Transmission Of Nerve Impulses Along Axon - Resting, Depolarisation, Repolarisation

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Transmission of nerve impulses along axon involves the formation of action potentials. An action potential is divided into several phases which are resting phase, depolarisation phase and repolarisation phase.

Resting phase

During resting phase, the potential difference which exists across axon membrane at rest is called resting potential. The resting potential for human cell is about -70mV. Sodium potassium pumps actively pump three sodium ions out of the axon and pumps two potassium ions into the axoplasm, therefore more positive ions are pumped out that pump in.

Depolarisation phase

During depolarisation phase, sodium gates open, positively charged sodium ions diffuse into axon and the charge inside axon changes from negative to positive.

Repolarisation phase

During repolarisation phase, potassium gates open, positively charged potassium ions diffuse out from axon and the charge inside axon changes from positive to negative.

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Transmission Of Nerve Impulses Across Synapse - Neurotransmitter, Acetylcholine

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Neurones are not continuous there is a gap between neurones. The gap between axon of one neurone and the dendrites on the next neurone is called a synapse.

At an axon terminal, there are tiny vesicles filled with neurotransmitters which are chemical substances used by one neurone to signal another neurone. An example of neurotransmitter is acetylcholine.

Transmission of nerve impulses across synapse

When a nerve impulse reaches the axon terminal, vesicles containing neurotransmitters (acetylcholine) move towards presynaptic membrane. Vesicles fuse with the membrane, releasing neurotransmitters (acetylcholine) into synaptic cleft through exocytosis. Neurotransmitters (acetylcholine) diffuse across synapse and bind to the receptors at postsynaptic membrane. Some of the neurotransmitters (acetylcholine) are broken down by enzymes (acetylcholinesterase), taken up again by the axon terminal and recycled, or simply diffused away.
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Factors Affecting Rate Of Transpiration - Temperature, Humidity, Wind Speed, Light Intensity

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Transpiration is evaporation of water through stomatal openings in leaves. Factors affecting rate of transpiration are temperature, humidity, air movement and light intensity.

Effect of temperature on rate of transpiration

The higher the temperature, the higher the rate of transpiration. The hotter it is, the more water molecules evaporate.

Effect of humidity on rate of transpiration

The higher the humidity, the lower the rate of transpiration. The more saturated the air, the less evaporation can take place.

Effect of wind speed on rate of transpiration

The higher the wind speed the higher the rate of transpiration. The more water molecules are blown away, the more evaporation can take place.

Effect of light intensity on rate of transpiration

The higher the light intensity, the higher the rate of evaporation. The more stomata open during photosynthesis, the more water molecules evaporate.
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Factors Affecting Rate Of Enzyme Reaction - pH, Temperature, Substrate Concentration, Enzyme Concentration

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Factors affecting rate of enzyme reaction are pH, temperature, substrate concentration and enzyme concentration.

Effect of pH on enzyme reaction

Enzymes are active within narrow range of pH. At optimum pH, the rate of enzyme reaction reaches its maximum. Pepsin works best in acidic environment, amylase works best in neutral environment and trypsin works best in alkaline environment.

Effect of temperature on enzyme reaction

At low temperature, the rate of enzyme reaction is low because enzymes are inactive. As temperature increases, the rate of enzyme reaction increases. At optimum temperature of 370C, the rate of enzyme reaction reaches its maximum. At high temperature, the rate of enzyme reaction decreases. At temperature above 600C, enzymes are denatured.

Effect of substrate concentration on enzyme reaction

As substrate concentration increases, the rate of enzyme reaction increases to a certain level, whereby the enzyme concentration becomes the limiting factor.

Effect of enzyme concentration on enzyme reaction

As enzyme concentration increases, the rate of enzyme reaction increases to a certain level, whereby the substrate concentration becomes the limiting factor.
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Effects Of Osmosis On Animal And Plant Cells - Hypotonic Solution, Isotonic Solution, Hypertonic Solution

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Hypotonic solution is a solution which has higher water potential than another solution.

Isotonic solution is a solution which has equal water potential as another solution.

Hypertonic solution is a solution which has lower water potential than another solution.

Effects of osmosis on animal cells

In hypotonic solution, more water molecules entering the cells than leaving the cells. The cells swell and burst (lysed).

In isotonic solution, water molecules entering and leaving the cells at equal amount. The size and shape of cells remain the same.

In hypertonic solution, more water molecules leaving the cells than entering the cells. The cells shrink (crenated).

Effects of osmosis on plant cells

In hypotonic solution, more water molecules entering the cells than leaving the cells. The cells swell and become turgid as the cell walls prevent the cells from bursting.

In isotonic solution, water molecules entering and leaving the cells at equal amount. The size and shape of cells remain the same.

In hypertonic solution, more water molecules leaving the cells than entering the cells. The cytoplasm shrinks away from cell walls (plasmolysed) and the cells become flaccid.
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Four Structural Levels Of Proteins - Primary, Secondary, Tertiary, Quaternary Structures

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The four structural levels of proteins are primary structure, secondary structure, tertiary structure and quaternary structure.

Primary structure

Primary structure is the number and sequence of amino acids or peptides. An example of primary structure is a polypeptide.

Secondary structure

Secondary structure involves coiling of a polypeptide chain in a helical or parallel structure which is stabilised by hydrogen bonds. An example of secondary structure is a fibrous protein.

Tertiary structure

Tertiary structure involves folding of polypeptide into a more compact, globular structure which is stabilised by disulphide, ionic and hydrogen bonds. An example of tertiary structure is a globular protein.

Quaternary structure

Quaternary structure involves combining two or more polypeptide chains to form a larger, more complex molecule which is stabilised by disulphide, ionic and hydrogen bonds. Non protein molecules are usually integrated onto this complex, forming a conjugated protein. An example of quaternary structure is haemoglobin.
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Types Of Neurones - Sensory Neurone, Relay Neurone, Motor Neurone

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Neurone or nerve cell is classified into three types which are sensory neurone (afferent neurone), relay neurone (interneurone) and motor neurone (efferent neurone).

Sensory neurone / Afferent neurone

Sensory neurone also known as afferent neurone, transmits nerve impulses from receptor to central nervous system (CNS) which consists of brain and spinal cord.

Relay neurone / Interneurone

Relay neurone also known as interneurone, transmits nerve impulses from sensory neurone to motor neurone. Relay neurone is found within the central nervous system (CNS).

Motor neurone / Efferent neurone

Motor neurone also known as efferent neurone, transmits nerve impulses from central nervous system (CNS) which consists of brain and spinal cord, to effectors which are either muscles or glands. In response to impulses, muscles contract and glands secrete.
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ABO Blood Group System Inheritance

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In ABO blood group system, an individual can be blood group A, B, AB or O based on the types of antigens on the surfaces of red blood cells.

A cross between an individual with blood group A and an individual with blood group B

=> A and B are codominant whereas O is recessive
=> For blood group A: IAIA or IAIO
=> For blood group B: IBIB or IBIO
=> For blood group AB: IAIB
=> For blood group O: IOIO

Parent phenotypes:    Blood group A   X   Blood group B
Parent genotypes:                    IAIO      X      IBIO
Gametes:                               IA      IO        IB      IO
F1 genotypes:                    IAIB   IAIO     IBIO   IOIO
F1 phenotypes:                    AB       A          B        O
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Sex-Linked Inheritance - Colour Blindness Inheritance

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Sex-linked inheritance is caused by a gene defect on the X chromosome. For example, colour blindness.

Females have two X chromosomes, therefore they need to have two copies of alleles to express the trait.

Males have one X chromosome, therefore they need only one copy of allele to express the trait.

A cross between a father who is normal and a mother who is a carrier of colour blindness

=> Normal is dominant whereas colour blind is recessive
=> R - normal, r - colour blind
=> For male: XRY (normal), XrY (colour blind)
=> For female: XRXR (normal), XRXr (carrier), XrXr (colour blind)

Parent phenotypes:        Father, normal  X  Mother, carrier
Parent genotypes:                    XRY           X          XRXr
Gametes:                              XR           Y            XR          Xr
F1 genotypes:                  XRXR      XRXr       XRY       XrY
F1 phenotypes:        Daughter,  Daughter,     Son,        Son,
                                    normal       carrier          normal    colour blind
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Monohybrid Inheritance - Monohybrid Cross

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A monohybrid cross is a cross involving one trait only. For example, plant height.

A cross between a tall plant (true breed) and a dwarf plant (true breed)

=> Tall is dominant whereas dwarf is recessive
=> T - tall, t - dwarf
=> F2 ratio is 3 : 1

Parent phenotypes:      Tall    X   Dwarf
Parent genotypes:         TT     X     tt
Gametes:                    T      T        t      t
F1 genotypes:            Tt     Tt      Tt    Tt
F1 phenotypes:       Tall   Tall   Tall   Tall

F1 self-crossed

F1 phenotypes:             Tall    X    Tall
F1 genotypes:                 Tt     X     Tt
Gametes:                    T      t        T      t
F2 genotypes:           TT    Tt      Tt     tt
F2 phenotypes:      Tall   Tall   Tall  Dwarf
F2 ratio:                      3 (Tall) : 1 (Dwarf)
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Transport Across Plasma Membrane - Active Transport, Diffusion, Osmosis

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The movement of substances across plasma membrane is classified into active transport and passive transport. Passive transport is further classified into diffusion and osmosis.

Active transport

Active transport is the movement of substances against concentration gradient, from a region of low concentration to a region of high concentration. Active transport involves carrier proteins, requires energy expenditure and the presence of membrane is not necessary. For example, movement of solutes and ions.

Diffusion

Diffusion is the movement of substances follows concentration gradient, from a region of high concentration to a region of low concentration. Diffusion does not require energy expenditure and the presence of membrane is not necessary. For example, movement of liquid or gas.

Osmosis

Osmosis is the movement of water molecules follows concentration gradient, from a region of high concentration to a region of low concentration. Osmosis does not require energy expenditure and the presence of semi-permeable membrane is necessary. For example, movement of water molecules.
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Lock-And-Key Hypothesis And Induced-Fit Hypothesis

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All enzymes are proteins, required in small quantities, reusable and denatured at high temperature. Enzyme reaction is reversible and specific. Two hypotheses about enzyme actions are lock-and-key hypothesis and induced-fit hypothesis.

Lock-and-key hypothesis

In lock-and-key hypothesis, enzyme acts as the lock and substrate acts as the key. Substrate binds to enzyme at active site, forming enzyme-substrate complex which lowers the activation energy. Products are formed and enzyme remains unchanged and reusable.

Induced-fit hypothesis

In induced-fit hypothesis, substrate binds to enzyme at active site, enzyme adjusts its shape to fit substrate, forming enzyme-substrate complex which lowers the activation energy. Products are formed and enzyme remains unchanged and reusable.
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Nitrogen Cycle - Nitrogen Fixation, Nitrification, Denitrification

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Nitrogen cycle is a continuous cycle by which nitrogen circulates in the air, soil, water and organisms. Nitrogen cycle involves three main processes which are nitrogen fixation, nitrification and denitrification.

Nitrogen fixation

Nitrogen fixation is a process which combines free atmospheric nitrogen with hydrogen to form ammonium compounds by nitrogen-fixing bacteria such as Azotobacter and Rhizobium, in root nodules of leguminous plants.

Nitrification

Nitrification is a process which converts ammonium compounds into nitrites and nitrates by nitrifying bacteria such as Nitrosomonas and Nitrobacter. Nitrosomonas converts ammonia into nitrites whereas Nitrobacter converts nitrites into nitrates.

Denitrification

Denitrification is a process which converts nitrates into free atmospheric nitrogen by denitrifying bacteria such as Pseudomonas.
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Neurone Structures And Functions - Cell Body, Axon, Myelin Sheath, Dendrites

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A neurone is the most basic functional unit of nervous system. A neurone consists of dendrites, dendrones, cell body, axon, myelin sheath and nodes of Ranvier.

Dendrites

Dendrites form connections with other neurones and transmit nerve impulses towards the cell body.

Dendrones

Dendrones transmit nerve impulses from dendrites to the cell body.

Cell body

Cell body which contains nucleus, controls the activities of the neurone.

Axon

Axon transmits nerve impulses away from the cell body.

Myelin sheath and nodes of Ranvier

Myelin sheath protects the axon from injuries; and together with nodes of Ranvier, they speed up the rate of transmission of impulses.
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Types Of Symbiosis - Commensalism, Mutualism, Parasitism

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Close living associations are called symbiotic relationships or symbiosis. Three types of symbiosis are commensalism, mutualism and parasitism.

Commensalism

Commensalism is the interaction between two organisms where one benefits and the other is unharmed. An example of commensalism is nitrogen-fixing bacteria, Rhizobium living in the nodules of leguminous plants convert atmospheric nitrogen into nitrates which are taken up by the plants to make plant proteins.

Mutualism

Mutualism is the interaction between two organisms where both benefit. An example of mutualism is lichen which composed of alga and fungus. The fungus depends on the photosynthetic algae for food, the algae depends on the fungus for anchorage, moisture and nutrients.

Parasitism

Parasitism is the interaction between two organisms where the parasite benefits and the host is harmed. An example of parasitism is tapeworms (parasites) which reside in human body (host).
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Comparisons Between Animal And Plant Cells

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Similarities Between Animal And Plant Cells

Both animal and plant cells have nucleus, cytoplasm, mitochondria, ribosomes, cell membrane, rough endoplasmic reticulum, smooth endoplasmic reticulum and Golgi apparatus.

Differences Between Animal And Plant Cells

Cell wall

In animal cell, cell wall is absent whereas in plant cell, cell wall is present.

Location of nucleus

In animal cell, nucleus is usually at the center of cell whereas in plant cell, nucleus is usually at the edge of cell.

Vacuole

In animal cell, vacuoles are usually small and numerous whereas in plant cell, vacuole is usually large and at the center of cell.

Storage of carbohydrate

In animal cell, carbohydrate is stored as glycogen whereas in plant cell, carbohydrate is stored as starch granules.

Chloroplast

In animal cell, chloroplast is absent whereas in plant cell, chloroplasts are present.

Centriole

In animal cell, centrioles are present whereas in plant cell centriole is absent.

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Comparisons Between Aerobic And Anaerobic Respiration

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Similarities Between Aerobic And Anaerobic Respiration

Both aerobic and aerobic respiration produce energy and are catalysed by enzymes.

Differences Between Aerobic And Anaerobic Respiration

Aerobic respiration occurs only in the presence of oxygen whereas anaerobic respiration does not require the presence of oxygen to occur.

Aerobic respiration is used by plants and animals whereas anaerobic respiration is used by a few bacterial groups that exist in anaerobic environment.

In aerobic respiration, the breaking down of glucose is complete whereas in anaerobic respiration, the breaking down of glucose in not complete.

Aerobic respiration uses oxygen as the final electron acceptor whereas anaerobic respiration uses a substance other than free oxygen as a final electron acceptor such as NO3-, SO43-, CO32- and others.

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ABO Blood Group System - Blood Group A, B, AB, O

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In ABO blood group system, an individual can be blood group A, B, AB or O based on the types of antigens on the surfaces of red blood cells.

Blood group A

An individual with blood group A has A antigens on the surfaces of red blood cells and antibodies against B antigens in plasma.

Blood group B

An individual with blood group B has B antigens on the surfaces of red blood cells and antibodies against A antigens in plasma.

Blood group AB

An individual with blood group AB has both A and B antigens on the surfaces of red blood cells. An individual with blood group AB is a universal recipient.

Blood group O

An individual with blood group O has both antibodies against A and B antigens in plasma. An individual with blood group O is a universal donor.

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Differences Between Nervous And Endocrine Systems

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Complexity

Nervous system is more structurally complex whereas endocrine system is less structurally complex.

Structure

Nervous system involves systems of neurones which branch throughout body whereas endocrine system involves endocrine glands which secrete hormones into the bloodstream and are carried to target organs.

Communication

In nervous system, neurones conduct electrical signals directly to and from specific targets whereas in endocrine system, hormones circulate throughout whole body in bloodstream.

Response time

In nervous system, transmission of nerve impulses is fast whereas in endocrine system, it may takes minutes, hours or days for hormones to be produced, carried in bloodstream to target organs and for responses to occur.
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Differences Between Respiration And Photosynthesis

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In respiration, energy is released whereas in photosynthesis, energy is stored as carbohydrates.

In respiration, oxygen is used, carbon dioxide and water are released whereas in photosynthesis, carbon dioxide and water are used, oxygen is released.

Respiration involves the breaking of carbohydrates whereas photosynthesis involves the synthesis of carbohydrates.

Respiration results in a loss of dry mass whereas photosynthesis results in a gain of dry mass.

Respiration takes place all the time in all cells, with or without light whereas photosynthesis takes place only in cells containing chloroplasts, in the presence of light.
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