EXCHANGE

FEATURES.

• large surface area to volume ratio
• thin - short diffusion pathway
• selectively permeable - allow selected materials across
• movement of environmental medium - maintains concentration gradient
• transport system t move internal medium - maintains concentration gradient

diffusion = (surface area x difference in concentration) / length of diffusion path

• small - large surface area to volume ratio
• oxygen absorbed across body surface via cell-surface membrane
• cardbon dioxide diffuses out across body surface

RESPIRATORY SYSTEM.

• internal network of tubes - tracheae - supported by strengthened rings to prevent collapse
• tracheae divide into tracheoles extending throughout al body tissues - no cell is far from a source of air/oxygen
• spiracles - pores in the body surface opened and closed by valves to control water loss

MOVEMENT OF RESPIRATORY GASES.

DIFFUSION GRADIENT - cells respire, oxygen used up, concentration decreases, oxygen diffuses in from atmosphere. Cells respire, carbon dioxide produced, concentration increases, carbon dioxide diffuses out to atmosphere. Takes place in gas phase - more rapid than liquid.

MASS TRANSPORT - muscle contraction squeezes tracheae allowing mass movement of air in and out

ENDS OF TRACHEOLES FILLED WITH WATER - anaerobic respiration during major activity causing lactate to build up. This lowers water potential of cells, causing water to move in from tracheoles by osmosis, decreasing volume of tracheoles causing air to be pulled in.

STRUCTURE OF THE GILLS.

• gill filaments stacked up
• gill lamellae at right angles to filaments, increasing surface area
• water forced over gills through mouth and out sides
• flow of water takes place in opposite direction to flow of blood - countercurrent flow

COUNTERCURRENT EXCHANGE PRINCIPLE.

• blood and water flow over gills in opposite direction
• blood already well loaded with oxygen meets water with maximum concentration of oxygen, diffusion takes place
• blood with little oxygen meets water with almost all oxygen removed, diffusion takes place
• maintains concentration gradient across whole width of lamellae 

PHOTOSYNTHESIS + RESPIRATION.

• photosynthesis takes place - most of CO2 obtained from external air, not respiration + some oxygen used in respiration but most diffuses out
• photosynthesis not occurring (e.g. in dark) - oxygen diffuses into leave for respiration and carbon dioxide diffuses out

STRUCTURE OF THE LEAF.

• many stomata - no cell is fair from a source of oxygen, short diffusion pathway
• numerous interconnecting air spaces throughout mesophyll - gases readily contact mesophyll cells
• large surface area of mesophyll cells - rapid diffusion

ENSURES no cell is fair from external air AND diffusion takes place in the gas phase, faster than in water.

STOMATA.

• pores opened + closed by guard cells, closed when water loss would be excessive

ADAPTATIONS.

• small surface area to volume ratio - minimises area over which water is lost
• waterproof coverings - outer skeleton of chitin covered in waterproof cuticle
• spiracles - can be closed to reduce water loss

• cannot have a small surface area to volume ratio - need to photosynthesise so need large leaves to capture light + exchange gases
• waterproof coverings + ability to close stomata

XEROPHYTES - plants adapted to living in areas where water is in short supply

• thick cuticle
• rolling up of leaves, hair leaves, stomata in pits or grooves - traps region of still air around the leaf which becomes saturated with water vapour so has high water potential, decreasing water potential between inside and outside of eaf so no movement of water
• reduced surface area to volume ratio of leaves - small, circular leaves (e.g. needle-like) reduces area over which water is lost

GAS VOLUMES.

• large volume of oxygen to be taken up and carbon dioxide to be removed
• mammals are large organisms - lots of cells, lots of respiration
• high body temperature - related to high metabolic + respiratory rates

LUNGS.

• located within bodu as air is not dense enough to support them and this would also cause massive water loss

RIBCAGE - bony box with muscles inbetween to allow the muscles to move and ventilate the lungs in a tidal stream of air

LUNGS - lobed structures made of highly branched tubules, bronchioles, ending in alveoli

TRACHEA - flexible airway supported by rings of cartilage to prevent collapse with walls of muscle lined w/ epithelial and goblet cells

BRONCHI - two divisions of the trachea supported by cartilage, one for each lung, with goblet cells to produce mucus to trap dirt w/ cilia cells to move mucus towards the throat

BRONCHIOLES - branching subdivisions of the bronchi made of muscle lines with ephithelial cells - muscle allows contraction to control flow of air in and out

ALVEOLI - minute air sacs at the end of the bronchioles with collagen and elastic fibres inbetween to allow stretching and recoil for uptake of oxygen and removal of carbon dioxide lined with epithelium - the gas exchange surface

INSPIRATION.

• external intercostal muslces contract, internal intercostal muscles relax
• ribs pulled up and outwards, increasing volume of thorax
• diaphgram muscles contract causing it to flatten, further increasing the volume
• increased pressure of thorax reduces pressure in lungs
• atmospheric pressure > pulmonary pressure, air forced into lungs down pressure gradient

EXPIRATION.

• internal intercostal muscles contract, external intercostal muscles contract
• ribs move downwards and inwards, decreasing the volume of the thorax
• diaphrgam muscles relax, pushing it up by the abdomen contents that were compressed during inspiration, further decreasing the volume of the thorax
• decreased volume of thorax increases pressure in lungs
• pulmonary pressure > atmospheric pressure, air forced out of lungs down pressure gradient

ROLES.

• lined with thin epithelial cells
• network of pulmonary capillaries of narrow diameter
• red blood cells slowed as they pass through capillaries, allowing more time for diffusion
• red blood cell flattened against capillary walls, short diffusion pathway
• thin alveolar + capillary walls to reduce diffusion pathway
• large total surface area of alveoli + capillaries
• breathing movements constnatly ventilate lungs + heart constantly circules blood to maintain concentration gradient

• oesophagus - carries food from mouth to stomach
• stomach - muscular sac with inner layer + glands to secrete enzymes
• ileum - long muscular tube that secretes enzymes from walls + glands, folded into villi with microvilli to increase surface area
• large intestine - absorbs water,, mostly from secretions of digestive glands
• rectum - faeces periodically stored before egestion
• salivary glands - pass saliva with salivary amylase into mouth
• pancreas - large gland that secretes pancreatic juice containing proteases, lipase and amylase

PHYSICAL BREAKDOWN.

• teeth mechanically break down food, stomach churns for further breakdown
• increases surface area for enzymes as well as making ingestion easier

CHEMICAL BREAKDOWN.

• hydrolysis of large, insoluble molecules into small, soluble ones carried out by enzymes

• saliva enters mouth and is thoroughly mixed with food
• contains salivary amylase - hydrolyses starch in ood to maltose, contains mineral salts to maintain neutral pH
• food swallowed and enters stomach, acidic conditions denature amylase and prevents further hydrolysis
• food passed into small intestine, mixed with pancreatic juice
• pancreatic amylase continues hydrolysis of starch to maltose w/ alkaline salts to maintain neutral pH
• muscles in intestinal walls push food along ileum, walls contain membrane-bound disaccharidase maltase which hydrolyses maltose into a-glucose. Sucrase and lactase may also be present.

LIPID DIGESTION.

• lipids split into tiny droplets - micelles - by bile salts from liver during emulsification to increase surface area
• lipases hydrolyse ester bond in trigylcerides to fatty acids and monoglycerides 

PROTEIN DIGESTION.

ENDOPEPTIDASES - hydrolyse peptide bonds between amino acids in central region, forming series of peptide molecules

EXOPEPTIDASES - hydrolyse peptide bonds on terminal amino acids of peptide molecules, releasing dipeptides and amino acids

DIPEPTIDASES - hydrolyse bond between two amino acids to form amino acids. Membrane-bound.

• folded wall
• wall posesses projections - villi - which extend into lumen

- increase surface area for diffusion

- thin walls reduce diffision pathway

- contain muscle so can move to continue flow of contents of ileum to maintain concentration gradient

- highly vascularied to maintain concentration gradient through blood flow

- posess microvilli (projections on cell surface membrane of epithelial cells) to further increase surface area

AMINO ACIDS + MONOSACCHARIDES.

• active transport + co-transport

TRIGLYCERIDES.

• monoglycerides and fatty acids remain in association with bile salts in micelles
• micelles come into contact with epithelial cells lining villi of ileum
• micelles break down, releasing monoglycerides + fatty acids to diffuse across cell-surface membrane (non-polar, lipid-soluble)
• monoglycerides + fatty acids transported to endoplasmic reticulum to be reformed as trigylcerides
• triglycerides associate with cholesterol + lipoproteins in the golgi apparatus to form chylomicrons
• chylomicrons move out of epithelial cells by exocytosis and enter lymphatic capillaries, lacteals
• chylomicrons pass via lymphatic vessels into blood system
• triglycerides hydrolyses by an enzyme in endothelial cells of blood capillaries before diffusing into cells