Unit 2

Capillaries are very thin (one cell thick), so they do not let many substances pass through. Only large substances with large molecules such as proteins,cannot escape the blood. This is an advantage as useful nutrients and gases are easily excanged (short diffusion pathway) and also water can freely leak out.

There is more water outside of the blood system than there is in it. The excess water outiside the blood produces a solution that fills up gaps between certain cells. This is called tissue fluid.

Water cannot just drain out of the blood vessels without being replaced. In the tissues there is a constant movement of water into and out of the capillaries. By the time blood enters the capillaries from the arterioles it is at a much lower hydrostatic pressure. However,the pressure is still large enough to froce water out of the capillaries into the tissue fluid. So, as the blood passes along the capillaries, the water content decreases.

Small solutes including glucose and ions also pass out, but the large soluable proteins cannot go through the walls of the capillaries. This therefore lowers the water potential inside the capillary. The result is that at the venule end of the capillary the water potential of the plasma is lower than the water potential of the tissue fluid. Hence, water goes back into the capillaries by osmosis.

Water enters a plant through its root hair cells. Water has to travel from the soil, through the root and into the xylem (plants system of taking water through the plant). The part of the root that abosrbs water contains many root hair cells, which increase the surface area, increasing the rate at which water is taken up.

Once the water has been absorbed, the water has to get through the cortex, including the endodermis, before it can reach the xylem.

The soil around the roots generally has high water potential, whereas the leaves usually low water potential as water is constantly evaporating. This creates a water potential gradient that keeps water moving from the soil into the plant, down a water potential gradient.

Water can travel through the roots into the xylem by two different paths:

1- Symplast pathway; goes through the living part of the cells (cytoplasm) and throught (plasmodesmata).

2- Apoplast pathway; goes through the non living parts of the roots - cell walls. The walls are very absorbent and can simply diffuse throught them, as well as passing through the spaces between them.

Water moves up a plant against the force of gravity.

Cohesion and Adhesion

• Water evaporates from the leaves at the top of the xylem
• This then creates tension, which pulls more water into the leaf.
• As water molecules are polar and cohesive, if once molecule of water is pulles the other molecules follow. This means that the whole colum of water in the xylem, from the leaves down into the roots move upwards.
• Water enters the stem from the roots.

Root pressure

• As water is transported in the xylem it creates a certain amount of pressure and  pushes water already in the xylem upwards. The pressure is weak, so is only useful in young smaller plants where the leaves are still in devlopment.

Transpiration is a loss of water from a plants surface, especially the leaves.

•  Water evaporates from the cell walls and accumulates in the spaces between cells in the leaf
•  When the stomata opens, it moves out of the leaf down the concentration gradient as there is more air inside the leaf than the air outiside.

There are more factors that can affect transpiration rate:

• Light: The lighter it is the faster the transpiration rate. This is because the stomata open when it is lighter and close when it is darker, so there is little transpiration rate when it is dark.
• Temperature: The hotter temperature is the faster the transpiration rate. This is because the molecules of water have more kinetic energy. This then increases the concentration gradient between the outside and the inside of the leaf, which means that water diffuses out of the leaf faster.
• Humidity: The lower the humidity, the faster the transpiration rate. If the air around the leaf is dry there is a concentration gradient, so water moves out of the leaf.
• Wind: The more wind there is, the faster the transpiration rate is. Air blows the water molecules away from the stomata. This therefore increases the concentration gradient, water moves out of the leaf.

Taxonomy: Is the science of classification, which involves naming organisms and organising them into groups based on their similarities and differences.

There are seven levels of groups:

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

Hierarchy: Are groups within other groups, with no overlaps.

Species: A group of similar organisms able to reproduce to give a fertile offspring.

Phylogenetics: The study of the evolutionary history of groups of organisms

All organims have a common ancestor, but some diverged to become a completly different organism. Phylogenetics tell us what organisms are related to who and by how close.

Defining organisms as disticnt species is very hard because we can't always see if organisms can reproduce to create a fertile offspring.

• They could reproduce asexually?
• They are extinct?
• There are practical and ethical issues?

DNA sequencing: Can be done by simply looking at the order of the bases of each organism that you want to test for. Closley related species a higher percentage of similarity. DNA sequencing comparison has allowed us to create new classification methods for plants as the classification for plants is almost entirely based on similarities and differences.

DNA hybridisation: This is the use of using heat to break apart the hydrogen bonds between the bases to find out if the DNA is related or not.

• DNA from two different species is collected, and separated into separated strands and mixed together.
• The strands are then heated.
• The base sequences of the same DNA will require more heat to break apart the hydrogen bods, the more the DNA bases hybridise together, the more alike DNA is.

Antibiotics are used to treat bacteria diseases as they are chemicals that kill or inhibit the growth of bacteria. There are different types of antibiotics eg; some stop the cells from forming  the cell wall, which helps for structure. This can lead to osmotic lysis

• The antibiotics inhibit the enzymes that are needed to make the chemical bonds in the cell wall of the bacterial cells
• This therefore stops cells from growing and dividing and weakens the cell wall
• Water therefore moves into the cell by osmosis down a concentration gradient
• The cell wall therefore cannot withstand the water pressure so burts.

Mutations in bacterial cells cause antibiotic resistance...

The genetic material in a bacteria cell is the same as any other organsism- DNA

Mutations occur, which therefore change the order of the bases in DNA sequence. So if a mutation occurs this may change the bacteria's characteristic, resulting in the bacteria becoming resistant to the antibiotic.

Vertical gene transmission

• Bacteria reproduce asexually, so each of their offspring is genetically identical to the origianal cell.
• Therefore each offspring has the EXACT genes of the parent cell, including the gene that is resistant to antibiotics.
• Genes for antibiotic resistance are found in the bacterial chromosomes or in the plasmid
• The chromosomes or plasmid are therefore passed onto the daughter cells during reproduction

Horizontal gene transmission

• Genes can also be passes on horizontally
• Two bacterial cells fuse together in the process called conjugation and an identical copy of the plasmid is passed into the bacterial cell without the antibiotic resistant gene.
• This can happen to the same member of the species or even to a different species of bacteria.

MRSA

MRSA has evolved to become resistant to common antibiotics including methicillin. The major problem is that nearly all the antibiotics are not effective against MRSA and it will take a long time for clinics to find a antibiotic that will kill it, but of course in this time people are dying.

Tuberculosis

The tuberculosis bacteria has evovled the become resistance to antibiotics as they were greatly used against it. This has led to a new breed of bacteria who have been naturally selected to be resistant against bacteria. Tuberculosis now needs to be killed by taking a combination of different antibiotics for a period of around 6 months depending on the severity.

Natural selection

• Some bacteria cells contain an allele that is resistant to antibiotics
• Antibiotics kill off the bacteria without the allele, leaving the antiobiotic resistant cells
• The allele is then passed onto the daughter cells, making all the cells resistant

• Some people believe that antibiotics should only be used in life threatning situations in order to increase resistance. Others believe that this would increase the likleyhood of disease and spreading of the disease.
• Other people believe that terminally ill patients should not receive antibiotics because they are going to die anyways. But withholding the antibiotics from terminally ill patients could reduce their survival length and quality of life.
• Some people believe that animals should not be given antibiotics as it increases antibiotic resistance, whereas others believe that this would lead to unnecessary suffering in animals.

Species diversity is the number of species present in a community

• The higher the species diversity of plants and trees in an area the higher the species doversity of animals, insects and birds. This is as there are more habitats for animals and larger and more varied food.
• Diversity can be measured to help us monitor ecosystems and identify areas where it has been dramatically reduced.

Species diversity is measured by the index of diversity  

• This is done by counting the number of species in an area

  N= total number of organisms of all species

  n= total number of one species

Woodland clearance: 

This is done by destroying habitats and food sources.

Hedgerow removal: 

This is the process of turning many small fields into a smaller number of larger fields. This decreases the diversity for the same reasons as deforestation and woodland clearance.

Monoculture

This is where farmers grow fields only containing one plant or crop. A single type of plant will support few species so diversity is reduced.

Pesticides

These are chemicals that kill organisms that feed on crops, this disrupts the food chain recducing the diversity as food is killed for predators.

Herbicides

These are chemicals that kills unwanted plants or weeds. This reduces plant diversity and could reduce organism diversity.