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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 CO2, 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 CO2, 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.
FeatureMonocotyledons  Dicotyledons 
SeedSeed containing one cotyledonSeed containing two cotyledons
LeavesLeaves containing parallel veinsLeaves containing branched veins
RootFibrous root system (adventitious)Tap root system with lateral roots


Animals

  • There are two main groups in the animal kingdom: the chordates and the invertebrates. The invertebrates consist of Nematodes, Annelids, Molluscs and Arthropods.

Comparison between Annelids, Nematodes and Molluscs.


Annelid Nematode Mollusc
Body coveringHard, slightly waterproofSoft, not waterproofSoft shell helps to save water
Segments visible?YesNoYes
MovementUse chaetae (bristles) to move from place to placeWriggles but lives in one placeCreeps on foot from place to place
Feeding methodHerbivoreMainly parasitesMainly herbivores, some carnivores
 

Arthropods are divided into insects, crustacians and arachnids.

 Insect CrustacianArachnids Myriapoda
ExamplesBees, butterflies, locust Crabs, lobsters, shrimpsSpider, scorpion Millipedes and centipedes
Body segment3 body segments: head, thorax, abdomen2 body segments: cephalothorax and abdomen2 body segments: cephalothorax and abdomen 2 body segments
Jointed legs3 pairsMore than 4 pairs4 pairsMany legs (1 or 2 pairs of legs on every body segment)
Antennae1 pair2 pairs which are sensitive to touch and chemicalsNone. Instead, there is a pair of chelicerae to hold prey 1 pair
Wings    1 or 2 pairsNoneNoneNone
EyesCompound and simple eyes1 pair of compound eyesSimple eyesSimple eyes
BreathingThrough tracheae "spiracles"GillsBook lungs 
 

Adaptation to insects on life on land
  • Body covered in flexible chateaus exoskeleton 
  • 1 or 2 pairs of wings 
  • Jointed legs for quick movement 
  • Can live on all food materials 
  • Can camouflage to hide from enemies

Chordates (or Vertebrates)

  • Chordates are vertebrates - animals with back bones
  • They consist of: Fish, Amphibians, Reptiles, Birds, and Mammals
  1. Fish 
    • Body covered in moist scales 
    • Has fins to swim and gills for gas-exchange 
    • Lays eggs in large amounts (eggs are soft with no shells) 
  2. Amphibians 
    • Moist, smooth and non-scaly skin. 
    • Some can camouflage e.g. frogs 
    • Young live in water & have gills & adults live on land & have lungs 
    • Have 4 limbs 
    • Lays soft non shell eggs 
    • Has an ear drum 
  3. Reptiles 
    • Covered in dry scaly skin to prevent water loss 
    • 4 limbs (except snakes) 
    • Lay water proof eggs with hard shells 
    • Has a third transparent eyelid for protection 
  4. Birds 
    • Body covered in feathers 
    • Beak for feeding 
    • 2 limbs and 2 wings 
    • Lays water proof hard shells
  5. Mammals
    • Body covered in hair
    • 4 limbs
    • Breathe through lungs
    • Milk from mammary glands
    • External ear pinna
    • 4 kinds of teeth: Incisors, Canines, Premolar, and molars
    • Have sweat glands
    • Have a diaphragm 

3. Cell Structure

  • A cell is the smallest part of an organism all cels consist of a membrane, cytoplasm and a nucleus.

Differences between plant and animal cells:

FeaturePlant CellAnimal Cell
Cell wallPresent and made of celluloseAbsent
ChloroplastPresentAbsent
VacuolePresentAbsent
Carbohydrate storageStarch, glucoseGlycogen
Protein storageCan store proteinCannot store protein
Fat storageOilFats
ShapeRegular shape because of cell wallIrregular shape
SizeLargeSmall

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
PlantAnimal
Root hair cell:
- It has long hairs to increase surface area of the cell.
- It has a large number of mitochondria to provide energy for active uptake.
Red blood cells:
- It transports oxygen from the lungs to tissues.
- It has no nucleus.
- It has haemoglobin which absorbs oxygen.
- Its shape gives it a high surface area
- it is small enough to pass through capillaries.
Xylem vessels:
- It transports water and minerals to the plant.
- The xylem cells are dead
- It is made of long hollow tubes running throughout the root, stem, and leaves.
- It has thick cell walls.
Nerve cells:
- They conduct electrical impulses which travel to and from the brain.
- They are very long and their chemical reactions cause impulses to travel through their fibers.
- They also have a layer of fat for insulation.
Phloem cells:
- They transport sugar and amino acids to the body from the place they were made (the leaf).
- The phloem cells are long cells joined together.
- The cell wall where 2 phloem cells join together has holes which allow the cytoplasm of both cells to communicate to pass the dissolved food.
White blood cells:
- These occur in large amounts in the blood stream.
- They get rid of bacteria and viruses.
- They are able to change shape and can penetrate the blood vassals to enter tissues.
- They also contain enzymes that kill microorganisms in their cytoplasm.
Guard cell (stomata):
- Allows O2 and CO2 to pass in and out the leaf.
- They can change their shape and thus can open and close their holes.
Ciliared cell:
- These have cilia (hairs) which can move mucus away from the lungs by a wavy motion.
  Muscle cells: 
- These can contract to move the body (they don’t relax but they return to their original shape by the influence of other cells)
- They contain a lot of mitochondria to provide the energy needed.

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 organAn 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.
 DiffusionOsmosisActive Transport
Selectiveness of substances to be transportedNot selectiveNot selectiveSelective, cells absorb what they need
Movement up/down a concentration graidentSubstances move down a concentration gradientWater moves down a concentration gradientSubstances move up concentration gradient
Energy required?NoNoYes
Partially permeable membrane required?NoYesYes

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

Type of foodMethodPositive ObservationNegative Observation
StarchAdd iodine solution (yellow brown colour) Turns blue black or dark blue colourThe colour remains yellow brown
Reducing sugar or simple sugarAdd Benedict's solution and heat (blue colour as it contains copper salts)
Precautions!
  • hold the tube with a holder
  • Direct the opening of the tube away from your face
  • Do not fill more than half of the tube to avoid splashed when the solution boils
  • Preferable to use a water bath
Orange red or brick red precipitate is formed. (gradual change in colour from blue to green, yellow, orange, then red)The colour remains blue
ProteinBiuret test: add potassium hydroxide, then add drops of copper sulphate, the colour turns bluePurple colour (mauve or lilac) The colour remains blue
Fats or lipidsEthanol emulsion test: add ethanol, fats dissolve in ethanol forming a clear solution. Add drops of water to the clear solutionMilky emulsion or turbid solution is formedThe solution remains clear

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

Minerals SourcesImportanceDeficiency symptoms
Calcium
  • Milk and its products
  • Many fruits and vegetables 
  • Necessary for formation of bones and teeth
  • Necessary for blood clotting
  • Necessary for lactation
  • Brittle (soft) bones
  • Brittle teeth
  • Slow dentition in children
Iron
  • Liver
  • Egg yolk
  • Red meat
  • Leafy vegetables
Necessary for formation of haemoglobin of the red blood cellsAnaemia (rapid tiredness and shortness of breath due to lack of haemoglobin which is used to carry oxygen to different parts of the body to be used for the production of energy by the process of aerobic respiration)

Vitamins

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

VitaminSourcesImportanceDeficiency symptomsProperties
C (aka ascorbic acid)
  • citrus fruits such as orange and lemon
  • fresh vegetables
  •  helps wounds to heal
  • keeps blood vessels healthy
  • keeps cement of tooth healthy
  • keeps gum and teeth healthy
  • helps the body to use iron
  • stimulates the immune system
  • lack of vitamin C causes a disease called scurvy
symptoms of scurvy:
  • pain in joints and muscles
  • bleeding from gum and other parts of the body
  • delayed healing of wounds
  • water solution vitamin therefore it cannot be stored in the body
  • spoils if food is heated or canned
  • destroyed by exposure to air eg if food is grated or minced as this activates enzymes in food which destroy vitamin C
  • refrigeration keeps the vitamin C content of the food but only to a certain limit
D (aka calciferol)
  • butter, eggs, cod liver oil
  • can be formed in the skin by exposure to sun rays
  •  helps absorption of calcium and phosphorus
  • helps deposition of calcium and phosphorus in bones and teeth
  •  rickets in children: causes bones to be soft and deformed
  • soft bones or osteomalacia in adults
  • slow dentition
  •  fat solution vitamin, so can be stored in the body (in the liver)

Digestion


The digestive system is made up of the alimentary canal and the associated organs. 
The alimentary canal is lined with epithelial, goblet, and muscle cells
  • Ingestion: taking food into a living organism 
  • Digestion: Breaking down large insoluble food molecules into small soluble ones 
  • Absorption: The process by which food molecules enter the blood stream 
  • Assimilation: Making use of the absorbed food substances 
  • Egesting: Getting rid of undigested materials. 

The mouth

  • Food is ingested and chewed. The teeth help to tear and grind the food into small pieces. This increases the surface area for the action of enzymes. 
  • The food is mixed with saliva which has two functions. 
    • The saliva contains mucus which is a slimy substance which helps the food to be swallowed.
    • It contains the enzyme amylase which begins the digestion of starch into the sugar maltose. As food does not remain in the mouth for very long, only a small amount of starch is digested here.
  • The food is then turned to a bolus shape by the action of the mouth and then swallowed.

Oesophagus

  • This tube pushes the food to the stomach by way of rhythmic contractions. 
  • There are two sets of muscles in the oesophagus. 
    • Circular muscles - these make the oesophagus narrower. 
    • Longitudinal muscles - these make the oesophagus wider. 
  • They work in conjunction with each other to force the food down to the stomach in a rhythmic wave called peristalsis. In this way, the food is moved all the way along the alimentary canal.
  • The moment the food is swallowed, a flap called the epiglottis closes so that the food would not enter the trachea.

Stomach

  • When the food reaches the stomach, gastric juice is released from the stomach lining. Gastric juice contains two substances. 
    • Pepsin - an enzyme which breaks proteins down into shorter chains called polypeptides. 
    • Hydrochloric acid - needed to help pepsin work and also helps to kill any ingested bacteria. 
  • The stomach has two rings of muscles at the top and bottom, called sphincter muscles which prevent food from leaving the stomach while it is being churned around. 
  • After a few hours, the food is now a mushy liquid called chyme
  • It is then allowed to continue on its journey a bit at a time.

Duodenum, Liver, Gall Bladder and Pancreas

  • When food enters the duodenum (the first 30cm of the small intestine) a number of secretions are added to it. 
  • Digestive enzymes from the wall of the duodenum and from the pancreas are added. There are a number of enzymes here which will complete the digestive process. 
  • Another substance is added from the gall bladder: Bile, made in the liver and stored in the gall bladder, contains no digestive enzymes but it contains bile salts which play a vital role in fat digestion. 
    • Fats and oils do not mix with water, but the enzyme lipase which digests them needs water in order to work. 
    • Bile salts break down the large fat drops into tiny droplets which can mix better with water to create an emulsion
    • This makes it easier for lipase digest the chemicals as it increases the surface area of the fat. 
  • The pancreatic secretions contain hydrogen carbonate ions to neutralize the stomach acid.
  • Enzymes of the small intestine work best in a slightly alkaline environment
Digestive enzymes in the small intestine
EnzymeType of food digestedProducts
AmylaseStarchMaltose 
MaltaseMaltose Glucose 
Protease (eg trypsin)Polypeptides Amino acids 
LipaseFats and oils Fatty acids and glycerol

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

  • The leaf has a waxy cuticle to prevent it from losing water and drying out.
  • The epidermis is a protective layer of cells and contains no chloroplasts.
  • The palisade layer contains the most chloroplasts as it is near the top of the leaf. It is here that photosynthesis takes place. The palisade cells are arranged upright to increases the chance of photosynthesis.
  • The spongy layer contains fewer chloroplasts, but enough to catch what the palisade layer cannot absorb.The spongy layer has air spaces to make it easier for gases to circulate in the leaf.
  • The vascular bundle provides the leaf with water via the xylem vessels. Food, such as sugar, made in the leaf is transported in the phloem vessels to the rest of the leaf.
  • The stomata (stoma - singular) are tiny pores that allow carbon dioxide to enter the leaf while oxygen leaves the leaf.
  • Guard cells can open or close the stomata pores to regulate how much gas can enter or leave the leaf. At night the pores close, opening in the daytime. 
Limiting Factors

There are factors that affect photosynthesis changing these factors are: Temperature, Light intensity, and concentration of carbon dioxide.

  1. Temperature
    • When the temperature rises the rate of photosynthesis rises also. This is because the particles in the reaction move faster and collide more. 
    • At optimum temperature, the rate of photosynthesis progresses as fast as it can, limited only by the other factors. 
    • Beyond this temperature the enzymes controlling the reaction become denatured and the reaction quickly comes to a halt. 
  2. Light intensity
    • The plant can photosynthesize faster as a result of a higher light intensity. 
    • As the light intensity decreases the rate of photosynthesis decreases. 
    • Light is a limiting factor at low light intensities. There will come a point where any extra light energy will not increase the rate of the reaction. 
    • This is because the enzymes controlling the reaction are working at maximum rate. At this point light is no longer a limiting factor. 
  3. Concentration of carbon dioxide
    • When the concentration of carbon dioxide is low the rate of photosynthesis is also low. This is because the plant has to spend a certain amount of time doing nothing, waiting for more carbon dioxide to arrive. 
    • Increasing the concentration of carbon dioxide increases the rate of photosynthesis. 
    • There is a point at which further addition of carbon dioxide will not increase the rate of photosynthesis. 
    • This is because the enzymes controlling the reaction are working at maximum rate, so the excess carbon dioxide has no effect. 
Plant mineral requirements
  • Plants need a number of minerals to be healthy. 
  • These mineral ions are needed to make certain chemicals or to make certain reactions work properly. 
  • Plant absorbs these minerals from the soil when water is absorbed.
ElementMineral saltWhy it is neededResult of deficiency
NitrogenNitratesto make proteinspoor growth, yellow leaves
SulphurSulphatesto make proteinspoor growth, yellow leaves
PhosphorusPhosphatesto make DNA and chemicals involved in respirationpoor growth, especially of roots
MagnesiumMagnesium saltsto make chlorophyllyellowing between veins of leaves
IronIron saltsto make chlorophyll; iron is not contained in chlorophyll but is needed for its manufactureyellowing in young leaves
PotassiumPotassium saltsto keep correct salt balance for cellsmottled leaves
 

8. Transport in plants

Transport is the movement or flow of different substances within a living organism. The transport system in plants is the vascular bundles (xylem and phloem). Cambium tissue contains cells which divide by mitosis to produce more phloem and xylem.

Xylem tissue

  • contains long xylem vessels adapted for the rapid transport of water and dissolved mineral ions.
  • movement is always up the stem

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.


Differences between xylem and phloem
     XylemPhloem
DescriptionConsists of non-living woody (lignified) cells (elements( joined together to form continuous tubes (vessels)Consists of living cells (sieve elements)
Substances carriedSap: water and mineral saltsSolution of organic molecules made by the plant, including hormones and products of photosynthesis
Direction of transportUnidirectional: mostly upwards, from roots to stems and leavesBidirectional: movement of substances occurs both downward and upward
Mechanisms of transportMostly passive processes including osmosis, capillary action and evaporationMostly active processes involving the use of energy. 

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. 
PhaseAtriaVentriclesCuspid valvesSemi-lunar valves
DiastoleRelaxedRelaxedClosedClosed
Atrial systoleContractingRelaxedOpenClosed
Ventricular systoleRelaxedContractingClosed Open

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. 
VeinsArteries
Blood travels to heart    Blood travels away from heart 
Large lumenNarrow lumen 
Thin wall with few elastic fibresThick wall with lots of elastic fibres 
Thin muscular layerThick muscular layer 
Valves present to prevent back flowNo valves
Blood travels constantlyBlood travels in pulses 
Blood under low pressureBlood under high pressure 
Blood moves slowlyBlood moves rapidly 

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
Aerobic RespirationAnaerobic Respiration
It is the breakdown of glucose in the presence of O2. In this process glucose is completely oxidized into carbon dioxide and water.

This process is slow and is controlled by many enzymes and the energy produced is not used immediately but stored as
ATP. The energy released from ATP can be used in many activities such as:
  • cell division
  • maintaining body temperature
  • active transport in the membrane
  • conduction of nerve impulses
This is the breakdown of glucose without using oxygen. In this process the energy produced is relatively small and the product is variable.

Alcohol can be produced when anaerobic respiration happens during fermentation in yeast.

In the human body,
lactic acid is a product of anaerobic respiration during heavy exercise. The lactic acid produced needs to be broken down further by oxygen. That is why we continue to breathe heavily after exercising. The oxygen required for the subsequent breakdown of lactic acid is called oxygen debt.

10. Gas Exchange

  • The lungs are located in the chest inside a lubricated membrane called the pleural membraneThis 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.

Smoking

Smoking causes a number of diseases, some of them are life-threatening.

  • Nicotine: 
    • This is the substance which makes smoking addictive. 
    • Nicotine is a stimulant which can make the heart beat faster and increase the amount of adrenaline released. 
    • It also makes the smoker more shaky and causes stress.
  • Carbon Monoxide: 
    • This is produced due to incomplete burning of the tobacco. 
    • This gas binds irreversibly to the haemoglobin in red blood cells preventing them from carrying oxygen. 
    • If the smoker is pregnant the baby will get less oxygen than usual.
  • Tar 
    • It is a mixture of many different chemicals. 
    • It prevents the cilia in the lungs from working and so the dirt and tar cannot be removed from the lungs. 
    • It also damages the alveoli and decrease the lungs' surface area. 

11. Excretion

  • Excretion is the removal of waste products of metabolism and substances in excess of requirements from organisms. (do not mix up with egesting). 
Mammalia excretory organs
OrgansExcretory products
LungsCO2, water vapour, "expired air" 
KidneysWater, urea, uric acid, excess salts, drugs and inactive hormone in the form of urine 
LiverBile pigments result from the destruction of old red blood cells released from bile 
SkinWater, salt and urea "sweat".

Note: Sweat is a response to rise in temperature and not due to  change in blood composition so it is not considered a real excretory organ

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 O2CO2, glucose, urine salts and amino acids enter). 
  • Most substances including O2, 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. 

 On a cold dayOn a warm day 
Detection by skin receptor and hypothalamus in the brainMessage is immediately sent out through nerves to switch on the warming mechanismMessage is immediately sent out through nerves to switch on the cooling mechanism
Blood vessels (arterioles)Vasoconstriction: arterioles become narrower so that less blood flows through the capillariesVasodilation: arterioles dilate so that more blood flows through the capillaries close to the skin surface, more heat lost from the skin
HairsHair erector muscles contract, causing hairs to stand. This traps a thick layer of air between the hairs, creating a insulator layerHair erector muscles relax, causing hairs to flatten. This layer of trapped air becomes thinner so heat is easily lost from the body by radiation and convection.
Sweat glandsLess active so that latent heat is not lost from the bodyVery active so more water is brought to the sweat glands during vasodilation, thus more sweat is secreted and more latent heat is lost when sweat evaporates. 
Metabolic rateIncreases. More heat is produced. Shivering due to involuntary contraction of the musclesDecreases. Less heat produced.


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. 
Bright lightDim light 
More photoreceptor cells in the retina are stimulated by an increase in light intensityFewer photoreceptor cells are stimulated due to decrease in light intensity 
Greater number of impulses pass along sensory neurons to the brainFew impulses pass along sensory neurons to the brain 
In the iris disphragm, circular muscle contracts and radial muscle relaxesIn the iris diaphragm, circular muscle relaxes and radial muscle contracts
Pupil contractsPupil dilates
Less light enters the eyeMore light enters the eye


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. 



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