2.4 Membranes Objectives 9/24/10 8:37 AM

Topic 2: Cells2.4 Membranes

Orange book: pg. Green book: pg.

2.4.1 Draw and label a diagram to show the structure of membranes (pg. 26,

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes

2.4.3 List the functions of membrane proteins

2.4.4 Define diffusion and osmosis

2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.

2.4.1 Membrane Structure 9/24/10 8:37 AM

2.4.1 Draw and label a diagram to show the structure of membranes

Orange book: pg. 26Green book: pg.

To do: Complete the colouring-in handout “The Fluid Mosaic Model” Colour and annotate the handout “The Cell Membrane”View the diagrams below illustrating membrane structure. Use your textbook, the diagrams below, diagrams/ pictures from the internet and your handouts to familiarize yourself with the cell membrane.In your green exercise books draw a clear labeled diagram of the structure of the cell membrane. The most important labels for you to identify and remember are: Phospholipids hydrophilic head and hydrophobic (fatty acid) tail Peripheral protein Integral (channel) protein Glycolipid Glycoprotein Cholesterol

Plasma membranes of two adjacent cells (electron micrograph).

Molecular structure of the plasma membrane

2.4.2 Phospholipids 9/24/10 8:37 AM

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes

Orange book: pg. 26Green book: pg.

To do: Complete the colouring-in handout “Evidence of Membrane Ultrastructure” Read the relevant sections in the green textbook. Read and highlight the key words in the text below. In your green exercise books (using diagrams to illustrate) describe the structure and properties of phospholipids.

Membrane StructureThe cell membrane (or plasma membrane) surrounds all living cells, and is the cell's most important organelle. The plasma membrane: defines the boundaries of the cell governs its interactions with other cells and controls how substances can move in and out of the cell.

The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown in this diagram.

The phospholipids form a thin, flexible sheet, while the proteins "float" in the phospholipid sheet like icebergs, and the carbohydrates extend out from the proteins.

Phospholipid StructureEach phospholipid is composed of a 3-carbon compound called glycerol. Two of the glycerol carbons have fatty acids attached. The third carbon is attached to a highly polar organic alcohol that includes a bond to a phosphate group. On the other hand, because the organic alcohol with phosphate is highly polar, it is soluble in water. This structure means that membranes have two distinct areas when it comes to polarity and water solubility. Once area is water soluble and polar, and is referred to as hydrophilic (water loving). This is the phosphorylated alcohol side. The other area is not water soluble and is non-polar. It is referred to as hydrophobic (water fearing).

Phospholipids and the Cell MembraneThe phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane. Because the fatty acid ‘tails’ do not strongly attract one another, the membrane tends to be fluid or flexible. This allows animal cells to have a variable shape and also allows the process of endocytosis.

2.4.3 Membrane Proteins 9/24/10 8:37 AM

2.4.3 List the functions of membrane proteins

Orange book: pg. 26Green book: pg.

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below. Visit the following website: Construction of the Cell MembraneThis is a short animation taking you through the basics of the cell membrane structure. It illustrates some of the proteins found in the cell membranehttp://www.wisc-online.com/objects/index_tj.asp?objID=AP1101Visit the following website:Membrane Structure – TutorialThis is an in-depth site on membrane structure – there are questions to complete as you go through the site. Some of it can be left out as it is too detailed, this is indicated below.http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swfThe summary will be completed in class from the board into your green exercise books

Proteins Proteins are a major component of cellular membranes and create extreme diversity in membrane function. Proteins of various types are embedded in the fluid matrix of the phospholipid bilayer. This creates the mosaic effect referred to in the fluid mosaic model. There are usually two major types of protein. Proteins which span from one side of the phospholipid bilayer to the other are called integral proteins, whilst those that sit on one of the surfaces are peripheral proteins. They can slide around the membrane very quickly and collide with each other, but can never flip from one side to the other. Proteins comprise about 50% of the mass of membranes, and are responsible for most of the membrane's properties.

Integral ProteinsIntegral proteins have both hydrophobic and hydrophilic regions in the same protein. The hydrophobic region, with non-polar amino acids, is in the mide-section of the phospholipid membrane, holding the protein in place. Their hydrophilic region is exposed to the water solutions on either side of the membrane.

Peripheral ProteinsPeripheral proteins do not protrude into the middle hydrophobic region, but remain bound to the surface of the membrane. Often these peripheral proteins are anchored to an integral protein.

Membrane Protein FunctionsIt is the membrane proteins that impart different functions to different membranes. There are many different proteins but hey tend to have six general functions: Receptors e.g. hormone binding sites Enzymes e.g. metabolic pathways and contact digestion Cell adhesion e.g. tight junctions between cells of intestine Cell-to-cell communication and identity e.g. glycoproteins to recognize

‘self’ Channels e.g. for passive transport Active transport e.g. sodium/ potassium ATPase

Receptors - proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding causes a change in the shape of the protein which then triggers other events in the cell. They may also be involved in cell signalling and cell recognition.

Enzymes – enzymes embedded in the plasma membranes of cells carry out the final stages of starch and protein digestion in the small intestine. They are involved in breaking down hormones and neurotransmitters once their job is done. Often the enzymes are grouped so that a sequence of metabolic reaction (metabolic pathway) may occur.

Cell Adhesion – provided by proteins when they hook together in various ways to provide permanent or temporary connections. These connections, referred to as junctions, may include gap junctions or tight junctions.

Cell communication and identity markers – usually a glycoprotein to allow a cell be recognized as ‘self’ and not ‘foreign’. It allows the body to recognize those cells that belong and those cells that do not.

Channel proteins - proteins that span the membrane are usually involved in transporting substances across the membrane. Materials will move from an area of high concentration to an area of low concentration

Active Transport – proteins shuttle a substance from one side of the membrane to another by changing shape. This process requires energy in the form of ATP. It does not require a difference in concentration to occur.

Membrane Structure – TutorialThis is an in-depth site on membrane structure – there are questions to complete as you go through the site. Some of it can be left out as it is too detailed, this is indicated below.http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf

Remember - Certain aspects of this tutorial are very detailed. You only need to pay particular attention to obtaining the correct answers to the following questions. When you answer – read the information provided and then translate into your own words as best as possible.

1. Fluid Mosaic ModelWhat are biological membranes mainly composed of?

How are membrane lipids organised?

How are the proteins arranged and what is their function?

Why are membranes described as being fluid?

Pop Quiz……………………………………… are the primary determinants of membrane structure, while …………………………………… carry out membrane function.* Remember to click on the figure to simulate membrane fluidity.

2. Internal MembranesWhich type of cells contain membrane bound organelles?

List some organelles which are surrounded by membranes.

What is the function of the cell membrane?

What is the significance of having cellular structures with their own membranes?

3. Membrane FunctionsDescribe each of the following membrane functions.Signaling

Barrier

Transport

Localisation

Communication

4. Three Classes of Membrane LipidsWhat are the general functions of lipids in living organism?

Name the three membrane lipids.

Describe the structure of phospholipid.

What is a glycolipid composed of?

What is the meaning of hydrophobic and which part of the phospholipid molecule is hydrophobic?

What is the meaning of hydrophilic and which part of the phospholipid molecules is hydrophilic?

Leave out the following two pages – they are too detailed:5. Examples of Common Membrane Lipids6. Examples of phosphoglycerides.

7. Self-assembly of a Lipid BilayerDescribe the hydrophobic effect.

Explain why polar molecules are soluble in water and non-polar molecules are not.

8. Membrane FluidityWhich two ways can membrane lipids move?

Explain why membrane fluidity can occur.

Leave out the following two pages – they are too detailed:9. Fluidity-O-Meter10.Membrane Asymmetry

11. Membrane ProteinsWhat is a peripheral membrane protein?

What is an integral membrane protein?

HW3 Membrane Questions 9/24/10 8:37 AM

2.4.1 Draw and label a diagram to show the structure of membranes

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes

2.4.3 List the functions of membrane proteins

?? Some of the following questions may need a little bit of extra research as we have not yet covered the section on Biological Molecules.35 marksPlease type the answers in blue so it can be clearly seen.

1. The table below refers to components of the cell surface membrane (plasma membrane) and to their roles in transporting substances across the membrane.Complete the table by inserting an appropriate word or words in the empty boxes. (6)

Component Subunits Chemical bond between subunits

Role in Transport

Phospholipids Fatty acids, glycerol and phosphates

Carbohydrate side chain

Receptor

Protein Peptide

2. The diagram below shows the structure of a molecule found in the cell surface membrane.

a. Name the type of molecule shown in the diagram. (1)

b. Name A and B as labelled on the diagram. (2)

c. Region X is said to be hydrophobic.What is meant by the term hydrophobic?(1)

d. Explain why the cell surface membrane is described as a fluid-mosaic. (2)

3. The diagram shows the structure of a phospholipid molecule,

a. Name the part of the molecule labelled Y. (1)

b. Describe how a phospholipid molecule differs in structure from a triglyceride molecule. (1)

c. Chitin is a nitrogen-containing polysaccharide. Name one chemical element present in a phospholipid which would not be present in chitin.(1)

4. An artificial membrane was made. It consisted only of a bilayer of phospholipid molecules. In an investigation, the permeability of this artificial membrane was compared with the permeability of a plasma membrane from a cell. Explain why:a. both membranes allowed lipid soluble molecules to pass through. (1)

b. only the plasma membrane allowed glucose to pass through.(2)

5. The following extract has been taken from a dictionary of biological terms.

cell membrane: a membrane found either on the outside of a cell or within it. Cell membranes are extremely thin. They are only about 7 nm thick and so cannot be seen with a light microscope. A transmission electron microscope however shows a cell membrane consists of three lines forming a sandwich. The two outer lines are dark in colour while there is a lighter one in between. As it is impossible, even with an electron microscope, to see how the actual molecules are arranged in a cell membrane, it is necessary to produce a model to explain the membrane’s properties. The most accurate model of membrane structure that has been developed is the fluid mosaic model and this can be used to describe most of the properties of a cell membrane. Cell membranes play a very important part in the biology of cells and they are particularly important in regulating the movement of substances into and out of cells.a. Describe the structure of a cell membrane. (5)

b. Describe two ways in which the appearance of a plant cell wall would differ from a cell membrane when viewed with an electron microscope. (2)

c. Describe the part played by cell surface membranes in regulating the movement of substances into and out of cells. (6)

d. Describe how the distribution of cell membranes in a prokaryotic cell such as a bacterium differs from that in a cell from a plant leaf.(4)

2.4.4 Diffusion and Osmosis 9/24/10 8:37 AM

2.4.4 Define diffusion and osmosis

Orange book pg. 27 & 31Green book pg. 23

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below.

Diffusion Websites:Simple Diffusion Animationhttp://www.wisc-online.com/objects/index_tj.asp?objID=AP1903

Diffusion Animationhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Watch the Brainpop animations: “Diffusion: Matter Likes to Mix”“Passive Transport: Getting Stuff in and out of Cells, the easy way”.

Osmosis Websites

Osmosis Animationhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

Hypo or Hyper Tonichttp://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Brief introduction to osmosis and includes isotonic, hypertonic and hypotonic.http://www.wisc-online.com/objects/index_tj.asp?objID=AP11003

Plasmolysis Illustratedhttp://ccollege.hccs.edu/instru/Biology/AllStudyPages/Diffusion_Osmosis/Elodeagif.swf

Summary in your green exercise books to consisting of definitions for:DiffusionOsmosis

Passive and Active TransportThere are two general means of cellular transport:• Passive transport• Active transport

Passive transport does not require energy (in the form of ATP), but active transport does. Passive transport occurs in situations where there are areas of different concentration of a particular substance. Movement of the substance occurs from an area of high concentration to an area of lower concentration. Movement is said to occur along a concentration gradient.

When active transport occurs, the substance is moved against a concentration gradient, so energy expenditure must occur.

Simple Diffusion (lipid diffusion)

A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is a passive process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

Diffusion is the movement of molecules from a high concentration to a low concentration down their concentration gradient.

Osmosis

Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells, the diffusion of water has its own name - osmosis. The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.

Osmosis is the diffusion of water from a high concentration of water molecules to a lower concentration of water molecules, down their concentration gradient through a semi-permeable membrane.

Cells and OsmosisThe concentration (or water potential) of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:Isotonic solution a solution of equal water potential (or

concentration) to a cell

Hypertonic solution a solution of more negative water potential (more concentrated) than a cell

Hypotonic solution a solution of less negative water potential (less

concentrated) than a cell

The effects of these solutions on cells are shown in the diagram:

These are problems that living cells face all the time. For example:Simple animal cells (protozoans) in fresh water habitats are surrounded by a hypotonic solution and constantly need to expel water using contractile vacuoles to prevent swelling and lysis.

Cells in marine environments are surrounded by a hypertonic solution, and must actively pump ions into their cells to reduce their water potential and so reduce water loss by osmosis.

Young non-woody plants rely on cell turgor for their support, and without enough water they wilt. Plants take up water through their root hair cells by osmosis, and must actively pump ions into their cells to keep them hypertonic compared to the soil. This is particularly difficult for plants rooted in salt water.

Interactive ActivityOsmosis Animation

2.4.5 Simple Diffusion 9/24/10 8:37 AM

2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion

Orange book pg. 28-30Green book pg. 23

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below.

Simple Diffusion Animationhttp://www.wisc-online.com/objects/index_tj.asp?objID=AP1903*If you check the above site at home it should load.

Diffusion Animationhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Facilitated Diffusion Animationhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Summarise in your green exercise books the similarities and differences between simple and facilitated diffusion.

Simple diffusion (lipid diffusion)

A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

Diffusion is the movement of molecules from a high concentration to a low concentration down their concentration gradient.

Passive Transport (Facilitated Diffusion)

Passive transport is the transport of substances across a membrane by a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. As the name suggests, this is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient. There are two kinds of transport protein:

Channel Proteins form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.

Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.

2.4.6 Active Transport 9/24/10 8:37 AM

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes

Orange book pg. 33Green book pg. 23

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below.

Active Transport Animationhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Give a concise definition of active transport in your green exercise book.

Active Transport (or pumping)

Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. The protein pumps are also ATPase enzymes, since they catalye the splitting of ATP ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.

The rate of diffusion of a substance across a membrane increases as its concentration gradient increases, but whereas lipid diffusion shows a linear relationship, facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate being limited by the number of transport proteins. The rate of active transport also increases with concentration gradient, but most importantly it has a high rate even when there is no concentration difference across the membrane. Active transport stops if cellular respiration stops, since there is no energy.

The sodium-potassium pump (Na+/K+ATPase) is a very common pump. At this stage you do not need to learn the details of ion exchange, it is more to demonstrate the use of energy to bring about a conformational change.1. A specific protein binds to three intracellular sodium ions

2. Binding of sodium ions causes phosphorylation by ATP

3. The phosphorylation causes the protein to change its shape, thus expelling sodium ions to the exterior

4. Two extracellular potassium ions bind to different regions of the protein and this causes the release of the phosphate group

5. Loss of the phosphate group restores the protein’s original shape thus causing the release fo the potassium ions into the intracellular space.

2.4.7 Endocytosis and Exocytosis 9/24/10 8:37 AM

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

Orange book pg. 34Green book pg. 24

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below.

Endocytosis and Exocytosis AnimationQuick and simple animation to illustrate endocytosis and exocytosis.http://www.wisc-online.com/objects/index_tj.asp?objID=AP11203

Endocytosis and Exocytosis AnimationAnimation with audio explanation.http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Give a concise definition of endocytosis and exocytosis in your green exercise book.

VesiclesThe processes described so far only apply to small molecules. Large molecules (such as proteins, polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.

Endocytosis is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane, which then pinches shut to form a closed vesicle. Strictly speaking the material has not yet crossed the membrane, so it is usually digested and the small product molecules are absorbed by the methods above. When the materials and the vesicles are small (such as a protein molecule) the process is known as pinocytosis (cell drinking), and if the materials are large (such as a white blood cell ingesting a bacterial cell) the process is known as phagocytosis (cell eating).

Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. Materials to be exported must first be enclosed in a membrane vesicle. Hormones and digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands. Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in by endocytosis at one end of a cell and passing out by exocytosis at the other end. Exocytosis usually begins in the ribosomes of the RER and progresses through a series of four steps until the produced substance is secreted to the environment outside the cell.

1. Protein produced by the ribosome of the rough ER enters the lumen of the ER

2. Protein exits the ER and enters the cis side or face of the Golgi apparatus; a vesicle is involved.

3. As the protein moves through the Golgi apparatus, it is modified and exits on the trans face inside a vesicle.

4. The vesicle with the modified protein inside moves to and fuses with the plasma membrane – this results in the secretion of the contents from the cell.

2.4.8 Membrane Fluidity 9/24/10 8:37 AM

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.

Orange book pg. 34Green book pg. 25

To do: Read the relevant sections in the green textbook. Read and highlight the key words in the text below.

In you green exercise books:describe the structure of membranes which lead to fluidity.Describe the role of cholesterol in membrane fluidity.

Dancing membrane animation http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane%20fluidity.swf

SummaryTakes you through simple diffusion, facilitated diffusion and osmosis.http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html

Membranes are fluid structures, rather like cooking oil, because most of the membrane lipids and many of the membrane proteins easily rotate and move sideways in their own half of the bilayer.

Membrane fluidity depends on both the number of double bonds in the fatty acid tails of the lipids that make up the bilayer and on the amount of cholesterol present. Each double bond puts a ‘kink’ in the fatty acid tail, which increases membrane fluidity because it prevents lipid molecules from packing tightly in the membrane. More cholesterol also increases membrane fluidity.

Due to the fluidity of membrane lipids, the lipid bilayer self-seals if it is torn or punctured. When a needle is pushed through the cell membrane and pulled out, the puncture site seals spontaneously, and the cell does not burst. This self-sealing also happens during exo- and endocytosis.

Summary of Membrane Transport

HW10 – Essay (25 marks) 9/24/10 8:37 AM

Title: Transport across the cell membrane (300 words).Describe the role of simple diffusion, facilitated diffusion, osmosis, active transport and endo/exocytosis in the transport of substances across the cell membrane.

For each you should include:A detailed description of the processAn example of the process.

You can use this space to plan your essay however the essay is to be hand written and submitted for marking.

HW11 – 2.4 Revision Questions 9/24/10 8:37 AM

Revision Questions* Answer in your green exercise books NOT on laptop1. The diagram below shows part of a cell surface membrane.

a. Name the molecules labelled A and B. (2)

b. Cell surface membranes can be broken into small pieces. These small pieces curl in on themselves to form membrane-bound spheres. These spheres are filled with liquid and are known as vesicles.An experiment was carried out into the movement of sodium ions across the membranes of these vesicles. The vesicles were immersed in a solution of sodium chloride. The concentration of sodium ions in the vesicles was measured over a period of five minutes.The procedure was then repeated with ATP added to the sodium chloride solution.During the experiment the temperature was kept constant at 23 °C. The results are shown in the graph below.

i. Compare the uptake of sodium ions by the vesicles with and without ATP. (3)

ii. What do these results suggest about the mechanisms of transport of sodium ions across these membranes? Explain your answer. (4)

c. State why the temperature was kept constant during this experiment. (1)

2. An experiment was carried out with cells of carrot tissue to determine the effect of temperature on the absorption of potassium ions.Slices of carrot tissue were immersed in a potassium chloride solution of known concentration. The changes in concentration of potassium ions in the solution were determined at intervals for 6 hours. From these measurements, the mass of potassium ions taken in by the carrot cells was found. The experiment was carried out at 2°C and 20°C. The solutions were aerated continuously.The results are shown in the graph below. Absorption of potassium ions is given as micrograms of potassium per gram of fresh mass of carrot tissue (μg g–1).

During the first hour, some of the potassium ions enter the cells by diffusion. a. State two conditions which are necessary for a substance to enter a cell by diffusion. (2)

b. Calculate the mean rate of absorption of potassium ions at 20°C, between 2 and 6 hours. Show your working. (3)

c. Compare the rates of absorption of potassium ions at 2°C and 20°C during this experiment. (3)

d. Suggest an explanation for the differences in the rates of absorption of potassium ions at the two temperatures. (3)

3. Read through the following passage on the cell surface membrane, then write on the dotted lines the most appropriate word or words to complete the passage. (5)All cells are surrounded by a cell surface membrane. This is made up of ....................................................... arranged as a bilayer. Embedded in this bilayer are ......................................................., some of which act as carriers. Some molecules in the cell surface membrane have glycoside side chains made up of ........................................... sub-units. These side chains act as ................................................. and enable the cell to form vesicles around extracellular substances, part of the process known as .................................................

4. The table below refers to four membrane transport processes: diffusion, facilitated diffusion, osmosis and active transport. If the statement is correct, place a tick ( ) in the appropriate box and if the statement is incorrect, place a cross (X) in the appropriate box. (4)

5.

Experiments were carried out to investigate the uptake of mineral ions by barley roots.In the first investigation, isolated barley roots were immersed in an aerated culture solution containing potassium ions (K+) and nitrate ions (NO3–). After ten hours, the roots were removed and the concentrations of these ions in the cell sap were determined. The results are shown in the table.

Process Takes place against aconcentration gradient

Requires energy in the form of ATP

DiffusionFacilitated diffusionOsmosisActive transport

Ion Concentration in culturesolution / mmol per dm3

Concentration in cellsap / mmol per dm3

Potassium

7.98 97.8

Nitrate 7.29 38.1a. Suggest why the culture solution was aerated. (2)

b. These results show that the concentration of potassium ions in the cell sap is 12.3 times greater than that in the culture solution. This is referred to as the accumulation ratio. Calculate the accumulation ratio for nitrate ions. Show your working. (2)

c. What do these results suggest about the mechanism for the uptake of potassium and nitrate ions? Explain your answer. (2)

In a further experiment, the effect of temperature on the uptake of potassium ions was investigated. Isolated barley roots were kept in aerated nutrient solutions at a range of temperatures, and the concentrations of potassium ions in the cell sap were measured after ten hours. The results are shown in the table below.

d. What effect does temperature have on the concentration of potassium ions in the cell sap? (2)

e. Suggest an explanation for these results. (2)

6. The table shows some similarities and some differences between osmosis, active transport and facilitated diffusion. Complete the table with a tick if the feature applies or with a cross if it does not apply. (3)

Feature Osmosis Active transport

Facilitateddiffusion

Requires energy from ATPRequires protein carrier moleculesCan take place against a concentration gradient

Temperature / °C Concentration of potassium ions in cell sap /mmol per dm3

612182430

35427095110

7. An investigation was carried out into the effects of temperature on the rate of diffusion of chloride ions. Slices of carrot were placed in distilled water at different temperatures. After 10 minutes the concentration of chloride ions in the distilled water was measured. The results are shown in the graph.

a. Explain the increase in the rate of diffusion between 14°C and 40°C. (2)

b. Suggest what caused the sudden increase in the rate of diffusion at 40°C. (2)

8. The diagram represents part of an animal cell which has been put in distilled water.

Use the diagram to:a. explain why the water potential of the distilled water is higher than the water potential of the cytoplasm of the cell. (2)

b. describe the property of the cell surface membrane which allows osmosis to take place. (1)

c. Osmosis has been described as a special case of diffusion. Describe two ways in which you would expect the movement of water into a cell by osmosis to be similar to the diffusion of oxygen into a cell. (2)

9. The graph shows the expected and actual results of an experiment to investigate the uptake of glucose by human red blood cells.

Curve B shows the result that would be expected if glucose enters the red blood cells by simple diffusion.Curve A shows the results obtained from the red blood cells. It shows that these cells took up glucose by facilitated diffusion.

Explain the shape of the curve at glucose concentrations:a. less than 2 mmol dm–3; (2)

b. greater than 5 mmol dm–3. (1)

10. A kidney consists of a large number of very small tubes called kidney tubules. Some of the cells which line these tubules are able to absorb glucose. The diagram shows how these cells absorb glucose from the contents of the tubule and secrete it into the blood.

a. Glucose moves into the cell by facilitated diffusion. Osmosis also takes place across the plasma membrane. Give two differences between facilitated diffusion and osmosis. (2)

b. Explain the link between active transport and the presence of large numbers of the organelles labelled A in this cell. (3)

c. Explain two ways, shown in the diagram, in which the structure and activities of this cell ensure efficient absorption of glucose from the inside of the kidney tubule. (2)

d. Explain what is meant by the following terms. i. Osmosis (3)

ii. Facilitated diffusion (3)