specialisation and organisation of animal cells into tissues, organs and systems with specific functions: digestive, endocrine and excretory

What this dot point is asking

VCAA wants you to describe how animal cells specialise into tissues, organise into organs and systems, and explain the structure and function of three specific systems: digestive, endocrine, excretory.

The answer

Hierarchy of organisation

Cells specialise to form tissues (groups of similar cells with a common function); tissues form organs (structures of several tissue types with a specific function); organs form organ systems (groups of organs working together).

The four primary animal tissue types

Epithelial tissue. Sheets of tightly packed cells that line and cover surfaces (skin, gut, blood vessels, glands). Function: protection, absorption, secretion, filtration. Varieties include simple squamous (thin, for diffusion), simple columnar (for absorption in the gut), and stratified (for protection in the skin).

Connective tissue. Cells in an extracellular matrix (fibres and ground substance). Includes loose connective tissue, adipose (fat), cartilage, bone, blood and lymph. Function: support, attachment, storage, transport.

Muscle tissue. Cells that contract. Three types: skeletal (voluntary, striated, attached to bones), cardiac (involuntary, striated, in the heart), smooth (involuntary, non-striated, in gut and blood-vessel walls).

Nervous tissue. Neurons (transmit impulses) and supporting glial cells. Function: communication and coordination.

The digestive system

The digestive system breaks food down mechanically and chemically into small absorbable molecules and excretes the indigestible remainder.

Structure (alimentary canal). Mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum), anus. Accessory organs: salivary glands, liver, gall bladder, pancreas.

Mouth. Teeth break food mechanically (mastication). Salivary glands secrete saliva containing amylase (starch to maltose).

Oesophagus. Smooth muscle moves the bolus to the stomach by waves of peristalsis.

Stomach. Smooth muscle churns food. Gastric glands secrete HCl (low pH, kills microbes, denatures proteins) and pepsinogen (activated to pepsin for protein digestion). The mucus layer protects the stomach wall.

Small intestine. Main site of digestion and absorption.

• Pancreatic juice delivers amylase, lipase and proteases (trypsin, chymotrypsin) plus bicarbonate to neutralise stomach acid.
• Bile from the liver (stored in the gall bladder) emulsifies fats into small droplets so lipase can work.
• The walls are covered in villi (finger-like projections) with microvilli (the brush border) increasing surface area enormously (about 250 square metres). Each villus has a capillary network (absorbs sugars and amino acids) and a lacteal (absorbs fatty acids).
• Final digestion is by brush-border enzymes (maltase, peptidases).

Large intestine. Absorbs water and ions; bacteria synthesise vitamins K and B. Forms and stores faeces.

Anus. Eliminates faeces.

The system depends on specialised tissues: smooth muscle for peristalsis, glandular epithelium for enzyme and acid secretion, absorptive columnar epithelium for nutrient uptake.

The endocrine system

The endocrine system uses hormones (chemical messengers) to coordinate slow, widespread, long-lasting changes.

Endocrine glands are ductless: they release hormones directly into the bloodstream. Hormones travel everywhere but only act on target cells with the matching receptor (often inside the cell for steroid hormones, on the surface for protein hormones).

Key glands and hormones:

• Hypothalamus. Releases hormones that control the pituitary.
• Pituitary gland. The "master gland". Releases growth hormone, ADH (antidiuretic hormone, controls water reabsorption), oxytocin, FSH, LH, TSH and others.
• Thyroid. Releases thyroxine (T4) and triiodothyronine (T3), which set metabolic rate.
• Parathyroid. Releases parathyroid hormone, controlling blood calcium.
• Adrenal glands. Adrenaline (fight-or-flight, fast) and cortisol (stress response, slower); aldosterone (sodium balance).
• Pancreas. Insulin (lowers blood glucose) and glucagon (raises blood glucose) from the islets of Langerhans.
• Gonads. Ovaries produce oestrogen and progesterone; testes produce testosterone.

Hormone action. Two main mechanisms:

• Steroid hormones (lipid-soluble, such as cortisol, oestrogen) diffuse through the plasma membrane and bind intracellular receptors; the receptor-hormone complex enters the nucleus and changes gene expression. Slow and long-lasting.
• Protein and peptide hormones (water-soluble, such as insulin, adrenaline) bind cell-surface receptors; trigger a signal transduction cascade (such as cAMP) inside the cell. Faster but still slower than nerves.

The endocrine system works with the nervous system: the hypothalamus links them.

The excretory system

The excretory system removes metabolic waste (especially nitrogenous waste from protein breakdown, such as urea) and regulates water and salt balance (osmoregulation).

Main organs:

• Kidneys. Filter blood and form urine.
• Ureters. Carry urine from kidneys to bladder.
• Bladder. Stores urine.
• Urethra. Carries urine out of the body.

Other organs also excrete: lungs (CO2), skin (water, salts, urea in sweat), liver (bile pigments via faeces).

The nephron. The functional unit of the kidney. Each kidney contains about one million nephrons.

1. Glomerulus and Bowman's capsule. Blood is forced under high pressure through a knot of capillaries; small molecules (water, ions, glucose, urea) are pushed out into the capsule as glomerular filtrate. Large molecules (proteins, blood cells) stay in the blood.
2. Proximal convoluted tubule. Most useful substances (glucose, amino acids, most water, some ions) are reabsorbed by active transport and osmosis.
3. Loop of Henle. Creates a salt concentration gradient in the medulla that allows further water reabsorption.
4. Distal convoluted tubule. Fine adjustment of ion balance, regulated by hormones.
5. Collecting duct. Final water reabsorption controlled by ADH (more ADH = more water reabsorbed = concentrated urine).

The filtrate that remains is urine: water + urea + excess ions + waste products.

Hormonal regulation:

• ADH from the pituitary increases collecting-duct permeability when the body needs to retain water.
• Aldosterone from the adrenal cortex increases sodium reabsorption.

Together these hormones link the excretory system to the endocrine system to maintain homeostasis.

Worked example

After a meal high in protein, your stomach uses pepsin (a protease) to digest the protein into peptides; pancreatic trypsin and intestinal peptidases finish the job into amino acids. Amino acids cross the villus epithelium into capillaries by active transport and travel to the liver. Excess amino acids are deaminated in the liver, producing urea as nitrogenous waste, which is dumped into the blood. The kidneys filter urea from the blood at the glomerulus, fail to reabsorb it, and excrete it in urine. Meanwhile, the rise in blood amino acids and glucose triggers the pancreas to release insulin; insulin tells liver, muscle and fat cells to take up glucose, lowering blood glucose. Three systems (digestive, excretory, endocrine) coordinate for one meal.

Common traps

Saying "tissue = cell". Tissue is a group of similar cells.

Confusing exocrine and endocrine glands. Exocrine glands secrete through ducts to a surface (sweat, saliva, digestive enzymes). Endocrine glands secrete hormones into the blood (no duct).

Saying the liver is part of the digestive system "because of bile". The liver is an accessory digestive organ (it makes bile) and also a metabolic and excretory organ.

Saying urine is "filtered out of blood". Urine is the final product after filtration plus reabsorption plus secretion across the whole nephron. Most of the original filtrate is reabsorbed.

Forgetting that the systems interact. The digestive system needs the endocrine system (insulin/glucagon) and the circulatory system (transport). The excretory system needs hormones (ADH, aldosterone). Real biology is coordination.

In one sentence

Animal cells specialise into four primary tissue types (epithelial, connective, muscle, nervous) that organise into organs and systems including the digestive system (mouth to anus, plus liver and pancreas, with villi for absorption), the endocrine system (ductless glands releasing hormones into blood for slow, long-lasting, target-specific responses), and the excretory system (kidneys filter blood through nephrons, reabsorbing useful substances and excreting urea, water and ions as urine, regulated by ADH and aldosterone).