MODULE 6 ORGANISMS RESPOND TO CHANGES IN THEIR INTERNAL AND EXTERNAL ENVIRONMENTS BIOLOGY NOTES
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INDEX TOPIC 1: STIMULI, BOTH INTERNAL AND EXTERNAL, ARE DETECTED AND LEAD TO A RESPONSE......... 5 1. Survival and Response........................................5 2. Receptors............................................... 9 3. Control of Heart Rate....................................... 11 TOPIC 2: NERVOUS COORDINATION................. 15 1. Nerve Impulses........................................... 15 2. Synaptic Transmission....................................... 18 TOPIC 3: SKELETAL MUSCLES ARE STIMULATED TO CONTRACT BY NERVES AND ACT AS EFFECTORS.............. 22 TOPIC 4: HOMEOSTASIS IS THE MAINTENANCE OF A STABLE INTERNAL ENVIRONMENT.................. 28 1. Principles of Homeostasis and Negative Feedback...................... 28 2. Control of Blood Glucose Concentration............................ 30 3. Control of blood water potential................................. 33 3
TOPIC 1 DNA, Genes and Chromosomes
Stimuli, Both Internal and External, are Detected and Lead to a Response 1 Survival and Response All organisms needs to maintain a limited set of conditions Need to respond to changes in external and internal environments This is because cellular activities rely on enzymes which require a specific set of conditions to work effectively Organs need to coordinate their activity to maintain optimal internal conditions that support survival Types of plant responses: Tropism Directional growth responses Phototropism Shoots grow towards light Enables them to photosynthesise Geotropism/gravitropism Roots grow towards the pull of gravity Anchors them in the soil Helps them to take up water as a raw material Chemotropism Occurs on flowers Pollen tubes grow down the style towards ovaries They are attracted by chemicals Thigmotropism Shoots out of climbing plants wind around other plants Gain support These are called trophisms/ taxes/ kineses and help maintain an organism in a favourable environment 5
Response to the Environment Hormones co-ordinate plant response Produced in a variety of cells Not in endocrine glands Often known as plant growth regulators Hormones move around the plant Diffusion Active transport Mass flow in phloem and xylem Nastic response = a non-directional response to stimuli, e.g. thigmonasty Mimosa pudica plant responds to touch by folding its leaves. Plant Hormones Plants are capable of producing a range of hormones. Some are synergic and amplify each other s effects. Others are antagonistic and oppose each other s effects. There are many kinds. In flowering plants, hormones are made in some regions of the plant that may be different to where they act to initiate a response to stimuli. Auxins Responsible for regulating plant growth Inhibits the growth of side shoots Inhibits leaf abscission Action: Causes cell elongation Increases stretchiness of cell wall by increasing the AT of hydrogen ions ATPase enzyme moves more ions through the plasma membrane, into the cell wall Low ph allows wall loosening enzymes to work These break bonds in the cellulose, allowing the cells to expand Cytokines Promote cell division Gibberellins Promotes seed germination Promotes growth of stems Abscisic Acid Inhibits seed germination Causes stromal closure when the plant is water stressed Ethene Promotes fruit ripening 6
Plant Growth Growth occurs by 2 process in the meristem tissue Cell elongation Cell division Apical Meristems are located behind shoots and are responsible for shoots getting longer Lateral bud meristems are found in buds and give rise to shoots Lateral meristems are found near the outside of shoots and root and make them wider 2 cell walls are formed Auxin Primary does not have uniformly arranged fibres Secondary has uniformly arranged fibres Cell elongation Inhibit growth of side-shoots Cause of Phototropism Shoot bends towards a light source Shaded side elongates faster than the lit side Light causes cells to actively unload IAA Unloaded from cells in light, towards those in shade Causes the shoot to bend Leaf Loss Cytokinins stop the leaves of deciduous plants from senescing (turning brown and dying) Makes sure the leaf acts as a sink from phloem transport Guaranteed to have a good supply of nutrients If cytokinin production drops, so will the supply of nutrient and senescence will begin Process: Leaf senescence causes auxin production at the top of the leaf to stop Makes abscission zone more susceptible to ethene Drop in auxin production causes an increase in ethane production Increases production of the enzyme cellulose Digests walls of cells in the abscission zone Causes petiole to separate from stem EXAM TIP Be clear about the exact roles of the key plant hormones. For example, auxins cause cell elongation not cell division. Ie, the number of cells remains the same. 7
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