Biology unit 2



  • Each nucletodie is made from a pentose sugar, a phosphate group and a nitrogenous base.
  • The sugar in DNA nucleotides is a deoxyribose sugar.
  • The four possible bases are adenine (A), thymine (T), cytosine (C) and guanine (G).
  • DNA nucletodies join together to form polynucleotide strands. They join between the phosphate group of one and the sugar of another. The 2 strands join together by hydrogen bonds.
  • Specific base pairing= A to T and C to G.
  • Eukaryotic DNA is linear, that exists as chromosomes. The DNA molecule is really long so it is wound around histone proteins to fit into the nucleus. 
  • Prokaryotic DNA is shorter and circular. It isnt wound around proteins, it fits into the cell by supercoiling.
  • Genes are sections of DNA, which code for proteins. Different proteins have a different number and order of amino acids. its the order of nucleotide bases that determines the order of amino acids in a particular protein. each amino acid is coded for by a sequence of 3 bases (a triplet).
  • The order of bases in each allele is slightly different, so they code for slightly different versions of the same characteristic. 
  • Mutations are changes in the base sequence of an organisms DNA. mutations produce new alleles. 
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Meiosis and genetic variation

1) The DNA unravels and replicates, so there are 2 copies of each chromsome, called chromatids. The DNA then condenses to form 2 double armed chromosomes, made from 2 sister chromatids.

2) Meiosis 1- the chromosomes arrange themselves into homologous pairs. These homologus pairs are then separated, having the chromosome number.

3) Meiosis 2- the pairs of sister chromatids that make up each chromosome are separted. Four haploid cells (gametes) that are genetically different from each other are produced.

  • Crossing over of chromatids- when the homologus pairs come together in meiosis 1, the chromatids twist around each other and bits of chromatids swap over. The chromatids still contain the same genes but now have a different combination of alleles.
  • Independent segregation of chromosomes- when the gametes are produced, different combinations of maternal and paternal chromosomes go into each cell.
  • Genetic bottleneck- an event that causes a big reduction in a population. This reduces the number of different alleles in the gene pool and so reduces genetic diversity.
  • Founder effect- when a few organisms from a population start a new colony.
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  • Haemoglobin is a large protein with a quaternary structure, it is made from 4 polypeptide chains.  red blood cells contain haemoglobin.
  • Haemoglobin has a high affinity for oxygen, each molecule can carry 4 oxygen molecules.
  • In the lungs, it joins with oxygen to form oxyhaemoglobin, this is a reversible reaction. 
  • Oxygen loads onto haemoglobin where there is a high pO2 (lungs) and it unloads oxygen where there is a lower pO2 (respiring tissues)
  • A dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure.
  • 100% saturation means that every haemoglobin molecule is carrying the maximum of 4 molecules of oxygen and 0% means it is carrying no oxygen.
  • Where pO2 is high (lungs) , high affinity for oxygen so there is a high saturation.
  • Where pO2 is low (respiring tissues), low affinity for oxygen so there is a low saturation.
  • When cells respire they produce cardon dioxide, which raises the pCO2. This increases the rate of oxygen unloading- the curve shifts down. the saturation is lower for a given pCO2 meaning more oxygen is being released. 
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Variation in carbohydrates and cell structure.

1) Starch- the main energy store in plants. plants store excess glucose as starch. starch is a mixture of 2 polysaccharides of alpha glucose- amylose and amylopectin. it is insoluable in water so it doesnt cause water to enter by osmosis which would make them swell.

2)Gylcogen- the main energy store in animals. animals store excess glucose as gylcogen- another polysaccharide of alpha glucose. it has lots of side branches coming off it which means that stored glucose can be released quickly. it is also very compact.

3) Cellulose- the major component of cell walls in plants. its made of long, unbranched chains of beta glucose. the bonds between the sugars are straight, so cellulose chains are straight and are linked together by hydrogen bonds to form strong fibres called microfibrils. 

Plant cells:

  • Ridig cell wall made of cellulose for support and strenght.
  • Permenant vacuole- pushed the chloroplasts to the edge of the cell
  • Chloroplasts- where photosynthesis occurs. surrounded by double plasma membrane. have membranes inside them called thylakoid membranes which are stacked into grana. 
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The cell cycle

1) Interphase->

  • G1- cell grows and new organelles and proteins are made.
  • S- cell replicates its DNA ready to divide by mitosis. DNA helicase breaks the hydrogen bonds between the 2 polynucleotide DNA strands and unzips the helix. each single strand acts as a template for free floating nucleotides to join to exposed bases- specific base pairing. DNA polymerase joins new strands together. Semi conservative. 
  • G2- cell keeps growing and proteins needed for cell divison are made. 

2) Mitosis-> growth and repairing damaged tissues. 

  • Prophase- chromosomes condense, centrioles start moving to opposite ends of the cell, forming fibres called spindles. nuclear envelope breaks down.
  • Metaphase- chromosomes line up along the middle of the cell and become attached to the spindle by their centromere. 
  • Anaphase- centromeres divide, separating each pair of sister chromatids. spindles contract pulling them to opposite ends of the cell.
  • Telophase- chromatids reach the edge of the cell and uncoil and become long and thin again.
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Gas exchange

Single celled organisms  absorb and release gases by diffusion through their outer surface. they have a large surface area, a thin surface and a short diffusion pathway, so oxygen can take part in biochemical reactions as soon as it diffuses into the cell.

Fish  water containing oxygen enters the fish through its mouth and passes out through the gills. each gill is made of lots of thin plates called gill filaments which give a big surface area. the gill filaments are covered in lamella which increase surface area more and have lots of blood capillaries and a thin surface layer of cells to speed up diffusion. blood flows through the lamellae in one direction and water flows over in the opposite direction. this is called a counter current system. it maintains a large concentration between the water and the blood- so as much oxygen as possible can diffuse from water into the blood.

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Gas exchange.

Insects microscopic air filled pipes called tracheae which they use for gas exchange. air moves into the tracheae through pores on the surface called spiracles. oxygen travels down the concentration gradient towards the cells. carbon dioxide from the cells moves down its down concentration gradient towards the spiracles to be released into the atmosphere. the tracheae branch off into the tracheoles which have thin permeable walls and go into individual cells.

Dicotyledonous plants the main gas exchange surface is the surface of the mesophyll cells in the leaf. theyre well adapted for their function- they have a large surface area. the mesophyll cells are inside the leaf. gases move in and out through special pores on the epidermis called stomata. These open to allow exchange of gases.

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The circulatory system

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Blood vessels

  • Arteries: walls are thick and muscular and have elastic tissue to cope with high pressure created by the heartbeat. the inner lining is folded allowing it to stretch also helping to cope with high pressure.
  • Arteroles: arteries divide into smaller arterioles. these form a network throughout the body. blood is directed to different areas of demand in the body by the muscles inside them. these contract to restrict blood flow and relax to allow blood to flow.
  • Veins: have a wider lumen, with little elastic or muscular tissue. they have valves to stop blood flowing backwards. blood flow is helped by the contraction of muscles around them.
  • Capillaries: arterioles branch into capillaries, the smallest of the blood vessels. substances like glucose and oxygen are exchanged between cells and capillaries , so theyre adapted for efficient diffusion. near cells= short diffusion pathway. walls are one cell thick- short diffusion pathway. large number= large surface area.
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Tissue fluid

Surrounds the cells in tissues. made from substances that leave the blood (e.g. oxugen, water and nutrients) take these substances in from tissue fluid and release metabolic waste into it.

Pressure filtration:

At the start of the capillary bed, nearest the arteries, the pressure inside the capillaries is greater than the pressure inside the tissue fluid. this difference is pressure forces fluid out of the capillaries and into the space around the cells, forming tissue fluid.

As fluid leaves, the pressure reduces in the capillaries- so the pressure is much lower at the end of the capillary bed thats nearest to the veins.

Due to the fluid loss, water potential at the end of the capillaries nearest the veins is lower than the water potential in tissue fluid- so some water re-enters the capillaries from the tissue fluid at the vein end by osmosis.

Any excess tissue fluid is drained into the lymphatic system.

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Water transport in plants

  • The soil around the roots has a high water potential and the leaves have a lower water potential. this creates a water potential gradient that keeps water moving from roots to leaves.
  • The symplast pathway: goes through the living parts of the cells- the cytoplasm of neighbouring cells which is connected by the plasmodesmata.
  • The apoplast pathway: goes through the non living parts of the root- the cell walls. the walls are very absorbent and water can simply diffuse through them,as well as passing through spaces.
  • When water in the apoplast pathway gets to the endodermis cells its path is blocked by the casparian *****, so the water takes the symplast pathway.
  • Cohesion and tension- help water move up the plant, from roots to leaves, against gravity. water evaporating from the leaves creates tension, which pulls more water to the leaf. water molcules are cohesive, so more water travels up the xylem.
  • Transpiration- evaporation of water from a plants surface. transpiration rate is faster when it is....1)lighter-> stomata open when it is light, 2)hotter-> more kinetic energy, concentration gradient, 3)windier-> blows away molecules, increasing concentration gradient and 4)not very humid-> dry air, increases concentration gradient.
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Principles of classification

  • Taxonomy: involves naming and organising organisms into groups based on their similarities and differences. this makes it easier for scientists to identify them. 7 levels of groups. hierarcy= groups within groups with no overlap. species= a group of similar organisms able to reproduce to give fertile offspring. 
  • Phylogenetics: the study of the evolutionary history of groups of organisms. tells us who is related to who and how closely related they are. 
  • Problems with defining organisms-> 1) you can't study their reproductive behaviour because they are extinct. 2) they reproduce asexually- they never reproduce together even if they belong to the same species. 3) practical and ethical issues involved. 
  • DNA hybridisation:  DNA from2 species is collected and separted into 2 single strands and mixed together. where the base sequences of the DNA are the same on both strands, hydrogen bonds form by specific base pairing. the more DNA bases that hybridise together, the more similar the DNA. DNA is heated to separate the hydrogen bonds, higher temperature is needed to separate the DNA that is more similar. 
  • Similar organisms will also have similar proteins. proteins can be compared by comparing amino acid sequence or immunological comparisons (antibodies).
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Antibiotic action and resistance

  • Antibiotics can be bacterioscidal (kill bacteria) or bacteriostatic (they inhibit growth, protein synthesis and metabolic reactions).
  • Osmotic lysis: some antibiotics inhibit enzymes that are needed to make the chemical bonds in the cell wall. this prevents the cell wall from growing properly and weakens the cell wall. water moves into the cell by osmosis. the weakened cell wall can't withstand the increase in pressure and bursts (lyses).
  • Mutations: in bacterial DNA cause antibiotic resistance. mutations are changes in the base sequence of an organisms DNA. if the mutation occurs in a gene it could change the protien and cause a different characteristic. this may mean that they are no longer affected by antibiotics, they have developed antibiotic resistance. 
  • Vertical gene transmission: each daughter cell has an exact copy of the parent cells genes, including any for antibiotic resistance. chromosomes and plasmids passed on.
  • Horizontal gene transmission:  2 bacteria join by a process called conjugation and a copy of a plasmid is passed from one cell to the other. passed from same or different species.
  • Populations develop antibiotic resistance through natural selection. population exposed to antibiotic, all the ones without it die, resistant reproduce and pass on DNA.
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