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Cell Biology > Membrane Structure > Flashcards

Membrane Structure Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

Describe why membranes have solid- and liquid- like properties.

A

Solid like to engulf cellular content

Liquid like to allow movement within and through membrane

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2
Q

How are lipid shape and lipid composition important for membranes?

A

Lipid shape influences the shape and properties of membranes
Lipid composition of different membranes in a human cell
Is dynamic, composition adapts to membranes function

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3
Q

Lipids assemble spontaneously into different structures in an aqueous environment. True or False?

A

True

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4
Q

Describe the structure of phospholipids.

A

Hydrophobic fatty acid tail region, hydrophilic phosphate head region.

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5
Q

How are the phospholipids orientated?

A

Polar head group interacts with water, while the fatty acid tails are usually turned away from aqueous environments, e.g. formation of a micelle (single layer)

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6
Q

What are liposomes?

A

Closed bilayer structures with head groups orientated outward and inward to aqueous environments with hydrophobic region of tails inbetween.

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7
Q

How can lipid shape be characterised?

A

The relative cross-sectional area/diameter of the head groups vs. the tails determine the lipid shape, ultimately determining membrane curvature.

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8
Q

What are cylindrical-shaped lipids?

A

Lipids where the head group has the same cross-sectional diameter as the tail, observed mostly in bilayer membranes

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9
Q

Which lipid shape is characteristic of micelle formation?

A

Cone-shaped: head group has greater cross-sectional diameter than fatty acid tails

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10
Q

Why is a planar phospholipid bilayer with edges energetically unfavourable?

A

A lipid layer wants to keep water from being exposed to the hydrophobic region, hence a planar layer is unstable.

A sealed compartment formed by a bilayer is the most energetically favourable because of this.

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11
Q

Curvatures of and within biological membranes can exist between the most energetically unfavourable and most favourable depending on the unique function of the region of the membrane. True or False?

A

True

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12
Q

Describe the membrane of vesicles.

A

Vesicles related to tubules: same-cross-section, 50-100nm in diameter and highly curved - likely cone shaped than cylindrical.

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13
Q

Describe the membrane of the Golgi apparatus

A

Highly curved membrane at rims, versus flat at centre - different shaped lipids throughout

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14
Q

Describe the membrane of the ER

A

Large sheets and tubular connections between sheets.
Different regions have different functions (protein synth/trafficking, lipid synth + vesicular transport network)

Smooth ER tubules - lipid synth, high curvature.
Rough ER - flat sheets studded with ribosomes - protein synthesis.

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15
Q

The transition temperature between gel and fluid membrane is dependent on the membrane composition. True or False?

A

True

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16
Q

What is the fluid-mosaic model?

A

The idea that the membrane is at the same time solid and liquid. It is comprised of small mosaics which can diffuse freely through and within the membrane.

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17
Q

Why is diffusion in membranes necessary?

A

Key for function of the membrane and its proteins

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18
Q

How can Ricin staining prove the membrane is asymmetrical?

A

Ricin, a lectin that binds to sugars on the membrane, stained onto rabbit erythrocyte membranes. Sugar polymers are always expressed towards the outside - not toward the cytosol, which is apparent in an electron micrograph of the stained membranes.

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19
Q

What is FRAP?

A

Fluorescent Recovery After Photobleaching. A modern tool to show free diffusion in the membrane.

Not every membrane component/domain can freely diffuse though.

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20
Q

What is lipid structure important for in membranes?

A

Lipid structure determines curvature, fluidity and thickness.

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21
Q

How does saturation of lipids influence membrane fluidity?

A

Straight chains = less fluid,

Unsaturated fatty acids = more fluid

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22
Q

How do sterols contribute to membrane properties?

A

Sterols, e.g. cholesterol = more thickness and more rigidity into the membrane

23
Q

Fine tuning the composition and properties of lipids allows control over…

A
  • Lipid and protein diffusion in the membrane
  • Membrane plasticity/deformability e.g. vesicular transport
  • Protein conformational change to influence functions, including transport protein, enzyme activity and signalling
24
Q

How are lipids made in the ER?

A
  1. Fatty acyl CoA + Glycerol phosphate -> phosphatic acids (catalysed by acyl transferases, e.g. GPAT and LPAAT)
  2. Phosphatases cleave phosphate groups from the phosphatic acid -> e.g. DAG
  3. Choline transferases for example can add choline groups (phosphate heads) onto the phosphatic acids.
25
Q

What makes the phospholipid diversity very large?

A

Number of possible different acyl chain and headgroup combinations

26
Q

Why do lipids have to be sequestered into storage components?

A

So that they are not toxic - lipid toxicity arises from their hydrophobicity.

27
Q

What is fat?

A

Proliferative tissue of lipids - adipocytes

28
Q

All cells produce lipids droplets. Just not as much as adipocytes. True or False?

A

True

29
Q

Describe the structure of lipid droplets

A

Lipid droplets surrounded by phospholipid monolayer, and inside filled with neutral lipids in order to pack tightly into a hydrophobic lumen. Outside studded with proteins to generate and maintain lipid droplet, allowing it to be dissolved in water.

30
Q

What are the two types of neutral lipids?

A

Triglycerides - fatty acids and glycerol

Sterolesthers - sterols that are estherified

31
Q

Where are sterolesthers and triglycerides found?

A

Made in ER then packaged into lipid droplets

32
Q

Which enzymes synthesise lipid droplets?

A

DGAT - Diacylglycerol acyl transferase

ACAT- Acetyl-CoA:cholesterol acyl transferase

33
Q

What is a function of lipid droplets?

A

To store and detoxify excess lipids, and to shunt phospholipids from ER

34
Q

Describe the membranes of the early secretory pathway

A

Interfacial voids, almost neutral surface charge

35
Q

Describe the membranes of the late secretory pathway

A

High molecular lipid packing, negative surface charge, inward transport of phosphatidyl serine, negative charge to head group

36
Q

Outline the extensive lipid remodelling in the secretory pathway

A

Unsaturated fatty acid chains are replaced with more saturated ones from ER to plasma membrane

More cholesterol and sphingolipids in the late secretory pathway

Membrane thickness increases and becomes more asymmetrical toward plasma membrane

37
Q

What are the three ways that lipids can be transported throughout a cell?

A
  1. Vesicles
  2. Lipid Transfer Proteins (LTPs) - extract lipids from membranes and transfer to new membranes
  3. Free diffusion - small lipids with a low degree of hydrophobicity
38
Q

What are Membrane Contact Sites (MCSs)?

A

The close proximity of the membranes of two organelles.

39
Q

Where can LTPs be found?

A

In (plasma) membranes / membrane contact sites

40
Q

What is the function of membrane contact sites?

A

To transfer lipids between organelles; influencing lipid composition, and also calcium concentration and therefore calcium signalling -> links to cytoskeletal dynamics.

Also aids in organelle fission and protein sorting within/between organelles.

41
Q

Where are some MCSs located?

A

Mitochondria and ER,
Vacuole and ER,
Nucleus and ER
Golgi and ER

42
Q

What is cholesterol?

A

A sterol - a type of lipid - with a rigid structure with rings, a short hydrocarbon tail. Headgroup (sticks out of plasma membranes) is a single hydroxyl group which is esterified when cholesterol is packaged into lipid droplets.

43
Q

What is the function of cholesterol?

A

Alters membrane fluidity, rigidity and thickness.

Utilised for membrane synthesis and for steroid hormones in mitochondria

44
Q

What are the two groups of cholesterol protein carriers?

A

Low Density Lipoprotein and High Density Lipoprotein

45
Q

Where is cholesterol synthesised?

A

Can be made in all cells but majority produced in the liver or taken up in digestion.

46
Q

What is the function of LDL?

A

To carry cholesterol from site of synthesis or uptake to peripheral tissues

47
Q

What is the function of HDL?

A

To carry cholesterol from periphery yo liver for degradation or excretion

48
Q

Explain how LDL leads to accumulation of cholesterol in peripheral cells.

A

LDL containing cholesterol arrives at tissues and binds to LDL receptors on membrane.

LDL receptors endocytosed with ligand (LDL containing cholesterol) into a lysosome and delivered to endosome.

At low pH (in the endosome) LDL receptors undergo conformational change releasing ligand from receptor.

Cholesterol esters in LDL are hydrolysed in cytoplasm and therefore accumulates in cell as required.

49
Q

Give one way that cholesterol accumulation/distribution in peripheral tissues can be regulated

A

LDL receptors will be synthesised snd trafficked to the membrane when a cell requires additional cholesterol.

50
Q

Describe how glycolipids are synthesised

A

Glycans are added onto hydroxyl of ceramide in ER and Golgi by stepwise addition of monosaccharides.

First monosaccharides added at cytosolic or lumenal faces of ER and Golgi.

Elaboration with additional, branched oligosaccharides structures (glycans) done in Golgi.

51
Q

Give an important example of glycans

A

Blood group antigens

52
Q

What are some functions of glycolipids?

A

Gangliosides, AKA glycosphingolipids, are essential in neuronal membranes.

Important in Lipids Rafts.

Face outside the cell; often used by pathogens for attachment

53
Q

Explain how cholera toxin uses GM1 to enter the cell

A

Cholera toxin is a pentamer, it’s B subunit binding 5x GM1 ganglioside, a surface glycolipid, allowing endocytosis of the toxin through secretory pathway, to ER then into cytosol. (Where is causes infection by altering G-alpha-s and adenylyl cyclase activity)

Cell Biology (9 decks)

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