Food sources

In a given ecosystem, food forms a web of interlocking chains with primary producers at the bottom and apex predators at the top.Other aspects of the web include detrovores (that eat detritis) and decomposers (that break down dead organisms).Primary producers include algae, plants, bacteria and protists that acquire their energy from sunlight. Primary consumers are the herbivores that consume the plants, and secondary consumers are the carnivores that consume those herbivores. Some organisms, including most mammals and birds, have diets consisting of both animals and plants, and are considered omnivores.The chain ends with the apex predators, the animals that have no known predators in its ecosystem.Humans are considered apex predators.

Humans are omnivores, finding sustenance in vegetables, fruits, cooked meat, milk, eggs, mushrooms and seaweed.Cereal grain is a staple food that provides more food energy worldwide than any other type of crop.Corn (maize), wheat, and rice account for 87% of all grain production worldwide. Just over half of the world's crops are used to feed humans (55 percent), with 36 percent grown as animal feed and 9 percent for biofuels.Fungi and bacteria are also used in the preparation of fermented foods like bread, wine, cheese and yogurt.

Photosynthesis

During photosynthesis, energy from the sun is absorbed and used to transform water and carbon dioxide in the air or soil into oxygen and glucose. The oxygen is then released, and the glucose stored as an energy reserve. Photosynthetic plants, algae and certain bacteria often represent the lowest point of the food chains,making photosynthesis the primary source of energy and food for nearly all life on earth.

Plants also absorb important nutrients and minerals from the air, natural waters, and soil.Carbon, oxygen and hydrogen are absorbed from the air or water and are the basic nutrients needed for plant survival.The three main nutrients absorbed from the soil for plant growth are nitrogen, phosphorus and potassium, with other important nutrients including calcium, sulfur, magnesium, iron boron, chlorine, manganese, zinc, copper molybdenum and nickel.

Microorganisms

Bacteria and other microorganisms also form the lower rungs of the food chain. They obtain their energy from photosynthesis or by breaking down dead organisms, waste or chemical compounds. Some form symbiotic relationships with other organisms to obtain their nutrients.Bacteria provide a source of food for protozoa,who in turn provide a source of food for other organisms such as small invertebrates.Other organisms that feed on bacteria include nematodes, fan worms, shellfish and a species of snail.

In the marine environment, plankton (which includes bacteria, archaea, algae, protozoa and microscopic fungi)provide a crucial source of food to many small and large aquatic organisms.

Without bacteria, life would scarcely exist because bacteria convert atmospheric nitrogen into nutritious ammonia. Ammonia is the precursor to proteins, nucleic acids, and most vitamins. Since the advent of the industrial process for nitrogen fixation, the Haber-Bosch Process, the majority of ammonia in the world is human-made.

Plants

Foods from plant sources

Plants as a food source are divided into seeds, fruits, vegetables, legumes, grains and nuts.Where plants fall within these categories can vary, with botanically described fruits such as the tomato, squash, pepper and eggplant or seeds like peas commonly considered vegetables.Food is a fruit if the part eaten is derived from the reproductive tissue, so seeds, nuts and grains are technically fruit.From a culinary perspective, fruits are generally considered the remains of botanically described fruits after grains, nuts, seeds and fruits used as vegetables are removed.Grains can be defined as seeds that humans eat or harvest, with cereal grains (oats, wheat, rice, corn, barley, rye, sorghum and millet) belonging to the Poaceae (grass) family and pulses coming from the Fabaceae (legume) family.Whole grains are foods that contain all the elements of the original seed (bran, germ, and endosperm).Nuts are dry fruits, distinguishable by their woody shell.

Fleshy fruits (distinguishable from dry fruits like grain, seeds and nuts) can be further classified as stone fruits (cherries and peaches), pome fruits (apples, pears), berries (blackberry, strawberry), citrus (oranges, lemon), melons (watermelon, cantaloupe), Mediterranean fruits (grapes, fig), tropical fruits (banana, pineapple).Vegetables refer to any other part of the plant that can be eaten, including roots, stems, leaves, flowers, bark or the entire plant itself.These include root vegetables (potatoes and carrots), bulbs (onion family), flowers (cauliflower and broccoli), leaf vegetables (spinach and lettuce) and stem vegetables (celery and asparagus).

The carbohydrate, protein and lipid content of plants is highly variable. Carbohydrates are mainly in the form of starch, fructose, glucose and other sugars.Most vitamins are found from plant sources, with the exception of vitamin D and vitamin B12. Minerals can also be plentiful or not. Fruit can consist of up to 90% water, contain high levels of simple sugars that contribute to their sweet taste, and have a high vitamin C content.Compared to fleshy fruit (excepting Bananas) vegetables are high in starch, potassium, dietary fiber, folate and vitamins and low in fat and calories.Grains are more starch based and nuts have a high protein, fiber, vitamin E and B content.Seeds are a good source of food for animals because they are abundant and contain fiber and healthful fats, such as omega-3 fats. Complicated chemical interactions can enhance or depress bioavailability of certain nutrients. Phytates can prevent the release of some sugars and vitamins.

Animals that only eat plants are called herbivores, with those that mostly just eat fruits known as frugivores,while leaf and shoot eaters are folivores (pandas) and wood eaters termed xylophages (termites). Frugivores include a diverse range of species from annelids to elephants, chimpanzees and many birds.About 182 fish consume seeds or fruit.Animals (domesticated and wild) use as many types of grasses that have adapted to different locations as their main source of nutrients.

Humans eat thousands of plant species; there may be as many as 75,000 edible species of angiosperms, of which perhaps 7,000 are often eaten.Plants can be processed into breads, pasta, cereals, juices and jams or raw ingredients such as sugar, herbs, spices and oils can be extracted.Oilseeds are pressed to produce rich oils – ⁣sunflower, flaxseed, rapeseed (including canola oil) and sesame.

Many plants and animals have coevolved in such a way that the fruit is a good source of nutrition for the animal, who then excretes the seeds some distance away, allowing greater dispersal.Even seed predation can be mutually beneficial, as some seeds can survive the digestion process.Insects are major eaters of seeds,with ants being the only real seed dispersers. Birds, although being major dispersers, only rarely eat seeds as a source of food and can be identified by their thick beak that is used to crack open the seed coat.Mammals eat a more diverse range of seeds, as they are able to crush harder and larger seeds with their teeth.

Animals

Various raw meats

Animals are used as food either directly or indirectly. This includes meat, eggs, shellfish and dairy products like milk and cheese.They are an important source of protein and are considered complete proteins for human consumption as they contain all the essential amino acids that the human body needs.One 4-ounce (110 g) steak, chicken breast or pork chop contains about 30 grams of protein. One large egg has 7 grams of protein. A 4-ounce (110 g) serving of cheese has about 15 grams of protein. And 1 cup (~240 mL) of milk has about 8 grams of protein. Other nutrients found in animal products include calories, fat, essential vitamins (including B12) and minerals (including zinc, iron, calcium, magnesium).

Food products produced by animals include milk produced by mammary glands, which in many cultures is drunk or processed into dairy products (cheese, butter, etc.). Eggs laid by birds and other animals are eaten and bees produce honey, a reduced nectar from flowers that is used as a popular sweetener in many cultures. Some cultures consume blood, such as in blood sausage, as a thickener for sauces, or in a cured, salted form for times of food scarcity, and others use blood in stews such as jugged hare.