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3.4.6Inquiry Process

Intracellular Receptors — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 131 of 160

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Lipid-Soluble Signals

Steroid hormones
  • Steroid hormones Oestradiol, progesterone and testosterone are made from cholesterol, are non-polar, and can diffuse across the phospholipid bilayer.
Transport in blood
  • Transport in blood Because they dissolve poorly in water, they are carried on plasma proteins until they reach a target tissue.
Intracellular receptor
  • Intracellular receptor The receptor lies in the cytoplasm or nucleus and is a protein with a hormone-binding site and a DNA-binding region.
Slower action
  • Slower action The effect needs transcription and translation, so it appears over minutes to hours but may persist for days.

Section 2 of 5Mechanism of Action

1
The steroid hormone diffuses through the plasma membrane into the cytoplasm of the target cell.
2
It binds its specific receptor, producing a hormone-receptor complex with a changed shape.
3
The complex moves into the nucleus if it was not already there.
4
It binds specific DNA sequences near target genes and acts as a transcription factor.
5
RNA polymerase transcribes the gene more or less often, so the amount of a particular protein in the cell increases or decreases.

Section 3 of 5Comparing Receptor Types

Comparing Receptor Types
Surface receptor
  • Surface receptor Binds hydrophilic ligands, uses a second messenger, and changes the activity of proteins that already exist.
Intracellular receptor
  • Intracellular receptor Binds hydrophobic ligands directly and changes which genes are switched on or off.
Example
  • Example Oestradiol binds receptors in target cells and switches on genes involved in development of female characteristics.
Specificity
  • Specificity Only cells containing the receptor respond, even though the hormone reaches every tissue in the blood.
FeaturePeptide hormoneSteroid hormone
Crosses membraneNoYes
Receptor locationCell surfaceInside cell
Messenger neededSecond messengerNone, receptor acts directly
Time to effectSeconds to minutesMinutes to hours

Section 4 of 5Worked Example

Question

  • A cell is treated with a drug that blocks transcription.
  • Predict the effect on its response to (a) epinephrine and (b) oestradiol, giving reasons.

  1. Epinephrine acts through a surface receptor and a cascade that modifies existing enzymes, so no new transcription is required.
  2. Oestradiol works by binding DNA and altering gene expression, which needs transcription of target genes.
  3. Blocking transcription would therefore prevent the oestradiol response but leave the epinephrine response intact.

Answer

  • (a) Epinephrine response continues;
    • (b) oestradiol response is lost.

Section 5 of 5Exam Checklist & Tips

3.4.6 – Key Points for the Exam

  • Steroid hormone A lipid-soluble hormone derived from cholesterol that diffuses into target cells.
  • Hormone-receptor complex Combination of hormone and its intracellular receptor that can bind DNA.
  • Transcription factor A protein that binds DNA and controls how often a gene is transcribed.
  • Gene expression The use of information in a gene to make a protein, controlled here by hormone signals.

Common Mistakes

  • Saying steroid hormones need a transmembrane receptor; they diffuse in.
  • Believing the hormone itself binds to DNA; it is the hormone-receptor complex.
  • Expecting an immediate response; changes in gene expression take time.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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