molecules+of+life+summary+notes

This section wil be used for making a set of summary notes of relevant concepts. Each of you will be given (in class) a specific area to cover. You will then be able to read and edit each others work.
 * IMPORTANT!** If you do edit someone elses work, then write a comment in the box at the bottom of the page (while in edit mode) or post a comment in the **discussion** section on this page and explain the reasons behind the change.

You have 2 days to complete the task and then post your summary on this page in the space below.
 * Task**
 * Analyse and scrutinise the examination-style question ( **the secret question** ) you have been given.
 * Do not reveal the question to anyone else.
 * Think about which part of the course the question relates to (refer to the study design).
 * Write a concise paragraph of cohesive notes that would allow someone to be able to successfully answer the question.
 * Give your paragraph a suitable heading.
 * Your notes must not directly contain any part of the question within it. THIS IS IMPORTANT!
 * Follow up task**
 * Read the summary notes created by others.
 * You can edit other peoples’ work if you feel it is required **BUT** you must post an explanation in the “discussion” section of the page.
 * If you are unclear about what someone else has written, then ask a question in the “discussion” section of the page.

Osmosis is a special case of diffusion which describes the net movement of free water molecules from an area of low solute concentration to an area of high solute concentration, across a partially permeable membrane. As an example, if there were more salt molecules dissolved in water inside the membrane than outside, then the overall movement of water would be into the cell. Keeping this in mind, it is important to note that water molecules will always move in and out of the cell because of the continuous random motion of the particles, however it is the net (or overall) movement that is considered.
 * Osmosis in living cells**

When there are more free water molecules (and therefore a low concentration of solute) in the surrounding fluid of a cell than the inside of it, the surrounding environment of the cell is described as hypotonic (‘hypo’ meaning below normal). Hence, water will diffuse into the cell via osmosis. Conversely, when there are more free water molecules inside the cell than in the surrounding fluid, the environment of the cell is said to be hypertonic. Hence, water will move out of the cell via osmosis. When the two solutions (inside and outside the cell) are of equal concentration, the fluid surrounding the cell is called isotonic to the cell and water moves equally in both directions. It does not simply stop moving.

Placing an animal in highly concentrated solution clearly demonstrates the effect of osmosis. Water molecules will move out of the cell, down their concentration gradient, into the surrounding fluid, thereby causing the cell to shrivel. Conversely if the cell is placed in a hypotonic environment, water will move into the cell via osmosis, causing it to swell and eventually lyse (burst open).

Osmosis is a universal process and therefore occurs in practically all cells, plant cells naturally included. Plant cells absorb or lose water depending on the concentration of solutes in their sap. When plant cells intake a lot of water they begin to swell (or become turgid), and the cellulose wall starts to stretch. When the cell wall can stretch no further, it becomes firm and described as being fully turgid. When there is plenty of water in the surrounding environment, a plant’s cells are usually turgid. This state of turgor enables plants to stay erect and firm, as well as maintain their shape.

If the concentration of solutes happens to be greater outside the plant cell, it will lose water via osmosis. Turgor pressure will then drop, and the cell will shrivel, causing the parts of the plasma membrane to pull away from the cell wall (which maintains its general shape). This process is called plasmolysis, and the result puts the plant cell in a flaccid state. Flaccid cells give no support to a plant, which then wilts and droops.


 * // Osmosis and Cells: //**

The amount of water in the external environment surrounding a cell affects the concentration of the solution within the cell, as it determines whether “free” water molecules will pass into or out of the cell by the process of osmosis.
 * If a cell is in a **hypotonic solution**, the fluid surrounding the cell is //more dilute// than the solution within the cell. Therefore, WATER WILL DIFFUSE INTO THE CELL
 * If a cell is in a **hypertonic solution**, the fluid surrounding the cell is //more concentrated// than the solution within the cell. Therefore, WATER WILL DIFFUSE OUT OF THE CELL
 * If a cell is in an **isotonic solution**, the fluid surrounding the cell and within the cell are of //equal concentrations// . Therefore, WATER DIFFUSES EQUALLY IN BOTH DIRECTIONS.

When placed in a hypotonic solution, animal cells are unable to support themselves if too much water diffuses by osmosis into them. They swell, and eventually burst open (lyse). Plant cells do not lyse when placed in a hypertonic solution, due to the presence of a cell wall. The water will diffuse into the plant cell by osmosis and will accordingly flow into the plant cell’s vacuole, which will increase in volume and push the plasma membrane against the cell wall. When no more water can be taken in and the cell wall is as stretched as possible, the cell is said to be **//turgid// .** When placed in a hypertonic solution, water will move out of the cell by osmosis and the animal cell will lose its rigidity and shape. It will shrink and shrivel. When plant cells are placed in a hypertonic solution, PLASMOLYSIS occurs. This process involves water diffusing out of the cell by osmosis. The vacuole will continue decreasing in volume, and the cytoplasm peels away from the cell wall, leaving a gap between the cell wall and the plasma membrane. At this stage, the cell is referred to as **//flaccid.//** In the isotonic solution, water will diffuse equally in both directions, into and out of the cell. Both plant and animal cells remain normal, retaining their usual shape and not shrinking or swelling.
 * HYPOTONIC SOLUTION: **
 * // Animal Cells //**
 * // Plant Cells //**
 * // Turgor //** is very important to plant cells, as it supports them and assists them in maintaining their shape and form.
 * HYPERTONIC SOLUTION: **
 * // Animal Cells //**
 * // Plant Cells //**
 * ISOTONIC SOLUTION: **
 * // Plant and Animal Cells //**


 * __ ACTIVE and PASSIVE movement across a Plasma Membrane __**


 * Passive Transport** involves particles diffusing from an area in which they are in high concentration to an area of low concentration through a selectively permeable membrane. This is often described as moving along a concentration gradient. Eventually a solution will have an even concentration of solute throughout, but this does not mean the particles have stopped moving. The movement is random so there is no __net__ movement of particles.


 * Key points:**
 * Passive**


 * There are three types of passive transport: Diffusion, osmosis and facilitated diffusion
 * The net movement of a substance from a region of high concentration of that substance to a region of lower concentration
 * Passive movement, therefore, does not require the input of energy.
 * Occurs directly through the phospholipid bi-layer (for osmosis and diffusion processes)
 * Occurs through protein channels (for facilitated diffusion)
 * Is a relatively slow process (for osmosis and diffusion), but is a faster process (for facilitated diffusion)
 * For osmosis and diffusion: I.e. small, non-polar (lipid soluble), and uncharged molecules
 * For facilitated diffusion: Large, polar (lipid insoluble) and charged molecules


 * Active Transport** is the pumping of molecules or ions, fuelled by energy (ATP) through a protein within the membrane against their concentration gradient . These ions and molecules move from an area of low concentration to an area of high concentration. When a cell expends ATP directly during active transport, the process is called primary active transport. Using another energy source, such as the potential energy stored in an ion gradient, is secondary active transport.


 * Key points:**
 * Active**
 * The net movement of dissolved substances into or out of the cells, against the concentration gradient
 * Is an energy-requiring process (ATP).
 * Involves particles moving through the membrane proteins.
 * Enables cells to maintain internal concentrations different from external surroundings
 * **I.e** . lipid insoluble, charged particles.

The plasma membrane is composed of a phospholipid bi-layer (two layers of phospholipid molecules), associated with other molecules including proteins, carbohydrates and cholesterol. Phospholipid molecules have one end that is hydrophobic (water- hating) and the other end hydrophilic (water-loving).
 * __ Cell membrane structure and its role in the permeability of the membrane __**

Therefore when in contact with an aqueous solution, phospholipid molecules line up with their hydrophobic tails pointing away from the solution. The impermeability of membranes to water-soluble (polar) molecules is due to the phospholipid bi-layer. Most other membrane functions are carried out by the proteins which are located throughout the membrane. All proteins contain carbon, hydrogen, oxygen and nitrogen. There are thousands of different proteins with various functions. Membrane proteins provide the channels through which water-soluble molecules and ions pass. Facilitated diffusion (passive movement) and active transport (requiring energy expenditure) occur through selective channels formed by the membrane protein.

// Inorganic molecules // are simple compounds that usually don’t contain carbon. Inorganic compounds include // ions // such as: - Sodium (Na+) - Potassium (K+) - Calcium (Ca2+) - Chlorine (Cl-) · Charged ions cannot diffuse freely across cell membranes due to the hydrophobic nature of the lipids that make up the lipid bi-layer. · These ions are therefore transported across the phospholipid bi-layer through protein-mediated process known as // facilitated // // diffusion // and // active transport //
 * __ The movement of IONS across a cell membrane __**

//** Facilitated diffusion: **// · Facilitated diffusion is the movement of particles from an area of //high// concentration to an area of //lower// concentration through // protein channels // (provided by the embedded // protein molecules.) // · This means molecules are moving along/down their concentration gradient. · Facilitated diffusion is a passive process, thus no energy is required

//** Active transport: **// -Active transport is the movement of particles from an area of relatively //low// concentration to an area of //high// concentration -This means molecules are moving against/up their concentration gradient -Active transport is an active process, thus it requires the input of energy in the form of ATP.

 A Protein structure(made up of amino acids) that allows the movement of charged ions and large water soluble molecules across the cell membrane from an area of low concentration to high concentration i.e. against concentration gradient. For this process to occur the protein must undergo a change in shape, to allow the substance in question through the membrane, this change in shape requires energy, which is supplied by the bonds between phosphate groups in the chemical ATP.
 * Movement of substances in and out of cells**

 The bulk of the cell membrane encasing our cells is a chemical known as a phospholipid. These molecules consist of a polar, hydrophilic head in the form of a single phosphate group which is in contact with extrastitual fluid surrounding cells. There is also a non polar end, a hydrophobic tail in the form of a lipid(glycerol connected to two fatty acid chains). These molecules, when immersed in water form a bilayer that insures the ‘hydrophobic ends are not in contact with the water through having the tail of a phospholipid on one layer in contact with the tail of the phospholipid on the other layer. This set up of molecules is then referred to as the phospholipid bilayer.

 Cells need many different molecules to function adequately however there is a limit to how much of each molecule the cell needs, if they exceed this limit the cell can no longer function properly. So if the cell has reached it’s capacity for a certain molecule yet there is still a higher concentration of this molecule outside then inside the cell the substance will be inclined to move into the cells through diffusion(with the concentration gradient). But if cells dont want these molecules inside of them they have to some how push them out. This is done by, utilising a process called active transport which allows the cells to move substances from areas of low concentration to high concentration, against the concentration gradient. This process is involves carrier proteins also referred to as protein pumps which are imbedded in the phospholipid bilayer.

 the movement of free moving water molecules from an area of high concentration to low concentration.

 If the concentration of free moving water molecules is greater on the out side of a cell than the inside, the water outside the cell is inclined to move into the cell due to osmosis. The cell now has a greater number of molecules inside it than it had before causing the cell to increase in size, swell. If the cell continues to take on water molecules it reaches a point where the cell membrane can no longer increase in size causing the cell to rupture(burst). In plant cells the same process applies although plant cells have evolved a cellulose wall that surrounds the outer cell membrane. This cellulose wall is much stronger than the cell membrane and ensure the cell does not rupture

=__Diffusion:__= - Non-polar (as they must be lipid soluble) molecules, Eg) alcohol. - Small, uncharged, polar molecules. Eg) Oxygen, Carbon dioxide molecules. __Osmosis:__ Diffusion doesn’t have to occur through the phospholipid bi-layer, it can also take place through proteins. – This is referred to as FACILITATED DIFFUSION. __Facilitated diffusion:__ - Larger sized molecules that cannot physically fit through the bi-layer. - Polar or charged substances that are not lipid soluble and therefore cannot otherwise pass through the phospholipid bi-layer. Eg) sugar, amino acids, ions and water. The movement of substances via proteins isn’t always from a high to low concentration, it can also occur from a low to high substance concentration via the expenditure of energy. –This is referred to as ACTIVE TRANSPORT. __Active transport:__ · Active transport is a protein mediated transport method. ·  It is the movement of substances from an are of low to high concentration and therefore cannot be passive ·  It relies on the use of energy (ATP- Adenosine triphosphate) to actively move ions and molecules via carrier proteins. Eg) Movement of sodium/ potassium against the concentration gradient in/out of the cell.
 * __Transport methods across the plasma membrane of the cell:__**
 * Diffusion is the passive transport of a substance from an area of hight concentration to an area of low concentration. (Down its concentration gradient.)
 * Within cells this process generally occurs through the phospholipid bi-layer of the membrane.
 * Substances that undergo diffusion via the phospholipid bi-layer are:
 * A type of diffusion that is specific for the movement of water molecules.
 * Whereby water molecules move passively down their concentration from a region of high to low concentration of free-water molecules.
 * It occurs via the phospholipid bi-layer.
 * Facilitated diffusion is the same process of diffusion just that it takes place through protein channels and transport proteins in the plasma membrane.
 * Substances that use proteins to diffuse in and out of the cell are:

** The rate of diffusion (the passive movement of molecules from an area of high concentration to an area of low concentration) is fundamentally affected by two factors: //Concentration// and //Temperature//. ** Concentration: ** Temperature: **
 * Factors Affecting the Rate of Diffusion **
 * The higher the concentratraion the faster diffusion will occur to achieve equilibrium. If a cell is in an environment which is hypertonic (more concentrated environment outside the cell) then the substance will enter the cell at a certain speed or rate. If there is a much larger proportion of a certain molecule outside the cell, then that molecule will diffuse relatively quickly across the plasma membrane to achieve an isotonic environment (equal concentration). As well as this if a certain molecule suddenly started to become scarce in the cell (for instance if a muscle cell is using up large quantities of oxygen to achieve cellular respiration) than the molecules outside would enter the cell relatively quickly as well. This is also true if the cell is in a hypotonic solution (more concentrated in the cell than outside it). If there is a large proportion of a molecule inside the cell compared to outside, that molecule will diffuse relatively quickly out of the cell (for this we can again utilize our cellular respiration example where large amount of carbon dioxide are produced as a byproduct and diffuse quickly out of the cell). If a cell is in an isotonic solution then molecules will still pass across the membrane but it will do so much slower. **
 * Even though diffusion is passive when temperatures are high there is more energy (heat energy) which can be used for diffusion. Energy in a sense pushes molecules and makes them move quicker meaning there is more spontaneous movement of molecules resulting in an increase in the rate that molecules move down their concentration gradient. This is also true in lower temperatures. Unlike in heat, energy is not manifest in cold so when temperatures are low there is no energy propelling the molecules so diffusion occurs rather slowly. **

__Different structures of lipids__ à The most common lipid molecule is made up of glycerol back bone with three fatty acid chains; this is referred to as a triglyceride. à The lipids found in the plasma membrane are structurally different than the common lipid found in fat cells (adipose), this is crucial to the life of all cells. à The lipids found in the plasma membrane are composed of a phosphate group added to the glycerol backbone, from which are connected to two fatty acid chains. This structure is referred to as a phospholipid à The phosphate group combined with the glycerol backbone compose the hydrophilic (water loving) region of the phospholipid à The two fatty acid chains form the hydrophobic (water hating) region of the phospholipid à Both types of lipids are not examples of a polymer bio-macromolecule, as they are not a chain of repeated monomers

__Movement across the plasma membrane__ à the plasma membrane is referred to as fluid due to its flexible nature; this elasticity is caused by the phospholipid bilayer à the plasma membrane allows substance in and out of the cell via active transport, diffusion, osmosis and facilitative diffusion à Cholesterol molecules with in the plasma membrane give the cell a certain level of rigidity à The only substances that are able to move through the phospholipid bilayer (diffusion) are small lipid soluble molecules such as oxygen and carbon dioxide; other substances are able to pass through the plasma membrane via facilitative diffusion, active transport and osmosis. à Proteins act as mediators, allows substances to pass through them (facilitate diffusion, active transport)