Biology Topic 3: Organisms exchange substances with their environment

how is the size of an organism related to its SA:volume

• large organisms have a low sa:vol

• small organisms have a high sa:vol

how are large organisms adapted to facilitate gas exchange?

• specialised exchange surfaces (alveoli, gills)

• transport systems

• thin, folded membranes within exchange surfaces

how is metabolic rate related to the sa:vol

higher sa:vol > higher metabolic rate

• smaller organisms lose heat more rapidly, so heat is generated via metabolic processes

[and vice versa]

how are exchange surfaces adapted in unicellular organisms?

• exchange occurs through the cell membrane

• high sa:vol ratio to maximise rate of diffusion

• concentration gradients in/out of the cell to transport substances via diffusion or active transport

do insects have a transport system?

no - oxygen is transported directly into respiring muscle cells

how is O₂ exchanged in insects?

• O₂ enters the trachea via the spiracles

• O₂ dissolves into tracheal fluid, allowing O₂ to be transported faster

• when all O₂ in the fluid is used up, muscle cells go into anaerobic respiration. this produces lactic acid, reducing the water potential of muscle cells

• water from tracheal fluid enters the muscle cells via osmosis, so no more fluid is left in the trachea

• this allows O₂ to diffuse directly into the muscle cells without dissolving. this shortens the diffusion path, increasing diffusion rate

how is the tracheal system adapted for gas exchange?

• spiracles can open or close to avoid water loss/regulate air flow

• tracheal fluid allows gases to dissolve, facilitating diffusion

• abdominal pumping increases pressure within the tracheal system, forcing air into the trachea from the spiracles, providing a rapid supply of O₂ and fast removal of CO₂

how does ventillation occur in fish?

ventillation achieves a unidirectional flow of blood

• fish pushes its tongue down, opening the buccal cavity floor. this allows water to enter the fish

• fish closes the mouth, raising buccal cavity floor and increasing internal pressure causing the operculum to open

• pressure gradient between mouth and operculum cavities causes water to move over the gill filaments

• O₂ from the water is absorbed into the blood via lamellae in the gill filaments

what is the counter-current principle?

• the lamellae capillary system ensures deoxygenated blood flows in the opposite direction to the flow of water

• this maintains a steep O₂ concentration gradient across the entire length of the capillary, so maximal O₂ is diffusing into the blood

how are dicotyledonous leaves adapted for gas exchange?

• stomata open and close, letting gases in/out of the leaf. stomata are close to the cells, reducing diffusion path

• spongy mesophyll has air spaces and a large surface area. mesophyll cells absorb CO₂ for photosynthesis, and release O₂ as a product.

how are xerophytes adapted to minimise water loss?

• sunken stomata - minimise water loss

• curled leaves + stomata hairs - trap moist air around the plant, minimising water loss via transpiration or osmosis

• thick waxy cuticle - prevents water from evaporating out of the cell by increasing diffusion distance

• thin leaves/spindles - reduce surface area, preventing photosynthesis and thus transpiration

how are insects adapted to prevent water loss?

• waterproof exoskeleton

• spiracle hairs - trap moist air around the spiracles, preventing water loss via osmosis

which structures form the human gas exchange system?

• alveoli

• bronchioles

• bronchi

• trachea

• lungs

alveoli

site of gas exchange in the lungs

bronchioles

• connect the alveoli to the bronchi

• enable to flow of air into/out of the lungs

• large bronicholes contain cartillage

• wall is composed of smooth muscle and elastic fibres

bronchi

• pair of large tubes which connect the lungs to the trachea

• enables air flow in/out of the lungs

• layers of smooth muscle surrounded by C-ring cartillage

  • inner surface of cartillage is composed of loose tissue

• epithelial layer with goblet/ciliated epithelial cells

trachea

• windpipe - connects the bronchi and nose/mouth

• wider than the bronchi

• enables air flow in/out of the lungs

• layers of smooth muscle surrounded by C-ring cartillage

  • inner surface of cartillage is composed of loose tissue

• epithelial layer with goblet/ciliated epithelial cells

lungs

organs of gas exchange

what is the role of the alveolar epithelium?

surface for gas exchange

adaptations of the alveolar epithelium

• one cell thick: short diffusion distance for faster gas exchange

• large surface area: increases the rate of gas exchange

• moist layer: gases can dissolve and diffuse across the membrane

• concentration gradients: low O₂ conc, so O₂ comes in. high CO₂ conc so CO₂ moves out

• oxygenated blood supply: O₂ moves into the alveoli via diffusion

describe the role of cartilage in the mammalian gas exchange system

• supports the trachea and bronchi

• prevents the lungs from collapsing if there’s a pressure drop during exhalation

describe the role of the ciliated epithelium in the mammalian gas exchange system

• found in trachea, bronchi and bronchioles

• moves mucus away from the lungs to the throat (to be swallowed), preventing lung infections (as mucus can trap pathogens)

describe the role of goblet cells in the mammalian gas exchange system

• found in the trachea, bronchi and bronchioles

• secrete mucus which traps bacteria and dust, preventing lung infection (lysozymes then digest the bacteria)

describe the role of smooth muscle in the mammalian gas exchange system

• contracts to constrict the airways, controlling the flow of air to the alveoli

describe the role of elastic fibres in the mammalian gas exchange system

• stretch during exhalation, recoil during inhalation, helping to control the flow of air

describe the process of inspiration (ventilation)

• diaphram and external intercostal muscles contract

• rib cage raises upwards

• this causes the volume inside the thoracic cavity to increase

• thoracic cavity pressure decreases to below atmospheric pressure

• pressure gradient between the lungs and atmosphere causes air to move into the lungs (air moves down the pressure gradient)

describe the process of expiration (ventilation)

• diaphram relaxes

• internal intercostal muscles contract

• rib cage is lowered

• volume in throacic cavity decreases, and pressure increases to above atmospheric pressure

• pressure gradient between the lungs and atmosphere causes air to move out of the lungs (air moves down the pressure gradient)

describe the interaction between the external and internal intercostal muscles

• antagonistic

• when the internal contracts, the external relaxes and vice versa

what is a spirometer?

• device used to measure lung volume

  • when lung volume is increased > inspiration

  • when lung volume is decreased > expiration

• can measure tidal volume, vital capacity and breathing rate

• for the graph - volume is on the y axis, time is on the x axis

what is tidal volume?

• the volume of air in a normal breath at rest

• usually 0.4 - 0.5dm³

what is the ventilation rate?

• number of breaths taken per minute at rest

• usually around 15

what is vital capacity?

maximum volume of air that can be breathed in and out of the lungs

what is forced expiratory volume?

• maximum volume an individual can expire in one second

• cannot be more than the total volume of gas in the lungs, as there’s always residual air in the alveoli ensuring they do not close

what is the residual volume?

the volume of air always present in the lungs

what is the expiratory reserve volume?

additional volume of air that can be exhaled on top of the tidal volume (e.g. during exercis)

pulmonary ventilation rate equation

PVR = tidal volume x breathing rate

how does lung disease affect gas exchange?

• damages the alveoli, reducing their population in the lungs thus decreasing the surface area available for gas exchange.

• can thicken the alveolar epithelium by increasing the amount of mucus in the lungs, increasing the diffusion distance for gas exchange

• decreases oxygen levels in the blood > fatigue

• increases CO₂ levels in the blood (hypercapnia)

how do pollution and smoking increase chances of lung disease?

• chemicals in cigarettes/pollutants damage the cilia. this means mucus cannot be swept from the lungs to the throat

• this causes a build up of mucus in the lungs, blocking or irritating the airways

• mucus build up can also damage the alveoli or thicken the alveolar wall, decreasing gas exchange

what occurs during digestion?

large biomolecules are hydrolysed to smaller molecules that can be absorbed across the cell membrane

how are carbohydrates digested?

• carbohydrates are broken down by amylase and membrane-bound dissacharidases

  • amylase hydrolyses starch into dissacharides like maltose

  • dissacharidases hydrolyse dissacharides into monosaccharides like glucos

where does the digestion of carbohydrates occur in the body?

• amylase is produced in the saliva and pancreas, and acts in the mouth, stomach and ileum

• dissacharidases are attached to cell-membranes of ileum epithelial cells, and act in the ileum

how are lipids digested?

• lipids are digested by lipases and bile salt action

  • bile salts bind to large lipid droplets and break them up into smaller droplets (emulsification). this increases the SA of lipid droplets, increasing the rate of digestion by lipase

  • lipase breaks down lipids into glycerol and fatty acids in the ileum

where does lipid digestion occur in the body?

• bile salts are produced in the liver and act in the duodenum

• lipase acts in the duodenum

describe the process of emulsification

1. bile salts are ampipathic

2. the hydrophobic end is inserted into the lipid droplets. since the lipid is also hydrophobic, the lipid and bile salt repel each other

3. this repeated repulsion breaks the lipid up into smaller droplets. this increases the surface area for lipase action

describe the role of micelles in lipid production

• micelles transport digested lipids through the small intestine to the surface membrane of intestinal cells

• at the cell membrane, lipids (fatty acids + monoglycerides) diffuse out of the micelles and into the intestinal cells

• inside the cells, fatty acids and monoglycerides are assembled into triglycerides, then packaged into chlyomicrons

• chylomicrons are released into the lymphatic system and deliver absorbed lipids around the body

describe the process of amino acid/glucose cotransport

1. sodium is actively transported out of ileum epithelial cells through action of the sodium-potassium pump. this creates a low concentration of sodium within the cell

2. sodium and glucose bind to the sodium-glucose cotransporter protein

3. concentration gradient between ileum lumen and the cell means sodium can move into the cell via diffusion, thus glucose also moves into the cell via co-transport (active)

4. once inside the cell, sodium and glucose dissociate from the protein. the protein undergoes a conformational change to its original shape

5. sodium moves out of the cell via active transport, glucose moves out of the cell via facilitated diffusion

how are proteins digested?

• proteins are digested by endopeptidases, exopeptidases and dipeptidases (protease enzymes)

  • endopeptidases hydrolyse interior bonds in the polypeptide chain, creating separate chains

  • exopeptidases hydrolyse exterior peptide bonds in polypeptide chains, creating individual amino acids. dipeptidases are a type of exopeptidase

  • membrane-bound dipeptidases are attatched to the cell membrane of ileum epithelial cells

what are haemoglobins?

• proteins with a quaternary structure (2 beta polypeptide chains and 2 alpha helixes)

• group of chemically similar molecules (complex containing a haem group)

how do haemoglobin and red blood cells transport oxygen?

1. when blood passes through the lungs, O₂ diffuses through the rbc and binds to the haem group of haemoglobin.

2. each haemoglobin can bind to 4 O₂ molcules. this forms oxyhaemoglobin (in a reversible reaction)

3. when the rbc reaches the tissue, the oxygen and haemoglobin separate, and oxygen is released into the cells

how is haemoglobin adapted to its function?

• polypeptide folding in the quaternary structure allows haemoglobin to have a hydrophilic exterior and hydrophobic core

• hydrophilic exterior means haemoglobin is soluble, and thus can easily be transported in the blood

• hydrophobic core prevents the oxidisation of Fe2+ in the haem group into Fe3+ (as Fe3+ cannot bind to the oxygen)

what does it mean when haemoglobin is saturated?

when all of the oxygen binding sites have an oxygen (when it binds to 4 oxygen molecules)

what is the partial pressure of oxygen?

• measure of oxygen concentration

• more oxygen dissolved in cells = higher partial pressure

what is oxygen loading?

the uptake of O₂ (by haemoglobin) in the lungs

what is oxygen unloading?

depositing of O₂ (by haemoglobin) at the tissue

what is affinity of oxygen?

the tendency (of haemoglobin) to bind to oxygen molecules

how does the affinity of oxygen change?

• during loading, the partial pressure of oxygen increases (so more oxygen)

• this increases the affinity of oxygen for haemoglobin, as more oxygen can bind to haemoglobin

• during unloading, the partial pressure of oxygen decreases (oxygen is used during cellular respiration)

• this decreases the affinity of oxygen for haemoglobin, as there’s less O₂ for the haemoglobin to bind to.

• thus, oxygen is released in the respiring tissue

where does oxygen association occur?

haemoglobin + oxygen > oxyhaemoglobin

happens in the lungs (where O₂ conc. is high)

when does oxygen dissociation occur?

oxyhaemoglobin > oxygen + haemoglobin

happens at the respiring cells (where O₂ conc. is low)

what does the oxyhaemoglobin dissociation curve show?

• variance in haemoglobin saturation with partial pressure

• saturation (%) - y axis

• partial pressure (kPa) - x axis

why is oxygen binding cooperative?

binding of the first O₂ molcule triggers a conformational change in haemoglobin, making further binding for the other 3 molecules easier

describe the oxyhaemoglobin dissociation curve

bottom of the graph:

• at low partial pressure, there is a low saturation as its difficult for the first oxygen molecules to bind

• low partial pressure - at respiring cells

central region:

• after the first molecule binds and protein undergoes conformational change, the saturation increases as its easier for the 2nd and 3rd molecules to bind (positive cooperativity)

top of the graph:

• the likelihood of the 4th oxygen finding a binding site is low, so graph begins to plateau

• high partial pressure - in the lungs

describe the Bohr effect

• when CO₂ is increased, the affinity for oxygen is decreased

• dissolve CO₂ is acidic, lowering of pH triggers a conformational change in haemoglobin

• this reduces the affinity of haemoglobin for oxygen as the oxygen can no longer bind

• thus oxygen dissociation increases

how does haemoglobin vary across organisms?

• species in areas with a low O₂ concentration have haemoglobin with a higher affinity for oxygen, so oxygen association is easier and dissociation is harder.

• fetal haemoglobin has a higher affinity for oxygen, as blood reaching the placenta has a lower oxygen saturation

how is blood circulated in a mammal?

mammals have a double circulatory system - blood passes through the heart twice in one complete circuit

describe the pattern of blood flow through the heart

1. deoxygenated blood enters the right atrium via the vena cava

2. passes from the right atrium through the triscuspid valve intro the right ventricle

3. from the right ventricle, blood is pumped out through the pulmonary artery to the lungs

[blood oxygenated via gas exchange in the lungs]

4. oxygenated blood returns to the left atrium via the pulmonary vein

5. left atrium contracts, and blood moves through bicuspid valve to the left ventricle

6. left ventricle contracts, pumping blood through the aorta to the rest of the body

describe the gross structure of the heart

• two muscular pumps: the right pumps deoxygenated blood, left pumps oxygenated blood

• 4 chambers (ventricles and atria) separated by valves

• atria have thin walls > only have to pump blood to the lungs

• ventricles have thicker walls > have to pump blood to the entire body/ withstand higher pressures

• tricuspid valve: right atrium > right ventricle

• biscupid valve: left atrium > left ventricle

describe the roles of the major blood vessels surrounding the heart

• coronary arteries: supply oxygenated blood to the heart

• pulmonary vein: transports oxygenated blood from the lungs to the left atrium

• pulmonary artery: transports deoxygenated blood from the right ventricle to the lungs

• vena cava: transports deoxygenated blood from the body to the right atrium

what is the role of the atrioventricular valves?

• bicuspid and tricuspid valves

• prevent the backflow of blood from the ventricles into the atria (maintain unidirectional flow) during ventricular systole

what is the role of the semilunar valves?

• valves between arteries and ventricles

• prevent backflow from arteries to ventricles during ventricular diastole

name the pressure changes that occur during the cardiac cycle

• cardiac diasole

• atrial systole

• ventricular systole

explain the process of an atrial systole

1. atrial walls contract, increasing atrial pressure

2. behind AV valve increases, pushing them open

3. blood moves into the ventricles, increasing ventricular volume and decreasing atrial volume

4. during this, ventricular diastole occurs, so ventricular pressure is decreased

this occurs for 0.1 seconds

explain the process of ventricular systole

1. atria relax and ventricles contract, increasing ventricular pressure

2. ventricular volume is increased, and the increase in pressure makes the semilunar valves open (AV valves shut)

3. blood flows into the arteries

4. as ventricular pressure decreases, semilunar valves start closing

5. atrial diastole is occuring, so atrial volume is increasing

this is 0.3 seconds

explain the process of ventricular diastole

1. ventricles relax, so ventricular pressure decreases.

2. semilunar valves close

3. once ventricular pressure is less than atrial pressure, the AV valves open again and the cycle repeats

this is 0.4 seconds

how is the structure of arteries related to their function?

• arteries carry oxygenated blood away from the heart (bar the pulmonary artery)

• thick, muscular and elastic walls > withstand high blood pressure

• elastic layers > arteries can stretch and recoil with changes in pressure

• smooth muscle > lined with endothelium to reduce friction for blood flow

• no valves needed > blood is transported at such high pressure that backflow isn’t possible

• small lumen > maintains high pressure

how is the structure of veins related to their function?

• transport deoxygenated blood from the body to the heart (bar the pulmonary vein)

• thin muscle layer > blood is transported at low pressure, vasoconstriction isn’t needed to control blood flow

• thinner elastic layer > blood is transported at low pressure, so no stretch/recoil is required to prevent bursting

• wide lumen > maximises the blood volume able to be transported to the heart

• valves > prevent backflow as blood is transported at low pressure

how is the structure of arterioles related to their function?

• transport blood from arteries to capillaries

• thick muscle layer > can constrict/relax to control blood pressure

• thinner elastic layer > blood is transported at a lower pressure

• smooth muscle > allows vasoconstriction and vasodilation to control blood flow

how is the structure of capillaries related to their function?

• exchange substances between the blood and body tissues (site of metabolic exchange)

• smallest blood vessel

• one cell thick walls > minimises diffusion distance to optimise rate of gas exchange

• highly branched/large surface area > optimises rate of gas exchange

• narrow diameter > flattens red blood cells against the side of the capillary, decreasing diffusion distance to optimise rate of gas exchange

• narrow lumen > blood flow slows down, allowing more time for diffusion

how is tissue fluid formed?

1. Capillaries have small pores in their walls

2. at the arterial end of the capillary, the hydrostatic pressure exceeds the osmotic pull

3. this causes small molecules and fluid to be pushed through the gaps out of the capillary, down the hydrostatic pressure gradient

4. large molecules (e.g. proteins) remain in the blood as they’re too big to pass out of the capillaries

how does tissue fluid return to the circulatory system?

1. at the venous end of the capillary, the osmotic pull is larger than the hydrostatic pressure

2. dissolved proteins lower the water potential of the blood, creating a water potential conc. gradient between the tissue fluid and capillary (low in the blood, high in the tissue fluid)

3. tissue fluid moves via osmosis into the capillary

what is the xylem?

tissue that transports water in the stem and leaves of plants

describe the cohesion-tension theory

1. water evaporates from the surface of the leaf due to transpiration. this lowers the water potential of the mesophyll cells

2. water molecules cohere to each other via hydrogen bonds between molecules, forming a continuous column of water (transpiration stream)

3. water is pulled up through the xylem via the transpiration stream. this creates tension in the xylem

4. for each water molecules lost through transpiration, another is pulled up through the roots/stomata

5. the water molecules also adhere to the walls of the xylem

what is the phloem?

tissue that transports organic substances (sucrose) in plants

describe the mass-flow hypothesis

• at the source, sugars like sucrose are actively loaded into the sieve tube elements from the companion cells.

• this decreases the water potential in the sieve tube elements

• lowered water potential enables water to move into the sieve tube elements from the xylem and companion cells via osmosis

• influx of water into the sieve tube elements increases the hydrostatic pressure at the source. this creates a pressure gradient between the source and sink

• sucrose + other solutes move down the pressure gradient to the sink

• at the sink, sucrose is actively transported out of the sieve tube elements. this increases the water potential at the sink

• water thus moves out of the sink via osmosis, decreasing hydrostatic pressure

• the process is maintained by loading at the source and unloading at the sink

explain how ringing experiments can be used to investigate transport in plants

demonstrates whether the phloem is responsible for transporting sugars

1. ring of bark (including phloem) is removed from the stem. the xylem remains in the stem

2. the stem swells above the area where the bark has been removed with fluid

3. fluid is tested for sugars

results:

• if the fluid has a high concentration of fluids, it shows that translocation occurs in the phloem

• also shows that sugars are transported from source to sink (leaves to roots)

explain how radiotracers can be used to investigate transport in plants

tracks the movement of specific substances within the plant

1. radioactive isotopes (like isotopes of carbon in CO₂) are supplied to a leaf

2. isotope is used in photosynthesis/metabolic processes to produce radioactive organic substances, like sucrose

3. movement of radioactive substance is traced through the plant via autoradiography

results:

• demonstrates the translocation occurs in the phloem

• demonstrates that sucrose moves from source to sink