Gas Exchange in Fish and Plants

• The need for specialised exchange surfaces arises as the size of the organism increases, which leads to an increase in the surface area to volume ratio.
• Single-celled organisms easily allow substances to enter the cell, because the distance required to cross is short.
• Multicellular organisms require specialised exchange surfaces for gas exchange of carbon dioxide and oxygen, due to higher surface area to volume ratio, which equates to a larger diffusion distance.

Features of an Efficient Exchange Surface

• Large surface area
• Thin to ensure a short diffusion distance
• Good blood supply/ventilation to maintain a steep gradient

Ventilation and Gas Exchange

• Ventilation and gas exchange occurs in fish, insects, and plants.

Fish

• Fish require a specialised gas exchange surface because they have a small surface area to volume ratio and an impermeable membrane.
• Bony fish have four pairs of gills supported by an arch on each with multiple projections called gill filaments, with lamellae responsible for gas exchange.
• Blood and water flow across the lamellae in a counter current direction to maintain a steep diffusion gradient for maximum oxygen diffusion.
• Water flow holds the gill projections apart, to prevent the gills from sticking together, therefore fish cannot survive out of water.
• Ventilation requires a continuous unidirectional flow and begins with the fish opening its mouth to lowering the floor of the buccal cavity, after the mouth closes, the buccal cavity floor raises.
• Pressure difference between the mouth cavity and opercular cavity forces water over the gill filaments.
• The operculum acts as a valve and pump.

Terrestrial Insects

• Insects transport oxygen directly to tissues undergoing respiration, as they don't possess a transport system.
• Spiracles (small openings of tubes), help achieve appropriate oxygen quantities in the body.
• Larger trachea, or smaller tracheoles run into the body of an insect and supply it with the required gases.
• Gases move in and out through diffusion, mass transport as a result of muscle contraction, and as a result of volume changes in the tracheoles.

Plants

• Plants adapt to gas exchange through adaptations in their leaves, such as stomata, which allow gases to enter and exit.
• A large number of stomata means no cell is far from a stomata (short diffusion distance).
• Air spaces allow gases to move around the leaf and contact mesophyll cells.

Mammalian Gaseous Exchange

• The lungs are structures with a large surface area located in the chest cavity, surrounded by the rib cage.
• A lubricating substance prevents friction between the rib cage and lungs during inflation and deflation.
• External and internal intercostal muscles between the ribs contract to raise and lower the ribcage.
• The diaphragm separates the lungs from abdomen area.
• Air enters through the nose, then the trachea, bronchi, and bronchioles, which enable passage of air into the lungs.
• Gaseous exchange takes place in the walls of alveoli (tiny sacs of air).
• Rings of cartilage (incomplete in the trachea) enable the trachea, bronchi, and bronchioles to enable airflow through and out of the lungs - these airways are held open.
• The incompleteness of the cartilage rings in the trachea allows passage of food down the oesophagus behind the trachea.
• Trachea and bronchi structure: composed of several layers making up a thick wall (mostly cartilage in the form of incomplete C rings), The inside surface of the cartilage is a layer of glandular and connective tissue, elastic fibres, smooth muscle, and blood vessels "loose tissue".
• The inner lining is an epithelial layer composed of ciliated epithelium and goblet cells.
• Bronchioles are narrower than the bronchi, only larger bronchioles contain cartilage, and have walls made of smooth muscle and elastic fibres.
• Smallest bronchioles have alveoli clusters at the ends.
• Alveoli are adapted for diffusion through being only one cell thick and are surrounded by capillaries
• The constant blood supply by capillaries maintains a steep concentration gradient.
• There are a large number of alveoli (~300 million), collectively giving a surface area of ~70m².

Structures and Functions of the Mammalian Gaseous Exchange System

• Cartilage supports the trachea and bronchi and prevents the lungs from collapsing in the event of pressure drop during exhalation.
• Ciliated epithelium moves mucus to prevent lung infection.
• Goblet cells secrete mucus to trap bacteria and dust, reducing the risk of infection with the help of lysozymes.
• Smooth muscle constricts the airway to control airflow.
• Elastic fibres stretch when inhaling and recoil when exhaling.

Ventilation

• Ventilation (flow of air in and out of the alveoli) is composed of inspiration and expiration, occurring with the help of intercostal muscles and diaphragm.
• Inspiration is when the external intercostal muscles contract (internal relaxes) causing the ribs to raise upwards, the diaphragm contracts and flattens - increasing the volume inside the thorax, lowering the pressure and drawing air into the lungs.
• Expiration is when the internal intercostal muscles contract (external relaxes) lowering the rib cage, the diaphragm relaxes and raises upwards - decreasing the volume inside the thorax, increasing pressure which forces air out of the lungs.

Spirometer

• Spirometer measures lung volume and traces a graph when breathing in and out of the airtight chamber.
• Vital capacity is the maximum volume of air that can be inhaled or exhaled in a single breath.
• Factors affecting vital capacity include gender, age, size, and height.
• Tidal volume is the volume of air breathed in and out at each breath at rest.
• Breathing rate (number of breaths per minute) = calculated from the spirometer trace by counting peaks or troughs in a minute.
• Residual volume = the volume of air that is always present in the lungs.
• Inspiratory reserve volume is when the tidal volume can be exceeded.
• Expiratory reserve volume is the additional volume of air that can be exhaled on top of the tidal volume.

Digestion and Absorption

• Digestion = the hydrolysis of large biological molecules into smaller molecules which can be absorbed across cell membranes.

• Carbohydrates: Amylases digest larger polymers in the mouth, maltases in the ileum break down monosaccharides, and sucrases and lactases break down dissacharides in the ileum.

• Lipids: Lipases hydrolyse the ester bond between the monoglycerides and fatty acid.

• Lipids are emulsified into micelles by bile salts released by the liver prior to being broken down in the ileum.

• Emulsification increases the surface area and speeds up the reaction.

• Proteins: peptidases are divided into 3 groups.

  • Endopeptidases hydrolyses peptide bonds between specific amino acids in the middle of a polypeptide.
  • Exopeptidases hydrolyses at the ends of a polypeptide.
  • Dipeptidases break dipeptides into individual amino acids.

• Products of digestion are absorbed by cells lining the ileum of mammals.

• Amino acids are absorbed by facilitated diffusion through specific carrier molecule in the surface membrane of epithelial cells.

• A diffusion gradient for Na+ is maintained by active transport through the base of epithelial cells where amino acids pass by facilitated diffusion.

• Monoglycerides and fatty acids are non polar so diffuse easily across the cell membrane into the epithelial cells lining the epithelium.

• They move out of the cells by vesicles into the lymph system, after transported to the endoplasmic reticulum where they are reformed into triglycerides.

Haemoglobin

• Haemoglobin: water soluble globular protein consisting of two beta polypeptide chains and two alpha helices, with each molecule forming a complex containing a haem group.

• Each Hb molecule can carry four oxygen molecules, because oxygen can bind to the haem (Fe2+) group

• The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen.

• Partial pressure increases, increases Haemoglobin affinity.

• In the lungs oxygen's affinity increases to haemoglobin, in the process of loading.

• During respiration, the partial pressure decreases thus decreasing the affinity of oxygen for haemoglobin, causing oxygen to be released.

• After the unloading process, Hb returns to the lungs where it binds to oxygen again.

• Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes.

• The higher the partial pressure equates a higher saturation of Hb of oxygen.

• Saturation can improve Hb affinity. For example, after binding to the first oxygen molecule, the affinity of Hb for oxygen increases due to a change in shape - making it easier for the other oxygen molecules to bind

• The oxygen disassociation curve shows that at first it is hard to bind an individual oxygen molecule.

• Once it has bound though it changes the shape making it easier for oxygen molecules two and three to bind, hence the steep increase. This is called positive cooperativity.

• Fetal Hb has a different affinity for oxygen because by the time oxygen reaches the placenta, the oxygen saturation of the blood is decreased.

• Fetal Hb must have a higher affinity for oxygen in order for the foetus to survive at low partial pressure.

• The affinity of haemoglobin for oxygen is also affected by the partial pressure of carbon dioxide.

• In the presence of carbon dioxide, the affinity of haemoglobin for oxygen decreases, thus causing it to be released. This is known as the Bohr effect.

• Decreased oxygen content can slightly acidify which change the shape of haemoglobin protein.

The Mammalian Circulatory System

• A circulatory system transports substances like O2 and glucose to cells in organisms with a small surface area to volume ratio.
• Common features: suitable medium, a means of moving the medium and a mechanism to control flow around the body (valves) and a closed system of Vessels
• In mammals the circulatory system is a closed double circulatory system.
• There are two pumps, one pumps bloods to the lungs to be oxygenated whilst the other is larger and stronger and pumps the oxygenated blood around the body to supply vital organs and tissues.

Structure and Cardiac Cycle of the Heart

• The heart has two pumps each with two chambers, made of:
  • Atrium (a thin walled and elastic area that stretches when filled with blood).
  • Ventricle (thick muscular wall to pump blood around the body or to the lungs).
• Two separate pumps are needed in order to maintain blood pressure.
• Between the atria and ventricles there are a set of valves, with the left being the atrioventricular valve (bicuspid valve) and the right being the atrioventricular valve (tricuspid valve).
• Aorta connects to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.
• Pulmonary artery connects to the right ventricle and carries deoxygenated blood to the lungs where it is oxygenated and the carbon dioxide is removed.
• Pulmonary vein connects to the left atrium and brings oxygenated blood back from the lungs.
• Vena cava connects to the right atrium and brings deoxygenated blood back from the tissues except the lungs.
• The cardiac cycle:

  • The heart can initiate its own contraction, called myogenic, in the wall of the right atrium (sinoatrial node), which initiates a wave of electrical stimulation, causing the atria to contract at roughly the same time.

  • The ventricles do not start contracting until the atria have finished. There is tissue which is unable to conduct the wave of excitation (septum.)

  • The electrical wave eventually reaches atrioventricular node (between the two atria), which passes on the excitation to ventricles down the bundle of His towards the apex of the heart.

  • The bundle of His branches into Purkyne fibres that cause the ventricles to contract and force blood upwards out of aorta and pulmonary artery.

  • There are 3 stages: Cardiac diastole, Atrial systole and Ventricular systole

  • Cardiac diastole is when atria and ventricles relax, and the elastic recoil of the heart lowers the pressure inside the heart chambers - blood returns and fill the atria and blood flows into ventricles, allowing the atrioventricular valves to open, and the semi-lunar valves are closed.

  • Atrial systole = the Aria contract and any remaining blood goes into the ventricles.

  • Ventricular systole is when the ventricles contract, which causes the atrioventricular valves to close, and the semi-lunar valves to open. Blood leaves the left ventricle through the aorta and right ventricle through the pulmonary artery.

Structure and Function of Blood Vessels

• Arteries are thick walled to withstand high blood pressure, and they contain elastic tissue to allow them to stretch and recoil, thus smoothing blood flow.
• Arteries also contain smooth muscle to vary blood flow, and they are lined with smooth endothelium to reduce friction.
• Arterioles branch off arteries - they have thinner and less muscular walls, and feed blood into capillaries.
• Capillaries are the smallest blood vessels; they are the site of metabolic exchange and are only one cell thick for fast exchange of substances.
• Venules are larger than capillaries but smaller than veins.
• Veins carry blood from the body to the heart, and they contain a wide lumen to maximise the volume of blood carried to the heart.
• Veins are thin walled (blood at low pressure), contain valves to prevent the back-flow of blood and have little elastic tissue or smooth muscle.

Function of Tissue Fluid

• Tissue fluid comprises of: dissolved oxygen, nutrients and solutes, that supply the tissues - in exchange for waste products.
• Hydrostatic pressure is what forces blood fluid out of the capillaries when blood is pumped along the arteries, into arterioles, and then capillaries.
• Capillaries allow substances that are small enough to escape through the gap, such as nutrients and ions.
• Actioned by oncotic pressure which pushes some fluid back into them.
• Both the tissue fluid and blood contain solutes, so they have a water potential.
• The tissue fluid is positive in comparison to the blood as it contains less solutes.
• Water moves down the water potential gradient from the tissue fluid to the blood by osmosis.
• Remaining volume is carried back to the lymphatic system, but fluid consists of a lower O2 fluid, that contains waste products.
• The lymph system has lymph nodes help with the immune system, which filter out bacteria and foreign material from the fluid with the help of lymphocytes which destroy pathogens.

Mass Transport in Plants

• Transpiration = water and dissolved minerals travel up xylem in plants. Water travels up plant by means of transpiration and translocation.
• Xylem enables water as well as dissolved minerals to travel up the plant (passive process of transpiration.)
• Phloem enables sugars to reach all parts of the plant with translocation.

Vessels in Plants

• The vascular bundle in the roots:
• Vascular bundles: xylem and pholem (providing structural support).
• X shape: made of xylem cells. An inside layer of mesothelial cells makes the pericycle
• X shape arrangement of xylem vessels is surrounded by endodermis which is an outer layer of cells which supply xylem vessels with water.
• The vascular bundle in the stem:
  • The xylem is present on the inside to provide support and flexibility to the stem (only in non-wooded plants)
  • Pholem is found outside the vascular bundle.
  • Cambium is a layer in between xylem and pholem, allowing for more xylem and pholem.
• The vascular bundle in the leaf:
  • Vascular bundles form the midrib and veins of a leaf.
  • Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards (transport and support).

Transpiration

• Plants absorb water through the roots, this water moves up the plant and is released into the atmosphere as H2O vapour through pores in leaves
• Transpiration stream provides:
  • the plant with water - enabling processes such as photosynthesis, growth and elongation.
  • required minerals
  • temperature control
• Osmosis occurs to move water from the xylem to the mesophyll cells.
• Evaporation from the surface of mesophyll cells into intercellular spaces.
• Diffusion of water vapour moves down a water vapour potential gradient out of the stomata.
• Transpiration can be investigated with a potometer (measures water lost by leaf and replaced by capillary tube).
• Factors Affecting Rate of Transpiration Rate:
• Number of leaves
• number/size or position of stomata
• presence of waxy cuticle
• amount of light present
• temperature
• humidity of the air
• air movement
• water availability
• Xerophytes are plants adapted to living in dry conditions (able to survive because minimise the water loss.)
• Adaptations: Smaller leaves reduce the surface area; densely packed mesophyll and thick waxy cuticle = prevent water loss from evaporation
• Lower epidermis exposure is reduced with rolling (trapping hair) from the air. The stomata closes to prevent loss.

Movement of Water

• Water enters through root hair cells and moves into xylem (centre root).
• Due to water potential gradient, more water enters, as the water potential is higher inside the soil than inside - due to cell sap (dissolved substances).
• Active transport absorbs the minerals as they need to be pumped against gradient concentration.
• Two pathways of water uptake via cortex of root through the xylem: the symplast pathway and the apoplast pathway.
• Symplast pathway where water is passed from cell to cell through channels which connect with plasmodesmata and the cytoplasm.
• The Apoplast Pathway where water enters the water cellulose in the cell wall and water can easily pass here, without much mineral ion salts.
• Casparian strip blocks water when at endodermis, which cannot be penetrated.
• Moving through the cell via symplast pathway is necessary to cross the endodermis.
• Once it moved across, then can easily pass again via water potential gradient across the cell.

Mass Flow and Translocation

• Water is removed from the xylem into the mesophyll cells down the potential gradient, it's pushed so more water enters.

• The action of bringing the xylem the endodermis is done via active transport.

• Surface tension between water and the attractive cohesive molecules in the mesophyll.

• Translocation: pholem features living cells with two types of cell:

• Tube elements transport nutrients in a tube.

• Companion cells have tubes made of sugar. These tubes move suga from sink cells to force the process of translocation.

• Sieve linked tubes comminute with pasmodesmata.

Steps for Translocation

• 1. ATP used to pull H+ ions creating a concentration gradient which creates sucrose, via creating sucrose and H+ concentration.
• 2. Now H+ will diffuse and go back, but go back with a co-trasnpirer protein pulling sucrose. Inside the cell, sucrose increases
• 3. Sucrose diffuses in with plasmodesmata. Now the potential increases.
• 4. As it diffuses to elemnts, this also increases water, as it has osmosis from the xylem. The hydrostatic pressure then increases.
• 5. Movement is high.
• 6. The sucrose is moved in elemts via active transport, in turn meaning the plant needs more and so it is removed to provide glucose. This reduces translocation.

• Mass flow theory: a mass of assimilates from assimilates go to sink. For:

• Sap is made from the plants

• Greater sucrose from source

• Glucose is good and gets sucrose levels

Against:

• Plate function

• Glucose should not move

• Low sucrose increases more

• Ringing involves taking off the tissues above and will swell, this is how sucrose transports in the pholem.

• Tracers = 14Carbon dioxide (radioactive). So those cells that move are radioactive, those are transporters.