GJHeducation CIE International Alevel Biology 2 2 g

GJHeducation CIE International A-level Biology 2. 2 (g) The relationship between the structure and functions of phospholipids

GCSE Biology was so 2 D! “Such a critical part of an eukaryotic animal cell but since the beginning of Science lessons, we’ve always drawn it as a simple thin line!” Start this lesson by trying to identify the part of the cell being described by the student and then discuss how this structure could be related to a lesson about lipids

The cell membrane is critical for the passage of substances in and out of the cell, and in line with this complex function, it probably won’t surprise you to learn that this structure is a little more complicated than a simple line. In fact, a high proportion of a cell membrane is made up of a type of lipid known as a phospholipid, and this lesson focuses on the structure and properties of this macromolecule, a very large, organic molecule just like the triglyceride that was covered in the previous lesson……

2. 2 (g) QUIZ R 1 Round 1 is called “A FAMILY AFFAIR” and introduces some of the structural similarities between a phospholipid and a triglyceride

As triglycerides and phospholipids are both lipids (and macromolecules), it makes sense that they would have a number of structural similarities. In “A FAMILY AFFAIR”, you will be challenged to use your knowledge of triglycerides to learn some of the structural features that are also true of a phospholipid. A statement or key term will appear at the bottom of the board and all you have to do is buzz in and state the structural feature which is common to both of these lipids. For example, if the term “molecule” appeared at the bottom of the slide, the 1 st team to buzz in and say “MACROMOLECULE” would get the 2 TEAM POINTS. Be a little wary though because any wrong answers will be met with a loss of 2 POINTS! A FAMILY AFFAIR!

Carbon, hydrogen and oxygen are found in a triglyceride Carbon, hydrogen and oxygen are found in a phospholipid A FAMILY AFFAIR! Contains these chemical elements

A molecule of glycerol is found in a triglyceride A molecule of glycerol is found in a phospholipid A FAMILY AFFAIR! Contains a 3 -carbon molecule

Fatty acid chains are found in a triglyceride Fatty acid chains are found in a phospholipid A FAMILY AFFAIR! Contain chains which can be saturated or unsaturated

Components linked by ester bonds in a triglyceride Components linked by ester bonds in a phospholipid A FAMILY AFFAIR! Chemical bond between components

Understanding check The different components of a phospholipid are joined together by ester bonds Although not a structural feature, state what will have been similar during the synthesis of a triglyceride and a phospholipid based on the formation of ester bonds. Verbalise your answer to a partner (1 mark) The ester bonds were formed during condensation reactions

“ 3” TO THINK ABOUT You have 3 minutes to discuss with 2 people A triglyceride consists of a glycerol molecule joined to fattystart acidsofby Based purely on 3 the 3 ester bonds. However, only two the names of these two fatty acids are found in a macromolecules, discuss whatrd phospholipid and in place of the 3 is likely to be the main one, there is a phosphoric acid structural difference molecule bonded to the glycerol Triglyceride vs Phospholipid

The molecular structure of a phosphoric acid molecule (H 3 PO 4) is shown on the right

Start by recalling the structure of the different components of a phospholipid (glycerol, fatty acids, phosphoric acid) and then use your knowledge of condensation reactions to draw the final structure of a phospholipid

A phospholipid

PHOSPHOLIPID KEY POINT A phospholipid is an example of a macromolecule which consists of one glycerol molecule bonded to two fatty acids and a phosphate group

This might just prove useful…… Once the phosphoric acid molecule is bonded to the glycerol, it is often referred to as a phosphate-containing group or the phosphate head. The fatty acid chains are referred to as the fatty acid tails

You’ve probably realised that the 16 Biological prefixes and suffixes that you were given earlier in topic 2 have been playing key roles in every lesson lately and here they are again mono poly mer philic hydro phobic ase glyco myo lysis macro tion cyte ose micro eu The two that have been highlighted relate to the fatty acid tails – take a few minutes to discuss what this key term now tells you about the tails

PHOSPHOLIPID KEY POINT In a phospholipid, the fatty acid tails are nonpolar and so are repelled by water. As such, these tails are referred to as being hydrophobic

Understanding check The phosphate head is the “polar” opposite of the fatty acid tails. Copy the passage below and then use the previous KEY POINT and the statement above to help you to select the most appropriate terms to go into the gaps (3 marks) When surrounded by water, the phosphate group has negative charge and is therefore ______. polar This a _____ means that the phosphate head is attracted to water hydrophilic and is referred to as being ______

This might just prove useful…… You will have encountered a phosphate group before when looking at the structure of a DNA nucleotide and it will be important to remember that this is negatively-charged when you come to topics like electrophoresis later in the course

2. 2 (g) QUIZ R 2 Round 2 is going to challenge your Biology, Maths and “English” skills as you have to convert FROM NUMBERS 2 LETTERS

FROM NUMBERS 2 LETTERS On the next slide there a 2 phospholipid-based statements which each have numerical answers and each of those numbers corresponds to a letter of the alphabet (e. g. if the number is 6, then the letter is F). Your challenge is to convert FROM NUMBERS 2 LETTERS and to be the 1 st to buzz in with another commonly-used prefix. The reward for a correct answer is 2 TEAM POINTS FROM NUMBERS 2 LETTERS

FROM NUMBERS 2 LETTERS ü Fatty acids in a phospholipid i b ü (Carbon atoms in a glycerol molecule) 2 FROM NUMBERS 2 LETTERS

2 TO THINK ABOUT You have 5 minutes to discuss as a class If the fatty acid tails in a phospholipid are hydrophobic, how is it possible for this macromolecule to be the main component of a cell membrane when there is an aqueous solution on both the inside and outside of the cell? Combine all of your brain power and use the prefix from the latest competition to help you to start to consider this question and by the end of this 5 minutes, try to have a class diagram showing how the cell membrane is arranged to overcome this problem

There are two rows of phospholipids, arranged into a bilayer. The tails point inwards and away from the water whereas the heads point outwards and into the solution The individual phospholipids are freely moveable in the layer but never in a way to expose the tails to water, which provides some membrane stability

At the very beginning of this lesson, we recalled that the function of the cell membrane was to control the passage of substances in and out of the cell. This is achieved by the membrane being selectivelypermeable to the point where only small, non-polar molecules are allowed to move through the tails in the bilayer. This is exemplified by the diffusion* of oxygen from the alveoli to the red blood cells in the capillaries and the diffusion of carbon dioxide in the opposite direction

This is actually fairly important…… * At A-level, this movement of substances through the tails in the bilayer, that goes with the concentration gradient, is known as simple diffusion

1 TO THINK ABOUT You have 5 minutes to discuss with 1 person In the next topic, you will be introduced to another form of diffusion known as facilitated diffusion. By thinking about the meaning of any similar sounding words in English, begin some initial discussions about how this form of diffusion may differ from simple diffusion

Facilitated diffusion is covered in detail in topic 4 and this is the movement of molecules across a cell membrane with the concentration gradient, just like simple TO T HE diffusion. However, the molecules do not go LINK through the tails in the phospholipid bilayer. A translation of facilitate is to make a process easier, and in line with this, polar or large substances are given assistance. Fto move into UTU the cell through the use of channel or. Rcarrier E proteins that span the membrane

Prior knowledge check (GCSE) Carrier proteins facilitate the diffusion of molecules across the cell membrane By verbalising your answers to a partner, name another method of movement that uses carrier proteins and then describe how this method differs from facilitated diffusion (2 marks)

Active transport moves molecules against the concentration gradient using carrier proteins

This might just prove useful…… Between 20 – 80% of plant and animal cell membranes are made of phospholipids and as you’ve just seen, some of the remainder are proteins. Another notable component of the membrane is cholesterol, and this is a type of lipid that is not made out of glycerol or fatty acids. A hydrophobic molecule, it sits between the fatty acid tails in the middle of the hydrophobic layer and regulates the fluidity of the membrane