3A: Exchange and Transport Substances

Cells need to take in O2 for aerobic respiration and nutrients, need to excrete waste products like CO2 and urea. Need to stay at same temperature, so heat needs to be exchanged.

SA:V ratio: Affects how quickly substances are exchanged. Smaller organisms have higher SA:V ratios than larger organisms.

SA of sphere: 4πr² ~ Volume of sphere: 4/3πr³ ~ Volume of cylinder: πr²h

In single celled organisms, substances diffuse directly in/out cell across CSM. Diffusion quick bc small distance. In multicellular organisms, diffusion too slow bc some cells deep in body, big difference between them and outside environment/larger animals have low SA:V ratio, difficult to exchange enough substances to supply large volume of animal through relatively small outer surface. Multicellulars need specialised exchange organs and efficient system to carry substances to/from cells, mass transport. Circulatory system, blood carries glucose, oxygen, hormones, antibodies, CO2. Plant transport, water, solutes in xylem, phloem. 

Metabolic activity inside cells creates heat. Body size: Big>small SA>harder to lose heat. Small>large SA> heat lost more easily. Smaller organisms need high metabolic rate, to generate enough heat to stay warm.

Body shape: Animals with compact shape>small SA:V ratio>minimises heat loss from surface. Animals with less compact shape>larger SA:V ratio>increases heat loss from surface.

Adaptations: Whether compact or not depends on temperature of environment - body shape adapted to suit environment. 

Animals with high SA:V ratio tend to lose more water as evaporates from surface. Problem for animals living in hot regions where water evapourates quickly. Some small desert mammals have kidney structure adaptations so they produce less urine to compensate.

Smaller mammals have thick layers of fur or hibernate when weather gets really cold. 

To support high metabolic rates, small mammals living in cold regions eat large amounts of high energy foods such as seeds and nuts.

Larger organisms living in hot regions, such as elephants and hippos, find it hard to keep cool as heat loss relatively slow. Elephants have large flat ears, increase SA, allows to lose more heat. Hippos have behavioural adaptation, stay in water.

Gas exchange occurs over gas exchange surface, boundary between outside and internal environment of organism. Need O2, CO2 to diffuse across gas exchange surfaces as quickly as possible. Most have to increase diffusion rate: 1. Large SA, 2. Thin, often 1 layer of epithelial cells, provides short diffusion pathway across GES. 3. Maintains steep concentration gradient.

Single-celled organisms: Diffusion, large SA, thin surface, short diffusion pathway.

Fish: Lower conc O2 in water than air. GES are gills. Water enters fishs mouth, passes out through gills. Made of lots of thin plates, gill filaments, give large SA for exchange of gases. These covered with lamellae, increase SA even more. These have lots of blood capillaries, thin layer of cells to speed up diffusion between water and blood.

Counter-current system: Blood flows through lamellae in 1 direction and water flows over them in opposite direction. Means that water with relatively high O2 conc always flows next to blood with lower conc of O2. Means steep conc gradient maintained between water, blood, so much O2 as possible diffuses from water into blood. Normal circulation of fish replaces oxygenated blood that leaves gill with more deoxygenated blood. Normal ventilation of fish ensures more water with relatively high O2 conc is taken in. Both help maintain steep conc gradient.

Plants need CO2 for photosynthesis, produces O2 as waste gas. Need O2 for respiration, produces CO2 as waste gas. Main GES is surface of mesophyll cells. Large SA. Inside the leaf. Gases move in/out through pores in epidermis, mostly lower - stomata. Stomata can open to allow exchange of gases, close if plant losing too much water. Guard cells control opening/closing.

Terrestrial insects have microscopic air-filled pipes called tracheae, used for gas exchange. Air moves into tracheae through pores, spiracles. O2 travels down conc gradient towards cells. Tracheae branch into tracheoles, have thin, permeable walls and go to individual cells. Means that O2 diffuses directly into respiring cells, insects circulatory system doesnt transport O2. CO2 from cells moves down own conc gradient towards spiracles to be released into atmosphere. Insects use rhythmic abdominal movements to move air in/out of spiracles.

If insects losing too much water, close spiracles muscles. Waterproof, waxy cuticle all over body, tiny hairs, reduce evaporation. Stomata open during day, allow gaseous exchange. Water enters guard cells, makes turgid, opens stomatal pore. Plant dehydrated, guard cells lose water, become flaccid, closes pore.

Xerophytes (warm, windy, dry): Stomata sunk in pits traps water vapour, reduces CG between leaf and air. Hairs. Curled leaves. Few stomata. Thicker waxy, waterproof cuticles.

Ventilation: Inspiration and Expiration. Inspiration: External intercostal and diaphram muscles contract. Ribcage moves upwards and outwards, diaphram flattens, volume of thoracic cavity increases. As it increases, lung pressure decreases to below atmospheric pressure. Air flows from area of higher pressure to lower pressure so down trachea into lungs. Active process.

Expiration: External intercostal and diaphram muscles relax. Ribcage moves downwards and inwards, diaphram curves upwards again, volume of thoracic cavity decreases, air pressures increases to above atmospheric pressure. Air forced down pressure gradient and out of lungs. Passive process. Forced expiration: External intercostal muscles relax, internal intercostal muscles contract, pulling ribcage further down and in. Movement is antagonistic.

Gas exchange occurs in alveoli. Single cell wall of thin, flat cells - alveolar epithelium. Walls contain protein called elastin, helps alveoli to recoil to normal shape after inhaling and exhaling air.

Air>Trachea>Bronchi>Bronchioles>Alveoli. Down pressure gradient. Alveolar epithelium>Capillary endothelium>Blood. O2 moves into blood where it can be transported around body, moving down diffusion gradient. CO2 moves down own diffusion and pressure gradients, but opposite to O2, so can be breathed out.

O2 diffuses out of alveoli, across alveolar epithelium, capillary endothelium into haemoglobin in blood. CO2 diffuses into alveoli from blood.

Thin exchange surface: One cell thick, short diffusion pathway. 

Large SA: Millions of alveoli. Large SA for gas exchange.

Steep CG of O2 and CO2 between alveoli and capillaries, which increases rate of diffusion. Constantly maintained by flow of blood and ventilation.

Affect both ventilation and gas exchange. 

Tidal volume: Volume of air in each breath. Usually 0.4/0.5dm3 for adults.

Ventilation rate: No. of breaths per minute. Healthy person at rest is 15.

Forced expiratory volume: Max volume of air that can be breathed out in 1 sec.

Forced vital capacity: Max volume of air to breathe forcefully out of lungs after really deep breath in

Common symptoms of lung disease: Coughing, shortness of breath, fatigue, chest pains.

All reduce rate of gas exchange in alveoli. Less O2 able to diffuse into bloodstream, body cells receive less O2, rate of aerobic respiration reduced. Less energy released, sufferers often feel tired and weak.