Greek Roots in Biology

Lipids

• Refers to a collection of organic molecules of varying chemical composition: fatty acids, glycerides, nonglyceride lipids, and complex lipids.
• Includes sterol derivatives.

Biological Functions of Lipids

• Energy source: each gram of fat releases 9 kcal of energy.
• Energy storage: in the form of triglycerides (TAG) in adipocytes.
• Cell membrane structural components: phosphoglycerides, sphingolipids, and steroids make up the basic structure of all cell membranes.
• Hormones: eicosanoids and steroid hormones.
• Vitamins: lipid-soluble vitamins (A, D, E, and K).
• Vitamin absorption: dietary fat serves as a carrier of lipid-soluble vitamins.
• Protection: fats as shock absorbers.
• Insulation: subcutaneous fat.

Eicosanoids

• Act as paracrine signals.
• Derived from arachidonic acid.
• Are drug targets.

Sterols and Derivatives

• Sterols are amphipathic (contain polar and nonpolar parts) and planar (flat).
• No rotation around carbon bonds.
• Cholesterol is a sterol derivative.
• Bile salts: derived from cholesterol, amphipathic, and enable fat absorption.

Synthesis of Lipids

• Lipids are built from acetate.
• Acetate is converted to Acetyl-Coenzyme A (Acetyl-CoA).
• Fatty acids and sterols are made from Acetyl-CoA.
• Fatty acids are built two carbons at a time.
• Palmitate is the precursor of fatty acids.
• Longer chain fatty acids are made by fatty acid elongation systems in the smooth ER.

Steroid Hormones

• Derived from sterol.
• Act as cellular messengers.
• Many drugs are steroid analogues.
• Examples: sex hormones (e.g., progesterone) and corticosteroids.

Vitamins

• Water-soluble vitamins: B class (1, 2, 3, 5, 6, 7, 9, 12) and C.
• Fat-soluble vitamins: A, D, E, and K.
• Vitamin A (Retinol): important in skin cells and vision.
• Vitamin D: made from 7-dehydrocholesterol, controls calcium metabolism.
• Vitamin K: used as an antidote against rat poison and as a treatment for anticoagulant overdose.

Cell Signalling

• Cell signalling is the communication between cells, involving a chemical messenger (signal/ligand) released by the signalling cell.
• The signal is detected by the responding cell (receptor), triggering intracellular reactions that influence the behaviour of the responding cell.

Tasks of Cell Communication

• Signal release: synthesis and excretion of the signalling molecule by the signalling cell.
• Signal detection: interaction of signal and receptor.
• Signal transduction: translation of detection to changes in cell physiology or gene expression.

Types of Signals

• Paracrine signals: act on a local set of cells.
• Autocrine signals: act on the secreting cell itself.
• Endocrine signals: long-range signals moving through the bloodstream.

Signalling and Control of Gene Expression

• Cell-cell signalling can change the repertoire of transcription factors in the responding cell, resulting in different gene expression.
• Some signals can form gradients, and cell fate depends on signal concentration.

Signal Entry into the Cell

• Two strategies for signal entry:
  • The signal can pass through the plasma membrane (e.g., nuclear receptors).
  • The signal can activate a membrane receptor (e.g., G protein linked receptors, Serine/threonine kinase receptors).

Nuclear Receptors

• Signals are steroids or retinoids.
• These signals can pass through the plasma membrane and encounter nuclear receptors in the cytoplasm.
• The receptor-ligand complex then enters the nucleus to activate genes.

Signalling through Nuclear Receptors

• Nuclear receptors like the glucocorticoid receptor are cytoplasmic proteins.
• In their inactive form, they are bound to Hsp chaperones.
• Ligand binding releases the Hsp, and the receptor-ligand complex moves to the nucleus to activate the transcription of target genes.

G-Protein Linked Receptors

• Transmembrane receptors are linked to a G-protein.
• Upon ligand binding, the G protein releases GDP and takes up GTP.
• Ga dissociates from Gbg and activates downstream 'second messengers' (e.g., cAMP).
• GTP is hydrolyzed to GDP, and Ga reassociates with Gbg.

Serine/Threonine Kinase Receptors

• Transmembrane receptors that bind the TGFb family of signals.
• Ligand brings together type I and type II receptors.
• Type II phosphorylates type I.
• Smads become phosphorylated and move into the nucleus to act as transcription factors.

Cell Signalling

• Cell signalling is the communication between cells, involving a chemical messenger (signal/ligand) released by the signalling cell.
• The signal is detected by the responding cell (receptor), triggering intracellular reactions that influence the behaviour of the responding cell.

Tasks of Cell Communication

• Signal release: synthesis and excretion of the signalling molecule by the signalling cell.
• Signal detection: interaction of signal and receptor.
• Signal transduction: translation of detection to changes in cell physiology or gene expression.

Types of Signals

• Paracrine signals: act on a local set of cells.
• Autocrine signals: act on the secreting cell itself.
• Endocrine signals: long-range signals moving through the bloodstream.

Signalling and Control of Gene Expression

• Cell-cell signalling can change the repertoire of transcription factors in the responding cell, resulting in different gene expression.
• Some signals can form gradients, and cell fate depends on signal concentration.

Signal Entry into the Cell

• Two strategies for signal entry:
  • The signal can pass through the plasma membrane (e.g., nuclear receptors).
  • The signal can activate a membrane receptor (e.g., G protein linked receptors, Serine/threonine kinase receptors).

Nuclear Receptors

• Signals are steroids or retinoids.
• These signals can pass through the plasma membrane and encounter nuclear receptors in the cytoplasm.
• The receptor-ligand complex then enters the nucleus to activate genes.

Signalling through Nuclear Receptors

• Nuclear receptors like the glucocorticoid receptor are cytoplasmic proteins.
• In their inactive form, they are bound to Hsp chaperones.
• Ligand binding releases the Hsp, and the receptor-ligand complex moves to the nucleus to activate the transcription of target genes.

G-Protein Linked Receptors

• Transmembrane receptors are linked to a G-protein.
• Upon ligand binding, the G protein releases GDP and takes up GTP.
• Ga dissociates from Gbg and activates downstream 'second messengers' (e.g., cAMP).
• GTP is hydrolyzed to GDP, and Ga reassociates with Gbg.

Serine/Threonine Kinase Receptors

• Transmembrane receptors that bind the TGFb family of signals.
• Ligand brings together type I and type II receptors.
• Type II phosphorylates type I.
• Smads become phosphorylated and move into the nucleus to act as transcription factors.