IGCSE Biology Notes

Thanks to Shahzeb for contributing the notes!

1. Characteristics of living things

Biology is the study of living organisms. For something to be alive it needs to perform all seven functions of living things: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.

1. Movement

   • Most organisms are able to move their whole body even plants can shift their stem towards the sunlight and their roots move towards healthy soil.

2. Respiration

   • It is the breakdown of food inside a living organism and is vital for survival.

   • 2 types:

     • Aerobic Respiration which involves O₂ & glucose breaking down to form CO_2, water & energy

     • Anaerobic Respiration which is the incomplete breakdown of food. Happens when there is not enough oxygen. Equation: Glucose & O₂ (not enough) to form CO_2, lactic acid or alcohol (depending on the organism) & a little energy.

3. Sensitivity

   • It is the ability to detect and respond to a stimulus.

4. Growth

   • It is the permanent increase in size and quantity of cells using materials absorbed from the environment.

5. Reproduction

   • It is forming new individuals of the same species either sexual (2 parents) or asexual (1 parent)

   • It is removal of harmful products of metabolism. Egestion is the removal of undigested products which haven’t entered the cell.

6. Nutrition

   • It is the intake of food material from the environment.

   • Autotrophic nutrition: Organisms that make their own food such as plants.

   • Heterotrophic nutrition: Organisms that need readymade food including herbivores, carnivores & omnivores.

2. Classification

Classification is sorting organisms into smaller groups based on their similarities which then allows us to make comparison between them.

Organisms are split into the following:

• Kingdom Many organisms, few features in common

• Phylum

• Class

• Order

• Families

• Genus

• Species Individual organisms, many features in common

A specie is a group of organisms that share the many similar appearances and can bread with each other.

Species are scientifically named by two names in Latin to avoid differences in languages. The first name is the name of the genus while the second name is the species name e.g. WOLF (Cannis Lupus) (must be italic and underlined)

The main groups of living are the 5 kingdoms. They don’t include virus since it doesn’t obey some characteristics of life.

The five kingdoms are: Bacteria, Protoctista, Fungi, Plants, and Animals.

Virus

• The size of a virus about 30-300 nm and it is only visible with an electron microscope.

• It has a protein coat around the DNA or RNA which sometimes have spikes.

• It has no cell structures.

How a virus multiplies

1. Virus ejects its DNA or RNA into the cell

2. The genetic material multiplies

3. New viruses are formed inside the cell and then burst out of the cell.

Bacteria

• The size of bacteria is about 0.2 to 10 um. It is only visible under high-powered microscopes.

• Bacteria reproduce asexually by binary fission every 20 min (if conditions are suitable)

• If conditions are not suitable some species can form spores for survival.

Structure:

• No nucleus

• No mitochondria

• No chloroplast in most of them

• They are either saprophytes or parasites

• cell wall (not made of cellulose)

Fungi

• Mostly multicellular (many cells) (yeast is an exception)

• Cell wall made of chitin

• It has cytoplasm and it may be a saprophyte or a parasite

• It reproduces asexually by spore formation or by budding (in yeast) but in poor conditions it reproduces sexually for survival

• Budding is when a yeast cell splits into two cells and this keeps repeating.

• A mushroom is an example of a parasitic fungus.

Plants

• Plants produce seeds from inside the flower.

• The plant kingdom is divided into algae, ferns, mosses, and seed plants. '

• Seed plants are divided into conifers and angiosperms.

• Angiosperms are divided into two groups: Monocotyledons and Dicotyledons.

Similarities between animal and plant cells

1. Cell membrane present

2. Nucleus present

3. Cytoplasm present

4. Organelles present: organelles are found in the cytoplasm each one has a specific job e.g. mitochondria

Main cell parts description

1. Cell wall: Non-living structure which is made of cellulose. It supports the plant from pressure and gives it a regular shape.

2. Cell membrane: A complex semi-permeable structure which allows substances to move in and out of the cell

3. Cytoplasm: jelly-like substance where most chemical reactions take place

4. Nucleus: Contains DNA. It controls the activities of the cell and carries genetic materials.

5. Vacuole: A fluid made of cell sap. It contains some useful materials and waste products

6. Chloroplast: Large bodies containing chlorophyll where photosynthesis takes place

7. Mitochondria: It consists of a double membrane and is the site of aerobic respiration.

Specialisation of cells

Tissues, organs and systems

• Cells are the structural unit of life. Many cells join together to form tissues.

• Tissues area group of cells working together to perform a function.

• Many tissues join together to make an organ. An organ is a group of tissues working together to perform a certain function.

• Organs join together to make systems which are groups of organs working together to perform a certain job.

• Systems join together to make an organism which is a living individual.

4. Diffusion, Active Transport, Osmosis

All the chemicals reacting in the cells need to move in and out either by a passive process (doesn’t need energy e.g. osmosis and diffusion) or an active process (requires energy e.g. active transport)

Diffusion

• It is the movement of a molecule from a region of high concentration to a region of low concentration, down the concentration gradient (the difference in concentration of the substance). The greater the difference, the higher the rate of diffusion.

• The rate of diffusion depends on :

  • Concentration gradient

  • Temperature

  • Size of molecules

  • Surface area

  • Permeability of membrane

Osmosis

• It is the movement of water from a region of high concentration (a dilute solution) to a region of low concentration (a concentrated solution) down the concentration gradient through a semi-permeable membrane.

• A hypertonic solution has higher concentration of salt; a hypotonic solution has a higher concentration of water and an isotonic solution as an equal concentration of water and salt.

Active Transport

• It is the uptake of substances from a region of low concentration to a region of high concentration, against the concentration gradient and requires protein carriers.

5. Enzymes

• Enzymes are proteins that act like biological catalysts which speed up reactions.

• Each enzyme is specific for one chemical reaction or a stage in a series reaction.

• Most enzymes act inside the cell but some act outside it.

General characteristics of enzymes:

• Catalyst: Speed up reactions

• Specific: their shape is specialized for 1 reaction only

• Temperature and pH :

  • Enzymes are sensitive to a certain temperature and pH.

  • They work best at an optimum temperature or pH. Each enzyme has different optimum temperature and pH.

  • If the pH is too high or too low the enzyme will be denatured and would not work anymore.

• Enzymes are also used in washing powders as they can remove stains such as (blood and milk).

6. Nutrition and Digestion

Nutrition is obtaining food materials from the environment for growth and repair.

Food classes:

1. Protein

2. Fats

3. Carbohydrates

4. Vitamins

5. Minerals

6. Fibers

7. Water

Food Tests

Carbohydrates

• Carbohydrates are made up of carbon, hydrogen and oxygen.

• Carbohydrates are very important because they produce energy.

• In plants cells they are stored as starch and in animal cells they are stored as glycogen.

• Carbohydrates are always stored as polysaccharides because this does not affect the osmotic pressure.

• Excess carbohydrates can be stored as fats under the skin.

• Monosaccharides:

  • they are the simplest carbohydrate units

  • they are soluble in water and have a sweet taste.

  • Their formula is C₆H₁₂O₆

  • E.g. glucose

• Disaccharides:

  • These are 2 monosaccharides joined together

  • they are sweeter than monosaccharides and dissolve in water.

  • Their formula is C₁₂H₂₂O₁₂

  • e.g. sucrose.

• Polysaccharides:

  • Made up of many mono and disaccharides

  • they are insoluble in water and don’t have a sweet taste.

  • Their formula (C₁₂H₂₂O₁₂)n.

  • e.g. starch.

Fats

• Fats are a source of energy.

• They produce double the amount of energy produced by carbohydrates

• They are made up of fatty acids and glycerol

• They are formed from atoms of carbon, hydrogen and oxygen. (the amount of oxygen in fats is about half that in carbohydrates).

• Fats form a part of the cell membrane and they form a waterproof layer under the skin.

Protein

• Proteins are made up of amino acids and contain carbon, hydrogen, nitrogen and sometimes sulphur.

• They are present in foods such as milk and meat.

• They are used in growth and repair and in enzymes and make up antibodies.

Fibre

• Fibre is present in all plant foods.

• They are not digested but give the stomach something to push against and work harder.

• They also clear all the remaining foods from the alimentary canal.

Minerals

Vitamins

Vitamins are organic substances and are only needed in small amounts in the body to perform specific functions.

Digestion

Ileum

• As food is digested the products are absorbed into the blood.

• There are a number of adaptations which increases the surface area for absorption.

  • The ileum is long and narrow which provides a larger surface area than a short broad tube.

  • The ileum is folded which increases the surface area.

  • The surface is covered with tiny (about 1mm long) fingerlike projections called villi.

  • The cells on the surface of the villi have tiny fingerlike projections on their cell membrane called micro-villi. These further increase the surface area for faster rate of absorption.

Colon

• By the time the food reaches the large intestine all nutrients have been absorbed.

• What remains is indigestible fibre, bile salts and water.

• Water is reabsorbed here.

• The remaining substances are passed along to the rectum before passing out through the anus.

7. Plant Nutrition

Green plants make their own food from sunlight, using the following process:

Test for starch

• A plant is left in a dark cupboard for a few days so it doesn’t produce any more starch

• A leaf is boiled in a beaker to kill all cells

• It is then put in a boiling tube of ethanol and then boiled to remove chlorophyll

• The leaf is then left in a beaker of water to remove the ethanol

• Iodine solution is added (blue-black means starch is present)

Structure of the leaf

Phloem tissue

• The leaf is the site of photosynthesis, where food is produced for the whole plant.

• These substances need to be transported to the parts of the plant which cannot make their own food. The chemicals are transported in phloem tubes.

• Sieve tube elements (the cells which make up phloem tubes) are arranged in long columns. Unlike xylem vessels they are filled with cytoplasm, but they have no nucleus.

• It is adapted for transport of organic products of photosynthesis (sugars - transported as sucrose, and amino acids). This transport is called translocation.

• The cell walls at each end of the phloem cell are perforated to form sieve plates.

• The phloem cells have associated companion cells which contain nucleus. The companion cells supplies the sieve tube elements with some required substances as the sieve tube element cannot make things like proteins on its own.

• A piece of apparatus called a potometer can be used to investigate water loss from a plant in different environmental conditions. The effect of temperature, humidity, wind speed and light intensity can therefore be studied.

Potometer

Stem

• vascular bundles are arranged in a ring with soft cortex in the centre, helping to support the stem.

Root

• root hairs are extended cells of the epidermis.

Transpiration

• Water enters the plant via the roots by osmosis. They are then carried up the xylem vessels and lost through transpiration which is when water is lost through the stomata.

• When water is lost through the stomata it forces the water to be sucked upwards.

• The xylem vessels are very thin tubes, like capillary tubes.

• They have very hard and waterproof walls (covered by lignin).

• The cells which make up the xylem vessels are hollow to produce a continuous column or tube.

Factors affecting rate of transpiration

• Temperature: the higher the temperature the faster the uptake

• Humidity: The higher the humidity the lower the uptake

• Air current: The higher the air current the higher the uptake

• Water availability in soil

• Surface area of the leaf

• Density of stomata

• Thickness of cuticle

• Number of leaves

Adaptation

• Plants with a thick waxy layer will reduce water loss through the leaves.

• Plants can have needle-like leaves. This reduces the surface area of the leaf and thereby decreases the numbers of stomata on each leaf.

• Hair-like fibre on the leaf traps air close to the leaf. It creates a microclimate around the leaf. As water is lost from the leaf the microclimate becomes very humid. The hairs prevent this humid air from being blown away. As humidity slows down the rate of transpiration decreases, allowing the leaf to conserve water.

• Leaves can be folded. Marram grass, which grows on sand dunes, is a good example. The leaf blade is curled in on itself so that the stomata are on the inside. This creates a humid micro-climate which slows down water loss.

9. Transport in humans

• Transportation in humans is done by the circulatory system which involves blood being pumped around the body by the heart.

• Humans have a double circulatory system which means that the blood is pumped twice around the body - once to the heart and another to the rest of the body.

• Blood transports O₂, CO₂, nutrients, hormones and waste products so the movement must be fast.

• The heart is really two pumps stuck together. There are two chambers to each side of the heart.

• The first chamber is called the atrium and is the smaller of the two chambers.

• The larger one is called the ventricle. This chamber is the more powerful of the two as it has to force blood out of the heart.

• The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs.

• The left side of the body receives oxygenated blood and pumps it around the body so its force must be stronger. (both sides of the heart are separated by valves so the blood doesn’t flow backwards).

• In the heart both sides pump together at the same time.

• The blood must flow through the heart in one direction.

• Blood enters the atria from the veins and is then forced into the ventricles.

• The ventricles force the blood into the arteries.

• There are a number of sphincter muscles and valves that prevent blood flowing in the wrong direction.

• The valves are a little like parachutes. When blood flows the wrong way the valves bulge out, blocking the path.

Heartbeat

Involves three distinct stages:

1. relaxation phase - diastole

2. atria contract - atrial systole

3. ventricles contract - ventricular systole

DIASTOLE

1. The atria and the ventricles relax.

2. The semi-lunar valves close, preventing back flow into the ventricles.

3. The elastic walls of the aorta and pulmonary artery contract, forcing blood towards the body and the lungs.

4. Blood from the veins flows into the atria, which begin to fill.

5. Deoxygenated blood enters the right atrium, and oxygenated blood flows into the left atrium.

ATRIAL SYSTOLE

1. The atria contract, forcing blood into the ventricles, which then fill up.

2. Sphincter (ring) muscles close off the venae cavae and the pulmonary veins prevents backflow of blood from the atria into the main veins.

VENTRICULAR SYSTOLE

1. The ventricles contract, forcing blood into the aorta and pulmonary artery.

2. The main heart valves (tricuspid & bicuspid) are forced shut to prevent backflow of blood into the atria. This happens because the pressure of blood in the ventricles is higher than the pressure in the atria. The valve cord prevents the valve from being pushed back too far.

3. The walls of the aorta and pulmonary artery expand.

Coronary Heart Disease

• The heart rate can be measured by the heart pace.

• There are muscles in the wall of the heart that receive hormones from the brain telling it to speed up or slow down e.g. adrenaline.

• The vessel supplying the heart with blood is called the coronary artery. This is one of the most important arteries in the body because it supplies the heart with all the nutrients it needs.

• If this artery is blocked the heart will slow down or stop causing a heart attack. This is how coronary heart diseases (CHD) happen - by the build up of fats inside the vessel.

• The more the amount of fats build up, the slower the heart pumps and the more easily the heart gets tired.

Reasons for CHD

1. Inheritance

2. Fatty diet: eating too much fat.

3. Smoking: it contains nicotine which increases the rate of fat deposition

4. Stress and lack of exercise

Veins & Arteries

• Blood vessels are tubes, which carry the blood around the body.

• There are different types of blood vessels.

  • Arteries carry blood away from the heart. These vessels split up into smaller ones called arterioles.

  • Arterioles split up into tiny blood vessels called capillaries. It is from these vessels that movement of particles to and from the blood takes place.

  • Capillaries join together to form larger vessels called venules which join together to form veins.

Blood

• The blood transports nutrients, gases, waste, hormones and heat

• There are about 5-7 litres of blood in an adult body

• The distribution is as follows:

  • 55% plasma which contains 90% water and 10% soluble materials

  • 45% are blood cells

• The blood is also the main defense against diseases as it has platelets that form clots and they have white blood cells which have phagocytes which engulfs bacteria and lymphocytes which produce antibodies.

Blood Clotting

• When we cut ourselves we not only lose blood but we also make it easier for bacteria to get inside our bodies.

• Therefore the body must stop the flow of blood and block the breach in the skin to prevent blood loss and infection. For this to be effective it needs to be quick.

• Platelets in the blood carry an enzyme, which is released into the plasma when the platelets come into contact with air or damaged cells.

• The enzyme changes the soluble plasma protein fibrinogen into the insoluble fibrin.

• Fibrin is sticky and forms long threads creating a net, which traps some red blood cells. This makes a plug called a blood clot.

• Phagocytes, attracted to the damaged site, engulf the pathogens.

• The clot hardens and becomes a scab. This protects the wound as the skin heals beneath.

White blood cells and immunity

• There are two types of white blood cell, phagocytes and lymphocytes. Their role in defence against disease is different.

• Phagocytes wander around the blood looking for foreign bodies. When these are encountered a phagocyte will surround the foreign body and engulf it. The phagocyte then digests the foreign body, killing it.

• There are two types of lymphocytes, B-lymphocytes and T-lymphocytes. They work in different ways.

• B-cells make special proteins called antibodies. These proteins will stick onto the surface of foreign bodies.

• T-cells hunt foreign cells, cells infected by viruses, and cancer cells. When they find them they inject them with toxins, which destroy them.

Immunity

• When you become ill due to a disease-causing organism you eventually recover as your body's defense system defeats the invading pathogen.

• When you encounter the pathogen again, your body remembers the past infection and is ready to fight it.

• The invader is usually defeated before you even get any symptoms of being ill. This is known as immunity.

• The cells responsible for immunity are the lymphocytes.

• All cells have surface proteins called antigens.

• The lymphocytes recognise the antigens which belong in the body, and detect all others as foreign.

Transplant

• Sometimes people get transplants from other people with organs that have different antigens so the body might attack the new organ.

• That is why donors tissues are checked to see if the antigens are close to the one of the transplant recipient. The closer the antigens, the lower the chance of a failure transplant.

Material Exchange

Blood travels via arteries until it reaches smaller vessels called capillaries. It is here that materials are exchanged between blood and the tissue cells.

1. The blood enters a capillary bed. These vessels are very leaky and are only wide enough for one cell at a time to pass through. The capillary walls are only 1 cell thick!

2. The blood pressure forces some of the blood plasma to leak out of the capillary. This fluid is high in nutrients and oxygen (from the red blood cells). Large objects like red blood cells and protein molecules cannot pass through the walls of the capillary. The fluid that is surrounding the tissue cells is called tissue fluid. It is from this fluid that materials will diffuse into the cells.

3. White blood cells are the only cells, which can leave the blood, so they can hunt down pathogens.

4. Waste materials like carbon dioxide and urea diffuse from the cells into the tissue fluid. This fluid is drawn back into the blood capillary by an osmotic pressure supplied by the large proteins in the blood.

5. Not all the tissue fluid flows back into the blood. If it did not return the tissues would swell with fluid. Sets of vessels, called lymph vessels, drain this tissue fluid and carry it away from the tissues. Eventually the fluid (called lymph) drains back into the blood.

6. The blood leaves the capillary beds and travels back to the heart via veins.

Respiration

• Respiration is the chemical breakdown of food molecules to release energy.

• Breathing is the mechanical movement to ventilate the respiratory surface. It includes inhaling and exhaling.

• Gaseous exchange is the diffusion of O₂ on a moist surface into an organism and the diffusion of CO₂ out of the organism.

• 2 types of respiration:

• Aerobic respiration

• Anaerobic respiration

10. Gas Exchange

• The lungs are located in the chest inside a lubricated membrane called the pleural membrane. This allows the lungs to move freely inside the pleural cavity.

• The lungs are connected to the outside via the trachea (windpipe). The trachea is a tube kept in a rigid shape due to rings of cartilage.

• The larynx or voice box is located at the top of the trachea while at the bottom end it branches into two bronchi. These lead into the lungs.

• The bronchi in turn branch off into smaller and smaller bronchioles. These end in tiny air sacs called alveoli.

• It is here that gaseous exchange takes place. The surface area of all these alveoli is very large so as to be able to absorb oxygen very quickly.

• The lungs are very delicate and can easily be damaged.

• The cells lining the airways have very tiny hair like structures called cilia on them. These cilia are coated with sticky mucus.

• The beating cilia force the mucus and any particles of dirt up out of the lungs.

Characteristics of the alveoli

• Large surface area: allow faster rate of diffusion O₂

• Thin walls so gaseous exchange can take place faster

• Rich supply of capillaries for faster rate of gaseous exchange

• Moist so gas can dissolve in the water

Diaphragm and breathing

• When we breathe in the diaphragm muscle contracts, pulling the sheet down.

• The intercostal muscles between the ribs also contract which pulls the whole ribcage upwards and outwards. These together increase the volume of the chest.

• Air is drawn into the lungs because the the pressure inside them is lowered as the chest volume is increased.

• When we breathe out the diaphragm relaxes as does the intercostal muscles. This decreases the volume of the chest, increasing the pressure.

• This forces air out of the lungs.

• So it is the changing volume of the chest which causes air to enter and leave the lungs. The lungs themselves are just like balloons which are inflated and deflated.

Gaseous Exchange

• The walls of the alveoli are very thin and so are the walls surrounding the alveoli so that is why diffusion of O₂ and CO₂ can take place.

• Note that other gases do not diffuse through the walls because the concentration of these gases inside and outside of the body are the same.

Urinary system

• The urinary system consists of 2 kidneys, 2 ureters, a bladder and a urethra.

• The job of the kidney is to purify the blood as it enters it.

• The blood enters the kidney via the aorta and is filtered.

• The clean blood then returns to the heart and the urine goes down the ureter and to the bladder and then to the urethra.

• The outside of the kidney is called the cortex and the inner part is called the medulla and the part connecting to the ureters is called the pelvis (the part in the middle).

• Urea is a harmful substance made in the liver. It is made when proteins are broken down.

Functions of the kidneys

• Regulation of blood water level

• Reabsorption of useful substances into the blood

• Adjustment of the level of salts and ions in the blood

• Excretion of urea and other metabolic wastes

Process

• A nephron is the smallest unit that filters the blood.

• Blood at high pressure entering the aorta passes through the walls of the bowman’s capsule except the blood cells and protein (only O₂, CO_2, glucose, urine salts and amino acids enter).

• Most substances including O_2, glucose, most of the water and some salts are absorbed at the tubules to join the renal artery.

• The rest of the substances then go down the loop of Henle.

• Then the rest of the unwanted substances are passed to the ureter and then out of the body.

• Most of the nephron is in the cortex only.

• The loop of Henle is in the medulla and the collecting duct heads to the pelvis and collected as the ureter (there are about 1 million nephrons in each kidney.)

Osmoregulation

• Osmoregulation keeps the water and salt levels constant in the blood.

• They are regulated by the hypothalamus.

• If the concentration of water is too low (e.g. during heavy exercise as a lot of water is removed by sweating) the blood becomes too concentrated so the hypothalamus detects too little water in the blood.

• A message is sent to the pituitary gland to release anti-diuretic hormone.

• This makes the membranes of the collecting ducts become more permeable to water so more water passes through.

• If the concentration of water in the blood is too high water moves into the cells by osmosis. This could cause the cells to burst so the water in the blood stops the hypothalamus signalling the pituitary.

• The membranes of the collecting ducts become less permeable to water and large amounts of dilute urine is produced.

• The concentration of urine depends on many factors e.g. diet, exercise and temperature.

Kidney failure

• This is when a kidney of a person fails then he has to either get a transplant or dialysis.

• Transplant:

  • This is when the diseased kidney is surgically removed and replaced by a fully functioning kidney from a deceased or a live donor.

  • It is only possible after a satisfactory tissue-match.

  • Even after a successful tissue-match the recipient's immune system has to be drugged or suppressed to stop it from rejecting the new kidney.

• Dialysis:

  • In the absence of a suitable donor kidney, the alternative solution is for the patient to be hooked-up to a dialysis machine every 2 - 3 days.

  • A dialysis machine mimics the functioning of the kidney.

  • Blood from an artery in the patient's arm is pumped into the kidney machine which removes urea and excess salts from it.

  • The blood is checked for air bubbles before being returned to a vein in the arm.

12. Homeostasis and Hormones

• Homeostasis is the maintenance of the conditions of the internal body environment.

• The conditions are maintained by hormones which are secreted by some organs.

• Hormones are chemical messages and chemicals released from an endocrine gland into the blood controlled by the brain.

• Negative feedback is when the hormone has done its effect and the brain orders it to stop.

Maintenance of Temperature

• Temperature can be maintained by the skin using the following ways:

  • Sweat glands: Release sweat which evaporates, taking heat away from the body and decreasing the body temperature.

  • Hair: Erects to trap air, acting as a layer of heat insulation.

  • Blood vessels: become narrower or wider to to decrease or increase the loss of heat respectively.

Maintenance of blood sugar levels

• Blood sugar is too high

• Messages sent to pancreas to produce insulin

• Insulin converts glucose to glycogen

• Sugar level maintained

• Message sent to pancreases to stop insulin.

• Blood sugar too low

• Messages sent to pancreas to produce glucagon

• Glucagon converts glycogen to glucose

• Messages sent to pancreas to stop glucagon.

Diabetic people cannot control their blood sugar level so they take insulin pills etc to try to maintain the blood sugar level. A symptom of this illness is the presence of glucose in urine.

Plants hormones

Tropisms

• Growing in response to a stimuli is called tropism.

  • Phototropism is growth in response to light. This is an example of a positive tropism, growing towards the stimulus.

  • Hydrotropism is a response to water whereby the roots grow towards it.

  • Geotropism is a response to gravity. Roots show positive geotropism while shoots show negative geotropism (in that they grow away from gravity).

Use of plant hormones

• Plant hormones including auxins have been used in agriculture and by horticulturalists for a number of purposes.

• Plant hormones are used in rooting powder to stimulate the development of roots from plant cuttings.

• They are also used in fruit ripening to make sure that all the fruit ripens at once to aid harvesting.

13. The nervous system

Any nervous action is a result of a stimulus

• A Stimulus

  • is any change either internal or external which leads to a response. This could be a noise, smell or the changes in blood sugar level.

• A Receptor

  • is a specialized cell which can sense the stimulus. There are lots of different types of receptors; some can sense light, while others can sense heat etc.

• A Coordinator

  • is a cell or organ which 'decides' what to do. It gives a message to the effectors to do something.

• The Effectors

  • is an organ which responds to the stimulus. This could be a muscle which contracts or organs like the liver which may perform a complex task like lowering the blood sugar levels after a meal.

• The Response

  • is what happens when the organism reacts to the stimulus.

  • A stimulus can be internal or external.

  • Example of internal stimulus: an increase in body temperature.

  • Example of external stimulus

    • External stimulus: light from the sun

    • Receptor: light sensitive cells on the retina

    • Coordinator: brain

    • Effecter: muscles of the iris

    • Response: narrowing of the iris

• The nervous system

  • The nervous system is made up of the brain, the spinal cord (CNS central nervous system) and nerves.

Nerve cells

Neurons

• Made up of a bundle of axons which are surrounded by myelin sheath.

• A synapse joins two neurons together. It contains chemical messages.

• Have branched ends that receives impulses.

• Types of neurons:

  • Motor neurons: transmit impulses from the CNS to the effecter muscle

  • Sensory neuron: transmits messages from the sensory neuron to the CNS

  • Relay neuron: Links the motor with the sensory neuron.

Motor neurons

• Efferent Neuron – Moving toward a central organ or point: Relays messages from the brain or spinal cord to the muscles and organs

• These neurons carry impulses away from the CNS towards effecter organs like muscles or glands.

• These cells have very long axons at the end of which are motor end plates where the nerve cell can stimulate the effecter organ.

Sensory neuron

• Afferent Neuron – Moving away from a central organ or point: Relays messages from receptors to the brain or spinal cord

• The sensory neuron gathers information from the senses and passes it on to the central nervous system (CNS).

• It is attached to special receptor cells or in some cases the nerve's end is a receptor itself.

• When stimulated it carries an electrical impulse along its length, passed the cell body and down the axon to the nerve endings.

• It is here that the cell meets with another neuron (or neuron) at a junction called a synapse.

• The cell bodies of sensory neurons can all be found together in a nerve.

• This causes a swelling called a ganglion.

Relay neuron (interneuron)

• Relays message from sensory neurone to motor neurone

• Make up the brain and spinal cord

The reflex arc

• A reflex action is usually quick passive action and is usually for protection.

• The spinal reflex does not need to pass through the brain but pass through a relay neuron.

• A stimulus happens and a receptor (the sensory organ) passes the impulse to the sensory neuron and then to the relay neuron and then to the motor neuron.

Synapse

• The synapse is a junction where two or more nerve cells meet.

• The synapse allows the nerve cells to pass their electrical impulse to another cell.

• The synapse is also a way of controlling the direction in which impulses travel. They can only travel one way through a synapse.

• When an impulse reaches the synaptic knob, it releases vesicles of a chemical called a neurotransmitter to be released into the synaptic cleft.

• They quickly diffuse across the gap and bind with receptors on the surface of the connecting neuron.

• When enough of the receptors have been filled an electrical impulse is triggered in this neuron.

14. The eye

• Sclera: a tough coat that protects the eye from the inside.

• Choroid: A layer that absorbs light so no internal reflection happens

• Retina: contains light sensitive cells.

  • The retina is the light sensitive layer which is responsible for 'seeing' light. The retina is composed of two types of light sensitive cells.

• Yellow spot: The highest concentration of rods and cones

• Blind spot: the least concentration of light sensitive cells

• Optic nerve: Transmits impulses generated by the retina.

• Vitreous humour: Helps maintain the shape of the eyeball

• Lens: Responsible for the refraction of light in the eye

• Suspensory ligament: Adjust the lenses shape

• Iris: Controls the entry of light.

• Pupil: The circular opening which lets light in

• Cornea: The outer layer of the eye (transparent)

• Aqueous Humour: Maintains the curvature of the choroid

• Rectus muscles: Adjust the position of the eyeball

• Tears: contain sodium chloride, water, sodium bicarbonate and lysozem

Rod cells

• These cells are capable of seeing only different degrees of light intensity and cannot distinguish colour. As a result they can only see in black and white.

• They are able to sense low levels of light and so are used for seeing in dim light such as at night but do not work in bright light.

Cone cells

• There are three different types of cone cell, each sensitive to a different colour of light.

• The three cells are sensitive to the three primary colours of light, red, green and blue.

• These cells need a great deal of light in order to work and as a result are not able to 'see' in dim light.

• The cells are mixed together so that different colours can be seen.

Accommodation

It means the ability of the lens to detect far and near objects by changing its thickness to be able to see far and near.