BIO - S2 - Transport Across Cell Membranes

4.1 Cell-Surface Membrane

They are a barrier between the cell and its environment  They are partially permeable --> they let some molecules through but not others  Fluid --> phospholipid molecules are constantly moving  Mosaic --> the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern 

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Phospholipids

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The molecules arrange themselves into a bilayer  The hydrophobic 'tail' --> repels water The hydrophilic ¡head' --> attracts water  Functions Allow lipid-soluble substances to enter and leave the cell Prevent water-soluble substances entering and leave the cell  Make the membrane flexible and self-sealing 

Cholesterol

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Cholesterol binds to the hydrophobic tails of the phopholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.  Gives stability to the membrane. Functions Reduce lateral movement of the other molecules including phospholipids  Make the membrane less fluid at high temperatures Prevent leakage of water and dissolved ions from the cell

Proteins

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Proteins - Functions Provide structural support  Allow active transport across the membrane through carrier proteins  Form cell-surface receptors for identifying cells  Help cell adhere together  Glycolipids - Functions --> A carbohydrate attached to a lipid  Help maintain the stability of the membrane  Help cells to attach to one another and so form tissues Glycoproteins - Functions --> A carbohydrate attached to a proteinAllow cells to recognise one another, for example lymphocytes can recognise an organism's own cells

Temperature and Membranes

Temperatures Below 0 ºC The phospholipids do not have much energy, so they cannot move very much They are packed closely together and the membrane is rigid  Channel and Carrier proteins denature, increasing the permeability of the membrane  Temperature between 0 and 45 ºC The phospholipids can move around and are not packed as tightly together The membrane is partially permeable  As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane  Temperatures above 45 ºC The phospholipid bilayer starts to melt and the membrane becomes more permeable  Water inside c¡the cell expands, putting pressure on the membrane  Channel and Carrier proteins denatured so they cannot control what enters or leaves the cell - this increases the permeability of the cell membrane 

4.2 Diffusion

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Caption: : Diffusion is the movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration

Diffusion is a passive process - no energy is needed for it to happen.Factor Affecting the Rate of Diffusion The concentration energy - The higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium. This means that diffusion slows down over time. The thickness of exchange surface - the thinner the exchange surface, the faster the rate of diffusion. The surface area - The larger the surface area, the faster the rate of diffusion

Facilitated Diffusion

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Caption: : Facilitated diffusion moves particles down a concentration energy, from higher to lower concentration

It is a passive process - it does not require energy  Large (amino acid, glucose)  and charged particles (ions and polar molecules) diffuse through carrier proteins or channel proteins  Factors Affecting The Rate of Facilitated Diffusion The concentration gradient --> The higher the concentration gradient, the faster the rate of facilitated diffusion. As equilibrium is reached, the rate of facilitated diffusion will level-off The number of channel or carrier proteins --> once all the proteins in a membrane are in use, facilitated diffusion cannot happen any faster, even if you increase the concentration energy.

Carrier Proteins

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A large molecule attaches to a carrier protein in the membrane. Then, the protein changes shape This releases the molecule on the opposite side of the membrane  

Channel Proteins

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Channel proteins form pores in the membrane for charged particles to diffuse through.  They allow specific water-soluble ions to pass through The channels are selective, each opening in the presence of a specific ion  If the particular ion is not present, the channel remains closed.

4.3 Osmosis

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Pure water has a potential of zero  Adding solutes to pure water lowers its water potential - so the water potential of any solution is always negative  The more negative the water potential, the stronger the concentration of solutes in the solution. Factors Affecting The Rate Of Osmosis The water potential gradient --> The higher the water potential gradient, the faster the rate of osmosis  The thickness of exchange surface --> The thinner the exchange surface, the faster the rate of osmosis. The surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis.

Caption: : Is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential

Isotonic

Isotonic

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water potential of external solution compared to cell solution is equal There is no change  Cells in an isotonic solution will not lose or gain any water 

Hypotonic

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Water potential of external solution compared to cell solution is higher (less negative) Net movement of water --> enter the cell The cells swells and bursts

Hypertonic

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Water potential of external solution compared to cell solution is lower (more negative) Net movement of water --> leaves the cell The cell shrinks

4.4 Active Transport

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Caption: : The movement of molecules or ions from an area of lower concentration to an area of higher concentration using ATP and carrier proteins

Active transport requires energyCarrier Proteins The carrier proteins span the plasma membrane The molecule or ion binds to the receptor sites on the carrier protein ATP undergoes a hydrolysis reaction, spliting into ADP and Pi (inorganic phosphate) The molecule or ion is then released to the other side of the membrane The Pi is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeate. The Pi then recombines with the ADP to form ATP during repiration.

Co-transporters

They bind two molecules at once The concentration energy of one molecule is used to move the other molecule against its own concentration energy

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Caption: : Co-transports are a type of protein

Co-transport and the absorption of glucose

Step 1 Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood This takes place in one type of protein (carrier protein) This creates a concentration gradient - there is now a higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells

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Step 2 Sodium ions diffuse into the epithelial cells, down their concentration gradient They do this via the sodium - glucose co-transporter proteins. The co-transporter carries glucose into the cell with the sodium As a result the concentration of glucose inside the cell increases.

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Step 3Glucose/amino acids diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion

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Factor Affecting The Rate Of Active Transport

The speed of individual carrier proteins - the faster they work, the faster the rate of active transport The number of carriers proteins present - the more proteins there are, the faster the rate of active transport The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport cannot take place.