Biology - Cell Biology and Biochemistry

1. All living organisms are composed of one or more cells
2. The cell is the basic unit of structure and function in all living organisms
3. All cells arise from pre-existing cells

The nucleus is the control centre of the cell. It contains:

• Nuclear envelope: Double membrane with nuclear pores that control the movement of substances in and out
• Nucleolus: Dark-staining region where ribosomal RNA (rRNA) is synthesised
• Chromatin: DNA wrapped around histone proteins; condenses into chromosomes during cell division

Functions:

• Controls cell activities through gene expression
• Stores genetic information (DNA)
• Site of DNA replication and transcription

Mitochondria are the sites of aerobic respiration, producing ATP through the Krebs cycle and Oxidative phosphorylation.

Structure:

• Outer membrane: Permeable to small molecules
• Inner membrane: Folded into cristae to increase surface area; site of the electron transport chain
• Matrix: Contains enzymes for the Krebs cycle, mitochondrial DNA, and ribosomes

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \to 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \mathrm{ATP}$

Rough ER (RER):

• Studded with ribosomes
• Involved in protein synthesis and transport
• Proteins enter the ER lumen for folding and modification

Smooth ER (SER):

• No ribosomes
• Involved in lipid synthesis, detoxification, and calcium storage

A stack of flattened membrane-bound sacs (cisternae) that modifies, sorts, and packages proteins and Lipids.

Functions:

• Modifies proteins (e.g., adding carbohydrate groups to form glycoproteins)
• Packages proteins into vesicles for transport
• Forms lysosomes (in animal cells)

Ribosomes are the sites of protein synthesis (translation).

• Free ribosomes: Float in the cytoplasm; synthesise proteins for use within the cell
• Bound ribosomes: Attached to the RER; synthesise proteins for secretion or for the cell membrane

Membrane-bound vesicles containing digestive enzymes (hydrolytic enzymes). Functions include:

• Breaking down worn-out organelles (autophagy)
• Digesting material engulfed by phagocytosis
• Releasing enzymes outside the cell (extracellular digestion)

The site of photosynthesis.

Structure:

• Double membrane: Outer and inner membrane
• Thylakoids: Flattened sacs containing chlorophyll; site of the light-dependent reactions
• Grana: Stacks of thylakoids
• Stroma: Fluid-filled space; site of the light-independent reactions (Calvin cycle)

$6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} \xrightarrow{\mathrm{light}} \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

The cell membrane (plasma membrane) is a phospholipid bilayer with embedded proteins.

Fluid Mosaic Model:

• Fluid: Individual phospholipid molecules can move laterally within the layer
• Mosaic: Various proteins are embedded in the bilayer, creating a pattern

Components:

• Phospholipids: form the basic bilayer structure
• Intrinsic (integral) proteins: span the entire membrane; involved in transport and signalling
• Extrinsic (peripheral) proteins: attached to the surface; involved in cell recognition and enzymatic activity
• Cholesterol: regulates membrane fluidity
• Glycolipids and glycoproteins: involved in cell recognition

Movement of molecules from a region of higher concentration to a region of lower concentration, Directly through the phospholipid bilayer.

• Only for small, non-polar molecules (e.g., $\mathrm{O}_2$, $\mathrm{CO}_2$Lipid-soluble substances)
• Net movement stops at equilibrium (dynamic equilibrium)
• Rate depends on: concentration gradient, temperature, surface area, distance

Movement of molecules down their concentration gradient through transport proteins.

• Channel proteins: Form hydrophilic pores for specific ions (e.g., $\mathrm{Na}^+$ $\mathrm{K}^+$, $\mathrm{Ca}^{2+}$)
• Carrier proteins: Bind to specific molecules, undergo conformational change, and release on the other side
• Used for larger or polar molecules (e.g., glucose, amino acids, ions)

The movement of water molecules from a region of higher water potential to a region of lower water Potential, through a selectively permeable membrane.

Water potential ($\Psi$) is measured in kilopascals (kPa). Pure water has $\Psi = 0 \mathrm{ kPa}$. Adding solutes decreases water potential (makes it more negative).

Osmosis in animal cells:

Osmosis in plant cells:

Movement of molecules against their concentration gradient (from low to high concentration), Requiring ATP and carrier proteins.

• Specific to certain molecules (e.g., $\mathrm{Na}^+/\mathrm{K}^+$ pump, absorption of minerals by root hair cells)
• Can become saturated at high concentrations (carrier proteins are limiting)
• Inhibited by metabolic poisons (e.g., cyanide) that block ATP production

The cell membrane engulfs material to bring it into the cell.

• Phagocytosis: “Cell eating” — engulfing solid particles (e.g., white blood cells engulfing bacteria)
• Pinocytosis: “Cell drinking” — engulfing liquid droplets

Vesicles fuse with the cell membrane to release contents outside the cell (e.g., secretion of Hormones, neurotransmitters).

General formula: $\mathrm{C}_x(\mathrm{H}_2\mathrm{O})_y$

Monosaccharides:

• Glucose ($\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$): primary energy source
• Fructose: found in fruits
• Galactose: component of milk sugar
• Ribose and deoxyribose: in RNA and DNA respectively

Disaccharides (formed by condensation, broken by hydrolysis):

Polysaccharides:

• Starch: Energy storage in plants (amylose: helical; amylopectin: branched)
• Glycogen: Energy storage in animals (highly branched)
• Cellulose: Structural in plants (beta-glucose, straight chains, hydrogen bonds)
• Chitin: Structural in arthropod exoskeletons and fungal cell walls

Tests for carbohydrates:

• Benedict”s test: Reducing sugars (e.g., glucose) produce a brick-red precipitate when heated with Benedict’s reagent
• Iodine test: Starch turns blue-black

Proteins are polymers of amino acids.

Amino acid structure:

Each amino acid has an amino group ($\mathrm{NH}_2$), a carboxyl group ($\mathrm{COOH}$), a hydrogen Atom, and an R group (side chain), all bonded to a central carbon atom.

Peptide bond formation:

Amino acids join by condensation reactions, forming peptide bonds ($\mathrm{-CO-NH-}$) and releasing Water.

• Dipeptide: 2 amino acids
• Polypeptide: many amino acids

Protein structure levels:

Tests for proteins:

• Biuret test: Add Biuret reagent (NaOH + CuSO$_4$); violet/purple colour indicates protein

Lipids are organic molecules that are insoluble in water but soluble in organic solvents.

Triglycerides:

• Formed from one glycerol + three fatty acids
• Joined by ester bonds (condensation reaction)
• Energy storage molecules
• Contain more energy per gram than carbohydrates

Phospholipids:

• Similar to triglycerides but one fatty acid is replaced by a phosphate group
• Amphipathic: hydrophilic head (phosphate) and hydrophobic tail (fatty acids)
• Form the cell membrane bilayer

Saturated vs unsaturated fatty acids:

• Saturated: no double bonds (C-C), solid at room temperature (animal fats)
• Unsaturated: one or more double bonds (C=C), liquid at room temperature (plant oils)

Test for lipids:

• Ethanol emulsion test: Dissolve in ethanol, pour into water; cloudy white emulsion indicates lipids

DNA (Deoxyribonucleic Acid):

• Double-stranded helix
• Sugar: deoxyribose
• Bases: adenine (A), thymine (T), guanine (G), cytosine (C)
• Base pairing: A-T (2 H-bonds), G-C (3 H-bonds)
• Stores genetic information

RNA (Ribonucleic Acid):

• single-stranded
• Sugar: ribose
• Bases: A, U (uracil replaces thymine), G, C
• Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)

Enzymes are biological catalysts produced by living cells. They are globular proteins that speed up Biochemical reactions without being consumed.

• Specific: each enzyme catalyses one particular reaction (or a small group of reactions)
• Not used up: can be reused many times
• Work in small amounts
• Affected by temperature and pH
• Protein in nature: denatured by extreme conditions

The substrate (reactant) fits into the active site of the enzyme like a key fits into a lock. The Active site has a specific complementary shape to the substrate.

A more refined model: the active site changes shape slightly when the substrate binds, improving the Fit. This lowers the activation energy of the reaction.

Temperature:

• Rate increases with temperature (up to the optimum)
• Beyond the optimum, the enzyme denatures (active site changes shape irreversibly)
• Human enzymes: optimum around $37^\circ\mathrm{C}$

pH:

• Each enzyme has an optimum pH
• Extreme pH causes denaturation by disrupting ionic and hydrogen bonds
• Pepsin: optimum pH 2 (stomach)
• Trypsin: optimum pH 8 (small intestine)

Substrate concentration:

• Increasing substrate concentration increases the rate (up to a point)
• At saturation, all active sites are occupied; rate plateaus (Vmax)

Enzyme concentration:

• Increasing enzyme concentration increases the rate (provided substrate is not limiting)

An enzyme has an optimum temperature of $40^\circ\mathrm{C}$. At $20^\circ\mathrm{C}$The reaction Rate is $0.3$ units/s. At $40^\circ\mathrm{C}$The rate is $1.2$ units/s. Calculate the Q10 (temperature coefficient).

$Q_{10} = \frac{\mathrm{Rate at }(T + 10)}{\mathrm{Rate at } T}$

Between $20^\circ\mathrm{C}$ and $30^\circ\mathrm{C}$: $Q_{10}$ might be approximately 2 (typical For biological reactions). Without the $30^\circ\mathrm{C}$ data, we can estimate the overall Effect:

From $20^\circ\mathrm{C}$ to $40^\circ\mathrm{C}$ (a $20^\circ\mathrm{C}$ increase):

$\mathrm{Rate increase factor} = \frac{1.2}{0.3} = 4$

This is consistent with $Q_{10} \approx 2$ (since $2^2 = 4$).

Competitive inhibition:

• Inhibitor has a similar shape to the substrate
• Competes with the substrate for the active site
• Can be overcome by increasing substrate concentration
• Example: malonate inhibiting succinate dehydrogenase

Non-competitive inhibition:

• Inhibitor binds to a site other than the active site (allosteric site)
• Changes the shape of the active site
• Cannot be overcome by increasing substrate concentration
• Example: heavy metal ions (lead, mercury)

1. Glucose (6C) is phosphorylated (uses 2 ATP)
2. Glucose is split into two molecules of triose phosphate (3C)
3. Triose phosphate is oxidised and dehydrogenated (produces 2 NADH)
4. Net production: 2 ATP, 2 NADH, 2 pyruvate (3C)

• Pyruvate (3C) is decarboxylated and dehydrogenated
• Forms acetyl CoA (2C) + $\mathrm{CO}_2$ + NADH

For each glucose molecule (two turns of the cycle):

• 2 $\mathrm{CO}_2$ released
• 3 NADH produced per turn (6 total)
• 1 FADH$_2$ produced per turn (2 total)
• 1 ATP produced per turn (2 total)
• Regenerates oxaloacetate (4C)

• NADH and FADH$_2$ donate electrons to the electron transport chain
• Energy released pumps protons across the inner membrane
• Protons flow back through ATP synthase, producing ATP
• Oxygen is the final electron acceptor, forming water

1. Light energy is absorbed by chlorophyll in Photosystem II
2. Water is split (photolysis): $\mathrm{H}_2\mathrm{O} \to 2\mathrm{H}^+ + 2e^- + \frac{1}{2}\mathrm{O}_2$
3. Electrons pass through the electron transport chain, generating ATP
4. Light is absorbed by Photosystem I; electrons are re-energised
5. Electrons reduce NADP$^+$ to NADPH

Products: ATP, NADPH, $\mathrm{O}_2$

1. $\mathrm{CO}_2$ is fixed by ribulose bisphosphate (RuBP, 5C) using the enzyme RuBisCO
2. Forms an unstable 6C compound that splits into two molecules of glycerate-3-phosphate (GP, 3C)
3. GP is reduced to triose phosphate (TP, 3C) using ATP and NADPH
4. Some TP is used to make glucose and other organic compounds
5. Most TP is used to regenerate RuBP (uses ATP)

For every 3 $\mathrm{CO}_2$ molecules fixed: 1 molecule of triose phosphate (3C) is produced. It Takes 6 $\mathrm{CO}_2$ molecules to produce 1 molecule of glucose (6C).

Life’s building blocks: Cells are like factories — the nucleus is the management office, mitochondria are power plants, and ribosomes are assembly lines. Biochemical tests identify which molecules are present.

Why it matters: From diagnosing diseases to understanding genetics, cell biology is the foundation of modern medicine and biotechnology.

The key insight: Structure determines function — the shape of a protein determines what it does, and mutations that change shape can cause disease.

flowchart TD
    A[1_Cell Biology And Biochemistry] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

• Always distinguish between prokaryotic and eukaryotic cells in comparison questions.
• For membrane transport, state whether energy is required and the direction of movement relative to the concentration gradient.
• In enzyme questions, always mention the effect on the active site when discussing denaturation.
• For photosynthesis limiting factor graphs, identify which factor is limiting at each point on the curve.
• When describing biochemical pathways (respiration, photosynthesis), name the specific location of each stage.

Explain how the fluid mosaic model of the cell membrane accounts for the following properties: (a) selective permeability, (b) ability of cells to communicate, (c) flexibility for phagocytosis.

If you get this wrong, revise: Cell Organelles — Cell Membrane

Mitosis is the process of cell division that produces two genetically identical daughter cells, each With the same number of chromosomes as the parent cell.

Functions of mitosis:

• Growth: increasing the number of cells in an organism
• Repair: replacing damaged or dead cells
• Asexual reproduction: in some organisms

A cell in the G2 phase of the cell cycle has 46 chromosomes. How many chromosomes and chromatids Will be present in each daughter cell after mitosis?

After mitosis: each daughter cell has 46 chromosomes (each consisting of one chromatid).

During S phase (before mitosis), each chromosome replicates to form two sister chromatids. So at the Start of mitosis, there are 46 chromosomes (92 chromatids). After anaphase, the chromatids separate, And each daughter cell receives 46 single-chromatid chromosomes.

Meiosis is a type of cell division that produces four genetically different daughter cells, each With half the number of chromosomes of the parent cell. It is essential for sexual reproduction.

Key differences from mitosis:

Sources of genetic variation in meiosis:

1. Crossing over: During prophase I, homologous chromosomes exchange segments of DNA. This creates new combinations of alleles on the same chromosome.
2. Independent assortment: During metaphase I, homologous pairs line up randomly at the equator. Different combinations of maternal and paternal chromosomes are distributed to daughter cells.
3. Random fertilisation: Any sperm can fertilise any egg, further increasing genetic diversity.

Meiosis I (reductional division):

• Prophase I: Homologous chromosomes pair up (synapsis); crossing over occurs
• Metaphase I: Homologous pairs line up at the equator
• Anaphase I: Homologous chromosomes separate (sister chromatids remain together)
• Telophase I: Two cells form, each with half the chromosome number (but each chromosome still has two chromatids)

Meiosis II (equational division):

• Prophase II: Chromosomes condense again
• Metaphase II: Chromosomes line up singly at the equator
• Anaphase II: Sister chromatids separate
• Telophase II: Four haploid daughter cells form

The cell cycle describes the sequence of events from one cell division to the next.

G1, S, and G2 together are called interphase, which accounts for approximately 90% of the cell Cycle.

The cell cycle is controlled by checkpoints:

• G1 checkpoint: Checks if the cell is large enough and DNA is undamaged before entering S phase
• G2 checkpoint: Checks if DNA has been replicated correctly before entering M phase
• Spindle checkpoint (M checkpoint): Checks if all chromosomes are properly attached to spindle fibres before anaphase

Cancer is a disease caused by uncontrolled cell division, resulting in the formation of tumours.

• Benign tumours: Grow slowly, remain localised, do not spread
• Malignant tumours: Grow rapidly, invade surrounding tissues, can spread to other parts of the body (metastasis)

Causes of cancer:

• Mutations in proto-oncogenes (become oncogenes, promoting cell division)
• Mutations in tumour suppressor genes (losing their inhibitory function)
• Exposure to carcinogens: UV radiation, tobacco smoke, certain chemicals, ionising radiation
• Some viruses (e.g., HPV, hepatitis B and C)

DNA replication is semi-conservative: each new DNA molecule consists of one original strand and one Newly synthesised strand.

1. Helicase unwinds and separates the double helix by breaking hydrogen bonds between complementary bases
2. DNA polymerase adds complementary nucleotides to each template strand, following the base pairing rules (A-T, C-G)
3. Leading strand: Synthesised continuously in the 5-prime to 3-prime direction
4. Lagging strand: Synthesised in short fragments (Okazaki fragments) that are later joined by DNA ligase
5. Each new DNA molecule contains one original strand and one new strand

DNA polymerase has a …/1-number-and-algebra/3_proof-and-logicreading function. If an incorrect nucleotide is added, it is removed and Replaced. This gives an error rate of approximately 1 in $10^9$ base pairs.

Transcription is the process of copying the genetic information from DNA to messenger RNA (mRNA).

1. RNA polymerase binds to the promoter region on the DNA template strand
2. The DNA double helix unwinds locally
3. RNA polymerase synthesises a complementary mRNA strand using the DNA template (A pairs with U, T pairs with A, C pairs with G, G pairs with C)
4. The mRNA molecule is released when RNA polymerase reaches the terminator region
5. In eukaryotes, the pre-mRNA is processed:

• A cap is added to the 5-prime end
• A poly-A tail is added to the 3-prime end
• Introns (non-coding regions) are removed by splicing
• Exons (coding regions) are joined together

Translation is the process of synthesising a polypeptide chain from the mRNA template.

1. The mRNA binds to a ribosome
2. The ribosome reads the mRNA in codons (groups of three bases)
3. Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome
4. Each tRNA has an anticodon that is complementary to the codon on the mRNA
5. Peptide bonds form between adjacent amino acids (catalysed by peptidyl transferase in the ribosome)
6. Translation stops when a stop codon (UAA, UAG, UGA) is reached
7. The polypeptide chain is released and folds into its functional 3D shape

• The genetic code is degenerate: most amino acids are coded for by more than one codon
• The genetic code is universal: the same codons code for the same amino acids in nearly all organisms
• The genetic code is non-overlapping: each base is part of only one codon

Cells communicate through chemical messengers called ligands that bind to specific receptors.

Types of cell signalling:

Cell-surface receptors: For large or hydrophilic signalling molecules that cannot cross the cell Membrane (e.g., insulin, adrenaline).

Intracellular receptors: For small or hydrophobic signalling molecules that can diffuse through The membrane (e.g., steroid hormones like testosterone, oestrogen).

When a ligand binds to a cell-surface receptor, it triggers a cascade of intracellular events:

1. Signal reception: The ligand binds to the receptor
2. Signal transduction: The signal is relayed inside the cell, often involving:

• G-proteins: Activate or inhibit enzymes
• Second messengers: Small molecules that amplify the signal (e.g., cAMP, $\mathrm{Ca}^{2+}$)
• Enzyme cascades: Kinase cascades that phosphorylate target proteins

1. Cellular response: The cell changes its activity (e.g., gene expression, metabolism, secretion)

First line of defence:

• Skin: physical barrier
• Mucous membranes: trap pathogens
• Stomach acid: destroys pathogens
• Tears, saliva: contain lysozyme (an enzyme that breaks down bacterial cell walls)

Second line of defence:

• Phagocytes (neutrophils, macrophages): engulf and digest pathogens by phagocytosis
• Inflammation: increased blood flow, swelling, heat, pain; brings phagocytes to the infection site
• Fever: raises body temperature, inhibiting pathogen growth and enhancing immune function
• Interferons: proteins produced by virus-infected cells that inhibit viral replication in neighbouring cells

Cell-mediated response (T cells):

• T helper cells: release cytokines that stimulate B cells and cytotoxic T cells
• Cytotoxic T cells (killer T cells): destroy virus-infected cells and cancer cells by inducing apoptosis
• Memory T cells: provide long-term immunity

Humoral response (B cells):

• B cells produce antibodies (immunoglobulins) specific to the antigen
• Plasma cells: short-lived cells that secrete large quantities of antibodies
• Memory B cells: provide long-term immunity; respond faster and more strongly upon re-exposure

An antibody (immunoglobulin) is a Y-shaped protein with:

• Two identical heavy chains
• Two identical light chains
• Variable regions: specific to the antigen (at the tips of the Y)
• Constant regions: the same for all antibodies of the same class

Explain how vaccination provides immunity against a disease.

A vaccine contains a weakened or dead form of the pathogen (or parts of it like antigens). When Introduced into the body, it triggers a primary immune response without causing the disease. B cells Produce antibodies and memory B cells are formed. If the person is later exposed to the actual Pathogen, the memory B cells quickly produce large quantities of antibodies in a secondary response, Destroying the pathogen before symptoms develop. This provides active artificial immunity.

Definition: Undifferentiated cells that can divide and differentiate into specialised cell Types.

Therapeutic uses:

• Treatment of leukaemia (bone marrow transplants)
• Potential for regenerative medicine (repairing damaged tissues)
• Research into disease mechanisms

Ethical considerations:

• Embryonic stem cells involve destruction of embryos
• Adult stem cells have more limited potential
• Induced pluripotent stem cells (iPSCs) offer an alternative

The water potential of a solution is:

$\Psi = \Psi_s + \Psi_p$

Where:

• $\Psi_s$ = solute potential (always negative or zero)
• $\Psi_p$ = pressure potential (positive in turgid plant cells, zero in animal cells)

For a solution with no pressure applied:

$\Psi = \Psi_s = -iCRT$

Where:

• $i$ = ionisation constant (number of particles per molecule)
• $C$ = molar concentration
• $R$ = gas constant ($8.314 \mathrm{ J mol}^{-1} \mathrm{ K}^{-1}$)
• $T$ = temperature in Kelvin

Calculate the water potential of a $0.3 \mathrm{ mol/dm}^3$ sucrose solution at $25^\circ\mathrm{C}$. ($i = 1$ for sucrose)

$\Psi_s = -iCRT = -(1)(0.3)(8.314)(298) = -743.3 \mathrm{ kPa}$

Since there is no pressure: $\Psi = -743.3 \mathrm{ kPa}$

Calculate the water potential of a $0.2 \mathrm{ mol/dm}^3$ $\mathrm{NaCl}$ solution at $20^\circ\mathrm{C}$. ($i = 2$ for $\mathrm{NaCl}$)

$\Psi_s = -(2)(0.2)(8.314)(293) = -974.4 \mathrm{ kPa}$

$\Psi = -974.0 \mathrm{ kPa}$

Problem 1: Describe three structural differences between prokaryotic and eukaryotic cells, and explain how each difference relates to the cell’s function.

If you get this wrong, revise: Cell Structure — Prokaryotic vs Eukaryotic Cells

Problem 2: A red blood cell is placed in a 0.5 mol/dm$^3$ sucrose solution. Predict what will happen to the cell and explain your reasoning. (The water potential of the red blood cell cytoplasm is approximately -700 kPa.)

If you get this wrong, revise: Membrane Transport — Osmosis; Water Potential and Osmosis Calculations

Problem 3: Explain the effect of increasing temperature on enzyme activity from 0 degrees C to 60 degrees C. Refer to kinetic energy, collision frequency, and enzyme structure in your answer.

If you get this wrong, revise: Enzymes — Factors Affecting Enzyme Activity

Problem 4: A cell in the G1 phase of the cell cycle has 46 chromosomes. How many chromosomes and DNA molecules are present at the end of S phase, during metaphase of mitosis, and after cytokinesis?

If you get this wrong, revise: Cell Division — Mitosis; The Cell Cycle

Problem 5: Compare the structure and function of DNA and mRNA.

If you get this wrong, revise: DNA Replication; Protein Synthesis

Problem 6: Describe the stages of mitosis and explain their significance in ensuring genetic continuity.

If you get this wrong, revise: Cell Division — Stages of Mitosis

Problem 7: Explain how vaccination provides immunity against a disease, referring to both the primary and secondary immune responses.

If you get this wrong, revise: Immune System — Specific (Adaptive) Immunity; Primary vs Secondary Immune Response

Problem 8: Describe the role of the Golgi apparatus in protein processing and transport.

If you get this wrong, revise: Cell Organelles — Golgi Apparatus

Problem 9: Calculate the water potential of a 0.15 mol/dm$^3$ NaCl solution at 25 degrees C. ($i = 2$ for NaCl.) Determine whether a plant cell with $\Psi = -800 \mathrm{ kPa}$ and $\Psi_p = 200 \mathrm{ kPa}$ would gain or lose water in this solution.

If you get this wrong, revise: Water Potential and Osmosis Calculations

Problem 10: Explain how the structure of the mitochondrion is adapted to its function in aerobic respiration.

If you get this wrong, revise: Cell Organelles — Mitochondria; Cellular Respiration

Glycolysis occurs in the cytoplasm and does not require oxygen. It is the first stage of both aerobic and anaerobic respiration.

Stages of glycolysis:

$\text{Glucose (6C)} + 2\mathrm{NAD}^+ + 2\mathrm{ADP} + 2\mathrm{P}_i \to 2\text{ Pyruvate (3C)} + 2\mathrm{NADH} + 2\mathrm{H}^+ + 2\mathrm{ATP} + 2\mathrm{H}_2\mathrm{O}$

Link reaction (pyruvate oxidation):

• Occurs in the mitochondrial matrix
• Each pyruvate (3C) is decarboxylated (loses $\mathrm{CO}_2$) and dehydrogenated (NAD$^+$ reduced to NADH)
• The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl CoA
• Per glucose: 2 pyruvate $\to$ 2 acetyl CoA + 2 $\mathrm{CO}_2$ + 2 NADH

Krebs cycle (citric acid cycle):

• Occurs in the mitochondrial matrix
• Each acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C)
• Through a series of reactions, citrate is converted back to oxaloacetate, releasing:
• 2 $\mathrm{CO}_2$ (decarboxylation)
• 3 NADH (dehydrogenation)
• 1 FADH$_2$ (dehydrogenation)
• 1 ATP (substrate-level phosphorylation via GTP)
• Per glucose: 2 turns of the cycle produce 4 $\mathrm{CO}_2$6 NADH, 2 FADH$\_2$2 ATP

• Occurs on the inner mitochondrial membrane (cristae)
• NADH and FADH$_2$ donate electrons to the electron transport chain
• Electrons pass through a series of carriers (complexes I-IV), releasing energy used to pump $\mathrm{H}^+$ into the intermembrane space
• $\mathrm{H}^+$ flows back through ATP synthase (chemiosmosis), producing ATP
• Oxygen is the final electron acceptor, combining with $\mathrm{H}^+$ and electrons to form water

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