Section 5 - Biological Membranes

What is the role of membranes at the surface of cells?

• To act as partially permeable barriers between the cell and its environment,

• Sites of cell communication (cell signalling)

What are the ways substances can move across the plasma membrane?

• Diffusion

• Facilitated Diffusion

• Osmosis

• Active Transport

• Bulk transport

What is the function of membranes WITHIN the cell?

The membranes around organelles divide the cell into different compartments - they allow for compartmentalisation

Give three examples of useful compartmentalisation/membranes

1. The substances needed for respiration (like enzymes) are kept inside a mitochondrion by a membrane.

2. Membranes can form vesicles to transport substances around the cell—Proteins are transported in vesicles from the RER to the Golgi body during protein synthesis.

3. Membranes on organelles are still partially permeable - RNA leaves the nucleus via the nuclear membrane.

Give an example of how membranes can be a S___ of C_______ R_______

Site of Chemical Reactions The inner membrane of the mitochondrion contains enzymes needed for respiration. It has a large surface area which increases number of enzymes and makes respiration more efficient

What are the key principles of the fluid mosaic model?

The phospholipid molecules form a continous bilayer and this bilayer is "fluid" because the phospholipids are constantly moving Protein molecules are scattered through the bilayer like a mosaic There are also glycoproteins and glycolipids There is cholesterol present

Label the diagram

What is the main function of the phospholipid bilayer?

They form a barrier to dissolved (water-soluble) substances

What are the two parts of the phospholipids

Hydrophilic head - attract waterHydrophobic tail - repels water

Describe the centre of the bilayer

The centre of the bilayer is hydrophobic, so it doesn't allow water substances (like ions and polar molecules) to diffuse through it

What kind of molecules CAN dissolve through the bilayer

• SMALL, UNCHARGED molecules

• Gases - CO2, O2

• Vitamins

• Fat-soluble substances

What does cholesterol do in the membrane?

Gives the membrane stability

How do cholesterol molecules regulate fluidity/ provide stability

They are small, so they fit between phospholipid molecules. At high temperatures they bind to the hydrophobic tails, causing them to pack more closely together - this makes the membrane LESS FLUID and MORE RIGID

At lower temperatures, it prevents phospholipids from packing too close together, so it increases membrane fluidity

What other special quality does cholesterol have

It also has hydrophobic regions to create a further barrier to polar substances

what is the primary role of the proteins

they control what enters and leaves the cell Channels - allow small charged particles through Carrier proteins transport larger molecules and charged particles across the membrane by active transport and facilitated diffusion.

What is a glycoprotein, and what is a glycolipid

• A protein with a carbohydrate attached

• A lipid with a carbohydrate attached

What are the roles of glycoproteins and glycolipids

• They stabilise the membrane by forming hydrogen bonds with surrounding water molecules

• They act as receptors for messenger molecules in cell signalling

• Sites where drugs, hormones and antibodies bind - are also antigens

What are the factors affecting membrane permeability

Solvents, Temperarture (super low, moderarte, super high)

Describe the effect of the solvent on permeability

Some solvents such as ethanol dissolve the lipids in the cell membrane, so the membrane loses its structure and becomes more permeable

So what is the general rule in terms of how solvent concentration affects permeability

Increasing the concentration of the solvent will also increase membrane permeability

How do temperatures below 0°C affect membrane permeability

The phospholipids don't have much energy, so they can't move very much.They're packed very closely together and the membrane is rigid Channel proteins and carrier proteins denature, increasing the permeability of the membrane

What may form at very low temperatures

Ice crystals may form and pierce the membrane, making it highly permeable when it thaws

How do temperatures between 0°C and 45°C affect permeability

The phospholipids can move around and arent as packed tightly together - the membrane is partially permeable As temperature increases, the phospholipids move more because they have more energy and this increases the permeability.

How do temperatures above 45°C affect permeability?

The phospholipid bilayer starts to melt (break down), and the membrane becomes more permeable. Water inside the cell expands putting pressure on the membrane Channel proteins and carrier proteins in the membrane denature so they cant control what enters or leaves the cell - this increases permeability

Describe how to investigate the effect of temperature on membrane permeability? PART 1

1. Use a scalpel to cut five equal sized pieces of betroot and rinse the pieces to remove any pigment released during cutting.

2. Add the five pieces to five different test tubes, each containing 5cm3 of water, using a pipette to measure the water.

3. Place each test tube in a water bath at different temperatures e.g; 10°C, 20°C, 30°C, 40°C, 50 °C for the same length of time

4. Remove the pieces of beetroot from the tubes, just leaving the coloured liquid.

Describe how to investigate the effect of temperature on membrane permeability? PART 2

Now, set up a colourimeter to the blue filter.Add distilled water to a cuvette then put it into the colorimeter and calibrate the machine to 0 Then, use a pipette to transfer a sample of the liquid from the first test tube to a clean cuvettePut the cuvette in the colorimeter and read and record the absorbance Repeat the last 2 steps for the rest of the test tubes

What will the results (of absorbance) tell you?

The higher the absorbance, the MORE pigment released, so the higher the permeability of the membrane.

Why do cells need to communicate and HOW do they communicate

To control processes inside the body and to respond to changes in the environment Cel signalling, which uses messenger molecules

How does cell signalling happen?

It starts when one cell releases a messenger molecule (e.g. a hormone). This molecule travels to another cell. The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane The binding then triggers a change in the cell

What role does the membrane play in cell signalling?

Proteins in the membrane act as receptors for messenger molecules These are called "membrane-bound receptors."

Why can only specific messanger molecules bind to the membrane bound receptors.?

Receptor proteins have specific shapes - only messanger molecules with a COMPLEMENTARY shape can bind to them.Different cells have different receptors as they respond to different messenger molecules

What is a target cell?

A cell that responds to a particular messanger molecule

What are most messenger molecules?

Hormones

What is an example of a messenger molecule that is a hormone?

FSH - A hormone released by the pituitary gland during the menstrual cycle. It binds to receptors on the cells in the ovaries, causing an egg to mature and be ready for ovulation

What is the role of drugs in this

Many drugs work by binding to receptor cells in membranes They either trigger a response in the cell or block the receptor and prevent it from working

What is an example of a drug that works by binding to receptors in the membrane?

Morphine - the body produces chemicals called endorphins to relieve pain. Endorphins bind to opioid receptors in the brain and reduce the transmission of pain signals. Morphine is a drug used to relive pain as it works by doing the same thing

Define diffusion

The net movement of particles from an area of high concentration to an area of low concentration Molecules will diffuse both ways until particles are evenly distributed

What are the factors affect diffusion?

1. The concentration gradient - the higher it is, the faster

2. The thickness of the exchange surface - the thinner (the shorter the distance of diffusion) the faster the rate

3. The surface area - the larger the SA, the faster the rate of diffusion

4. The temperature - the warmer it is, the faster the diffusion rate because the particles have more KE so they move faster.

What is the chemical used to investigate diffuision

Phenolphthalein - it is a pH indicator, pink in ALKALINE solutions and colourless in ACIDIC solutions

So, how is this chemical used to investigate diffusion

If you Place cubes of agar jelly containing phenolphthalein and an alkali i.e (NaOH) in an acidic solution and leave themThey will eventually turn colourless as the acid diffuses into the agar jelly and neutralises the NaOH

How to investigate the effect of concentration gradient

Make up some agar jelly with phenolphthalein and dilute NaOH. This will make the jelly pink!Prepare 5 test tubes containing HCl, in increasing concentrations - 0.2M, 0.4M, 0.6M, 0.8M, 1MUsing a scalpel, cut out 5 equal-sized cubes from the agar jelly Put one cube into the first test tube and use a stopwatch to time how long it takes to turn colourlessDo the same for the rest of the tubes

How to investgate the affect of SURFACE AREA

Make up some agar jelly with phenolphthalein and dilute NaOH. This will make the jelly pink!Using a scalpel, cut out five different sized cubes from the agar jelly and work out their surface area to volume ratio Place all the 5 cubes into the same conc. of HCL and time how long it takes each one to go colourlessYou would expect the cube with the largest SA:V ratio to go colourless first

How to investigate the effect of temperature

Make up some agar jelly with phenolphthalein and dilute NaOH. This will make the jelly pink!Cut agar jelly into 5 equal sizes Prepare 5 test tubes containing the same concentration of HClPut the boiling tubes into water baths at different temperatures and when it has reached the desired temperature Put one cube into each test tube and use a stopwatch to time how long it takes to turn colourless. The highest temp should go colourless first

Define osmosis

The net movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane.

What is water potential

The likelihood of water molecules diffusing into or out of a solution. Pure water has a water potential of 0 (highest possible)

How might you lower water potential

Adding solutes to pure water lowers its water potential - so the water potential of a solution is always negative The more negative the water potential, the stronger the concentration of solutes in solutions

How are cells affected by Water potential

They are affected by the water potential of the surrounding solutionHow much water moves in and out via osmosis depends on the water potential of the surrounding solution compared to the inside of the cell

What are isotonic solutions and describe the trend in water movement

If two solutions have the same water potential, they are ISOTONIC Cells in an isotonic solution won't lose or gain any water as there's no difference in water potential between the cell and the surrounding solution.

What is a hypotonic solution

Solutions with a higher water potential compared with the inside of the cell

What is expected to happen when a cell is in a hypotonic solution.

Water will move into the cell by osmosisAn animal cell will swell and could eventually burstIn a plant cell, the vacuole will swell and the vacuole and cytoplasm will push against the cell wallThis causes the cell to become turgid (swollen)BUT - the cell wont burst because the inelastic cell wall is able to withstand the increase in pressure

What is a hypertonic solution

Solutions with a lower water potential than the inside of the cell.

What is expected to happen when a cell is in a hypertonic solution

Water will move out of the cell by osmosis. An animal cell will shrink. A plant cell would become flaccid (limp), and the cytoplasm and the plasma membrane would eventually pull away from the cell wall - called PLASMOLYSIS

How to investigate the effect of water potential on plant cells

1. Prepare sucrose solutions of the following concentrations: 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 Μ.

2. Use a cork borer to cut potatoes into identically sized cylinders. Divide the cylinders into groups of three and measure the mass of each group using a mass balance.

3. Place one group into each of your sucrose solutions and leave the cylinders in the solutions for at least 20 minutes (making sure that they all get the same amount of time).

4. Remove the cylinders and pat them dry gently with a paper towel. Weigh each group again and record your results.

5. Calculate the percentage change in mass for each group.

How to calculate percentage change?

• Percentage Change = Final Value - Initial Value /Inital value x100

• A positive result = potato gained mass Negative result = potato lost mass

What are the two types of proteins involved in facilitated diffusion, and what kind of things do they diffuse?

Carrier proteins Channel proteinsLarger molecules (amino acids, glucose) and Charged particles (ions and polar molecules)

Which carrier protein helps transport glucose?

GLUT1

How does a carrier protein work

First, a large molecule attaches to a carrier protein in the membrane Then the protein changes shape This releases the molecule on the opposite side of the membrane

What do channel proteins do

Form pores in the membrane for smaller ions and polar molecules to diffuse down their concentration gradient

What is active transport, and how does it differ from facilitated diffusion?

It uses energy to move molecules and ions across plasma membranes against a concentration gradient. This involves carrier proteins -> it is the same process but energy from ATP is used

What is endocytosis

• A cell can surround a substance with a section of its plasma membrane

• The membrane can then pinch off to form a vesicle inside the cell containing the ingested substance

• It requires energy from ATP

What is an example of how endocyotis is used

Some white blood cells (phagocytes) use endocytosis to take in microorganisms and dead cells so that they can destroy them

What is exocytosis

Some substances produced by the cell need to be releasedVesicles containing this substance pinch off from the Golgi body and move towards the plasma membraneThe vesicles fuse with the plasma membrane and release their contents outside the cellUses energy / ATP

When might this process of exocytosis differ a bit

Some substances (like membrane proteins) arent released outside the cell - instead they are inserted straight into the plasma membrane.