General Adaptations for Nutrition

Digestive Enzymes: Digestive enzymes like amylase, protease and lipase break down complex food molecules into simpler ones. These enzymes work optimally at specific pH and temperature, which vary throughout digestive tract.
Specialised Structures: Specialised structures in digestive system, such as mouth, stomach, and intestines, each plays a unique role in digestion and absorption of food.

Adaptations in Unicellular Organisms

Phagocytosis: Single-celled organisms, like amoeba, take in solid food particles through a process called phagocytosis. The food particles are engulfed and enclosed in a food vacuole.
Extracellular Digestion: Some unicellular organisms secrete enzymes to break down food outside their cell body, followed by absorption of these simpler molecules.

Roots and Root Hair Cells: Roots absorb water and minerals from the soil. Root hair cells increase the surface area for absorption.
Leaves and Stomata: Green leaves possess chloroplasts for photosynthesis. Stomata on leaves allow for gas exchange necessary for photosynthesis.
Veins (xylem and phloem): Veins transport absorbed nutrients and water throughout the plant. Xylem transports water and minerals from roots to leaves, while phloem distributes sugars from leaves to other plant parts.

Teeth and Salivary Glands: Teeth are specialised to mechanically breakdown food; salivary glands secrete saliva containing amylase to begin the digestion process.
Stomach and Gastric Juice: The stomach secretes acidic gastric juice, which contains pepsinogen, to begin protein digestion. The acidity also kills most ingested microorganisms.
Small Intestine and Villi: The small intestine has large surface area due to the presence of villi and microvilli, allowing for optimal nutrient absorption. Various digestive enzymes and bile also act in the small intestine.
Large Intestine: The large intestine absorbs water, salts, and some vitamins produced by resident gut bacteria. Its microbiome plays a key role in human health.
Liver and Pancreas: The liver produces bile which emulsifies fats for easier digestion, while the pancreas secretes various digestive enzymes.