Case 2 BIOCHEM - biological membranes

Question 1

Role of biological membranes in the life of a cell

Answer 1

1. acts as barrier which separates the inside of the cell from extracellular environment - preserve homeostasis
2. selectively permeable - control movement of substances in and out of the cell
3. allow for the flow of information between cells and their environment: binding to signalling molecule & sending chemical or electrical signals to other cells

Question 2

Name the three major components of a membrane

Answer 2

lipids, proteins & carbohydrates

Question 3

Explain where lipids are found in the membrane and their functions

Answer 3

• found in the phospholipid bilayer. phosphate heads are hydrophilic and lipid tails are hydrophobic. Hydrophobic tails congregate at he interior of the membrane, may contain cholesterol and other minor lipids.
• fn. prevent solar solutes from diffusing across membrane and allows for passive diffusion of hydrophobic molecules.

Question 4

Name and describe the types of proteins found in biological membranes

Answer 4

• broad categorisation: peripheral vs. integral proteins

types:
1. anchoring protein: attach plasma membrane to other structures and stabilise its position.

2. recognition protein: mainly glycoproteins, allows for immune cells from distinguishing self from non-self
3. enzymes: may be integral of peripheral proteins, can catalyse reactions in ECF or cytosol.
4. Receptor proteins: sensitive to binding of ligands
5. carrier proteins: binds solutes and transport them across plasma membrane
6. channel proteins: some integral proteins contain central pores that form passageways across the membrane. Allows for movement of water and small molecules.

Question 5

Describe the structure and functions of membrane carbohydrates

Answer 5

• forms glycolipids or glycoproteins
• carbohydrate portions of glycolipids & glycoproteins extend beyond the outer surface of the membrane, forming a layer called the GLYCOCALYX

functions:

1. lubrication & protection
2. anchoring and locomotion
3. provide specificity in binding: recognition by immune system

Question 6

Properties of biological membrane and why they are important

Answer 6

1. stability - prevent leakage
2. selective permeability - keeps intracellular materials inside whilst allowing passage of wanted material & elimination of wastes
3. extendibility - for cell division, membrane fusion/fission
4. flexibility - for shape and movement of cell

Question 7

Functions of biological membranes

Answer 7

1. protection - glycocalyx protects epithelial cells against abrasion & acidic environments (e.g. intestinal lumen)
2. binding - cell surface receptors recognise receptors on ligands
3. signalling - membrane receptors can trigger intracellular signalling cascades (e.g. GPCR - cAMP cascade)
4. adhesion - allows for cell to cell adhesion and cell-substratum adhesion
5. shape & movement - actin filaments are bound to specific membrane proteins that lie just beneath the cell membrane which confers shape and rigidity to the cell surface.
6. transport - selective permeability

Question 8

Explain how molecules cross biological membranes

Answer 8

• small and lipophilic molecules cross by simple diffusion (O2, N2, CO2, H2O)
• ions, sugars, amino acids and hydrophilic molecules cannot diffuse fast enough to meet cells’ metabolic needs thus are transported by integral membrane proteins (carrier and channel proteins)

Question 9

Outline the types of membrane transport that are facilitated by membrane protein

Answer 9

1. facilitated transport - does not require energy, substances moving down concentration gradient. For movement of fat insoluble or large molecules (e.g. GLUT)
2. active transport - require energy and molecules move against concentration gradient. (e.g. Na/K ATPase)

Question 10

Mechanisms of membrane transport processes

Answer 10

1. Passive transport by simple diffusion
2. Carrier mediated transport including passive diffusion and active transport
3. Vesicular transport

Question 11

Explain the biochemistry mechanism of ORT

Answer 11

1. Na membrane chemical potential is created by the Na/K ATPase at the base of the enterocyte. This protein pumps 3K into the cell and 2Na out of the cell. This results in the Na concentration inside the enterocyte lower than that of the intestinal lumen.
2. Na is transported into the, cell coupled by glucose via SGLT-1. This symporter only works in presence of both glucose and Na. Without Na, glucose cannot be transported into the cell due to a lack of driving force, and without glucose Na cannot be transported into the cell because the inherent mechanism of SGLT-1.
3. at the base of enterocyte, glucose enters interstitial fluid by the action of GLUT2 pumps, and Na is pumped into the interstitial fluid by the Na/K ATPase.

Question 12

Functions of the various components of biological membranes

Answer 12

1. lipid & cholesterol contribute to the structure of membrane
2. proteins contribute to function
3. sugars attach to proteins and lipids forming carbohydrates (glycoproteins/glycolipids) that give unique “identities” to cells.

Question 13

Which aspect of the structure of biological membranes aids the self-assembly of phospholipid bilayer?

Answer 13

the bulkiness of the hydrophobic tails. Therefore two tails are attached to a single hydrophilic head

Question 14

Structure of wax

Answer 14

ester bond forming between a fatty acid and a fatty alcohol

Question 15

Structure of triglyceride

Answer 15

glycerol backbone (3C) linked to 3 hydrocarbon chains

Question 16

How to generate an amphipathic molecule from a triglyceride

Answer 16

add a phosphate group to the glycerol backbone by removing one of the hydrocarbon chains. This forms phosphatidic acid.

Question 17

How to make phosphatidic acid compatible with biological membranes

Answer 17

by partially neutralising the phosphate head by adding a choline. this structure contains a 3C glycerol backbone, a phosphate, choline, and 2 acyl chains, known as phosphatidyl choline (a type of glycerophospholipid).

Question 18

What is cardiolipin

Answer 18

• a type of glycerophospholipid derived from phosphatidic acid. consists of 2 phosphatidic acids attached to glycerol (4 fatty acid chains instead of 2).
• Normally found in mitochondrial membrane in metabolically active cells

Question 19

2 Types of phospholipids

Answer 19

1. Glycerophospholipid

2. Spingophospholipid

Question 20

Structure of sphingophospholipid

Answer 20

1. consists of sphingosine (instead of glycerol), 2 fatty acid chains, phosphate and choline
2. Sphingomyelin: insulate axons in nervous system

Question 21

Difference between glycolipids and phospholipids

Answer 21

• Glycolipids also contain glycerol or sphingosine linked to fatty acids but have sugars as polar head groups, instead of phosphate.

Question 22

Structure of cholesterol and their role in membrane structure

Answer 22

1. 4 ringed steroid together with a short hydrocarbon side chain
2. polar head slightly hydrophilic (OH)
3. rigid planar steroid ring structure
4. non-polar tail
5. fits into membrane bilayer with its hydroxy group close to the phosphate head and ring steroid structure interact with neighbouring regions of lipid tails - stiffens and rigidify them

Question 23

Motions of biological membranes

Answer 23

1. lateral diffusion
2. flexing of acyl tails
3. transverse diffusion
4. rotational diffusion

Question 24

Permeability coefficient

Answer 24

measure of how easily a particular substance can permeate bilayer

Question 25

Define “transition temperature”

Answer 25

temperature at which bilayer melts

Question 26

Factors affecting membrane fluidity

Answer 26

1. temperature: at low temperature hydrocarbon tails pack closely together to form ordered arrangement. at high temperature lipid molecules vibrate more rapidly, causing bilayer to “melt”
2. cholesterol content
3. fatty acid composition