Nutrient and Gas Requirements

Autotrophs and Heterotrophs

We can loosely classifying all living things as either heterotrophs or autotrophs. heterotrophs are organisms that must consume other organisms for energy. Autotrophs are organisms that can produce their own energy.

Organisms require both organic and inorganic nutrients

Introduction

Heterotroph: 'Hetero' - other; 'troph' - nourishment, so organism that nourishes itself with other organisms. An organism that cannot produce its own food and has to take in all of its nutrients from other organisms. This includes herbivores, carnivores and omnivores.

Autotroph: 'Auto' - self; 'troph' - nourishment, so an organism that nourishes itself. An organism that produces it own nutrition ether through photosynthesis (plants) or chemosynthesis (bacteria).

There are many substances that all living organisms require in order to function efficiently. These substances can be classed as either organic or inorganic substances. These substances can act as ither nutrients that are required to give the organism energy or as raw materials to be used in the building structure of cells and living tissues.

Organic substance: Substances that are made by living things and contain carbon and hydrogen atoms in chains (hydrocarbons). Includes glucose, amino acids, fatty acids, glycerol, nucleotides and vitamins. Autotrophs produce these but Heterotrophs do not.

Inorganic substance: Substances that are part of the non-living world and do not contain carbon and hydrogen atoms in chains (hydrocarbons). Includes minerals such as phosphates, sodium ions and chloride ions, and water. Both Autotrophs and Heterotrophs must obtain these from external sources.

Organic and inorganic compounds differ in structure

Wolves have adapted to their carnivorous diet

Comparing requirements

Autotrophs and heterotrophs have a fundamental difference in their functioning which is reflected in the different methods by which they have to obtain their nutrients and gases. However, both types of organisms require a lot of the same important nutrients and gases for life, they just obtain them differently. We will be looking at the specific processes over the next few lessons but lets start by introducing them and setting up a quick comparison.

Heterotrophs do not photosynthesise and so do not require carbon dioxide and water reactants. Instead these organisms must ingest glucose in some way, typically by eating an autotroph. This is also true for other organic molecules, because they cannot make them themselves, heterotrophs have to obtain them from the environment. When they ingest carbohydrates and other organic molecules, heterotrophs begin to break the molecules down into the components they need. So looking at this picture of sugar and starch again, a heterotroph might ingest a potato which is high in starch. But to use the energy in this potato, the heterotroph has to break it down into individual sugar molecules. So the starch molecule could be broken down into 9 individual sugar molecules.

The process of breaking down the glucose molecule to release the energy is cellular respiration. Cellular respiration requires certain reactants to be present for the reaction to happen. In particular, sugar and oxygen are required.

The chemical equation for cellular respiration

Both autotrophs and heterotrophs require cellular respiration in order to access energy from glucose. This means that they both need access to sugar, which the autotroph makes and the heterotroph gets by eating, and oxygen, which plants make through photosynthesis and if they have excess will release into the atmosphere. Heterotrophs gain this oxygen by breathing and plants will either use the oxygen they have just produced, or if they don't have enough will pull in from the atmosphere through their leaves (we will look at this in detail later).

So to summarise, autotrophs and heterotrophs both require inorganic and organic substances, water and oxygen gas. Autotrophs also require carbon dioxide so that they can conduct photosynthesis.

Chemosynthesis is similar to photosynthesis but uses chemical energy rather than solar energy.

Plants are able to produce their own energy

Autotrophs carry out the process of photosynthesis: they transform energy from the sun into chemical energy in the form of food, sugar specifically. They store this energy in the high-energy bonds between the atoms that make up glucose. They store this energy in their body and access and release it for use when needed. The energy can be stored as simple a simple sugar molecule, or it can bind several sugar molecules together to make more complex molecules that are organic molecules like lipids (fats) or complex sugars which are called carbohydrates.

For example: starch is referred to as a complex sugar. This is because it is made up of multiple glucose molecules bound together.

To carry out photosynthesis plants need certain reactants (left side of the equation) to produce the required products (right side of the equation).

The chemical equation for photosynthesis

Plants use photosynthesis to produce organic compounds

Organisms have different methods of obtaining energy

Glucose is a monomer and joins together to form polymer molecules such as starch

Autotrophs and heterotrophs have different methods/source of obtaining nutrients

Plants are the primary example of autotrophs

Autotroph structure

There are many substances that all living organisms require for their efficient functioning. Inorganic and organic substances are essential nutrients for both autotrophs and heterotrophs. These nutrients are required to supply energy to the organism and to provide the raw materials to be used in building the structure of cells and living tissues. Heterotrophs need to take in all of their nutrients from external sources. Autotrophs can produce their own organic nutrients and need to obtain water, minerals, and gases from external sources. The majority of autotrophic organisms are plants.

The vascular bundles in celery are visible

Vascular bundles

Most plants are described as vascular plants because they possess a transport system to move substances from one part of the plant to another. This is very similar to mammals which have vascular bundles that transport blood.

Plants have specialized cells that are grouped together into tissues. These tissues will link up with other tissues to form organs which carry out particular functions to support the effective and efficient functioning of the plant. These tissues all have specific functions within the plant, including transport of substances, conducting photosynthesis, reproduction and gas exchange.

By having such a structure, vascular plants ensure that each organ – the leaves, stem, roots, flowers and seeds – receives what it needs. These organs are part of the ‘body’ systems found in plants – the roots, the shoot and the vascular system.

In a vascular plant, there are specialized cells and tissues that distribute organic compounds, water, minerals and gases around the plant. The vascular system is made up of tube-like structures which are involved in the transport of substances in plants. They are composed of two different types of tissues: xylem and phloem.

Xylem: (pronounced zeye-lem) Responsible for the transport of water, along with water-soluble (dissolvable) nutrients and minerals, absorbed from the soil through the root system.

Phloem: (pronounced flow-em) Responsible for the transport of sugars in the form of dissolved sucrose (2 glucose molecules bonded together) and other plant products from one part of a plant to another.

The circulatory system in also called the vascular system

A small number of plants are termed non-vascular plants because they do not possess this transport system. These plants include mosses and liverworts and they have a very simple structure which all nutrients and wastes moved in and out of the organism by diffusion and osmosis on the surfaces of the plant.

Liverwort is a non-vascular plant

Mosses are non-vascular plants

Strawberry plans produce visible runners

Macroscopic investigations

There is a wide variety of ways to observe the structures of plants, but the amount of detail that is obtained will vary based on the technique.

Macroscopic investigations can include just observing a plant and making identifying structures and considering their applications.

For example, below we can see a strawberry plant reproducing. The mature strawberry plant is sending out runners that grow horizontally and then produce buds that develop into roots and shoots. It is possible is possible to use these plants to observe their root system by digging the plant up and washing off the soil. We can see that the daughter plant has its own roots. So if the runner between the mother plant and daughter plant is severed, the daughter plant will still be able to obtain nutrients and water from the soil.

Observations with the naked eye show that strawberry plants can reproduce with runners

We can also observe and compare different plants to see how some of their features have been specialized. For example we can see a cabbage plant and a potato plant below. In the cabbage the stem has specialized to be smaller but quite thick, while the leaves a very large and broad. In the potato plant the leaves appear relatively small, the stem is long and thin, while the roots have large sections, the potato itself. Both of these plans have similar structures, roots, stem and leaves but they are very different in where they store their energy.

Cabbages roots are not often observed

Consider the famous Venus fly trap which is a carnivorous plant. Watch the video below to find out how and why these plants eat bugs.

Venus fly trap leaves are specialised to consume insects but they still have roots to anchor them to the soil

Venus fly trap trigger hairs inside the leaf

Mangroves prefer estuary environments

Furthermore, we can observe strange structures in mangrove trees. These are very important plants that work to protect shorelines from erosion and large waves. These trees grow where salt water meets freshwater and they have structures called pneumatophores (below left) which are specialized roots that are involved in gas exchange. They are sometimes called 'breathing roots'. They also have some adaptations that help them deal with their salty environment such as depositing salt into sticks, bark and leaves which later drop off and remove salt from the plant.

Mangrove breathing roots

Mangroves excrete salt through their leaves

Mangroves have several adaptations to help them survive the harsh salty conditions of the estuaries that they grow in

When microscope slides are prepared, a transverse or longitudinal sample may be taken

Microscopic investigations

Microscopic investigations often involve taking sections of plants or squashing a plant to obtain a thin specimen that light can pass through. Depending on where the section was taken from and which type of cut the structure observed will be different, particularly in vascular plants.

There are two types of cutting sections (seen below). A trick to remembering is that longitudinal allows you to get a longer section.

In plants the three parts (root, stem and leaf) look very different when a transverse section is taken. We will be looking at these in practicals as well, but you will need to be familiar with the differences between the three so that you are able to identify them (seen below). Longitudinal sections are harder to differentiate as it very much depends on where in the stem/root/leaf the section has been taken.

Vascular tissue arrangement in different plant organs

Plant stomata

Xylem and phloem tissue light micrograph

Labelled light micrograph of epidermal onion cells

Plant roots inside a pot

Imaging technologies

Imaging technologies are primarily observed through secondary sources as the tech can be quite expensive to purchase and use so we rely on data provided by other people who do have access.

The development of technologies that are much more advanced than light and electron microscopes has led to a greater level of understanding of not only plant structure but also their functioning.

We can create 3D images using photos from different angles. Or we can grow plants in clear containers so tat the structure of roots can be studied by MRI. These structures can then be analyzed in greater detail than through simple observation. For example the image below shows the roots of two plants (barley left, sugar beet right) and how their roots compete for space in a pot, meaning they cannot reach their full potential compared to in soil.

It is important to recognize the difference between diagrams and actual electron micrographs. Transmission electron micrographs (TEMs) will be two-dimensional and usually black and white. We use these to observe different organelles in a high level of detail.

Mitochondria can be observed using highly magnified electron micrographs where you can see the outer membrane and the highly folded inner membrane. The highly folded inner membrane portion is called crista (cristae is plural). The material enclosed by the inner membrane is called the matrix and it contains ribosomes, DNA and granules.

Mitochondria electron micrograph and labelled diagram

Chloroplast can also be observed but depending on the magnification you may be able to see the double membrane (outer and internal) running parallel around the chloroplast. Stacks of darkened lines joined by thin lines are the internal features. The darkened stack (a thylakoid grana stack) or granum (grana is plural) is made up of layers of thylakoid discs where each disc has a membrane and internal space called a lumen. The thinner lines joining the stack are lamella and they hold the stacks in place to maximize light absorption. Stroma fills the internal space and it contains enzymes, ribosomes and DNA.

Chloroplast electron micrograph and labelled diagram

Other technologies like PET (positron emission tomography) and NT (neutron tomography) can also provide not only greater detail but also functional information about transport and processes. Both of these technologies involve the detection of radiation produced by a radioisotope.

Plant root neutron tomography image

Plant stems grow from the apical bud at its tip and axillary buds along the stem

Plant roots and stems

The structure of a vascular plant ensures that each organ - the leaves, stems, roots, flowers and seeds - receives what it needs. The organs are part of the 'body' system - the root, the shoot and the vascular system.

Dicotomous plants have a tap root whilst monocot plants have a fibrous root

Roots

The main functions of the root system are anchoring the plant and absorbing water and inorganic nutrients from the soil. The root system is therefore usually underground.

The roots have a very large surface area due to their thin but widespread manner of spreading through the soil. This allows for a very large surface area that allows water and inorganic mineral salts to be absorbed efficiently. The epidermis is the outermost layer of the root and it is through specialized epidermal cells that water and minerals are absorbed.

There are two main types of roots, tap roots and fibrous roots.

Root systems can increase their surface area in the following ways:

Extensive branching of root systems in the mature region increases the surface area of the root for absorption, and also provides good anchorage for the plant. In other words, more small, thin off shoots of roots are produced.
The root hair zone is in the younger part of each root, near the tip. In this region, the epidermal cells protrude outwards into the surrounding soil, as microscopic extensions called root hairs. Their presence increases the surface area of a root up to 12 times.
Water enters the root through the epidermal cells across the entire surface of the root system. The flattened nature of these cells increases their exposed surface, but the surface area of general epidermal cells is smaller than that of root hair cells and so less water is absorbed per cell than in the root hair zone.

Movement of substances into the roots:

Water moves into the roots by osmosis while mineral ions usually move into the roots by diffusion. However this is only if the concentration is higher in the soil than in the plant. If the movement by diffusion is too slow or the concentration gradient is not high enough, facilitated diffusion and active transport may also be involved.

Cells of the root cannot photosynthesis because they do not contain chloroplasts and are not exposed to sunlight. They do not carry out aerobic cellular respiration like all living cells. Oxygen diffuses into the cells from the air pockets in the soil and carbon dioxide diffuses out. This is the opposite to leaves as cells in the roots do not require carbon dioxide for photosynthesis but do require oxygen for cellular respiration.

Aquatic plants obtain water from their environment so the roots are primarily involved in anchoring the plant and obtaining minerals.

If we have a look at the diagram below that we saw last lesson we can see that the transverse root section has more xylem than the stem and the leaf do. Xylem transports water and dissolvable minerals so the roots have a thicker layer to help with this absorption process. The Xylem then streamlines as it travels up through the stem and eventually spread out across the area of the leaves.

Plant roots send out projections to increase the surface area to volume ratio

Microscopic root hairs cells

Root hair cells in the epidermal layer

Water and gas diffusion in roots

Vascular tissue in plant organs

Stems

The two main structures that make up the shoot system are the stems and the leaves. The stem provides both structural support and a transport pathway between the roots and the leaves. The stem, roots and leaves all contain dermal tissue, vascular tissue and ground tissue. The three types of tissue carry out functions such as storage, photosynthesis and extra support for the plant.

Light micrograph of plant vascular tissue arrangements

Vascular tissue arrangement is different in monocot and dicot stems

The dermal tissue makes up the outer layer of the steam and provides waterproofing as well as protection and control of gas exchange.
The vascular tissue is composed of xylem and phloem tissues that are arranged in the stem in structures called vascular bundles and reach from the roots to the leaves. These provide structural support and enable transport of materials. The water and mineral ions that are absorbed by the roots and have moved into the xylem continue their journey to the leaves through the stem. The products of photosynthesis move from the leaves to all parts of the plant through the phloem tissue in the stem. The arrangement of vascular tissues varies between different plant species such as monocots vs dicots (you don't have to stress too much about this.
Ground tissue in the stem fills in around the vascular tissue.

Plant leaves are vital in photosynthesis

Plant Leaves

The main function of a leaf is to absorb sunlight and carbon dioxide and produce the organic compound glucose in the process of photosynthesis. Leaves are also the site of transpiration, a process by which water evaporates from the leaf. This aids in the movement of water from the roots to the leaves and also cools the plant. The structure of a leaf greatly assists it to carry out these functions in an efficient and effective manner.

Plants in rainforests have large leaves to maximise access to sunlight

Absorbing Sunlight

The leaves of plants are adapted to absorb the maximum amount of sunlight possible to provide the energy to break the bonds in water molecules during the first stage of photosynthesis. The thin, flat structure of most leaves is very well suited o this function. A large surface area allows maximum absorption of light energy by the chlorophyll inside the chloroplasts of the cells.

The thin structure of the leaf means that no internal cell is too far from the surface to receive light. The outermost layer of cells, the epidermis, is transparent, allowing the sun to penetrate through to the photosynthetic cells beneath.

Leaves have a specific structure which can vary a little but generally follows the same pattern. The top layer is the cuticle which is a waxy layer that helps protect the leaf from heat. In hot, dry habitats, plants risk losing a lot of water through evaporation. Under this is the epidermis layer which helps protect the mesophyll layer.

The cells that occur in the mesophyll layer or middle layers of the leaf, are responsible for most of the plant's photosynthesis. Two main types of cell make up the mesophyll. They are palisade cells and spongy cells.

Palisade cells are elongated cells that have lots of chloroplasts and are the main photosynthetic cells in leaves. They sit vertically, directly under the upper epidermis, so they are exposed to the maximum amount of sunlight. The large number of chloroplasts in these cells ensures a maximum rate of photosynthesis.
Spongy mesophyll cells are the second most important photosynthetic cells. These cells sit under palisade cells. They have fewer chloroplasts than the palisade cells and their shape and distribution is very irregular.

Plant leaves are made of layers of specialised cells

Vascular bundles are embedded throughout the leaf

Plants that live in different environments have different leaf shapes.

Rainforest plants that grow on the ground below the canopy receive very little light because it is blocked out by the leaves above. So these plants have very large, thin and flat leaves that are dark in color so they absorb as much sunlight as possible without reflecting the light away. Because of the high humidity in these environments they are less worried about maintaining their water levels so they may not have a cuticle layer or it may be quite thin.

Plants that live in hot, dry environments have large amounts of sunlight but very little water and evaporation rate is high. They often have leaves that are very small so that their surface area is small enough to help reduce water evaporation but they can still maximize their sunlight absorption.

Plants in deserts have small leaves to minimise water loss

Gas exchange

The epidermis layer covers the surface of leaves but sits under the cuticle. These cells are simple flattened cells on the top and bottom of leaves (called upper and lower epidermis). Epidermal cells protect the delicate inner tissues and are actually the source of the cuticle as they secrete it to prevent the evaporation of water from the increased surface area of leaves.

Epidermal cells are transparent so that light is able to pass through them to the mesophyll layer below.

Within the epidermal layer there are specialized guard cells that control both the exchange of gases and the loss of water through the leaves.

Guard cells are bean shaped cells that occur in pairs surrounding a pore known as a stoma. Stomata usually occur on the under surface of the leaf, but can occur on both the upper and lower surfaces of the leaf.

Stomata open and closed in response to water availability in the guard cell vacuoles

The opening of the stomata depends on the vacuoles in the guard cells. When the stomata are full of water they force each other apart, opening the pore. When they start to lose this water such as on a hot day, the vacuoles empty, the stomata stop pushing each other apart and the pore closes. This way the plant is able to prevent water from being lost on a hot day but is able to get rid of excess water if needed.

Carbon dioxide gas enters the plant through the stoma and oxygen leaves the plant through the stomata. Plants adapted to hot environments will have the majority on the underside of the leaf and will move the leaf throughout the day to keep the heat off the stomata, preserving water, but allowing carbon dioxide to travel through.

Stomata are the primary site of gase exchange

Stomata arrangement varies from plant to plant and demonstrates an adaptation to the plants environment to account for water availability

Nutrients must be transported in plants

Transport

The vascular tissue in the center of the root is continuous, passing up the stem and into the leaves as 'veins' in the leaf, and serves as the main transport tissue in the plant. The main vein in the leaf is called the midrib and many smaller veins branch out from it. The veins contain xylem and phloem tissue. The distribution of vascular tissue throughout the leaf ensures that no leaf cells are two far away from a source of transport. Vascular tissue also plays an important role in supporting the leaf.

Plants 'sweat' water via transpiration

The diagram above shows a simplified explanation of transcription. We will look at this process in more detail in the next part of the syllabus concerned with transport. But for now it is enough to understand that water is absorbed into the plant through the roots, travels up through the stem in the xylem and leaves the plant through the stomata in the leaf by transcription.

Photosynthesis and cellular respiration are complementary processes

Cellular respiration

It is important to remember that plants carry out cellular respiration as well as photosynthesis. Cellular respiration is a function of all living cells; in leaves it is simply masked by photosynthesis and so the exchange of gases by plants differs from that at night.

Cellular respiration occurs both during the day and the night whereas photosynthesis primarily occurs during the day, with the exception of the light independent component that we looked at last term.

The oxygen required for cellular respiration comes from the oxygen produced as a by-product of photosynthesis. Photosynthesis occurs at a greater rate than cellular respiration during the day so while some oxygen is used up, the majority is released into the environment. At night the plant will use up any remaining oxygen but will pull in oxygen from the environment if needed. Roots, which are below the soil, pull in oxygen from the soil throughout the day and night in order to use the energy they need to grow.
The carbon dioxide released as a result of cellular respiration during the day is used in photosynthesis. When the rate of photosynthesis is high it often means the plant quickly uses up the carbon dioxide produced by cellular respiration and so it has to pull some in from the air.

Plant stomata open and close based on water availability in guard cells

Gas exchange in plants

Gas exchange is an important process in all organisms. All living cells require a constant supply of oxygen and the removal of carbon dioxide. Both plants and animals possess specialized structures that allow the efficient movement of gases into and out of the organism.

The surface that the gases cross is called the respiratory surface. In unicellular organisms the entire cell membrane is the respiratory surface while in multicellular organisms specific surfaces need to be devoted to gas exchange. We will be focusing on plants.

A longitudinal light micrograph of plant stomata

Stomata

Plant leaves are covered with the waxy cuticle layer that is produced by the epidermis layer. This layer stops both gas and water from entering the leaf. So, leaves have pores in the epidermis layer through which the gases oxygen and carbon dioxide can move in and out of the plant. We mentioned these pores last lesson and identified them as stomata.

Stomata are found mostly on the underside of the leaf in the lower epidermis. There will be some found in the upper epidermis but in much fewer numbers.

Plants have their stomata arranged in different ways. For example, Australian Eucalypts have their leaves have vertically to help minimize their exposure to the heat of the sun. These leaves have their stomata distributed on both the upper and lower epidermis layers. Plants that float on water usually have stomata only on their upper surfaces, while some completely aquatic plants have no stomata.

Eucalyptus plant leaves hand vertically to minimise exposure

Stomata are arranged differently in different plants

Stomata specifically refers to the pore in the epidermis layer. The two cells that enclose the pore are called guard cells. Unlike the other cells that make up the epidermis, guard cells contain chloroplasts. Therefore these are the only cells in the epidermal layer that can photosynthesis to make energy. There are several theories as to why this is the case.

Plants have to balance the exchange of gases without losing too much water. Having the stomata opens means gas can be exchanged but it also means water can escape.

Labelled longitudinal diagram of plant stomata

Which can be hazardous to the plant, particularly if it is in a hot environment. So stomata are able to open and close. Now here is where it gets alittle backwards. In order to close the stomata, the guard cells need to fill up with water. When they fill up the two points where they touch each other they force each other apart, opening the pore. When the guard cells fill with water and become turgid, their vacuoles store the water and the thin outer walls are able to stretch outwards, but the inner walls are thicker and not able to stretch so the guard cell is pulled into a curved shape.

Vacuoles in guard cells store water, forcing the pore to open

When the guard cells lose water their vacuoles empty, the outer walls no longer have to stretch and the inner walls are able to relax as well, closing the pore.

What causes water to move into and out of the guard cells is still being researched, but current theories suggest it is linked to the movement of potassium (K) ions.

The opening and closing of stomata depend on a number of environmental factors. Light is the main factor. Generally stomata open during the day and close at night. When the stomata are open, water is lost from the plant. Plants need to balance their gas exchange requirements while minimizing water loss. If more water is lost than is taken up be the plant, the water content of the plant falls. This causes the stomata to close and the exchange of gas to cease.

In some plants this is visible, for example, the Peace Lily is probably one of the easiest house plants to keep alive because it shows you when it needs to be watered. Look at it. It looks sad. You made it sad by not watering it. You should feel bad.

The Peace Lily visibly wilts when it is dehydrated

If gas exchange stops and water content is decreased, photosynthesis may be limited and the concentration of carbon dioxide inside the plant will rise. This also causes stomata to close so that carbon dioxide does not increase even further.

Plants often do well in humid environments like jungles and greenhouses because the level of water in the plant does not decrease but the stomata are able to stay open.

Lenticels in stems allow for gas exchange

Lenticels are pores through which gas exchange occurs in the woody parts of plants, such as the trunks and branches of trees and woody shrubs. They appear as small dots to the naked eye, but on microscopic examination it can be seen that they are clusters of loose cells in the cork layer of bark. The diffusion of oxygen, carbon dioxide and water vapour through lenticels is relatively slow.

Melvin Calvin

Tracing products of photosynthesis

Now that we've looked at the reactants involved in photosynthesis we can start considering the products. Now its easy for me to sit here and tell you what we know about photosynthesis but it is important to understand how we know these things.

The mechanism by which the process of photosynthesis occurs has been investigated over a long period time. It was originally thought that sunlight acted on carbon dioxide in the initial stages of photosynthesis.

Melvin Calvin used a radioactive tracer, Carbon-14, to show that sunlight actually acts on the chlorophyll to begin the manufacturing of glucose. He was awarded the Nobel Prize in 1961 for his work.

Basically he identified chloroplasts and the chlorophyll within, as the site for photosynthesis.

Isotopes are different forms of the same element. Radioisotopes are isotopes that emit radiation - they are said to be radioactive because they emit radioactive waves or particles to try to achieve a stable state. The emission of these radioactive particles or waves can be measured using a number of methods. Radioisotopes are very useful as tracers - technologies can be used to trace their paths in physical, chemical and biological systems.

Tracing the journey of oxygen in photosynthesis

Radioisotopes were used to determine whether the oxygen released during photosynthesis originated from the oxygen atom in water or in carbon dioxide. Plants that were given water that contained radioactive atoms of oxygen showed that all of the radioactive oxygen atoms from the water molecules were released as oxygen gas, showing that water was the source of the oxygen gas during photosynthesis, not carbon dioxide.

The pathway of the glucose produced in photosynthesis can also be traced using radioisotopes. To do this, Carbon-14 is added to the carbon dioxide supply of a plant. This carbon-14 then takes part in the reactions of photosynthesis and is used to make glucose. The pathway of glucose molecules can be traced using radiation being emitted by the carbon-14 and recorded in an autoradiograph. This can be achieved by placing the experimental plant against a photographic film; the dark areas on the film show where the carbon-14 has accumulated.

Modern computer software can convert the information into 3D images so that investigators can 'see' where the radioactively labelled chemicals are moving during photosynthesis. Real-time radioactive imaging system (RRIS) is a non-destructive imaging system that visualizes the movement of substances in the phloem.

PlanTIS is a PET scanner for plants. It is used to trace the radioisotope carbon-11, which in the process of decay emits radiation that is detected by the scanner. This traces the movement of carbon-11, which has been incorporated into the glucose molecule.

The lungs are the site of gas exchange in mammals

Respiration and gas exchange in mammals

Gas exchange occurs in all animals and involves the movement of gases between the internal and external environments by diffusion across cell membranes. Gases required by the organism to carry out normal cell functioning move into the cells, while the waste gases produced as result of these reactions diffuse out.

Cellular respiration chemical equation

Molecules in the cellular respiration reaction

Gas exchange in animals

Oxygen is essential for all cells to carry out cellular respiration to release energy from the nutrients they have consumed. As a result of this process, carbon dioxide is produced and must be removed, as it is toxic if its concentration is too high, changing the pH of cells and interfering with enzyme functioning.

The lungs feature alveolus which increase the SA:V ratio to maximise the rate of diffusion

Carbon dioxide is a waste product that forms carbonic acid when dissolved in water or blood plasma, causing toxicity symptoms

The respiratory system enables the exchange of gases between an organism and its environment. It contains organs made up of specialized tissues that allow an organism to take in oxygen and to remove carbon dioxide from its body.

Different animals possess different respiratory organs: mammals possess lungs, gills are present in fish and insects have what is known as trachea system. In microscopic organisms, gas exchange occurs by diffusion across their cell membrane. In larger terrestrial animals, the gas exchange system is inside the body to prevent dehydration of the gas exchange surfaces.

All gaseous exchange structures share common characteristics to ensure efficient functioning and maximum exchange of gases.

They all:

Have a large surface area that has been enhanced by folding, branching or flattening, depending on the structure. This large surface area allows a faster rate of diffusion to supply oxygen and to remove carbon dioxide.
Have a moist, thin surface to ensure that the oxygen and carbon dioxide dissolve for easier diffusion - thinness decreases the distance that the gases have to travel.
Are in close proximity to an efficient transport system that will transport the gases to and from all cells in the body.
Have a greater concentration of required gas on one side of the membrane than the other, so that a concentration gradient is maintained.

The respiratory systems of terrestrial animals are internal to reduce the loss of water from the respiratory surface. The gaseous exchange surfaces in mammals are located in the lungs. These structures are known as alveoli. Each thin-walled alveolus is composed of an air sac that is connected to the external environment and is surrounded by tiny thin-walled blood vessels called capillaries.

The circulatory system works closely with the respiratory system to transport gases around the body

Animals use different types of gas exchange organs

Gas exchange in humans

The alveoli

The alveoli in the lungs have all the features that allow for efficient gas exchange:

Increased surface area is achieved by the approximately 300 million microscopic alveoli that are supplied by 280 million capillaries. Each alveolus has folding of the thin lining, thus further increasing surface area.
Each alveolus has a thin lining made of flattened cells that are in a single layer, facilitating the efficient diffusion of gases across a very small distance.
The surface of all parts of the respiratory system is moist. The air inside the alveoli is saturated with water vapor and the mucus-lined epithelium reduces the evaporation of this water. This ensures that the oxygen and carbon dioxide that diffuse across the gaseous exchange surface are in a dissolved form, enhancing the efficiency of diffusion.
The numerous blood capillaries that closely surround the outside of each alveolus ensure that all alveoli are in close contact with the blood.

The alveoli are delicate and easily damaged

The movement of gases between the air in the alveoli and the bloodstream occurs by diffusion across the concentration gradient. Inhaled air contains approximately 20% oxygen and 0.04% carbon dioxide. Exhaled air contains approximately 15% oxygen and 4% carbon dioxide.

Oxygen in the incoming alveolar air is in a high concentration than in the bloodstream, so oxygen diffuses from the air sacs into the bloodstream; carbon dioxide is in higher concentration in the bloodstream, so it diffuses along a concentration gradient from the capillaries through the alveolar lining and into the alveolar air, from where it is exhaled.

The branching structures of the lungs and the interweaving circulatory system

Fish breathe using their gills

Gas exchange in fish

Aquatic animals also need to obtain oxygen and remove carbon dioxide from their bodies to function effectively and efficiently. Gases have a low solubility in water so their concentration in water is much lower than their concentration in air. The gas exchange structures (gills) that fish possess have characteristics that can extract the maximum possible amount of oxygen from the water. Gills have all the characteristics of respiratory surfaces in animals, but are very different in structure.

They require water flowing over them to ensure maximum oxygen uptake. The water flows in only one direction, entering when the fish opens its mouth as it swims. This causes the water to enter and flow over the fills and then leave the fish through the gill slit or slits. As the water flows over the gills, gaseous exchange takes place.

Insects breathe using their tracheoles

Gas exchange in insects

The terrestrial habitat of the insect presents a challenge to the insect to reduce the loss of water from its internal respiratory surfaces. Insects take in an expel air through structures called spiracles, which are in effect breathing pores. To ensure that these spiracles are not continually exposed to the drying effects of the environment, they have valves to regulate their opening and closing. As well as this, little or no gaseous exchange can occur through their body coverings.

Insects do not have lungs or blood capillaries. Because they are small organisms, they can achieve the exchange of gases using a much simpler system. This system involves branching air tubes called tracheal tubes, which carry air directly to the cells of the body. Air that enters the spiracles is drawn into these tracheal tubes or tracheae. These are kept open by spiral rings of a touch supportive substance called chitin. Tracheae branch into smaller tubules called tracheoles, which create a very large surface area for gaseous exchange. Tracheoles bring the air directly to and from cells of the insect.

The respiratory surface in insects differs from all other internal respiratory systems in that it has no blood or blood capillaries involved in the transport of gases. The ends of the tracheoles are filled with a watery fluid in which the gases dissolve. Oxygen from the air, dissolved in this fluid, diffuses directly into the cells and carbon dioxide diffuses directly out of the cells into the tracheoles. The number of open and closed spiracles generally controls the rate of respiration in insects - more are open when the insect is active. Muscular movements of the thorax and abdomen during movement and general body movements when flying also help to ventilate the tracheal system.

The stomach

Mammalian digestion

Heterotrophs are living things that need to take in or eat all of their nutrient requirements to supply energy and the building blocks for organic compounds. Complex foodstuffs are eaten, or ingested, and are broken down by our digestive system into simpler molecules that can be absorbed into the bloodstream.

The digestive system contains many organs

Process of digestion

Digestion is the breaking down of large and complex food particles into much smaller and simpler particles. There are two types of digestion: mechanical and chemical. The overall aim of digestion is to break down the particles into substances that are small enough to be absorbed through the intestinal walls into the bloodstream.

Mechanical and chemical digestion occur in the digestive system. Different organs of the digestive system are specialized to perform different functions to achieve digestion of the food that we eat.

Peristalasis is the physical churning of food in the digestive organs

Mechanical vs Chemical

This involves the physical breakdown of food particles. Mechanical digestion begins in the mouth when the different types of teeth break food into smaller pieces by cutting, tearing, chewing and grinding the food. The churning motion of the stomach continues the process of mechanical digestion. The aim of this mechanical digestion is to start the process of breaking food into smaller pieces so that its surface area is increased and it can then be acted on by enzymes in chemical digestion.

Chemical digestion is the process of using digestive enzymes to chemically break down the large, complex molecules in the food that has been ingested into their smaller, simpler forms. Some of the simple substances obtained are glucose from complex carbohydrates, amino acids from proteins, glycerol and fatty acids from lipids, and nucleotides from nucleic acids.

Enzymes work to break down specific substrates

The digestive system is adapted to have a high SA:V ratio

Path of digestion

The mouth is where food enters and digestion begins

Mouth

After food enters the mouth, mechanical digestion begins the process of the breakdown of the food. Teeth break the food up into smaller pieces with greater surface area for the more efficient action of enzymes. Salivary amylase is released into the mouth, and is mixed with food by the tongue and the action of chewing. This enzyme begins the chemical breakdown of the complex carbohydrate starch into the simpler sugar maltose.

Once the food has been chewed into small pieces and mixed with saliva, the tongue forms it into a ball shape called the bolus. This is then swallowed and enters the oesophagus.

Chewing increases the SA:V ratio of food

The oesophagus transport food away from the mouth

Oesophagus

Once the bolus enters the oesophagus, it travels along the soft-walled, muscle-ringed tube to the stomach. As it passes the entrance to the trachea, a flap of skin, the epiglottis, closes over this entrance to prevent the entry of food into the respiratory system. The bolus of food does not move down the oesophagus just due to gravity. Muscular contractions also move the bolus by a process called peristalsis. The chemical digestion of starch continues during movement along the oesophagus.

The Epiglottis prevents food from entering the trachea when swallowing

The stomach uses both mechanical and chemical digestion

Stomach

At the entry and exit of the stomach, there are narrow openings whose opening and closing are controlled by circular sphincter muscles. This controls the movement of substances into and out of the stomach. Once inside the stomach, relaxation and contraction of the stomach walls continue mechanical digestion. The bolus breaks up into pieces that combine with gastric juices contained within the stomach to form a mixture known as chyme. Gastric juices, secreted from the wall of the stomach, contain water, hydrochloric acid, pepsinogen and pepsin. The acid causes the pH of the interior of the stomach to be 2.0-3.0. Mucus lining the stomach prevents the acid from 'eating away' the walls of the stomach.

The enzyme pepsinogen is converted into an active form called pepsin in the acidic environment and begins the chemical breakdown of the long-chained proteins into shorter chained peptides. Pepsin also breaks down nucleic acids (DNA and RNA) in the food to their component nucleotides. The chyme remains in the stomach for about 6 hours.

The digestive system of animals differs based on their diet, particularly in ruminants

The small intestine is thin but very long

Small Intestine

The chyme from the stomach enters the small intestine gradually though a small muscular opening, the pyloric sphincter. The highly folded small intestine is approximately 7m long in an adult and contains 3 main regions: the duodenum (at the start of the small intestine), the jejunum (middle section) and the ileum (end region).

As the chyme enters the duodenum, it stimulates the release of a hormone, which in turn stimulates the release of pancreatic juices into the area. Pancreatic juices are secreted by the pancreas and contain a mixture of the digestive enzymes amylase, trypsin and lipase, as well as bicarbonate ions. The bicarbonate ions act to neutralise the acidic chyme leaving the stomach. Amylase and trypsin continue the chemical breakdown of carbohydrates and proteins.

When there are lipids present in the chyme, bile is released into the duodenum. Bile is produced by the liver and is stored in the gall bladder. Bile is not a digestive enzyme. It acts in the same way as detergent acts on fats when washing a greasy saucepan - it breaks down (emulsifies) the fats into smaller pieces or fat droplets. This increases the surface area for the action of the digestive enzyme lipase to chemically break down the lipids into fatty acid and glycerol molecules.

From the duodenum, food enters the jejunum where most of the absorption of the digestive products occurs.

The villi have a close blood supply and use diffusion to take in various nutrients

Absorption in the large intestine

The absorption of substances mostly occurs in the jejunum section of the small intestine. Some substances, such as alcohol and drugs, are absorbed quickly in the stomach. The products of digestion, including amino acids, glucose, fatty acids and glycerol, move into the transport systems of the body in the small intestine. These products are moved by diffusion or active transport through tiny projections, called villi, which line the intestinal wall. These projections greatly increase the surface area for much more efficient diffusion. Villi walls are moist and are one cell thick. They have a rich blood supply in the tiny capillaries that are wrapped around a lacteal. Lacteals are connected to another transport system in the body - the lymph system. Glucose and amino acids are absorbed into the capillaries, while fatty acids and glycerol move into the lacteal. Some water absorption will also occur here.

The small intestines have many villi

The liver careful controls glucose levels in the blood

Liver

Digested food, once absorbed into the bloodstream, travels to the liver, which is the centre of food metabolism. It plays an important role in keeping sugars, glycogen and protein levels in balance in the body. It also detoxifies the blood.

The lymphatic and circulatory systems are closely linked with the digestive system to transport nutrients and manage infections respectively

The large intestine is wide but short

Large Intestine

When all the required digestive products have been absorbed in the small intestine, the remaining undigested material moves to the large intestine. This material is composed of substances such as water, salts and dietary fibre. The large intestine has two main sections: the colon and the rectum. In the colon the water and some salts are absorbed back into the bloodstream, with the undigested material compacting into a more solid substance. Vitamins A and K, which are produced by bacteria in the colon acting on the undigested matter, are also absorbed into the bloodstream. The remaining waste material, known as faeces, is moved into the rectum by peristalsis and then egested, or eliminated, from the body through the anus.

Polymers must be broken down into monomers in order to for cells to access important molecules

Fate of food

The end products of digestion can be built up by the body into useful substances, as either new biological material or an energy source. For example, in mammals such as humans, blood transports the products of digestion to where they are needed in the body. They can then be reassembled by the cells of the body into structural parts (for example, lipids and proteins form a structural part of cell membranes, and protein fibres in muscle tissue) or into energy storage (for example, fatty tissue or fat beneath the skin, or the carbohydrate glycogen, a form of 'animal starch', in the liver and muscles). Protein cannot be stored.

Organs and their role in digestion

Nutrient and Gas Requirements:

Inquiry question: What is the difference in nutrient and gas requirements between autotrophs and heterotrophs?

investigate the structure of autotrophs through the examination of a variety of materials, for example:
- dissected plant materials
- microscopic structures
- using a range of imaging technologies to determine plant structure
investigate the function of structures in a plant, including but not limited to:
- tracing the development and movement of the products of photosynthesis
investigate the gas exchange structures in animals and plants through the collection of primary and secondary data and information, for example:
- microscopic structures: alveoli in mammals and leaf structure in plants
- macroscopic structures: respiratory systems in a range of animals
interpret a range of secondary-sourced information to evaluate processes, claims and conclusions that have led scientists to develop hypotheses, theories and models about the structure and function of plants, including but not limited to:
- photosynthesis
- transpiration-cohesion-tension theory
trace the digestion of foods in a mammalian digestive system, including:
- physical digestion
- chemical digestion
- absorption of nutrients, minerals and water
- elimination of solid waste
compare the nutrient and gas requirements of autotrophs and heterotrophs

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