IB Biology HLCarbohydrates & LipidsPaper 1 & 2~11 min read
Phospholipids
Take a triglyceride, swap one fatty acid for a phosphate group, and you get a molecule with two ends that want opposite things. Drop a lot of them into water and they arrange themselves into a membrane without being told to.
📚 What you need to know
Phospholipids are formed from glycerol and fatty acids, but contain only two fatty acids — the third has been replaced by a phosphate ion.
The phosphate is polar, so it is soluble in water, or hydrophilic. The fatty acid tails are non-polar, so they are hydrophobic.
Having both regions makes phospholipids amphipathic.
In water they form monolayers and bilayers. A phospholipid bilayer is the basic structure of the cell membrane, with tails inwards and heads outwards.
The bilayer acts as a barrier to most water-soluble substances, so sugars, amino acids and proteins cannot leak out.
Small, non-polar molecules such as O2 and CO2 dissolve in the bilayer and diffuse across quickly, with no transport proteins needed.
Larger non-polar molecules such as steroid hormones also cross. They contain cholesterol, which has both hydrophobic and hydrophilic regions.
One swap changes everything
Phospholipids are also formed from glycerol and fatty acids. Unlike triglycerides, they contain only two fatty acids bonded to the glycerol, because the third has been replaced by a phosphate ion.
The purple block is glycerol in both molecules. Only the top attachment has changed, and that is enough to change what the molecule does in water.
The phosphate is polar, so it is soluble in water, or hydrophilic.
The fatty acid tails are non-polar and therefore insoluble in water, or hydrophobic.
Molecules with both polar and non-polar regions are said to be amphipathic.
Monolayers and bilayers
Because they have both hydrophobic and hydrophilic parts, phospholipid molecules arrange themselves when placed in water.
At a water surface, the hydrophilic phosphate heads orient towards the water and the hydrophobic tails orient away from it. The result is a phospholipid monolayer.
Mixed into water, two-layered structures known as phospholipid bilayers form. This is the basic structure of the cell membrane: hydrophobic tails face inwards and hydrophilic heads face outwards, towards the water on both sides.
Nothing organises the bilayer. The tails are not “trying” to hide from water — it is simply that any arrangement exposing them to water is less stable, so the molecules settle into the arrangement that is. Membranes assemble themselves, which is why they can also reseal themselves.
What can cross, and what cannot
The amphipathic nature of phospholipids means the bilayer acts as a barrier to most water-soluble substances. The non-polar fatty acid tails in the middle prevent polar molecules or ions from passing between them.
This is why water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell, and unwanted water-soluble molecules cannot get in.
Nothing physically plugs the gap. Polar molecules are blocked because they cannot dissolve in the hydrophobic middle layer.
Molecules that get through
Substance
Crosses the bilayer?
Why
O2 and CO2
Yes, quickly
Small and non-polar, so soluble in the lipid bilayer; no transport proteins needed
Steroid hormones
Yes
Larger but non-polar, because they contain cholesterol
Glucose, amino acids, ions
No
Polar or charged, so they cannot pass between the non-polar tails
Cholesterol and steroid hormones
Steroid hormones contain cholesterol, a type of lipid. The hydrocarbon region of cholesterol is non-polar, which is what allows it to cross lipid bilayers — although cholesterol also has a hydrophilic region, so it too is amphipathic.
Oestradiol and testosterone are two examples of steroid hormones formed from cholesterol, produced by gonadal tissues in the reproductive organs. Because of their lipid structure they can cross the lipid bilayer and travel readily into and out of cells and nuclei. Inside the nucleus, these hormones alter and direct the process of transcription.
Why that detail matters. Most hormones must bind to a receptor on the outside of a cell because they cannot get in. Steroid hormones can, which is why they act directly on the genes rather than through a chain of messengers inside the cell.
Worked examples
WE 1
Explaining bilayer formation
Explain why phospholipids form a bilayer when placed in water. (3 marks)
Point 1: the two regions
Phospholipids are amphipathic: the phosphate head is polar and hydrophilic, the fatty acid tails are non-polar and hydrophobic.
Point 2: what each region does
The heads orient towards the water and the tails orient away from it.
Point 3: the result
With water on both sides, the molecules form two layers with tails facing inwards and heads facing outwards.
Heads to the water, tails away from it, on both sidesthe word amphipathic is usually the first marking point — use it early
WE 2
Why glucose needs help
Explain why oxygen can diffuse straight through a cell membrane but glucose cannot. (3 marks)
Point 1: oxygen
Oxygen is small and non-polar, so it is soluble in the lipid bilayer and can diffuse across quickly without transport proteins.
Point 2: glucose
Glucose is polar and water-soluble, so it cannot dissolve in the non-polar fatty acid tails at the centre of the bilayer.
Point 3: the consequence
Glucose therefore requires a transport protein to cross the membrane.
The hydrophobic core is the filter, and it filters by polaritysay “non-polar tails” specifically; “the membrane blocks it” is not an explanation
WE 3
Steroid hormones
Explain how a steroid hormone such as oestradiol is able to affect transcription inside a cell. (3 marks)
Point 1: the structure
Steroid hormones are formed from cholesterol, and their hydrocarbon region is non-polar.
Point 2: getting in
Being non-polar, they are soluble in the lipid bilayer and can cross both the cell surface membrane and the nuclear membrane.
Point 3: the effect
Once inside the nucleus they alter and direct the process of transcription.
Lipid-soluble, so it reaches the DNA itselfcontrast with water-soluble hormones, which have to bind to receptors on the outside
💡 Exam tips
State that a phospholipid has two fatty acids, not three, and that a phosphate replaced the third.
Use the word amphipathic whenever you explain bilayer or monolayer formation.
Explain the barrier through the non-polar core, not through “the membrane is tight”.
Learn the two examples of steroid hormones: oestradiol and testosterone.
Remember O2 and CO2 need no transport protein.
⚠ Common mistakes
Drawing three tails on a phospholipid. That is a triglyceride.
Saying the tails are hydrophilic. The heads are; the tails are hydrophobic.
Saying phospholipids are polar. They are amphipathic — polar at one end only.
Saying the bilayer blocks everything. Small non-polar molecules pass through easily.
Saying steroid hormones bind to surface receptors. They cross the membrane and act in the nucleus.
That completes Carbohydrates & Lipids. The seven notes run as one argument: carbon makes four bonds, those bonds build monomers, condensation links monomers into macromolecules, and the exact structure of each one — which isomer, where the branches are, whether the tail is kinked — is what decides the job it does.
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