The Transfer of Energy
Life depends on continuous transfers of energy.
In this unit we start with photosynthesis. Splitting it into the light dependent reactions (LDR), and the Light Independent reactions (LIR).
LDR take place in the thylakoid membranes of the chloroplasts. Light energy is absorbed by chlorophyll, and results in electron transfer, which in turn creates a proton gradient, and the enzyme ATP synthase to produce ATP. The LDR also produces reduced NADP, which together with carbon dioxide enter the Calvin cycle and produce triose phosphate which can be converted to other organic molecules such as glucose. There are also three required practicals on the cross board CPAC linked to photosynthesis.
In aerobic respiration, the breakdown of glucose as a respiratory substrate starts with glycolysis in the cytoplasm, which then links to the mitochondria. Krebs cycle occurs in the mitochondrial matrix, finishing off with the Electron Transport Chain producing ATP.
This unit extends to the transfer of energy between organisms, as Net Primary Productivity (NPP) . this is Gross Primary Productivity ( GPP) minus respiratory losses. This extends to animals ( consumers) when N ( Net production) is equal to chemical energy ingested, minus losses in faeces, urine and respiration. Farming practices are designed to make these transfers most efficient.
Finally, nutrients alo cycle through the ecosystem, and at A Level biology , these focus on the Nitrogen cycle and the Phosphorus cycle. This is extended to A level by the role of saprobionts and mycorrhizae.
- U5T1 – The Light Dependent Reactions of Photosynthesis
- U5T2 – The Light Independent Reactions of Photosynthesis
- U5T3 – Limiting Factors and Photosynthesis Practicals
- U5T4 – Respiration: Glycolysis and Link Reaction
- U5T5 – Respiration: Krebs Cycle
- U5T6 – Respiration: The Electron Transfer Chain
- U5T7 – Anaerobic Respiration & Respiration Experiments
- U5T8 – Biomass, Gross and Net Primary Productivity
- U5T9 – The Nitrogen Cycle & Phosphate Cycle
Responding to Change
A stimulus is a change in the internal or external environment, which can be detected by a receptor, and an effect brought about by an effector. In this unit, we look at how different organisms detect and coordinate these responses.
Plants respond to stimuli via growth factors. At A Level, this is based around Indole Acetic Acid (IAA). the responses are slow, growth responses called tropisms. There have been many experiments on plant responses, and some of these are tested in here.
Animals ( such as many invertebrates) can have simple responses to stimuli that can enable a motile organism, to move to a favourable environment. These are either taxes , which are directional responses, or kineses, which are non-directional responses.
Nervous coordination includes the establishment and maintenance of a negative resting potential,and the generation of an action potential due to changes in membrane permeability to sodium and potassium ions. Once again, we extend GCSE basic understanding of synapses, to consider summation and inhibition too. Receptors are cells that detect specific stimuli, and cause the establishment of a generator potential. In a pacinian corpuscle, the stimulus is pressure. We also use the examples of rod and cone cells as receptors that detect light.
Skeletal muscle is an effector. The arrival of an action potential causes biochemical changes which results in cycles of actinomyosin bridge breaking and formation.
Homeostasis is the maintenance of a constant internal environment. Specifically we look at the control of blood glucose via insulin and glucagon. Also the action of adrenaline, and the second messenger model. We also look specifically at the control of water potential of the blood via ADH on the kidney.
- U6T1 – Responses in Plants
- U6T2 – Taxes and Kinesis
- U6T3 – Nerves: The Resting Potential
- U6T4 – Nerves: Action Potentials
- U6T5 – Nerves: Conduction Along Axons
- U6T6 – Nerves: Synaptic Transmission
- U6T7 – Nerves: Receptors (Pacinian Corpuscles and Eye)
- U6T8 – Muscle Contraction
Populations, Evolutions and Genetics
All individuals of a population show variation in their phenotype, caused by the environment and genetics. In this unit, we look at mendelian ratios derived from monohybrid and dihybrid crosses. We also look at sex linkage, autosomal linkage and epistasis.
The Hardy Weinberg equation is a quadratic equation which allows us to calculate allele frequency in a population. The Hardy Weinberg equation assumes a population to be large, with random mating, and no immigration, no emigration, no mutation . To use the Hardy Weinberg, you must first identify what is the recessive allele, and then if the information on that allele is about whole organisms ( in which case use ‘q2’) or number of alleles in which case use ‘q’. Variations due to meiosis and mutation, and ensuing differential reproductive success give rise to new alleles and changes in allele frequency.
This can lead to evolution of new species, called speciation. Speciation can be either allopatric ( geographically separated) or sympatric (reproductively isolated without geographical barriers). This unit also looks at the study of populations in ecosystems. Here we recap terms such as ‘community’ and ‘niche’ and interspecific and intraspecific speciation.
Techniques such as random sampling, belt transects and mark-release-recapture are also covered here. Succession is the process where an ecosystem changes overtime ( this is not the same process as evolution). Pioneer species ( often lichen) are the first species to grow on bare rock, creating a thin soil, so bigger species are then able to take root. Lastly conservation is also covered in this unit.
Populations, Evolutions & Genetics
- U7T1 – Definitions, Monohybrid, Dihybrid and Codominance
- U7T2 – Sex Linkage, Autosomal Linkage and Epistasis
- U7T3 – Hardy Weinberg & Allele Frequency
- U7T4 – Variation, Evolution and Speciation
- U7T5 – Populations in Ecosystems
- U7T6 – Succession
Control of Expression
Although all cells within an organism carry the same genetic code, different parts of the code can be ‘turned on’ , and used by different cells. This ‘turning on’ of genes is called gene expression. It is the different expression of genes which allows cells to have different features and functions.
Mutations are re-visited in this unit ( also in unit 4) , but extended to look at inversion, duplication and translocation of bases. Some mutations will lead to a change in the entire reading frame after that point, and this is called a frame shift. Mutation of tumour suppressor genes and proto-oncogenes can lead to cancer.
Cancer can also be influenced by epigenetics. Epigenetics is the marking of DNA or histone proteins, which affects the likelihood of that section of DNA being expressed or not. Epigenetic control is perhaps the biggest discovery this century in the field of Biology. The marks are in the form of acetylation or methylation, and have been found to have strong roles in disease states such as cancer.
Another recent discovery in the control of expression has been the discovery of Small Interfering RNAs ( siRNA) . Here RNAs bind to mRNA’s causing them to be degraded by enzymes, and therefore preventing that mRNA from being translated.
Gene technologies are ever advancing, and this is taken into account with the acknowledgement that sequencing methods for example are constantly improving . Recombinant DNA technologies are revisited from GCSE, and advanced by looking at the Polymerase Chain Reaction ( PCR) as a method of in vitro DNA replication. The use of DNA probes and hybridisation, along with genetic finger-printing are also included in this section.