Responding to the Environment Flashcards
Why do plants need to respond to their environment?
To avoid predation and abiotic stress: shoots grow to light to allow photosynthesis; roots grow downwards to anchor plant and allow take up of water and minerals.
Define tropism
A directional growth response in which the direction of the response is determined by the direction of the external stimulus. E.g. shoots show positive phototropism (grow towards light)
How do plants grow (generally)?
Plants can only grow from certain, immature tissues called meristems, as otherwise the cell wall inhibits growth. Cells divide at the meristem, and cell elongation (resulting in plant growth) happens just behind this. This is because auxins are produced in the meristem that then travel backwards, causing elongation. The auxins stimulate elongation by increasing the stretchiness of the cell wall. Hydrogen ions are actively transported by an ATPase enzyme in the plasma membrane into the cellulose cell wall, which simultaneously disrupts hydrogen bonds in the cellulose as well as providing the low optimum pH of wall-loosening enzymes (expansins). Cell can therefore expand as it takes on water. Cell elongation is proportional to auxin concentration
Where are tissues capable of dividing in plants found?
Meristem tissues are found:
Apical meristems - in tips/apices of roots and shoots, allowing them to get longer
Lateral bud meristems - found in the buds, allowing side shoots to grow
Lateral meristems - found in a cylinder near the outside of roots and shoots, allowing them to get wider
How does the positive phototropic response work?
Shoot grows towards a light source because more auxins are found on the shaded side of the plant, meaning the shaded side grows faster. How auxins are redistributed has something to do with enzymes phototropin 1 and 2, whose activity is promoted by blue light - lots on light side, progressively less on shaded side.
What are the hormones involved in deciduous plants shedding their leaves, and how do they work?
Cytokinins stop leaves from senescing (ageing) by making sure the leaf acts as a sink for phloem transport - leaf has good nutrient supply
If cytokinin production drops, senescence begins, and leaves may shed - abscission
Auxins inhibit abscission by acting on cells in abscission zone (at base of leaf, where joins to stem)
But: leaf senescence causes auxin production to drop, so cells in abscission zone are more sensitive to another growth substance, ethene. Drop in auxin concentration increases ethene production, which increases production of cellulase which digests walls of cells in abscission zone, separating the leaf from the stem.
What is apical dominance?
The growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot. Lateral buds only grow if the shoot is cut.
What is the experimental evidence for the role of auxins in apical dominance?
Auxins are produced by shoot tip; upon removal (and therefore decreased concentrations of auxins), lateral buds grow (but this effect could be observed if cut end produced a hormone on exposure to oxygen…)
Auxin inhibitors ringing the shoot below the apex cause lateral buds to grow without having to cut the shoot (but the 2 variables - auxin conc. and growth inhibition - could be unrelated, but both be affected by a third variable)
Auxin concentration in lateral buds increases when shoot tip is cut off - disproves direct link
2 other hormones involved: abscisis acid and cytokinins. Auxins keep abscisis acid levels high; when no more auxins, abscisis acid levels drop and bud grows. Cytokinins promote bud growth as direct application to lateral buds causes growth. High auxin concentrations make the shoot tip act as a cytokinins sink; when cut, are more evenly spread through plant.
What is the experimental evidence for the role of gibberellin in the control of stem elongation?
When applying gibberellins (originally found in a fungus that made plants grow) to plants, they grow taller (just because this effect can occur, doesn’t mean it naturally occurs in nature)
Compare concentrations of gibberellins in tall and dwarf pea plants: higher concentrations in the tall plants (doesn’t show directly causes stem growth)
Graft plant that has a mutation that prevents production of gibberellin by blocking metabolic pathway early on to roots of a dwarf pea plant (that doesn’t have enzyme for final conversion) - plant grew tall, confirming that gibberellin does cause stem elongation
How are auxins used commercially?
Used to prevent leaf and fruit drop and promote flowering.
Taking cuttings: dipping cutting in auxins encourages root growth
Seedless fruit: auxins promote ovule growth, triggering auxin production in tissues, completing developmental process
Herbicides: artificial auxins used to kill weeds. Transported to all parts of plants where they stick around because not a close fit for enzymes that break them down. Promote shoot growth so much that stem can’t support itself, buckles and dies.
How are gibberellins used commercially?
Delay senescense in citrus fruits
Act with cytokinins in apples to elongate them, improving shape
Make grape bunches less compact, allowing individual grapes to grow more
Speeds up production of maltose in brewing (triggers release of amylase enzymes)
Stimulates growth between nodes in sugar cane where sugar is stored
Induce seed formation in young trees, allowing selective breeding to occur faster
Stopping plants producing gibberellins keeps them short and stocky - good for preventing lodging in crop plants (plant gets too tall and falls over)
How are cytokinins used commercially?
Prevent yellowing of lettuce leaves after being picked
In tissue cultures to mass produce plants as promote bud and shoot growth
How is ethene used commercially?
Applied through 2-chloroethylphosphonic acid as it is a gas naturally
Speed up fruit ripening in apples, tomatoes and citrus fruits
Promoting fruit drop in cotton, cherry and walnut
Promoting female sex expression in cucumbers, reducing chance of self-pollination
Promoting lateral growth in some plants
Restricting ethene (low temps, no oxygen, much carbon dioxide) prevents fruit ripening
Why do animals need to be able to respond to the environment?
Coordinate responses such as running from a predator, control of balance, posture and temperature, responding to stress situations
Describe the organisation of the nervous system
Have the central nervous system, CNS, which is the brain and spinal cord, made up of grey (no myelin) matter and white matter
Have the peripheral nervous system, PNS, which contains sensory neurones and motor neurones. Motor neurones are divided into the somatic and autonomic systems
Somatic motor neurones carry impulses from CNS to skeletal muscles under voluntary control. Myelinated neurones; 1 neurone per connection to effector
Autonomic motor neurones carry impulses from the CNS to cardiac muscle, smooth muscle and glands that are not under voluntary control. Non-myelinated neurones; 2 neuones per connection to effector (ganglion). Autonomic nervous system is subdivided into the sympathetic and the parasympathetic system
How is the autonomic nervous system organised?
Non-myelinated nerves; 2+ connections per effector with neurones joining at ganglion. Subdivided into sympathetic and parasympathetic system.
What are the differences in function between the sympathetic and parasympathetic subsystems?
P most active in sleep and relaxation; S most active in times of stress
P has effects of action including decreased heart rate, pupil constriction, decreased ventilation rate, sexual arousal
S has effects of action including increased heart rate, pupil dilation, increased ventilation rate and orgasm
What are the differences in structure between the sympathetic and parasympathetic subsystems?
P has neurone pathways linked at a ganglion within the target tissue, so the pre-ganglionic neurones vary in length
S has neurones of a pathway are linked at a ganglion just outside the spinal cord, so the pre-ganglionic neurones are very short
P has post-ganglionic neurones secreting acetylcholine
S has post-ganglionic neurones secreting noradrenaline
Describe the gross structure of the brain
Person looking to left as we see it
Cerebrum in arch over the top
Corpus callosum in gap (middle below)
Thalamus below corpus callosum
Hypothalamus below and slightly to left of thalamus
Pituitary gland below and to left of thalamus (sticking out into face slightly)
Cerebellum is the cauliflowery bit bottom right
Medulla oblongata is in the brainstem, in line with bottom of cerebellum
What is the function of the cerebrum?
Responsible for the elements of the nervous system that are associated with being ‘human’, including thought, imagination and reasoning. Also controls conscious decision to move voluntary muscles. It is subdivided into areas responsible for specific activities and body regions
Sensory areas: receive impulses indirectly from receptors
Association areas: compare input with previous experiences in order to interpret what the input means and judge appropriate response
Motor areas: send impulses to effectors (muscles and glands)
What is the function of the cerebellum?
Coordinates fine control of muscular movements that require a significant level of nonconscious thoughts, e.g. walking, riding a bike, playing and instrument, driving. More specifically, it involves:
Responding to remain balanced and upright
Sensory activities such as judging the position of objects and limbs
Feedback information on muscle position, tension
Operating antagonistic muscles to coordinate contraction and relaxation
What is the function of the medulla oblongata?
Controls non-skeletal muscles. Basically means it is in control of the autonomic nervous system. Amongst other things, contains the cardiac centre that regulates heart rate and the respiratory centre, which controls rate and depth of breathing
