Autotrophic Nutrition

This is the synthesis of organic compounds from inorganic sources.

There are two types of autotrophic nutrition.

• Chemosynthesis is the synthesis of organic compounds from carbon dioxide and water using energy from the oxidation of inorganic materials such as hydrogen sulphide, ammonia, and iron II.
• Photosynthesis is the synthesis of organic compounds from carbon dioxide and water using energy from light.

Importance of Autotrophic nutrition

1. Sugars resulting from autotrophic nutrition constitute structures of plants and animals
2. Sugars obtained from autotrophic nutrition are sources of energy
3. Hyman depend on photosynthesis for energy-containing fuel, which have developed over millions of years
4. Photosynthesis maintains a constant concentration of carbon dioxide and oxygen in the atmosphere. Oxygen is added to the atmosphere as a result of photosynthesis while carbon dioxide is removed.

Conditions necessary for photosynthesis

(a) Carbon dioxide: is obtained from the atmosphere and water in case of aquatic plants. It is the source of carbon in carbohydrates.

Experiment to show that carbon dioxide is necessary for photosynthesis

The experiment is set up as shown above

1. Two younger seedlings growing in pots are kept in the dark for 3 days (to remove starch) and a leaf from each plant is tested to confirm the absence of starch.
2. In experiment I the burning candle supplies carbon dioxide while in experiment II carbon dioxide is absorbed by caustic soda.
3. After 3 hours, a leaf is taken from each plant and tested for the presence of starch. Starch is found in the leaf of the experiment I which had carbon dioxide.

(b) Water provides hydrogen for the reduction of carbon dioxide to form the carbohydrates

(c) Light usually from the sun is the source of energy for photosynthesizing tissues.

The wavelength of light for photosynthesis

An action spectrum is a plot of the rate of a physiological activity plotted against wavelength of light. It shows which wavelength of light is most effectively used in a specific chemical reaction. Some reactants are able to use specific wavelengths of light more effectively to complete their reactions.

The action spectrum of chlorophyll is shown in the graph below

From the graph of the action spectrum of chlorophyll, it observed that chlorophyll is much more efficient at using blue-violet light (wavelength (550-400nm) and orange (600-700nm). 

However, Bacteriochlorophyll of purple sulphur bacteria absorbs ultraviolet (350nm) and infrared (750nm) part of the spectrum. The difference in wavelengths optimum for photosynthesis both in plants and bacteria enables bacteria to survive under waterweeds in the pond. i.e. bacteria find the spectrum useful after the plants have filtered out light for photosynthesis.

(d) Chlorophyll: This is a green pigment that helps to trap light energy for photosynthesis.

Experiment to show that chlorophyll is necessary for photosynthesis

When a variegated leaf is picked from a plant on a sunny day and tested for starch. Starch is found only in parts that originally contained chlorophyll.

(e) Suitable temperature

Photosynthesis proceeds by a series of chemical reactions controlled by enzymes that are sensitive to temperature. By comparing the rates of photosynthesis, at different temperatures, optimum temperatures for various plants are obtained.

 Limiting factors

The main factors affecting the rate of photosynthesis are light intensity, carbon dioxide concentration, and temperature. In any given situation any one of these may become a limiting factor, in other words, the factor that is in short supply.

In 1905, when investigating the factors affecting the rate of photosynthesis, Blackman formulated the Law of limiting factors.

This states that the rate of a physiological process will be limited by the factor which is in the shortest supply. Any change in the level of a limiting factor will affect the rate of reaction.

For example, the amount of light will affect the rate of photosynthesis. If there is no light, there will be no photosynthesis. As light intensity increases, the rate of photosynthesis will increase as long as other factors are inadequate supply. As the rate increases, eventually, another factor will come into short supply.

The graph below shows the effect of low carbon dioxide concentration. It will eventually be insufficient to support a higher rate of photosynthesis, and increasing light intensity will have no effect, so the rate plateaus.

If a higher concentration of carbon dioxide is supplied, light is again a limiting factor and a higher rate can be reached before the rate again plateaus. If carbon dioxide and light levels are high, but the temperature is low, increasing temperature will have the greatest effect on reaching a higher rate of photosynthesis. 

The leaf

In higher plants, the major photosynthetic organ is the leaf.

Functions of the leaves

1. Carry out photosynthesis with subsequent production of organic materials
2. Carry out gaseous exchange through the stomata
3. Transpiration takes place mainly through the leaves resulting in the cooling of plant and absorption of mineral salts and water
4. Some leaves such as those of peas are modified by tendrils for support.
5. Some leaves suck those bryophyllum are modified for storage
6. Some leaves such as for bryophyllum are modified for vegetative reproduction
7.  Some leaves possess itching hairs, thorns, spikes, and poisonous substances for protection of the plant.
8. Some leaves e.g. Venus flytrap are modified for capturing and extracting nitrogen from animals
9. Some produce substances that kill parasites.

Functions of tissues in dicotyledonous leaves

Tissue	Structure	Function
Upper and lower epidermis	One cell thick External walls covered with cutin (waxy substance) Contain stomata (pores surrounded by guard cells)	ProtectiveCutin is waterproof and reduces water loss from the leaves. Allow gaseous exchange
Palisade mesophyll	Column-shaped cells with numerous chlorophyll in a thin layer of cytoplasm	Main photosynthetic tissue. Chloroplast may move towards the light
Spongy mesophyll	Irregularly shaped cells fitting loosely to leave large air space	Photosynthetic, but fewer chloroplast than palisade cells. Gaseous exchange can occur through the large air space via stomata.
Vascular tissue	Extensive finely branching network through the leaf	Conduct water and mineral salts to the leaf in xylem Remove photosynthetic products through the phloem Provide a supporting skeleton to the lamina, aided by turgidity

Adaptation of leaves to photosynthesis

1. The leaf is only a few cells thick for easy penetration of light and gases.
2. Wax cuticle prevents excessive water loss
3. The palisade cells that contain numerous chloroplasts are well-positioned to receive light.
4. Presence of xylem that supplies the leaf with water for photosynthesis
5. The spongy mesophyll has many air spaces. Gases can readily diffuse to all photosynthetic cells
6. They are broad and flat to offer a large surface area exposed to light and air
7. Stomata control passage of gases
8. The arrangement of leaves to the plant is such a way that each leaf receives light.

Modification of leaves to their functions

1. Flattened surface to reduce the diffusion distance
2. Shiny waxy cuticle to reduce water loss
3. Mosaic arrangement to increase light absorption
4. modified into tendrils to support weak climbing stems e.g. Gloriosa, peas
5. Some leaves are modified into  spines in order to reduce water loss. Spines also protect the plant from herbivorous animals
6. Some leaves are modified into scale leaves (thin, membranous, dry colorless structures) for protection. E.g. onion outer leaves
7.  Phyllode : It is a green, expanded structure formed by the modification of petiole or rachis of leaf. Many xerophytes reduce the size of their leaves to minimize water loss. Such plant develops phyllodes to carry out photosynthesis e.g., Acacia, Melanoxylon, and Parkinsonia.
8. Some leaves are flesh to store water and food
9. Some leaves develop vegetative bud for reproduction, e.g. bryophyllum
10. In some rootless, aquatic plants, the submerged leaves are modified into root-like structure to absorb water and mineral salts. Such modified leaves are called absorbing leaves. e.g., Utricularia.
11. Some leaves are brightly colored to attract insect pollinator, e.g. Bougainville
12. Some leaves are modified to trap insects.

Measuring the rate of photosynthesis

Method

1. The apparatus is filled with carbonated water to enrich it with carbon dioxide.
2. The setup is allowed to adjust to light intensity for 2-3 minutes. The rate of bubbling should be adequate such as 10 bubbles per minute.
3. Light intensity is varied by increasing the distance, d, between the light source and the apparatus such as 10, 15, 20, 40, and 45cm. the bigger the distance, d, the lower the light intensity.
4. The rate of formation of bubbles per minute at different distances, d, is an empirical measure of photosynthesis at a given light intensity.
5. Temperature change is buffered by using a beaker of a bigger capacity.

The chloroplast is biconcave (to increase the surface area for absorption of light)

It is bound by a double membrane within which are numerous structures called thylakoids.

Each thylakoid consists of a pair of membranes close to each other with narrow spaces between them. 

In places, the thylakoid is arranged in a neat stack, rather like a pile of a coin called granum (pl. grana). The grana are connected to each other by a less regular arrangement of the inter grana thylakoids.

The thylakoid system is embedded in a protein-rich matrix called the stroma

Adaptations of chloroplast to their functions

1. The outer membrane is semi-permeable to regulate the entry and exit of substances for maintaining the internal chloroplast environment.
2. Contains chlorophyll to trap light for photosynthesis
3. Abundant enzymes catalyze photosynthetic reactions in the stroma.
4. Many grana provide a large surface area for photosynthetic pigments, electron carriers, and ATP synthase enzymes. 
5. Grana surrounded by the stroma so the products from the light-dependent reaction (which occur across the thylakoid membrane which makes up the grana) can readily pass into the stroma for the light-independent reaction. 
6.  Narrow intermembrane space enables H⁺ ion concentration gradient to be rapidly established for chemiosmosis to occur
7. The inner membrane contains molecules for electron transport pathway
8.  DNA presence codes for protein synthesis, including enzymes. 
9.  Many ribosomes for protein synthesis to reduce importing proteins from the cytoplasm
10. The outer membrane is permeable to gases like carbon dioxide which is a raw material for photosynthesis

Differences between cellular respiration and photosynthesis

	Cellular Respiration	Photosynthesis
Production of ATP	Yes; theoretical yield is 38 ATP molecules per glucose but the actual yield is only about 30-32.	Yes
Reactants	C6H12O6 and 6O2	6CO2 and 12H2O and light energy
Requirement of sunlight	Sunlight not required; cellular respiration occurs at all times.	Can occur only in the presence of sunlight
Chemical Equation (formula)	6O2 + C6H12O6 → 6CO2 +6H2O + ATP (energy)	6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O
Process	Production of ATP via oxidation of organic sugar compounds. Glycolysis: breaking down of sugars; occurs in cytoplasm Krebs Cycle: occurs in mitochondria; requires energy. Electron Transport Chain proteins in mitochondria; converts O2 to water.	The production of organic carbon (glucose and starch) from inorganic carbon (carbon dioxide) with the use of ATP and NADPH produced in the light-dependent reaction
The fate of oxygen and carbon dioxide	Oxygen is absorbed and carbon dioxide is released.	Carbon dioxide is absorbed and oxygen is released.
The energy required or released?	Releases energy in a stepwise manner as ATP molecules	Requires energy
Main function	Breakdown of food. Energy release.	Production of food. Energy Capture.
Chemical reaction	Glucose is broken down into water and carbon dioxide (and energy).	Carbon dioxide and water combine in the presence of sunlight to produce glucose and oxygen.

Stages	4 stages: Glycolysis, Linking Reaction (pyruvate oxidation), Krebs cycle, Electron Transport Chain (oxidative phosphorylation).	2 stages: The light-dependent reaction, light-independent reaction. (also called light cycle & Calvin cycle)
What powers ATP synthase	H+ proton gradient across the inner mitochondria membrane into a matrix. High H+ concentration in the intermembrane space.	H+ gradient across the thylakoid membrane into the stroma. High H+ concentration in the thylakoid lumen
Products	6CO2 and 6H2O and energy(ATP)	C6 H12 O6 (or G3P) and 6O2 and 6H2O
What pumps protons across the membrane	Electron transport chain. The electrochemical gradient creates energy that the protons use to flow passively synthesizing ATP.	Electron transport chain
Occurs in which organelle?	Mitochondria	Chloroplasts
Final electron receptor	O2 (Oxygen gas)	NADP+ (forms NADPH )
Occurs in which organisms?	This occurs in all living organisms (plants and animals).	This occurs in plants, protista (algae), and some bacteria.
Electron source	Glucose, NADH + , FADH2	Oxidation H2O at PSII
Catalyst – A substance that increases the rate of a chemical reaction	No catalyst is required for respiration reaction.	The reaction takes place in the presence of chlorophyll.
High electron potential energy	From breaking bonds	From light photons.

Chemistry of photosynthesis

In the process of photosynthesis obtain energy from sunlight is trapped by chlorophyll and used for the manufacture of carbohydrates from carbon dioxide and water.

The process is summarised by the following equation

    Photosynthesis takes place in 3 stages

• Light-harvesting
• Electron transport
• Reduction of carbon dioxide

Stage 1: Light-harvesting

Pigment molecules in the thylakoid membrane absorb and collect solar energy and funnel it to special molecules of chlorophyll a known as the reaction center chlorophyll molecule. On striking this molecule an electron in its orbit is raised to a higher energy level and picked by an electron acceptor.

There are two kinds of reaction centers in plants called photosystem I (PSI) and photosystem II (PS II) that differ in structure and function. PSI absorbs light at 700nm and PSII at 690nm. Whereas many chlorophyll molecules are involved in collecting energy, only a few make up the reaction centers.

The transport of electron through the two centers comprise the second stage of photosynthesis.

Stage 2: electron transport

This stage occurs in thylakoid of the chloroplast and produces ATP and reduced nicotinamide adenine phosphate, (NADPH) to be used in the reduction of carbon dioxide in the third stage.

The events of electron transport are described in the figure below

(i) Formation of NADPH

When light is absorbed by a chlorophyll molecule PSI an electron is displaced and transferred to an electron acceptor which donates it to a protein ferredoxin. Ferredoxin then passes the electron to NADP which is reduced to NADPH.

The electron lost by PSI is restored from PSII. When PSII absorbs light energy, an electron is displaced from it and passes through an electron carrier system which includes cytochromes.

The electron lost from PSII is then restored through the photolytic splitting of water (Hill reaction)

(ii) ATP formation

(a) Non cyclic photophosphorylation

The flow of electrons from PSII to PSI in the thylakoid membrane causes accumulation of H⁺ inside the thylakoid space creating a gradient. The passage of H⁺ out of the thylakoid provides energy to synthesize ATP.

(b) Cyclic photophosphorylation

Here when PSI is struck by light, an electron is excited, passes through an electron carrier system, and then returned to PSI; that is PSI is the electron donor and acceptor. H⁺ builds up in the thylakoid space creating a gradient. The passage of H⁺ out of the thylakoid provides energy for synthesizing ATP. It is useful to synthesize ATP when the synthesis of NADPH is not necessary.

Differences between cyclic and non-cyclic photophosphorylation

	Cyclic photophosphorylation	Non-cyclic photophosphorylation
1	It occurs only in PS I	It occurs only in PS I and PS II
2	It involves the synthesis of ATP	It involves the synthesis of ATP and NADPH
3	Photolysis of water does not occur	Photolysis  occurs and oxygen is liberated
4	Electron motion is cyclic	Electron motion is noncyclic

Stage 3: Reduction of carbon dioxide

This involves a cyclic chain reaction called the Calvin cycle. It occurs in the stroma of chloroplast where NADPH and ATP from stage 2 provide reducing power and energy for the synthesis of sugars.

1. In the first step, carbon dioxide combines with a 5-carbon organics compound called ribulose bisphosphate (RUBP) with the aid of an enzyme ribulose bisphosphate carboxylase giving unstable 6-carbon compound.
2. The unstable 6-carbon compound immediately splits into two molecules of 3-carbon compound glycerate-3-phosphate, (GP)
3. The GP is reduced to form a 3-carbon sugar, glyceraldehyde-3-phosphate (GALP). The hydrogen for reduction comes from NADPH which also supplies most of the energy, the rest coming from ATP.
4. The 3-carbon sugar is built into a 6-carbon sugar which may be converted into starch for storage.
5. Some of the 3-carbon sugar is used to regenerate ribulose bisphosphate to be used in the Calvin cycle.

ADAPTATIONS TO PHOTOSYNTHESIZE IN SUN AND SHADE

Shade plant	Sun plant
1. Abundant chlorophyll b (low chlorophyll a to chlorophyll b ratio) which gives leaves dark green color to increase light absorption in the dark;	1. Abundant chlorophyll a (high chlorophyll a to chlorophyll b ratio) to increase light absorption;
2. Palisade/ spongy mesophyll ratio low to allow maximum light penetration;	2. Palisade/ spongy mesophyll ratio high to minimize light penetration;
3. Mesophyll cell surface/leaf area ratio low to maximize light-trapping;	3. Mesophyll cell surface/leaf area ratio high to minimize excessive light and transpiration;
4. Leaf orientation horizontal to maximize light-trapping;	4. Leaf orientation erect to minimize light-trapping;
5. Reddish leaf undersides to enhance reflectance back up through the photosynthetic tissue; giving the plant a second chance to utilize the light.	5. Stomatal density high to avoid overheating;
6. Stomatal density low to avoid overcooling;	6. Much carotenoids prevent damage to chlorophyll from very bright light.
7. Thin leaves to maximise light penetration;	7. Thick leaves to minimise light penetration;
8. Stomatal size large to allow loss of excess water;	8. Stomatal size small to minimise water loss;
9. Elongated internodes for increased access to light;	Other features (i) RuBISCO and soluble protein content /mass higher (ii) Chlorophyll / soluble protein ratio high (iii) Chloroplast size small

C3 and C4 plants

In C3 plant such as tomato, and rice, carbon dioxide is fixed by ribulose bisphosphate and the first stable intermediate in the production of sugar is a 3-carbon compound, glycerate-3-phosphate. However, the C4 plants such as sugar cane and maize fix carbon dioxide using phosphoenol pyruvic acid (PEP) and the immediate products of CO₂ fixation is a 4-carbon compound oxaloacetic acid.

The oxaloacetic acid formed in C4 plants is subsequently converted into maleic acid from which CO₂ is fed into the Calvin cycle to form carbohydrates.

Advantages of C4 plants over C3 plants

1. C4 plants ably photosynthesize at very low CO₂ concentration (e.g. in dense tropical vegetation) because PEP carboxylase enzyme has a very high affinity for carbon dioxide.
2. Concentric arrangement of mesophyll cell produces a smaller area in relation to volume for better utilization of available water and reduces the intensity of solar radiations.
3. Photorespiration, which inhibits growth in C3 plants is avoided / reduced in C4 because

(i) the CO₂ fixing enzyme PEP carboxylase does not accept oxygen

(ii) RuBISCO enzyme inside the bundle sheath cells is shielded from high oxygen concentration by the ring of palisade cells.

4. The CO₂ fixing enzymes in C4 plants are more active at hot temperature and high illumination, therefore photosynthesis occurs rapidly at low altitude, hot and brightly lit tropical conditions than in C3 plants.

5. The productivity of C4 almost four times greater than in C3 because of the increased rate of CO₂ uptake caused by

(i) large internal leaf surface area

(ii) short CO₂ diffusion distance

(iii)CO₂ steep diffusion gradients

6.. The bundle sheath cells in which dark reactions occur have

(i) a large photosynthetic surface area enabled by un-usually large chloroplasts

 (ii) lack of grana on which O₂ would be produced, so no photo-respiration.

7. The palisade cells in which light reactions occur have large grana to increase the photosynthetic surface area.

Disadvantage of C4 plants over C3 plants

The CO₂ fixing enzymes in C4 plants are less active at cool, moist and low illumination conditions, therefore photosynthesis occurs slowly at high altitude with cool temperature and in low light intensity of temperate conditions.

NB: C4 plants grow better under hot, dry conditions when plants must close their stomata to conserve water – with stomata closed, CO₂ levels in the interior of the leaf fall, and O₂ levels rise

CAM Plant e.g. pineapple

These are C4 plants that live especially in arid environment. Because they are at risk of severe water loss, they have accustomed by opening their stomata at night. Thus as opposed to other C4 plants, in CAM plants, the initial fixation of CO₂ by PEP and the conventional Calvin cycle are separated in time. The stomata are generally closed during the day so there is little loss of water; the CO₂ is fixed during the night when the stomata are open.

Photorespiration

Photorespiration refers to the process whereby oxygen is added to ribulose bisphosphate (RuBP) thereby breaking it down into carbon dioxide and water.

The process thus uses up oxygen, leads to loss of carbon, like carbon dioxide, and energy.

However, no carbon dioxide is used up, it is instead produced.

Photorespiration occurs only in C3 plants due to high oxygen and low carbon dioxide partial pressure and / or high temperature. In such conditions, oxygen out-competes carbon dioxide for the enzyme RuBP carboxylase, which then takes up the oxygen and releases the fixed carbon dioxide.

Difference between C3, and C4 plant

	C3 pathway	C4 pathway
1	Photosynthesis occurs in mesophyll cells	Photosynthesis occurs in mesophyll cells and bundle sheath cells
2	The CO2 molecule acceptor is RuBP	The CO2 acceptor molecule is phosphenol pyruvate
3	The first stable product is a 3C compound Called Glycerate-3-phosphate (PGA)	The first stable product is a 4C compound oxaloacetic acid (OAA)
4	The photorespiration rate is high and leads to the loss of fixed CO2. Decrease CO2 fixation rate	Photorespiration is negligible and is almost absent. Hence high CO2 fixation rate
5	Optimum temperature is 200C to 250C	Optimum temperature is 30 to 450C.
6	Photosynthetic rate is low	Photosynthetic rate is high
7.	Examples of C3 plants are rice, wheat and tomato	Examples of C4 plants are maize, sugar cane

Definition

Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C₂ photosynthesis) refers to a process in plant metabolism where the  enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants.

Nutrition in bacteria

Bacteria are classified into 2 two groups

1. Autotrophic bacteria are classified according to source energy, carbon and hydrogen

No.	Nutritional type	Source of energy	Source of carbon	Source of hydrogen	examples
(a)	Photoautotrophs	Sunlight	CO2	H2S	Green sulphurbacteria
(b)	Photoautotrophs	sunlight	CO2	H2O	cyanobacteria
(c)	photoheterotroph	Light	Organic compounds	Organic compounds	Purple nonsulphur bacteria
2	heterotrophs	Organic compound	Organic compounds		Most bacteria

2. Heterotrophic bacteria derive energy and raw materials from already manufactures organic compounds.

Importance of bacteria

(a) Harmful effects

• Bacteria are agents of disease of animals and plants such as syphilis, cholera, tetanus
• Bacteria cause milk and food deteriorate
• Denitrifying bacteria such as Thiobacillus lead to deterioration of soil fertility.

(b) Useful effects

• Saprotrophic bacteria cause decay of waste materials and lead to the formation of humus and recycling of minerals. E.g. Bacteria (Nitrosomonas and nitrobacter) convert organically combined nitrogen (e.g. protein) to nitrate which are absorbed by plants.
• Nitrogen fixing bacteria e.g. Azobacter fix nitrogen into the soil.
• Aerobic and anaerobic bacteria are useful in digestion of sewage into harmless substances. Methane gas given off is sometimes used for fuel.
• Mutualistic bacteria in ruminants’ assist ruminants to digest cellulose, and synthesis of vitamin K and B group.
• Some bacteria on the skin of humans enable it to obtain protection again inversion of pathogenic organism
• Bacteria and fungi are useful for making cheese.
• Used for production of useful chemicals in genetic Engineering such as insulin

Effects of Light on organisms

1. Effect of light on protoplasm: sun rays may penetrate such covers and cause excitation, activa­tion, ionization, and heating of protoplasm of different body cells. Ultraviolet rays are known to cause mutational changes in the DNA of various organisms.
2. Effect of light on metabolism: The metabolic rate of diffe­rent animals is greatly influenced by light. The increased intensity of light results in an increase in enzyme activity, general metabolic rate, and solubility of salts and minerals in the protoplasm. Solubi­lity of gases, however, decreases at a high light intensity. Cave dwe­lling animals are found to be sluggish in their habits and to contain a slow rate of metabolism.
3. Effect of light on pigmentation: Light influences pigmenta­tion in animals. Cave animals lack skin pigments. If they are kept out of darkness for a long time, they regain skin pigmentation. The darkly pigmented skin of human inhabitants of the tropics also indicates the effect of sunlight on skin pigmentation. The skin pig­ment’s synthesis is dependent on the sunlight.

Light also determines the characteristic patterns of pigments of different animals that serve the animals in sexual dimorphism and protective coloration. Animals that dwell in the depths of the ocean where the environ­ment is monotone, though pigmented do not show patterns in their coloration.

4. Effect of light on animal movements:

Some animals move in response to light called phototaxis. Positively phototactic animals such as Euglena, move towards the source of light, while negatively phototactic animals such as planarians, earthworms, slugs move away from the source of light.

5.Growth response such as phototropism

6. Photoperiodism and biological clocks: Regularly occurring daily cycles of Light (day; and darkness (night) have been known to exert a profound influence on the behavior and metabolism of many organisms. Underlying such environmental rhythms of light and darkness are the movements of the earth relative to the sun and the moon.

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Compiled by Dr. Bbosa Science