Movement across the Membrane

Transcription

1 Chapter 8. Movement across the Membrane

2 Cell membrane Cells have an inside & an outside Cell membrane is the boundary Can it be an impenetrable boundary? NO! Why not? The cell needs materials in & products or waste out! 2

3 Semi-permeable membrane Need to allow passage through the membrane But need to control what gets in or out membrane needs to be semi-permeable So how do you build a semi-permeable membrane? 3

4 The Cell s needs What kinds of molecules need to get through? sugars & carbohydrates peptides & proteins lipids nucleic acids water 4

5 Cell membrane structure Basic structure of cell membrane is phospholipid bilayer What molecules can go directly through? 5

6 Cell membrane permeability Which molecules can go directly through phospholipid membrane? carbohydrates - No, large & hydrophilic proteins - No, large & many hydrophilic lipids - Yes, hydrophobic (lipid-soluble) nucleic acids - No, hydrophilic water - No, hydrophilic So what gets through? Lipid-soluble molecules (non-polar, hydrophobic) 6

7 Cell membrane permeability So how do you get polar, hydrophilic molecules through a phospholipid membrane? Put holes in it! But the cell needs control Put selective holes in it! How? PROTEINS! 7

8 Transport proteins Globular proteins serve as channels to move molecules through cell membrane open channel conformational changes Cellular Donuts! Each transport protein is specific as to the substances that it will translocate (move). For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer. Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.these channel proteins allow fast transport. For example, water channel proteins, aquaprorins, facilitate massive amounts of osmosis. Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport. 8

9 Getting through cell membrane Passive transport diffusion of hydrophobic molecules high low concentration gradient Facilitated transport diffusion of hydrophilic molecules through a protein channel high low concentration gradient Active transport diffusion against concentration gradient low high uses a protein pump requires ATP 9

10 Passive transport is diffusion 2nd Law of Thermodynamics governs biological systems Universe tends towards disorder Diffusion movement from high low concentration of that item Movement from high concentration of that substance to low concentration of that substance. 10

11 Diffusion of 2 solutes Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances Things tend to get mixed up evenly. 11

12 Facilitated diffusion Movement of molecules down concentration gradient via transport protein high low concentration protein channel 12

13 Gated channels Some channel proteins open only in presence of stimulus (signal) stimulus usually different from transported molecule ex: ion-gated channels when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na + ions to enter nerve cell ex: voltage-gated channels change in electrical charge across nerve cell membrane opens Na + & K + channels When the neurotransmitters are not present, the channels are closed. 13

14 Gated channels 14

15 Active transport Cells may need molecules to move against concentration gradient need to pump against gradient protein pump requires energy (usually ATP) conformational change in transport protein 15

16 Active transport examples Na + K + pump in neurons H + pumps in mitochondria & chloroplasts nitrate & phosphate pumps in plant roots Plants: nitrate & phosphate pumps in roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer Supplying these nutrients to plants Replenishing the soil since plants are depleting it 16

17 The sodium-potassium pump actively maintains the gradient of sodium (Na + ) & potassium ions (K + ) across the membrane an animal cell has higher concentrations of K + and lower concentrations of Na + inside the cell sodium-potassium pump uses the energy of one ATP to pump 3 Na + ions out and 2 K + ions in 17

18 Transport summary 18

19 How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = cellular eating pinocytosis = cellular drinking receptor-mediated endocytosis exocytosis exocytosis 19

20 Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by ligand signal 20

21 Movement of water across the cell membrane

22 The special case of water Water is critical to biological systems, so we talk about water separately Osmosis diffusion of water down its concentration gradient 22

23 Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water water hypotonic hypertonic net movement of water 23

24 Managing water balance Cell survival depends on balancing water uptake & loss 24

25 Managing water balance Isotonic animal cell immersed in isotonic solution blood cells in blood no net movement of water across plasma membrane water flows across membrane, at same rate in both directions volume of cell is stable 25

26 Managing water balance Hypotonic animal cell in hypotonic solution will gain water, swell & burst Paramecium vs. pond water Paramecium is hypertonic H 2 O continually enters cell to solve problem, specialized organelle, contractile vacuole pumps H 2 O out of cell = ATP plant cell turgid 26

27 Water regulation Contractile vacuole in Paramecium 27

28 Managing water balance Hypertonic animal cell in hypertonic solution will loose water, shrivel & probably die salt water organisms are hypotonic compared to their environment they have to take up water & pump out salt plant cells plasmolysis = wilt 28

29 Any Questions?? 29