Biology Edexcel Notes - B2

8/10/2019 Biology Edexcel Notes - B2

1/30

Topic 1The building blocks of cells

1.1Plant and animal cells

Most cells are too small to be seen with the naked eye therefore we use light microscope. In a light

microscope, a beam of light passes through the cells.

Electron microscopes pass a beam of electrons through the cells. They enable us to see in much

more detail.

You can work out the magnification of a light microscope using:

Total magnification = magnifying power of eyepiece lens x magnifying power of objective lens

The greater the resolving power, the clearer the image. Electron microscopes have a better

resolving power than light microscopes.

Resolving power = wavelength / 2

Animal cells

Cytoplasm Where most of the cellschemical reactions takeplace

Cell membrane Controls the movement ofchemicals in and out of thecell

Nucleus Contains chromosomes.Controls gene expression

and mediates thereplication of DNA duringthe cell cycle.

Mitochondria Where sugars are brokendown releasing energy.Includes a biochemicalprocess where respirationand energy productionoccurs.

Plant cells

Cell wall Made out of cellulose to strengthen the cell

Vacuole Filled with cell sap to keep the cell turgid

Chloroplasts Contain chlorophyll, which absorbs light energy for photosynthesis.


2/30

Bacterial cells

Do not have mitochondria, chloroplasts or a nucleus (chromosomal DNA and loops of DNA called

plasmid lie loose in the cytoplasm).

Cell wall not made out of cellulose

Some bacteria have a flagellum, which propels the cell through liquid.

1.2DNA

DNAdeoxyribonucleic acid

DNA consists of two molecules that are arranged in a ladder-like structure called a Double Helix

The two strands are held together by hydrogen bonds between pairs of bases.

When a cell grows and divides into two, it first has to make a duplicate copy of each DNA molecule.

This is done by the bonds breaking between the two strands, the strands unwinding, and then new

bases joining each old strand to make new strands.

A molecule of DNA is made out of several subunits called Nucleotides.

Each nucleotide consists of:

1) Phosphate groupwhich vitamins help form2) Pentose sugarfrom carbohydrates

3) Nitrogenous basewhich proteins help form

To crack the genetic code found in DNA, we need to look at the sequence of bases. Bases are

arranged in triplets called codons.

Bases consists of A (adenine), T (thymine), G (guanine) and C (cytosine). A will always join with T

and G will always join to C to form complementary bases pairings.

A gene is a section of DNA that codes for a protein. It is these proteins and combination of proteins

which give us our phenotype.

DNA can be easily extracted from cells by:

Use a detergent/salt mixture to break up the membrane of cells and release the chromosomes

Use a protein-digesting enzyme to break down the protein part of the chromosomes, releasing their

DNA

Add cold methanol, which precipitates the DNA. The strands are now clearly seen.


3/30

Discovery of DNA

In the early 1950s two scientists, Rosalind Franklin and Maurice Wilkins, studied DNA using x-rays.

Franklin produced an x-ray photograph that allowed two other researchers, James Watson and

Francis Crick to work out the 3D structure of DNA.

The structure of DNA was found to be a double helix.

The Human Genome Project

The genetic information in an organism is called its genome.

The Human Genome Project began in 1989. It was very ambitious and had several aims, including:

To work out the order or sequence of all the three billion base pairs in the human genome

To identify all the genes

To develop faster methods for sequencing DNA

The sequencing project was finished in 2001, and work continues to identify all the genes in the

human genome.

The project involved scientists from 18 different countries and showed how scientists collaborate.

The scientists broke up the chromosomes of cells to get their DNA, and placed them in sequencers

to display the most likely order of this base.

Powerful computer were used to help match the base sequences of genes with the proteins for

which are the code.

Knowing the human DNA profile raises some ethical questions, but many consider this knowledge to be of

benefit:

Improved genetic testing

Location of genes that might be linked to increased chances of inheriting a disease

New gene therapy treatments

New knowledge of how humans have evolved

Personalised medicines

1.3Genetic engineering

Differences between Genetic engineering and cloning

Cloning Genetic engineering

Produces exact copies Produces a unique set of genes

Genes copied within the same species Genes can be swapped across species


4/30

How it works

A strand of DNA from the cell carrying a gene which enable cells to produce useful proteins is cut

out using a restriction enzymein order to isolate the gene.

Bacteria contains circular DNA called plasmid DNA which is also cut out using a restriction enzyme

The gene is inserted into the plasmid using ligase enzyme.

The recombinantplasmid is then placed back into the bacterial cell

The bacteria multiplies and produces millions of identical clones, with the DNA coding for required

protein. The bacteria grows in fermenters. The end product is removed from the fermenter.

Uses

GM insulin

Natural insulin can be taken from the pancreas of a pig or cow. It is used to treat diabetes but is

limited in supply and doesn't suit all people.

Modern practice is to create insulin synthetically using genetically modified bacteria.

The gene for insulin secretion is cut from a length of human DNA and inserted into the DNA of a

bacterium.

Advantages of GM insulin

Cheap to produce and available in large quantities

It is (human insulin), so the users do not experience any allergic reactions or tolerances.

Does not use animal products so it is not a problem for religious groups or vegetarians.

Wild rice

Scientists have added a gene to wild rice that makes it produce beta carotene. This changes the

colour of the wild rice to a golden colour.

Beta carotene is needed by humans in order to make Vitamin A.

Advantages Disadvantages

Can be used in areas where Vitamin A

deficiency is common and so can helpprevent blindness.

Not meant to be the only solution to VitaminA deficiency

Fears that it will crossbreed with andcontaminate wild rice Worries that GM organisms might harmpeople Beta carotene levels aren't high enough tomake a difference


5/30

GM organisms can be expensive

Herbicide resistant crops

Scientists have added genes to crop plants that make them resistant to herbicides.

This reduces the quantity of herbicide that needs to be used.

Potential disadvantages of this genetic modification include:

The potential development of herbicide-resistant weeds

Loss of biodiversity as fewer weed species survive as a food and shelter source for animals

GM crops

Grow in places with low rainfall

Produce their own chemicals to kill insects to damage them

Resist diseases and herbicides

Worries are:

Not natural

Eating GM food may affect our health

Crops may harm wildlife

1.4Mitosis and meiosis

There are two types of cell division.

Mitosis is used for growth and repair and produces diploid cells identical to each other and the

parent cell.

Meiosis is used for sexual reproduction and produces haploid cells different to each other and theparent cell.

Mitosis

Mitosis is the type of cell division used for growth, repair and asexual reproduction.

Mitosis occurs wherever new cells are needed. It produces two daughter cells that are identical to

each other from the parent cell.

In mitosis each chromosome is copied exactly.

The new chromosomes are moved to opposite sides of the cell, before the cell divides leaving one

complete set of 46 chromosomes in each of the two new cells.


6/30

Meiosis

Meiosis is a different kind of cell division. It is used to produce male and female gametes.

A human body cell contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or

eggs, and only contain half as many chromosomes (23).

This is why meiosis is sometimes called reduction division.

Fertilisation

In humans all reproduction is sexual. It involves joining together haploid gamete cells from each

parent with half the normal number of chromosomes to make a diploid zygote.

The cells from each parent that combine to form the zygote are called gametes. In humans, the

male gamete is called sperm, and the female gamete is called an egg.

When the gametes join they form a cell called a zygote. Human sperm and eggs contain 23

chromosomes. Human zygotes contain 46 chromosomes.

The type of cell division that produces gametes with half the normal chromosome number is called

meiosis. It is used to produce cells for repair and asexual reproduction

Gametes contain different genetic information to each other and to the parent cell. Meiosis is

responsible for causing genetic variation.

1.5Cloning

Asexual reproduction

Plants can make identical copies (clones) of themselves. Many plants have ways of increasing their

numbers by asexual reproduction


7/30

New plants are created by repeated cell division:

A potato plant can produce many tubers, each of which can grow into a new plant.

Strawberry plants and spider plants produce long stems with tiny plants on the end. These runners

can produce several new plants from one parent.

It's fairly easy to artificially produce new plants by taking a cutting, and waiting for the cutting to

develop new roots and leaves.

Process of cloning

The nucleus of a body cell of the animal to be cloned is transferred to anenucleatedegg cell (one

that has had its original nucleus removed).

The cell is then stimulated to start dividing to form an embryo.

This is implanted into the uterusof the surrogate motherwho is a different individual to the parent

A new individual develops and is genetically identical to the animal which wished to be cloned.

Ethical issues

Animal cloning raises ethical issues about how far humans should be allowed to interfere in the

production of new life.

Cloning plants is easier than cloning animals. Cloning expensive food crops has been carried out for

many years, and causes the public fewer ethical and moral concerns than animal cloning.


8/30

How to clone cows using embryo transplants

This technique could be used to make many copies of cows that have a high milk yield.

It would produce a herd of cows much faster than if the original cow was used for breeding in the

normal way. Sexual reproduction is still involved and the calves are not identical to either parent.

CloningAdvantages Disadvantages

All the new plants are geneticallyidenticalthey will all have thedesired characteristics.

Organisms that are difficult orslow to breed normally can bereproduced quickly. Some plantvarieties do not produce seeds,others have seeds that aredormant for long periods.

If a clone is susceptible todisease or changes inenvironment, then all the cloneswill be susceptible.

It will lead to less variation, andless opportunity to create newvarieties in the future.

Stem cells

During the development of an embryo most of the cells become specialised. They cannot later

change to become a different type of cell.

But embryos contain a special type of cell called stem cells. These can grow into any type of cell

found in the body. They are not specialised.

Stem cells are extracted from a 5-7 day old blastocyst.

Stem cells can divide in culture to form more of the same kindthis is called a stem cell line.


9/30

Stem cells could be used for:

Making new brain cells to treat people with Parkinson's disease

Repairing damaged immune systems

Making replacement heart valves

Therapeutic cloning

If you were to receive medical treatment with cells grown from stem cells, your body's immune

system would recognise the cells as foreign, and they would be rejected and die.

But this would not happen if you received cells with the same genes as you.

This could be done by cloning one of your cells to produce an embryo, then taking stem cells from

this.

Here are the steps involved:

Nucleus taken out of a human egg cell

Nucleus from a patient's cell put into the egg cell

Egg cell stimulated to develop into an embryo

Stem cells taken from the embryo

Stem cells grown in a container of warm nutrients Stem cells treated to develop into required cell types

Stem cell research

For Against

Research groups such asESCR (Embryonic StemCell Research) fulfils theethical obligation toalleviate human suffering

Since excess IVFembryos are discardedanyway, wouldnt it bebetter to use it forvaluable research?

Therapeutic cloningproduces cells in a petridish, not a pregnancy.

In ESCR, stem cells aretaken from a humanblastocyst, which is thendestroyed. This amountsto murder.

There is a risk ofcommercial exploitation ofparticipants of ESCR.


10/30

1.6Protein synthesis

Peptideschains of 220 amino acidsPolypeptideschains of 2150 amino acids

Ribonucleic acid (RNA) is a chemical like DNA. However, RNA are single stranded and has U baseinstead of T

RNA has two roles in protein synthesis:

Messenger RNA (mRNA)carries the protein-making information from the DNA inside the nucleusof the cell to the ribosomes in the cytoplasm, where the protein is made.

Transfer RNA (tRNA)carries the amino acids needed to form the protein to the ribosomes

There are two stages: transcription (which occurs in the nucleus) and translation (which occurs in the

cytoplasm).

Transcription1) The strands of DNA separate2) Strands of mRNA form as the bases of RNA nucleotides combine with their complementary bases

of the single stranded DNA.3) The strands of mRNA separate from their respective strands of complementary strands of DNA.

They pass from the nucleus through gapsTranslation

4) Each strand of mRNA binds to a ribosome creating an mRNA-ribosome complex.5) Each type of tRNA molecule binds to its particular type of amino acid dissolved in the cytoplasm,

depending on the codon it carries.

6) tRNA/amino acid combinations pass to the mRNA-ribosome complex. The exposed bases of tRNAbind to the complementary bases of mRNA. Chemical bonds form between the amino acids next toeach other.

7) Once the bonds form, each tRNA separates from its amino acid and the mRNA strand.8) The linked amino acids form a polypeptide.

1.7 - Enzymes

Enzymes are soluble protein molecules that can speed up chemical reactions in cells.

These reactions include respiration, photosynthesis and making new proteins. For this reasonenzymes are sometimes called biological catalysts.

Enzymes speed up (catalyse) chemical reactions occurring inside and outside of living cells. This includes:

DNA replication(DNA polymerasebreaks down the double helix before DNA replication, alsoinvolved in checking the copying of the DNA strand.)

Protein synthesis (Speeds up the rate of the joining together of individual amino acids)


11/30

Digestion (Different enzymes break down large food molecules in the mouth, stomach and smallintestine to smaller ones so they can be absorbed into the cells).

Each enzyme will only speed up one reaction as the shape of the enzyme molecule needs to matchthe shape of the molecule it reacts with (the substrate molecule).

The part of the enzyme molecule that matches the substrate is called the active site.

Enzymes and temperature

At low temperatures, enzyme reactions are slow.

They speed up as the temperature rises until an optimum temperature is reached. After this point

the reaction will slow down and eventually stop.

Enzymes and pH

Most enzymes work fastest in neutral conditions. Making the solution more acidic or alkaline will

slow the reaction down. At extremes of pH the reaction will stop altogether.

Some enzymes, such as those used in digestion, are adapted to work faster in unusual pH

conditions and may have an optimum pH of 2 (very acidic) if they act in the stomach.


12/30

Enzymes and substrate concentration

Enzymes will work best if there is plenty of substrate available. As the concentration of the substrate

increases, so does the enzyme activity.

However, the enzyme activity does not increase without end. This is because the enzyme can't work

any faster even though there is plenty of substrate available.

The lock and key mechanism

Enzymes work best at their optimum temperature. This is why homeostasis is important - to keep

our body temperature at a constant 37C.

As the temperature increases, so does the rate of chemical reaction. This is because heat energy

causes more collisions, with more energy, between the enzyme molecules and other molecules.

However, if the temperature gets too high, the enzyme is denaturedand stops working.

Enzymes are specific. Only molecules with the correct shape can fit into the enzyme. Just like only

one key can open a lock, only one type of enzyme can speed up a specific reaction. This is called

the lock and key model.

Denaturing the enzymes

The important part of an enzyme is called the active site. This is where specific molecules bind to

the enzyme and the reaction occurs.

Anything that changes the shape of the active site stops the enzyme from working. The shape of the active site is affected by pH. This is why enzymes will only work at a specific pH,

as well as a specific temperature. Change the pH and the enzyme stops working.

Increasing the temperature to 60C will cause a permanent change to the shape of the active site.

This is why enzymes stop working when they are heated. We say they have become denatured.

Topic 2Organisms and energy

2.1Respiration

Respirationseries of chemical reactions that oxidise glucose, releasing energy. In the presence of

oxygen, glucose is oxidised to carbon dioxide and water.

Aerobic respiration

Respiration is not the same thing as breathing. That is more properly called ventilation.


13/30

In aerobic respiration, oxygen is used to release energy from molecules such as glucose.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process

happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygen carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. (In plants,

for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids canthen join together to make proteins.)

The energy is used to:

To allow muscles to contract in animals

To maintain a constant body temperature in birds and mammals

The circulatory system

Blood carries oxygen and nutrients to the body's cells, and waste products away from them.

The diagram outlines the circulatory system. Oxygenated blood is shown in red, and deoxygenated blood in

blue.

A process called diffusion takes place in the capillaries.

Diffusionis where particles of a high concentration move to an area of low concentration.

Glucose and oxygen diffuse into the cells from the capillaries. Carbon dioxide diffuses out of thecells into the blood in the capillaries.

Anaerobic respiration

When exercising very hard, the heart cannot get enough oxygen to the muscles. Anaerobic

respiration does not need oxygen. It releases energy from glucose but the amount is much lower.

It happens when there is not enough oxygen for aerobic respiration.

Here is the word equation:

glucose lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration. The lactic acidthat forms causes muscle fatigue and pain.

Effect of exercise on breathing


14/30

During exercise, the muscle cells respire more than they do at rest. This means:

Oxygen and glucose must be delivered to them more quickly

Waste carbon dioxide must be removed more quickly

This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected

by measuring the pulse rate.

The stroke volume also increasesthis is the volume of blood pumped each beat

The total cardiac output can be calculated using the equation:

Cardiac output = stroke volume x heart rate

During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells.

When this happens, anaerobic respiration takes place, as well as aerobic respiration.

Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate

is likely to return to normal more quickly after exercise.

The after effect of exercise

During hard exercise when anaerobic respiration occurs with aerobic respiration, an oxygen debt

builds up. This is now known as Excess Post-exercise Oxygen Debt or EPOC.

This is because glucose is not broken down completely to form carbon dioxide and water. Some of

it is broken down to form lactic acid.

Panting after exercise provides oxygen to break down lactic acid.

The increased heart rate also allows lactic acid to be carried away by the blood to the liver, where it

is broken down.

2.2Photosynthesis

A process in which plants absorb sunlight, carbon dioxide and water to produce glucose and oxygen.

Structure of a leaf

The equation for photosynthesis is:

Carbon dioxide and water glucose and oxygen

Adaption Purpose

Large surface area To absorb more light

Thin Short distance for carbon dioxide to diffuse into leafcells


15/30

Chlorophyll Absorbs sunlight to transfer energy into chemicals

Network of veins To support the leaf and transport water andcarbohydrates

Inside of a leaf

Adaption Purpose

Epidermis is thin and transparent To allow more light to reach the palisade cells

Thin cuticle made of wax To protect the leaf without blocking out light

Palisade cell layer at top of leaf To absorb more light

Spongy layer Air spaces allow carbon dioxide to diffuse throughthe leaf, and increase the surface area

Factors affecting photosynthesis

Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water

and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.


16/30

Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there

is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.

If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets

too hot.

Farmers can use their knowledge of these limiting factors to increase crop growth in greenhouses.

They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a

higher-than-normal light intensity.

The use of paraffin lampsinside a greenhouse increases the rate of photosynthesis because the

burning paraffin produces carbon dioxide, and heat too.

2.3Transpiration

Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem

cells without the use of a pump.

Transpirationthe movement of water and mineral salts from roots to leaves in plants.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out

of the leaf.

More water is drawn out of the xylem cells inside the leaf to replace what's lost.

As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a

drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. When

water is scarce, or the roots are damaged, it increases a plant's chance of survival if the

transpiration rate can be slowed down.


17/30

Plants can do this themselves by wilting, or it can be done artificially, like removing some of the

leaves from cuttings before they have chance to grow new roots.

Factors that affect transpiration rate

Factor Description Explanation

Light In bright light transpiration

increases

The stomata (openings in the leaf)

open wider to allow more carbon

dioxide into the leaf for

photosynthesis

Temperature Transpiration is faster in higher

temperatures

Evaporation and diffusion are

faster at higher temperatures

Wind Transpiration is faster in windy

conditions

Water vapour is removed quickly

by air movement, speeding up

diffusion of more water vapour out

of the leaf

Humidity Transpiration is slower in humid

conditions

Diffusion of water vapour out of

the leaf slows down if the leaf isalready surrounded by moist air

2.4Plant transport

They use two different systems for transport:

Xylem moves water and solutes from the roots to the leaves

Phloem moves food substances from leaves to the rest of the plant.

Both of these systems are rows of cells that make continuous tubes running the full length of the plant.

Xylem

Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the

plant.

For this reason, the transport systems are arranged differently in root and stem:

In the root it has to resist forces that could pull the plant out of the ground (resist stretching forces)

In the stem it has to resist compression and bending forces caused by the weight of the plant andthe wind.


18/30

Comparison between xylem and phloem

Tissue Process What is moved Structure

Xylem Transpiration Moves water and

minerals from roots to

leaves

Columns of hollow,

dead reinforced cells

Phloem Translocation Moves food substances

from leaves to rest of

plant

Columns of living cells

2.5Root hair cells and osmosis

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large

surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it is used for different

purposes:

It is a reactant used in photosynthesis

It supports leaves and shoots by keeping the cells rigid

It cools the leaves by evaporation

It transports dissolved minerals around the plant

Leaves

Leaves are adapted for photosynthesis by having a large surface area, and contain openings, called

stomata to allow carbon dioxide into the leaf.

Although these design features are good for photosynthesis, they can result in the leaf losing a lot of

water.

The cells inside the leaf have water on their surface. Some of this water evaporates, and the water

vapour can then escape from inside the leaf by diffusion

To reduce loss the leaf is coated in a wax cuticle to stop the water vapour escaping through the

epidermis.

Leaves usually have fewer stomata on their top surface to reduce this water loss.


19/30

Reducing water loss

Stomata

Plants growing in drier conditions tend to have small numbers of tiny stomata and only on theirlower leaf surface, to save water loss.

Most plants regulate the size of stomata with guard cells.

In low light the guard cells lose water and become flaccid, causing the stomata to close. They would

normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Turgidity

Most plant cells are turgid at all times. This supports the weight of the plant, which is especially

important where there is no woody tissue, such as leaves, shoot and root tip.

If the plant loses water faster than it can be absorbed the cells lose turgor pressureand begin to

wilt.

Osmosis

Osmosisis the movement of water molecules from an area of high concentration of water to an

area of lower concentration of water through a partially permeable membrane.

An example is the flooding of plants by sea water. Sea water contains many chemicals in solution,

such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the

stronger solution.

2.6Fieldwork techniques

Sampling

Ecologists will want to know certain information about the species present:

Where an organism is found (distribution)

The number of that organism present (population)

Whenever a scientist studies an area it is usually not possible to look at the entire environment indetail. Therefore, the scientist samples a section or small portion.

Sampling several small sections is representative of the whole area. The sampling technique useddepends on the habitat and type of organisms present.


20/30

Pooters

A pooter is used to catch small insects.

The user breathes in through the mouthpiece which has a piece of net covering the end.

The insects are sucked into the holding chamber via the inlet tube.

Sweep nets

Sweep nets are used in areas of long grass to catch organisms. They can also be used in ponds.

Pitfall traps

Pitfall traps are used to catch small, crawling insects. They can be set up and left overnight to catch nocturnal species.

All organisms caught should be released unharmed.

Quadrats


21/30

They are used to sample all the plant species in a particular area.

Environmental factors

Device Description

Light intensity can be measured using a light

intensity meter.

pH is measured with a pH probe

Temperature is measured with a temperature

probe

Sometimes a single probe can be used to read

both pH and temperature


22/30

Topic 3Common systems

3.1Fossils and evolution

Fossilsthe preserved remains of organisms that lived a long time ago

Fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex

organisms in the newest rocks. This supports the theory of evolution, which states that simple life

forms gradually evolved into more complex ones.

Certain environmental conditions drastically slow down the decaying process, helping to preserve the

tissues. Examples of this are:

Insufficient oxygen, e.g when an organism becomes trapped in amber

Low temperatures, e.g when an organism becomes frozen in a glacier

High soil acidity, e.g when an organism falls into a peat bog

If these conditions are not present, the remains will not be fossilised.

This makes tracing the story of evolution of any one species challenging.

In most cases there are big gaps in fossil records. Problems also arise as soft tissues decayresulting in scientists having to estimate what the organism was like. Finally, there are also lots of

fossils that we haven't yet found.

The pentadactyl limb

Many vertebrates have a very similar bone structure despite their limbs looking very different on the

outside. This structure is known as the pentadactyl limbfive fingered.

This suggests that many vertebrates descended from the same common ancestor.

The evolution of the horse

Time period Height

Modern horse

1 million years ago 1.6 metres


23/30

Time period Height

Pliohippus


Merychippus


Mesohippus


Eohippus


3.2Growth

Humans are made of millions of cells. This has a number of benefits:

Cells can be specialised to do particular tasks.

Groups of cells can function as organs, making a more efficient but complex organism.

The organism can grow very large

Growth- Growth can be defined as an increase in size, length and mass.

Growth in animals and plants

Feature Plant Animal

Where growth occurs Mainly at shoot and root tips and in

special growth zones like buds

New cells can be made by most

tissues

How growth occurs Size increase often caused by

increasing the size (elongation) of

cells by absorbing water into the

vacuole

Size increase is brought about by

increasing the number of cells

Cell specialisation Most plant cells can differentiate

into different cell types

Only stem cells can differentiate into

different cell types. Other animal cell

functions remain fixed


24/30


25/30

White blood cells

Have a nucleus

Lymphocytesmake antibodiesbind to microorganisms that cause disease and destroy them

Phagocytessurround and destroys foreign cells that enter the body

Contain a cytoplasmallows them to access tissues so they can protect the body by attacking and

destroying bacteria and viruses.

Plasma

Straw-coloured liquid

Transports dissolved substances around the body, such as hormones, nutrients and waste

substances.

Platelets

Fragments of cells with no nucleus. They also contain protein

When platelets are damaged by a cut, they release a substance that starts a chain of chemical

reactions in the blood.

Fibrogenproduced soon turns into insoluble fibrin as the scab formed dries out.

3.4The heart

Cells, tissues and organs

A living tissue is made from a group of cells with a similar structure and function, which all work

together to do a particular job.

An organis made from a group of different tissues, which all work together to do a particular job.

An organ systemis made from a group of different organs, which all work together to do a

particular job.

Heart


26/30

1. A vena cava brings deoxygenated bloodfrom the body into the right atrium.

2. When the atrium is full, muscles in the wall contract and force blood through the values into the

right ventricle. The values have tissue to prevent backflow.

3. When the ventricle is full, the ventricle wall contracts forcing blood through the pulmonary artery

where it picks up oxygen from the lungs.

4. The oxygenated bloodreturns enter the left atrium through the pulmonary vein.5. When the atrium is full, the muscle wall contracts and forces blood through the left ventricle.6. When ventricle wall contracts, this forces the blood out of the aortato respiring cells in the body.

The left ventricle exerts more pressure than the right ventricle, and so it has a thicker moremuscular wall.

The atria (plural of atrium) exert less pressure than the ventricles so they have a thinner muscularwall.

3.5The circulatory system

Blood vesselsthat tube shaped organs that carry blood. There are three types: arteries, veinsand capillaries.

Arteries

Carry blood away from the heart.

The blood in arteries has to be under pressure so that it can reach all parts of the body. Therefore,

arteries have strong, thick walls.

Capillaries

Allow substances to diffuse into and out of the blood, into the cells in tissues.

To help this process, they have very think walls


27/30

Veins

Have wide passages inside them to carry blood to the heart.

They have to be wide because the blood flows relatively slowly under low pressure.

3.6The digestive system

The digestive system is made out of the alimentary canala muscular tube running through thebody from the mouth to the mouth of the anus and several other organs that make chemicals fordigestion (including enzymes)

Process

1) Mouth

Food is broken into smaller pieces during chewing. This increases the surface area for digestiveenzymes to work on.

The bolus (formed by the tongue) is lubricated with saliva in order to make it easier to swallow.

Salivary amylase breaks down the starch in the food

2) Oesophagus

A muscular tube between the mouth and the stomach.

Peristalsis occursmuscles contract in waves to push the food towards the stomach

3) Stomach

A muscular tube that makes hydrochloric acid and some enzymes (such as proteaseto digest

proteins) Churns the food up with these juices by peristalsis to make a thick paste.

4) Small intestine

A long, coiled muscular tube in while large insoluble food molecules are turned to smaller solubleones.

Contains digestive enzymes made by the pancreas and makes its own digestive enzymes as well.

Molecules of digestive food are absorbed into blood using villi which contain capillaries.Food ismoved on using peristalsis.

Pancreasthis organ makes digestive enzymes and releases them to the first part of the small intestine.

5) Large intestine

Undigested food passes into this wide, thin-walled tube.


28/30

Water is diffused back into the blood leaving faeces behind.

6) Anus

Undigested food is passed out of the body

Liver

Digestive food is absorbed by the small intestine and dissolves in the blood plasma. Once in theblood, it is taken to the live to be processed. Some of the molecules are broken down even more.Some are built into large molecules again.

The liver makes bilewhich helps in digestion of fats.

Gall bladder

A small organ that stores bile made by the liver and releases it to the small intestine when needed.

3.7Breaking down food

Digesting carbohydrates

Digestive enzymes that break down carbohydrates are called carbohydrases.

Amylase is a carbohydrase that break down starch into sugarswhich can then be absorbed bythe small intestines or broken down into glucose by other carbohydrases.

An amylase is present in saliva. Another amylase is present in the pancreas and released into thesmall intestine.

Digesting proteins

Proteases are enzymes that digestproteinsbreaking them into shorter chains then amino acids.

Pepsinis a protease made in the stomach. It works well in acidic conditions. The acid produced inthe stomach is pH 2-3. This is the optimum pHfor pepsin to break down protein as fast as possible.

However, the contents in the small intestine is alkaline so the proteases released into the smallintestine work best at pH 8.

Digesting fats

Lipases digest fatsand break down the fat molecules into fatty acidsand glycerol.

Fat and water do not mix so fats and oils form globules in the watery digestive juices. Bile is

secreted into the small intestine, where it emulsifiesfats. This is important, because it provides alarger surface area in which the lipases can work.


29/30

3.8Villi

The inside wall of the small intestine is thin, with a large surface area. This allows absorption tohappen quickly and efficiently.

To get a big surface area, the inside wall of the small intestine is lined with tiny villi. These stick outand give a big surface area.

They also contain blood capillaries to carry away the absorbed food molecules.

The villi have a rich blood supply. The blood supply has a lower concentration of food moleculesand this steep concentration gradientbetween the small intestine and the blood means diffusionoccurs quickly.

Thin wallone cell thick.

3.9Probiotics and prebiotics

Probiotics

Contain live, beneficialbacteria which are usually Lactobacillusand Bifidobacteriathat producelactic acid in the gut.

Manufacturers of probiotic foods (such as yogurt) claim that these make you healthier by improvingyour digestive system helping your body protect itself against diseases and reducing allergies.

However, in 2010, scientists at the European Food Safety Agency looked at the evidence for 180health claims for probiotics. 10 were rejected completely whilst there was not enough evidence tosupport the other 170 claims.

Prebiotics

Substances that the body cannot digest. They act as food for the beneficialbacteria in the gut andencourage their growth.


30/30

Found in tomatoes, bananas, onions which all contain oligosaccharides, a common form ofprebiotic. Can be also found in dairy products or sold in capsules.

Plant stanol esters

Oily substances found in plants. Scientists have discovered that these can stop the small intestinefrom absorbing cholesterol, lowering the levels of cholesterol in the blood. High cholesterol leads toan increased risk of heart disease.

Used in many food such as spreads, drinks, etc. There is clear evidence that this has an effect.

Documents

Biology Edexcel Notes - B2