Levels of Organisation and Regulation in Multicellular Organisms

Levels of Biological Organisation in Multicellular Organisms

• Multicellular organisms are organized into a hierarchical structure to ensure successful functioning and reproduction.

• Cells: These are specialized units with a specific function.

• Tissues: Groups of similar cells working together to carry out a particular function.

• Organs: Formed by two or more tissues that work together to perform one or more specialized tasks.

• Systems: A group of organs that work together to perform a vital task.

• Organisms: Systems work together and contribute to the successful functioning and reproduction of the whole organism.

• This progression is often referred to as "The World Tour" of biological levels.

Understanding the Internal Environment and Regulation

• All living things must regulate their internal environment to keep cells functioning efficiently.

• Internal Environment: Defined as everywhere inside the organism where molecules must cross membranes to reach.     - Exclusions: The inside of cells is generally not included in the definition of the internal environment.     - External Continuity: The digestive tract, urinary tract, reproductive tract, and respiratory tract are all continuous with the external environment and are generally not considered part of the internal environment.

• Control Mechanisms: In humans, internal regulation is primarily controlled by the brain, which utilizes the endocrine and nervous systems to maintain internal stability.

• Factors Regulated: Regulation includes temperature, solute concentration (water volume), oxygen levels, and carbon dioxide levels.

• Tolerance Range: Each species has an optimal level for various factors and a range in which functioning can be maintained.     - Functioning is compromised at both ends of this range.     - Death occurs if a factor goes beyond the tolerance range.     - The breadth of this "normal activity" band varies between species.

Homeostasis and Feedback Loops

• Homeostasis: The tendency of biological systems to maintain relatively constant conditions in the internal environment while continuously interacting with and adjusting to changes originating within or outside the system.

• Detection:     - Receptors: Used to detect changes.     - Interoceptors: Detect changes in the internal environment.     - Exteroceptors: Detect changes in the external environment.

• Stimulus-Response Model: Once a change (stimulus) is detected, a response is initiated via effectors to maintain internal stability.

• Negative Feedback Loops: The primary mechanism for homeostasis.     - A change in a given direction causes a change in the opposite direction.     - Example: An increase in a substance's concentration triggers feedback that causes the concentration to decrease.     - These loops result in small fluctuations around an ideal or set point.     - Fluctuations occur due to the time lag required for a response to occur, be registered, and for the stimulus to be reduced.

The Mammalian Endocrine System

• Signalling Molecules: Produces hormones that allow widely separated tissues to act in a coordinated manner.

• Glands: Endocrine glands are ductless, releasing hormones into tissue fluids and the blood.

• Hormone Types and Transport:     - Protein hormones: Made of protein, travel dissolved in plasma. They cannot cross the cell membrane; receptors are on the outside of the plasma membrane.     - Lipid-based hormones (e.g., oestrogen, testosterone): Rely on carrier proteins to travel in blood. They enter the cell; receptors are in the cytosol or nucleus.

• Specificity: Only cells with the appropriate receptor respond. Different cells may have different responses to the same hormone.

• Brain Regulation:     - Hypothalamus: Part of the brain producing neurohormones.     - Pituitary Gland: Known as the "master gland"; it produces hormones that control other endocrine glands (e.g., the Thyroid).

Table of Human Endocrine Glands and Functions

Glucose Regulation and Diabetes

• Glucose is essential for cellular respiration.

• Blood glucose levels ($BGLs$) must be maintained in a narrow range to prevent coma or death.

• Pancreatic Islets of Langerhans:     - Beta ($\beta$) cells: Release insulin to lower $BGL$.     - Alpha ($\alpha$) cells: Release glucagon to raise $BGL$.

• Dynamic Storage: Excess glucose is stored as glycogen or fat. Glycogen can be rapidly broken down back into glucose.

• Diabetes Mellitus:     - Type 1: Autoimmune disease where the immune system destroys $\beta$ cells. No warning for development. Cells cannot absorb glucose and use fats instead, producing toxic ketones. Ketones cause a "ripe fruit" smell. Treatment: monitored $BGL$ and insulin injections.     - Type II: Use of insulin is ineffective because cells stop responding (receptor resistance). Treatment: Injecting more insulin or using medications to increase cell sensitivity.     - Gestational Diabetes (GD): Occurs during pregnancy; usually resolves after birth but increases risk of future Type II diabetes.

• Hyperglycaemia: Persistent high $BGL$. Causes nerve/blood vessel damage and glucose appearing in urine. Symptoms include excessive thirst and urination.

• Hypoglycaemia: Condition of low blood sugar. Can be life-threatening. Treated with sugar-rich foods or a glucose IV drip. Symptoms: dizziness and irritability.

Thyroid Hormones and Metabolism

• Thyroxine ($T_3$ & $T_4$): Controls metabolic rate, foetal brain development, muscle/bone development, and other body activities.

• Control Pathway:     - Hypothalamus produces Thyrotrophin-Releasing Hormone ($TRH$).     - $TRH$ acts on the Anterior Pituitary to produce Thyroid-stimulating hormone ($TSH$).     - $TSH$ acts on the Thyroid Gland to release Thyroxine.

• Negative Feedback: Hypothalamus monitors stimuli like body temperature. Lower body temperature triggers more thyroxine to increase metabolic heat production.

• Malfunctions:     - Hypothyroidism (too little thyroxine): Results in cretinism (mental retardation in infants), weight gain, cold sensitivity, and goitre (often caused by lack of iodine).     - Hyperthyroidism (too much thyroxine): Results in weight loss, heat sensitivity, heart palpitations, protruding eyeballs, and goitre.

Thermoregulation

• Heat Exchange: Occurs through Conduction, Convection, Radiation, and Evaporation.

• States:     - Hyperthermia: Body temperature higher than optimal. Symptoms: dizziness, delirium, coma.     - Hypothermia: Body temperature lower than optimal. Symptoms: shivering, fumbling, confusion, death.

• Mechanisms to Increase Temperature:     - Increased metabolic rate/respiration (liver and brown fat) via thyroxine/adrenaline.     - Shivering: Skeletal muscle contraction.     - Arrector pili muscles: Contraction causes hairs to stand up (insulation layer).     - Vasoconstriction: Skin arterioles constrict to reduce blood flow/heat loss.     - Behavioural: Huddling or adding clothing.

• Mechanisms to Decrease Temperature:     - Reduced metabolic rate.     - Vasodilation: Skin arterioles dilate to increase blood flow/heat loss.     - Sweating: Loss through evaporation (inefficient in high humidity).     - Behavioural: Sprawling (increased surface area) and reduced activity.

The Mammalian Excretory System

• Excretion: Removal of materials that have been part of cells (Urinary system, Lungs for $CO_2$, Skin for sweat).

• Wastes: Urea, ammonia, creatinine, excess salts, water, and chemicals like penicillin.

• Deamination: Process where excess proteins are broken down into glucose and nitrogenous wastes. Nitrogenous wastes are highly toxic.

• Urinary Anatomy:     - Kidneys: Filter blood to remove wastes, excess salt, and water.     - Renal Artery: Brings unfiltered blood.     - Renal Vein: Takes filtered blood away.     - Ureter: Connects kidney to bladder.     - Urethra: Exit tube for urine.

• The Nephron (Active Unit of Kidney):     1. Glomerulus: Mass of capillaries where filtration occurs under pressure.     2. Bowman's Capsule: Collects filtrate (salts, water, glucose, urea). Large molecules/cells stay in blood.     3. Proximal Convoluted Tubule: Reabsorption of water (osmosis), ions (diffusion), and glucose/amino acids (active transport).     4. Loop of Henle: Capillaries reabsorb water. Longer loops are found in species from drier environments.     5. Distal Convoluted Tubule: Active uptake of ions. Substance here is now called urine (approx. $1\%$ of original filtrate).     6. Collecting Duct: Final collection of urine.

Osmoregulation (Water Balance)

• Water Roles: Cytosol, interstitial fluid, blood plasma, and lymph fluid.

• Inputs: Drinks, food, and metabolic water (from respiration).

• Outputs: Urine, sweat, faeces, tears, exhaling, evaporation, and lactation.

• Consequences of Imbalance:     - Too much fluid: Build-up around organs (e.g., heart) or lungs.     - Insufficient fluid: Loss of blood pressure.

• Hormonal Regulation:     - Aldosterone: Prompts distal tubule to pump salts into tissues, causing water retention.     - Antidiuretic hormone ($ADH$): Increases permeability of the collecting duct to water for retention.     - Alcohol Influence: Alcohol inhibits $ADH$ release, leading to dehydration.

Questions & Discussion

• Duct Connections: A question asks to identify the correct diagram of duct connections between the liver, gall bladder, pancreas, and intestine. (Diagram options A, B, C, D provided in transcript).

• Glucose and Insulin Table Inquiry:     - Group X (0 min): Plasma Glucose $5.3\,mmol/L$, Plasma Insulin $68\,mmol/L$.     - Group X (60 min): Plasma Glucose $13.0\,mmol/L$, Plasma Insulin $66\,mmol/L$.     - Group Y (0 min): Plasma Glucose $5.3\,mmol/L$, Plasma Insulin $69\,mmol/L$.     - Group Y (60 min): Plasma Glucose $7.8\,mmol/L$, Plasma Insulin $380\,mmol/L$.     - Question: What is the purpose of the control group, and which group (X or Y) included people with diabetes based on the data?

• Temperature Homeostasis Diagram:     - Components labeled X (Stimulus), Y (Transmission), Z (Effector) result in decreased body temperature. The control centre is the hypothalamus.

• Desert vs. Rainforest Scientists:     - Desert: Core Temp $37.3\,^\circ C$, Air Temp $36.0\,^\circ C$, Moisture $9.0\%$, Wind $12.0\,m/s$.     - Rainforest: Core Temp $38.9\,^\circ C$, Air Temp $35.5\,^\circ C$, Moisture $92.0\%$, Wind $3.0\,m/s$.     - Question: Use this information to explain the difference in mean core body temperature (Hint: Humidity and evaporation efficiency).