Biology AS

The Cell Cycle and Mitosis

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The Cell Cycle

The cell cycle is the process that all body cells from multicellular organisms use to grow and divide.

1) The cell cycle starts when a cell is produced by cell dividion and ends with the cell dividing to produce two identical cells.

2) The cell cycle consists of a period of cell growth and DNA replication, called interphase, and a period of cell division, called mitosis.

3) Interphase (cell growth) is sub-divided into 3 seperate growth stages. These are called G1, S and G2.

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Mitosis (Four Stages)

Interphase - The cell carries out normal functions, but also prepares to divide. The cell's DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increased (ATP provides the energy needed for cell division).

Prophase - The chromosomes condense, getting shorter and fatter. The centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope (the membrane around the nucleus) breaks down and chromosomes lie free in the cytoplasm.

Metaphase - The chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere.

Anaphase - The centromeres divide, separating each pair of sister chromatids.

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Mitosis Core Practical

1) Cut the tip from a growing root. Your root tip should be about 5 mm long.

2) Place the root tip on a watch glass (small, shallow bowl) and add a few drops of hydrochloric acid.

3) Add a few drops of stain so that the chromosomes become darker and so easier to see under a microscope. There are loads of different stains - toluidine blue, acetic orcein...

4) Warm the watch glass by passing it through a Bunsen burner flame.

5) Place a root tip on a microscope slide and use a mounted needle to break it open and spread the cells out thinly.

6) Add a few more drops of stain and then place a cover slip over it.

7) Squash the cover slip down gently and warm the slide for a few seconds to intensify the stain.

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The Cardiac Cycle

Ventricular diastole, Atrial systole: The ventricles are relaxed. The atria contract, decreasing the volume of the chamber and increasing the pressure inside the chamber. This pushes the blood into the ventricles. There's a slight increase in ventricular pressure and chamber volume as the ventricles recieve the ejected blood from the contracting atria.

Ventricular systole, Atrial diastole: The atria relax. The ventricles contract increasing their pressure. The pressure becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow. The pressure in the ventricles is also higher than in the aorta and pulmonary artery, which forces the SL valves open and blood is forced out into these arteries.

Ventricular diastole, Atrial diastole: The ventricles and the atria both relax. The higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back-flow into the venticles. Blood returns to the heart and the atria fill up again due to the higher pressure in the vena cava and pulmonary vein. In turn this starts to increase the pressure of the atria. As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open. This allows blood to flow passively into the ventricles from the atria. The atria contract and the whole process begins again.

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CVD Atheroma

Atheroma Formation

  • The wall of an artery is made up of several layers.
  • The endothelium is usually smooth and unbroken.
  • If damage occurs to the endothelium there will be an inflammatory responce - this is where white blood cells move into the area.
  • These white blood cells and lipids from the blood, clump together under the endothelium to form fatty streaks.
  • Over time, more white blood cells, lipids and connective tissue build up and harden to form an atheroma.
  • This plaque partially blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase.
  • The hardening of arteries, caused by atheromas is called atherosclerosis.
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CVD Thrombosis

  • Atheromas develop within the walls of the arteries.
  • An atheroma can rupture the endothelium of an artery, damaging the wall and leaving a rough surface.
  • This triggers thrombosis (blood clotting) - a blood clot forms at the site of the rupture.
  • This bood clot can cause a complete blockage of the artery, or it can become  dislodged and block a blood vessel elsewhere in the body.
  • The blood flow to tissues supplied by the blocked blood vessel will be severely restricted, so less oxygen will reach those tissues, resulting in damage.
  • Heart attack, stroke and deep vein thrombosis are three forms of cardiovascular disease that can be caused by blood clots.
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CVD Blood Clots

Thrombosis is used by the body to prevent lots of blood being lost when a blood vessel is damaged. A series of reactions occus that leads to the formation of a blood clot.

  • A protein called thromboplastin is released from the damaged blood vessel.
  • Thromboplastin triggers the conversion of prothrombin (a slouble protein) to fibrin ( an enzyme).
  • Thrombin then catalyses the conversion of fibrinogen (a soluble protein) to fibrin (solid insolube fibres).
  • The fibrin fibres tangle together and form a mesh in which platelets and red blood cells get trapped - this forms the blood clot.
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CVD Blood Clots 2

Blood clots can cause heart attacks...

  • The heart muscle is supplied with blood by the coronary arteries.
  • This blood contains the oxygen needed by the heart muscle cells to carry out respiration.
  • If a coronary artery becomes completely blocked by a blood clot an area of the heart muscle will be totally cut off from its blood supply so it wont get any oxygen.
  • This causes a myocardial infarction - more commonly known as a heart attack.
  • A heart attack can cause damage and death of the heart muscle.
  • Symptoms include pain in the chest and upper body, shortness of breath and sweating.
  • If large areas of the heart are affected complete heart failiure can occur, which is often fatal.
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Plant Cell Structure 1

Cell Wall - Surrounds plant cells and supports them.

Middle Lamella - Outermost layer of the cell and acts as an adhesive, sticking cells together and giving the plant stability.

Plasmodesmata - Channels in the cell walls that link cells together and allows for transport of substances.

Pits - Regions where the wall is thin and also allows for transport of substances.

Chloroplasts - Flattened structure surrounded by a double membrane. The site where photosynthesis takes place. Some parts of photosynthesis happen in the grana and other parts in the stroma.

Amyloplasts - A small organelle enclosed by a membrane. They contain starch granules. They also convert starch back to glucose for release when the plant requires it.

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Plant Cell Structure 2

Xylem Vessels

The function of xylem vessels isto transport water and mineral ions up the plant, and provide support.

They're very long, tube-like structures formed drom dead cells joined end to end. The tubes are found in bundles.

The cells have a hollow lumen and have no end walls.

This makes an uninterrupted tube allowing water and mineral ions up the middle easily.

Their walls are thickened with a substance called lignin which is impregnated and supports the plant.

Water and mineral ions move into and out of the vessels through pits in the walls where there's no lignin.

Xylem vessels are found throughout the plant but particularly around the centre of the stem.

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Plant Cell Structure 3

Sclerenchyma Fibres

The function of the sclerenchyma fibres is to provide support.

Like xylem vessels, they're made of bundles of dead cells that run vertically up the stem.

The cells are long and also have a hollow lumen and no end walls.

Their cell walls are also thickened with lignin. They have more cellulose than other plant cells.

They're found throughout the stems of plants but partiularly around the outer edge.

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Plant Cell Structure 4


Cells get energy from glucose, Plants store excess glucose as starch (when a plant needs more glucose it breaks down the starch to release the glucose).

Starch is a mixture of 2 polysaccharides of alpha glucose - amylose and amylopectin.

  • Amylose - a long unbranched chain of &-glucose. The angles of the glycosidic bonds give it a coiled structure. This makes it compact and good for storage.
  • Amylopectin - a long branched chain of &-glucose. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly.

Starch is insoluble in water, so it does not cause water to enter cells by osmosis. This makes it good for storage.

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Plant Cell Structure 5


Cellulose is made of long unbranched chains of beta-glucose, joined by glycosidic links.

The glycosidic bonds are straight, so the cellulose chains are straight.

Between 50 and 80 cellulose chains are linked together by a large number of hydrogen bonds to form stong threads called microfibrils.

The strong threads mean cellulose provides structural support for cells.

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Membrane Transport

Diffusion is the passive movement of particles.

Diffusion is the net movement of particles from an area of high concentration to an are of low concentration.

Molecules will diffuse both ways, but the net movement will be to the are of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas.

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Cells and Organelles

Cells can be Eukaryotic or Prokaryotic

1) Eukaryotic cells are complex and include all animal and plant cells.

2) Prokaryotic cells are smaller and simpler.

Rough ER, Vesicles and Golgi Apparatus are involved in Protein Transport.

1) Proteins are made at the ribosomes.

2) The ribosomes on the rough ER make proteins which are attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.

3) New proteins produced at the rough ER are folded and processed in the rough ER.

4) Then they're transported from the ER to the golgi apparatusin vesicles.

5) At the golgi, the proteins may undergo further processing.

6) The proteins enter more vesicles  to be transported around the cell.

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Cell Organisation

Similar cells are grouped together into tissues. Tissues are organised into organs. An organ is a group of different tissues that work together to perform a particualr function.

The leaf is an example of a plant organ. It 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 - Carries water to the leaf
  • Phloem - Carries sugars away from the leaf.
  • Upper Epidermis - Covered in a waterproof waxy cuticle to reduce water loss.

The lungs are an example of an animal organ. They're made up of the following tissues:

  • Squamous epithelium tissue - Surrounds the alveoli.
  • Firbous connective tissue - Helps to force air back out of the lungs when exhaling.
  • Blood vessels - Capillaries surround the alveoli.
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Biodiversity is the Variety of organisms in and area. It includes:

Species diversity - the number of different sepcies and the abundance of each species in a given area. For example, a wood could contain many different species of plants, insects, birds and mammals.

Genetic diversity - The variation of alleles within a species. For example, human blood type is determined by a gene with 4 different alleles.

Conservation is needed to help maintain biodiversity

Endemism is when a species is unique to a single place - e.g. the giant tortoise is endemic to the Galapagos Islands - it can only be found there.

Conservation is really important for endemic species because they're particularly vunerable 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.

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Biodiversity 2

The species diversity in a habitat can be measured.

It is important to be able to measure species diversity so you can compare different habitats, or study how a habitat has changed over time. You can measure species diversity in different ways..

Count the number of different species in an area. The number of different species in the area is called the species richness. The higher the number of species, the greater the species richness. But species richness gives no indication of the abundance of each species.

Count the number of defferent species AND the number of individuals in each of them. 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 3

When measuring a species diversity, it is usually too time-consuming to count every individual organism in a habitat. Instead, a sample of the population is taken. Estimates about the whole habitat are based on the sample. Sampling involves:

Choose an area to sample - a small area within the habitat being studied.

To avoid bias in your results, the sample should be random. For example, if you were investigating the species of plants in a field you could pick random sample sites by dividing the field into a grid and using a random number generator to select coordinates.

count the number of individuals of each species in the sample area. How you do this depends on what you're counting, for example:

  • For plants you'd use a quadrat.
  • For flying insects you'd use a sweepnet.
  • For ground insects you'd use a pitfall trap.
  • For aquatic animals you'd use a net.

Repeat the whole process - take as many samples as possible. This gives a better indication of the whole habitat.

Use the results to estimate the total number of individuals  or the total number of different species  in the habitat being studied.

When sampling different habitats and comparing them, always use the same sampling techniques.

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Biodiversity 4

The genetic diversity within a species can also be measured. We can measure diversity within a species by looking at genetic diversity.

1) Diversity within a species is the variety shown by individuals of that species.

2) individuals of the same species vary because they have different alleles.

3) Genetic diversity is the variety of alleles in the gene pool of a species or population.

4) The gene pool is the complete set of alleles in a species or population,

5) The greater the variety of alleles, the greater the genetic diversity. For example, animals have different alleles for blood group. In humans there are three alleles for blood group, but gorillas have only one so humans show greater genetic diversity for blood group than gorillas.

You can investigate the changes in the genetic diversity of a population over time, or how two populations of the same species show different diversity.

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Biodiversity 5


  • Phenotype describes the observable characteristics of an organism.
  • Different alleles code for slightly different versions of the same characteristics.
  • 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.
  • For example, humans have different eye colours due to different alleles. Humans in northern europe show a variety  of blue, grey, green or brown eyes. Outside this area, eye colour shows little variety - they're usually brown. There's greater genetic diversity in eye colour in northern europe.


  • Samples of an organism's DNA can be taken and the sequence of base pairs analysed.
  • The order of bases in different alleles is slightly different.
  • By sequencing the DNA of individuals of the same species, you can look at similarities and differences in the alleles within a species.
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Production of Gametes

DNA is passed to new offspring by Gametes.

Gametes are the male and female sex cells found in all organisms that reproduce sexually.

They join together at fertilisation to form a zygote, which divides and develops into a new organism.

In animals, male gametes are sperm and the female gametes are the egg cells (ova).

In plants, the male gametes are contained in pollen grains and the female gametes are contained in the ovules.

Normal body cells of plants and animals contain  the full number of chromosomes . Humans have two sets of 23 chromosomes - one set from each parent, giving a body cell 46 chromosomes.

Gametes contain half the number of chromosomes as body cells - they only contain one set (23 in total for humans).

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Production of Gametes 2

Mammilian Gametes are specialised for their function.

Sperm Cell

  • Lots of mitochondria provide energy for tail movement
  • Flagellum allows sperm to swim towards the egg cell
  • Acrosome contains digestive enzymes to break down the egg cell's zona pellucida.

Egg Cell

  • Cell membrane
  • Follicle cells form protective coating
  • Zona pellucida - protective layer that sperm have to penetrate
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Production of Gametes 3

Cell Division by Meiosis produces Gametes

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