Biology Chapter on Hormones and Diffusion

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Questions and Answers

What stimulates the release of tropic hormones from the anterior pituitary?

Diffusion barriers

Releasing hormones (correct)

Concentration gradients

Inhibitory factors

Which factor increases the rate of diffusion across biological membranes?

Smaller concentration gradients

Increased distance of travel

Larger surface area (correct)

Thicker barriers

What characteristic of a molecule influences its permeability according to Fick’s Law of Diffusion?

Molecular weight

Lipid solubility (correct)

Charge of the molecule

Electrical polarity

Which of the following statements about inhibitors of tropic hormone release is true?

They prevent the release of tropic hormones. Signup and view all the answers

According to Fick’s Law, which scenario would lead to the slowest rate of diffusion?

A thicker barrier with low permeability Signup and view all the answers

What does osmolality specifically measure in a solution?

The concentration of all dissolved particles Signup and view all the answers

Which condition describes a solution where the solute concentration is higher outside the cell?

Hypertonic Signup and view all the answers

In what scenario does a cell experience no net movement of water?

When it is in an isotonic solution Signup and view all the answers

What characteristic does oncotic pressure primarily relate to?

Presence of large molecules Signup and view all the answers

Which of the following best describes homeostasis?

The active maintenance of a stable internal environment Signup and view all the answers

What is the primary mechanism of negative feedback in homeostatic reflexes?

Detecting changes and initiating a corrective response Signup and view all the answers

Which term describes the behavior of cells based on non-penetrating solute concentrations?

Tonicity Signup and view all the answers

What is the formula for molarity in diluted solutions?

Mol/L = M Signup and view all the answers

What is the primary role of the afferent pathway in a homeostatic response?

It carries messages to the integrating/control centre. Signup and view all the answers

Which part of the body serves as the integrating/control centre for homeostatic reflexes?

Hypothalamus Signup and view all the answers

In a negative feedback system, what is the effect of increased thyroid hormone (TH) levels?

Decreased secretion of TRH and TSH. Signup and view all the answers

What type of feedback mechanism do birth contractions exemplify?

Positive feedback Signup and view all the answers

What does the efferent pathway do in the homeostatic process?

Carries messages to target cells. Signup and view all the answers

Which gland is primarily involved in hormone secretion in the negative feedback loop for thyroid hormone regulation?

Anterior Pituitary Gland Signup and view all the answers

What role do chemical signals play in homeostasis?

They facilitate communication between cells. Signup and view all the answers

Which of the following correctly describes the role of the hypothalamus?

It regulates autonomic nervous system functions. Signup and view all the answers

In the example of body temperature regulation, what action is taken to lower high body temperature?

Vasodilation and increased sweating. Signup and view all the answers

Which type of cell transmits neurotransmitter signals to other nerve cells?

Nerve cells Signup and view all the answers

What is the primary outcome of the cleavage stage in embryonic development?

Equal divisions of the zygote with no growth Signup and view all the answers

What structure is formed from the inner cell mass of the blastocyst?

Embryonic structures Signup and view all the answers

During which process does the primitive streak form, defining the body axes?

Gastrulation Signup and view all the answers

What is the role of the notochord in early embryonic development?

It guides the formation of the neural tube. Signup and view all the answers

What is the fate of the epiblast cells during the 3-layer stage?

They differentiate into all three germ layers. Signup and view all the answers

What leads to the bloodstream formation during embryogenesis?

Vasculogenesis Signup and view all the answers

Which mesodermal component ultimately forms the kidneys and gonads?

Intermediate Mesoderm Signup and view all the answers

What happens during the process of somitogenesis?

Creation of somites from mesenchymal cells. Signup and view all the answers

How are left-side cells affected in the asymmetry process during development?

They experience cilia rotation. Signup and view all the answers

What is the primary contribution of the sclerotome during embryonic development?

Formation of skeletal elements. Signup and view all the answers

During which week of embryonic development does the embryo undergo significant folding?

4th week Signup and view all the answers

Which type of transition do neural crest cells undergo during development?

Epithelial-Mesenchymal Transition (EMT) Signup and view all the answers

What is a result of cilia activity at the primitive node?

Regulation of left-right asymmetry Signup and view all the answers

What structure is formed by the lateral folding of the embryo?

Gut tube Signup and view all the answers

Which part of the stomach develops faster and causes curvature?

Dorsal part Signup and view all the answers

What are the branching structures that develop from the primary tracheal buds?

Bronchi Signup and view all the answers

Which embryonic structures give rise to skeletal structures in the head?

Neural crest cells Signup and view all the answers

What is the primary function of transmembrane proteins in the cell membrane?

To connect intracellular and extracellular environments Signup and view all the answers

What is the role of Na+/K+ ATPases in maintaining ion distribution?

To create high Na+ concentration outside and high K+ concentration inside Signup and view all the answers

What causes the negative charge on the inner membrane during resting membrane potential?

Efflux of K+ ions through channels Signup and view all the answers

Which membrane property allows ions to move selectively into the cell?

Facilitated diffusion via gated channels Signup and view all the answers

During lung development, what is the outcome of the first bifurcation of primary tracheal buds?

Division into left and right bronchi Signup and view all the answers

What defines the resting membrane potential of a cell?

Difference in charge across the membrane due to K+ leakage Signup and view all the answers

What component forms the mesenchymal core of pharyngeal arches?

Mesoderm and neural crest cells Signup and view all the answers

Which type of growth describes the development of the respiratory diverticulum?

Ventro-caudal growth Signup and view all the answers

What drives the movement of ions through selective gated channels?

Electrical stimulation Signup and view all the answers

What primarily influences K+ movement out of the cell?

Concentration gradient established by Na+/K+ ATPase Signup and view all the answers

Study Notes

Concentrations, Osmolarity, and Tonicity

Concentration refers to the number of specific solute particles in a given volume of solution.
Osmolarity refers to the number of all dissolved particles in a given volume of solution, regardless of whether they are penetrating or non-penetrating solutes.
Tonicity describes the osmotic pressure gradient between two solutions, impacting the cell's behavior.
- Isotonic: Equal solute concentration on either side of the membrane, resulting in zero net water movement.
- Hypertonic: Higher solute concentration in the solution; water leaves the cell.
- Hypotonic: Lower solute concentration in the solution; water enters the cell.
Oncotic pressure refers to the osmotic pressure generated by large molecules, such as proteins.
Cell osmolarity typically averages 300 mOsM, indicating the balance of osmolarity across cell membranes to ensure stability.

Physiology and Homeostasis

Physiology examines how cells, organs, and the entire body function, and how these functions are maintained.
Homeostasis maintains a stable internal environment, despite changes in the external environment.
Homeostatic reflexes work to maintain stable internal conditions.
Negative feedback is a closed-loop system (self-regulating) with these steps:
- Stimulus occurs.
- Sensory/receptor detects the change.
- Afferent pathway transmits the message to the integrating/control center.
- Integrating/control center compares the current value to the intended value.
- Efferent pathway transmits the message to the response system.
- Effector/target is the response system.
- Response aids in returning the variable to homeostatic conditions.
Positive feedback is an open-loop system, amplifying the feedback loop.
- Example: Birth contractions. The baby’s head against the cervix stimulates stretch receptors that signal the brain, leading to the release of oxytocin from the pituitary gland. Oxytocin causes uterine contractions, which further push the baby's head against the cervix, creating a positive feedback loop that amplifies contractions until birth.

Homeostatic Control Mechanisms and the Hypothalamus

Chemical signals utilize various cell types:
- Endocrine cells release hormones through secretory vesicles into the bloodstream, delivering them to target cells with corresponding receptors.
- Neurosecretory cells transmit hormone molecules into the bloodstream, aiming for specific target cells.
- Nerve cells transmit neurotransmitter signals to other neurons.
Hypothalamus: A brain region crucial for integrating homeostatic reflexes:
- Regulates the autonomic nervous system.
- Releases neurosecretory cells, including:
  - Oxytocin-producing cells.
  - ADH/vasopressin-producing cells.
  - Tropic releasing hormones.
Posterior Pituitary Gland: Neurosecretory region producing oxytocin and ADH.
Anterior Pituitary Gland: Responsible for hormone secretion.
Negative Feedback - Thyroid Hormone: A high concentration of thyroid hormone (TH) inhibits the release of TRH and TSH, creating a negative feedback loop.
- Hypothalamus secretes TRH.
- Anterior pituitary secretes TSH.
- Thyroid gland secretes TH.
- TH stimulates target cells and inhibits the hypothalamus and anterior pituitary gland.
Neurosecretions from the Hypothalamus to the Anterior Pituitary:
- Releasing hormones stimulate the release of tropic hormones from the anterior pituitary gland.
- Inhibitory factors (statins) inhibit the release of tropic hormones from the anterior pituitary gland.
- Tropic hormones stimulate hormone release in other glands.

Diffusion Across Membranes: Fick's Law

Diffusion is a passive process where molecules move down their concentration gradient (from high to low concentration)
Fick's Law of Diffusion governs the rate of diffusion:
- Concentration Gradient: A larger gradient → faster diffusion.
- Barrier Permeability:
  - More permeable molecules → faster diffusion.
  - Permeability is determined by lipid solubility and molecule size.
- Distance of Travel: A thinner barrier → faster diffusion.
- Surface Area: A larger surface area → faster diffusion.

Embryonic Development

Cleavage: Initial stage of development involving equal divisions of the zygote, without growth.
Morula: A ball of cells that enters the uterus.
Blastocyst: A fluid-filled cavity with a single-cell layer, formed from the morula.
Implantation: The blastocyst implants on the uterine lining, leading to the formation of germ layers.
Blastocyst Development:
- Trophoblast: Forms extra-embryonic structures (placenta).
- Inner Cell Mass: Forms embryonic structures.
- Implantation: Occurs between 5-10 days after fertilization.
Two Germ Layer Stage:
- Inner cell mass splits into epiblast and hypoblast: This is the differentiation process, forming two distinct cavities (cavity formation).
- This stage results in the formation of the embryonic disc.
Gastrulation: The process where the three germ layers are formed.
- The primitive streak forms, defining the body axes.
- The primitive streak is a line of cells on the epiblast that invaginates to form the primitive groove.
- The primitive streak does not reach the cranial end.
3 Layer Stage:
- Epiblast cells move medially and into the primitive groove, forming the three germ layers.
  - First cells move into the hypoblast, forming the embryonic endoderm.
  - Later cells move between the epiblast and endoderm, forming the embryonic mesoderm.
  - Remaining epiblast cells form the embryonic ectoderm.
Asymmetry (Left-Right determination) Occurs during the formation of the primitive groove.
- The primitive node at the end of the groove contains cilia that rotate left, creating a fluid flow.
- The cilia on the left edge bend, releasing Ca2+ ions, leading to leftward organ placement.
- The cilia on the right edge do not bend or release Ca2+, leading to rightward organ placement.
- Situs invertus is a condition where organs are mirrored, potentially causing other problems.
Notochord and Neural Tube:
- Notochord: A cartilage-like, transient structure that aids in forming the neural tube.
  - It is a cranial midline extension from the primitive node, forming a hollow tube.
  - Teratomas can occur in newborns if primitive streak cells remain.
- Neural Plate and Neural Tube:
  - The ectoderm above the notochord thickens, forming the neural plate.
  - The neural plate folds to form the neural tube.
Neural Crest Cells:
- Cells released during neuralation through epithelial-mesenchymal transition (EMT).
- Migrate to differentiate into:
  - Dorsal root ganglia.
  - Enteric ganglia.
  - Schwann cells.
  - Melanocytes (pigment cells).
  - Sympathetic and parasympathetic ganglia.
  - Dentin.
Segmentation of the Neural Tube:
- The cranial end swells, forming brain vesicles.
- The remainder forms the spinal cord.
Embryo Folding:
- Initially, the embryo is a flat disc.
- During the 4th week, rapid growth of the embryonic disc and amnion, alongside minimal growth in the yolk sac, leads to folding to generate a body form.
Mesoderm Development:
- Paraxial Mesoderm: Located next to the neural tube.
  - Forms somites in the trunk region, which differentiate into muscle, bone, and dermis.
- Somitogenesis:
  - Somites form in pairs below the head region (cranial → caudal).
  - Mesenchymal cells undergo MET, forming a somite.
  - Somites split into dermamyotome (which further splits into dermatome and myotome) and sclerotome.
  - Dermatome → dermis, Myotome → muscle, Sclerotome → skeletal elements.
- Intermediate Mesoderm:
  - Forms the urogenital system (kidneys, gonads, and associated ducts).
  - The pronephros (first kidney) degenerates.
  - The mesonephros (second kidney) becomes the gonads and ducts.
    - Genital ridges develop into either testes (SRY → SOX9 ←→ FGF9) or ovaries (WNT4/RSPO1, FOXL2).
    - FGF9 and WNT4/RSPO1 suppress each other.
    - DMRT1 and FOXL2 suppress each other.
  - The metanephros (third kidney) forms the kidney.
- Lateral Mesoderm:
  - Consists of the somatic/parietal and splanchnic/visceral mesoderm.
  - Forms the ventro-lateral body wall (connective tissue), bones of limbs, heart and vasculature, and the gut wall.
Cardiovascular System Development:
- Vasculogenesis: Blood vessel formation from mesodermal cells.
- Heart Development:
  - Begins as a simple tube.
  - Folding and partitioning create the ventricles.
Endoderm Development:
- Gut Tube: Formed by lateral folding of the embryo.
- Parietal/Somatic Mesoderm: Folds laterally and fuses, lining the embryonic coelom.
- Visceral/Splanchnic Mesoderm: Becomes the mesodermal lining of the gut.
Regions of the Gut Tube:
- Foregut: Forms the mouth, pharynx, esophagus, stomach, and proximal duodenum.
- Midgut: Forms the distal duodenum, jejunum, ileum, cecum, and appendix.
- Hindgut: Forms the colon, rectum, and anal canal.
Stomach Development:
- Dilation and enlargement of the distal foregut ventro-dorsally.
- Dorsally growing faster leads to stomach curvature.
- Rotation 90° left and superiorly (up) bends the duodenum into a C-shape.
Other Endodermal Organs:
- Endoderm thickens:
- Cell proliferations form buds:
- Lengthening and branching
- Lungs:
  - A ventral out-pocketing of the endoderm forms the respiratory diverticulum (which forms the trachea).
  - Growth occurs in the ventro-caudal direction.
  - Branching leads to:
    - 1st bifurcation → Right and left primary tracheal buds → Bronchi.
    - 2nd bifurcation → Secondary bronchial buds (3 right, 2 left) → Lung lobes.
    - 3rd bifurcation → Tertiary bronchial buds → Bronchopulmonary segments.
  - 14 additional branching → Terminal bronchioles.
Head Development:
- Pharyngeal Arches:
  - Four main arches in the human embryo.
  - Covered by ectoderm (pharyngeal cleft).
  - Mesenchymal core from mesoderm and neural crest cells.
  - Inner lining of endoderm (pharyngeal pouch).
  - Each arch has:
    - Cartilaginous skeletal element from neural crest.
    - Muscle from head mesoderm.
    - Cranial nerve.
    - Arch artery from mesoderm-derived endothelial cells.

Biological Basis of Membrane Potential

Key Rules:
- Molecules move down their gradients, with faster movement for steeper gradients (passive process).
- Opposites attract, like charges repel.
Cell Membrane:
- Creates distinct intracellular and extracellular environments by regulating the entry and exit of molecules.
- Composed of hydrophilic and hydrophobic regions, allowing simple diffusion and facilitated diffusion.
- Transmembrane proteins connect the intracellular and extracellular environments.
- Ions, being hydrophilic, require selective gated channels (proteins) to cross the membrane in the presence of a stimulant.
K+ and Na+ Distribution:
- High Na+ outside the cell, high K+ inside the cell due to Na+/K+ ATPases, creating diffusion gradients.
- The membrane is more permeable to K+.
- K+ leaves via specific K+ leak channels, creating a negative charge on the inner membrane.
- Some K+ re-enters the cell due to the negative membrane potential.
- Resting membrane potential is reached when K+ movement in and out is equal.
Membrane Potential: The charge difference across the membrane, measured in millivolts (mV).
- An increase in membrane potential (more positive) is known as depolarization.
- A decrease in membrane potential (more negative) is known as hyperpolarization.
Resting Membrane Potential: The membrane potential when cell permeability is determined by leak channels.

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Test your knowledge on the key concepts of hormones, diffusion, and homeostasis in this biology quiz. This covers topics like tropic hormones, osmolality, and Fick's Law of Diffusion. Challenge yourself and understand the principles that govern biological processes.