thebiotutor.com AS Biology Unit 2 Exchange & Transport

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1 thebiotutor.com AS Biology Unit 2 Exchange & Transport 1

2 Exchange of materials Oxygen and Carbon dioxide are obtained passively by simple diffusion Fick s law The rate of diffusion = concentration difference X surface area distance travelled Unicellular organisms like Amoeba and small multicellular oganisms like Hydra no not need special gas exchange structures because the distance for diffusion is very small. Oxygen and carbon dioxide diffuse across their surface membranes. Multicellular organisms need a special gas exchange surface Look at the diagrams below and explain why this is the case. Explain the relationship between surface area and volume and why this is significant for multicellular organisms. Features of specialized exchange surfaces Large surface area Short diffusion distance Partially permeable Ventilation movements Blood / transport medium Gas exchange in insects 2

3 Explain 1. how the diffusion gradient is maintained 2. ventilation movements Gas exchange in fish Explain the counter current principle (see diagram on next page) 3

4 Gas exchange in a leaf 4

5 State 5 ways in which the leaf is adapted for gas exchange Mammalian circulatory system 5

6 Blood vessels Arteries and veins Explain the functions of the following: collagen (fibrous layer) elastic tissue endothelium smooth muscle 6

7 Capillary Make a table below to summarise the differences between arteries, veins and capillaries Blood, tissue fluid and lymph Blood components: Blood cells - red blood cells, phagocytes and lymphocytes Platelets - cell fragments involved in blood clotting Blood proteins - albumen and globulin, large molecules which exert osmotic potential Plasma - straw coloured liquid component of the blood carrying dissolved nutrients such as glucose and amino acids, urea, hormones and antibodies 7

8 NB: only plasma and small solute molecules can normally escape through the pores in the capillary. Study the simplified diagram below to show how tissue fluid is formed and how it returns to the blood at the venous end of the capillary bed. 8

9 Questions What is tissue fluid? How is tissue fluid formed? Explain pressure filtration How does tissue fluid return to the blood? Explain hydrostatic pressure and water potential differences. Explain the role of lymph Hydrostatic pressure Water potential Lymph What is edema and how might it be caused? 9

10 The role of hemoglobin in oxygen and carbon dioxide transport. The role of hemoglobin in carrying oxygen What is hemoglobin (Hb)? Make notes here on structure Fe Fe Fe Fe Oxygen affinity The hem groups have an attraction or affinity for oxygen Each one can loosely attach to one molecule of oxygen (O2) The combination is called oxyhemoglobin Equation: Question: How many oxygen molecules can be carried by one single HB molecule? Answer: % saturation of Hb If all the Hb molecules in a blood sample are carrying their maximum load of oxygen (4 molecules of O2 each) then the blood is fully (100%) saturated. Oxygen association / dissociation Processes by which the hem groups on Hb take up / release the oxygen molecules. This is entirely dependent upon the amount (partial pressure) of oxygen in the surrounding fluids. 10

11 Oxygen dissociation curve Make notes on dissociation curve here Q: Where in the circulatory system is the po2 greatest? A; Q: Where is the po2 least? A: What is the advantage of the S shaped curve? 11

12 Fetal Hemoglobin Fetal Hb has a higher affinity for oxygen than adult Hb Q: How does this benefit the fetus? (Illustrate on your graph) The role of Hemoglobin in carbon dioxide transport. Diagram illustrating the chain of events which lead to CO2 being excreted and O2 released to respiring cells. 12

13 CO 2 is released from respiring tissues and is dissolved in tissue fluid It diffuses into red blood cells Red blood cells have an enzyme called carbonic anhydrase which speeds up the reaction of CO 2 with water to from carbonic acid (H2CO3) Equation: Carbonic acid dissociates into HCO3 - (hydrogen carbonate) ions and H + ions HCO3 - ions move out of the red blood cells and are carried, dissolved in the plasma to the lungs where CO 2 is excreted Extension question: What is the significance of the chloride shift in this process? The Bohr shift H + ions compete with oxygen molecules and attach more readily to the available hem groups. Notice the effect this has on the dissociation curve. Why does the presence of CO2 cause a shift to the right? What is the advantage of the Bohr shift : Exam tip: BOHR shift RIGHT / FETAL Hb shift LEFT 13

14 Plant cell structure Palisade cells Explain the function of Thylakoid membrane Stroma DNA loop (not visible in diagram) Starch grain Cell wall Starch and cellulose (plant polysaccharides) Starch is a polymer made up of a mixture of two types of polysaccharide chain, amylose and amylopectin. Both consist of the monosaccharide sub unit alpha glucose. Starch is a storage molecule. It is Insoluble Compact Can be hydrolysed to glucose 14

15 Why does it twist up in this way? Glycogen Is very similar to starch, also made up from alpha glucose chains but the chains are shorter and very much more branched so it is more easily hydrolysed. Cellulose - a structural polysaccharide Cellulose is made up of repeating subunits of beta glucose This structure gives the molecule completely different properties it does not coil up H bonds are formed between adjacent chains of glucose sub units This makes the molecules line up side by side in long microfibrils Cellulose is insoluble and indigestible by humans It forms fibre, or roughage, in the diet passing right through the gut, giving bulk to the food to aid movement and digestion. 15

16 Water transport in plants root structure Water transport in plants Water enters the root Notes on active uptake of mineral ions, water potential and osmosis Transpiration - the loss of water by evaporation from the leaves of a plant. Factors affecting the rate of transpiration Leaf area Number and position of stomata 16

17 Cuticle thickness Temperature Humidity Wind currents Light Measurement of water loss from a plant. - The potometer Explain how this works and what it measures. Why can you not say that this apparatus measures the rate of transpiration? What is the function of the reservoir? What precautions do you need to take in setting it up? What other approach could you use to measure water loss in a plant? 17

18 How does water get from root to leaf? You now know how water gets into the root hair cells and how it leaves through the stomata but what of the processes between? Remember that the tubes through which it travels are non-living xylem cells, i.e. dead tubes and some trees can be over 100m tall. This requires a great deal of lifting power. To understand the process you need to consider the following forces: The pulling force of transpiration The cohesive forces sticking water molecules together The adhesive forces sticking water to the sides of xylem cells The osmotic force created by active uptake of minerals by roots (root pressure) Transpiration pull As water is lost from the spongy cells of the leaf by evaporation it is replaced by water drawn from xylem cells in the leaf vein. Water molecules are not easily pulled apart because they are polar and have cohesive properties. Cohesion Water molecules are attracted to each other by small electrostatic forces (see how a drop of water hangs from a tap.) The xylem tubes are very narrow and there are no air leaks so there is a continuous stream of water from root to leaf in the xylem tissue, held together by cohesion. This is called the transpiration stream. Adhesion Water sticks to the sides of xylem cells by adhesive forces. (it also sticks to the glass walls of capillary tubes and tends to rise, producing a meniscus). Adhesive forces prevent the column of water slipping backwards down the stem. The cohesion tension theory The combination of cohesion and adhesion produce an unbroken column of water up the xylem which is in a state of tension. It is this tension which is believed to sustain the transport of water up tall trees though it is impossible to reproduce the effect using fine glass tubes (hence the use of the word theory ) Root pressure How is the diffusion gradient created? 1. Active ion uptake by root hair cells 18

19 2. Active ion pump from endodermal cells 3. The role of the Casparian strip Apoplastic or symplastic pathway? Apoplastic pathway Symplastic pathway 19

20 Xerophytes - plants with special features for reducing water loss. Transpiration is essential for water transport and water is necessary for photosynthesis and to maintain turgor. Plants living in very dry regions, however, have a need to conserve water and reduce loss. Marram grass (beach grass) lives in dry sand at the edge of the ocean. The diagram below shows a sectional view through its coiled up leaves. Leaf section Special adaptations: Marram grass Other xerophytic features: 20