Detailed biology essay plans for AQA A-Level [Paper 3]. Essay titles include:
- THE IMPORTANCE OF ORGANISMS BEING ABLE TO RESPOND TO THEIR INTERNAL AND EXTERNAL ENVIRONMENTS
-IMPORTANCE OF ENERGY TRANSFERS WITHIN AND BETWEEN ORGANISMS
- IMPORTANCE OF TRANSFERS OF SUBSTANCES WITHIN ORGANISMS & B...
THE IMPORTANCE OF ORGANISMS BEING ABLE TO RESPOND TO THEIR INTERNAL
AND EXTERNAL ENVIRONMENTS
TOPIC = Homeostasis: blood glucose concentration TOPIC = TROPISMS & GROWTH FACTORS IN PLANTS
[negative feedback loop] ● Tropism is the growth of a part of a plant in response
to a directional stimulus
● If blood glucose concentrations increase for ● Positive phototropism is growth towards the stimulus
example, following ingestion of food or drink
i.e light which occurs at the tip of shoots
containing carbohydrates
● This is detected by the beta cells in the islets of ● Cells in the tip of shoots produce IAA which is
Langerhans in the pancreas transported down the shoot
● Beta cells release insulin ● If there’s unilateral light, the IAA will diffuse towards
● Insulin binds to specific receptors on the cell the shaded slide of the shoot
surface membrane of liver and muscle cells & ● IAA concentration increases on the shaded side
increase the permeability of membrane to which promotes cell elongation
glucose by increasing the number of channel
● causes the plant to bend towards the light source
proteins which increases uptake of glucose from
blood by facilitated diffusion ● They also respond to gravity. In roots, the IAA
● It also activates enzymes in liver & muscle cells moves to the lower side of the roots in response to
that convert glucose to glycogen which is called gravity so that the upper side elongates more and
glycogenesis the roots bends down towards gravity and anchors
● This decreases blood glucose concentrations the plant in it. This is because IAA inhibits cell
elongation in roots. This is positive gravitropism
❖ Blood glucose concentrations falls following
exercise of if you have not eaten
❖ Detached by the alpha cells in the islets of IMPORTANCE OF:
Langerhans ● importance of gravitropism - not only anchors plant
❖ Alpha cells release glucagon and adrenal glands in the soil but is also puts the plant in better position
release adrenaline to gain nutrients & mineral ions e.g phosphate &
❖ Secondary messenger model nitrate which are important biological molecules -
❖ Glucagon binds to specific receptors on cell
used to synthesise amino acids, nucleic acids e.g
surface membrane of liver cells which activates
adenylate cyclase which converts ATP into cyclic DNA & RNA which are important in many metabolic
AMP [cAMP] which is the secondary messenger reactions such as protein synthesis, respiration &
❖ cAMP activates protein kinase A which ultimately without these the growth of the plant will be stunted
results in glycogenolysis ● Positive phototropism = plant bending towards light
means that it would receive light to carry out LDR.
This is important as without light, photoionisation
IMPORTANCE OF/LINKS:
wouldn’t occur & electrons wouldn’t be able to pass
● It’s important to respond to changes in blood
glucose concentrations as if the blood glucose down the ETC & lose energy. This means that
concentrations are too low which is protons wouldn’t receive energy to be actively
hypoglycemia than not enough glucose is transported from stroma into the thylakoid so an
being produced for respiration as there’s less electrochemical gradient wouldn’t be created &
glucose so lower rate of glycolysis which means protons wouldn’t be able to move down via
that less ATP is produced which means that facilitated diffusion through ATP synthase so ATP
there’s less ATP for metabolic processes such as
isn’t created & NADP isn’t reduced
active transport and less ATP for muscle
contraction as less ATP available to bind to ● without these products LIR can’t occur as ATP &
myosin head causing it to detach from actin NADPH is used to convert GP into TP & ATP is used
binding site and attach to different binding site. to regenerate RuBP
Also less ATP available to release energy for ● Calvin Cycle is important as after each round of the
recovery stroke. This means that muscles can’t cycle 1 carbon from TP is used to convert into useful
contract properly
organic substances e.g glucose which is important
● Too high blood glucose concentration results in
hyperglycemia. Therefore, it's important for as without this respiration wouldn’t occur = growth of
organisms to respond to these changes as blood the plant will be stunted
would have lower water potential due to
, increased solute i.e glucose. This means that
water is lost from tissue to blood via osmosis and
kidneys can’t absorb all glucose resulting in
more water lost in urine causing dehydration
● Diabetes
AO1: AO1:
● Immune response ● Control of heart rate or haemoglobin
● A phagocyte is a macrophage that’s found in the ● Baroreceptors & chemoreceptors located in aorta
blood & in tissues and carries out phagocytosis and carotid arteries
● Phagocytes detect and have many receptor ● Baroreceptors stimulated by high/low blood pressure
binding points on the surface which enables to ● Low BP – more frequent impulses to medulla – more
attach to chemicals or antigens on the frequent impulses sent to SAN along sympathetic
pathogens’ membrane neurones – more frequent impulses sent from SAN
● The phagocyte changes shape to move around – cardiac muscle contracts more frequently so heart
and engulf the pathogen and its able to change rate increases
its shape due to its cell membrane ● High BP – more frequent impulses to medulla –
● Once engulfed the pathogen is contained with a more frequent impulses sent to SAN along
phagosome vesicle parasympathetic neurones – less frequent impulses
● A lysosome within the phagocyte will fuse with sent from SAN – cardiac muscle contracts less
the phagosome & release its contents frequently so heart rate decreases
● The lysozyme enzyme is released into the ● Chemoreceptors stimulated by blood pH / CO2 conc
phagosome. This is a lytic enzyme which (related to exercise)
hydrolyses the pathogen thus destroying it ● High blood CO2 conc. / low pH – more frequent
● The soluble products are absorbed & used by impulses to medulla – more frequent impulses sent
the phagocyte to SAN along sympathetic neurones – more frequent
● The antigen is placed on the cell surface impulses sent from SAN – cardiac muscle contracts
membrane of the phagocytes and the phagocyte more frequently so heart rate increases
becomes an antigen presenting cells ● Low blood CO2 conc. / high pH – more frequent
impulses to medulla – more frequent impulses sent
to SAN along parasympathetic neurones – less
IMPORTANCE OF/LINKS: frequent impulses sent from SAN – cardiac muscle
● This is important as without phagocytosis cell contracts less frequently so heart rate decreases
mediated and humoral response wouldn't occur
● This is because without the presence of an
antigen presenting cell, specific helper T cell IMPORTANCE OF/LINKS:
wouldn’t be able to bind on the antigen on the ● PH = enzyme activity - denature enzymes e.g ATP
surface of the antigen presenting cell so can’t synthase or ATP hydrolase = less energy released
divide by mitosis to form clones which stimulate so processes such as muscle contraction can’t take
B cells for the humoral response, stimulate place = cant respond to stimuli
cytotoxic T cells to kill infected cells by producing
perforin and stimulate phagocytes to engulf
pathogens by phagocytosis
● Also the humoral response wouldn't be able to
occur as specific B cells that are bound to the
antigen presenting cells and are stimulated by
helper T cells which releases cytokines, can’t
divide rapidly by mitosis to form clones [clonal
expansion] which means that they can’t
differentiate into plasma cells or memory B cells
so antibodies aren’t produced as plasma cells
secrete antibodies and memory B cells divide
rapidly by mitosis into plasma cells during the
secondary exposure to the pathogen to make
large numbers of antibodies rapidly so without
this the organism suffers from the effects of the
antigen as agglutination of pathogens would not
occur as efficiently due to the lack of antibodies
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