cell membranes and transport

All living cells are surrounded by a very thin membrane – cell surface membrane.

• controls exchange of material such as nutrients and waste product between cell and environment.
• regulates transport across membranes of organelles
• enables cells to receive hormone messages  

Phospholipid – a lipid containing a phosphate group in its molecule.
Phospholipids – little bags inside membrane where chemicals can be isolated from the external environment. They are membrane-bound compartments that are known as cells and organelles.
If phospholipid molecules are spread over the surface of water, they form a single layer with their heads in the water because they are polar (hydrophilic) and tails out wards  since they are non-polar (hydrophobic). Polar refers to the uneven distribution of charge which occurs in some molecules.
If they are shaken up with water, ball-like structures can form called micelles. All hydrophilic heads face outwards into the water, shielding the hydrophilic tails, which point towards one another.
Bilayers can also be formed, these are two-layered structures arranged in sheets. This phospholipid bilayer is the basic structure of membranes.

Structure of membranes
Phospholipid bilayer is visible with a electron microscope at very high magnification. It is 7nm wide
Singer and Nicolson put together a hypothesis for membrane structure – fluid mosaic model. Fluid because both phospholipids and the proteins can move about by diffusion. The phospholipids move sideways, mainly in their own layers. Some of the protein molecules move about, other remain fixed to structures inside and outside the cell.  Mosaic describes the patterns produced by scattered protein molecules when surface of membrane is viewed from above.

Features of the fluid mosaic model
The membrane is a double layer of phospholipid molecules. Individual phospholipid molecules move about by diffusion within their own monolayers. The phospholipid tails point inwards facing each other and forming a non polar hydrophobic interior. The phospholipid heads face the aqueous medium.
The more unsaturated the phospholipid tails are, the more fluid the membrane, because the unsaturated fatty acid tails are bent and therefore fit together more loosely. Fluidity also affected by tail length, the longer the tail the less fluid the membrane. As temperature decreases, membrane becomes less fluid, some organisms can regulate their own temperature (bacteria, yeast), by increasing the proportion of unsaturated fatty acids in their membranes.
Proteins are found embedded within the membrane – intrinsic (integral) proteins. Found in the inner, outer layer, if spanning the whole membrane – transmembrane proteins, the hydrophobic regions which cross the membrane are often made up of one or more α-helical chains. Intrinsic proteins have hydrophilic and hydrophobic regions. They stay in the membrane because the hydrophobic regions made from hydrophobic amino acids are next to the hydrophobic fatty acid tails and are repelled by watery environment on both sides of membrane. The hydrophilic regions are made of hydrophilic amino acids and are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment or line the hydrophilic pores that pass through the membrane.
Extrinsic proteins (peripheral) are found on the inner or outer surface of the membrane. Many are bound to the intrinsic proteins. Some are held by binding to molecules inside or outside the cell,  or to the phospholipids.
Proteins and lipids have short, branching carbohydrate chains attached to the side of molecule which faces the outside of membrane, forming glycoprotein or glycolipids. Molecules of cholesterol are also found in the membrane.

Role of phospholipids
Phospholipids for a bilayer which is the basic structure of the membrane. Because tails of phospholipids are nonpolar it is difficult for polar molecules or ions to pass through membranes – they act as a barrier for most water soluble substances. Sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot enter the cell.
Some phospholipids can be modified chemically to act as signaling molecules, the move about in the bilayer activating other molecules like enzymes. They can also be hydrolyzed to release small water-soluble  glycerol-related molecules. These diffuse through the cytoplasm and bind to specific receptors. One such system results in release of calcium ions from storage in the endoplasmic reticulum which brings exocytosis of digestive enzymes from pancreatic cells.

Role of cholesterol
It is a relatively small molecule, it has hydrophilic heads and hydrophobic tails. They fit between the phospholipid molecules with their head at the membrane surface. In animal cells, cell surface membranes contain as much cholesterol as phospholipids. It is less common in plant cells and absent in prokaryotes. There are other compounds with similar function.
At low temperature, cholesterol increase the fluidity of membrane, preventing it from becoming too rigid This is because it prevents close packing of phospholipid tails. Increased fluidity means cells can survive at lower temperature. Interaction of phospholipid tails with cholesterol molecules helps stabilise cells at high temperature when the membrane could be too fluid. Cholesterol is also important for the mechanical stability of membranes. WIthout it, the membranes quickly break and cells burst open. The hydrophobic regions of the cholesterol molecules help prevent ions or polar molecules from passing through the membrane.This is important in the myelin sheath (made from many layer of cell surface membrane) around the nerve cells where leakage of ions would slow down nerve impulses.

Roles of glycolipids, glycoproteins and proteins
Many lipids on the outer surfaces of cell surface membranes and all proteins have short carbohydrate chains attached to them. These ‘combination’ molecules are known as glycolipids and glycoproteins.  The carbohydrate chain stick out like antennas into the watery fluid surrounding the cell. There, they form hydrogen bonds with the water molecules and help stabilise the membrane structure. The carbohydrate chains for a sugary coating called glycocalyx. In animal cells in is formed mainly from glycoproteins, in plant from glycolipids.
The carbohydrate chains glycoproteins and glycolipids to act as receptor molecules. Different cells have different receptors, depending on their function.

• Signaling receptors – part of the signaling system that coordinates the activities of the cell. They recognize messenger molecules like hormones and neurotransmitters (chemicals that allow nerve impulses to pass from one cell to the other). When messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell, ex glucagon in liver cells.
• Endocytosis receptors – bind to molecules that are parts of the structures to be engulfed by cell the cell surface membranes.
• Binding receptors – bind cells to others (cell adhesion) in tissues and organs of animals.

Some glycolipids and glycoproteins act as cell makers or antigens, allowing cell-cell recognition. Each type of cell has its own type of antigen.
Many proteins act as transport proteins providing hydrophilic channels for ions and polar molecules to pass through the membrane. Two types – channel protein and carrier proteins.  Each transport protein is specific to a particular type of ion or molecule therefore the types can be controlled when enter or leave.
Other membrane proteins are enzymes that catalyse the hydrolysis of molecules such as disaccharides.
Some proteins on the inside of cell surface membrane are attached to a system of protein filaments inside the cell – cytoskeleton. They help decide and maintain the shape of the cell. They are also involved in the change of shape when cell moves.
Proteins play an important role in the membranes of organelles, membranes of mitochondria and chloroplasts are involved in the process of respiration and photosynthesis .

Cell signaling
Getting a message from one place to another. Cells need it to be able to respond appropriately to their environments. This is possible because of a complex range of signalling pathways which coordinate the activities of cells, even at large distances.
Signaling pathway includes receiving a stimulus or signal, transmitting the message and making an appropriate response.  Transduction – conversion of original signal to a message that is then transmitted. Transmitting the message involves crossing barriers such as cell surface membranes. Signalling molecules are usually very small for easier transport.

Movement of substances into and out of the cell
Phospholipid bilayer make a very effective barrier around the cells, against the water soluble molecules and ions. The aqueous contents of the cell are prevented from escaping. There are five basic mechanisms by which exchange is achieved: diffusion, facilitated diffusion, osmosis, active transportation and bulk transportation.

Diffusion
The net movement as a result of random motion of the molecules of a substance from regions of high to lower concentration. The molecules or ions move down a concentration gradient. The random movement is caused by the natural kinetic energy of the molecules or ions.  As a result of diffusion, molecules or ions tend to reach an equilibrium where they are evenly spread within the volume of space. Some molecules or ions can pass through living cell membrane by diffusion.
Can be shown with visking tube (dialysis tubing) because it is partially permeable. Has molecular sized pores which let through small molecules like glucose but not starch. Fill with mixture of starch and glucose. Suspend in a tube of boiling water, test water for glucose with Benedict.
Rate at which substances diffuse across membrane depends on factors such as:

• The steepness of the concentration gradient. The greater the difference in concentration, the greater the difference in the number of molecules passing the two directions and the faster the rate of diffusion.
• Temperature – at high temperatures, molecules and ions have more kinetic energy, they move around faster and therefore diffusion takes place faster.
• The surface area across which diffusion is taking place. The greater the surface area, the more molecules can cross it simultaneously which results in faster diffusion. Surface area can be increased by foldings such as microvilli in intestine or cristae in mitochondria.

The surface area: volume ratio decreases as the size of a 3d object increases.

• The nature of molecule or ion. Large molecules require more energy to get them moving than small ones. Non polar molecules diffuse easier through cell surface membrane because they are soluble in the non-polar phospholipid tails. Water molecules are very small so diffuse across phospholipid bilayer.      

Investigate effect of size on diffusion
Different sizes of agar cubes (2×2, 1×1, 0,5×0,5), should be made with sodium hydroxide solution and universal indicator, put in an ice tray and cover with diffusion solution like hydrochloric acid. Measure time taken for the acid to completely change color or distance travelled in a given time.

Demonstrating diffusion
Piece of beetroot, placed into water of different temperatures or alcohol of different concentrations. Change in color of surrounding solution will be caused by diffusion of red pigment from the central vacuole where it is high concentration to low. A colorimeter can be used to compare the colors.     

Facilitated diffusion
Diffusion of a substance through transport proteins in a cell membrane; proteins provide hydrophilic areas that allow the molecules or ions to pass through membrane which would otherwise be less permeable to them.
Large molecules such as glucose or amino, or ions such as natrium or chloride acids cannot diffuse through the phospholipid bilayer. These can only cross the membrane with the help of certain protein molecules. There are two types of protein involved – channel and carrier proteins, they are highly specific. The proteins provide hydrophilic areas that allow the molecules to pass through the membrane which would otherwise be less permeable.
Channel proteins – water-filled pores, allow charged particles, usually ions to diffuse through membrane. Have a fixed shape. Most are ‘gated’, part on the inside surface of membrane can move to close or open, allowing exchange of ions, found in nerve cell membrane. One type allows sodium ions for action potential, the other allows potassium ions needed during recovery phase – repolarisation. Some channels occur in a single protein, others are formed by several combined.
Carrier proteins flip between two shapes, the binding site is alternately open to one side of the membrane, then the other.  When molecules diffuse across the membrane, direction of movement will depend on the relative cc on each side of the membrane; they will move up and down a cc gradient from higher to lower.
Rate at which facilitated diffusion will take place depends on how many channel or carrier proteins there are, in case of channel proteins – if they are open or not.
Fibrosis is caused by defect in a channel protein and it doesn’t let the chloride ions move out.       

Osmosis
Diffusion only involving water. Solute + solvent = solution; sugar + water = sugar solution.
Two solutions are separated by the partially permeable membrane.
Solute molecules are too large to get through the membrane, only water molecules can pass. Water molecules will tend to spread evenly between the solute molecules.
Movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane is osmosis. Net movement of water from region of higher water potential to region of lower potential through a partially permeable membrane as a result of random motion (diffusion).

Water potential
The tendency of water to move out of a solution depending on, noted by letter Ψ.

• How much water there is in relation to solutes
• How much pressure is being applied to it

Water always moves down a potential gradient until water potential is the same throughout the system, equilibrium has been reached. Pressure on a liquid increases water potential. Concentration of solution decreases water potential. Potential of pure water is 0, a solution has a negative value.

Solute and pressure potential
Solute potential – the extent to which the solute molecules decrease the water potential of solution.  The more solute there is, the lower the tendency if water to move out of the solution. Adding more water to a solution decreases its water potential. The greater the cc of solution the more negative the value, psi symbol used to show the solute potential, with a subscript -ψs, s is added.
Pressure potential – contribution of pressure to water potential. Increasing the pressure exerted on solution results in increased tendency of water to move out of it so it increases the water potential Ψs.

Osmosis in animal cells
It is important to maintain a constant water potential inside the bodies of animals. For ex if a red blood cell is surrounding by a solution with high water potential, the water will diffuse inside the rbc and it will burst; if the water potential is too low, water will diffuse outside of the cell and it will shrink.

Osmosis in plant cells
Water or solution has higher water potential than the plant, it enters the cell through cell surface membrane by osmosis; cell volume increases and the plant cell pushes back against the expanding protoplast (living part of cell inside the cell wall) so pressure starts to build up – pressure potential, it increases the water potential of cell until the water potential inside cell equal water potential outside the cell and equilibrium is reached. Cell wall prevents cell from bursting. When a plant cell is inflated with water it becomes turgid.
For plant cells, water potential is the solute potential combined with pressure potential  ψ=ψs+ψp
Plant cell wall is fully permeable. When water diffuses out of the cell by osmosis, protoplast shrinks and begins to pull away from the cell wall – plasmolysis, the cell is plasmolysed.
Point where pressure potential reached 0 and plasmolysis is about to occur is incipient plasmolysis.

Observing osmosis
Epidermis strips placed in a range of different sucrose solutions or sodium chloride. Strips then place on glass slides and observed with microscope. Plasmolysis may take several minutes.

Determining water potential of plant.
Samples of potato tissues allowed to come into equilibrium with different water potential solutions, changes of wither mass or volume should be recorded. Graph plotted allows to see which solution caused no change in volume or mass, this solution has the same water potential as in the plant tissue.

Active transportation
There is a concentration gradient in cells, with lower cc outside and higher inside. Ions in such cells must have accumulated against a concentration gradient. Active transport is responsible for that.
Specific carrier proteins use ATP (adenosine triphosphate) to change their shape, transferring ions across the membrane against the concentration gradient. Example of carrier protein is the sodium-potassium pump, which pumps three sodium ions out and allows two potassium ions into the cell for each ATP molecule used. Ions are positively charged so net result is that the inside of the cell become more negative than the outside – potential difference created across the membrane.
Active transportation – energy consuming transport of molecules or ions across the membrane against the concentration gradient, energy provided by ATP from cell respiration. It is important for reabsorption  in kidneys , absorption of products in the gut, load sugar from photosynthesising cells to leaves into phloem tissue for transport around the plant, load inorganic ions from soil into root cells.

Bulk transport
Active transport involving large nr of materials in (endocytosis) and out (exocytosis), requires energy.
Endocytosis, engulfing of material by cell surface membrane to form a endocytic vacuole. Phagocytosis is bulk uptake of solid materials, phagocytes white blood cells engulf bacteria and lysosomes fuse with phagocytic vacuole and digest the bacterium. Pinocytosis – bulk uptake of liquid, vesicles are often formed which are extremely small, process called macropinocytosis.
Exocytosis – material are removed from cell, secretion of digestive enzyme by the pancreas.