3.3 Organisms exchange substances with their environment

surface area to volume ratio

Ratio of a organism's outside area to its internal volume. It affects how quickly substances are exchanged; Smaller organisms have larger SA:V and larger organisms have smaller SA:V.

Adaptations to increase SA:V

1. villi and microvilli
2. alveoli and bronchioles
3. spiracles and tracheoles
4. gill filaments and lamellae
5. thin wide leaves
6. many capillaries

Volume of a cylinder

V\=πr²h

Volume of a sphere

4/3πr³

single celled organisms

diffuse directly across cell-membrane due to small distance and large SA:V

Fick's Law of Diffusion

the rate of diffusion is proportional to both the surface area and concentration difference and is inversely proportional to the thickness of the membrane

structure of the human gas exchange system

alveoli, bronchioles, bronchi, trachea, lungs

Ventilation

inspiration and expiration, controlled by the diaphragm, intercostal muscles and rib cage.

inspiration

1. external intercostal muscle contracts
2. diaphragm contracts
3. rib cage moves up and out
4. lung volume increases
5. lung pressure decreases below atmospheric pressure
6. air flows in, down pressure gradient

expiration

1. internal intercostal muscle contracts
2. diaphragm relaxes
3. rib cage down and in
4. lung volume decreases
5. lung pressure increases above atmospheric pressure
6. air flows out, down pressure gradient

pulmonary ventilation equation

pulmonary ventilation \= tidal volume x ventilation rate

alveolar epithelium

1. gas exchange between alveolar epithelium and the blood in capillaries through capillary endothelium
2. millions of alveoli = large surface area
3. thin = short diffusion pathway
4. surrounded by large network of capillaries = maintains concentration gradient

Gas exchange in insects

* Occurs by diffusion directly between the atmosphere and the insect's body cells as cells respire so use oxygen and produce carbon dioxide which maintains concentration gradient.
* Two main tracheae run the length of the insect's body, have rings around them for strength
* Spiracles along its body open from the atmosphere into the tracheae
* The tracheae branch into smaller tracheoles that carry air directly into the body cells
* Large number of tracheoles creates a large surface area and their thin walls ensure a short diffusion distance between cells and the atmosphere
* Some larger insects can speed this process up by contracting and relaxing their abdomens, which moves air along the tracheae by mass transport
* Flying insect's muscle cells respire anaerobically and produce lactate which lowers water potential of cells and moves water from tracheoles into cells via osmosis and decreases volume in the tracheoles causing air from atmosphere to flow in.

Limiting water loss in insects

* small surface area to volume ratio
* waterproof exoskeleton
* spiracles can open and close

Gas exchange in fish

1. Lower concentration of oxygen in water so fish have special adaptations to maintain concentration gradient - gills
2. Water containing oxygen enters fishes mouth and passes out of the gills.
3. Each gill is made of gill filaments that increase SA
4. Gill filaments are covered in lamellae that have lots of capillaries and thin surface layer which increase SA
5. They maintain concentration gradient by the counter-current exchange where blood and water flow in opposite directions across the whole length of the gill lamellae where water will always have a higher concentration of oxygen than the blood

Gas exchange in dicotyledonous plants

1. Gases move in and out through the stomata (controlled by the guard cells) in the epidermis to the mesophyll cells which have a large surface area for gas exchange.
2. Oxygen diffuses out if not respiring
3. carbon dioxide diffuses in for photosynthesis
4. to reduce water loss the stomata close at night when photosynthesis isn't occurring.

Xerophytic plants

1. They are adapted to reduce water loss and thus live in very dry conditions
2. stomata sunken into pits to trap moisture and increase humidity
3. layer of hairs to trap water vapour
4. Curled leaves to protect from wind and to trap moisture and increase humidity
5. less stomata
6. thick waxy waterproof cuticle to reduce evaporation

lung function

1. tidal volume: volume of air in each breath
2. ventilation rate: number of breaths per minute
3. forced expiratory volume: maximum volume that can be exhaled in 1 second
4. forced vital capacity: maximum volume of air that can be forcefully exhaled after 1 deep breath in

lung diseases

1. Tuberculosis: caused by bacteria forms small hard lumps that reduce tidal volume by killing tissue
2. Fibrosis: scar tissue which reduces tidal volume and FVC
3. Asthma: inflamed and irritated airways, mucus produced and causes constriction of airways
4. Emphysema: loss of elastin in alveoli so can't recoil and smaller SA

Digestion

large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.

Digestion of Carbohydrates

1. amylase in salivary glands and pancreas which releases into duodenum
2. Hydrolyses polysaccharide starch into disaccharide maltose by hydrolysing glycosidic bonds
3. Membrane-bound disaccharidases hydrolyse disaccharides into monosaccharides such as maltose into glucose so they can be absorbed across epithelial cells

Digestion of proteins

1. Endopeptidases - hydrolyses bonds peptide bonds within protein
2. Exopeptidases - hydrolyses peptide bonds at end of proteins
3. Dipeptidases - hydrolyses peptide bonds between 2 amino acids

Digestion of lipids

1. Lipase produced in pancreas and released in the small intestine hydrolyses ester bonds in triglycerides to form monoglycerides and fatty acids so they can be absorbed
2. Bile salts produced in the liver emulsify lipids to form tiny droplets called micelles as it increases SA for lipase

absorption of monosaccharides

1. Glucose and galactose are pumped into the absorptive cells along with sodium by active transport via a co-transporter.
2. Fructose is absorbed via facilitated diffusion.

Absorption of monoglycerides and fatty acids

1. Micelle moves monoglycerides and fatty acids towards the epithelium
2. monoglycerides and fatty acids are lipid-soluble, non-polar and small so can simply diffuse across cell membrane
3. reform into triglycerides inside of the endoplasmic reticulum and golgi apparatus

Absorption of amino acids

The absorption of amino acids is dependant on the sodium ion gradient and is co-transported with sodium through transport protein (similar to glucose and galactose).

Haemoglobin

Quaternary structure protein in red blood cells that transports oxygen, 4 haem groups that contain iron ion that can associate with oxygen to form oxyhaemoglobin

cooperative nature of oxygen binding

the change in shape of haemoglobin caused by binding of the first oxygens makes the binding of further oxygens easier.

Oxyhaemoglobin dissociation curve

A graph showing the relationship between pO2 and percentage saturation of haemoglobin.


1. Oxygen is loaded in high partial pressure of oxygen - high affinity
2. Oxygen is unloaded in low partial pressure of oxygen - low affinity

Bohr effect

High carbon dioxide concentration causes oxyhaemoglobin curve to shift to the right, the affinity to oxygen decreases and disassociates because the acidic carbon dioxide changes the shape of haemoglobin slightly. This is useful in respiring cells.

Low oxygen environment haemoglobin

Low concentration of oxygen, increases affinity for oxygen so dissociation curve moves left to load

High activity level haemoglobin

High oxygen demand, low affinity for oxygen, dissociation curve moves right to release

Small mammals haemoglobin

small organism so high SA:V, lose heat quickly, high metabolic rate, high oxygen demand, low affinity to oxygen, so dissociation curve moves right to release

Double circulatory system

Blood passes through the heart twice in one complete circuit of the body, delivers blood to the lungs and delivers blood to rest of the body. Manages pressure of blood flow - low pressure in the lungs to prevent damage to capillaries and allows more time for gas exchange. Higher pressure to the rest of the body to ensure blood reaches all respiring cells.

Blood vessels

1. Coronary arteries: supplies blood to heart
2. Aorta: oxygenated blood from heart to body
3. Pulmonary artery: deoxygenated blood from heart to lungs
4. Vena cava: deoxygenated blood from body to heart
5. Pulmonary vein: oxygenated blood from lungs to heart
6. Renal artery: blood from body to kidney
7. Renal vein: blood from kidney to vena cava

Artery structure

* Thick wall (high blood pressure/oxygenated blood)
* Elastic walls
* Narrow lumen

Arteriole structure

Smaller branches from arteries that contract and relax to control blood flow the different areas of demands in the body

veins structure

* thin-walled
* Larger lumen than arteries
* Valves to stop backflow of blood

Capillaries

* Smallest blood vessels branch from arterioles
* specialised for gas exchange as short diffusion pathway
* large number to increase SA

Structure of the heart

* Four chambers - left atrium and ventricle, right atrium and ventricle
* two atrioventricular valves, two semilunar valves
* Three layers - epicardium, myocardium, endocardium
* ventricles have thicker muscle walls for bigger contraction to create high pressure as travel further distances. Left is thicker- to the body
* septum separates deoxygenated and oxygenated blood to maintain high oxygen concentration gradient
* The two AV valves are located at the entrance into the ventricles. They are called the tricuspid valve and the bicuspid (mitral) valve. The tricuspid valve is located between the right atrium and the right ventricle; the bicuspid (mitral) valve is located between the left atrium and the left ventricle.
* The semilunar valves are located at the exit of each ventricle at the beginning of the great vessels. They are known as the pulmonic valve and the aortic valve. The pulmonic valve is located at the entrance of the pulmonary artery as it exits the right ventricle. The aortic valve is located at the beginning of the ascending aorta as it exits the left ventricle.

Formation of tissue fluid

1. high hydrostatic pressure in arterial end of capillary bed. Hydrostatic pressure so fluid is pushed out into surrounding tissues, forming tissue fluid. Most of plasma is pushed out except for RBC's and plasma proteins as too large
2. Diffusion takes place between blood and cells via tissue fluid.
3. in venous end of capillary bed due to plasma proteins generating low water potential in the blood. Hydrostatic pressure is low. 95% tissue fluid moves back into capillary via osmosis. remaining 10% move back into lymphatic tissue.

cardiac cycle

1. Diastole: atria and ventricles relaxed, blood enters atria via veins which increases pressure in both atria
2. Atrial systole: atria contract, increases pressure and causes atrioventricular valves to open and blood flows into ventricles, ventricles are relaxed
3. ventricular systole: ventricles contract after short delay, increase pressure above atria so atrioventricular valves shut and semi lunar valves open so blood flows into arteries

cardiac output

heart rate x stroke volume (volume of blood that leaves heart each beat)

cardiovascular disease

a disease of the heart and blood vessels


1. Coronary heart disease: coronary arteries have lots of atheroma which restricts blood flow.
2. Atheroma formation: clump of white blood vessels under lining of vessels and hardens to form plaque
3. Aneurysm: atheroma weaken linings of vessels so pushes inner layer out which can burst
4. Thrombosis: blood clot blocks vessel
5. Myocardial infarction: blood flow to heart obstructed and deprives oxygen

Risk factors for cardiovascular disease

1. high blood pressure
2. smoking
3. high cholesterol and bad diet

Xylem structure

* xylem vessel elements which are aligned end to end to form continuous xylem vessels as no end walls
* walls contain lignin which waterproofs the cell and is strong to provide structural support
* Pits (areas which not thickened with lignin) allow water to move transversely

Transpiration

1. water evaporates from the leaves out of stomata
2. water column pulled up xylem as cohesive and adhesive to walls
3. this creates tension
4. water enters roots via osmosis

factors affecting transpiration

1. Temperature: more heat--> more evaporation--> increase in transpiration
2. Wind speed: transpiration increases--> wind removes humidity around leaf


1. Humidity (water vapour in air): a rise in humidity--> larger concentration of water vapour in the air--> decrease in transpiration rate as higher concentration on the outside than inside.
2. Light: transpiration increases--> the stomata will open more in light, more SA for evaporation

structure of phloem

* Companion cells (Many mitochondria for ATP production for active transport)
* Sieve tube elements (Little cell contents for ease of flow)
* Sieve plates (connect the elements together to allow ease of flow)

Translocation

1. At the source, sucrose is actively transported into the phloem by companion cells that use ATP to transport hydrogen ions into the surrounding tissue, creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells via co-transporter proteins with sucrose molecules, causing the concentration of sucrose in the companion cells to increase.
2. sucrose lowers the water potential causing water to enter via osmosis from the xylem, increasing the hydrostatic pressure.
3. As a result water moves down the sieve tube from an area of high hydrostatic pressure to an area of low hydrostatic pressure.
4. Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube. This in turn means that water leaves the sieve tube by osmosis back into the xylem, and as a result reduces the pressure in the phloem at the sink.

Mass flow evidence

radioactive labelling and tree ringing