Biology B2

Biology B2- genes, alleles and DNA

  • Most genes come in several different forms, called alleles.
  • e.g, a hair colour genes might have one alleles that gives black hair, another for brown and so on.
  • Humans have two sets of chromosome in each cell- one from each parent.
  • Everybody's DNA is slightly different
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Biology B2- how genes affect characteristics

  • In rabbits there might be a gene for fur colour- one allele of the gene for black fur, another allele for white fur.
  • Complete sets of chromosomes= 2 copies of each gene
  • If an allele for black fur is B, and white fur, b, ten there are three possible combinations:BB,Bb,bb
  • Bb=black fur because the B allele is dominant, the b allele is recessive.
  • when a dominant and a recessive allele are together, only the dominant allele will have an affect.
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Biology B2- inheriting chromosomes and genes

  • Genetic diagrams show how alleles of genes are inherited
  • half of the sperm cell will have allele B and the other half with have allele b
  • All female eggs with contain allele b- bb
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Biology B2- how genes work

  • A gene is a length of DNA that carries the code for making protein
  • Each strand of a DNA molecule is made up of a sequence of four bases.
  • The sequence of bases in the DNA determines the sequence of amino acids in the protein that is made.
  • Mutations in the DNA form new alleles of genes.
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Biology B2- genetic disorders

  • Disorders are cause by a dominant allele, you only need to inherit one of these to have a condition.
  • Cystic fibrosis- a disorder of cell membranes- caused by a recessive allele.
  • A person witht the dosorder will have inherited two recessive alleles, someone with one recessive allele is called a carrier.
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Biology B2- photosynthesis

  • Equation for photosynthesis: carbon dioxide + water ---> energy from sunlight glucose + oxygen
  • In photosynthesis, light energy is stored in glucose molecules.
  • The energy is transfered to animals when they eat the plants.
  • Glucose can be converted into starch and stored for later use, they cannot diffuse out of cells. 
  • Light energy is absorbed by chlorophyll found in the chloroplast of plant cells.
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Biology B2- limiting factors

  • Light is a limiting factor for photosynthesis.
  • when the rate of photosynthesis does not increase anymore, even when the plant is getting more light, this could be because there isn't enough carbon dioxide or the temperature is too low.
  • limiting factors of photosynthesis:
  • - light intensity
  • - carbon dioxide
  • - temperature
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Biology B2- structure and functions of tissues

  • The cells that make up tissues are adapted for particular roles.
  • Muscular tissue is specialised to produce movement.
  • Glandular tissue is made up of cells which secrete (release) useful substances such as enzymes or hormones.

Plant tissues and organs

  • The organs in a plant are made up of tissues.
  • The whole plant is covered in a layer of epidermis. This helps to protect underlying cells, stops the leaves from producing too much water and prevents pathogens from entering the plant.
  • Most of the cells in a leaf are mesophyll cells. This is where photosynthesis takes place.
  • Xylem and Phloem tubes run through the entire plant.
  • These are tubes that make up the plants transport system:
  • Xylem carries water from the roots to the leaves; sugars are transported around the plant in phloem.
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Biology B2- enzymes

  • An enzyme molecule is a long chain of amino acids, folded into a ball. 
  • There is a dent in the ball which means another molecule can fit in
  • The dent is an active site of the enzyme
  • The molecule that fits inside another is called its substrate
  • The enzyme makes the substrate react, changing it into a new substrate
  • Once the active site has lost its shape, the substrate no longer fits
  • When the enzyme is permanently changed, it is said to be denatured.
  • when the temperature reacts, the molecules move very quickly and there is more chance they will collide.
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Biology B2- enzymes and digestion

  • Digestive enzymes pass out of the cell in the glandular tissue
  • Go into the space inside the gut and become mixed up with the food
  • They cut large food molecules into smaller ones
  • There are three main groups of enzymes:

Acid and alkali

  • Liver produces bile which is stored in the gall bladder
  • Bile mixes with the food in the bile duct of the small intestine
  • Bile is an alkali- alkaline optimum for enzymes in small intestine
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Biology B2- enzymes in industry

  • Baby foods- some baby food manufacturers add protease to their products,
  • These enzyme break down large protein molecules into amino acids.
  • when the baby eats the pre-digested food, it can absorb the amino acids.
  • Sugar syrup- this is used in making sweets and sports drinks.
  • The starch can be changed into sugar syrup by adding carbohydrase enzymes such as amylase.
  • Slimming foods- these contain very sweet sugar called fructose instead of glucose.
  • Fructose is made from an enzyme using an enzyme called isomerase.
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Biology B2- aerobic respiration

  • Cells release energy by combining glucose molecules with oxygen
  • The word equeation is: glucose + oxygen ---> carbon dioxide + water (+energy)
  • Oxygen enters the blood in the lungs.
  • It is transported  in the blood to all the cells in the body 
  • The carbon dioxide that the cells make is carried back into the blood to the lungs
  • Aerobic respiration takes place in the mitochondria of cells
  • They contain all the enzymes that are needed to make the reactions of respiration happen quickly.
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Biology B2- anaerobic respiration

  • Anaerobic respiration is incomplete breakdown of glucose to lactic acid: glucose ---> lactic acid (+a little energy)
  • It releases less energy than aerobic respiration
  • lactic acid build up in the muscles
  • It can make the muscles feel tired and can cause cramps
  • They stop contracting efficiently 
  • Once you have stopped exercising, you have to get rid of the lactic acid
  • You continue to breathe heavily to allow extra oxygen in (the oxygen debt)
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Biology B2- cell division- mitosis

  • More cells are made when existing cells divide into 2- Mitosis
  • It is important because it provides cells for growth and to replace dead or damaged cells.
  • Normal body cells have 23 pairs of chromosomes.
  • Before it divides by Mitosis, it first copies each chromosome.
  • When the cells are divided, the chromosomes are shared equally between the 2 new cells.

How mitosis happens

  • Chromosomes are copied 
  • identical copies of each chromosome split apart and move to oposite ends of the cell.
  • chromosomes now form into two nuclei
  • new cell membranes form and 2 new cells have been made.
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Biology B2- cell division- meiosis

  • Before meiosis happens, the chromosomes in the cell that are going to divide are copied- same as mitosis.
  • The cell divides TWICE. This means that FOUR cells are produced.
  • Each new cell gets half the original number of chromosomes.
  • This is how gametes are made.
  • It only happens in the testes and the ovaries.
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Biology 2- Quadrats

  • Measure the distribution of organisms in a habitat.
  • Quadrats are placed randomly.
  • After, you can:
  • - count the numbers of each one
  • - estimate the percentage of the area inside the cell the quadrat that each species occupies
  • Placing the qaudrat randomly ensures the results are valid.
  • calculating the mean will increase reliability
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Biology B2- stem cells

  • The fertilised egg divides over and over again to produce an embryo
  • For the first few days all of the cells stay as stem cells
  • each cell has the potential to develop into any kind of cell in the human body
  • Most cells can differentiate and form specialised cells
  • A few remain as stem cells and can continue to divide and specialise thorught out adult life.
  • In plants, many cells can differentiate throughout their lives
  • Stem cells from embryos and adults particulary bone marrow stem cells can be used to damagereplace d tissues in the future.
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Biology B2- new species

  • Many new species aries through a series of steps.
  • Geographical isolation
  • - if a few lizard for example, drift away on a log and end up somewhere else. They are isolated from the rest of the species.
  • Genetic variation
  • - In both mainland and island lizards, there are many different alleles of genes which lead to variation.
  • Natural selection
  • - the enviroment, predators etc on the mainland are different to those on the island. This is because natural selection takes place and are adapted to their own enviroment.
  • Speciation
  • - Over time, more and more differences build up between the 2 populations of lizards until they are no longer the same species.
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Biology B2- key words

  • Homozygous- 2 identical alleles, for example bb
  • Heterozygous- 2 different alleles, for exapmple Bb
  • Genotype- the combination of alleles, for example Bb
  • Phenotype- a characteristic, for example black fur
  • Species- a group of organisms that share similar characteristics and can breed together to produce fertile offspring.
  • Differentiate- change of cell from general purpose to a specialised cell to carry out a particular function
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