This page contains the GCSE AQA Biology Hormonal Coordination Questions and their answers for revision and understanding Hormonal Coordination.

B11: GCSE AQA Biology Hormonal Coordination

B11.1 Principles of Hormonal Control

Page no. 161

Intext Questions:

1 (a)

hormone is any member of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behavior.

(b)

Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct.

Nervous control is very rapid and integral for immediate action, coordination and control, and receiving and interpreting messages from your environment. The rapid control is achieved via a network of neurons in the central and peripheral nervous systems (CNS and PNS).

To understand rapid control you must understand how the neurons function. The neurons connect onto each other and a rapid electrical impulse is transmitted from one to the other (the basis of which is ion channels opening and closing and changing the potential difference across the membrane of the neuron relative to its surroundings).

These events occur speedily and efficiently- going from sensory neurons in your skin (all part of the PNS) or sense organs straight to the CNS (brain and spinal cord). There the information is processed, and sent back along motor neurons (also in the PNS) to the effectors (usually muscles, but can be glands or organs) in order to elicit a response from them. This occurs in a matter of milliseconds.

On the other hand, the endocrine system controls functions which take much longer to come into effect. This includes growth and development, homeostasis (which is keeping the systems balanced in the body), and signalling for longer term responses. You do not grow all at once, or reach puberty in a millisecond.

The way in which the endocrine system controls functions over a long period pf time is via the release of specific hormones from specific glands and organs which target the cells or organs for which the effect is meant.

The types of functions controlled by the endocrine system include maturation of an individual during puberty, milk production and release in female mammals, blood glucose and sodium levels, and growth of all tissues approaching, during, and after puberty.

The pituitary controls the function of most other endocrine glands and is therefore sometimes called the master gland. In turn, the pituitary is controlled in large part by the hypothalamus, a region of the brain that lies just above the pituitary. By detecting the levels of hormones produced by glands under the pituitary’s control (target glands), the hypothalamus or the pituitary can determine how much stimulation the target glands need.

4(a)

pituitary gland doesn’t produce enough growth hormone, growth can slow. A lack of growth hormone causes growth hormone deficiency. This can cause a child to be very short to have very low bone density and muscle strength.

(b)

In adults, excessive growth hormone for a long period of time produces a condition known as acromegaly, in which patients have swelling of the hands and feet and altered facial features. These patients also have organ enlargement and serious functional disorders such as high blood pressure, diabetes and heart disease. Over 99% of cases are due to benign tumors of the pituitary gland, which produce growth hormone. This condition is more common after middle-age when growth is complete so affected individuals do not get any taller.

B11.2 The Control of Blood Glucose Levels

Page no. 163

1 Define the terms:

(a)

hormone produced in the pancreas by the islets of Langerhans, which regulates the amount of glucose in the blood. The lack of insulin causes a form of diabetes.

(b)

a disease in which the body’s ability to produce or respond to the hormone insulin is impaired, resulting in abnormal metabolism of carbohydrates and elevated levels of glucose in the blood

(c)

Polysaccharide that is the chief carbohydrate storage material in animals, being converted to glucose by depolymerization; it is formed by and largely stored in the liver, and to a lesser extent in muscles, and is liberated as needed.

2(a)

In response to an increase in blood glucose level above the normal level, the pancreas produces a hormone called insulin which is released into the bloodstream. Insulin causes glucose to move from the blood into cells, where it is either used for respiration or stored in liver and muscle cells as glycogen.

(b)

When blood sugar drops too low, the level of insulin declines and other cells in the pancreas release glucagon, which causes the liver to turn stored glycogen back into glucose and release it into the blood.

(c)

Many people who live with diabetes don’t feel any particular symptoms, unless they are experiencing hyperglycemia (glucose level is too high) or hypoglycemia (glucose level is too low). Hyperglycemia can cause significant damage to some organs, which then leads to complications of diabetes. These include:

cardiac or vascular event, such as myocardial infarction (heart attack) or stroke;
kidney problems that may require dialysis;
eye problems, which may lead to loss of vision (blindness);
sexual issues, such as erectile dysfunction;
problems with circulation and scarring, which can lead to amputation.

To avoid the complications of diabetes, you must control your blood glucose very well to minimize the risk of hyperglycemia. This will allow you to prevent the complications of diabetes.

differences between type 1 and type 2 diabetes
Type 1 Diabetes	Type 2 Diabetes
Often diagnosed in childhood	Usually diagnosed in over 30 year olds
Not associated with excess body weight	Often associated with excess body weight
Often associated with higher than normal ketone levels at diagnosis	Often associated with high blood pressure and/or cholesterol levels at diagnosis
Treated with insulin injections or insulin pump	Is usually treated initially without medication or with tablets
Cannot be controlled without taking insulin	Sometimes possible to come off diabetes medication

Insulin and glucagon have both similarities and differences. Both are hormones secreted by the pancreas but they are made from different types of cells in the pancreas. Both help manage the blood glucose levels in the body but they have opposite effects. Both respond to blood glucose levels but they have opposite effects.

During digestion, foods that contain carbohydrates are converted into glucose. Most of this glucose is sent into your bloodstream, causing a rise in blood glucose levels. This increase in blood glucose signals your pancreas to produce insulin.

The insulin tells cells throughout your body to take in glucose from your bloodstream. As the glucose moves into your cells, your blood glucose levels go down. Some cells use the glucose as energy. Other cells, such as in your liver and muscles, store any excess glucose as a substance called glycogen. Your body uses glycogen for fuel between meals.

Glucagon works to counterbalance the actions of insulin.

About four to six hours after you eat, the glucose levels in your blood decrease, triggering your pancreas to produce glucagon. This hormone signals your liver and muscle cells to change the stored glycogen back into glucose. These cells then release the glucose into your bloodstream so your other cells can use it for energy.

This whole feedback loop with insulin and glucagon is constantly in motion. It keeps your blood sugar levels from dipping too low, ensuring that your body has a steady supply of energy.

B11.3 Treating Diabetes

Page no. 165

Question:

Differences between type 1 and type 2 diabetes
Type 1 diabetes	Type 2 diabetes
Symptoms usually start in childhood or young adulthood. People often seek medical help, because they are seriously ill from sudden symptoms of high blood sugar.	The person may not have symptoms before diagnosis. Usually the disease is discovered in adulthood, but an increasing number of children are being diagnosed with the disease.
Episodes of low blood sugar level (hypoglycemia) are common.	There are no episodes of low blood sugar level, unless the person is taking insulin or certain diabetes medicines.
It cannot be prevented.	It can be prevented or delayed with a healthy lifestyle, including maintaining a healthy weight, eating sensibly, and exercising regularly.

2 (a)

For type 1 diabetes, insulin has long been the essential treatment method. Blood glucose monitoring, frequent insulin injections, even insulin pumps are used to help diabetics control their glucose levels and avoid dangerous spikes and dips in their blood sugar. it is found that transplanting purified human pancreatic islet cells into type 1 diabetics can lead to nearly normal glycemic control and no longer being reliant on insulin.

(b)

People with type 2 diabetes don’t respond normally to insulin anymore, so glucose stays in the bloodstream and doesn’t get into the cells. This causes blood glucose levels to go too high.

4
High blood sugar levels can make teens with type 2 diabetes feel sick, so their treatment plan involves keeping their blood sugar levels within a healthy range while making sure they grow and develop normally. To do that, they need to:

eat a healthy, balanced diet and follow a meal plan
get regular exercise
take medicines as prescribed
check blood sugar levels regularly

The good news is that sticking to the plan can help people feel healthy and avoid diabetes problems later.

B11.4 The Role of Negative Feedback

Page no. 167

Negative feedback is a regulatory mechanism in which a ‘stimulus’ causes an opposite ‘output’ in order to maintain an ideal level of whatever is being regulated.

There are many negative feedback pathways in biological systems, including:

Temperature regulation
Blood pressure regulation
Blood sugar regulation
Thyroid regulation
Photosynthesis in response to increased carbon dioxide
Predator/prey population dynamic

Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. Thyroid hormones play vital roles in regulating the body’s metabolic rate, heart and digestive functions, muscle control, brain development and maintenance of bones.

This hormone production system is regulated by a negative feedback loop so that when the levels of the thyroid hormones, thyroxine and triiodothyronine increase, they prevent the release of both thyrotropin-releasing hormone and thyroid stimulating hormone. This system allows the body to maintain a constant level of thyroid hormones in the body.

4 (b)

Adrenaline is a hormone released from the adrenal glands and its major action, together with noradrenaline, is to prepare the body for ‘fight or flight’.

Adrenaline is released mainly through the activation of nerves connected to the adrenal glands, which trigger the secretion of adrenaline and thus increase the levels of adrenaline in the blood. This process happens relatively quickly, within 2 to 3 minutes of the stressful event being encountered. When the stressful situation ends, the nerve impulses to the adrenal glands are lowered, meaning that the adrenal glands stop producing adrenaline.

B11.5 Human Reproduction

Page no. 169

Hormones are the drivers of human reproduction, responsible

For secondary sexual development (puberty)

controlling the menstrual cycle.

Oestrogen in female produced by ovary. Testosterone in male produced by testes and stimulates sperm production

Males and females both grow body hair in their pubic area, as well as under the arms and on the legs.
Boys and girls both may put on weight, although in different places
Both boys and girls as they deal with their changing bodies and may experience feelings of awkwardness and embarrassment.
Although boys and girls may be affected differently, feelings such as confusion, fatigue and lack of emotional control are common to both genders.

Differences between boys and girls at puberty

The physical changes for females during puberty experience are marked by the following features of growth:

Puberty in girls is marked by the start of the menstruation cycle, commonly referred to as periods.
A major physical development is that of the breast that grows during the teenage years and attains full growth by the age of 18 years.
Hair growth in the pubic area and the armpits are also observed. The normal cycle of hair growth that is seen in adults is reached by the average age of 14 years.
The adolescent years are also marked by a rapid growth spurt. In girls this growth spurt starts at the age of 11 or usually around the time she reaches menarche and slows down by the age of 16.

The physical changes during puberty for males that occur are different in a number of ways. The developments that a boy undergoes during adolescence are:

In boys, the scrotum and the testicles start to grow when they reach puberty.
The penis also increase in length and reaches the proper adult size and shape by the age of 17 or 18.
There is a hair growth observed in the pubis area, armpits as well as the chest and the face. This usually starts around the age of 12; by the time the boy reaches 18 years of age, the pattern of hair growth resembles those of adults.
The growth spurt starts at about 13 years and continues on to about 18 years of age. After that, the growth slows down.
Another physical change that is observed in adolescent boys is the change in their voices. Their vocal cords grow and as a result, the voice pitch changes into a heavier tone.
Physical development and changes at such a sudden and fast rate is quite something for the adolescent kid to deal with. This is the time when the child is in constant need of support and care.

The menstrual cycle is an approximately 28-day cycle which results in the release of a mature egg from the ovary. This egg may then go on to become fertilized or may be released, unfertilized, along with the lining of the uterus. The latter may take place with monthly bleeding called menstruation — day 1 of the 28-day cycle. This delicate cycle results from a complicated interplay among several hormones.

Follicle-Stimulating Hormone (FSH)

This hormone stimulates the development of new follicles as well as the production of the hormone estrogen. During this phase, called the follicular phase of the menstrual cycle, an increase in FSH occurs. This increase stimulates the growth and development of new follicles, one of which will develop into the ovulated egg.

Estrogen

Estrogen is responsible for the continuing development of follicles within the ovaries. However, the effects of estrogen are not limited to within the ovaries. In the uterus, the rising levels of this hormone play an important role in thickening the endometrium — a layer of the uterus. It also causes the mucus within the cervix to become thicker. Finally, estrogen release acts as a suppressor of its own release — called a negative feedback loop. It also acts to suppress the production of LH, until just before ovulation. Afterward, estrogen actually stimulates the release of large amounts of LH in what is called the mid-cycle LH surge.

Luteinizing Hormone (LH)

LH peaks in the middle of the 28-day cycle. This is typically called the LH surge and serves as a signal that ovulation — the release of the mature egg from one of the two ovaries — is about to occur. During this peak of LH release, concentration of this hormone becomes ten times higher than usual. Ovulation generally occurs within 9 hours of the LH surge. The egg releases from the ovary, able to be fertilized for about 1-2 days after it releases. If it does not become fertilized, it begins to disintegrate or releases along with the inner lining of the uterus as part of the monthly menstruation cycle.

Progesterone

Once ovulation has occurred, the hormone progesterone releases from a structure called corpus luteum. Progesterone makes the mucus around the entrance of the uterus thick and sticky, preparing for a potential pregnancy. If the released egg becomes fertilized, it will become implanted in the wall of the uterus and the fetus will begin to grow.

Size

Egg: One of the largest cells in the female body.

Sperm: One of the smallest cells in the female body.

Produced in

Egg: Produced in the ovary of a female.

Sperm: Produced in the Testicles of a male.

Appearance

Egg: Round shaped and consists of a large amount of cytoplasm before conception.

Sperm: Oval head on the top and has a rigid middle with a slimly tail in the back to allow it to swim.

Amount

Egg: Only a single egg cell is produced during once menstruation cycle.

Sperm: Millions of cells are released during a single ejaculation.

Temperature

Egg: Requires warm body temperature for sustenance.

Sperm: Requires approximately two degrees lesser than body temperature.

Life-Span

Egg: Have a short life span 12-24 hours.

Sperm: Longer life span surviving for 3-5 days.

Storability

Egg: Cannot be stored.

Sperm: Can be frozen and stored.

B11.6 Hormones and the Menstrual Cycle

Page no. 171

The four hormone that control the menstrual cycle are:

FSH:

Cause eggs to mature
Stimulate the ovary to produce Oestrogen

LH:

Triggers ovulation

OESTROGEN:

Cause the linning of the uterus to develop
Inhibits the release of FSH
Stimulate the release of LH

PROGESTERONE:

Maintains the lining of the uterus
Inhibits the release of both FSH and LH.

The interaction of four hormones control the maturing and release of an egg from the ovary and the buildup of lining of the uterus in the menstrual cycle

2 (a)

On 28^th day women have menstrual cycle

(b)

LH and FSH are the hormones that encourage ovulation. Both LH and FSH are secreted by the pituitary gland in the brain. At the beginning of the cycle, LH and FSH levels usually range between about 5-20 mlU/ml. Most women have about equal amounts of LH and FSH during the early part of their cycle. However, there is a LH surge in which the amount of LH increases to about 25-40 mlU/ml 24 hours before ovulation occurs. Once the egg is released by the ovary, the LH levels goes back down.

(c)

Oestrogen hormone control the buildup of lining of uterus

The menstrual cycle is a recurring process which takes around 28 days. During the process, the lining of the uterus is prepared for pregnancy. If pregnancy does not happen, the lining is then shed. This is known as menstruation.

The length of menstrual cycle has been assumed to be 28 days (which is the average among women). The entire duration of a Menstrual cycle can be divided into four main phases:

Menstrual phase (From day 1 to 5)
Follicular phase (From day 1 to 13)
Ovulation phase (Day 14)
Luteal phase (From day 15 to 28

Menstrual phase (day 1-5)

Menstrual phase begins on the first day of menstruation and lasts till the 5th day of the menstrual cycle. The following events occur during this phase:

The uterus sheds its inner lining of soft tissue and blood vessels which exits the body from the vagina in the form of menstrual fluid.

Follicular phase (day 1-13)

This phase also begins on the first day of menstruation, but it lasts till the 13th day of the menstrual cycle. The following events occur during this phase:

The pituitary gland secretes a hormone that stimulates the egg cells in the ovaries to grow.
One of these egg cells begins to mature in a sac-like-structure called follicle. It takes 13 days for the egg cell to reach maturity.
While the egg cell matures, its follicle secretes a hormone that stimulates the uterus to develop a lining of blood vessels and soft tissue called endometrium.

Ovulation phase (day 14)

On the 14th day of the cycle, the pituitary gland secretes a hormone that causes the ovary to release the matured egg cell. The released egg cell is swept into the fallopian tube by the cilia of the fimbriae. Fimbriae are finger like projections located at the end of the fallopian tube close to the ovaries and cilia are slender hair like projections on each Fimbria.

Luteal phase (day 15-28)

This phase begins on the 15th day and lasts till the end of the cycle. The following events occur during this phase:

The egg cell released during the ovulation phase stays in the fallopian tube for 24 hours.
If a sperm cell does not impregnate the egg cell within that time, the egg cell disintegrates.
The hormone that causes the uterus to retain its endometrium gets used up by the end of the menstrual cycle. This causes the menstrual phase of the next cycle to begin.

Several hormones control this cycle – for example, they are involved in controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining.

Hormone	Produced	Role
FSH (follicle stimulating hormone)	Pituitary gland	Causes an egg to mature in an ovary. Stimulates the ovaries to release oestrogen
Oestrogen	Ovaries	Stops FSH being produced (so that only one egg matures in a cycle). Repairs and thickens the uterus lining. Stimulates the pituitary gland to release LH.
LH (luteinising hormone)	Pituitary gland	Triggers ovulation (the release of a mature egg)
Progesterone	Ovaries	Maintains the lining of the uterus during the middle part of the menstrual cycle and during pregnancy.

Hormone levels during the menstrual cycle
If a woman becomes pregnant, the placentaproduces progesterone. This maintains the lining of the uterus during pregnancy and means that menstruation does not happen.

B11.7 The Artificial Control of Fertility

Page no. 173

Contraception (birth control) prevents pregnancy by interfering with the normal process of ovulation, fertilization, and implantation. There are different kinds of birth control that act at different points in the process.

2 (a)

All the form of contraceptive uses the progesterone.

(b)

The pill uses hormones to prevent pregnancy. The combination pill contains estrogen and progestin. Birth control pills prevent pregnancy by stopping your ovaries from releasing an egg each month. The hormones thicken the cervical mucus, which makes it harder for sperm to swim to the egg. The hormones also alter the lining of the uterus, so that if an egg does get fertilized, it will be unable to implant in the uterus.

Contraceptive

The patch contains the same hormones as the pill, estrogen and progestin. The patch works just like the pill. The hormones prevent an egg from being released and change both the cervical mucus and uterine lining..

The implant is made of medical plastic that is sterile and soft. This contraceptive rod is 40mm (1.5 inches) long and 2mm (0.08 inches) in diameter. The implant, once inserted, is effective for a maximum of three years. Once the birth control implant is inserted, it begins releasing small doses of the synthetic progesterone. it is released slowly and steadily over the course of the three year period.

(c)

Contraceptive implant is most effective of all of these because the contraceptive implant can last up to three years. A tiny tube is inserted under the skin by the doctor and slowly release progesterone. This 99.95% effective

Birth Control Methods Comparison Charts

The Implant :-Provides protection from pregnancy for up to 3 years. It is 99.9% effective.
-Inserted and removed by a healthcare professional so leaves little room for error.
-Remains in place; allows sexual spontaneity.
-Easily reversible. Most females can return to fertility within 3 months of removing the implant.
-It is safer than the pill for females who are older than 35, smoke, have high blood pressure, are overweight or have a history of blood clots.
-Does not have the estrogenic side effects of combination pills (including nausea and vomiting, abdominal pain, back pain and decreased vaginal lubrication.)
-Most females will experience shorter and lighter menstrual periods within the first year of use.
Gives females control over their own pregnancy prevention.

Barrier method:

Abstinence

Most effective method of STI prevention if oral, anal and vaginal sex are all abstained from. To prevent STI transmission, latex condoms and dental dams should be used every time the user decides to engage in oral, anal and vaginal sex, even if penetration does not occur. It is 100% effective.
Can still get sexual pleasure through masturbation.
Can focus on alternate activities with your partner.

Surgical method:

Surgical “irreversible” methods of contraception.

These include:

Female Sterilisation: Tubal Ligation, With this, the fallopian tubes are cut each side, and a small piece is taken out, making it impossible for the egg and sperm to meet. This is done in theatre and is more complicated than a Vasectomy. It’s very difficult to reverse, and if it is reversed it can lead to an increased risk of ectopic pregnancies, which is when pregnancy occurs outside of the uterus.
Male Sterilisation: During a Vasectomy the tubes that take sperm from the testicles are cut, which prevents the sperm from getting into the ejaculate. It’s a very quick procedure and takes around 30 minutes. The advantage here is that it can be reversed more successfully than a tubal ligation. It’s also important to remember that the man will continue to produce sperm, it just won’t be in the ejaculate. This means that if he ever wanted to have more children it be achieved fairly easily.

B11.8 Infertility Treatments

Page no. 175

In vitro fertilisation (IVF) is a process of fertilisation where an egg is combined with sperm outside the body, in vitro (“in glass”). The process involves monitoring and stimulating a woman’s ovulatory process, removing an ovum or ova (egg or eggs) from the woman’s ovaries and letting sperm fertilise them in a liquid in a laboratory. The fertilised egg (zygote) undergoes embryo culture for 2–6 days, and is then transferred to the same or another woman’s uterus, with the intention of establishing a successful pregnancy.

(a)

Artificial hormone can be used to help people overcome infertility and conceive naturally.artificial FSH can be used as a fertility drug . it stimulate the eggs in the ovary to mature and also triggers oestrogen production. An artificial LH can then be used to triggers ovulation. If women who is not ovulating as a result of a lack of her own FSH is treated in this way she may be able to get pregnant naturally.

(b)

Fertility drugs are also used in IVF (In vitro fertilisation).

IVF is a form of fertility treatment used if the oviducts have been damaged or blocked by infection, if a donor egg has to be used, or if there is no obvious cause for long-term infertility.

They give the mother synthetic FSH to stimulate the maturation of a number of eggs at the same time, followed by LH to bring the eggs to the point of ovulation.
They collect the eggs from the ovary of the mother and fertilise them with sperm from the father outside the body in the laboratory.
The fertilised eggs are kept in special solutions in a warm environment to develop into tiny embryos.
At the stage when they are minute balls of cells, one or two of the embryos are inserted back into the uterus of the mother. In this way they bypass the faulty tubes.

Advantage of artificial hormone to control the fertility:

Highly effective reversible contraception.
Menstrual cycle regulation.
Less severe menstrual cramps. Birth control pills can offer significant relief to women with painful menstrual cramps (dysmenorrhea)
Decreased risk of iron deficiency (anemia).
Reduce the risk of ovarian cysts
Protection against pelvic inflammatory disease
Improved acne
Reduces the risk of symptomatic endometriosis. Women who have endometriosis tend to have less pelvic pain and fewer other symptoms when they are on the Pill.
Improves fibrocystic breasts.
Improved excess hair (hirsutism). Women with excessive facial or body hair may notice an improvement while taking the Pill, because androgens and testosterone are suppressed by oral contraceptives

Risks and disadvantages

Heart attack. The chances of birth control pills contributing to a heart attack are small unless you smoke.
Women who take oral contraceptive and have a history of migraines (particularly migraines with aura) have an increased risk of stroke compared to nonusers with a history of migraine4.
Blood pressure. Women taking birth control pills usually have a small increase in both systolic and diastolic blood pressure, although readings usually remain within the normal range.
Nausea and vomiting.This side effect usually goes away after the first few months of use or can be prevented by taking the pill with a meal.
Breast tenderness.
Chloasma (spotty darkening of the skin on the face). Darkening of the skin on the upper lip, cheeks, forehead, or under the eyes (chloasma).

B11.9 Plant Hormones and Responses

Page no. 177

1 (a)

Phototropism: the orientation of a plant or other organism in response to light, either towards the source of light ( positive phototropism ) or away from it ( negative phototropism ).

(b)

Gravitropism: Gravitropism (also known as geotropism) is a turning or growth movement by a plant or fungus in response to gravity. It is a general feature of all higher and many lower plants as well as other organisms

A ‘tropism’ is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow.Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth.

Tropisms are directional movement responses that occur in response to a directional stimulus. Plants are not able to relocated if they happen to start growing where conditions are suboptimal. However, plants can alter their growth so they can grow into more favorable conditions, To do so requires the ability to detect where the conditions are better and then alter their growth so they can “move” in the appropriate direction. One of the most commonly observed tropic responses in plants is phototropism, in which plant stems grow towards light. As anyone who has grown plants near a window knows, the plants tend to lean towards the window where the light is usually stronger than inside the room. Another commonly observed tropic responses is gravitropism, where a plant will grow so that it stays oriented relative to the source of gravity (the earth). Thus, if a plant is knocked down the shoot will grow faster on the lower side until the shoot is more-or-less standing up again.

Tropic responses result from differential growth. Phototropism is a blue-light-dependent response controlled by the action of specific blue light photoreceptors called phototropins. Gravitropism is dependent on the presence of starch-filled plastids (amyloplasts) in specialized cells. When the orientation of the cells changes, the mass of the starch-filled plastids causes them to sink to the lower end of the cell. The tumbling of the amyloplasts triggers, through unknown mechanisms, differential growth that causes curvature to develop.

B11.10 Using Plant Hormones

Page no. 179

1 (a)

Plant hormones (also known as phytohormones) are chemicals that regulate plant growth. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, moved to other locations, in other functional parts of the plant

(b)

Rooting hormone acts as a catalyst for the new roots and protects the cuttings from fungus and disease that may have been introduced during the cutting process. A rooting hormone is used in plant propagation to grow new roots on cuttings.

Auxin can be used to make effective weed killers. If auxin solution spray on to the leaves of plant, the hormone is absorbed. The extra auxin can send the plant into rapid uncontrolled growth, killing them. Selective weedkillers kill some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants. The selective weedkiller contains growth hormone that causes the weeds to grow too quickly. The weedkiller is absorbed in larger quantities by the weeds than the beneficial plants.