Nutrition in Living Organisms: Autotrophs, Digestion and Circulation

T.9: Nutrition

Every living organism must exchange matter and energy with its environment, and its nutritional mode determines how it does so.

Types of nutrition

Autotrophs: These organisms use inorganic matter from the environment (water, carbon dioxide and mineral salts) to produce their own organic matter.

• Photosynthetic: obtain energy from light (algae, plants).
• Chemosynthetic: obtain energy from the oxidation of inorganic compounds (some bacteria).

Heterotrophs: These organisms depend on the organic compounds they obtain from other organisms, which they break down to obtain energy. Examples include animals, fungi and most bacteria.

Stages of the nutritional process

• Ingestion: Animals incorporate organic matter and extract the nutrients they need, while plants incorporate inorganic matter through absorption.
• Digestion: In animals, organic molecules are degraded into simpler molecules that can cross cell membranes.
• Gas exchange: Oxygen and carbon dioxide are exchanged through the respiratory apparatus.
• Transport: Nutrients are distributed throughout the body by the circulatory system.
• Metabolism: Nutrients are chemically transformed, releasing energy and producing substances usable by the cells.
• Excretion: Waste substances produced by metabolism are eliminated to the exterior by the excretory system.

Incorporation of nutrients in plants and algae

Thallophytes (algae): Algae absorb nutrients directly through their cell membrane, since they have no specialised transport tissues.

Cormophytes (higher plants): These plants have specialised structures for absorption and transport:

• Roots: absorb inorganic substances from the soil.
• Leaves: carry out photosynthesis, transforming inorganic matter into organic matter.
• Stem: transports inorganic substances from the roots to the leaves, and the products of photosynthesis from the leaves to the rest of the plant.

Absorption of water and mineral salts

Most land plants absorb water and minerals through their root hairs, which increase the surface in contact with the soil.

• Water enters the root by osmosis.
• Mineral salts enter by active transport, which requires energy supplied by ATP and specific transport proteins.

Internal structure of the root

• Epidermis: covers the surface of young root hairs, absorbs water and minerals from the soil, and protects the inner tissues.
• Cortex: occupies almost the whole root and is formed by cortical parenchyma, parenchymatous cells with large intercellular spaces, and the endodermis, a layer of cells without intercellular spaces whose walls thicken with a waterproof substance (suberin) forming the Casparian strip, which blocks the passage of water.
• Pericycle: located inside the endodermis, it is the tissue from which lateral roots originate.
• Vascular cylinder: contains the xylem (woody vessels) and the phloem (liber vessels). It fixes the root firmly to the soil and stores reserve substances.

Transport of raw sap:

Raw sap rises upward through the xylem, whose vessels are formed by dead, elongated cells whose walls are joined end to end, forming a long tube, from the roots to the leaves.

Mechanisms of sap ascent

The physical phenomena that produce the ascent of raw sap are:

• Root pressure: water entering the root by osmosis creates a pressure that pushes the raw sap upward.
• Transpiration: the main force, consisting of the loss of water from the leaves by evaporation. This creates a suction force that draws water from the roots up through the xylem.
• Cohesion-tension: water molecules are bonded to each other by hydrogen bonds, which allows the water column to rise continuously.

Gas exchange:

Plants need the atmospheric gases oxygen and carbon dioxide for respiration and photosynthesis. Gases enter the plant through:

• Stomata: the main route, through which most carbon dioxide enters and oxygen exits.
• Root hairs: gases dissolved in soil water enter the root.
• Lenticels: openings in the woody parts of the stem.

Opening and closing mechanism of the stomata

Stomata open and close through changes in the turgor of the guard cells. These changes depend on:

• Ion concentration: fundamental in regulating the opening and closing of stomata.
• Light: stomata generally open during the day and close at night to prevent water loss.
• Temperature: affects the opening and closing of stomata, with values above 35 ºC causing closure.

Light uptake and leaf structure

Leaves allow photosynthesis. They are thin, broad and numerous, and are formed by two tissues:

• Parenchyma: contains chloroplasts in its cells, especially in the palisade parenchyma.
• Conducting tissues: transport substances; they include the xylem (woody vessels) and the phloem (liber vessels), which together form a dense network of veins throughout the leaf.

Importance of photosynthesis

Photosynthesis takes place in chloroplasts, where pigments absorb the energy of sunlight. It is a fundamental anabolic process:

• It transforms inorganic matter into organic matter.
• It transforms light energy into chemical energy.
• It releases oxygen as a residual product.

Elaborated sap

During photosynthesis, raw sap becomes elaborated sap, formed by sugars (sucrose) and some nitrogenous amino acids. It is moved through the phloem by translocation, from the sources (leaves) to the sinks, which may act as sources too (potatoes).

Other forms of plant nutrition: symbiotic plants

Some plants live in association with other organisms. There are two types: rhizobia (root nodules) and mycorrhizae (fungal associations).

• Parasitic plants and carnivorous plants obtain nutrients from other organisms or from captured animals; they do not perform photosynthesis in the same way.

Destination of organic matter

Organic matter obtained through the nutritional process is transformed by chemical reactions in the metabolism. It results from the interaction of anabolism and catabolism.

Anabolism: A set of chemical reactions in which simple substances are combined to form more complex compounds. The main example is photosynthesis, in which starch (a reserve substance in plants, formed from sugars), cellulose (cell walls), proteins (part of membranes), lipids (part of cell membranes) and enzymes are synthesised. Food sources include potatoes (underground tubers), sugar cane, wheat and rice (seeds).

Catabolism: A set of chemical reactions in which complex organic compounds are degraded into simpler substances. The main process is cellular respiration, in which energy is obtained and used for vital functions. Cells can also transform fats into glucose, as in oil-producing plants (sunflower, olive).

T.10: The digestive process

Food undergoes transformations in the digestive apparatus. The stages are: ingestion, digestion, absorption and egestion.

Apparatuses involved in nutrition

Digestive, circulatory, respiratory and excretory.

Types of digestive systems

There are two models:

• Gastrovascular cavity: a central cavity in the animal communicates with the exterior through a single opening that serves as both mouth and anus (cnidarians, some flatworms).
• Digestive tube: has two openings, mouth and anus. It is the more advanced model, found in most invertebrates and vertebrates.

Digestive systems of some invertebrates

Most have a common tube structure: mouth, pharynx, oesophagus, stomach and intestine. Each group has specialised structures:

• Annelids: crop and gizzard.
• Molluscs: the digestive gland (hepatopancreas), which secretes digestive enzymes.
• Echinoderms: mouth with Aristotle's lantern.
• Arthropods: salivary glands in the mouth (insects).

Digestive system of vertebrates

It consists of a digestive tube and associated glands. The tube has muscular walls, differentiated regions, considerable length, and glands. The associated glands are the salivary glands, pancreas and liver.

Types of ingestion

Passive ingestion: occurs in sessile aquatic animals, which use structures such as cilia and flagella to capture food particles (sponges with flagellated cells; bivalve molluscs with cilia on their gills).

Active ingestion: the animal moves and uses adaptations to capture and hold its food:

• Radula (molluscs), tentacles and suckers.
• Teeth: cephalopods (beaks), vertebrates.
• Mouthparts (arthropods).

Types of digestion

Depending on where it takes place:

• Intracellular digestion (sponges): food particles are engulfed into vacuoles inside cells, where lysosomes release digestive enzymes that break them down.
• Mixed digestion: has two stages, intracellular and extracellular (cnidarians, flatworms).
• Extracellular digestion: occurs in a digestive tube with two transformations: mechanical and chemical.

Digestion in humans

Oral digestion: starts in the mouth. The mechanical phase involves the teeth (mastication). The chemical phase involves saliva (insalivation), produced by the parotid, submaxillary and sublingual glands. Saliva contains mucin, water and amylase, which begins the breakdown of starch into maltose. The food bolus is pushed by the tongue into the pharynx and then into the stomach by peristaltic movements.

Gastric digestion: takes place in the stomach in two phases. In the mechanical phase, the stomach walls contract to mix the food. In the chemical phase, gastric juice (water, mucin, HCl and pepsin) is secreted and forms chyme.

Intestinal digestion: is completed in the intestine, which is formed by two parts:

• Small intestine: in the duodenum, pancreatic juice (amylase and protease) breaks down starch into sugars and proteins into amino acids. Bile (bile acids and bile pigments) emulsifies fats. Intestinal juice contains maltase, sucrose, lactose, lipase and enterokinase. After these peptidases act, chyme is transformed into chyle, and nutrients are absorbed into the blood and lymphatic system. Monosaccharides and amino acids pass into the blood, while fatty acids and glycerol enter the lymphatics.
• Large intestine: undigested substances and faeces reabsorb water in the rectum and are expelled through the anus.

Transport of nutrients

Circulatory system: transports nutrients to the cells. It consists of:

• Circulatory fluid: invertebrates have haemolymph or hydrolymph (echinoderms), while molluscs and crustaceans have haemolymph. Vertebrates have blood (with haemoglobin) and lymph.
• Blood vessels: arteries (carry blood away from the heart), veins (return blood to the heart) and capillaries (thin vessels that connect arteries and veins).
• Heart: a muscular, accessory or septated tubular organ with atria and ventricles.

Types of circulatory systems

• Open: the fluid bathes the organs directly and forms sinuses or lacunae (bivalve molluscs, gastropods).
• Closed: the blood is always contained within vessels (vertebrates, cephalopods and annelids). These can be simple, with one circuit (blood not mixed), or double, with two circuits, which can be incomplete (blood partly mixed, as in amphibians and most reptiles) or complete (blood not mixed, as in crocodiles, birds and mammals).

Cardiac cycle

The cycle consists of contraction (systole) and relaxation (diastole):

1. Atrial systole (ventricular diastole): the atria contract and push blood into the ventricles, which are dilated.
2. Ventricular systole (atrial diastole): the ventricles contract and blood is pushed into the pulmonary artery and the aorta.
3. General diastole: the heart relaxes.

Each cardiac cycle lasts about 0.8 seconds.

External signs of the heartbeat

• Heart sounds: two sounds are produced: the first by the closure of the atrioventricular valves (mitral and tricuspid), and the second by the closure of the semilunar valves (pulmonary and aortic).
• Heartbeat: the impact of the apex of the heart against the chest wall during ventricular systole.
• Pulse: the rhythmic wave of pressure felt when an artery is pressed. It originates from the contraction of the heart, which pushes blood into the arteries, and travels along them. Normal rate: about 70 beats per minute.
• Blood pressure (arterial pressure): has two values, the maximum (systolic) and the minimum (diastolic), for example 14/7 (in kPa or in the older unit of mmHg, depending on the convention used).