Topic 3: Organisms exchange substances with their environment

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surface area to volume and size relationship

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surface area to volume and size relationship

as size increases, surface area to volume ratio decreases

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surface area to volume and metabolic rate relationship

higher the surface area to volume ratio, higher the rate of heat loss so a higher metabolic rate to maintain body temperature

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Fick’s law

rate of diffusion = concentration gradient x surface area / diffusion pathway length

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gas exchange in plants: maintain concentration gradient

CO2 into the plant, O2 out the plant and CO2 is used in photosynthesis

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gas exchange in plants: surface area

millions of stomata where the gas can enter and leave

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gas exchange in plants: diffusion pathway length

leaves are thin and flat

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xerophytic plants limit water loss

hairy leaves

rolled leaves

sunken stomata

= traps humid air and reduces water potential gradient

pine needle leaves

= lower surface area to volume ratio so less evaporation

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gas exchange in insects

spiracles (openings of small tubes)

trachea into tracheoles

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gas exchange in insects: ventilation movements

abdomen pumping increases pressure

forcing stale air out

= maintaining (increasing) concentration gradient

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terrestrial insects limit water loss

waxy layer

spiracles can close

hairs around spiracles

= reduce water potential gradient

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gill structures

gill arch, gill filament, gill lamellae

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gas exchange in fish: maintain concentration gradient

counter-current flow (blood flows in opposite direction to oxygen/water)

maintaining diffusion gradient across whole gill surface

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gas exchange in fish: surface area

gill filament’s folds increase surface area and on top of that there are gill lamellae folds to increase surface area further

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structures of lungs

lung, trachea, bronchi, bronchioles, alveoli

diaphragm, internal/external intercostal muscles, ribcage

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external intercostal muscles contract pushing ribcage up and out

diaphragm contracts down

increases the volume (in the thoracic cavity)

decreasing the pressure

fresh air moves down the pressure gradient into the lungs

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external intercostal muscles and diaphragm relax (domed shape)

internal intercostal muscles contract

decrease in volume (in thoracic cavity)

increasing the pressure

air moves down the pressure gradient our of the lungs

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gas exchange in alveoli: maintain concentration gradient

low O2 concentration air is continually replaced by breathing

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gas exchange in alveoli: surface area

lots of small balls/spheres

extensive capillary network

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gas exchange in alveoli: diffusion pathway length

one cell thick alveolar epithelium and capillary epithelium

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production of amylase

in salivary glands and pancreas

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function of amylase

to break down starch into maltose (polysaccharide into disaccharides)

hydrolyses glycosidic bonds

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function of membrane-bound disaccharidase

hydrolyse disaccharides into monosaccharides

attached to cell-surface membrane of epithelial cells in ileum

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production of lipase

made in pancreas

secreted in small intestines

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function of lipase

breaks down lipids into monoglycerides and fatty acids

hydrolysis of ester bonds

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bile salts

produced by the liver to emulsify lipids into smaller droplets

forming micelles

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digestion and transportation of lipids (mark scheme)

  1. micelles contain bile salts and fatty acids/monoglycerides

  2. makes fatty acids/monoglycerides more soluble in water

  3. fatty acids/monoglycerides absorbed by diffusion

  4. triglycerides reformed in cells

  5. vesicles move to cell-surface membrane

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function of endopeptidase

hydrolyse internal peptide bonds

producing more ends

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function of exopeptidase

hydrolyse external peptide bonds

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function of membrane-bound dipeptidase

hydrolyses dipeptides into amino acids

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absorption of amino acids

via co-transport with sodium ions (same as glucose)

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structure of haemoglobin

quaternary globular proteins containing 4 haem groups, alpha and beta chains, and Fe2+ ions

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function of haemoglobin

ability to bind and unbind with oxygen to form oxyhaemoglobin

transport molecule

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shape of oxyhaemoglobin dissociation curve


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axes of oxyhaemoglobin dissociation curve

y-axis = percentage saturation of haemoglobin

x-axis = partial pressure of oxygen

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explaining shape of oxyhaemoglobin dissociation curve

initially it takes a large increase in partial pressure of oxygen to bind the first oxygen

once the first oxygen is bound the shape of the haemoglobin protein changes, allowing for the second and third oxygen molecules to bind easily (with little increase in partial pressure of oxygen)

then, the shape changes again so it is difficult to bind the fourth and final oxygen molecule, requiring a greater increase in partial pressure of oxygen

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Bohr shift: adaptation to exercise

due to the increase in temperature from respiration and decrease in pH due to CO2 and lactic acid, the oxyhaemoglobin dissociation curve shifts to the right

this makes the haemoglobin proteins worse at binding to oxygen so more is released into the tissue to respire with

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affinity to oxygen definition

the ability of haemoglobin to bind to oxygen

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affinity to oxygen: higher altitudes/foetal haemoglobin

greater affinity to oxygen (steeper curve)

because there is a lower partial pressure of oxygen at higher altitude/lower concentration of oxygen at the placenta

which is advantageous as enough O2 will be supplied to all cells/tissue

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function of arteries

carry oxygenated blood away from the heart under high pressure

(except pulmonary artery)

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structures of arteries


elastic tissue

folded epithelium

thick muscle wall

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function of arteriole

narrower arteries connecting to capillaries

higher proportion of smooth muscle cells so they can contract and partially cut off blood flow (narrowing the lumen)

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function of veins

carry deoxygenated blood back to the heart under low pressure

(except pulmonary vein)

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structure of veins

wide lumen


thin muscle wall

pocket valves

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function of elastic tissue

to stretch and recoil smoothing out spikes in blood pressure so less stress of blood vessels

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function of folded endothelium

smooths lining of lumen, prevents plaque building up

folds allow lumen to open up

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function of muscle wall

narrows or widens lumen to regulate blood pressure

(in veins) contract to push blood up vein

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function of pocket valve

prevents back flow of deoxygenated blood

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function of capillaries

exchange products with tissue

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features of capillaries

one cell thick endothelium

one red blood cell thick diameter/lumen

capillary beds between tissue/cells

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pulmonary prefix

related to the lungs

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renal prefix

related to kidney

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coronary prefix

related to heart

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structures of the heart

vena cava

pulmonary artery

pulmonary vein


atrioventricular valve

semi-lunar valve

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cardiac cycle

  1. contraction of atria

  2. blood forced into ventricles

  3. contraction of ventricles

  4. atrioventricular valves shut

  5. blood forced out the heart

  6. semi-lunar valves shut

  7. relaxation of ventricles and atria

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heart rate equation

cardiac output = stroke volume x heart rate

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circulation of blood (valves)

ie. when the pressure before the valve is greater than after, the valve opens (vice versa)

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tissue fluid

  1. plasma forced out the arteriole end due to high hydrostatic pressure, forms tissue fluid

  2. tissue fluid bathes the cells, exchanging products with them

  3. fluid moves back via osmosis, down the water potential gradient

  4. excess water returns by lymphatic system

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lymph vessel

returns water to blood

drains excess water

transports fats and plasma proteins

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dissection practical: skills

lab coats, gloves, and goggles to avoid contamination with biological material

scalpel must be sharp for fine and precise cutting; always cut away from the body and keep fingers away from blade

scissors to cut large sections of tissue

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dissection practical: limitations

difficult to see smaller, finer structures

specimens do not reflect how the tissue would look in a living organism

dissecting one specimen so anomalies may be ignored/glossed over

must be same age and same species as others

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dissection practical: ethical concerns

questions on whether the dissected animals were ‘raised to be killed’

against some religious beliefs

must be from a reputable source and disposed in the correct manner

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dissection practical: scientific drawing

make sure drawings are large with detail and labels

no shading

use single and continuous lines

no colouring

label lines should be drawn with ruler

label lines should not cross

label lines should not be arrows

label all structures

include magnification/scale

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xylem tissue

transports water up the plant

made of dead cells forming completely unbroken tube through plant (no end wall or air bubbles)

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  1. water evaporation

  2. tension

  3. cohesion

  4. water enters

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water evaporation

water evaporates through stomata

kinetic energy causes water to evaporate

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cohesion-tension theory

cohesion means that water leaving through the stomata pulls the molecule behind it with it, creating a whole unbroken column of water to move upward

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cohesion-tension theory

tension means there is adhesion between the water molecule and cellulose of xylem, pulling the walls inward

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factors affecting rate of transpiration

light intensity




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movement of solutes around the plant by generating pressure gradient between source and sink

energy-requiring process that occurs in the phloem

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features of phloem

companion cells

sieve plate

sieve tube elements

thin layer of cytoplasm

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function of companion cells

carry out the living functions for sieve cells

a companion cell for each sieve cells

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function of sieve tube elements

living cells that form a tube for transporting solutes

no nucleus and few organelles

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mass flow hypothesis

  1. in source sugars actively transported into phloem

  2. by companion cells

  3. lowers water potential of sieve cells and water enters by osmosis from xylem tissue

  4. increase in hydrostatic pressure causes mass movement towards sink, down pressure gradient

  5. sugars removed into sink cells via diffusion and active transport, raising water potential in phloem

  6. sugars used for respiration/converted to starch for storage in sink

  7. water moves back to xylem, reducing pressure

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ringing experiment

evidence for mass flow hypothesis

remove strip of bark containing all phloem and creates a sugary bulge above ring

shows sugar moves downward

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radioactive tracers

evidence for mass flow hypothesis

can track substances down the plants ie carbon dioxide-14 autoradiography

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aphid experiment

evidence for mass flow hypothesis

shows pressure gradient as sap travels further higher up the plant

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measure rate of water uptake

make sure no air gap under bung and cut the plant in water

open the tap to move the air bubble to 0

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potometer doesn’t measure rate of transpiration, why?

turgidity of cell uses water too

photosynthesis uses up water too

probably not 100% sealed

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