2B: Cell Membranes


Cell Membranes - Basics

All cells surrounded by membranes. Many organelles surrounded by membranes.

Cell-surface membranes (CSM) are a barrier between cell and environement, control which substances enter and leave cell through osmosis, active transport, diffusion. Partially permeable. Sometimes called plasma membrane.

Around organelles divide cell into different compartments - barrier between organelle and cytoplasm. DNA cant leave nucleus because of this.

CSM structure: Lipids (phospholipids), proteins, carbohydrates. 1972, fluid mosaic model suggests phospholipid (ppl) molecules form continous double layer. Bilayer fluid because ppl constantly moving.

Proteins scattered through bilayer, channel proteins, carrier proteins, allow large molecules and ions to pass through. Receptor proteins on CSM allow cell to detect chemicals released from other cells. Chemicals signal cell to respond in same way.

Some proteins can move sideways, some are fixed. Some have carbohydrate attached - glycoprotein. Lipid with carbohydrate attached = glycolipid. Cholsterol there too.

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Membrane Components

Phospholipids: Form barrier to water-soluble substances. Hydrophilic head, hydrophobic tail. Automatically arranged into bilayer. Centre is hydrophobic so no water-solubles can diffuse through. Small, non-polar, and water can diffuse through

Cholesterol: Stability. Present in all cell membranes except bacteria. Fits between ppl. Bind to hydrophobic tails, pack more closely together. Restricts movement, less fluid, more rigid. Maintains shape of animal cells, because no cell wall. Hydrophobic regions create further barrier to polars.

Below 0C: Not much energy, cant move very much. Packed closely together, membrane is rigid. Channel proteins, carrier proteins denature, increasing permeability of membrane. Ice crystals may form and pierce membrane, making highly permeable when thaws.

0 to 45C: Ppl can move around, not packed as tight together, partially permeable. as temp increases ppl move more because more energy, more permeability.

45C+: Ppl bilayer melts, membrane more permeable. Water inside cell expands, more pressure on membrane. Channel proteins, carrier proteins denature, no control over what enters/leaves cell, increases permeability.

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Required Practical 4

Beetroot cells contain pigment betalain, higher permeability, more leaks out.

1. Cut 5 equal sizes of beetroot with scalpel on cutting board. Rinse to remove any pigment released when cutting. 2. Add to different test tubes, each containing 5cm3 water. Use pipette to measure water. 3. Place each test tube at water bath at different temperature for same length of time. 4. Remove beetroot, leaving coloured liquid. 5. Colorimeter passes light of specific wavelength through liquid and measure how much light absorbed. 6. Switch on and allow 5mins to stabilize. Use blue filter or wavelength 470nm. 7. Add distilled water to cuvette so 3/4 full. Put inside colorimeter not on frosted side. Calibrate to 0. 8. Use pipette to transfer sample of beet water to clean cuvette. 9. Put in colorimeter and record absorbance. 10. Repeat for other 4. 11. Higher absorbance, more pigment released, higher permeability of membrane.

Could do with solvent concentration instead of temperature.

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Net movement of particles from area of higher to lower concentration down concentration gradient. Will diffuse both ways, but net movement to area of lower until equillibreum. Passive process. Particles can diffuse across cell membranes as long as can move freely. When molcules diffuse directly is simple diffusion. 

O2, CO2 diffuse easily bc theyre small, so can pass through spaces between phospholipids. Non-polar, so soluble in lipids, so dissolve in hydrophobic bilayer.

Concentration gradient: Higher=faster. As diffusion occurs, difference in conc decreases until equilibreum. Diffusion slows over time.

Thickness of exchange surface: Thinner=shorter distance particles travel, faster rate of diffusion.

Surface area: Larger surface area, faster rate of diffusion.

Epithelial cells have microvilli which give larger SA , means more particles exchanged in same amount of time, increased rate of diffusion. 

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Facilitated Diffusion

Larger molecules, amino acids, glucose, diffuse extremely through ppl bilayer bc theyre so big. Charged particles, ions, polars, diffuse slowly bc theyre water-soluble, and centre of bilayer is hydrophobic. They go through carrier/channel proteins. Passive process.

Carrier proteins (CP): Move large molecules across membrane, down conc gradient. Different CP facilitate diffusion of different molecules. Large molecule attaches to CP in membrane. Protein changes shape. Releases molecule on opposite side of membrane.

Channel proteins (ChP): Form pores in membrane for charged particles. Different channel proteins facilitate diffusion of different charged particles. Aquaporins allow FD water through cell membranes. Kidney cells. Allows reabsorption of water otherwise excreted by body.

Concentration gradient: Higher=faster until equilibreum, then levels off

No. of Ch/CP: Once all in use, FD cant occur any faster, even if CG increases

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Diffusion of H2O across partially permeable membrane from area of higher to lower water potential (lots of h2o to low h2o). WP: potential of water molecules to diffuse out of or into solution.

Pure water has 0. Adding solutes to pure water lowers WP, WP of any solution always negative. More negative=stronger conc of solutes in solution. 

Isotonic: Same WP, no net movement

HypErtonic: Solution with lower WP than cell. H2O net movement EXITS cell. Lysis, Crenation.

HypOtonic: Solution with higher WP than cell. H2O net movement INTO cell

WP gradient: Higher gradient, faster rate of osmosis. As osmosis takes place, difference in WP on either side decreases, so rate levels off over time.

Thickness of exchange surface: Thinner, faster

SA of exchange surface: Larger, faster

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Investigating Water Potential - RP3

1. Serial dilution: Set of solutions that decrease in conc by same factor each time.

1. Initial 2M of sucrose, diluting by factor of 2. Line up 5 test tubes in rack. 2. Add 10cm3 2M to 1st test tube, 5cm3 distilled water to other 4. 3. Use pipette and draw 5cm3 of 1st test tube, add to 2nd and mix. This is 1M. 4. Repeat 3x to create solutions 0.5M, 0.25M, 0.125M.

2. Measuring change in mass

1. Cork borer cuts potatoes into identical size chips, 1cm. Divide into groups of 3, measure mass of each group. 2. One group in each test tube, 20mins+. 3. Remove and pat dry gently with paper towel. Weigh each group again, record. Calculate % change. They gain mass in higher WP than chips, lose in solutions with lower WP.

3. Calibration Curve

Plot percentage change in mass against conc of sucrose solution. Then use to determine WP of potato cells.

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Active Transport (AT)

AT uses energy to move molecules and ions across plasma membrane, against conc gradient, low to high conc. Carrier proteins, co-transporters.

CP: Molecules attaches to CP, protein changes shape and moves molecule across membrane, releasing on other side.

Requires energy, ATP. Undergoes hydrolysis, into ADP and Pi. Releases energy so solutes can be transported.

CT: Type of CP. Bind 2 molecules at a time. CG of 1 molecule used to move other molecule against its own CG. 

Co-Transport and Absorption of Glucose: Glucose absorbed into bloodstream in small intestine. In mammalian ileum, conc of glucose too low for glucose to diffuse into blood, so glucose absorbed from lumen by co-transport.

1. Na ions AT'd out of epithelial cells into blood via NaK pump. Creates CG, now higher conc of Na ions in lumen than inside cell. 2. Na ions diffuse into epithelial cell, down CG, via Na-glucose co-transporter proteins. Co-transporter carries glucose into cell with Na. Conc of glucose inside cell increases. 3. Glucose diffuses out of cell into blood, down CG through PC via FD.

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AT (2)

Speed of individual CP - Faster they work, faster rate of AT

No. of CP present - More, faster

Rate of respiration in cell and availability of ATP. If respiration inhibited, AT cant take place 

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