Unit 3 summary

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Growth is the irreversible increase in the dry mass of an organism.
2. In plants growth is restricted to regions called meristems
3. Primary plant growth occurs at apical meristems (root and shoot tips) where newly formed
cells become elongated, vacuolated and differentiated .
4. Secondary growth occurs in a perennial plant at a lateral meristem called cambium.
5. Cambial activity produces annual rings of secondary xylem and phloem. Each ring of
secondary growth contains a region of large vessels called spring wood and a region of
smaller vessels called autumn wood.
6. Animals do not possess meristems. Instead growth occurs all over a developing animal's
7. Flowering plants (angiosperms) have extensive powers of regeneration whereas
mammals have only limited powers.
8. Investigations into growth often involve measuring a variable factor such as fresh weight,
height , length etc.
9. A graph of the results normally takes the form of a sigmoid (Sshaped) growth curve.
10. Growth patterns vary from one type of organism to another.
The characteristics features of a cell are controlled by its genes.
2. To prevent resources being wasted, some genes can be switched on and off as required.
3. The lac operon (JabobMonod) hypothesis states that a structural gene remains switched
off while its operator gene is combined with a repressor from a regulator gene. The structural
gene becomes switched on and codes for its protein when an inducer prevents the repressor
combining with the operator gene.
4. All chemical processes that occur in a living organism are known collectively as its

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Each stage in a metabolic pathway is controlled by an enzyme.
6. Production of each metabolite is controlled by a particular gene (or group of genes).
7. A mutated gene is unable to code the information needed to produce its enzyme. Lack of this
enzyme may lead to an inborn error of metabolism such as PKU.
8. During differentiation, cells become specialised to perform specific functions.
9. Within a differentiated cell, only certain genes continue to operate and the other genes are
switched off.
10.…read more

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Drugs such as nicotine, alcohol and thalidomide harm a developing human foetus.
8. A plant grown in darkness develops a weak elongated stem and small curved leaves and is
said to be etiolated.
9. Green plants show positive phototropism by growing towards a source of light from one
10. In order to flower, long day plants require a number of hours of light above a certain critical
level. Short day plants need a critical number of hours of light.…read more

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Some of these factors affect the growth of the population in a manner independent of
population density. Other factors operate in a densitydependent.
8. In a natural ecosystem, a population is kept relatively stable by densitydependent factors
effecting negative feedback control (homeostasis).
9. The population numbers of many wild plants and animals are monitored by humans.
10.…read more


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