Chapter 5


Plasma membrane structure

Membranes are structures that seperate the contents of cells from their enviroment and areas within a cell, the formation of seperate membrane bound areas is called compartmentalisation and is vital to a cell as metabolism involves many incompatible reactions, so containing them in different parts of a cell allows the specific conditions of each reaction to be met.

All membranes have the same basic structure, the outer membrane of a cell is called the plasma membrane, membranes are formed from a phospholipid bilayer, the hydrophillic phosphate heads form both the inner and out surface of the membrane, sanwhiching the fatty acid tails of the phospholipids tp form a hydrophobic core. Bilayers are perfectly suited for as membranes as the hydrophillic heads can interact with water and as they are on both sides of the bilayer these membranes can seperate aqueous enviroments.

The theory of cell membranes is called the fluid mosiac model, in which proteins occupy various positions and the phospholipids are free to move within the layer relative to each other.  

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Intrinsic proteins

Intrinsic (integral) proteins are transmembrane proteins that are embedded in both layers of the  membrane, they have amino acids, hydrophobic r-groups on their external surfaces to interact with the core to keep them in place. Channel and carrier proteins are both involved in transport across the membrane.

  • Channel proteins provide a hydrophillic channel that allows diffusion of polar molecules down a concentration gradient, they are held in position by interactions between the hydrophobic core and the hyrdrophobic r-goups on the porteins surface.
  • Carrier proteins have an important role in both passive and active transport into cells, which can involve the shape of the carrier protein changing.

Glycoproteins are intrinsic proteins that have attached sugar chains of varying shapes and sizes that play a role in cell adhesion and in cell signalling, examples include: Receptors for neurotransmitters at nerve synapses, binding prevents or triggers an impulse in the next neurone. Receptors for peptide hormones, including insulin and glucagpn.

Glycolipids: These are similar to glycoproteins which act as cell markers and antigens and can by recognised by the body's immune system as self or non self

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Extrinsic proteins and cholesterol

Extrinsic (peripheral) proteins are present in one side of the bilayer. They normally have hydrophillic r-groups on their outer surfaces and interact with the polar heads of the phospholipids or intrinsic proteins, can be present in either or between layers. 

Cholesterol is a lipid with a hydrophillic end and hydrophobic enf and regulates the fluidity of membranes. Cholesterol molecules are positioned between phospholipds in a bilayer, the hydrophillic end interacting with the the tails pulling them together. In this way cholesterol adds stability to membranes without making them too rigid, these molecules prevent the phospholipid molecules from grouping too closely together and crystallising.

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Factors affecting membrane structure

Temperature: phospholipids in a cell membrane are constantly moving, when temperature is increased they will have more kinetic energy so will move more, making the membrane more fluid as it begins to lose its structure, if the temperature continues to increase the cell will eventually break down completely. This loss of structure increases the permeability of membranes making it easier for particles to cross it. Carrier and channel proteins in the membrane will be dentatured so membrane permeability will be affected.

Solvents: water, a polar solvent is essential for the formation of the bilayer, many organic solvents are less polar than water eg benzene. Organic solvents will dissolve membranes, disrupting cells, whihc is why alcohols are used as antiseptic. when the membrane is disrupted it becomes more fluid and permeable, some cells need intact membranes for their function, eg transmission of nerve impulses, which is what affecrs someones behaviour when they are drunk.

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Investigating membrane permeability

Beetroot cells contain betalain which is a red pigment that gives them their distinct colour, because of this they are useful for affecting how temperature affects membrane permeability as the pigment is released when the membrane is disrupted, the amount of pigment released in a solution is related to the disruption. To investigate this you can cut equal squares of beetroot, wash and dry them, and place them in water and put each one in a water bath of a different temperature and then measure the absorbance of each sample using a colorimeter.

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Passive movement utilises energy from the natural motion of particles rather than energy from another source. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, it is a passive process and will continue until reaching equilibrium. particles move at high speeds and are constantly colliding, meaning over short distances diffusion is fast, but is slow over longer distances, which is why cells are generally microscopic as in order for diffusion to be an efficient method of transport it needs to be over short distances. 

Factors affecting diffusion:

  • Temperature: the higher the temperature the higher the rate of diffusion as the particles have a greater kinetic energy so move faster.
  • Concentration difference: The greater the difference in concentration between two regions the faster the rate of diffusion becuase the overall movement will be larger.

A concentration difference is said to be a concentration gradient which goes from high to low concentration, diffusion takes place down a concentration gradient, it takes much more energy to move up a concentration gradient. 

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Diffusion across membranes

Diffusion across membranes involved particles passing thriugh the phospholipid bilyaer and can only happen if the membrane is permeable to the particles, non polar molecules like oxygen can diffuse easily down a concentration gradient, the hydrophobic interior repels substances with a positive or negative charge, polar molecules can diffuse but at a slow rate, small ones faster than large ones.The rate at which particles diffuse across membranes is affected by surface area and the thickness of the membrane.

Facilitated diffusion: Membranes contain channel proteins through which polar and ionic molecules can pass through, it can also involve carrier proteins chaning shape when a specific molecule binds to them and doesn't require external energy. The rate of facilitated diffusion is dependant on the temperature, concentration gradient, membrane surface area, thickness and the number of channel and carrier proteins present.

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Dialysis tubing

Dialysis tubing is used as a substitute membrane in practical investigations, it is partially permeable wiht pores similar to those in membranes, smaller molecules can diffuse through but larger molecules such as starch cannot. A model cell can be stimulated by tying one end up and filling it with a solution, and placing it in another solution containing different concentrations of solute molecules. The change between the solutions can be timed and used to calculate the rat eof diffusion.

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Active Transport

Many biological processes depend on a concentration gradient which in order ofr it to be maintained, particles must  be moved up it faster than the rate of diffusion, this is called active transport. Active transport is the movement of particles from a region of lower concentration to a region of higher concentration. This process requires energy and carrier proteins. External energy is needed because the particles are being moved up a concentration gradient, metabolic energy is supplied by ATP. Carrier proteins span the membranes and act as 'pumps'. 

  • Molecule or ionto be transported binds to receptors in the channel of the carrier protein on the outside of the cell..
  • On the inside of the cell ATP binds to the carrier protein and is hydrolysed to ADP and phosphate.
  • Binding of the phosphate molecule to the carrier protein causes it to change shape, opening up to the inside of the cell.
  • The molecule or ion is released to the inside of the cell.
  • The phosphate molecule is released from the carrier protein and recombines with ADP.
  • The carrier protein returns to its normal shape.
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Bulk Transport

Bulk transport is another form of active transport, large molecules such as enzymes and hormones cannot fit through carrier proteins so are moved by cell bulk transport, via endo and exo cytosis.

  • Endocytosis: Bulk transport of material into cells, pinocytosis for liquids and phagocytosis for solids. First the cell surface membrane bends inwards when it comes into contact with the material to be transported, the membrane enfolds the material until it becomes a vesicle as the membrane fuses around it. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing.
  • Exocytosis: Bulk transport of materials out of the cell, vesicles, usually formed in the Golgi apparatus, move towards and fuse with the cell surface membrane, the contents of the vesicle are hten released outside of the cell.

Energy in the form of ATP is required for vesicle movement along the cytoskeleton, changing the shape of the cells to engulf materials and the fusuin of membranes and vesicles. 

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Osmosis and water potential

Osmosis is a specifc type of diffusion of water across a partially permeable membrane and is a passive process.

A solute is a substance dissolved in a solvent forming a solution. The amount of solute in a certain volume of aqueous solution is the concentration. Water potential is the pressure exerted by water molecules as they collide with a membrane or container, the symbol is Ψ  and is measured in pascals and kilopascals. Oure water is defined as having a water potential of 0 kPa, this is the highest possible value for water potenital, as the presence of a solute lowers water potenital, the more concentrated the solution, the lower the water potential. When solutions of different concentrations ae seperated by a partially permeable membrane the water molecules can move between the solutions but the solute cannot. There will be a net movement of water from the solution of higher water potential (less concentrated) to the solution wiht lower water potential (more concentrated). This will continue until the water potential is equal on both sides of the membrane.

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Effects of osmosis on animal cells

The diffusion of water into a solution leads to an increase in volume which results in an increase in hydrostatic pressure and has the same unit as water potential, at a cellular levekl this change in pressure is large and can be damaging.

If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis increasing the hydrostatic pressure until the cell surface membrane cannot withstand the pressure and breaks as the cell bursts - cytolysis.

If an animal cell is placed in a solution with a lower water potential than its cytoplasm it will lose water to the solution by osmosis down the water potential gradient which will cause a reduction in volume and cause the cell surface membrane to pucker - crenation.

To prevent either cytolysis or crenation, multicellular animals usually have control mechanisms to make sure their cells are constantly surrounded by aqueous enviroments with an equal water potential. 

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Effects of water potential on plant cells

Like animal cells, plant cells contain a variety of solutes, mainly dissolved in a large vacuole, however unlike animals plants are not able to control the water potential of the fluid around them. Plants have a strong cellulose wall surrounding the cell surface membrane, when water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell wall, this pressure against the cell wall is called turgor. As the turgor pressure increases it resists the entry of water and is said to be turgid.

When plant cells are placed in a solutoin with a lower water potential than their own, water is lost from the cells by osmosis. This leads to a reduction in the volume of of the cytoplasm, which eventually pulls the cell surface membrane away from the cell wall - plasmolysed.  

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