4. CELL MEMBRANES AND TRANSPORT Flashcards

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1
Q

Examples of exocytosis

A
  • secretion of digestive enzymes from the cells of the pancreas
  • plants use exocytosis to get their cell wall building materials to the outside of the cell surface membrane
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2
Q

Why is ATP required for active transport

A

because substances are moved against a concentration gradient

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3
Q

How is ATP required for active transport

A
  • the energy is used to make the carrier protein change shape
  • transferring the molecules or ions across the membrane
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4
Q

Ions in high concentration in cells than outside

A

potassium and chloride ions

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5
Q

Role of the sodium-potassium pump

A
  • for each molecule of ATP used

- three sodium ions are pumped out of the cell and two potassium ions into the cell

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6
Q

Result of the sodium-potassium pump

A
  • the inside of the cell becomes more negative than the outside
  • a potential difference is created across the membrane
  • significant in nerve cells
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7
Q

Sites where active transport takes place

A
  • reabsorption of certain molecules or ions in the kidneys after filtration
  • absorption of some products of digestion in the gut
  • in plants, active transport is used to load sugar from photosynthesising cells into the phloem tissue
  • in plants, active transport is used to load inorganic ions from the soil into root hairs
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8
Q

Why is it important to maintain a constant water potential inside the bodies of animals

A
  • if the water potential of the solution surrounding the cell is too high, red blood cells bursts (haemolysis)
  • if the water potential is too low:
    red blood cell gets crenated (shrinks)
    plant cell gets plasmolysed (cytoplasm pulls away)
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9
Q

What happens when a plant cell is placed in a hypotonic solution

A
  • water enters the cell by osmosis and the volume of the cell increases
  • the cell wall pushes against the expanding protoplast and pressure starts to build up rapidly
  • this pressure potential increases until the water potential inside the cell equals the water potential outside the cell and equilibrium is reached
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10
Q

Method of preparing membranes to split open the bilayer

A

freeze-fracturing

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11
Q

Why is the membrane more fluid when there are more unsaturated fatty acid tails

A
  • the unsaturated fatty acid tails are bent

- so they fit together more loosely

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12
Q

Factors that increase fluidity of the membrane

A
  • more unsaturated fatty acids
  • shorter tail
  • higher temperature
  • more cholesterol
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13
Q

How do organisms respond at a lower temperature to live

A
  • membranes become less fluid
  • so bacteria and yeasts increase the proportion of unsaturated fatty acids in their membranes
  • so the membrane becomes more fluid
  • can also increase the amount of cholesterol in the membrane
  • this prevents close packing of the phospholipid tails so increases fluidity
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14
Q

In transmembrane proteins, what are the hydrophobic regions that cross the membrane made of

A

alpha-helical chains

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15
Q

Why do intrinsic proteins stay in the membrane

A
  • because the hydrophobic regions, made from hydrophobic amino acids, are next to the hydrophobic fatty acid tails
  • and are repelled by the watery environment either side of them
  • because the hydrophilic regions, made from hydrophilic amino acids, are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment inside or outside the cell
  • or line hydrophilic pores which pass through the membrane
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16
Q

Types of lipid present in the cell membrane

A

phospholipids
cholesterol
glycolipids

17
Q

Types of protein in the cell membrane

A

protein

glycoprotein

18
Q

Why can polar (charged) molecules not pass through the cell membrane

A
  • because the fatty acid tails of phospholipids that make up the membrane are non-polar (hydrophobic)
  • so they act as a barrier to most water-soluble substances
19
Q

How can phospholipids act as signalling molecules

A
  • some phospholipids can be modified chemically to act as signalling molecules
  • they can move about in the phospholipid bilayer, activating other molecules such as enzymes
  • they can be hydrolysed to produce small, water-soluble glycerol related molecules that can diffuse into the cytoplasm and bind to specific receptors
20
Q

How do cholesterol molecules fit in the phospholipid bilayer

A

like phospholipids, they have a hydrophilic head and hydrophobic tail so they fit in with their heads at the membrane surface

21
Q

Presence of cholesterol in cell membranes

A
  • almost as much as phospholipids in animal cells
  • much less common in plant cells
  • absent from prokaryotic cells (compounds similar to cholesterol serve the same function)
22
Q

How does more cholesterol increase the fluidity of the membrane

A
  • prevents close packing of the phospholipid tails
23
Q

Functions of cholesterol

A
  • prevent the membrane from becoming too rigid at low temperatures by preventing close packing of the phospholipid tails
  • interactions between phospholipid tails and cholesterol molecules prevent the membrane from becoming too fluid at high temperatures
  • important for the mechanical stability of membranes, as without it membranes quickly break and cells brust open
  • hydrophobic regions of cholesterol prevent polar molecules or ions fro escaping. particularly useful in the myelin sheath around nerve cells
24
Q

Functions of glycolipids/glycoproteins

A
  • carbohydrate chains project into the watery fluids surrounding the cell and form hydrogen bonds with water molecules, stabilising the membrane structure
  • carbohydrate chains form a sugary coating outside the cell called the glycocalyx
  • carbohydrate chains help the glycoproteins and glycolipids to act as receptor molecules, which bind with particular substances at the cell surface membrane
  • some glycolipids and glycoproteins act as antigens, allowing cell-cell recognition
25
Q

Molecules responsible for the stability of the membrane structure

A

cholesterol: without it, membranes quickly break and cells burst open
carbohydrate chains on lipids/proteins: form hydrogen bonds with water molecules

26
Q

What is the glycocalyx

A
  • carbohydrate chains on lipids/proteins form a sugary coating outside the cell
  • animals: mainly glycoproteins
  • plants: mainly glycolipids
27
Q

What determines the types of receptors on cells

A

the function of the cell

28
Q

Functions of proteins in the cell membrane

A
  • transport proteins
  • others may be enzymes
  • some proteins on the inside of the cell surface membrane may be attached to a system of protein filaments inside the cell, known as the cytoskeleton
  • involved in processes in membranes such as respiration and photosynthesis in mitochondria and chloroplasts