Endogenous Pathway of Lipoprotein Metabolism (June 2018)

In an integrated curriculum, which our Medical School deploys, temporal distribution of related topics can be an issue which may seriously affect students' learning. This problem can go unnoticed by both lecturers and students. It is a worry when scheduling related topics far apart. They would be close together in a conventional curriculum.


Let me give you an example of what I mean.

1. The topic Exogenous Pathway of Lipoprotein Metabolism was covered in two ways - guided self-learning (GSL5) and a class lecture. However, these two inputs were spaced far apart on the time-table.
2. The GSL5 module was made available to students at the e-learning portal on 21 Sept 2017
3. The lecture Lipoprotein Metabolism was delivered in the lecture hall on 28 February 2018
4. Time difference between GSL5 and the lipoprotein lecture was 5 months plus 1 week
5. A semester exam was conducted on 25 June 2018 (9 months after GSL5 and 4 months after the lipoprotein lecture)

Module:
CELL AND TISSUE
COURSE (GMT 101)
PHASE I MD 2017/2018
GUIDED SELF-LEARNING (GSL) ON
STRUCTURE AND FUNCTION OF LIPIDS
Prepared by
PROF. FARIDAH ABDUL RASHID
DEPARTMENT OF CHEMICAL PATHOLOGY
SCHOOL OF MEDICAL SCIENCES
KUBANG KERIAN, KELANTAN
Updated on
11 September 2017
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Medical Doctor (MD) Programme Phase I
Academic Session 2017/2018
Cell & Tissue Course (GMT 101)
Week 2
GSL 5: Structure and Function of Carbohydrates, Lipids, Amino Acids
and Nucleic Acids (Biomolecules) ... uploaded at e-learning portal
KNSS, IZA, FAR
21 Sept 2017
Contents:
PLASMA LIPOPROTEINS (page 6)

1, Describe the structure and function of plasma lipoproteins.
2. Describe the exogenous pathway of plasma lipoprotein metabolism.
3. Describe the endogenous pathway of plasma lipoprotein metabolism.
4. Describe reverse cholesterol transport.
5. Describe how lipids can cause heart disease.

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Phase I
Year 1 Sem 2
MEDICAL DOCTOR (MD) PROGRAMME PHASE I (YEAR 1)
Academic Session 2017/2018
CVS Course (GMT 107)
Week 3
Lecture
Lipoprotein Metabolism
28 February 2018

Even though the lecture on Lipoprotein Metabolism was given late in first year (Sem 2), in the CVS Course (GMT 107), it should not be an excuse for students who are unable to connect with or recall earlier topics (Learning Issues, LI, or Learning Objects, LO).

Students may not be able to recall or connect that the same topic and essay questions have appeared earlier - in the self-learning module or guided self-learning (GSL), in their first week of Medical School (Sem 1, Cell & Tissue Course (GMT 101).

This shows that students often do not revise, re-study or check back on what they have learned very early in their medical course. There is a gap and topics early in their medical studies may have been forgotten altogether. However, they only need to revise, re-study or check back. They may not be able to connect past topics if they did not re-check, revise or re-study the topics that they have learned many months before to exam.

There are 3 possible essay questions concerning the topic Lipoprotein Metabolism. Any of these 3 essay questions can pop up on any exam - Term and Professional. Therefore, students should be aware that there are these 3 options. They should be prepared, rather than not be aware and be unprepared or go blank during exam.

For June 2018 exam, though half the students passed (73/134 or 54%), I am quite disappointed that quite a large number of students (61/134 or 46%) still failed a repeat essay question on lipoprotein metabolism, which had been made available to students in their first week of Medical School since 2014 (and even earlier). What a shame!

The essay question below was taken directly from GSL5, that was made available online on the e-learning portal on 21 Sept 2017, to all incoming first-year medical students. Students either saw it but overlooked it, or took it seriously, studied and attempted to answer it! There is no excuse for failing this essay question.


Essay
Describe the endogenous pathway of lipoprotein metabolism. (10 marks)

How often has this question been used?
This question has been used repeatedly since 2014.

Proposed Model Answer

(i) Diagram

Is a diagram required?
No and it is not necessary.
A diagram is not required since it is not specifically asked.
However, you can draw a labelled diagram or flowchart if it helps and guides you to answer the question.
Refer to any diagram on "endogenous pathway of lipoprotein metabolism" in any textbook or Google it. 

Be careful when drawing for an essay question. You can use simple ball and arrow diagram and label everything using abbreviations. But students often go wrong here because they cannot recall the lipoprotein abbreviations correctly, and which way the LDL cascade should go. Many remember it wrongly. There are 3 distinct pathways of lipoprotein metabolism, and they do not merge. Be extra careful. 

(ii) Text

	Any 10 Statements pertaining to the following, or other statements relevant to the topic.	Max. 10 Marks
1.	Endogenous pathway happens all the time.	1
2.	It involves VLDL-LDL metabolism or “LDL cascade” and LDL receptor-mediated uptake in liver.	1
	Synthesis of VLDL:
3.	The liver synthesizes VLDL. Hepatocytes release nascent VLDL into the Space of Disse àVLDL enter sinusoids à VLDL enter systemic circulation.	1
	VLDL have several fates:
4.	a)      VLDL transport triglycerides from liver to peripheral tissues for utilization.	1
5.	b)      When VLDL reach vascular/capillary beds, they undergo hydrolysis (lipolysis) by LPL where their triglycerides contents are hydrolysed to free fatty acids (FFAs) and glycerol.	1
6.	c)      VLDL remnants are taken up by liver via LDL (B,E) receptors.	1
7.	d)     The VLDL are reduced in size (but increased density) and are converted into VLDL remnants and IDL, as a result of lipoprotein lipase (LPL) activity, hydrolysis or delipidation, which requires apolipoprotein C-II as cofactor.	1
	Unesterified FFAs have several fates:
8.	a)      During lipolysis, some FFA can be carried by plasma albumin and dispersed in plasma for delivery to other cells. Not much of this happens.	1
9.	b)      Normally, FFA can enter underlying adipocytes by simple diffusion. Inside the adipocytes, the FFA are re-esterified to form triglycerides (TG) once more. Adipocytes store TG until required (as an energy source during fasting or starvation). A majority of FFA are stored in adipocytes following lipolysis.	1
10.	c)      In times of starvation, when blood glucose is low and glycogen reserves are low, triglycerides stored in adipose tissues are hydrolysed by hormone sensitive lipase (HSL) and the FFAs are released from adipose tissue. FFAs then attach to circulating albumin and are brought to liver for beta-oxidation, for continued energy supply.	1
	IDL have several fates:
11.	a)      IDL can be converted into LDL by LPL in blood (intravascular).	1
12.	b)      IDL can be taken up by hepatic receptors.	1
13.	c)      IDL can be converted into LDL by hepatic lipase (HTGL) in liver.	1
	LDL have several fates:
14.	a)      Normally, LDL are taken up by hepatic LDL (B,E) receptors. The contents of LDL are broken down into FC, CE, PL, TG/ DG/ MG/ FFA and amino acids. These are kept within the hepatocytes or recycled for use by other cells. Hepatic contents of FC and CE are regulated by acyl cholesterol acyltransferase (ACAT). Few things can happen if cholesterol is low, specifically in the cell or in the blood. If there is low cellular FC, stored cholesterol (as cholesteryl ester, CE) is broken down to free cholesterol (FC) by ACAT. If there is low plasma FC, the liver cell makes more FC via increased HMG-CoA reductase activity. The 2 enzymes, ACAT and HMG-CoA reductase, are sensitive to cellular and blood cholesterol levels, and combined, they regulate cholesterol levels in cells and blood.	1
15.	b)      LDL can also deliver its contents directly to cells, eg adrenal glands, for synthesis of steroid hormones.	1
16.	c)      Under abnormal levels of LDL in the blood (eg hyperlipidaemia), LDL can be taken up by the scavenger receptors present on extrahepatic tissues (EHT).	1
17.	d)     LDL apoB-100 will be oxidized due to the prolonged presence of LDL in blood. Also, the LDL particle is now smaller and apoB-100 is unstable at this stage. ApoB-100 becomes easily oxidised. Oxidised apoB-100 has higher affinity for macrophages. Thus, oxidised LDL will be taken up by macrophages in EHT.	1
18.	e)      Macrophages contain scavenger receptors on their surface. Macrophages are present on blood vessel walls. Oxidised LDL will attach to the receptors and be internalised by macrophages.	1
19.	f)       The oxidised LDL are hydrolysed into component cholesterol, fatty acids, glycerol and amino acids, which are stored within the macrophages.	1
20.	g)      This uptake of oxidised LDL is unregulated (ie, uncontrolled). The macrophages take up as much LDL as they can until they stop functioning and die - becoming foam cells.	1

OUTCOMES

Students' English proficiency

1. Some can write well
2. A few cannot write correct English sentences and seemed to struggle
3. Not knowing how to properly use some words in sentences makes it difficult to comprehend what students are writing or trying to convey. Use of certain words seem difficult for students: constituent, composition, comprise, compose; retained, remained, degrade, bind, combine, uptake, consumed, utilised, undergo, etc
4. Understanding of reciprocal relationships - lipoprotein density increases as size decreases
5. Practice will make essay writing perfect, but there is lack of written assignments in first-year Medical School.
6. There may be greater underlying problems such as learning the relevant science subjects in Bahasa Malaysia in primary and secondary schools. Subjects such as Biology, Chemistry, Physics, may have been taught in Bahasa. They should be taught in English if students have to master English at university level, and especially in a medical course.
7. Students have tried their best to answer in English.

Students' answering difficulties

1. No schema or outline to use to answer the question
2. Careless writing 
3. Haphazard writing 
4. Don't know how & where to begin
5. No proper writing style
6. Page improperly utilised
7. Considerable amount of scribbling at edges
8. Diagrams in pencil and not ink 
9. Thinking and writing at full speed 
10. No knowledge to even write anything
11. Blank or stuck
12. Unexpected essay question
13. No time to check answers

Students' handwriting

• Handwriting was terrible for a majority of the scripts.
• Most handwriting were tiny and had to be read close-up.
• 1 script was intelligible (hard to read and comprehend)

Students' creativity

• Abbreviations for VLDL, IDL and LDL were various and incorrect
• Swapped functions of LDL and HDL
• New abbreviations for lipoproteins were created!  
• New terminologies were created! 
• One student wrote on ADP and ATP

Diagrams drawn by students

• Interesting diagrams, but some were incomplete or wrong
• There were a few little diagrams of sorts. 

PROBLEMS FACED BY STUDENTS
26 students (19.4%) scored 0 marks in June 2018

Breakdown of problems faced by students who scored 0 marks:

• 18 students handed in empty answer scripts (completely no answer; nothing written).
• 1 student answered: formation of Chylomicron (with a smiley) [False]
• 1 student answered wrongly on Chylomicron Metabolism [Chylomicron Metabolism is Exogenous pathway of lipoprotein metabolism.]
• 1 student wrote: In endogenous pathway, lipoprotein is metabolised into chylomicrons. [False. This never happens!]
• 2 students wrote on Bile Metabolism / Cholesterol Synthesis and HMG-CoA [wrong answers]
• 2 students wrote on Lipid Digestion / Micelles and Chylomicron / Stomach Functions and Chylomicron [wrong answers]
• 1 student wrote: Lipoprotein mechanism is breakdown of lipoprotein into amino acids. [False]

FOCUS of this essay question

Since this essay question is specifically about VLDL-IDL-LDL metabolism or endogenous pathway, any answer on chylomicron metabolism or exogenous pathway will be incorrect. Since HDL metabolism is not asked, there is no need to touch on HDL metabolism, except its associated role in the endogenous pathway.

Other answers such as stomach contents and functions, lipid digestion (formation of micelles, enzymic digesiton of lipids), lipid absorption, formation of chylomicron in enterocytes, passage of chylomicron from lacteals via lymphatics to liver are all irrelevant.

Other processes in the liver such as cholesterol synthesis are irrelevant as this question is specific and focused on the metabolism of lipoproteins containing apo B-100.


FALSE Statements

1. This VLDL is major transported [transporter?] of cholesterol [False].
2. LDL send the phospholipid to cell for cell integrity or to the cell that uses it for the production of energy [False].
3. VLDL is the major transporter of cholesterol from adipose tissues [False]. 
4. Lipoproteins are used to generate energy. [False]
5. Endogenous pathway of lipoprotein metabolism is metabolism of chylomicrons [False].

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IRRELEVANT Answers

The essay question specifically asked about endogenous pathway (VLDL-IDL-LDL metabolism), which occurs all the time in the body, whether we are fasting or not. There is no need to write about other topics. There is no need to write about carbohydrate metabolism (polysaccharide digestion, glucose and rbc metabolism). There is no need to write about GIT (stomach contents and functions). There is no need to write about lipid digestion and absorption (bile, lipases, micelles, lacteals) Stay focussed on the topic.

(a) Glucose & rbc metabolism

1. Glucose is converted into pyruvate which will form the cholesterol.
2. When rbc is damaged, or aged, lipoprotein will metabolised to be store into the body as a source of heat, membrane fluidity.

(b) Lipid digestion and absorption 

1. So lipoprotein metabolism functions to break down lipid and protein to their respective basic units for better absorption.
2. Lipid is broken down to its simplest form which are fatty acids and 3 glycerols with the help of ADH for energy.
3. Lipid will enter lacteals (specialised lymph in the villi) to be utilised by the body again (as chylomicra).

(c) Carbohydrate digestion 

1. From complex polysaccharide, it is broken down into disaccharide by removal of water. Then disscharide is broken down to amino acids which is the simplest form of protein.

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Student Answers on

ENDOGENOUS PATHWAY OF LIPOPROTEIN METABOLISM

[F] = False statement

[?] = examiner's query

(a) Students' understanding of the different lipoprotein pathways

1. There are 2 pathway(s) involved in lipoprotein mechanism which are through endogenous pathway and exogenous pathway
2. Endogenous pathway is about VLDL
3. Endogenous pathway of lipoprotein is when it is synthesis from cholesterol inside the liver.
4. Endogenous Pathway is occur within circulation. Endogenous Pathway of lipoprotein metabolism consist of VLDL-LDL metabolism, LDL metabolism and LDL-mediated receptor (uptake).

(b) Ambiguous statements written by students

1. Lipoprotein metabolism occur in liver. It is combination of lipid and protein.[Abstract writing!]
2. Cholesterol is transported from the GIT to the liver in both estercifed [esterified] and free form in the liver. [Meaning?]
3. Cholesterol under go oxidation estirification and compannded with protiens Apo proteins to form lipoproteins which are sent to the bloodstream as VLDL. [Meaning?]

(c) Lipoprotein structure

1. Lipid basic structure and triglyceride and fatty acids
2. For proteins, its amino acids
3. Cholesterol combines with apolipoprotein
4. Cholesterol, apolipoprotein, phospholipid and triglyceride will form VLDL and also empty HDL
5. Lipoprotein is constituent of both lipid and protein.
6. Triglyceride has about 60% in VLDL [This is better written as VLDL contains approximately 60% triglycerides.]
7. VLDL has Apo B-100 molecule [This is better written as Each VLDL has an apo B-100 molecule.]
8. IDL is high in cholesterol

(d) Classes of lipoproteins

1. There are 2 lipoprotein(s) that (are) present in (the) body which are high density lipoprotein (HDL) and low density lipoprotein (LDL). [False] [There are 4 major classes of lipoproteins.]
2. The last one is HDL(?)

(e) Function of lipoproteins

1. LDL is described as bad cholesterol
2. LDL is major transporter of cholesterol. It send the cholesterol (to) the cell of adipose and liver to be stored.
3. HDL is described as good cholesterol
4. The function of HDL is to transport excess cholesterol to the liver to be metabolised. The HDL is the could be recycle for the next  use.

(f) Lipoprotein conversions

1. In lipoprotein metabolism, chylomicron is converted into very low density lipoprotein (VLDL) [F]. VLDL in then convert into intermediate low density lipoprotein (ILDL) [F]. ILDL will form low density lipoprotein (LDL) which then converted into high density lipoprotein (HDL) [F].
2. Chylomicron will bind to Apo E and C to become high density lipoprotein [F]. Then it become  intermediate density lipoprotein, low density lipoprotein and very low density lipoprotein [F]. 
3. After the VLDL will converted into IDL or LDL
4. Triglyceride in VLDL is further dehydrolysed leads to conversion of VLDL to IDL.
5. Very low density lipoprotein (VLDL) is degraded by IDL [F]. [VLDL is degraded by LPL to IDL.] Then, forming Low density lipoprotein (LDL) and exocytosed [endocytosed?] either via (scavenger) receptor-mediated or via hepatic (receptors). Then, LDL is taken up to the hepatocytes [taken up by hepatocytes?]. LDL converted to free cholesterol in liver.

(g) Lipoprotein movements

1. Lipid (from circulation) will flow into the liver to (be) metabolised.
2. VLDL will transport cholesterol from tissue to liver for excretion and HDL will transport cholesterol from liver to tissue [F]. [What does VLDL transport?]
3. When it enters the blood circulation, VLDL attaches to Apo C and Apo E
4. Upon entering the circulation, Apo CII activates lipoprotein lipase (LPL). Lipoprotein lipase hydrolyses triglyceride into free fatty acid and glycerol
5. IDL is taken up by the hepatic cells. This is because hepatic cell recognise Apo E on IDL. Apo C and Apo E is retained from IDL [retained with IDL?]. IDL is hydrolysed [to what?]

(h) Products of lipoprotein hydrolysis by LPL

1. Free fatty acid is used for energy, milk lactating
2. Glycerol is needed in glucogenesis and gluconeogenesis

(i) Lipoprotein uptake

1. Macrophages engulfing lipoprotein becomes foam cell

(j) Depth of students' understanding of the topic asked

1. LDL is low density lipoprotein which collects cholesterol from liver to body
2. HDL is good cholesterol which collects cholesterol from body to the liver
3. Liver will produce VLDL with attachment of Apolipoprotein B-100 which is produced by hepatocyte
4. VLDL will combine with Apo E and Apo CII
5. CII will activate lipoprotein lipase to breakdown triglyceride into fatty acid and glycerol
6. Fatty acid can be used in synthesis of lipid, energy storage, production of milk fat in mammary gland
7. The breakdown of triglyceride reduce the VLDL into IDL (VLDL remnant)
8. IDL will transport into the hepatocyte by binding of Apo E on the surface of liver (LDL receptor)
9. IDL will also degrade into LDL with the help of HTL (hepato ... lipase) [HTL or HTGL is hepatic triglyceride lipase.]
10. LDL is more dense and reduce in size [Meaning?] [LDL is smaller and denser than IDL.]
11. It contain more cholesteryl ester
12. LDL can enter the liver by binding of Apo B-100 at the LDL receptor. [Meaning?] [Where is apo B-100 located?] [LDL apo B-100 binds to LDL receptors present on hepatocytes. OR LDL binds to LDL receptors via its apo B-100.]

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CORRECT answers attempted by students (10 marks)
Text only. Associated diagrams are not included here.

Example 1

1. VLDL (very low density lipoprotein) is synthesized in the liver and consist of triglycerides and Apo B-100. [F] [VLDL contains 4 lipid classes and apo B-100.]
2. Then, VLDL is goes out of the liver into blood circulation.
3. Inside the circulation, it receives two apolipoprotein from HDL which is apo C and apo E.
4. Then, VLDL is hydrolysed by lipoprotein lipase which hydrolyse the triglycerides in(to) fatty acids and monoglycerides.
5. This make VLDL size and density decrease [F].[VLDL size is reduced, but its density increases after lipolysis by LPL.]
6. Before transported back into liver, apo C return back to HDL and VLDL remnants diffuse into liver by lipoprotein receptor and by endocytosis. [Diffuse into liver or taken up by liver?]
7. The VLDL remnants become IDL but IDL does not remain for a long time before it converted into LDL (low density lipoprotein). [IDL exists transiently in blood.]

Example 2

1. VLDL (very light density lipoprotein) is form(ed) endogenously in liver. 
2. It consist(s) of free amino acid, cholesterol, cholesteryl ester, phospholipid, triacylglycerol and apo B-100. [VLDL cannot contain free amino acids! Apo B-100 is a large protein!]
3. It is released into circulation.
4. It then take up apo C and apo E from HDL (High density lipoprotein).
5. Apo C will activate Lipoprotein lipase in cappilary epithelium to break down triacylglycerol in VLDL into fatty acid and glycerol. [It should be capillary endothelium.]
6. Now VLDL decrease in volume increase in density and now convert to IDL (intermediate density lipoprotein).
7. IDL bind to lipoprotein receptor at hepatocyte  via apo E through endocytosis. [IDL are internalized by endocytosis.]
8. In the hepatocyte, triacylglycerol in IDL (undergo) further break down by hepatic lipase.
9. Now the compound is increase in density and rich in cholesterol.
10. It is now LDL (low density lipoprotein).
11. It will (be) released by the hepatocyte.
12. After that LDL (is) either absorb(ed) by the liver or posthepatic tissues to (be) further metabolize(d).

Example 3

1. In the lipoprotein metabolism, it consist the exogenous pathway, endogenous pathway and the reverse metabolism pathway. [F] [There is no such thing as reverse metabolism pathway! There is reverse cholesterol transport.]
2. During the endogenous pathway it is started in the liver.
3. From the very low density lipoprotein (VLDL) which is present inside our body will produce in the liver, 
4. It is function to bring the cholesterol to the tissue cell for storage.
5. When it enter the circulation it will undergo lypolysis and converted into the Intermediate density lipoprotein (IDL). 
6. This IDL is appear in the blood just for a while. [Transient]
7. Next the IDL will undergo the lipolysis and then being the low density lipoprotein (LDL).
8. This LDL will deposit into our tissue.
9. Plus the LDL is very anthrogenic [atherogenic], thus LDL will lead into the formation of the atherosclerotic plaque and lead to other disease.
10. This plaque prone to forming more at the abdominal aorta than the thoracic aorta.
11. This LDL that deposit inside the tissue will be taken up back into the liver via the high density lipoprotein (HDL) during the reverse lipoprotein pathway. [Reverse cholesterol transport]
12. The LDL serve as the bad cholesterol inside our body.
13. {Diagram}

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WRONG answers attempted by students (0 marks)

Example 1

1. Chylomicrons are found in intestinal mucosa from dietary lipid. 
2. During the endogenous pathway, high density lipoprotein (HDL) serves as reservoir for apo-C and apo E. [F] [Chylomicron metabolism is exogenous pathway.]
3. The apo-C and apo-E will bind to chylomicrons.
4. As chylomicrons enters the blood circulatory system, lipoprotein lipase (LPL) will hydrolyse the triglycerides into free fatty acid and glycerol.
5. As the triglycerides are broken down, the size of chylomicrons decreases, becomes smaller but its density increases.
6. The Apo-I in chylomicrons will restore back to high density lipoprotein before reaching liver  while the apo-E will stay with the chylomicrons. [Which apolipoproteins go where?]
7. The remnants of chylomicrons reach liver, and will be metabolized as source of energy, heat energy. [F] [What do chylomicron remnants contain?]

Example 2

1. Endogenous pathway of lipoprotein metabolism begins with the absorption of chylomicrons. [F]
2. The chylomicron binds with Apo B-II. [F] [There is no such thing as Apo B-II.]
3. The chylomicron then travels through the lymphatic system while being metabolised by lipoprotein lipase .
4. After a few stages of digestion, the chylomicron enters the liver.

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WRONG answers mixed with correct answers

Example 1 (2 marks)

1. Endogenous pathway.
2. The lipoprotein is transported as VLDL. [1 mark]
3. Then, it changes into LDL. [1 mark]
4. The LDL form chylomicron. [F]
5. Chylomicron is absorb(ed) by lacteal. [irrelevant]
6. Then it moves into blood. [irrelevant]

Example 2 (4 marks)

1. When dietary lipids and cholesterol enter intestine, they (are) converted to tryglyceride, monoglyceride  and fatty acids by intestinal lipase. [The correct spelling is triglyceride.]
2. They are absorbed into intestinal cells which (are) then converted into tryglyceride by golgi apparatus [Golgi apparatus].
3. Tryglyceride, cholesterol will be bind forming chylomicrons (CM). 
4. Chylomicrons are too big to enter blood vessels so it will enter lacteals which then enter blood vessels.
5. CM are utilised by lipase that (are) present in blood vessels forming smaller lipids to be transported to muscle cells for energy and to liver for storage,
6. CM are then transported to liver.
7. Cholesterol will bind to Apo B to form very low density lipoprotein (VLDL). [1 mark]
8. VLDL will be transported in blood vessels where it  (is) utilised by tissues converting it to Intermediate density lipoprotein (IDL). [1 mark]
9. IDL (is) transported to liver forming Low density lipoprotein (LDL). [1 mark]
10. LDL is then again circulated in blood to be utilised by tissues for energy. [1 mark]

End