2.1.5 Biological Membranes

What are the roles of membranes

• Partially permeable barriers between the cell and its environment, between organelles and the cytoplasm within organelles

  • Also separates them from the cytosol (the liquid portion of the cytoplasm)

• Acts as the interface for cell communication & signalling

• Sites of chemical reactions

• Uses diffusion, osmosis and active transport to move substances across it

What is the structure of phospholipids

• Only 2 fatty acids (the third has been replaced by a phosphate ion)

• Contains a phosphate group, glycerol and 2 fatty acid tails

• All held together by ester bonds

What is the structure of the plasma membrane

• Formed by a phospholipid bilayer

  • The hydrophilic phosphate heads of the phospholipids form both the inner and outer layer of the membrane

  • The hydrophobic fatty acid tails form the centre - they orient away from water and towards each other to minimise contact with aqueous surroundings

• Fluid - phospholipids and proteins are free to move within the layer (mostly sideways) via diffusion

• Mosaic - The proteins embedded in the layer vary in shape, size and position

What is a micelle

• If a phospholipid is mixed/shake with water they form spheres called a micelle

• Hydrophilic head faces outwards

• Hydrophobic fatty acid tails face inwards

Name the parts of the phospholipid bi-layer protein

• A = Glycolipid

• B = Extrinsic/peripheral protein

• C = Glycoprotein

• D = Carbohydrate part of the glycoprotein

• E = Phospholipid

• F = Cholesterol

• G = Transport/Intrinsic/Integral Protein

What is cholesterols role in the phospholipid bi layer

• Regulates the fluidity of the membrane by being between the phospholipids and regulating their movement

• They prevent phospholipids from packing to closely together and crystallising

• Also allows membrane to be impermeable to ions, to increase strength & stability

What is the structure of cholesterol

Lipid with a hydrophilic end and a hydrophobic end

What makes membranes less fluid

• More straight saturated fatty acid chains, pack together tightly, high number of intermolecular forces between the chains

  • Meaning there’s less space and therefore less movement

• Lower temperatures, molecules have less energy, not moving as freely, structure becomes more closely packed

What makes membranes more fluid

• More unsaturated fatty acid chains, pack tether less tightly, less molecular forces between the chains

• Higher temperatures, molecules have more kinetic energy, phospholipid move more freely, structure becomes more fluid

  • When too high, proteins denature which create gaps and increase permeability

Method: Factors affecting membrane structure and permeability - Temperature

• Using a scalpel or cork borer, cut five equal-sized cubes of beetroot

  • As this could affect the rate at which the pigment leaks out

• Weigh the pieces using a digital balance to ensure they have the same mass

• Rinse the beetroot pieces - to remove pigment released during cutting

• Add the beetroot pieces to five different test tubes, each containing the same volume of water e.g. 5cm³

• Put each test tube in a water bath at an increasing temperatures (e.g. 10℃, 20℃, 30℃, 40℃, 50℃) for the same length of time e.g. 30 mins

• Remove the beetroot pieces

• Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid

  • The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability

How do organic substances such as ethanol affect membrane structure

• Dissolve phospholipids, disrupting the bilayer structure and increasing membrane permeability

• For example in beetroot - It dissolves the phospholipids in the tonoplast and plasma membrane

What are the roles of glycolipds and glycoproteins in the membrane

Can act as:

• Receptor molecules for hormones and neurotransmitters

• Receptor molecules that detect signalling molecules - enabling communication & signalling

• Antigens

• Can be used for recognition/identification of cells

• Receptor/binding site on transport proteins

• Can be used for cell adhesion

• Attaches to water to stabilise the cell

What is the structure of a glycoprotein

• Intrinsic proteins embedded in the cell surface membrane

• Carbohydrate chains of varying lengths and shapes are attached to the protein

What is the structure of a glycolipid

Lipids with attached carbohydrate chains

What are the roles of intrinsic proteins

• Channel (pore) proteins: Create hydrophilic channels which allow ions and polar molecules to travel through membranes

  • Always transport down the concentration gradient - facilitated diffusion

• Carrier proteins - the protein changes shape to allow this to happen and bins to the molecule

  • Can transport up or down the concentration gradient - active transport or diffusion

• Each transport protein is specific to what it is transporting

• The more intrinsic proteins inside a membrane, the faster the rate of diffusion

• They are embedded through both layers of a membrane

What are receptor proteins/membrane bound receptor

• They bind to specific signalling molecules (hormones) and trigger a response inside the cell

• They (the signalling molecules) must bind to receptors proteins first as they aren’t lipid soluble and cannot pass through the phospholipid bi-layer, so they bind to receptors on the surface to transmit their signal

How do membranes act as a site of chemical reactions

• The enzymes of photosynthesis are found on membrane stacks within the chloroplasts

  • Also contains enzymes for respiration

• The proteins must be in particular positions for this to work

• E.g. the electron carriers and the enzyme ATPase must be in the correct positions within the cristae for the production of ATP in respiration

What is the purpose of a centriole

They help to move chromosomes during cell division

What is diffusion

The net movement of a substance from a region of its higher concentration to a region of its lower concentration

Why does diffusion happen

• Molecules move down a concentration gradient caused by natural kinetic energy

• Eventually particles become evenly spread due to random movement of particles

What are the factors affecting the rate of diffusion

• Steepness of the concentration gradient

• Temperature

• Surface area

• Properties of molecules/ions (size, charge)

• The presence of carrier/channel proteins

Method: Factors affecting diffusion rates - Concentration

1. Make some agar jelly with phenolphthalein and dilute sodium hydroxide. This will make the jelly pink

2. Prepare 5 test tubes containing Hydrochloric acid (HCI) in increasing concentrations e.g. 0.2M, 0.4M, 0.6M, 0.8M and 1.0M

3. Using a scalpel, cut out 5 equal size cubes from the agar jelly

4. Put one of the cubes into the first test tube and use a stopwatch to time how long it takes for the cube to turn colourless

5. Then repeat for the other concentrations, using a new cube each time.

6. The highest concentration of HCl will have the fastest colour change as it has the steepest concentration gradient.

Method: Factors affecting diffusion rates - Surface Area

1. Make some agar jelly with phenolphthalein and dilute sodium hydroxide. This will make the jelly pink

2. Using a scalpel cut the cubes into different sizes and work out the SA:V ratio

3. Prepare 5 test tubes containing equal amounts and concentrations of hydrochloric acid

4. Place one cube into one guest tube of the hydrochloric acid - time how long it takes to go colourless using a stopwatch

5. Repeat for the other cubes

6. The cube with the largest SA:V ratio will go colourless the fastest

Method: Factors affecting diffusion rates - Temperatures

• Make some agar jelly with phenolphthalein and dilute sodium hydroxide. This will make the jelly pink

• Cut the cubes into equal sizes - same SA & V

• Prepare several boiling tubes with the same concentration of Hydrochloric acid

• Put the boiling tubes in water baths with different temperatures

• The higher temperatures will make the cubes go colourless faster

How do the properties of molecules or ions affect the rate of diffusion

• Large molecules diffuse slower than smaller ones as they require more energy to move

• Uncharged and non-polar molecules diffuse directly across the phospholipid bi-layer

• Non-polar molecules diffuse quicker then polar molecules as they are soluble in the non-polar phospholipid bi-layer

What is facilitated diffusion

• Diffusion that uses Chanel and carrier proteins to assist certain substances that can’t use standard diffusion

• These include

  • large polar molecules like glucose and amino acids

  • Ions (charged particles) like sodium and chloride

What is osmosis

The net movement of water molecules from a region of high water potential to a region of low water potential across a partially permeable membrane

What is water potential

• A measure of the tendency of water molecules to move from one area to another;

• Pure water has a water potential of 0

How does osmosis of water into a plant cell work

• Plants placed in a dilute / HWP solution - water enters the plant cells cytoplasm & vacuole by osmosis - volume of cell increases

• Plant cells have protoplasts (living portion of a plant cell)

  • These push up against the cell wall Which increases pressure which makes the cell become turgid

• Turgidity provides support and strength for a plant - e.g. allows them to stand up straight

How does osmosis of water out of a plant cell work

• Plant cell placed in a concentrated solution

• Water leaves the plant cell’s cytoplasm and vacuole by osmosis - volume of cell decreases

• Protoplast shrinks and membrane pulls away from cell wall - decreases pressure - cell eventually becomes plasmolysed

How does having no cell wall affect animals cells during osmosis

• They feel the effects more severely

• When placed in high water potential solutions

  • Water enters the cell by osmosis - might lyse

• When placed in low water potential solutions

  • Water leaves the cell by osmosis - cell shrinks and becomes crenated (wrinkled)

What is a hypotonic solution

• The outside solution is less concentrated with solutes

• The net movement of water inwards

• Cells swell - and may burst

What is a hypertonic solution

• The outside solution is more concentrated with solute

• Net movement of water out of cell

• The cell shrivels

What is an isotonic solution

• Outside solution and cytoplasm a similar solute concentration

• No movement

Method: Investigating effects of different water potential on plant cells

The required number of potato cylinders are cut - one for each of the solutions you are testing

• Cut them to the same length and, blot dry to remove any excess moisture, measure initial mass and record before placing into the solutions

• Leave in solutions for a set amount of time (eg. 30 minutes) in a water bath - at around 30^o

• Removed and dry to remove excess liquid

• Calculate the final length and mass of each potato cylinder is then measure and record

• Calculate the percentage change in mass (final - initial/ initial x 100)

What is active transport

• The movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using ATP

• Requires energy from ATP to change the shape of carrier proteins

What is co-transport

The coupled movement of substances across a cell membrane via a carrier protein (facilitated diffusion and active transport combined)

E.g.

• NA & glucose ions are transported into epithelial cells via diffusion (Facilitated)

• NA is then actively transported out of the cell, into the blood (which helps maintain a conc. gradient for sodium)

• Glucose exits the cell and enters the blood again via facilitated diffusion

What organelles are adapted for faster transport

• Neurone & muscle cells - Cell membrane read have specific channel proteins for sodium, potassium and calcium ions

• Kidney cells - Contain very high number of Aquaporins

  • Aquaporins = specific channel proteins that allow facilitated diffusion

What is endocytosis

• The process by which cells engulf substances by infolding of the plasma membrane to form vesicles requiring ATP

• Energy is needed to move the membrane and form vesicles around the material being engulfed

What is exocytosis

• The process by which vesicles fuse with plasma membrane to release their contents outside the cell requiring ATP

• Contents incl. hormones (insulin) or digestive enzymes