Biological systems are interdependent

Living things have basic needs

Living things, including humans and other animals, have needs that must be met for them to stay alive. Some needs, such as food, shelter, gases (e.g. oxygen) and water, are common to all living things, although relative needs may vary. For example, a koala rarely needs to drink fresh water, while a fish cannot survive without it.

While most living things can survive in a wide range of conditions such as temperature, most will have a narrower range of preferred conditions in which they can thrive. For example, humans can live in a variety of conditions from the Antarctic to the Sahara desert. The human population living in these extreme environments is much smaller than in the narrower range of preferred climates. This is due to the limited amount of food, shelter and water present. This is true for animals other than humans.

The word ‘animals’ includes groups such as mammals (including humans), fish, insects, sponges, corals, amphibians, reptiles, birds, spiders, crabs, snails, clams, leeches and worms. All of these animals have different characteristics and abilities which allow them to meet their basic needs in different ways. For more information on animals, see Form and features of living things.

Some needs depend on the type of organism and the environment in which it lives. For example, some plants need to acquire nitrogen from external sources, such as well-fertilized soil. Other plants have root nodules in which special bacteria live that provide them with nitrogen.

Growth and repair of tissues, movement and reproduction are processes that require energy and nutrients. Carbohydrate and fat molecules (‘food’) are stores of chemical energy. Plants and animal cells can break down the complex molecules to release the energy. When they use oxygen to do this it is called aerobic respiration.

Plants can use the energy in the Sun’s rays to produce carbohydrates (sugars). This process is called photosynthesis and is carried out by specialised parts of plant cells called ‘chloroplasts’. Chloroplasts contain chlorophyll pigments which are generally green in colour. The process uses carbon dioxide gas and water and produces oxygen and glucose (sugars), and needs to occur in sunlight. The sugars are stored in plants’ tissues or converted into other molecules, for example, cellulose, to build cell walls. When a plant is creating more sugar molecules than it is breaking down, it produces more oxygen than it consumes.

One of the reasons plants need water is to transport nutrients from where they were absorbed, generally the soil, to where they are needed, generally the leaves. Our heart pushes blood (including water and nutrients) through our arteries and veins, but plants rely on different processes. One process is to open up pores (stomata) on their leaves. Water then evaporates into the air, cooling the leaves and causing water, and its dissolved nutrients, to rise up through small tubes in the stem called xylem vessels. This process is known as transpiration. Sap, which includes dissolved sugars produced by the leaves, circulates through a system of live tissue called phloem.

The timing of the supply of water, in particular having adequate water supply during flowering, is important for getting good grain yields. There are visible effects of over and under watering a plant that indicates that a plant is under stress. For example, when plants are over watered their leaves can turn yellow because roots need air in the soil to function; too much water displaces all the air in the soil and the roots drown. When plants are underwatered they wilt and close their stomata, which means they can no longer capture energy from sunlight using photosynthesis as they no longer get carbon dioxide from the air.

Living things interreact

No living thing can survive in isolation. Living things need food, water, air, and shelter to survive. Every species relies on other living and non-living things to fulfill these needs. For example, spiders attach their webs to trees and rocks, and micro-organisms obtain energy by decomposing fallen tree leaves. Sometimes the relationship between the living things is only beneficial for one of the organisms, for example, tall trees preventing sunlight from reaching smaller plants.

Living things need energy for movement and other body functions. Many living things obtain energy by eating other living things. The movement of energy through a community can be described in the form of food chains. Most food chains start with light energy from the Sun, which is transformed into chemical energy by a producer (plant) before it is eaten by consumers (animals). Most food chains are 4-5 steps as energy is transformed into movement, heat, and chemical energy at each step.

A decomposer is an organism (living thing) that gain their energy from the chemical energy left in dead organisms. Decomposers are linked to each part of the food chain as all organisms eventually die.

Not all relationships involve one living thing eating another living thing. Pollination and the formation and dispersal mechanisms of seeds are important components of the flowering plant's life cycle. Plants have developed relationships with animals to attract pollinators to visit them and animals to help spread their seeds. When living things have relationships with one another it is called symbiosis (meaning ‘living together’ in Greek). There are a number of different types of symbiotic relationships between living things.

Mutualism and cooperation are symbiotic relationships where two organisms—plant and animal, plant and plant, plant and fungi or animal and animal—mutually gain benefit from their interactions. In a cooperative relationship the organisms can survive in the absence of one of the organisms. Bees and flowering plants have a beneficial relationship due to the fertilisation of the plant when the bee is seeking nectar. Some orchids have a mutualistic relationship as they can only be pollinated by one species of bee. Many plant and bee relationships are cooperative as the flower can be pollinated by other pollinators and the bees can harvest pollen from different flower species. Ants may benefit some plants when they disperse their seeds, and the seed helps the ant by providing packets of food for the ant to consume.

Living things are affected by their environment

Living things generally require a specific set of physical or environmental conditions to survive, such as temperature ranges, rainfall, energy source (food or sunlight) and shelter. Even when plants or animals live in the same or similar conditions, their needs vary. For example, in the deserts of Australia, brown snakes need to shelter on and around northeast-facing rocks in the desert, which hold heat to help warm their cold bodies in a morning, while bilbies need soft, sandy soil to dig deep, cool burrows to sleep in during the day.

Animals and plants have adaptations to help them survive in their specific environments. For example, Eucalyptus leaves hang straight down to help reduce exposure to the harsh midday sun. This leaf adaptation prevents water loss from heat, while ensuring the leaves capture enough sunlight for photosynthesis and growth.

Living things can require very specific environmental conditions to survive, without which they might stop growing, or become unhealthy from accumulation of minerals or toxins, or even die. Some living things are more tolerant to variations in environmental conditions, for example, cockroaches can thrive in a number of extreme conditions. Many species can survive in sub-optimal conditions, but they do not grow as big or strong and might not reproduce successfully. The health of certain living things (bioindicators) can help indicate the health of an ecosystem.

Salt can have adverse effects on plant health and growth. When water available to plants contains higher concentrations of salt, the roots of the plant absorb less water. If concentrations of salt are high enough, the salt water around the roots will draw water out of the plant. The whole plant loses moisture and suffers stress. Symptoms of high salt damage are similar to those of high moisture stress: leaf tip dieback, leaf edges yellowing, scorching and turning brown or black, followed by leaf fall of dead leaves. Salt can also accumulate in the leaves, causing them to die.

Living things also rely on others for survival. A change in physical conditions might not directly affect them but it may still affect their food sources. For example, bilbies do not need to drink water to survive (they get all their water content from the food they eat), so a change in rainfall does not directly affect them. However, rainfall does affect the plants that they eat which can become less nutrient-dense, stop growing, or even dry out and die.