A-level Biology/Printable version

A-level Biology

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Biology Foundation

Biology foundation provides both an introduction to biology and also covers all the basic concepts that will be used throughout most of the other units. This unit is essential to the course structure.

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Human Health and Disease

Human health and disease provides everything to do with health, diseases, smoking and immunity.

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Transport provides everything to do with the transport exam, i.e. transport systems in mammals and plants.

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Central Concepts

Central concepts is the primary A2 module and is not optional. It builds on concepts learned in AS year.

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Photosynthesis   (Jun 11, 2008)

Energy and Respiration   (Jun 13, 2008)

Control, coordination and homeostasis   (Jun 13, 2008)

Meiosis, genetics and gene control   (Jun 12, 2008)

Classification, selection and evolution   (Jun 11, 2008)

Populations and interactions   (Jun 02, 2008)

Microbiology and Biotechnology

Introduction to Microbiology edit

Microbiology is the study of the organisms in the surrounding environment which are too small to be clearly visible with our naked eye when observed individually. These organisms are called '"microorganisms"' and can only be observed through microscopes. Microorganisms are less than 0.1mm in size, which means that they are smaller in size than the visual range of humans. Microorganisms can be found everywhere on the planet Earth. They are the most abundant and diverse group of organisms on Earth. Many different groups of microorganisms currently inhabit the Earth. They possess different adaptations, specialized and differentiated functions, various forms and structures, different shapes and sizes, and numerous habitats to ensure their survival and existence on Earth. Some of the different groups of microorganisms are Bacteria, Archaea Bacteria, Cyanobacteria or Blue Green Bacteria, Protists, Algae, Fungi, Mollicutes (Mycoplasmas and Phytoplasmas), Viruses, Viroids, and Prions.

The units which are used to measure the microorganisms and their structural components are nanometers and micrometers. Due to the larger sizes of some microorganisms, they are more readily visible than the other smaller types of microorganisms. Microorganisms are found everywhere on the earth including in soil, water, air, in interior surfaces and on exterior surfaces of the earth.

Most of the microorganisms act as the primary producers of the food chains in the fresh and marine water habitats, as most of them possess the ability to perform photosynthesis. Therefore, they form the basis of the food chains in the aquatic water bodies.

Saprotrophs and Saprophytes edit

The microorganisms that live in the soil, which are commonly known as the soil microorganisms possess the ability to decompose and digest the waste products and materials in the environment and also assist in the recycling of the chemical elements and compounds in between the soil, water, air and the living organisms. These soil microorganisms decompose complex organic matter into simple organic matter and then into simple organic and inorganic molecules and after that, into their constituent most basic elements either by physical breakdown or by biochemical transformation of these complex compounds. These organisms, called Saprotrophs or Saprophytes, cannot produce their own food and instead feed on the non-living, dead, organic, decaying and waste matter in order to survive. Most of the Fungi such as Rhizopus, Mucor, Agaricus, Penicillium, Aspergillus, Morchella and Saccharomyces (Yeast), some bacteria such as soil bacteria, some protozoans and some algal species such as photosynthetic algae, like green algae, exhibit saprophytic mode of nutrition.

Bioaerosols and Pathogens edit

Some microorganisms are suspended and released into the atmosphere as bioaerosols, possessing the capability to travel and spread long distances with the help of the wind current and precipitation. These bioaerosols are composed of bacterial cells, their cellular fragments and remnants, fungal spores and their hyphae, viral particles, viroids, mollicutes, prions, their metabolic waste and by-products. Therefore, the pathogenic bioaerosols are capable of transmitting and spreading of most of the infectious diseases in living organisms. Only a trace number of microorganisms which are associated with the other living organisms such as plants, animals and human beings are pathogenic and cause infectious diseases. The majority of the microorganisms are beneficial and harmless to other organisms, but some are very harmful, causing infectious diseases.

Extremophiles edit

Some of the microorganisms are well adapted to inhabit the extreme environmental conditions which are unfavourable and can even be lethal for the other living organisms. These microorganisms are known as extremophiles. These extremophiles can be discovered inside the crust of the Earth, in the deepest seas at high pressures, in extremely acidic or in extremely basic conditions, in hydrothermal and alkaline vents, in frozen sea water, in volcanoes, in hot water springs, in the upper atmosphere and in outer space.

Mammalian Physiology and Behavior

Mammalian physiology and behaviour is the module which is chosen by the student (usually the school) at A2, and assumes that you have completed central concepts.

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Cell Structure edit

Cell Organelles edit

Organelles are parts of cells. Each organelle has a specific function.

1. Nuclear membrane 2. Nuclear pore 3. Rough endoplasmic reticulum (REM) 4. Smooth endoplasmic reticulum 5. Ribosome attached to REM 6. Macromolecules 7. Transport vesicles 8. Golgi apparatus 9. Cis face of Golgi apparatus 10. Trans face of Golgi apparatus 11. Cisternae of Golgi apparatus 12. Secretory vesicle 13. Cell membrane 14. Fused secretory vesicle releasing contents 15. Cell cytoplasm 16. Extracellular environment

Nucleus edit


  • Largest organelle
  • Surrounded by a nuclear envelope, which contains pores (holes)
  • Contains chromatin and the nucleolus


  • Store genetic material
  • Controls cell activities
  • Pores allow substances to move between the nucleus and the cytoplasm
  • The nucleolus produce ribosomes (see below)
The parts of a cell nucleus

Mitochondria edit


  • Oval-shaped
  • They have a double membrane - the inner one is folded to form structures called cristae
  • Inside is the matrix, containing enzymes


  • They are the site of aerobic respiration
  • Makes energy in the form of ATP (adenosine triphosphate) as a source of energy for the cell's activities
  • Cristae give a bigger surface area so more enzymes can fit in
The diagram shows a section of a eukaryotic cell's mitochondrion.

Endoplasmic Reticulum edit


  • Smooth endoplasmic reticulum is a system of membranes which enclose a fluid-filled space
  • Rough endoplasmic reticulum is similar, but covered in ribosomes


  • Smooth endoplasmic reticulum synthesizes and processes lipids and carbohydrates
  • Rough endoplasmic reticulum folds and processes proteins that have been made at the ribosomes, transports proteins around the cell.

Golgi Apparatus edit


  • A group of fluid-filled, flattened sacs


  • Processes and packages new lipids and proteins
  • Once finished, it makes vesicles which transport the molecules to the edge of the cell for ejection
  • Makes lysosomes and also modification of the chemical components of the cell
  • Involved in cellular secretions

Ribosome edit


  • Very small
  • Either floats free in the cytoplasm or is attached to rough endoplasmic reticulum


  • The site where protein synthesis takes place

Lysosome edit


  • Round
  • No clear internal structure


  • Contains digestive enzymes which can be used to digest invading cells or break down worn-out organelles (autolysis)

Microvilli edit


  • These are folds in the plasma (cell surface) membrane
  • Found in cells involved in absorption
  • Stereotypically found on the villi in the small intestine


  • Increase the surface area of the plasma membrane

Cell-surface Membrane edit

  • Found on the surface of animal cells, it's mainly made of lipids and proteins. It controls the movement of substances in and out of the cell.

Chloroplast edit

  • Inner membrane is folded to form stacks of grana
  • Molecules of chlorophyll are on the grana in form of thylakoids . In it membranes the light dependent reactions occur. In its stroma the light independent reactions occurs.


  • Chlorophyll captures photons of light used for photosynthesis
Animal cell diagram
Plant cell diagram

Refer to the below table for the differences between plant and animal cells.

Table 1: Comparison of structures between animal and plant cells
Typical animal cell Typical plant cell
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough endoplasmic reticulum (ER)
  • Smooth ER
  • Ribosomes
  • Cytoskeleton
  • Golgi apparatus
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Lysosome
  • Centrosome/Centrioles
  • Many small vacuoles
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough ER
  • Smooth ER
  • Ribosomes
  • Golgi apparatus
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Chloroplast
  • One large vacuole
Additional structures
  • Plasma membrane
  • Cilia
  • Plasma membrane
  • Cell wall
  • Plasmodesmata

Prokaryotes and Eukaryotes edit

Eukaryotic cells are complex, and include all animal and plant cells. Prokaryotic cells are smaller and simpler, like bacteria.

The table below is a comparison of prokaryotic and eukaryotic cells:

Table 2: Comparison of features of prokaryotic and eukaryotic cells
  Prokaryotes Eukaryotes
Typical organisms bacteria fungi, plants, animals
Typical size ~ 1-10 µm ~ 10-100 µm (sperm cells, apart from the tail, are smaller)
Type of nucleus none nucleus with double membrane
Genetic material ring of DNA, plasmids chromosomes
Ribosomes Smaller (18 nm) Larger (22 nm)
Cytoplasmatic structure very few structures highly structured by endomembranes and a cytoskeleton
Mitochondria none one to several thousand (though some lack mitochondria)
Chloroplasts none in algae and plants
Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells
Cell division Binary fission (simple division) Mitosis

Analysis of Cell Compounds edit

Units of Size in Microscopy edit

  • The basic biological unit of measure is the micrometer (µm).
  • 1000 µm = 1mm.

Calculations in Microscopy edit

Size in real life = Size in image ÷ Magnification.
Magnification = Size in image ÷ Size in real life.

  1. Measure the size of the image in millimetres.
  2. Convert to micrometers by multiplying by 1000.

Example: A micrograph shows mitochondrion 210mm magnified 2500x. What is its size in real life?
210mm x 1000 = 210,000 µm
210,000 µm ÷ 2500 = 84 µm

Example: An object is 130 µm in real life and 52mm in an image. What is the magnification?
52mm x 1000 = 52,000 µm
52,000 µm ÷ 130 µm = 400x

Microscopes edit

Light Microscopes

  • Light rays travel through the specimen and 2 lenses
  • The objective lens provides the initial magnification of the image
  • The eyepiece lens magnifies and focuses the image

Electron Microscopes

  • Transmission electron microscopes (T.E.M.) pass a beam of electrons through the specimen to produce an image on a fluorescent screen.
  • Scanning electron microscopes (S.E.M.) scan a beam of electrons over the specimen.
  • Electromagnets focus the image.
  • Electrons are produced from a tungsten filament at the top of a column.
  • The column is a vacuum. As a result, living specimens cannot be used.
  • The preparation of specimens for microscopes can be drastic, and can produce artefacts. Artefacts are things you see under a microscope, but aren't actually there in real life. This could be due to something like an air bubble.
  • Magnification refers to how much bigger the image is than the actual specimen.
  • Resolution refers to how well a microscope distinguishes two different points that are close together. If a microscope can't separate two objects, then increasing the magnification won't help.

Below is a table comparing the different types of microscope.

Table 3: Comparison of microscopes
Property Light S.E.M. T.E.M.
Resolution Low Medium High
Depth of focus Low High Medium
Field of view Good Good Limited
Ease of specimen preparation Easy Fairly skilled Skilled
Speed of specimen preparation Rapid Quite rapid Slow
Relative cost Cheap Expensive Expensive
Living specimen Yes No No

Cell Fractionation edit

  • Cell fractionation breaks apart cells and separates its organelles.

Step 1: Homogenisation edit

  • This breaks open the cells.
  • Usually done by vibrating the cells, or grinding them up in a blender.
  • A homogenizer can also be used to do this. It bashes the cells around roughly, causing the cell walls and/or membranes to break.
  • It is done with a cold, isotonic buffer:
    • cold to slow down and stop organelle activity, particularly the hydrolytic enzymes in lysosomes
    • isotonic to prevent the movement of water in and out of organelles by osmosis
    • a buffer to prevent changes in pH levels.

Step 2: Filtration edit

  • Filter the solution through a gauze to remove debris e.g. large cell debris or tissue debris.

Step 3: Ultracentrifugation edit

  • Spin the solution in a centrifuge at a low speed.
  • The heaviest organelles (nuclei, chloroplasts) fall the to the bottom.
  • The rest of the organelles stay suspended in the fluid above this sediment. This is the supernatant.
  • The supernatant is drained off, poured into another tube, and spun again at a higher speed.
  • This time, organelles like mitochondria and lysosomes fall to the bottom.
  • Again, the supernatant is drained off, poured into another tube and spun at a higher speed.
  • Finally, the lightest organelles remain.

Plasma Membranes edit

Plasma membranes are located around the edge of animal cells and surround the cytoplasm and other organelles. They are made up of a phospholipid bilayer which consists of two layer of phospholipids with the hydrophilic heads on the outer layers and the hydrophobic tails on the inner layers. lipid soluble molecules can diffuse straight through the bilayer, water can osmosise through as well. The phospholipid bilayer contains intrinsic and extrinsic proteins. The intrinsic proteins go all the way through the bilayer whereas extrinsic proteins only go through the outer phospholipid layer. Extrinsic Proteins are used for recognition of the cell and usually have a glycoprotein attached for the recognition. Intrinsic Proteins are for allowing molecules through. Ways the proteins are designed to allow molecules through are: Protein pump, Protein Channel, Gated protein channel.

Cholera edit

Cholera is a disease caused by a bacterium and is commonly found in contaminated water. Once ingested, the bacterium sits in the endothelium of the gut and sets up an cotransporter with Na+ (sodium) ions. This lowers the water potential of the gut, and means that water moves in down the concentration gradient. The person is constantly dehydrated and has wet, loose faeces. These effects can be countered with oral rehydration therapy and antibiotics.