Ch6 - Exchange

- Can exchange passively (diffusion, osmosis) or actively (active transport)

- Smaller organism = Large SA:V = effective gas exchange 

- Hence for inactive, single-celled organisms (bacteria/viruses) simple diffusion across cell surface membrance suffices as it has a small SA:V. Hence, they have a flattened shape.

- FIX'S LAW = (SA * CONC. GRADIENT)/DIFFUSION DISTANCE 

1. Large SA:V - increases rate of exchange 

2. Thin - short diffusion distance = cross rapidly 

3. Selectively permeable 

4. Movement of enviromental medium - maintains gradient 

5. Transport system - movement of internal medium - maintains gradient 

- Can't be too thin as may be damaged/dehydrated, hence, located INSIDE organism.

- Large SA for gas exchange conflicts with reserving water 

- Internal network or tubes called trachea (supported by cartilage so don't collapse)

- Trachea divided into tracheoles. Hence, O2 brought directly to respiring cells as short diffusion pathway. Gases move in and out in 3 ways:

1. Along diff. gradient - cells respiring = O2 used up = conc in tracheoles decrease = diff. gradient made as O2 from air enters tracheoles. CO2 producd by cells in respiration = diff. gradient in opposite direction. Diff. in air is faster than in water.

2. Mass transport - ventilation. Contracting muscles squeeze trachea = low volume = high pessure = air forced out + vice versa.

3. Ends of tracheoles filled with water - during major activitiy muscle cells respire ANAEROBICALLY = produces lactate = lowers water potential. Water goes from tracheoles to cells by osmosis. Water in tracheoles decreases = more drawn in from the air. Hence final diff. path is gases (faster) which increases the rate where air goes into the tracheoles. 

- Spiracles are tiny openings. Opened and closed by valve - increases gas exchange but also increases water loss.

- Length of pathway needs to be short due to relying on diff. Hence ants size LIMITED to being small.

- Fish have a waterproof, gas-tight covering. Large = small SA:V

- Made up of gill filaments stacked in a pile. At right angles to these are gill lamellae which increase SA.

- Water taken in by mouth, forced OVER gills + out 

- Flow of water over the lamella + flow of blood are in opposite directions = COUNTERCURRENT FLOW.

- Maintaining O2 diff. gradient = ventillation + mass transport in blood

- Blood and water flow in opposite directions so:

1. Blood with high O2 conc. meets water with maximum conc. of O2. Hence diffusion from water to blood.

2. Blood with low O2 conc. meets water which had most O2 removed. Still diffusion from water to blood. 

- Hence, 80% of O2 absorbed. If parallel, diff. gradient only maintained at parts 50% absorbed.

- Plants photosynthesise and taken in CO2 and release O2. This O2 is used for respiration, hence less gas exchange with air. 

- STOMATA: minute pores on leaves underside. Each stomata surrounded by pair of guard cells which open and close stomatal pore - control rate of gas exchange + water loss by evapouration.

INSECTS:

To reduce water loss:

1. Small SA:V = minimalise area where H2O is lost

2. Waterproof covering = rigid = made of chitin + waterproof cuticle 

3. Spiracles = openings are CLOSED at rest to reduce water loss

PLANTS:

- Can't have small SA:V as photosynthesis needs to capture light. To reduce water loss:

1. Waterproof covering + close stomata. Some plants limit through transpiration = xerophytes

2. Thick cuticle - waxy + thick = less H2O leaves

3. Rolling up leaves - stomata rolls = traps air = region surrounded with H2O = high water potential = no gradient = no loss

4. Hairy leaves - traps still MOIST air = reduces water potential gradient

- Aerobic organisms = constant supply of O2 needed to relase energy through ATP during respiration + CO2 removed.

- Lungs inside because:

1. Air isn't dense enough to support + protect lungs

2. Body would lose H2O

- Lungs supported + protected by ribcage. Ventilated by tidal stream of air which is constantly replenished.

- Trachea: flexible airway supported by cartilage rings = prevents collapsing when air pressure falls during inspiration. Walls made of muscle, ciliated epithelial + globlet cells. 

- Bronchi: 2 divisions of trachea. Produce mucus to trap dirt + cilia move dirty mucus to throat 

- Bronchioles: walls made of muscle + epithelial cells. Muscles = contract - control flow of air 

- Alveoli: Air sacs. Between alvoeli = collagen + elastic fibres (stretching) and lined with epithelial 

- Air moving in + out of lungs = breathing/ventillation 

- When air pressure > lung pressure = INSPIRATION

- When lung pressure > air pressure = EXPIRATION

- 3 sets of muscles: Diaphragm, internal intercostal (contract = EXpiration), external intercostal (contract = INspiration)

- Inspiration: Active process. External contract + internal relax. Ribs pulled upwards and outwards = increases volume of thorax. Diaphragm contracts and flattens = increases volume. Hence pressure decreases. Air pressure > lung pressure, air forced INTO lungs 

- Expiration: Passive process. Externa relax + internal contract. Ribs move downwards + inwards = less volume. Diaphragm relaxes, pushed up = less volume. Hence pressure increases. Lung pressure > air pressure, air forced OUT. 

- Site of gas exchange in mammals = alveoli epithelium 

- Diff. gradient must be maintained. Exchange surgaces are: thin, partially permeable + large SA

- Has to have movement of enviromental + internal medium 

ALVEOLI:

- Large SA of pulmonary capillaries (RBCs squeeze = reduce diff. distance) + air sacs. 

- Each alveoli lines with epithelial cells = 1 cell thick 

- Capillaries - endothelial cells - 1 cell thick 

- Circulation and breathing maintains steep conc. gradient

- Made up long muscular tube + glands that produce enzymes that hydrolyse large molecules into small molecules for absorption = increases SA

- Oesophagus: carries food from mouth to stomach

- Stomach: Muscular sac that produces enzymes to store + digest proteins

- Ileum: further digestion by glands + enzymes. Inner walls folded into villi (increase SA). SA increased by tiny projections - microvilli on epithelial cells of each vilus. 

- Large intestine: absorbs water from food + secretions from glands 

- Rectum: storage of faeces. Removed via anus through egestion 

- Salivary glands: contain amalyase. Hydrolyses starch into maltose 

- Pancreas: large gland. Produces amalyase, protease + lipase

- Physical breakdown: large food broken into smaller pieces with teeth + churning food in stomach = increases SA 

- Chemical breakdown: hydrolyses large insoluble molecules to small soluble molecules. Carried out by specific enzymes:

1. Amalyase: carbs to monosaccharides

2. Lipase: lipids to f.a + glycerol

3. Protease: proteins to amino acids

- Amalyase produced in mouth + pancreas. Hydrolyses ALTERNATE GLYCOSIDIC BONDS of starch to produce maltose

- Maltase produced by ileum lining hydrolyses maltose to a-glucose 

- Sucrase hydrolyses SINGLE GLYCOSIDIC BONDS in sucrose to glucose + fructose 

- Lactase hydrolyses SINGLE GLYCOSIDIC BONDS in lactose to glucose + galactose

Process:

1. Saliva from salivary glands mixed with food. Amalyase hydrolyses starch to maltose, contains mineral salts that maintain pH at neutral. 

2. Stomach = acidic = denatures amalyase. Small intestine mixes with pacreatic amalyase and continues hydrolysis with ALKALINE SALTS to maintain pH7.

3. Epithelial lining produces maltase (membrane bound dissacharide) but not released into lumen, part of cell surface membrane.

- Lipase produced in pancrease hydrolyses the ESTER bond in triglycerides to form f.a + monoglycerides. 

- Lipid splits into tiny droplets called MICELLES by bile salts produced by liver. Process of emulsification increases SA. 

- Micelles come in contact with epithelial cell lining the villi of the ileum, break down into monoglycerides + f.a (NON-POLAR), hence, easily DIFFUSE across cell surface membrane to epithelial cells. They are then transported to ER then Golgi apparatus (recombines).

- Triglycerides + cholestrol + lipoproteins form chylomicrons that transport lipids. Move out of epithelial cells into lymphatic capillaries then to the bloodstream.

DIAGRAM IN TEXTBOOK

- Hydrolyed by peptidases. 3 types:

1. Endopeptidases - hydrolyse peptide bonds in between amino acids in CENTRAL region to form a peptide molecule 

2. Exopeptidases - hydrolyse peptide bonds on TERMINAL amino acids of peptide molecules to form dipeptides + single amino acids 

3. Dipeptidases - hydrolyse bond between 2 amino acids (membrane bound - found in epethilial cells in ileum lining)

- Absorbed by CO-TRANSPORT

- Folded wall of ileum = villi = increases SA. Adapted by:

1. Increased SA for diffusion

2. Thin wall = short diff. pathway

3. Contains muscle = movement = maintains conc. gradient as mixes the contents, hence as products absorbed, new materials replace it

4. Good supply of capillaries = carries away absorbed material = maintains diff. gradient 

5. Epithelial lining has MICROVILLI = futher increases SA