Nitrogen cycle

Nitrogen gas in the atmosphere is turned into nitrogen compounds that a plant can use. The plant is eaten or decayed by decomposers forming ammonia compounds either from decay or the urea of animals. The ammonia is then turned into nitrates by nitrifying bacteria. Nitrates are then converted back into nitrogen gas in the atmosphere by de-nitrifying bacteria.

Human influences on the environment

CO2 and other greenhouse gases: CO2, Methane, water vapour

1. Released from deforestation
2. Respiration
3. Burning of fossil fuels

The greenhouse effect is the trapping of heat within the atmosphere which increases global warming. It then radiates the heat back down towards earth.

SO2 is released from the burning of fossil fuels and released from sewage treatment plants. Can cause acid rain and erosion of rock i.e. limestone.

Carbon monoxide is released from incomplete combustion. It can bind irreversibly to haemoglobin thus preventing oxygen from being transported around the body.

Ammonia [from fertilisers] can be leached into rivers and streams.

Leaching is when excess dissolved nutrients and minerals from fertilisers run into streams and rivers and pollute water. This causes algae to bloom in immense quantities. Algae prevents sunlight from reaching plants on the river bed hence prevents plants from photosynthesising. The plants would die and the quantity of dead matter in the water. This pollutes the water even more. Bacteria respire aerobically to decompose the matter. This uses up all the oxygen in the water hence the organisms cannot respire. Nitrates and phosphates.

Natural selection

Natural selection occurs when a mutation arises in members of a species.

If the mutation is beneficial to the organism, it will live longer and reproduce more.
More of its offspring would have inherited the characteristic and too will survive.
This will continue over generations until the mutation becomes a common gene in the species in which those without will not be able to compete and will die.

Mutations

Occur when a gene has been copied incorrectly. This is not done on purpose and this faulty gene can be passed down through generations.

Stages of mitosis

1. Prophase is when the dna begins to condense and become more visible. The nucleus has also disappeared.
2. Metaphase is when the spindle of fibres have formed across the length of the cell and are prominent.
3. Anaphase is when the fibres have split in half and are pulled to the opposite sides of the cell.
4. Telophase is when the chromosomes have reached the opposite sides of the cell and the nuclear membrane has begun to reform. Cytokinesis has occurred and two identical daughter cells have been created.

Cytokinesis is the split of the cell physically.

Meiosis and Mitosis [haploid and diploid]

Meiosis occurs in the ovaries and in the testes. It is when one cell divides to produce four haploid cells i.e. gametes. A haploid cell is one with one chromosome i.e. half the genetic makeup. As a result, gametes are genetically different.

Mitosis occurs during growth and development. It is when one cell divides to produce two diploid cells. Diploid cells are those with a pair of chromosomes i.e. the full genetic makeup.

Offspring are genetically different to their parents as they have different genotypes as a result of the receipt of different chromosomes.

Note that it is at the stage of a foetus where cells begin to specialise.

DNA structure and purpose

DNA is found in the nucleus of any cell [except red blood cells which have none]. They must be able to do two things:

1. Contain instructions which control protein production hence which control the characteristics of an organism.
2. Be able to replicate in a way that preserves all the genetic information required. It should be able to be transferred from a parent to a child.

DNA is in the form of a double helix, with two strands forming the shape of a twisted ladder. Each 'rung' of the ladder consists of a base, Adenine, Thymine, Cytosine and Guanine. Bases are held together by intermolecular forces.

A sequence of bases contains a code that informs the cell of what protein to make. There are millions of sequences and a vast number of proteins that can be produced. A section of DNA that contains the instructions to create on complete protein is called a Gene.

DNA = sentence
Gene= Paragraph
Chromosome= Chapter

We have a pair of each chromosome so that there we have two copies of each gene which results in more variation within our species. A gene can come in different forms such as alleles. Alleles can be either dominant or recessive. We may have a chromosome from our mother who has the gene for brown hair and a chromosome from our father who has a gene for blue eyes. We have characteristics from both parents. A child who looks very much like their parent means their alleles are most likely dominant.

Source: BBC Bitesize

A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

A dominant allele is the one that will be made.
A recessive allele is one that will be masked.
A homozygous genotype is one that contains two of the same allele.
A heterozygous genotype is one that contains two different alleles, one dominant and one recessive.
Phenotype: A set of characteristics in an organism that originate from a combination of a genotype and interaction with the environment.
Genotype: Set of genes in DNA responsible for a particular trait.

Placenta and amniotic fluid

The placenta is significant for the transfer of molecules from the mother's blood to the embryo's blood stream. This is imperative as the embryo cannot respire, digest or excrete.

The placenta is spread out over a large surface area so that the diffusion of molecules occurs quickly.

Molecules transferred include:

1. Oxygen
2. Carbon dioxide
3. Glucose
4. Urea
5. ANTIBODIES [provides specific immunity against various diseases].

The amniotic fluid provides support, protection and allows movement. It also regulates body temperature and absorbs pressure to the uterus hence protecting the embryo.

Coagulation is the mix of different blood types. Although the placenta must be situated close to the embryo to ensure diffusion occurs quickly, it is important that the mother's blood and embryo's blood do not mix.

Menstrual cycle

The overall function of the menstrual cycle is to release an ovum into the reproductive system and for it to be successfully fertilised, implant and grow to term.

There are three main stages of the menstrual cycle:

1. Preparation: When the egg matures in the ovaries and the uterus wall lining has an increasing blood supply.
2. Ovulation: The release of an ovum from the ovaries to fallopian tubes.
3. Maintenance: When the blood supply of the uterus wall lining is maintained so that the egg may be fertilised and implant into the wall.

FSH [follicle stimulating hormone] is released from the pituitary gland when there are no significantly high levels of hormones in the bloodstream. It causes an egg to mature within a follicle in the ovaries.
Oestrogen is released from the ovaries as the egg begins to mature. It increases the blood supply to the uterus thus thickening the wall lining. It causes FSH to cease and LH to be released from the brain. LH causes the egg to be released from the follicle and causes the follicle to change structure into a corpus luteum. The corpus luteum is yellow. The corpus luteum produces progesterone. This maintains the blood supply to the uterus until the corpus luteum decays.

Oestrogen: develops secondary sexual characteristics in females.
Testosterone: ' in males.

1. Breast development
2. Increased body hair + pubic hair
3. Regulation of body mass - redistributed to hips and breasts
4. Development of sexual organs
5. Voice deepens

1. Development of sexual organs
2. Voice deepens
3. Pubic hair
4. Increased muscle mass

Structure of human reproductive systems

Female:

Ovaries: Where eggs are produced along with oestrogen [follicles] and progesterone [corpus luteum].
Fallopian tubes: Where ovum[a] are fertilised. They contain cilia which beat rhythmically to move the egg or zygote along into the uterus.
Uterus: This is where a zygote would implant itself and develop during the gestation period [pregnant period].
Cervix: This is a powerful muscle that prevents bacteria from entering the uterus. It produces mucus and several alkaline solutions.
Vagina: The walls secret a fluid that acts as a lubricant. Its muscular walls contract during childbirth.

Male:

Penis: Contains a vast number of nerve endings which allows it to be stimulated. It also allows muscle contraction for when the sperm is released.
Testicle: This is where sperm are produced. They are kept outside of the body so that the sperm can develop under cooler conditions.
Epididymis: This is where sperm is stored. As it is coiled, the area in which the sperm can be kept increases hence a larger volume of sperm can be stored.
Vas Deferens: A tube with muscular walls that contracts to push sperm.
Prostate gland: Along with the seminal vesicle, this gland produces a liquid called semen which sperm can swim in and gain nourishment from.
Bladder: Where urine is stored.
Urethra: The muscular tube in the penis that contracts to push sperm [in semen] out of the penis.

Cloning and selective breeding

Cloning can be viewed as artificial asexual reproduction. It is the process by which genetically identical copies of organisms are made. The characteristics of the cloned organism are identical to that of the original natural organism as the DNA in both is the same.

Micropropagation: 

The process of growing plantlets in tissue culture then planting them out.

1. Clippings of a plant are placed in a growth medium to produce shoots.
2. The plantlets are then transferred to a root medium to encourage the growth of roots.
3. The plant will then be transferred into soil where it will grow into a plant with identical characteristics as the natural plant.

Humans have been able to clone plants for thousands of years through their own natural process of asexual reproduction. However, humans have started to clone animals. This is done by splitting an embryo into several embryos hence artificially recreating the formation of twins.

Selective breeding: 

The breeding of two organisms with the desired characteristics for human use.

Organisms with the desired characteristics are selected.
These organisms are bred.
The offspring with the desired characteristics are selected.
These specific offspring are bred.
The process is repeated over several generations.

Advantages:

A large number of organisms with the desired characteristics and domesticated organisms.

Disadvantages:

There is no variation and as a result, disease would affect all of these organisms, potentially killing all of them.

Plants:

1. Big fruits
2. Nice colour
3. Desirable scents and flavours
4. Tolerant to a specific temperature
5. Resistant to a particular disease.

Animals:

1. Large muscle mass
2. Domesticated [lack of aggression]
3. Certain animals may produce more fur or wool

Natural methods of asexual reproduction in plants

Runners from plants such as strawberries extend towards the ground. They then specialise into specific cells such as root cells and a whole new plant develops.

There are artificial methods also such as cuttings:

A branch is cut off from a plant and the leaves are removed. It is then placed in soil and covered in a CLEAR plastic bag to retain the warm temperature and keep it moist. After a few weeks, new roots develop and a new plant begins to grow.

Seed germination

After fertilisation, seeds need to be removed from the plant and grow elsewhere so they are not in direct competition for resources such as minerals, water, sunlight etc. Therefore there needs to be an effective dispersal method of the seeds.

1. Through fruit. An animal may eat the fruit and the seed, inside, is resistant to the digestive juices hence it passes out through faeces.
2. Some seeds have small hooks which stick to the fur of animals.
3. Other seeds are very light so that they can be carried by the wind.

Germination is the process by which a plant emerges from the seed and begins growth. There are various conditions required for this to happen at a good rate:

1. Air supply
2. Water supply
3. Correct temperature to ensure the area is suitable for growth.

The food store in a seed is large. It is called the cotyledon. It contains starch which is a store of glucose. The glucose is utilised in respiration which enables mitosis to occur. [mitosis is the division of cells].This continues until the plant is large enough to start producing glucose from photosynthesis.

The embryo is where the cells of the seed are situated. These multiply through mitosis hence the seed will grow.

The testa is the seed coat. Water causes the testa to split which enables the radicle to grow downwards to form roots and the plumule to grow upwards to form the plant.

Fertilisation in a plant

When the pollen grain has landed on the stigma, it sticks as the stigma produces two chemicals. The stigma also produces a hormone which enables the pollen to produce an enzyme. This enzyme allows the pollen to digest part of the style and form a tube leading down to the ovaries. When the pollen grain reaches the ovaries, it fuses with a ovum. The ovum produces a hormone which directs the pollen grain in the direction to go during its journey down the style.

Fertilisation occurs as the pollen fuses with the ovum to form a zygote. After this happens, the zygote begins to divide by mitosis. As it divides, it produces a hormone which catalyses various changes in the structure of the plant.

1. The sepal and petals may wither away and fall off as no more insects need to be attracted to the plant.
2. Similarly, the stamen, style and stigma would wither away and fall off.
3. The carpel [stigma, style and ovary] may harden and form a fruit.

The resulting fruit has one job: to ensure the dispersal of the seeds is effective.

The carpel is the female reproductive organ of the plant.

Note that meiosis does not occur in asexual plants to form gametes. Mitosis only occurs to produce genetically identical cells and so all genetic data is copied.

Structure of a plant [reproduction]

Pollination is the transfer of pollen from an anther to a stigma.

Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of another plant.

Self pollination is the transfer of pollen from the anther to the stigma of the same plant.

Sepals: Strong modified leaves that wrap around the flower as it is developing and forming.
Petals: Modified colourful leaves that are typically found on insect pollinated plants as they attract insects. They vary in size and colour.
Stamens: Consist of the anther and the filament.
Anthers: Male part of the plant where the male gametes, pollen are produced through meiosis.
Filament: This is the stalk that supports the anthers and holds them up. They are typically longer and grow out of the plant in wind pollinated plants.
Stigma: This is the female part of the plant. It produces a two chemical substances which enable pollen to stick to it. It also contains a hormone which enables the pollen to release an enzyme which can allow it to digest part of the style and form a tube down to the ovary of the plant. It has a large surface area in wind pollinated plants.
Style: The style is the large stalk that supports the stigma. It is longer in wind pollinated plants so that it is more accessible and easier for the pollen to stick to it.
Ovary: This is where the ovules are found.
Nectaries: Can be located anywhere on a plant yet they provide glucose to insects in insect pollinated plants who need it for respiration.

Insect pollinated:

1. Large petals
2. Nectaries
3. Powerful scents

Wind pollinated:

1. Stigma with large surface area.
2. Stamens typically stick out of the plant.

Sometimes it is beneficial for the male parts of the flower to develop prior to the female parts. Self-pollination is prevented and the flower will have to do cross pollination. This results in more genetic variation in the species hence a greater chance of it surviving.

An advantage of self-pollination: less energy is required as no petals or nectaries have to be created.

Types of reproduction

Asexual/ sexual reproduction

Sexual reproduction is when two organisms from the same species are required to produce  non-idential offspring where characteristics are acquired from both parents. This is typical in most animals.

Asexual reproduction on the other hand is when one organism is required to produce genetically identical offspring. This occurs in some plants and in bacteria [binary fission].

A cell is diploid if it contains two copies of each chromosome [sexual].
A cell is haploid if it contains one copy of each chromosome [asexual].

Gametes are sex cells and and they are only utilised in sexual reproduction.
Fertilisation is the fusion of two gametes to produce a zygote which undergoes cell division and forms an embryo.

Structure of circulatory system

There are several significant arteries and veins leading away from important organs in the body such as:

Heart - Aorta/ Vena Cava
Lungs - pulmonary
Liver - hepatic
Kidneys - renal
Stomach - gastric

Effect of exercise on the heart

How the heart rate changes during exercise:

When you exercise, your muscles are moving more and at a greater rate hence the muscle cells respire more. Respiration requires a continuous flow of oxygen in order to release the maximum amount of energy as possible from glucose. Therefore the heart rate needs to increase so that the oxygen is pumped at a greater rate around the body to all the respiring cells primarily the muscle cells. More oxygen is breathed in and blood is pumped at a greater rate to and from the lungs where it is oxygenated. More carbon dioxide needs to be removed from cells also. As a result the heart rate increases.

Adrenaline

Adrenaline is produced in the adrenal glands above the kidneys. It stimulates receptors in the heart which increases the rate at which it works at. 

Immune system

Pathogens are harmful bacteria/ viruses that disrupt the normal functioning of tissues or organs or an entire organism. However, if the rate of their reproduction is less than the rate at which they are killed, one does not feel the effects of it and they show no symptoms.

If the rate of reproduction is higher than the rate at which they are killed, symptoms arise and one would feel ill.

Pathogens are destroyed by white blood cells. There are two different types:

1. Phagocytes
2. Lymphocytes

Phagocytes kill pathogens by engulfing them. However, this does not necessarily occur at a high rate and most of the time, lymphocytes are involved.

There are two types of lymphocytes: T cells and B cells.

Sequence of specific immunity

Phagocytes encounter a pathogen and begin to engulf it. Whilst doing so, it displays the pathogen's antibodies which alerts a t cell.
The t cell in turn alerts a b cell with the specific antibodies needed to target this pathogen.
The b cell begins to divide and produce antibodies in huge quantities.
The antibodies neutralise the toxins produced and cause the pathogens to clump together so that the phagocyte may engulf them all at the same time.
Some of the b cells are stored in the lymph nodes incase this pathogen is encountered again.

This sequence of events is called the specific immune response as it is targeting specific pathogens through the use of antibodies.

Vaccinations

Vaccinations work by injecting a weak version of a pathogen into the body. The antibodies specific to these pathogens are then produced by b cells. These increase the rate of destruction of these pathogen then they are stored in the lymph nodes where they are kept until the same pathogen is encountered again and a faster response is required. Future antibody production would be quicker and in a larger quantity thus targeting the pathogen faster.

Structure of the heart

There are four chambers in the heart:

1. Right atrium
2. Right ventricle
3. Left atrium
4. Left ventricle

The atria are where blood flows into the heart from [lungs/ body] i.e. from veins.
The ventricles have thicker walls as they generate the pressure to pump blood away from the heart. The left ventricle generates more pressure to pump blood around the entire body.

The atrio-ventricular valves are valves situated between the atria and ventricles to prevent blood from flowing in the wrong direction.

Tendinous chords prevent the valves from turning inside out in the event of high blood pressure.

The main blood vessels are:

1. Aorta
2. Vena Cava
3. Pulmonery artery
4. Pulmonery vein
___________

1. Arteries
2. Veins
3. Capillaries

1. The Aorta is the main artery taking oxygenated blood away from the heart to the rest of the body. It has thick muscular walls and a narrow lumen to deliver blood under a high pressure.

2. The Vena Cava is the main vein taking deoxygenated blood to the heart from the rest of the body. It has thin walls and a wide lumen. It delivers blood under a low pressure but contains valves to prevent blood from flowing the wrong way.

3. The pulmonary artery is the blood vessel that takes deoxygenated blood away from the heart to the lungs. It has thick muscular walls and a narrow lumen to increase the blood pressure.

4. The pulmonary vein takes oxygenated blood away from the lungs to the heart under low pressure. This is a result of the blood passing through small capillaries where it loses its pressure.

5. Arteries in general transfer oxygenated blood to tissues and organs all over the body. They have thick muscular walls and a narrow lumen to increase the blood pressure. Therefore they have no valves - the blood already flows at a high pressure. They have elastic walls to allow the artery to expand and recoil.

6. Capillaries are have walls one cell thick to decrease the diffusion distance for gases during gas exchange. They allow the exchange of substances into and out of the blood.

7. Veins have thin walls and a large lumen. They contain blood under a low pressure therefore they have valves to prevent it from flowing in the wrong directions. They contain deoxygenated blood except the pulmonary vein which transfers oxygenated blood from the lungs to the heart.


Note that smoking can severely increase blood pressure resulting in blood vessels from bursting. The bad cholesterol levels are raised due to the chemicals in tobacco smoke and they block the lumen.

Composition of the blood

Transport tissue

Blood is a transport tissue despite it containing a vast number of different types of cells. Blood cells transport oxygen around the body. However, all other components in the blood stream are being transported around the body in plasma. This is a yellow liquid containing water with dissolved solutes in it. Platelets are dead red blood cells that are significant in getting wounds to clot.

Components of the blood stream and where they are transported to/ from

White blood cells/ memory cells are transported from the lymph nodes to anywhere around the body that may have been infected with a pathogen. They consist of phagocytes and lymphocytes. Lymphocytes are divided into t and b cells for specific immunity.

Oxygen is transported from the alveoli to all cells in the body that respire.

Carbon dioxide is transported from all cells [as a waste product of respiration] to the alveoli where it is removed through diffusion.

Glucose is transported from the capillaries in the villi to everywhere in the body, to cells that respire.

Urea is transported as a waste product from all cells to the kidneys where it is removed as part of excretion. In particular, it is a waste product from the breakdown of proteins.

Hormones are transported from glands, the ovaries to specific cells.

How is a red blood cell adapted to transport as much oxygen as possible

A red blood cell is a bioconcave shape. This means it can travel through very small blood vessels such as capillaries.

It is enucleate. They have no nucleus to make space for more haemoglobin to absorb as much oxygen as possible.

Haemoglobin is made from iron.

Blood cells respire anaerobically as to not take any oxygen hence they have no mitochondria.

Their shape is essentially a flat disk with a dip in the middle to increase the surface area. A large surface area means a large volume of oxygen that can be transported. The diffusion distance is also decreased with this structure enabling oxygen to diffuse quickly into cells.

Transpiration

Transpiration is the evaporation of water from the surface of a leaf.  A plant loses around 90% of its water from transpiration.

Water is on the spongy mesophyll cells in a leaf. However, when it evaporates, it diffuses out of the leaf into the atmosphere. This is because the area on the outside of the leaf has a lower water potential than that inside of the leaf where the water vapour is currently situated. The xylem is a continuous tube from the roots to the leave therefore, as the water is being lost, more water is being drawn up through the xylem to replace it thus creating a flow of water.

Note: the water continues up the xylem due to intermolecular forces.

There are various factors that influence the rate of transpiration such as:

1. Light intensity
2. Temperature
3. Humidity
4. Wind intensity

1. If the light intensity increases, the stomata open to absorb carbon dioxide for photosynthesis. They do this as the guard cells [of stomata] gain water through osmosis. This results in them becoming turgid and opening the stomata. When the stomata are opened, water vapour is lost hence the transpiration rate increases.

2. If you increase the temperature, more water would be evaporated from inside the leaf on the spongy mesophyll. Transpiration is the evaporation of water from the surface of a plant hence greater heat = more evaporation = greater rate of transpiration. Remember, water can only escape from leaves in a gas form.

3. If you increase the humidity of the atmosphere this means that the volume of water vapour in an atmosphere increases. Therefore there is a smaller difference in water potential between the inside of the leaf and in the atmosphere. As a result, less water evaporates and diffuses into the atmosphere. Hence the rate of transpiration decreases.

4. If you increase the wind intensity of the atmosphere, more water molecules are going to be removed from the outside of the leaf therefore decreasing the water potential. To balance the concentration gradient,  more water diffuses into the atmosphere to replace the water vapour hence the rate of transpiration increases.

Prevention

Sometimes, the rate of transpiration is too great for a plant and they are losing a lot of water. To prevent this from happening, leaves are covered in wax. There are also much fewer stomata on the top of the leaf than at the bottom to restrict the water vapour from diffusing. The top of the leaf is more exposed to the light intensity,temperature, wind intensity more than the bottom side of the leaf. When flaccid, the guard cells will close. Every cells'  'turgor pressure' is lost meaning the plant is no longer supported and will die.

Turgor pressure and plant support

Remember, turgid cells are those that contain a large volume of water. They have a large turgor pressure which supports the plant and keeps it growing upwards and not wilting. Turgor pressure is the pressure of the water contained within the cell acting upon its cell wall. If all cells are turgid they are strongly pushing towards each other thus propelling the plant to grow upwards. A cell with a lack of water would have lost its turgor pressure and would wilt.

Experiments to investigate the role of environmental factors in the rate of transpiration

A potometer is simply used to measure the rate of water taken up through a plant.

Set up a basic potometer comprising of a beaker of water with a known volume, a capillary tube and a plant and rubber tubing.

Using a hairdryer, desk lamp and candle, situate each object one at a time near to the plant for a time of an hour.

Then measure how much water has been taken up by the plant by subtracting the new volume of water in the beaker by the new volume.

To ensure no water has been lost by evaporation [from the beaker] place a layer of oil on top.

Transport

Unicellular organisms

Simple unicellular organisms rely on diffusion as the means by which molecules move in and out of their cells. This is because they have a high surface area to volume ration which results in diffusion occurring quickly.

Multicellular organisms

Multicellular organisms are simply too large for diffusion to occur efficiently. They have a small surface area to volume ratio. Therefore, multicellular organisms have elaborate systems such as the respiratory system and circulatory system to transport molecules around the body to support themselves.

Water moving into the roots

Water moves into the root hair cells through osmosis. When soils take in nutrients and minerals from the soil by active transport, a low water potential is created. This low water potential means that water would diffuse into the roots. The roots have a large surface area to volume ratio, to absorb minerals and water at the greatest rate possible.

Lack of minerals in the soil, waterlogged soil and leaves removed

Without minerals in the soil such as nitrates, a plant would wilt. Nitrates make up amino acids which form proteins. Without protein, a plant cannot grow and support itself hence it wilts. Without magnesium, chlorophyll cannot be produced and a plant cannot photosynthesise to create glucose for growth also. The plant would also turn yellow.

A water logged soil would cause nutrients to become diluted. Diluted nutrients are not concentrated hence they cannot be taken up into the plant by active transport. There is no oxygen in waterlogged soil for plants to respire from.

A plant with its leaves removed would not be able to absorb water through osmosis in the root hair cells as transpiration is not occurring and providing a low water potential.

Minerals, nutrients, dissolved solutes and water are stored in the vacuole in cells before they are taken up through the xylem and distributed to other areas of the plants. Glucose is transported through the phloem from the leaves [where it is produced from photosynthesis] to other areas of the plant.

Phloem and Xylem

There are two types of transport tissue in a plant:

1. Phloem
2. Xylem

1. The phloem transports glucose from the leaves of a plant [and any green area with chlorophyll] to the other areas. The phloem vessels are thin, permeable so that glucose can diffuse in and out. They transport glucose in both directions as every cell requires it for respiration. The thin walls are made from lignin. The phloem is involved in TRANSLOCATION. This defines the movement of food substances from the stem to growing tissues around the plant.

1. On the other hand, the xylem only flows upwards [one direction]. It consists of hollow tubes constructed of dead cells. The walls are thick and made from cellulose. They are impermeable to allow no water or minerals to diffuse out. They transport the water, minerals and nutrients from the roots to the leaves.

Gas exchange [plants]

The role of diffusion in gas exchange is simple:

Gases move from an area dense in gas to an area less dense in gas. This is prevalent in gas exchange in the lungs. The oxygen in the alveoli diffuse into the bloodstream as it is less dense with oxygen. On the other hand, the carbon dioxide in the bloodstream diffuses into the alveoli as they are less dense with carbon dioxide.

Respiration is the conversion of oxygen and glucose into carbon dioxide and water. Oxygen has been exchanged with carbon dioxide.

On the other hand, photosynthesis is the conversion of carbon dioxide and light into oxygen and glucose.

Equation for photosynthesis:

Carbon dioxide and light = glucose and oxygen

The rate of photosynthesis is affected in three ways:

1. Light intensity
2. Temperature
3. Concentration of carbon dioxide in the air.

Temperature would affect photosynthesis as the enzymes involved in the chemical reactions during photosynthesis would not work efficiently in cool temperatures while becoming denatured in hot temperatures.

A high concentration of carbon dioxide would maximise the rate of photosynthesis.

A high light intensity would increase the rate of photosynthesis.


Image courtesy of BBC Bitesize.

Photosynthesis v respiration	Overall result
Dark	Respiration No photosynthesis	Oxygen taken in Carbon dioxide given out
Dim light	Photosynthesis rate equals respiration rate	Neither gas is taken in or given out, as each cancels the other out
Bright light	Photosynthesis rate greater than respiration rate	Carbon dioxide taken in Oxygen given out

Plants and how they are adapted for gas exchange

Plants are thin, enabling a short diffusion distance for carbon dioxide to be absorbed. There are stomata situated at the bottom of the leaf to allow the gases to diffuse into the leaves. When the guard cell (of the stomata) is turgid, it bends and opens, allowing the gas to diffuse into the leaf. 

Stomata in gas exchange

Photosynthesis produces oxygen as a byproduct. However, this oxygen is used in respiration. But, photosynthesis can only work in light therefore gas exchange only occurs during the day. As photosynthesis only occurs in the day, guard cells only absorb water during light hours. When they absorb water, they become turgid and the stomata opens. [stoma for singular]. During the night, the guard cells lose water through respiration, as only respiration continues during the night. Therefore, the guard cells become flaccid and close the stomata, preventing any gas from diffusing into the leaves.

An experiment to investigate the effect of light on the net release of carbon dioxide from a plant using hydrogen-carbonate indicator

Concentration	Indicator turns
Highest	Yellow
Higher	Orange
Atmospheric level	Red
Low	Magenta
Lowest	Purple

A leaf is placed in a stoppered boiling tube containing somehydrogen carbonate indicator solution. The effect of light intensity can then be investigated.

The table shows some typical results.

	Tube 1	Tube 2	Tube 3	Tube 4
Light turned on	✓	✓	✓	✓
Paper on tube	Black paper	Tissue paper	None	None
Leaf	Living	Living	Living	Dead (boiled)
Indicator colour at the end	Yellow	Magenta	Purple	Red
Carbon dioxide concentration	Highest	Low	Lowest	Atmospheric level
Respiration	✓	✓	✓	✗
Photosynthesis	✗	✓	✓✓	✗

• Tube 4 was a control. The results in tubes 3 and 4 show that the leaf has to be alive for the carbon dioxide concentration to change.
• Tubes 1, 2 and 3 show the effect of increasing the light intensity. The black paper stopped light reaching the leaf in tube 1, so only respiration could happen.
• The tissue paper stopped some of the light reaching the leaf in tube 2, and the leaf in tube 3 received the most light.
• Photosynthesis happened as well as respiration in tubes 2 and 3, so there was a net absorption of carbon dioxide.
• The rate of photosynthesis was greatest in the leaf in tube 3, and it had the greatest net absorption of carbon dioxide.

From this experiment we can see that the concentration of carbon dioxide produced fluctuated due to the light intensity varying. Gas exchange occurs at the greatest rate when respiration and photosynthesis occur at the same time.

Experiment to investigate photosynthesis

An experiment to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll.

Remember: Photosynthesis experiment = pondweed

This experiment will involve pondweed and a white leaved plant.

1. Add pondweed to a beaker of water with a known volume.
2. Add the white leaved plant to another beaker of water with the same volume.
3. Add the same volume of baking powder to increase the carbon dioxide levels.
4. Place two desk lamps 10cm away from each beaker.
5. Leave for five minutes so that the plants can accustom to the new light intensity.
6. After five minutes, count how many bubbles are emitted from each plant during a period of a minute.
7. After noting this down, move the desk lamps further away by 10cm.
8. Wait for five minutes then count how many bubbles are emitted from each plant for a period of a minute.
9. Repeat this process, moving the desk lamp back by 10cm then counting the number of bubbles.
10. Continue until the lamp is 50cm away from the plant[s].

Independent variable = light intensity
Dependent variable = bubbles omitted.
Controlled variables = size of each plant and temperature of the water.

Structure of a leaf for photosynthesis

In order for photosynthesis to occur efficiently in a plant and be most effective, the factors that influence photosynthesis must not be inhibited by a plants structure. A leaf is specifically adapted for photosynthesis in various ways both internal and external.

The veins that wrong along the leaf are for support and to transport water and the carbohydrates produced.

Light

1. The leaf is large and has a large surface area.
2. The leaf is covered in wax [gas exchange] this is transparent hence it does not reduce the intensity of the light entering the leaf.
3. The palisade cells are situated close to the top of the leaf and contain chlorophyll to ensure the maximum amount of light is absorbed.
4. The epidermis layer is thin and transparent to not block out the light.

Carbon Dioxide

1. The leaf is thin for a short diffusion distance.
2. The spongy mesophyll contains large air spaces for the carbon dioxide to diffuse through the leaf.
3. The stomata are situated near the bottom to ensure gas exchange is effective.
 

Mineral ions

These are significant for the growth of a plant. They are situated in the soil and are taken into the plant through active transport. Active transport is the taking in of molecules against the concentration gradient using energy released from respiration.

Nitrates are taken up to gel construct amino acids for protein. Without it, the plant will wilt.
Magnesium is used in chlorophyll. Without it, leaves would turn yellow.

Nutrition [plants]

Photosynthesis is the conversion of carbon dioxide and water into glucose and oxygen using light energy. The light energy is absorbed in the chlorophyll and divides the water into oxygen and hydrogen ions. The hydrogen ions bond with the carbon dioxide to form glucose.

The chemical energy in glucose is essential in growth and used in active transport. It is used in cellulose for cell walls also.

Carbon dioxide + water = glucose + oxygen

6CO2(g) + 6H2O(l) = C6H12O6(s) + 6O2(g)

Carbon dioxide relies upon the light intensity, carbon dioxide concentration and temperature in order to occur efficiently.

The following is credited to BBC Bitesize.

Limiting factors

Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.

• 
  

  Light intensity
  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.
• 
  

  Carbon dioxide concentration
  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.
• 
  

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

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

Balanced diet

A balanced diet is the different components present in appropriate proportions needed to maintain a healthy body. It should inlude carbohydrates, lipids, proteins, vitamins a,c,d, iron, calcium, dietary fibre and water.

Carbohydrates: Constructed mostly of starch and glucose hence it is a source of immediate energy. Found in cereal, potatoes, rice and pasta. Note that starch is a store of glucose.

Proteins: Constructed of a unique sequence of amino acids. They are required for the growth and repair of tissue. Found mostly in eggs, lentils, pork and soy.

Lipids: Constructed of fatty acids and glycerol. This is used as a store of energy in the long term. It is primarily used to insulate the body and stop the loss of heat i.e. homeostasis. It is found in dairy products, cheese, milk, fish etc.

Vitamin A: Essential for the reproduction of cells - maintains immune system. It is an antioxidant found in fatty fish, milk, cheese and mango.

Vitamin C: Another antioxidant that helps fight infection. Found in orange, grapefruit and kiwi.

Vitamin D: Maintains healthy bones and teeth by aiding the absorption of calcium. Remember that absorption is the taking in of minerals, ions and other soluble molecules into the bloodstream. It is found in milk and other dairy products i.e. yoghurt. Eggs and fish are also rich in vitamin d. Sunlight is helpful in encouraging the body to produce vitamin d.

Calcium: Is essential to the growth and repair of bones and heart function. Found in dairy products.

Iron: A component of haemoglobin in red blood cells. This is what oxygen binds to. It is common in red meats.

Water: Useful in the chemical reactions of cells. Found in the sea, clouds, me, you, in glaciers and in your tap.

Dietary fibre: Keeps the bowels functioning well and reduces the risk of bowel cancer. Found in cereals and rice.

Activity levels, age and pregnancy requirements for energy:

When people are young, they are more active and are growing. As a result they require more energy than an elderly person who is less active and no longer grows. With an active lifestyle, more energy is utilised in movement and the development of cells. As a result, more energy is required as the cells are respiring more.

When a woman is pregnant, she is supporting not only her life but her baby's. Therefore she needs to consume the appropriate amount of energy for both of them. The baby's growth and development requires significant amounts of energy.

An experiment to investigate the energy content in a food sample

1. Get a food sample.
2. Weigh the food sample and record the mass.
3. Fill a boiling tube with 10cm^3 of cold water.
4. Using a thermometer, measure the temperature of the water.
5. Burn the food sample using a bunsen burner until it catches fire.
6. Hold the burning food under the water for 2 minutes.
7. Extinguish the fire.
8. Measure the new temperature of the water using the thermometer and calculate the temperature rise.
9. Using the equation [mass of water x 4.2J/oC x temperature rise] calculate the total energy transferred.
10. Divide the total energy transferred by the original mass of the sample to calculate the energy present in one gram.

Human digestion

Key terms:

1. Mastication
2. Peristalsis
3. Absorption
4. Assimilation
5. Ingestion
6. Digestion
7. Egestion
8. Excretion

1. Mastication is the physical break-up of food in the mouth. It is the chewing and crushing of the food in order to obtain a higher surface area. A higher surface area enables enzymes to break down the food more efficiently.

2. Peristalsis is the muscular contractions that move food along the gut i.e. small intestine. Peristalsis occurs to ensure food is moved along the gut. It first occurs in the oesophagus yet it continues in the small intestine.

3. Absorption is the taking-in of small, soluble molecules into the bloodstream [at the end of the small intestine called the ileum]. It is performed efficiently as the ileum is lined with villi. These increase the surface area greatly, allowing the soluble molecules to be absorbed at a high rate. They are one cell thick and also contain blood capillaries to ensure the diffusion distance is small. The blood has a lower concentration of the food molecules therefore diffusion occurs quickly. The bloodstream transports the molecules all around the body to various tissues.

4. Assimilation is the taking in of molecules into the cells where they are then utilised.

5. Ingestion is the consumption of food by an organism.

6. Digestion is the breakdown of large food molecules into smaller, soluble ones.

7. Egestion is the removal of undigested semi-solid waste as faeces.

8. Excretion is the removal of all waste products from the body. Not necessarily as faeces.

Structure of the human alimentary canal

1. Mouth is where the physical breakdown of food occurs. Mastication increases the surface area greatly and this aids the enzymes. Amylase is an enzyme released with saliva. It breaks down starch into maltose. The saliva itself lubricates the food to help swallowing.

2. The oesophagus is situated next to the trachea and it connects the mouth to the stomach. Food never enters the trachea as it is covered with the epiglottis when food is swallowed. Therefore food only enters the oesophagus. Peristalsis occurs in the oesophagus to ensure food is moved along.

3. The stomach contains hydrochloric acid. This is a powerful acid with a pH of 1-2. As a result, it breaks down food even further to maximise the surface area more. The food here is now chyme, a liquid solution.

4. At the start of the small intestine, enzymes are released. These enzymes catalyse reactions to break down large food molecules into smaller, soluble ones. Each enzyme is specific to the reaction of a particular food molecule. Enzymes work most efficiently in the right temperature, pH and when there is a large surface area. There is more about enzymes further down the page.

5. The end of the small intestine is called the ileum. This is where absorption occurs. The ileum is lined with millions of small lumps called villi. These maximise the surface area hence they increase the rate of diffusion of soluble molecules into the bloodstream.

6. The large intestine is where water from chyme is absorbed and transported around the body.

7a. The pancreas produces enzymes essential to the chemical breakdown of food molecules such as amylase, lipase and protease. It is connected to the start of the small intestine [where enzymes are released to catalyse the breakdown of food] by the common bile duct.

7b. Bile is produced in the liver and stored in the gall bladder. It emulsifies lipids to increase the surface area. This makes their breakdown more efficient and occur at a greater rate. Bile is released with the pancreatic juices i.e. the enzymes through the common bile duct. Bile is also essential in creating alkaline conditions for the enzymes to work in. Note that it is not released specifically to neutralise stomach acid, it simply neutralises the acidic food molecules which have come out of the stomach. 

The digestive enzymes

These enzymes catalyse the reactions of the chemical breakdown of food molecules into smaller soluble molecules. They are made from proteins and are specific to the type of molecule they catalyse the reaction for. Amylase is produced in the salivary glands and is released in the mouth along with saliva. It breaks down starch into maltose. Maltose is then broken down by maltase into glucose at the start of the small intestine.

Protease breaks down proteins into amino acids. It is produced in the pancreas and released through the common bile duct along with the other enzymes at the start of the small intestine.

Lipase breaks down lipids into fatty acids and glycerol. It is produced in the pancreas and released along with bile from the common bile duct at the start of the small enzymes. Bile emulsifies the lipids, increasing the surface area so that the enzyme can work more efficiently.