BIOLOGY O LEVEL(FORM THREE) NOTES – COORDINATION -2

COORDINATION -2

Plants respond to a variety of stimuli in their environment. Unlike animals, plants cannot move from one place to another. However, they can move by forces of wind or water. Movement in plants in response to a stimulus is continuous and very slow. Movement of plants can be grouped into two:

• Growth movements
• Turgor movement

1. Growth Movement

These are the movements that take place in the meristematic regions due to unequal permanent growth. Growth movements can be classified into two categories namely:

• Autonomic movements
• Paratonic movements

(a) Autonomic Movements

These are self-controlled movements, for instance growth in the meristematic regions i.e. tips of stems and roots.

(b) Paratonic Movements

These are the plant movements induced by external stimuli. These stimuli include:

• Light
• Moisture
• Gravity
• Chemicals
• Touch

Paratonic movements include tropic and nastic movements.

Tropic Responses

These are growth movements that are caused by a wide range of stimuli. In this case, the plant grows either towards or away from the stimulus. If the response is towards the stimulus it is referred to as positive. If the response is away from the stimulus it is referred to as negative. Tropic movements are mediated through plant hormones. Tropisms are growth movements by plant organs in response to a unilateral stimulus, in which the direction of the movement is related to the direction of the stimulus.

Plant Hormones

The first plant hormones were discovered by a Dutch botanist Fritz Went in the year 1928. Fritz Went called these hormones auxin or more accurately, indoleacetic acid (IAA). This hormone has an extremely powerful effect on growth. Like animal hormones, plant hormones act in very low concentrations. A solution of 0.001 milligram in a litre of water applied to the side of a shoot is enough to cause bending.

Apart from auxins, plant hormones also include gibberellins and cytokinins. Auxins are synthesized from the amino acid tryptophan in meristematic tissues such as the shoot tips, buds, young leaves, and germinating seeds. Auxins increase cell wall elasticity by loosening the bond between the cellulose fibres. Auxins promote cell division, cell elongation, and cell differentiation.

Effects of Auxin Concentration on Growth

Experiments have revealed that higher concentrations of auxins stimulate growth in shoots while lower concentrations stimulate growth in roots. Amounts of auxins which stimulate shoot growth normally inhibit root growth.

Experiments demonstrating that a hormone regulates growth in shoots and roots.

Tropisms

A tropism is a movement by a plant organ in response to a unilateral stimulus, in which the direction of the movement is related to the direction of the stimulus. Tropisms are named according to the nature of the stimulus.

Types of Tropisms

(i) Geotropism

Geotropism is also known as gravitropism. This is the growth movement of plant parts in response to the direction of the force of gravity. The roots grow towards the direction of the force of gravity which means they are positively geotropic (gravitropic). The shoot grows away from the force of gravity which means it is negatively geotropic (gravitropic).

If a seedling is placed horizontally, the plumule will eventually grow vertically upwards while the radicle will grow vertically downwards. The above observation can be explained as follows:

• When the seedling is placed in a horizontal position, more auxin settles on the lower side of the root and shoots due to the pull of gravity.
• Shoots respond to a higher concentration of auxin than roots. In this case, the lower side of the shoot grows faster than the upper side, resulting in a growth curvature that makes the shoot grow vertically.
• Root growth is inhibited by high concentrations of auxins. Thus, the lower side of the root grows at a slower rate than the upper side where there is less auxin concentration. Consequently, this results in a growth curvature that makes the root grow vertically downwards.

The effect of gravity on the growth of roots and shoots.

(ii) Phototropism

This is the growth movement of plant organs in response to a unilateral source of light. In an experiment, it was revealed that auxins are directly involved in phototropism. If a shoot is exposed to light from one direction only, the shoot bends towards the source of light. Light causes an unequal distribution of the hormone (auxin). Light causes auxins to migrate to the darker side. In this case, the auxins are more concentrated on the darker side than on the side where the light is coming from. The cells on the dark side grow faster and elongate than the ones on the side where the light is coming from. As a result, the shoot bends towards light. Shoots are positively phototropic because they grow toward the light. Some roots grow away from light, which means they are negatively phototropic. However, many roots are not sensitive to light.

Effects of light on shoots.

(iii) Hydrotropism

This is the growth movement of plant organs in response to a unilateral source of water or moisture. In hydrotropism, the root grows toward the source of water, meaning the roots are positively hydrotropic. On the other hand, the shoot either grows away from the source of water, meaning it is negatively hydrotropic, or shows no response, meaning it is neutral.

Root is positively hydrotropic.

(iv) Thigmotropism

The term thigmo comes from a Greek word thigma meaning touch. Thigmotropism is also referred to as haptotropism. In plants such as Passiflora and Gloriosa with tendrils which curl around and cling to stems, auxins also play a major role. When climbing stems or tendrils come into contact with a hard object, the contact causes them to curve and coil around the hard object.

This is caused by the migration of the auxins from the point of plant contact and the hard object. In this case, the part in contact with the hard object has a lower auxin concentration than the outer part. Higher auxin concentration promotes faster growth in shoots. Therefore, greater auxin concentration in the outer part causes faster growth than the part in contact with the object, hence the shoot continues to round the object.

Thigmotropism.

(v) Chemotropism

This is the growth movement of plant organs in response to a unilateral source of chemicals. For instance, pollen tubes grow through the style towards the ovary and finally towards the ovules.

(vi) Thermotropism

This is the growth movement of plant organs in response to a unilateral source of heat as shown by movement of sunflower orienting itself towards the sun. However, there is an overlap between thermotropism and phototropism and sometimes a combination of both tropisms.

(vii) Rheotropism

This is the growth movement of plant organs in response to a unilateral source of air currents.

Importance of Tropisms

1. Phototropism: exposes the leaves of the plant to trap maximum sunlight for photosynthesis.
2. Haptotropism: enables plants with weak stems to obtain mechanical support.
3. Geotropism: enables the roots of the plant to grow deep in the ground to provide anchorage.
4. Chemotropism: enables the growth of the pollen tube in flowering plants to facilitate fertilization.
5. Hydrotropism: enables roots of the plant to obtain water.

Nastic Responses

These are non-directional movements of plant organs in response to diffuse stimuli, such as folding of leaves in warm weather, opening and closing of flowers in response to intensity of light and the closing of leaves when touched. Such movements occur as a result of changes in turgor pressure in certain cells.

Types of Nastic Responses

(a) Nyctinasty

This is a plant movement in response to temperature changes. This is a thermostatic movement; therefore nyctinasty is referred to as thermonasty.

(b) Photonasty

This is a plant movement in response to a change in light intensity. Some flowers in certain plants open in presence of light and close in its absence.

(c) Seismonasty

This is plant movement in response to shock or vibration.

(d) Hydronasty

This is plant movement in response to changes in atmospheric humidity.

(e) Haptonasty

This is plant movement in response to contact. The sensitive plant Mimosa pudica responds to touch by folding up its leaves.

(f) Chemonasty

This is a plant movement in response to chemical stimuli.

Tactic Movement

This is the movement of whole organism in response to an external stimulus. If the movement is toward stimulus the tactic is positive, when the movement is away from the stimulus, the tactic is negative. Tactic movement is known as taxis.

Types of Tactic Movement

1. Phototaxis – locomotory response to light
2. Chemotaxis – locomotory response to chemical
3. Aerotaxis – locomotion response to variation in oxygen concentration
4. Rheotaxis – locomotory response to direction of water current
5. Magnetotaxis – locomotory response to magnetic field
6. Osmotaxis – locomotory response to variations in osmotic pressure
7. Thermotaxis – locomotory response to temperature changes

Other Effects of Auxins

(a) Apical Dominance

This refers to the inhibition of lateral bud development by the terminal bud. If the terminal bud is removed, lateral buds develop into side branches. This is because when the apical bud is cut and removed, the apical dominance is reduced. However, if the apical bud is cut and then a substance containing auxin is applied to the cut end, lateral buds do not sprout or develop. This experiment clearly indicates that apical dominance is brought about by auxins. The principle of apical dominance is applied in pruning. Removal of the terminal bud encourages the sprouting of side branches causing the plant to grow sideways instead of upwards.

(b) Development of Adventitious Roots

Adventitious roots are the roots that develop from a stem cutting. Plant cuttings which do not develop roots readily may be dipped in rooting auxins e.g. Indole Butyric Acid (IBA) and Naphthalene Acetic Acid (NAA).

(c) Storage

NAA is used to increase the period of dormancy in tubers and bulbs so that they can be stored for a longer period of time.

(d) Parthenocarpy

This is the formation of fruits without fertilization. Parthenocarpy can be induced by treating unpollinated flowers with auxin. This phenomenon is applied in the development of seedless fruit varieties.

(e) Falling of Leaves and Fruits

Falling of leaves and fruits is brought about by a reduction in the concentration of auxins. Premature falling of fruits occurs due to the failure of the plant to produce adequate amount of auxins. This situation can be reversed by application of auxins.

(f) Weed Killer

In higher concentrations, auxins interfere with normal plant growth and can cause death. In this case auxins are used as herbicides or selective weed killers. For instance 2,4-dichlorophenoxyacetic acid (2,4-D) can be used as a weed killer (herbicide) killing broad-leaved plants.

Other Plant Hormones

(a) Gibberellins

These are a mixture of chemical compounds which have an effect on plant growth. A common example of gibberellins is Gibberellic acid. Gibberellic acid causes stem elongation in plants. It stimulates rapid growth in dwarf varieties of certain plants by increasing the length of the internodes. Also used in breaking seed dormancy and inducing parthenocarpy.

(b) Ethylene

Speeds up ripening of fruits such as citrus.

(c) Abscisic Acid (ABA)

Regulates fruit drop at the end of the season.

(d) Cytokinins

These are active growth substances which promote growth in plants in the presence of auxins. Cytokinins promote cell division by inducing growth of roots, leaves, callus tissue and repair of wounds in plants.

Phytochromes

These are pale blue-green compounds consisting of a pigment, which absorbs light energy. Phytochrome exists in two interconvertible forms. One absorbs red light at a wavelength of 665 nm while the other one absorbs far red light at a wavelength of 725 nm. These two forms of phytochrome are designated as Pr and Pfr respectively. When Pr absorbs red light it is rapidly converted into Pfr and when Pfr absorbs far-red light it is rapidly converted into Pr.

The two phytochromes, that is Pr and Pfr have the following effects:

1. Elongation of the stem is stimulated by far-red light but inhibited by red light.
2. Leaf expansion is stimulated by red light but inhibited by far-red light.
3. Lateral root growth is stimulated by far-red and inhibited by red light.
4. Seed germination is stimulated by red light but inhibited by far-red light.

Photoperiodism

This is a flowering response in plants relative to lengths of day and night. When a plant is exposed to light, phytochrome absorbs light energy and Pfr accumulates. Pfr initiates the formation of a flowering hormone known as florigen, which is transported to the stem apices to promote flowering.

With reference to photoperiodism, plants can be classified into three groups:

• Short day plants
• Long day plants
• Day neutral plants

Short-day Plants

These are the plants that require short-length illumination but longer night periods to flower. Examples include chrysanthemum and poinsettias.

Long-day Plants

These are the plants that require longer day-length illumination but shorter night periods in order to flower. Examples include wheat and lettuce.

Day-neutral Plants

These are the plants that flower irrespective of day-length or night periods. Examples of day-neutral plants include cotton and tomatoes.

EXCRETION

This is a process of getting rid of waste products from the body of living organisms formed during metabolic processes. Metabolic processes include all chemical reactions taking place inside living systems. Example: respiration.

During the process of respiration, carbon dioxide is one of the products. Therefore, carbon dioxide is known as an excretory product and the organs that get rid of them are called excretory organs.

Example

IMPORTANCE OF EXCRETION

1. It is important that all unwanted products be removed from the body of a living organism, because if they are allowed to remain in the body, they would soon become harmful and poisonous to the living.
2. Sometimes materials that are taken into the body from outside may be in excess of what is required. If so, they will have to be removed as waste. E.g. proteins.
3. In some cases, excretory products undergo detoxification in order to make them less toxic to the organism before they are removed from the body.

EXCRETION IN UNICELLULAR ORGANISMS

Unicellular or single-celled organisms such as amoeba and paramecium get rid of their waste products simply by diffusion through the surface of their bodies.

These waste substances diffuse from cytoplasm where they are at high concentration to outside of the body where concentration is low. Another method of excretion is by use of contractile vacuole.

EXCRETION IN HIGHER ANIMALS

Excretion in higher animals is carried out by an elaborate system made up of specialized tissue and organs. This is because their bodies are complex and have a greater number of cells such that simple diffusion will not suffice.

FORMS OF WASTE PRODUCTS IN ANIMALS

1. Nitrogenous Waste Products

Excess amino acids/proteins cannot be stored in the body; instead, they are broken down to form ammonia. Nitrogenous waste products can be removed in 3 forms:

a. Ammonia

Ammonia is highly poisonous and dissolves in water; it is removed in soluble form. It can be rapidly and safely removed if diluted in a sufficient volume of water. E.g. fish.

b. Urea

Ammonia with carbon dioxide forms a less toxic form of waste product. Urea is formed in the liver and is soluble in water. Urea is excreted by aquatic mammals and terrestrial animals e.g. man.

c. Acidic Urea/Uric Acid

Ammonia is also excreted as uric acid; uric acid is insoluble and non-toxic. This is excreted by animals living in shortage of water e.g. insects, birds, and reptiles. Uric acid is excreted in the form of crystals.

2. Carbon Dioxide

Carbon dioxide is produced during respiration. It is excreted through gaseous exchange.

3. Excess Water

Excess metabolic water from chemical breakdown of glucose is lost either as water vapour, sweat, or urine.

THE KIDNEY

These are dark red bean-shaped organs located at the back of the abdominal cavity. There are two kidneys in the human body: the right kidney and the left kidney. Above each kidney are adrenal glands which secrete hormones that stimulate reabsorption of sodium ions. There are two blood vessels connected to the kidney: one is the RENAL ARTERY, which supplies blood to the kidney, and the other is the RENAL VEIN, which takes blood away from the kidney.

A tube called the URETER runs from each kidney to the bladder. Urine passes through the ureter from the kidney to be stored in the bladder. From there, it is released periodically through a tube called the URETHRA.

When the bladder is nearly full, the stretching stimulates sensory nerve endings in its wall so that nerve impulses are relayed to the brain and the urge to urinate develops. The sphincter muscle located at the base of the bladder relaxes and the urine is released via the urethra. This tube is contained within the penis in mammals.

COMPOSITION OF KIDNEY

The kidney is composed of three regions namely:

1. Cortex
2. Medulla
3. Pelvis

INTERNAL STRUCTURE OF MAMMALIAN KIDNEY

i) Cortex

Is the outer zone which is dark in colour and contains a dense network of blood capillaries that form the glomeruli of the nephron, which are the functional units of the kidney.

ii) Medulla

This part lies between the cortex and the pelvis. The surface of the medulla facing the pelvis is folded to form projections called PYRAMIDS.

iii) Pelvis

Pelvis narrows to form the ureter. Pelvis is a collecting space leading to the ureter which takes the urine to the bladder.

THE NEPHRON

The nephron is the functional unit of the kidney. The nephron performs both functions of OSMOREGULATION and EXCRETION (Osmoregulation – maintains constant osmotic pressure of body fluids).

Each nephron consists of a long tubule closed at one end and open at the other.

The Nephron is divided into four parts:

1. Bowman’s capsule
2. Proximal convoluted tubule
3. Loop of Henle
4. Distal convoluted tubule

i) Bowman’s Capsule

This is a round cup-shaped part of the closed end of the tubule and encloses the glomerulus, which is a network of blood capillaries. The glomerulus is formed from the afferent blood vessels, a branch from the renal artery.

ii) Proximal Convoluted Tubule

Is the coiled part of the tubule next to the Bowman’s capsule. It lies in the cortex.

iii) Loop of Henle

The portion of the tubule which extends from the proximal convoluted tubule and dips into the medulla, from the medulla it bends back into the cortex to form a U-shaped loop.

iv) Distal Convoluted Tubule

This is the coiled part next to the open end of the tubule which joins with a collecting duct (ureter). The whole length of the nephron is surrounded by a network of capillaries.

MECHANISMS OF EXCRETION

Excretion takes place in three types:

1. Filtration
2. Reabsorption
3. Removal

i) Filtration

Kidney receives blood at high pressure through renal artery (branch of Aorta).

• The blood is rich in nitrogenous waste such as urea, dissolved food substances, plasma, protein, mineral ions, hormones, and oxygen.
• The afferent vessels entering the glomerulus are wider than the efferent vessels leaving the glomerulus. The narrowness of the efferent vessels produces resistance to blood flow and thus creates pressure in the glomerulus.
• Due to high pressure in the glomerulus, the liquid part of the blood and dissolved substances of small molecular sizes are forced out of the glomerulus into the Bowman’s capsule (urea, glucose, salt, and amino acids).
• Large sized molecules such as proteins and blood cells are not filtered because the walls of the capillaries of glomerulus and Bowman’s capsule have very small pores.

Hence the blood which remains is rich in plasma and has very little water.

This process is known as ultrafiltration and the filtrate formed is called glomerular filtrate.

ii) Reabsorption

As the glomerular filtrate moves along tubules, useful substances are selected and reabsorbed back into the blood.

Most of the reabsorption occurs in the proximal convoluted tubule through the process of active transport.

For efficient reabsorption of substances, the proximal convoluted tubule is adapted in several ways:

• The cells of the tubule have mitochondria which provide necessary energy in form of ATP.
• The cells have microvilli to increase surface area for reabsorption.
• The tubule is long and coiled to provide large surface area for reabsorption.
• Well supplied with blood capillaries.
• The tubule is coiled to slow down the speed of the flow of the filtrate.

COMPLICATIONS AND DISORDERS OF EXCRETORY SYSTEM

URINARY SYSTEM

Certain disorders of the body can be diagnosed by examining the contents and measuring the quantity of urine.

1. For example, if the urine contains glucose then this indicates the disease DIABETES MELLITUS.

– Diabetes Mellitus occurs because the pancreas does not produce enough of the hormone (insulin) which controls blood sugar level.

– It can be treated with injections of insulin.

2. Another type of diabetes is DIABETES INSIPIDUS, which results from large quantity of dilute urine being produced.

– This happens because the sufferer cannot produce enough Anti-Diuretic Hormone (ADH), which is responsible for regulation of the amount of water in the blood.

• Diabetes insipidus can be treated by nasal sprays which contain ADH.

NEPHRITIS

Nephritis is the general term for any infection or inflammation of the kidney.

• In one type, the glomeruli fail to function normally and allow protein to filter through into the tubules.
• It is diagnosed by the presence of protein in the urine.

Causes of Nephritis

1. Could be allergic reaction.
2. It may be blood vessel disorders or high blood pressure.
3. Damage of the kidney.

Treatment

• It is dependent on the cause.

KIDNEY STONES

Small stones can sometimes be formed in the pelvic region of the kidney.

– These stones may be made of:

• Uric acid
• Calcium oxalate or (mixture of calcium)
• Magnesium
• Ammonium phosphate

– They form as a result of obstruction of urine flow or excess of certain chemicals in the bloodstream.

– There are often no symptoms of kidney stones unless stones move from their original position.

– If a stone moves into the ureter, it causes severe pain (renal pain) which can be felt in the lower back to the groin, accompanied by vomiting and sweating.

– There may also be blood in the urine.

Treatment

1. Can be treated by X-rays; some small stones may pass down the ureter and out through the bladder without the need for treatment.
2. Larger stones may have to be removed surgically or they can be shattered into fragments by treatment.
3. The small fragments are passed out harmlessly in the urine.

NB: An untreated kidney stone may obstruct urine and lead to nephritis.

CYSTITIS

Cystitis is an inflammation of the bladder caused by infection.

Symptoms

1. Frequent painful urination and blood in urine.
2. Slight increase in the frequency of urination accompanied by a burning sensation.
3. If the infection spreads to the kidney, it may cause fever, blood in urine, and backache.

Causes

1. It may be caused by bacterial infection of the bladder usually from the urethra.

KIDNEY FAILURE

Kidney failure is a condition where one or both kidneys cease to function.

• If it happens to both kidneys it is fatal if not treated.
• It can happen suddenly as a result of high blood pressure.
• It is possible to live with one kidney only, but the only treatment for failure of both kidneys is DIALYSIS or a KIDNEY TRANSPLANT.
• In dialysis, the patient’s blood supply is linked to a kidney machine. The machine performs the functions of a normal kidney by filtering the blood and removing excess salts and water.
• The patient spends several hours in a week linked to the machine.
• A kidney transplant involves surgically inserting a healthy kidney from a donor to replace a diseased kidney.
• The kidney has to be compatible to avoid problems with rejection.
• Sometimes a healthy person will donate a kidney to save the life of a close relative suffering from kidney failure.

EXCRETION IN PLANTS

• Plants manufacture all their organic needs according to demand.
• They only make as much protein as they need at any one time, for example.
• Therefore, they do not excrete urea from excess amino acids because they do not usually have any.
• They do, however, respire and photosynthesize and as a result produce waste products.
• However, waste products of one process may be the raw material for the other e.g. carbon dioxide and waste produced by respiration are recycled during daylight hours when light intensity is usually high enough to produce oxygen faster than its use by respiration.
• Therefore, the oxygen is not used up by respiration and the excess is excreted.
• Some plants produce tannins and other organic acids from nitrogen and carbohydrate metabolism.
• These are passed into leaves where they build up and are lost from the plant when the leaves fall off.
• Bitter substances such as tannins and other organic acids have a protective role in deterring leaf-eating animals from feeding on the plants.
• Trees produce various gums, resins, and latexes which can be collected from the tree and have a wide range of industrial uses.
• Products such as turpentine, paints, varnishes, soap, cosmetics, food, surgical items, golf balls, bubble gum, and rubber are manufactured from these plant products.

Mechanisms through which plants remove their waste products

1. Diffusion
2. Abscission
3. Degradation