4.1 - structure of the cell surface membrane Flashcards

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1
Q

What are the membranes around and within cells known as

A

Plasma membranes

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2
Q

What is the cell-surface membrane

A

The name given to: the membrane that surrounds the cell, forming a boundary between the cell cytoplasm and the environment

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3
Q

What are the 2 basic functions of the cell-surface membrane

A
  • allows different conditions to be established inside and outside the cell
  • controls movement of substances in and out the cell
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4
Q

What are the different molecules that form the structure of the cell-surface membrane

A
  • Phospholipids
  • proteins
  • cholesterol
  • glycolipids
  • glycoproteins
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5
Q

Describe how the phospholipids form the phospholipid bilayer

A
  • The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
  • The hydrophobic tails of both phospholipid layers point to the centre of the cell-surface membrane repelled by water on both sides
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6
Q

What type of material moves through the membrane via the phospholipid portion, and how does this relate to the functions of phospholipids in the membrane

A
  • lipid-soluble material
    FUNCTIONS
    —> allow lipid-soluble substances to enter and leave the cell
    —> prevent water-soluble substances entering and leaving the cell
    —> make the membrane flexible and self-sealing
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7
Q

How are proteins embedded in the phospholipid bilayer

A

Proteins are dispersed throughout the membrane: there’s 2 types
- Extrinsic/peripheral proteins
- Intrinsic/integral proteins

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8
Q

What are extrinsic proteins

A
  • occur in the surface of the bilayer and never extend completely across it
  • Either: give mechanical support to membrane, or act as cell receptors for peptide hormones (role in cell recognition)
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9
Q

What are intrinsic proteins

A
  • completely span the phospholipid bilayer from one side to the other
  • act as: protein channels = form water-filled tubes to allow water-soluble ions to diffuse across the membrane
  • act as: carrier proteins = bind to ions or molecules (like glucose and amino acids), then change shape in order to move these molecules across the membrane
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10
Q

What are the functions of the proteins in the membrane

A
  • structural support
  • act as channels, transporting water-soluble substances across the membrane
  • allow active transport across the membrane through carrier proteins
  • form cell-surface receptors for identifying cells
  • help cells adhere together
  • act as receptors, e.g. for hormones
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11
Q

Where do cholesterol molecules occur

A
  • present in all cell membranes (except bacterial)
  • fit between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid
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12
Q

What are the functions of cholesterol molecules in the membrane

A
  • add strength to the membranes, maintaining the shape (important for cells that aren’t supported by other cells, e.g. red blood cells)
  • very hydrophobic = play role in preventing loss of water and dissolved ions from the cell
  • fits between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid —> reduce lateral movement
  • make the membrane less fluid at high temps
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13
Q

How do the phospholipids molecules form a bilayer

A

They automatically arrange themselves into a bilayer

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14
Q

Why can’t water-soluble substances pass through the phospholipid bilayer

A
  • the centre of the bilayer is where hydrophobic tails meets = centre is hydrophobic
  • act as as a barrier to these dissolved substances
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15
Q

What are glycolipids made up of

A

A carbohydrate covalently bonded with a lipid

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16
Q

Describes the structure of glycolipids

A
  • the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals
  • e.g. the human ABO blood system operates as a result of glycolipids on the cell-surface membrane
17
Q

What are the functions of glycolipids in the membrane

A
  • act as recognition sites
  • help maintain the stability of the membrane
  • help cells to attach to one another and so form tissues (cell adhesion)
18
Q

What are glycoproteins

A
  • Carbohydrate chains are attached to many extrinsic proteins on the outer surface of cell membrane
  • act as cell-surface receptors, more specifically for hormones and neurotransmitters
19
Q

What are the functions of glycoproteins in the membrane

A
  • act as recognition sites
  • help cells to attach to one another and so form tissues
  • allows cells to recognise each one another, e.g. lymphocytes can recognise an organism’s own cells
20
Q

What is the functions of membranes within cells

A
  • control entry and exit of materials in discrete organelles such as mitochondria and chloroplasts
  • separate organelles from cytoplasm so that specific metabolic reaction can take place within them
  • provide an internal transport system, e.g. endoplasmic reticulum
  • isolate enzymes that might damage the cell, e.g. lysosomes
  • provide surfaces on which reactions can occur, e.g. protein synthesis using ribosomes on rough endoplasmic reticulum
21
Q

Why might most molecules not be able to freely diffuse across the cell-surface membrane

A
  • they’re not soluble in lipids: therefore can’t pass through the phospholipid layer
  • too large to pass through the channels in the membrane
  • they’re the same charge as the charge on the protein channels = even if they’re small enough, they’re repelled anyway
  • they’re electrically charged (polar) = have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
22
Q

What is the fluid-mosaic model

A

The model that shows the arrangement of all the molecules that create the cell-surface membrane

23
Q

Why is it called the Fluid-Mosaic model

A
  • fluid: individual phospholipid molecules can move relative to one another = gives membrane a flexible structure that’s constantly changing
  • mosaic: the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way that the stones/tiles in a mosaic do
24
Q

What’s the function of the phospholipid bilayer

A
  • allow movement of lipid soluble molecules, e.g. steroid hormones
  • prevent water soluble molecules leaving the cell
  • make membrane flexible and self-sealing
  • forms a barrier to dissolved substances
25
Q

What’s the function of the fatty acid tails

A
  • forms (water) impermeable barrier to water-soluble substances, and allows non-polar molecules to pass through
  • allows cell to maintain different conc. either side
  • makes membrane self-sealing/able to fuse other membranes/flexible
26
Q

Fill in the blanks

A
27
Q

Sketch a CSM diagram

A
28
Q

Describe what the phospholipid bilayer is formed from

A
  • the molecules automatically arrange themselves into a bilayer
  • phospholipid molecules have a hydrophilic ‘head’ that points out towards the water either side of the membrane
  • and a hydrophilic ‘tail’ that points inwards. This makes the centre hydrophobic = so doesn’t allow water-soluble substances (like ions) through it - acting like a barrier
29
Q

What is cholesterol and where is it

A
  • A type of lipid
  • present in all cell membranes (except bacterial cell membranes)