state that cells use ATP from respiration for energy-requiring processes

Cells – The Basic Units of Living Organisms

Learning Objectives

• Explain how cells obtain ATP from respiration and use it to power energy‑requiring processes.
• Identify the main structural features of prokaryotic and eukaryotic cells, including organelles, cell walls and plasmodesmata.
• Describe the major classes of biological molecules, their properties and how they are tested for.
• Summarise enzyme structure, mechanism and factors that affect activity.
• Explain the fluid‑mosaic model of the plasma membrane and the different modes of transport (passive, active, bulk).
• Outline the stages of the mitotic cell cycle and the role of the spindle apparatus.
• Describe the structure of nucleic acids and the processes of DNA replication, transcription and translation.
• Compare transport mechanisms in plants (xylem, phloem, apoplast/symplast) and mammals (circulatory system, capillaries, red blood cells).

1. The Microscope in Cell Studies

1.1 Components of a Light Microscope

• Eyepiece (ocular lens) – usually 10 ×.
• Objective lenses – 4 ×, 10 ×, 40 ×, 100 × (oil immersion).
• Stage, condenser, diaphragm – hold the slide, focus light and control intensity.
• Light source – LED or halogen.
• Focus knobs – coarse and fine.

1.2 Preparing a Wet Mount

1. Place a drop of distilled water on a clean slide.
2. Add the specimen (e.g., onion epidermis, cheek cells).
3. Gently lower a coverslip to avoid air bubbles.
4. Secure the slide on the stage.

1.3 Calculating Total Magnification

Total magnification = eyepiece power × objective power

Example: 10 × eyepiece × 40 × objective = 400 × total magnification.

1.4 Using Scale Bars

Calibrate the eyepiece graticule with a stage micrometer, then draw a scale bar on every cell diagram (e.g., 10 µm).

2. Cell Structure

2.1 Major Organelles of Eukaryotic Cells

2.2 Prokaryotic vs. Eukaryotic Cells

2.3 Viruses – Are They Cells?

• Non‑cellular entities: nucleic acid (DNA or RNA) surrounded by a protein capsid; some have a lipid envelope.
• Cannot carry out metabolism or reproduce independently – require a host cell.
• Because they lack cellular structure, viruses are excluded from the definition “cell = basic unit of life”, although they are biologically important.

2.4 The Cell Theory (Cambridge Syllabus)

• All living organisms are composed of one or more cells.
• The cell is the smallest unit that can carry out all the processes of life.
• All cells arise from pre‑existing cells.

3. Biological Molecules

3.1 Carbohydrates

• General formula (CH₂O)n; monomers = monosaccharides (e.g., glucose, fructose).
• Disaccharides (maltose, sucrose, lactose) formed by glycosidic bonds (condensation reaction).
• Polysaccharides – starch (plant storage), glycogen (animal storage), cellulose (plant cell wall).
• Test: Iodine solution – blue‑black colour indicates starch.

3.2 Lipids

• Hydrophobic molecules: fatty acids, triglycerides, phospholipids, steroids.
• Key properties – insoluble in water, soluble in non‑polar solvents.
• Test: Lipid stain (Sudan III or Oil Red O) – red droplets in cells.

3.3 Proteins

• Polymers of amino acids linked by peptide bonds.
• Four structural levels:

  1. Primary – sequence of amino acids.
  2. Secondary – α‑helix or β‑pleated sheet (hydrogen bonding).
  3. Tertiary – three‑dimensional folding (hydrophobic interactions, disulfide bridges).
  4. Quaternary – association of multiple polypeptide chains.

• Test: Benedict’s solution – reducing sugars give a colour change; Biuret test – peptide bonds give a violet colour.

3.4 Water

• Polar molecule; extensive hydrogen‑bonding network.
• Properties essential for life: high specific heat, high heat of vaporisation, cohesion, adhesion, surface tension, excellent solvent.

4. Enzymes

4.1 Enzyme Structure & Mechanism

• Proteins (or RNA – ribozymes) with a specific active site that binds substrate(s).
• Lock‑and‑key model – exact fit; induced‑fit model – active site moulds around substrate.
• Transition‑state stabilisation lowers activation energy (Eₐ).

4.2 Factors Affecting Enzyme Activity

4.3 Michaelis–Menten Kinetics

\$v = \frac{V{\max}[S]}{Km + [S]}\$

• Vmax – maximum rate when all enzyme molecules are saturated.
• Km – substrate concentration at which the rate is ½ Vmax (indicator of affinity).

5. Cell Membranes & Transport

5.1 Fluid‑Mosaic Model

• Phospholipid bilayer with embedded proteins (integral & peripheral), cholesterol (animals) and glycolipids.
• Membrane is fluid; proteins can move laterally.

5.2 Passive Transport

• Simple diffusion – movement of small, non‑polar molecules down a concentration gradient.
• Facilitated diffusion – carrier or channel proteins assist polar molecules/ions.
• Osmosis – diffusion of water through a semi‑permeable membrane.

5.3 Active Transport

• Requires ATP (or an electrochemical gradient) to move substances against their gradient.
• Examples: Na⁺/K⁺‑ATPase, H⁺‑ATPase (plant plasma membrane), Ca²⁺‑ATPase.

5.4 Bulk Transport

• Endocytosis – uptake of extracellular material (phagocytosis, pinocytosis, receptor‑mediated).
• Exocytosis – secretion of vesicles; important for hormone release and membrane repair.

6. The Mitotic Cell Cycle

6.1 Overview

6.2 Role of the Spindle Apparatus

• Centrioles (animal cells) nucleate microtubules that become the mitotic spindle.
• In plant cells, microtubule‑organising centres (MTOCs) replace centrioles.
• Spindle fibres ensure accurate segregation of chromosomes.

7. Nucleic Acids & Protein Synthesis

7.1 DNA & RNA Structure

• Polymers of nucleotides (sugar‑phosphate backbone + nitrogenous base).
• DNA: deoxyribose, bases A, T, C, G; double‑helix (antiparallel).
• RNA: ribose, bases A, U, C, G; usually single‑stranded.

7.2 DNA Replication (Semi‑Conservative)

1. Helicase unwinds the double helix.
2. DNA polymerase adds complementary nucleotides (5′→3′) using existing strands as templates.
3. Leading strand synthesized continuously; lagging strand formed as Okazaki fragments.
4. DNA ligase joins fragments; topoisomerase relieves supercoiling.

7.3 Transcription (Nucleus)

1. RNA polymerase binds to promoter region.
2. DNA strand is read (template strand) and a complementary mRNA is synthesised.
3. RNA processing – 5′ cap, poly‑A tail, intron removal (splicing).

7.4 Translation (Cytoplasm)

1. mRNA attaches to a ribosome (small + large subunit).
2. tRNA molecules bring specific amino acids; anticodon pairs with codon.
3. Peptide bonds form; ribosome translocates – elongation cycle.
4. Termination occurs at a stop codon; polypeptide chain released and folds.

7.5 Gene Mutations

8. Transport in Plants

8.1 Xylem – Water & Mineral Transport

• Conducts water from roots to shoots by cohesion‑tension (transpiration pull).
• Structure: dead, lignified vessels (angiosperms) or tracheids (gymnosperms).

8.2 Phloem – Transport of Solutes

• Transports photosynthates (mainly sucrose) from sources (leaves) to sinks (roots, fruits).
• Pressure‑flow (mass‑flow) hypothesis: loading creates high osmotic pressure, unloading creates low pressure, driving bulk flow.
• Living elements: sieve‑tube elements + companion cells.

8.3 Apoplast vs. Symplast Pathways

• Apoplast – extracellular space & cell walls; water moves by diffusion.
• Symplast – cytoplasmic continuum connected by plasmodesmata; selective transport.

8.4 Example: Uptake of Mineral Ions

Root hair cells use H⁺‑ATPase to pump H⁺ out, generating an electrochemical gradient that drives the symport of nutrients (e.g., nitrate‑H⁺ symporter).

9. Transport in Mammals

9.1 Circulatory System Overview

• Closed system – heart pumps blood through arteries, capillaries and veins.
• Arteries: high pressure, thick muscular walls.
• Veins: contain valves, lower pressure.
• Capillaries: single‑cell walls (endothelium) allow exchange of gases, nutrients and waste.

9.2 Red Blood Cells (RBCs)

• Biconcave discs – increase surface‑area‑to‑volume ratio for gas exchange.
• Lack nuclei & mitochondria – rely on glycolysis for ATP.
• Contain haemoglobin (protein) that binds O₂ and CO₂.

9.3 Transport Mechanisms Across the Mammalian Membrane

• Na⁺/K⁺‑ATPase maintains membrane potential; essential for nerve impulse transmission.
• Glucose uptake in intestinal epithelium – Na⁺‑glucose symporter (secondary active transport).
• Kidney tubules – active reabsorption of ions and water via ATP‑driven pumps.

10. ATP – The Universal Energy Currency

10.1 Structure & Energy Release

• adenosine (adenine + ribose) + three phosphate groups.
• High‑energy phosphoanhydride bonds between the terminal phosphates.
• Hydrolysis: \$\text{ATP} + \text{H}2\text{O} \rightarrow \text{ADP} + Pi + \text{energy}\$ (≈‑30 kJ mol⁻¹).

10.2 Production of ATP – Cellular Respiration

Overall aerobic equation:

\$C6H{12}O6 + 6\,O2 \;\longrightarrow\; 6\,CO2 + 6\,H2O + \text{≈30–32 ATP}\$

Key Stages (ATP Yield Approx.)

1. Glycolysis (cytoplasm) – net 2 ATP (substrate‑level) + 2 NADH (≈3–5 ATP).
2. Pyruvate oxidation (mitochondrial matrix) – 2 NADH (≈5 ATP).
3. Krebs cycle (matrix) – 2 GTP (≈1 ATP), 6 NADH (≈15 ATP), 2 FADH₂ (≈3 ATP).
4. Electron transport chain & oxidative phosphorylation (inner mitochondrial membrane) – ≈20–22 ATP via chemiosmosis (proton gradient drives ATP synthase).

10.3 Anaerobic Pathways (Optional)

• Fermentation (e.g., lactic acid in muscle, ethanol in yeast) – regenerates NAD⁺; net gain of 2 ATP per glucose.

11. How ATP Powers Cellular Processes

11.1 General Principle

Energy‑requiring (endergonic) reactions are coupled to the exergonic hydrolysis of ATP, making the overall process spontaneous.

11.2 Major ATP‑Dependent Processes

11.3 Example: Active Transport of Glucose into a Plant Cell

1. Plasma‑membrane H⁺‑ATPase hydrolyses ATP to pump H⁺ out of the cell.
2. This creates an electrochemical proton gradient (high [H⁺] outside).
3. A glucose‑H⁺ symporter uses the downhill flow of H⁺ back into the cell to transport glucose against its concentration gradient.
4. Thus the energy from ATP hydrolysis is indirectly used to import glucose.

12. Links to Other A‑Level Topics

• Topic 4 – Membranes & Transport: active transport, ion pumps and secondary active transport all depend on ATP.
• Topic 5 – The Cell Cycle: DNA replication, mitosis and cytokinesis require large amounts of ATP.
• Topic 6 – Nucleic Acids & Protein Synthesis: transcription and translation are ATP‑driven.
• Topic 7 – Transport in Plants: H⁺‑ATPase in the plasma membrane powers nutrient uptake and phloem loading.
• Topic 8 – Transport in Mammals: Na⁺/K⁺‑ATPase and glucose‑Na⁺ symport rely on ATP.
• Topic 12 – Metabolic Control: regulation of glycolysis, the Krebs cycle and oxidative phosphorylation is governed by cellular energy status (ATP/ADP ratios).

13. Practical Skills

14. Key Terms

• ATP – Adenosine triphosphate: universal energy carrier.
• Cellular respiration: metabolic pathway converting glucose and O₂ into CO₂, H₂O and ATP.
• Active transport: movement of substances against a concentration gradient using energy.
• Oxidative phosphorylation: ATP synthesis driven by electron flow through the mitochondrial ETC.
• Substrate‑level phosphorylation: direct transfer of a phosphate group to ADP during glycolysis or the Krebs cycle.
• Enzyme inhibition: competitive, non‑competitive or irreversible reduction of enzyme activity.
• Plasmodesmata: cytoplasmic channels linking plant cells.
• Mass‑flow (pressure‑flow) hypothesis: mechanism for phloem transport of sugars.

15. Suggested Diagrams

• Flowchart of cellular respiration showing glycolysis (cytoplasm), pyruvate oxidation, Krebs cycle (mitochondrial matrix) and the electron transport chain (inner mitochondrial membrane) with ATP yields indicated at each stage.
• Diagram of the fluid‑mosaic membrane with embedded proteins, cholesterol and carbohydrate chains.
• Illustration of the mitotic spindle and chromosome alignment at metaphase.
• Plant transport diagram: water movement in xylem (cohesion‑tension) and sucrose movement in phloem (pressure‑flow).
• Mammalian circulatory system schematic highlighting arteries, capillaries, veins and red blood cells.

16. Sample Exam Question

Question: Explain how the ATP produced in cellular respiration is used to drive the active transport of glucose into a plant cell.

Answer outline (5 marks):

1. Glucose is taken up by a glucose‑H⁺ symporter in the plasma membrane.
2. The symporter relies on the electrochemical proton gradient established by a plasma‑membrane H⁺‑ATPase.
3. The H⁺‑ATPase hydrolyses ATP to pump H⁺ out of the cell, creating a higher external H⁺ concentration.
4. When the symporter allows H⁺ to flow back into the cell down its gradient, the coupled movement of glucose occurs against its concentration gradient.
5. Thus the energy released from ATP hydrolysis is indirectly transferred to the transport of glucose.