Cell membranes and transport

• You should be able to:• describe the structure of cell membranes• explain the functions of the molecules that• make up cell membranes• explain how substances enter and leave cells• find the water potential of plant tissues• through experiment

Liposome

• A liposome is a tiny bubble (vesicle), made out of the same material as a cell membrane. Liposomes can be filled with drugs, and used to deliver drugs for cancer and other diseases.

• a microscopic spherical particle formed by a lipid bilayer enclosing an aqueous compartment.

• The name liposome is derived from two Greek words: 'Lipos' meaning fat and 'Soma' meaning body.

• Liposomes were first described by British haematologist Dr Alec D Bangham FRS in 1961 (published 1964), at the Babraham Institute, in Cambridge.

Cell membrane It is composed of four different types of molecules:

• Phospholipids• Cholesterol• Proteins• Carbohydrates

Phospholipids

• Phospholipids form the bilayer, which is the basic structure of the membrane.

• Because the tails of phospholipids are non-polar, it is difficult for polar molecules, or ions, to pass through membranes, so they act as a barrier to most water-soluble substances.

• For example, water-soluble molecules such as 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.

• They may move about in the phospholipid bilayer, activating other molecules such as enzymes.

• Alternatively, they may 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 the release of calcium ions from storage in the ER, which in turn brings about exocytosis of digestive enzymes from pancreatic cells.

Micelles

• If the phospholipids are shaken up with water, they can form stable ball-like structures in the water called micelles.

• Here all the hydrophilic heads face outwards into the water, shielding the hydrophobic tails, which point in towards each other.

Structure of membranes

• The phospholipid bilayer is visible using the electron microscope at very high magnifications of at least × 100 000.

Cholesterol • Cholesterol molecules are made up of four rings of

hydrogen and carbon atoms. They are hydrophobic and are found among the hydrophobic tails in the lipid bilayer.

• Cholesterol molecules are important for maintaining the consistency of the cell membrane.

• They strengthen the membrane by preventing some small molecules from crossing it.

• Cholesterol molecules also keep the phospholipid tails from coming into contact and solidifying. This ensures that the cell membrane stays fluid and flexible.

Channel protein

• A channel protein is a protein that allows the transport of specific substances across a cell membrane.

• They are intrinsic proteins, so span across the whole membrane They basically make a channel/pathway/hole for stuff to go through

• The channel that the channel proteins make, is full of water. This means only water soluble substances can pass through.

Carrier proteins.

• carrier proteins bind solute on one side of a membrane, undergo conformational changes, and release them on the other side of the membrane.

• These proteins can mediate both active and passive transport.

• carrier proteins are glycoproteins.

Fluid mosaic model.

• https://www.youtube.com/watch?v=Qqsf_UJcfBc

• In 1972, two scientists, Singer and Nicolson, used all the available evidence to put forward a hypothesis for membrane structure. They called their model the fluid mosaic model.

• It is described as ‘fluid’ because both the phospholipids and the proteins can move about by diffusion

• The phospholipid bilayer has the sort off fluidity.• The phospholipids move sideways, mainly in their own

layers. • Some of the protein molecules also move about within

the phospholipid bilayer, like icebergs in the sea.• Others remain fixed to structures inside or outside the

cell.• The word ‘mosaic’ describes the pattern produced bythe scattered protein molecules when the surface of the membrane is viewed from above.

• The 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 that surrounds the membranes.

• If the lipids are more unsaturated the more fluid the membrane.

• This is because the unsaturated fatty acid tails are bent and therefore fit together more loosely.

• Fluidity is also affected by tail length: the longer the tail, the less fluid the membrane.

• As temperature decreases, membranes become less fluid,

Some organisms which cannot regulate their own temperature, such as bacteria and yeasts, respond by increasing the proportion of unsaturated fatty acids in their membranes.

Intrinsic and extrinsic proteins

• Extrinsic membrane proteins are entirely outside of the membrane.

• Intrinsic membrane proteins are embedded in the membrane. Many of them extend from one side of the membrane to the other and are referred to as transmembrane proteins.

• Most of the intrinsic protein molecules float like mobile icebergs in the phospholipid layers, although some are fixed like islands to structures inside or outside the cell and do not move about.

• second type of protein molecule is the extrinsic protein (or peripheral protein). These are found on the inner or outer surface of the membrane.

Glycoproteins and Glycolipids

• Lipid and proteins on the cell membrane surface often have short carbohydrate chains protruding out from the cell surface, known as glycolipids and glycoproteins.

Glycocalyx.

• The carbohydrate chains form a sugary coating to the cell, known as the glycocalyx.

• It provides cushioning and protection for the plasma membrane, and it is also important in cell recognition. Based on the structure and types of carbohydrates in the glycocalyx, your body can recognize cells and determine if they should be there or not. The glycocalyx can also act as a glue to attach cells together.

Endocytosis

• https://youtu.be/DuDmvlbpjHQ• https://youtu.be/JnlULOjUhSQ

Antigens,

• Some glycolipids and glycoproteins act as cell markers or antigens, allowing cell–cell recognition.

• Each type of cell has its own type of antigen, rather like countries with different flags.

Cells communicate by sending and receiving chemical and electrical signals. This molecular conversation allows the cells in your body to coordinate their activities.

• Cell-cell signaling involves the transmission of a signal from a sending cell to a receiving cell. However, not all sending and receiving cells are next-door neighbors.

• Cells communicate by means of extracellular signaling molecules that are produced and released by signaling cells.

• These molecules recognize and bind to receptors on the surface of target cells where they cause a cellular response by means of a signal transduction pathway.

• Cells have proteins called receptors that bind to signaling molecules and initiate a physiological response.

• Different receptors are specific for different molecules.

• Typical signaling pathway starts with the signal arriving at a protein receptor in a cell surface membrane.

• The receptor is a specific shape which recognizes the signal. Only cells with this receptor can recognize the signal.

• The signal brings about a change in the shape of the receptor.

• Changing the shape of the receptor allows it to interact with the next component of the pathway, so the message gets transmitted.

• G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior.

• G proteins acts as a switch to bring about the release of a ‘second messenger’,

• Apart from the secretion of chemical signals, direct cell-cell contact is another mechanism of signaling.

• This occurs, for example, during embryonic development and when lymphocytes detect foreign antigens on other cells.

Movement of substances intoand out of cells

• A phospholipid bilayer around cells makes a very effective barrier, particularly against the movement of water-soluble molecules and ions.

• The aqueous contents of the cell are therefore prevented from escaping.

• There are five basic mechanisms by which this exchange is achieved:

• Diffusion• Facilitated diffusion• Osmosis• Active transport and bulk transport.

Diffusion

• The process by which molecules spread from areas of high concentration, to areas of low concentration in order to reach equilibrium.

• The rate at which a substance diffuses across a membrane depends on a number of factors.

• The ‘steepness’ of the concentration gradient - The greater the difference in concentration, the greater the difference in the number of molecules passing in the two directions, and hence the faster the rate of diffusion.

• Temperature. At high temperatures, molecules and ions have much more kinetic energy than at low temperatures.

• They move around faster, and thus diffusion takes place faster.

• The surface area across which diffusion is taking place.

• The greater the surface area, the more molecules or ions can cross it at any one moment, and therefore the faster diffusion can occur.

• The nature of the molecules or ions - Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules.

• Non-polar molecules, such as glycerol, alcohol and steroid hormones, diffuse much more easily through cell membranes than polar ones, because they are soluble in the non-polar phospholipid tails.

• The respiratory gases – oxygen and carbon dioxide – cross membranes by diffusion.

• They are uncharged and non-polar, and so can cross through the phospholipid bilayer directly between the phospholipid molecules.

• Water molecules, despite being very polar, can diffuse rapidly across the phospholipid bilayer because they are small enough.

Facilitated diffusion

• Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them.

• There are two types of protein involved, namely channel proteins and carrier proteins. Each is highly specific, allowing only one type of molecule or ion to pass through it.

• Channel proteins are water-filled pores. They allow charged substances, usually ions, to diffuse through the membrane.

• Most channel proteins are ‘gated’. This means that part of the protein molecule on the inside surface of the membrane can move to close or open the pore, like a gate.

• This allows control of ion exchange.

• channel proteins have a fixed shape, carrier proteins can flip between two shapes.

• As a result, the binding site is alternately open to one side of the membrane, then the other.

• The rate at which this diffusion takes place is affected by how many channel or carrier protein molecules there are in the membrane, and, in the case of channel proteins, on whether they are open or not.

Cystic fibrosis

• is caused by a defect in a channel protein that should be present in the cell surface membranes of cells lining the lungs.

• This protein normally allows chloride ions to move out of the cells.

• If the channel protein is not• correctly positioned in the

membrane, or if it does not• open the chloride channel as and

when it should, then the• chloride ions cannot move out.

Osmosis

• Osmosis is a special type of diffusion involving water molecules only.

• This movement of water molecules from a dilute solution to a concentrated solution, through a partially permeable membrane, is called osmosis.

Water potential

• The tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action.

• Water potential is the 'measure of the ability of water molecules to move freely in solution'.

• All this means is that is a solution of pure water where there is no solute, all of the water molecules are free to move, so the water potential is high.

• If a solute is added to the solution, it is attracted to the water molecules, so those water molecules can no longer move freely and the water potential is lower.

• In osmosis, water molecules move down the water potential gradient, from a high water potential to a lower water potential.

Factors influencing the water potential

• The major factors influencing the water potential are

• Concentration• Pressure• Gravity

• Water always moves from a region of high water potential to a region of low water potential

• This will happen until the water potential is the same throughout the system, at which point we can say that equilibrium has been reached.

• A solution containing a lot of water (a dilute solution) has a higher water potential than a solution containing only a little water (a concentrated solution).

Redefining osmosis

• Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Solute potential Ѱs

• The presence of solute molecules in a solution lowers its water potential.

• The greater the concentration of solutes the lower is the water potential.

• This change in water potential as a consequence of the presence of solute molecules is called Solute potential.

Pressure potential Ѱp

• Pressure potential is based on mechanical pressure.• Pressure potential increases as water enters a cell.• As water passes through the cell wall and cell

membrane, it increases the total amount of water present inside the cell, which exerts an outward pressure that is opposed by the structural rigidity of the cell wall.

• By creating this pressure, the plant can maintain turgor, which allows the plant to keep its rigidity.

• Without turgor, plants will lose structure and wilt.

Osmosis in animal cells

• If the water potential of the solution surrounding the cell is too high, the cell swells and bursts.

• If it is too low, the cell shrinks, This shows one reason why it is important to maintain a constant water potential inside the bodies of animals.

Osmosis in Plant cells

• Plant cells are surrounded by rigid cellulose walls, (unlike animal cells), but plant cells still take in water by osmosis when placed in pure water.

• However, plant cells do not burst because their cellulose cell walls limit how much water can move in. The cell walls exert pressure, called turgor pressure, as the cells take up water. This is useful as plants do not have a skeleton.

• For plant cells water potential is a combination of solute potential and pressure potential.

• This can be expressed in the following equation:

• ψ = ψs + ψp

Protoplast

• The living part of the cell in plant cells I.e., excluding cell wall.

Plasmolysis

• https://youtu.be/zVvHn6Sj9PQ• The contraction of the protoplasm of cells

within plants due to the loss of water through osmosis. It is when the cell membrane peels off of the cell wall and the vacuole collapses when placed in a hypertonic environment.

Incipient plasmolysis.

• The stage of plasmolysis at which the first sign of shrinkage of cell contents from cell wall becomes detectable is called incipient plasmolysis. At incipient plasmolysis the protoplast has just ceased to exert any pressure against the cell wall, so the cell is flaccid.

Active transport

• Active transport describes what happens when a cell uses energy to transport something.

• Active transport is against concentration gradient, means they are pumping something from areas of lower to higher concentration.

• The energy required for active transport is supplied by the molecule ATP.

• The energy is used to make the carrier protein change its shape, transferring the molecules or ions across the membrane in the process.

sodium–potassium (Na+ – K+) pump

• The Sodium-Potassium Pump uses energy to transport Sodium and Potassium ions in and out of the cell. It plays a vital role in maintaining a cell's homeostasis.

• The pump is powered by a molecule of ATP. The ATP allows the shape of the pump to change, emptying its contents either into or out of the cell.

• Three sodium ions form inside the cell bind to the pump.

• The Phosphate group form a molecule of ATP binds to the pump.

• The pump changes shape and the sodium ions are released outside the cell.

• Two potassium ions bind to the pump. • The phosphate group is released form the pump and

the pump again changes shape and releases the ions into the inside of the cell.

• Active transport is important in reabsorption in the kidneys, where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules.

• It is also involved in the absorption of some products of digestion from the gut.

• In plants, active transport is used to load sugar from the photosynthesizing cells of leaves into the phloem tissue for transport around the plant and to load inorganic ions from the soil into root hairs.

Bulk transport

Endocytosis

PinocytosisPhagocytosis

Exocytosis

• Exocytosis is the reverse of endocytosis and is the process

• by which materials are removed from cells

• https://youtu.be/c8a3e7sCr24