Biology Edexcel AS Unit 2

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  • Created by: Raeesa
  • Created on: 30-03-13 13:40


Nucleus is a large organelle surrounded by a nuclear envelope (double membrane) which contains many pores. The nucleus contains chromatin and a nucleolus. Chromotin is made up from proteins and DNA. The pores allow substances to move between nucleus and the cytoplasms. The nucleolus makes ribosomes.

Lysomes are a round organelle surounded by a membrane with no clear internal structure. Contains digestive enzymes. These are kept seperate from the cytoplasm by the surrounding membrane but can be used to digest invading cells or to break down worn out components of the cells.

Vesicles are a small fluid filled sac in the cytoplasm, surrounded by a membrane. Transport substances in and out of the cell. Some are formed by the golgi apparatas or the endoplasmmic recticulus. Others are formed at the surface of the cell. 

Ribosomes are small organelles that float free in the cytoplasm or is attached to the rough ER. This is the site where proteins are made.

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Smooth ER: A system of membranes enclosing a fluid-filled space. It synthesises and processes lipids.

Rough ER: Similar to SER, but has ribosomes. It folds and processes proteins.

Golgi: A group of fluid filled sacs. Processes and packages new lipids and proteins, and makes lysosomes.

Centriole: Hollow cylinders containing a ring of microtubules. Involved with the seperation of chromosomes.

Mitochondria: They're usually oval shaped. They have a double membrane - the inner one is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration. The site of aerobic respiration where ATP is produce. They're found in large numbers in cells that are very active and require a lot of energy.

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Prokaryotic/Eukaryotic Cells

Cells can be Eukaryotic or Prokaryotic: Eukaryotic cells are complex and include and include all animal and plant cells. Prokaryotic cells are smaller and simpler. They include bacteria and blue-green algae.

Eukaryotes: Larger cells (2-200 diameter). DNA is linear. Nucleus present - DNA is inside nucleus. No cell wall (in animals) cellulose cell wall (in plants) or chitin cell wall (in fungi). Many organelles, mitochondria present. Large ribosomes. Example: Human liver cells.

Prokaryotes: Small cells (less than 2 diameter). DNA is circular. No nucleus - DNA free is cytoplasm. Cell wall made of a polysaccharide, but not cellulose of chitin. Few organelles, no mitochondri, smaller ribosomes. Example: E. coli bacterium.

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Protein Transport

Proteins are made at the ribosomes. 

The ribosomes on the rough ER make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm. 

New proteins produced at the rER are folded and process (e.g. sugar chains are added) in the rER. 

Then they're transported from the ER to the golgi apparatus in vesicles. 

At the golgi apparatus, the proteins may undergo further processing. 

The proteins enter more vesicles to be transported around the cell. E.g. extracellular enzymes move to the cell surface to be secreted. 

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Cell Organisation

Similar Cells are organised into tissues: Xylem tissue is a plant tissue with two jobs - it transports water around the plant and it supports the plant. It contains xylem vessel cells and parenchyma cells.

Tissues are organised in to organs: The leaf is made up of the following tissues.

Lower epidermis - contains stomata to let air in and out for gas exchange.

Spongy mesophyll - full of spaces to let gases circulate.

Palisade mesophyll - most photosynthesis occurs here.

Xylem - carriers water to the leaf.

Phloem - carries sugars away from the leaf.

Upper epidermis - covered in a waterproof waxy cuticle to recude water loss. 

Organs are organised into systems. 

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The Cell Cycle

G1: Cell grows and new organelles and proteins are made. 

Sythesis: Cell replicates its DNA ready to divide.

G2: Cell keeps groing and proteins needed for cell divisions are made. 


Interphase: Cell prepares to divide. DNA unravells and replicates, organelles also replicate, and ATP content is increased.

Prophase: Chromosomes condense. centrioles move to opposite ends of cell forming spindle fibres. Nuclear envolope  and nucleolus breaks down.

Metaphase: Chromosomes line up in the middle and spindles attach to centromers. 

Anaphase: Spindles contract, centromeres divide, seperating sister chromatids, centromeres first to opposite ends of the cell.

Telophase: Chromotides become long and thin again. Nuclear envolope reforms and the cytoplasm divides.

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Cells that divide by meiosis have the full number of chromosomes to start with but the cells that are formed from meiosis have half the number.

1) The DNA replicates so there are two identicle copies of each chromosome, called chromotids.

2) The DNA condenses to form double armed chromosomes made from two sister chromotids.

3) The chromosomes arrange themselves into homologous pairs.

4) First division - the homologous pairs are seperated, halving the chromosome number.

5) Second division - the pairs of sister chromotids are seperated.

6) Four new cells that are genetically different from each other are produced.

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Before the first division of meiosis, homologous pairs of chromosomes come together and pair up.

Two of the chromotids in each homologous pair twist around each other. The twisted bits break off their original chromotid and rejoin onto the other chromotid, recombining their genetic material.

The chromatides still contain the same genes but they now have a different combination of alleles.

This means that each of the four new cells formed from meiosis contains chromatids with different alleles.

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Mammal Fertilisation

The sperm swim towards the egg cells in the oviduct.

Once one sperm makes contact with the zona pellucida of the egg cell, the acrosome reaction occurs - this is where digestive enzymes are realeased fromt eh acrosome of the sperm. 

These enzymes digest the zona pellucida so that the sperm can move through it to the cell membrane of the egg cell.

The sperm head fuses with the cell membrane of the egg cell. This triggers the cortical reaction - the egg cell releases the contents of vesicles called cortical granules into the space between the cell membrane and the zona pellucida.

The chemicals from the cortical granules make the zona pellucida thicken, which make is inpenetrable to other sperm. This makes sure only one sperm fertillises the egg cell. 

Only the sperm nucleus enters the cell, the tail is discarded.

The nucleous of the sperm fuses with the nucleus of the egg cell. This is fertilisation,

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Plant Fertilisation

A pollen grain lands on the stigma of a flower. The grain absorbs water and splits open. A pollen tube grows out of the pollen grain down the style. There are three nuclei in the pollen tube. One tube nucleus at the tubes tip and two male gamete nuclei behind it. The tube nucleous make enzymes that digest surrounding cells, making a way through for the pollen tube.

When the tube reaches the ovary it grows through the microphyle and into the embryo sac within the ovule. In  the emryo sac, the tube nucleus disintergrates and the tip of the pollen tube bursts, releasing the two male nuclei. 

One male nucleus fuses with the egg nucleus to make a zygote. This divides by mitosis to become the embryo of the seed. The second male nucleus fuses with twi other nuclei at the centre of the embryo sac. This produces a cell with a large nucleus, which divides to become a food store for the mature seeds. 

So a double fertilisation has taken place (two male nuclei have fused with female nuclei).

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Cell Differentiation

Stem cells divide by mitosis to become new cells which them become specialised. The process by which a cell becomes specialised is called differentiation. In humans the stem cells are foundi nt he embryo, and in adult tissue e.g. stem cells in the bone marrow. Totipotency: the ability to produce all cell types. Pluripotency: the ability of a stem cell to produce all the specialised cells. Totipotent cells only present in the early life of an embryo.

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Stem Cells in Medicine

Obtaining stem cells from embryos created by IVF raises ethical issues because the procedure results in the destruction of an embryo thats viable. Many people believe that at the moment of fertilisation a genitiacally unique individual is formed that has a right to life. so they believe it is wrong to destroy embryos. Some people have fewer objections to stem cells being obtained from unfertilised embryos. They cant survive past a few days. 

Regulatory Authorities jobs:

Looking at proposal of research to decide whether they should be allowed. This makes sure research isnt unneccesarily repeated.  

Licensing and monotaring centres involved in embryonic research.

Producing guidlines and codes of practice

Providing information and advice to goverment and professionals. 

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Height is polygenic and affected by environmental factors, especially nutrition. E.g. tall parents usually have tall children but if the children are undernourished they won't grow to their maximum height (because protein is required for growth 

Monoamine Oxidase A is an enzyme that breaks down monoamines in humans. Low levels of MAOA have been linked to mental health problems. MAOA production is controlled by a signle gene (it is monogenic) but taking anti-depressants or smoking tobacco can reduce the amount produced.

Cancer is unctrolled division of cells that leads to lumps of cells (tumours) forming. The risk of feveloping some cancers is affected by genes, but environmental factors such as diet can also influence the risk.

Animal hair colour is polygenic, but the environment also plays a part in some animals. E.g. some arctic animals have dark hair in summer but white hair in winter. Environmental factors like decreasing tempurature trigger this change but it couldn't happen if the animal didn't have the genes for it.

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Adaptation and Evolution

The niche a species occupies within its habitat includes its interaction with other living organisms - e.g. the organisms it eats, and those its eaten by. Its interactions with the non-living environment - e.g. the oxygen an organism breathes in, and the carbon dioxide it breathes out. Every species has its own niche - a niche can only be occupied by one species. If two species try to occupy the same niche they will compete with each other. One species will be more successful until onlly one of the spcies is left. 

Adaptations can be:

Behavioural - ways an organism acts that increases its chance of survival and reproduction. For example male pipistrelle bats making mating calls to attract female common pipistrelle bats.

Physiological adaptation - processes inside an organisms body that increases its chance of survival. For example pipistrelle bats lower their metabolism in order to hibernate over winter. This allows them to conserve energy.

Anatomical adaptations - Structural features of an organism's body that increases its chance of survival, for example pipistrelle bats have light, flexible wings that allow them to hunt fast-flying insects.

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Adaptation and Evolution

Adaptations become more common by evolution. Individuals within a population show variation in their phenotypes (their characteristics.)

Predation, disease and competition create a struggle for survival.

Individuals with better adaptations are more likely to survive, reproduce and pass on their advantageous adaptations to their offspring.

Over time, the number of individuals with the advantageous adaptations increase.

Over generations this leads to evolution as the favourable adaptations become more common in the population.

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Kingdom --> Phylum --> Class --> Order --> Family --> Genus --> Species.

A species is a group of similar organisms able to reproduce to give fertile offspring.


Prockaryotae: Bacteria

Protoctista: Algae, protozoa 

Fungi: Moulds, yeasts, mushrooms

Plantae: Mosses, ferns, flowering plants

Animilia: nematodes, molluscs, insects, fish, reptiles, birds, mammals

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Plant Cell Structure

  • Cell Wall - A rigid structure that surrounds plant cells. It's made mainly of the carbohydrate cellulose and supports plant cells,
  • Middle lamella - The outermost layer of the cell. This layer acts as an adhesive, sticking adjacent plant cells together. It gives the plant stability.
  • Plasmodesmata - Channels in the cell walls that link adjacent cells together. Allow transport of substances and communication between cells.
  • Pits - Regions of the cell wall where the wall is very thin. They're arranged in pairs - the pit in one cell is lined up with the pit in the adjacent cell. Allow transport of substances between cells.
  • Chloroplasts - A small flattened structure. its surrounded by a double membrane, and also has membranes inside called thylakoid membranes. These membranes are stacked up in some parts of the chloroplast to form grana. Grana are linked together by lamellae. The site where photosynthese takes place.
  • Amyloplast - A small organelle enclosed in a membrane. They contain starch granules. They also convert starch back into glucose for release when needed,
  • Vacuole and tonoplast - The vacuole is a compartment surrounded by a membrane called the tonoplast. The vacuole contains the cell sap which is made up of water, enzymes, minerals and waste products. Vacules keep cells turgid this stops plants wilting.
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Xylem & Sclerenchyma

The function of xylem vessels is to transport water and mineral ions up the plants and provide support. They're very long, tube like structure formed from dead cells, joined end to end. The tubes are found together in bundles. The cells are longer than they are wide, they have a hollow lumen and no end walls. This makes an uninterrupted tube alllowing water and mineral ions to pass up through the middle easily. Their walls are thickened with a woody substance called lignin which helps to support the plant. Xylem vessels are found throughout the plant but particularly around the centre of the stem.

The function of sclerenchyma fibres is to provide support. Like xylem vessels, they're also made of bundles of dead cells that run vertically up the stem. The cells are longer that thy are wide and also have a hollow lumen and no end walls. Their cell wallsa re onlso thickened with lignin. They have more cellulose than other plant cells. They're found throughout the stems of plants but particularly around the outer edge.

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Cellulose and Fibres

Cellulose is made of long, unbranche chains of beta-glucose, joined by glycosidic bonds. The glycosidic bonds are straight so the cellulose chains are straight. Between 50 and 80 celluclose chains are linked together by a large number of hydrogen bonds to form strong threads called microfibrils. The strong threads mean cellulose provides structural support for cells.

Plant fibres are strong because of the arrangement of cellulose in the cell wall. The cell wall contains cullulose microfibrils in a net-like arrangement. The strength of the microfibrils and their arangement in the cell wall gives plant fibres strength.

When some structural plant cells have finished growing thry produce a secondary cell wall betweem the normal cell wall and the cell membrane. The secondary cell wall is thicked than the normal cell wall and usually has more lignin. The growth of a secondary cell wall is called secondary thickning. Secondary thickening makes plant fibres even stronger.

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Inorganic ions

Water is needed for photosynthese, to maintain structural rigidity, transport minerals and regulate tempurature. 

Magnesium ions are needed for the production of chlorophyll - the pigment needed for photosynthesis.

Nitrate ions are needed for the production of DNA, proteins and chlorophyll. They're requited for plant growth, fruit production and seed production.

Calcium ions are important components in cell walls. They're required for plant growth.

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Drug Testing

Phase 1: This involves testing a new drug on a smal group of healthy individuals. Its done to find out things like safe dosage, if there are any side effects and how the body reacts to the drug.

Phase 2: If a drug passes Phase 1 it will then be tested on a larger group of people this time patients to see how well the drug actually works.

Phase 3: During this phase is compared to existing treatments. It involves testing the drug on hundreds or even thousands of patients. Patients are randomly split intp two groups - one group recieves the new treatment and the othergorup recieves the existing treatment. This allows the scientists to tell if the nw drug is better than existing drugs. 

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Biodiversity and Endemism

Biodiversity is the variety of living organisms in an area. It includes species diversity, the number of different species and the abundance of each species in an area. Genetic diversity - the variation of alleles within a species.

Conservation is needed to help maintain biodiversity. Endemism is when a species is unique to a single place (isn't naturally found anywhere else in the world.) Conservation is really important for endemic species beacuse they're particularly vulnerable to extinction. They're only found in one place so if their habitat is threatened they can't usually migrate and their numbers will decline.

You can measure a species diversity in different ways. Could the number of different species in an area. The number of different species in the area is called species richness. The higher the number of species the greater the species richness. But species richness gives no indication of the adundance of each species.

Count the number of different species and the number of individuals in each species, then use a biodiversity index to calculate the species diversity. This takes into account the number and abundance of each species.

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Biodiversity and Endemism

Sampling: To reduce bias, sample should be random. Count the number of individuals of each species in the sample area. For plants you use a quadrat, for flying insects a sweepnet, for ground insects a pitfall trap, for aquatic animals a net.

To measure genetic diversity of a species you can look at:

Phenotype: By looking at the different phenotypes in a population of a species, you can get an idea of the diversity of alleles in that population. The larger the number of different phenotypes the greater the genetic diversity.

Genotype: By sequencing the DNA of individuals of the same species, you can look at similarities and differences in the alleles within a species. You can measure the number o different alleles a species has for one characteristic to see how genetically diverse the species is. The larger the number of different alleles the greater the genetic diversity.

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Seedbanks: It is cheaper to store seeds than to store fully grown plants. Larger numbers of seeds can be stored than grown plants because they need less space. Less labour is required to look after seeds than plants. Seeds can be stored anywhere as long as its cool and dry. Plants would need the conditions from their original habitat. Seeds are less likely to be damaged by disease, natural disaster or vandalism. However testing seeds for viability can be expensive and time consuming. it would be too expensive to store all types of seed and regularly test them all for viability. It may be difficult to collect seeds from some plants as they may grow in remote locations. 

Zoos have captive breeding programmes to help endangered species. Animals can have problesm breeding outside their natural habitat. Some people think its wrong to keep animals in captivity.

Oragnisms from zoos and seedbanks can be reintroduced into the wild to save them from extinction and help species that rely on them. However this could bring diseases, or reintroduced animals may not know how to behave in the wild.

Seedbanks and zoos contributw to scientific research. 

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