OCR Biology A2 Unit 2 Module 4 Notes-Responding To The Environment

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TROPISM: a directional growth response in which the direction of the response is determined by the direction of the external stimulus.

BIOTIC: living, ABIOTIC: non-living.

PHOTOTROPISM: growth towards light (positive), or away from light (negative).

GEOTROPISM: growth towards the pull of gravity, anchors in the soil and helps for taking up water (needed for support to keep cells turgid, for photosynthesis and to cool the plant), minerals e.g. nitrates for the synthesis of amino acids.

CHEMOTROPISM: pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.

THIGMOTROPISM: winding around other plants or solid structures and gaining support.

HORMONES: chemical messengers that coordinate plant responses to environmental stimuli, act in target cells/tissues, plant growth regulators, not in endocrine glands, bind to receptors on the plasma membrane, only bind to specific receptors with complementary shapes, use active transport, diffusion, mass flow, some amplify each others effects (synergy), cancel out each others effects (antagonism).

Auxins: promote cell elongation, inhibit growth of side shoots, inhibit leaf abscission.

Cytokinins: promote cell division.

Gibberellins: promote seed germination and growth of stems.

Abscisic acid: inhibits seed germination and growth, causes stomatal closure when the plant is stressed by low water availability.

Ethene: promotes fruit ripening.

Plant cells have a limited ability to divide and expand because of their cell walls. Growth only happens in particular places in the plant, where there are groups of immature cells that are still capable of dividing (MERISTEMS).

APICAL-tips of roots and shoots, make them longer.

LATERAL BUD-in buds, give rise to side shoots.

LATERAL MERISTEM-found in cylinders near the outside of roots and shoots, are responsible for them getting wider. 

INTERCALARY-between nodes-shoot gets longer.

Cell division happens happens closest to the apex and then cell elongation happens just behind the apex. Auxins are produced at the apex. The auxin travels, either by diffusion or active transport, to the cells in the zone of elongation, causing them to elongate, making the shoot grow. Auxins stimulate shoot growth by causing cell elongation. The extent to which cells elongate is proportional to the concentration of auxins, it promotes the active transport of hydrogen ions by an ATPase enzyme on the plasma membrane, into the cell wall. The low pH provides optimum conditions for wall-loosening enzymes (expansins) to work. These enzymes break bonds within the cellulose (at the same time the increased hydrogen ions also disrupt hydrogen bonds within cellulose) so the walls become less rigid and can expand as the cell takes in water.

Phototropic response-shoot bends towards light source, because its shaded side elongates faster than the illuminated side, pushing the end of the shoot towards the light. Light shining on one side of the shoot causes the auxins to be transported to the shaded side, where they promote an increase in the rate of elongation. Two enzymes: phototropin 1 and phototropin 2, activity promoted by blue light, lots of phototropin 1 activity on light side but less on dark. This gradient is thought to cause the…


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