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Bio IGCSE (2)
2 - Structure & Functions in Living Organisms
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(a) Level of Organisation
The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently. Cells are often called the "building blocks of life”.
| Level | Description |
|---|---|
| Organelles | Cell structure specialised with a specific function |
| Cells | Basic functional & structural units in a living organism |
| Tissues | Groups of cells of the similar structure carrying out the same function |
| Organs | Made from different tissues working together to perform specific functions |
| Organ Systems | Groups of organs working together to carry out body functions |
Organelles - Cell structure specialised with a specific function
Cells - Basic functional & structural units in a living organism
Tissues - Groups of cells of the similar structure carrying out the same function
Organs - Made from different tissues working together to perform specific functions
Organ Systems - Groups of organs working together to carry out body functions
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(b) Cell Structure
Both animal & plant cells have:
- Cell Membrane – you can imagine this as a balloon filled with water (actually cytoplasm) which contains everything else. The cells are not flat. Its main function is to control what goes in and out.
- Cytoplasm – is what the cell is full of and is where chemical reactions happen.
- Nucleus – The control centre or brain, controls what the cell does and contains the DNA which is the blueprint for reproduction
- Mitochondria - releases the most energy in respiration, sometimes called the powerhouse of the cell.
- Ribosomes - this is where protein is synthesised.
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But only plant cells have:
Cell Wall – Primarily structural, remember plants do not have a skeleton to keep them rigid and upright.
Vacuole – has numerous functions. It can contain waste, store water, isolate harmful substances and keep the pressure of the cell at the correct level. This can be called turgor. This helps maintain the structure of the tissues of the organism and keeps the cell acid inside.
Chloroplasts – plants use photosynthesis to feed, animals do not. More of this later but essentially they absorb light from the sun and convert the energy in the sunlight into chemical energy in the form of carbohydrates.
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Cell Differentiation
• Specialised cells are those which have developed certain characteristics in order to perform particular functions
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- • Cells specialise by undergoing differentiation: a process that involves the cell gaining new structures within the cell in order for it to be suited to its function
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- • Cells can either differentiate once early on or have the ability to differentiate their whole lives (these are called stem cells)
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- • In animals, most cells only differentiate once, but in plants, many cells retain the ability
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Stem Cells
- Many tissues in the human body contain a small number of undifferentiated cells
- These are called stem cells and their function is to divide and produce newly differentiated cells within the tissue for growth and repair
- Adult stem cells from bone marrow are quite useful in medicine as they can be directed to differentiate into several different cell types (unlike adult stem cells from other tissues which can only differentiate into related cell types)
| Advantages | Disadvantages |
|---|---|
| Can be used to replace damaged cells, for example in people with Type 1 diabetes and those paralysed by spinal cord injuries or multiple sclerosis | There are ethical concerns regarding using and then destroying embryos |
| Bone marrow transplants are an example of an adult stem cell transplant and can be used to treat leukaemia and other blood cell cancers | Mutations can occur in stem cells cultured for several generations |
| Whole organs could be grown for transplants without the need to find a donor | It is impossible to obtain embryonic stem cells from a patient as they have to be collected before birth |
| If organs are made patients on stem cells they would be no need for tissue typing and no chance of organ rejection | Difficulty in finding suitable stem cell donors |
Stem cells are also found in great numbers in embryonic tissue as there is a large amount of development and differentiation going on during foetal development
Scientists can clone these embryonic stem cells and direct them to differentiate into almost any cell type
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(c) Biological Molecules
There are 3 main types of organic molecule.
Carbohydrate
Protein
Lipid
Which as you probably know is pretty much all we eat, Lipids being the fats and oils.
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Carbohydrates
They are made of Carbon, Oxygen and Hydrogen. 
Glucose is sugar and sugar is energy.
There are:
- 6 carbon atoms
- 12 hydrogen atoms
- 6 oxygen atoms
One sugar molecule - monosaccharide
Two sugar molecules joined together - disaccharide
Many sugar molecules joined together - polysaccharide.
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Proteins
They are composed of Carbon, Oxygen, Hydrogen and Nitrogen and follow less repetitive patterns, hence becoming so complicated.
Protein molecules are much more complicated.
Proteins are made up of strings of simpler amino acids
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Lipids
Lipids are made of Hydrogen, Carbon and Oxygen, but with much less oxygen than in glucose.
Differences can be seen between saturated and unsaturated fats.
Lipids are formed from smaller blocks of fatty acids and glycerol
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Practical Food Tests
| \n | Method | \n |
|---|---|---|
| Test for Glucose | Add Benedict’s solution into the sample solution in the test tubeHeat at 60 – 70 °c in a water bath for 5 minutesTake test tube out of water bath and observe the colourA positive test will show a colour change from blue to orange or brick red | |
| Starch Test | Add drops of iodine solution to the food sampleA positive test will show a colour change from brown to blue-black | |
| Protein Test \n | Add drops of Biuret solution to the food sampleA positive test will show a colour change from blue to violet/purple | |
| Lipid Test | The food sample is mixed with 2cm3 of ethanol and shakenThe ethanol is added to an equal volume of cold waterA positive test will show a cloudy emulsion forming |
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NoteStudied by 2 peopleNoteStudied by 434 peopleNoteStudied by 14 peopleNoteStudied by 7 peopleNoteStudied by 9 peopleNoteStudied by 6 people