO₂ to the Calvin cycle in bundle-sheath cells, reducing photorespiration.

– e.g., sugarcane and corn.

CAM plants

– adapted to arid conditions.

– temporal separation: CO₂ enters through open stomata and is fixed into organic acids at night. During the day, these acids release CO₂ for the Calvin cycle.

– open stomata during the night and close stomata during the day.

– e.g., cacti and pineapples.

H. Cellular respiration

: metabolic pathways that release energy from nutrients and capture some of that energy in ATP.

Aerobic respiration: glycolysis in the cytosol, followed by pyruvate oxidation, the TCA cycle, and oxidative phosphorylation in mitochondria in eukaryotes. Fermentation is an alternative that regenerates NAD⁺ so glycolysis can continue without an electron transport chain.

C₆H₁₂O₆ + 6O₂→ 6H₂O + 6CO₂+ Energy (ATP)

1. Glycolysis

: a metabolic pathway that converts one glucose molecule into two pyruvate molecules in the cytosol.

Consumes 2 ATP and produces 4 ATP, yielding a net gain of 2 ATP per glucose.
Also produces 2 NADH, 2 H⁺, 2 H₂O, and 2 pyruvate per glucose.

Glucose + 2NAD⁺ + 2Pi + 2ADP → 2Pyruvates + 2NADH + 2ATP(NET) + 2H⁺ + 2H₂O

2. Fermentation

: Fermentation regenerates NAD⁺ from NADH without an electron transport chain, allowing glycolysis to continue producing ATP.

Glycolysis converts glucose to pyruvate, which can be reduced to lactate or converted to ethanol and CO₂ during fermentation.
Lactate fermentation occurs in muscle cells and some microorganisms.
Alcohol fermentation occurs in yeast cells.
In the Cori cycle, lactate travels to the liver and is converted back to glucose through gluconeogenesis.
Regeneration of NAD⁺

3. Mitochondria

: Mitochondria are double-membrane organelles that carry out much of aerobic cellular respiration and produce ATP.

Aerobic respiration

a. Structure of mitochondria

Double membrane
Cristae
Matrix
Krebs cycle

b. Acetyl-CoA

In eukaryotes, pyruvate oxidation in the mitochondrial matrix links glycolysis to the Krebs cycle.
Per glucose, two pyruvates yield 2 acetyl-CoA, 2 CO₂, and 2 NADH (plus 2 H⁺).

c. Krebs cycle (Citric acid cycle, TCA cycle)

In eukaryotes, the Krebs cycle occurs in the mitochondrial matrix and depends indirectly on oxygen to regenerate NAD⁺ and FAD.
Acetyl-CoA combines with oxaloacetate to form citrate.
Producing 6 NADH, 2 FADH₂, 2 ATP (2 GTP), and 4 CO₂ per 2 pyruvates (1 glucose molecule = 2 pyruvates)

d. Metabolism of various macromolecules

Gluconeogenesis is an anabolic pathway that synthesizes glucose from noncarbohydrate precursors such as glucogenic amino acids, glycerol, and lactate.

e. Electron transport

Oxidative phosphorylation couples the electron transport chain to ATP synthesis by chemiosmosis across the inner mitochondrial membrane.
Ubiquinone is a lipid-soluble mobile electron carrier in the inner mitochondrial membrane; cytochrome c is a water-soluble mobile electron carrier on its intermembrane-space side.
The electron transport chain pumps protons from the mitochondrial matrix to the intermembrane space, building an electrochemical proton gradient.
The proton gradient drives ATP synthesis as H⁺ flows through ATP synthase.
Oxidative phosphorylation typically yields about 26–28 ATP per glucose in eukaryotic cells (approximately 2.5 ATP per mitochondrial NADH and 1.5 ATP per FADH₂); the yield varies with the shuttle used for cytosolic NADH and coupling efficiency.
O₂ is the terminal electron acceptor in the electron transport chain and is reduced to H₂O.

4. Summary of photosynthesis and cellular respiration

a. Chloroplasts

Light reactions: PS II releases O₂ from water; electron transport establishes the proton gradient for ATP synthase.
PS I supplies electrons for NADPH production. ATP and NADPH from the light reactions support the Calvin cycle.
Calvin cycle: six turns fix six CO₂ and consume 18 ATP and 12 NADPH. Twelve G3P (PGAL) molecules form; ten regenerate RuBP, and two are the net output used to make one glucose equivalent.

b. Mitochondria

Krebs cycle: 4 CO₂, 6 NADH, 2ATP, and 2 FADH₂/ per 2 pyruvates
Oxidative phosphorylation: about 26–28 ATP per glucose; O₂ is reduced to H₂O

c. Typical ATP yield per glucose in eukaryotic cellular respiration: about 30–32 ATP

Glycolysis: 2 ATP
Krebs cycle: 2 ATP
Oxidative phosphorylation: about 26–28 ATP

d. Mitochondria and chloroplasts

Semiautonomous organelles containing their own DNA and ribosomes
Originated through endosymbiosis