Eukaryotic and Prokaryotic Cells

• Eukaryotic cells have membrane-bound organelles and have genetic material contained in the nucleus
• An organelle is a part of a cell that carries out a specific function
• Plant and animal cells are examples of eukaryotic cells
• Eukaryotic cells are typically between 10-100 μm in size
• 1 μm is equal to 1 x 10-6 m
• All eukaryotic cells have a nucleus, mitochondria, ribosomes, cytoplasm and a cell membrane. Plant cells also have a cell wall, vacuole and chloroplasts
• Mitochondria are the site of aerobic respiration which releases energy for cellular processes
• Ribosomes are the site of protein synthesis
• Chloroplasts contain chlorophyll to allow for photosynthesis
• The cell membrane controls the movement of substances in and out of a cell
• The cytoplasm is the site of chemical reactions within a cell
• The cell wall provides support and rigidity to plant cells and is made of cellulose
• The vacuole contains cell sap and provides rigidity to plant cells
• Prokaryotic cells do not contain membrane-bound organelles
• Prokaryotic cells are much smaller than eukaryotic cells. They are typically between 1-10 μm in size.
• Prokaryotic cells are approximately 10 orders of magnitude smaller than eukaryotic cells
• Prokaryotic cells contain genetic material in small rings called plasmids, or in larger loops
• Prokaryotic ribosomes are smaller than eukaryotic ribosomes

Aseptic Technique

• Petri dishes are used to produce cultures of bacteria and other micro-organisms
• Cultured bacteria are grown on a nutrient medium in controlled conditions
• Aseptic techniques must be used to prepare cultures to prevent contamination of the culture and the growth of harmful bacteria
• Petri dishes, inoculating loops and culture media must be sterilised before use. A flame can be used to sterilise equipment
• An inoculating loop is a piece of equipment used to transfer bacteria to the petri dish
• The lid of a Petri dish should be partially secured with tape to ensure bacteria cannot escape but conditions remain aerobic
• The Petri dish must be stored upside down to prevent condensation affecting bacterial growth
• In school laboratories, cultures should generally be incubated at 25 °C to prevent the growth of harmful bacteria
• A cotton wool swab can be used to transfer a sample to a Petri dish to investigate bacterial growth
• Bacteria on a Petri dish divide rapidly whilst the nutrient supply is rich. Every time the bacteria reproduce, the number doubles. The total number of bacteria can be calculated using the following formula: Final number of bacteria = Initial number of bacteria x 2number of divisions

Microscopes

• Microscopy is the field of using microscopes to view samples that cannot be seen with the naked eye
• Microscopy has developed over time
• Light microscopes allow us to see the largest organelles, including the nucleus, cell membrane, cell wall and cytoplasm. A stain is often used to make the organelles clearer
• The parts of a light microscope include the eyepiece lens, objective lenses, stage, coarse focusing wheel, fine focusing wheel, light/mirror
• A sample used with a light microscope must be very thin to allow light to pass through
• The specimen to be viewed under a microscope is placed on the stage and secured with stage clips
• The eyepiece lens and objective lens are used to increase the size of the image
• The coarse focusing wheel is used to move the stage and get the cells into frame
• The fine focusing wheel is used to sharpen an image
• The total magnification of a microscope can be calculated using the following equation: Total magnification = Objective lens x eyepiece lens
• Electron microscopes have a greater magnification and resolution than light microscopes. They are much more expensive than light microscopes
• Magnification is the number of times larger an image is than the object
• Resolution is the ability to distinguish between two points
• Electron microscopes allow are to see more organelles and study cells in greater detail
• Magnification can be calculated using the following equation: Magnification= Size of imageSize of object
• A scale bar can be used to calculate the magnification of an irregular object
• Magnification does not have a unit because it is a ratio

Diffusion

• Diffusion is the spreading out of particles, of a gas or liquid, resulting in net movement from an area of high concentration to low concentration
• Diffusion of some substances can happen through the cell membrane
• In gas exchange, oxygen and carbon dioxide diffuse between the alveoli and the blood
• Urea is a waste product made by cells that needs to be excreted by the kidneys. Urea diffuses from cells into blood
• The rate of diffusion is increased by: an increase in temperature, an increases in the difference in concentrations (concentration gradient) and by a greater surface area
• Unicellular organisms have a relatively high surface area to volume ratio allowing for sufficient transport of all required substances
• Large, multicellular organisms have adaptations to increases the surface area to volume ratio to allow for efficient exchange of substances
• The cell membrane is very thin so provides a short diffusion path
• In plants, the structure of leaves and roots increases the surface area for diffusion
• The lungs of mammals, birds and reptiles are well ventilated, have a large surface area and an efficient blood supply to maximise the rate of diffusion
• The gills of fish have a large surface area and efficient blood supply to maintain a high concentration gradient for diffusion

Osmosis

• Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Only water can move by osmosis
• A partially permeable membrane is a membrane that lets particular substances pass through it, either into or out of the cell
• A hypertonic solution is one in which the external solution has a higher concentration of solute than the cell. Water always moves out of a cell that is placed in a hypertonic solution, causing the cell to shrivel or become flaccid
• Tissue placed in hypertonic solutions decreases in mass
• A hypotonic solution is one in which the external solution has a lower concentration of solute than the cell. Water always moves into a cell that is placed in a hypotonic solution, causing the cell to swell or become turgid
• Tissue placed in hypotonic solutions increases in mass
• An isotonic solution is one in which the external solution has the same concentration of solute as the cell. Water will not move in or out of cells placed in an isotonic solution so their size will stay constant
• Guard cells open and close due to the movement of water by osmosis
• The mass of plant tissue can be measured before and after being placed in a solution of known concentration to calculate the percentage change in mass due to osmosis

Active Transport

• Active transport is the movement of substances from a more dilute solution to a more concentrated solution, requiring energy from respiration
• Active transport works against the concentration gradient
• Some substances are moved into a cell by both diffusion and active transport
• Active transport is used in the small intestine/gut to transport glucose into the blood for transport to cells for respiration
• Active transport is used in root hair cells to absorb mineral ions from the soil that are essential for plant growth
• Plants growing in waterlogged soils cannot absorb mineral ions because the cells do not have access to oxygen for respiration

Cell Growth and Division

• Both eukaryotic and prokaryotic cells undergo cell division
• Cells increase in number by dividing into two
• The eukaryotic cell cycle contains a growth phase where the cell grows to double sub-cellular structures (such ribosomes and cell membrane) and DNA, then the cell splits into two during mitosis
• The cell cycle of different cells lasts different lengths of time
• A microscope can be used to observe cells in different stages of the cell cycle
• The length of time in a certain stage of the cell cycle can be calculated using the following formula: observed number of cells at that stagetotal number of cells observed x total length of time of cell cycle
• The mass of DNA in a cell doubles during the growth phase of the cell cycle
• The mass of DNA in a cell can be measured in picograms
• 1 picogram = 1 x 109 g
• During mitosis DNA (arranged into chromosomes) is pulled to separate ends of the cell ready for division
• The final part of the cell cycle is when the cell membrane splits to produce two identical cells
• Mitosis is used by eukaryotic organisms for growth and repair
• Mitosis is used by eukaryotic organisms that asexually reproduce
• Mitosis does not occur in prokaryotic cells because they do not possess a nucleus
• Checkpoints in the cell cycle control the rate of cell division
• Cancer is caused by uncontrolled cell division
• A tumour is a mass of cells caused by uncontrolled cell division
• Benign tumours are a mass of cells contained in one area
• Malignant tumours are formed of cancer cells that invade other tissues and spread around the body where they form secondary tumours
• A risk factor is a gene or lifestyle choice that can increase the likelihood of a person developing a disease
• Lifestyle risk factors for cancer include poor diet, lack of exercise, smoking, UV exposure
• Genetic risk factors for cancer include gene mutations

Stem Cells

• Specialised cells arise from stem cells
• Stem cells are cells that are capable of differentiating into other types of cell
• When a cell differentiates, it acquires specific structures needed for that cell type
• Most animal cells differentiate at an early stage of development
• Embryonic stem cells can differentiate into all human cell types
• Adult bone marrow contains stem cells that can differentiate into different types of blood cell
• Embryonic stem cells can be used to study and treat diseases. There are religious and ethical objections to using embryonic stem cells in scientific research
• Plants contain meristem tissue at the tips of shoots and roots that retains the ability to differentiate throughout a plant’s life

Disciplinary Knowledge

• Change the subject of an equation
• Use percentages
• calculate percentage increase and decrease
• Identify in a given context:
• the independent variable as the one that is changed or selected by the investigator
• the dependent variable that is measured for each change in the independent variable
• Outline a simple ethical argument about the rights and wrongs of a new development, discovery or technology
• Explain that there are hazards associated with science-based technologies which have to be considered alongside the benefits
• Suggest a hypothesis to explain given observations or data
• Explain why a certain hypothesis was chosen, with reference to scientific theories and explanations
• Identify and assess risks to health related to lifestyle habits and the risk of disease
• Suggest sensible precautions to reduce risk

Practical Skills

• Application of aseptic technique
• Prepare a slide with cells for viewing under the light microscope
• Obtain a clear image using a light microscope
• Measure mass accurately

Biodiversity

• Biodiversity is the variety of all the different species in an ecosystem
• The biodiversity of a habitat can be measured by using sampling techniques to count the abundance of difference species
• High biodiversity in an ecosystem makes it stable because one species will not depend on another species alone
• Many human activities are reducing biodiversity on Earth
• Recent rapid growth in the global human population means that more resources are being used and more waste is produced
• Humans reduce the amount of land available for biodiversity by building, deforestation, quarrying, farming and waste disposal
• Deforestation happens in tropical areas to provide land for cattle, rice fields and to grow crops for biofuels
• Introducing non-indigenous species can reduce biodiversity if the species out-competes or kills indigenous species
• Peat bogs are a habitat that is being destroyed because peat is taken for garden and farming compost. Although we need peat compost for farming, this is also reducing biodiversity
• Peat is a fossil fuel. Decay or the burning of peat releases carbon dioxide into the atmosphere
• Scientists and citizens are using various programmes to reduce the negative impact humans have on biodiversity, including: breeding programmes for endangered species, protecting rare habitats, reducing how many forests are cut down, reforestation, recycling resources to reduce landfill waste and growing hedgerows on farms where previously there was only one crop growing

Pollution

• Rapid growth in the human population and an increase in the standard of living mean that increasingly more resources are used and more waste is produced. Unless waste and chemical materials are properly handled, more pollution will be caused.
• Pollution can occur in water, from sewage, fertiliser or toxic chemicals, in air, from smoke and acidic gases, on land, from landfill and from toxic chemicals
• Pollution is caused when human waste isn’t properly handled, for example: air pollution from smoke, land pollution from landfill rubbish and water pollution from sewage and fertilisers
• Pollution kills animals and plants which reduces biodiversity
• Indicator species can be used to monitor the level of pollution in a habitat
• Levels of carbon dioxide and methane in the atmosphere are increasing and contributing to global warming
• The biological consequences of global warming include loss of habitats, changing breeding patterns and changing migratory patterns which all affect biodiversity

Pyramids of Biomass

• Biomass is lost at each stage of a food chain
• Producers are mostly plants and algae which transfer about 1% of the energy from light into new plant biomass during photosynthesis
• Only approximately 10% of the biomass from each trophic level is transferred to the next trophic level
• Biomass is lost from a food chain when it is excreted as waste. This includes the egestion of undigested material in faeces, the loss of water and urea in urine, and the loss of carbon dioxide and water in respiration
• Life processes, including movement and regulation of temperature, require energy from glucose. Energy released during respiration, used to sustain these processes, is not transferred to the next trophic level
• Percentage efficiency transfer can be calculated using the following equation: Percentage efficiency transfer=biomass in higher trophic levelbiomass in lower trophic levelx 100
• The number of organisms at higher trophic levels is often lower because the efficiency of biomass transfer decreases

Food Security

• Food security means that all people on Earth have access to sufficient, safe, and nutritious food that meets their food preferences and dietary needs for an active and healthy life
• Food security is being threatened due to a range of different biological factors in different countries. These include increasing birth rates leading to a rise in populations, changing diets in developed countries, new pests and pathogens that affect farming, increased costs of farming, conflict over resources and environmental changes that affect biodiversity
• Food security can be increased by making food production more efficient
• Individuals can contribute to improving food security. This can be done by reducing meat intake, increasingly eating food products from producers and eating local, seasonal produce where possible
• The efficiency of food production can be improved by restricting energy transfer from livestock (animals bred for food) to the environment. This can be done by limiting their movement and by controlling the temperature of their surroundings. There are ethical implications to these farming methods
• Livestock can be fed high protein foods to increase growth, and given antibiotics to prevent disease
• Fish stocks in the oceans are declining. It is important to maintain fish stocks at a level where breeding continues or certain species may disappear altogether in some areas
• Fish stocks can be controlled by using nets with larger holes. This prevents smaller fish from being caught so they can continue breeding
• Fishing quotas limit the number of fish that can be caught by each country. The fishing of some endangered species of fish is banned
• Alternative food sources have been developed to enhance food security. Modern biotechnology techniques enable large quantities of microorganisms to be cultured for food
• The fungus Fusarium is useful for producing mycoprotein, a protein-rich food suitable for vegetarians. The fungus is grown on glucose syrup, in aerobic conditions, and the biomass is harvested and purified

Disciplinary Knowledge

• Explain why data is needed to answer scientific questions, and why it may be uncertain, incomplete or not available.
• Understand the principles of sampling as applied to scientific data
• Recognise that scientific methods and theories change over time
• Describe and explain specified examples of the technological applications of science.
• Describe and evaluate, with the help of data, methods that can be used to tackle problems caused by human impacts on the environment.
• Outline a simple ethical argument about the rights and wrongs of a new development, discovery or technology.

Types of Reproduction

• Cells in reproductive organs divide by meiosis to form gametes
• Meiosis halves the number of chromosomes in gametes
• When a cell divides to form gametes: copies of the genetic information are made and the cell then divides twice to form four gametes, each with a single set of chromosomes
• All gametes are genetically different from each other
• Gametes join at fertilisation to form a zygote with the normal number of chromosomes
• After fertilisation, the new cell divides by mitosis and the number of cells increases
• As the embryo develops, cells differentiate
• Organisms use either sexual or asexual reproduction to reproduce
• Sexual reproduction involves the joining (fusion) of male and female gametes: sperm and egg cells in animals, pollen and egg cells in flowering plants
• In sexual reproduction there is mixing of genetic information which leads to variety in the offspring
• Asexual reproduction involves only one parent and no fusion of gametes
• There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved
• Advantages of sexual reproduction: It produces variation in the offspring. If the environment changes, variation gives a survival advantage by natural selection, natural selection can be speeded up by humans in selective breeding to increase food production
• Advantages of asexual reproduction: only one parent needed, more time and energy efficient as do not need to find a mate, faster than sexual reproduction, and many identical offspring can be produced when conditions are favourable
• Some organisms reproduce by both methods depending on the circumstances. Malarial parasites reproduce asexually in the human host, but sexually in the mosquito
• Many fungi reproduce asexually by spores but also reproduce sexually to give variation. Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils

Gene Theory

• Our current understanding of genetics has developed over time
• In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged. The importance of Mendel's discovery was not recognised until after his death
• In the late 19th century behaviour of chromosomes during cell division was observed
• In the early 20th century, it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes
• In the mid-20th century, the structure of DNA was determined, and the mechanism of gene function worked out
• This scientific work by many scientists led to the gene theory being developed

DNA, Genes and Chromosomes

• DNA is a polymer. It is made of two strands which form a double helix. The DNA is contained in structures called chromosomes
• A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein
• The genome of an organism is the entire genetic material of that organism
• The monomers of DNA are called nucleotides
• DNA contains four bases, A, C, G and T. A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein
• Every chromosome is one of a pair so there are two copies of each gene in every genome
• Different versions of genes are called alleles
• Some characteristics, for example fur colour in mice or red-green colour blindness, are controlled by one gene
• The set of particular alleles present is called the genotype
• The genotype (e.g., brown allele of the fur colour gene) is expressed to make the phenotype (e.g., brown fur)
• A dominant allele is always expressed when present even when only one copy is present
• A recessive allele is only expressed when there are two copies of it (i.e., no dominant allele)
• If the two alleles present are the same, either both dominant or both recessive, then this is described as homozygous
• If the one allele is dominant and one is recessive, then this is described as heterozygous

Predicting Inheritance

• Most characteristics are the result of the interaction of many genes
• Punnett square diagrams can be used to predict the genotypes of offspring from a single gene cross
• Direct proportion and simple ratios can be used to describe the outcomes of a genetic cross
• Capital letters are used to denote dominant alleles and lower-case letters are used to denote recessive alleles when drawing a Punnett square
• Polydactyly is an inherited disorder where sufferers have extra digits; it is caused by a dominant allele
• Cystic fibrosis is an inherited disorder where sufferers have lung problems due to a faulty cell membrane protein; it is caused by a recessive allele
• An individual can be a carrier of a recessive disorder, but not of a dominant disorder
• Family trees show over several generations which individuals had a particular phenotype. This can be used to derive the genotype
• Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex. In females the sex chromosomes are the same (XX). In males the chromosomes are different (XY)
• A Punnett square can be used to demonstrate that there is a 1:1 ratio of offspring being male or female. The probability of this remains the same, regardless of the number of previous children of a particular sex a couple has had

DNA codes for Proteins

• Watson, Crick, Wilkins and Franklin played a part in the development of the DNA model
• The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases
• The DNA polymer is made up of repeating nucleotide units
• In the complementary strands a C is always linked to a G on the opposite strand and a T to an A
• Proteins are synthesised on ribosomes, according to a template. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order
• When the protein chain is complete it folds up to form a unique shape. This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen
• Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.
• A change in DNA structure may result in a change in the protein synthesised by a gene
• A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength

Disciplinary knowledge

• Recognise that scientific methods and theories change over time
• Understand simple probability
• Use percentages - calculating % time spent in different stages of the cell cycle
• Use prefixes and powers of ten for orders of magnitude (e.g., tera, giga, mega, kilo, centi, milli, micro and nano).

Practical skills

• Safe use of equipment to separate mixtures using filtration