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Biology A Level > Transport in Cells > Flashcards

Transport in Cells Flashcards

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1
Q

Describe the two types of cell membranes ?

A
  1. Cell surface membranes
  2. Membranes around organelles

(note these membranes have the same structure)

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2
Q

What is the function of the cell-surface membrane ?

A

Act as a barrier between the internal cell environment and external environment

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3
Q

What is the function of organelle membranes ?

A

They separate organelles from the cytoplasm to allow for compartmentalisation (and therefore specialisation in organelle function)

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4
Q

What is the general function of cell membranes ?

A

They are partially permeable to allow for substances to move in and out of the cell/organelle

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5
Q

What are the main components of a membrane ?

A
  1. Phospholipid bilayer
  2. Cholesterol
  3. Channel/carrier proteins
  4. Glyolipids/glycoproteins
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6
Q

Describe the structure of the phospholipid bilayer ?

A

The phospholipid bilayer consists of phospholipids. These form a bilayer because the polar phosphate heads are hydrophilic (water loving) and the hydrocarbon tails are repelled by water (hydrophobiic)

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7
Q

What is the function of the phospholipid bilayer ?

A

It helps to make the membrane partially permeable to lipid soluble molecules and small polar molecules (CO2, H2O, O2) only.

This means that it also prevents any unwanted movement of water soluble/lipid soluble molecules

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8
Q

How does the structure of the phospholipid bilayer help its function ?

A

As the tails are non-polar, they attract other non-polar, lipids. The tails being non-polar also means that water soluble molecules cannot move in or out of the cell membrane through the phospholipid bilayer

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9
Q

Describe the structure of cholesterol ?

A

A type of lipid that can fit between phospholipid molecules

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10
Q

In what type of cells in cholesterol not found ?

A

Bacterial cells - this is because bacterial cells contain a cell wall, which provides enough strength for the cell to remain a constant shape

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11
Q

What is the function of cholesterol ?

A

Cholesterol regulates the fluidity of the cell membrane

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12
Q

How does cholesterol carry out its function when the temperature is too high?

A

Cholesterol binds to hydrophobic tails so that they pack closely together. This prevents the membrane from becoming too fluid, which would mean gaps in the bilayer would be too large, leading to leakages and the cell bursting

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13
Q

How does cholesterol carry out its function when the temperature is too low?

A

Cholesterol prevents phospholipids from binding too closely to each other in order to prevent freezing or fracking.

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14
Q

What are the two types of membrane proteins ?

A

Intrinsic (span across the whole bilayer)
Extrinsic (do not span across the whole bilayer)

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15
Q

What is the function of an extrinsic protein ?

A

They provide mechanical support to the membrane, or help to form glycoproteins

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16
Q

What is the function of an intrinsic protein ?

A

These proteins form carrier and channel proteins, which allow for the movement of molecules that would not be able to otherwise pass through the phospholipid bilayer

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17
Q

How does a channel protein carry out its function ?

A

A channel protein is structured so that its hydrophilic R groups are pointing inwards, allowing for small, polar molecules to pass through the bilayer

(facilitated diffusion, so movement is bidirectional based on the concentration gradient)

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18
Q

How does a carrier protein carry out its function (active transport)

A

Channel proteins are used to transport larger molecules. The molecules bind to the protein, which is then able to change shape for the molecule to move through the bilayer.

(active transport, so the hydrolysis of a molecules of ATP is needed for movement)

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19
Q

Describe the structure of a glycolipid and a glycoprotein ?

A

Glycolipid - a carbohydrate chain attached extrinsically to the surface of a lipid

Glycoprotein - a carbohydrate chain attached extrinsically to the surface of a protein

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20
Q

What is the function of glycolipids and glycoproteins ?

A

They act as cell surface receptors

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21
Q

Describe the “Fluid mosaic model”

A

Proposed in 1972, the model suggests that the cell membrane is fluid as phospholipids can move sideways within their monolayer.

The membrane is suggested to be like a mosaic because proteins are randomly placed in the bilayer, making them look like a mosaic from above.

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22
Q

What is the thickness of the cell membrane ?

A

Around 7nm

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23
Q

What are the main types of transport across a cell membrane ?

A

Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Co-transport
Endocytosis
Exocytosis

24
Q

Define simple diffusion ?

A

The net movement of particles from an area of high concentration to an area of low concentration (down the concentration gradient) through a partially-permeable membrane.

25
Q

Why do we describe transport in terms of net movement ?

A

This is because particles are moving in both directions, but the favoured direction is known as the net movement

26
Q

What is a concentration gradient ?

A

A difference between two different areas of concentration

27
Q

Does diffusion continuously occur ?

A

No, it occurs until an equilibrium is established between two areas of concentration

28
Q

What is equilibrium ?

A

A dynamic process (continuously occurring) where particles continue to randomly move across a membrane . However, the overall net movement of the particles is zero.

29
Q

When can simple diffusion occur across a partially-permeable membrane ?

A

If a substance is small enough and non-polar it can diffuse through the small gaps in the phospholipid bilayer. This is because the substance is not repelled by the hydrophobic tails, and because the membrane is also fluid

30
Q

Which substances cannot easily pass through the phospholipid bilayer and why ?

A

Any polar molecules (not dependant on size) - they will be repelled by the hydrophobic tails in the centre of the bilayer and not diffuse through. If they do then this occurs very slowly

Large molecules - the gaps in the phospholipid bilayer are too small to allow large molecules to diffuse through. If they did, they would diffuse very slowly

31
Q

What are the factors affecting the rate of diffusion ?

A

Steepness of the concentration gradient
Thickness of exchange surface
Surface area of exchange surface

32
Q

How does the concentration gradient affect the rate of diffusion ?

A

The steeper the concentration, the greater the rate of diffusion. However, as an equilibrium is reached, the rate of reaction will decrease over time

33
Q

How does the thickness of an exchange surface affect the rate of diffusion ?

A

The thinner an exchange surface, the shorter the distance that particles have to travel across. This therefore increases the rate of diffusion

34
Q

How does surface area increase the rate of diffusion ?

A

The larger the surface area, the greater the rate of diffusion

35
Q

What is facilitated diffusion ?

A

The net movement of particles from an area of high concentration to an area of low concentration (down the concentration gradient) through transport proteins in the plasma membrane

36
Q

What are the two types of transport proteins in the phospholipid bilayer ?

A

Carrier protein
Channel protein

37
Q

What is the purpose of channel proteins in facilitated diffusion ?

A

They are bidirectional (dependant on conc. gradient) and allow charged particles that are small enough and hydrophilic to pass through.

38
Q

What is the purpose of carrier proteins in facilitated diffusion

A

They are bidirectional (dependant on the concentration gradient) and allow for larger molecules to diffuse through the bilayer. They are specific to a complementary molecule

39
Q

What are the differences between carrier and channel proteins ?

A
  1. Channel is for charged particles, but carrier is for larger molecules
  2. Channel is less specific than carrier
  3. Channel is less controlled than carrier
40
Q

What factors affect the rate of facilitated diffusion ?

A

Steepness of concentration gradient
Number of transport proteins in the membrane
Temperature

41
Q

How does the steepness of the concentration gradient affect the rate of facilitated diffusion ?

A

The steeper the concentration gradient, the faster the rate of diffusion up to the point of equilibrium is reached. As this point is reached, the rate of diffusion slows down.

42
Q

How does the number of transport proteins affect the rate of diffusion ?

A

As the number of transport proteins increases, the rate of diffusion increases as there are greater proteins available to transport molecules.

However, this is up to a certain point if the concentration remains constant as eventually a point of saturation will be reached/ maximum number of embedded proteins in the membrane

43
Q

How does temperature affect the rate of facilitated diffusion ?

A

As the temperature increases, the rate of diffusion will increases as particles gain more kinetic energy, so molecules move faster through transport proteins.

Past the optimum temperature, the rate of diffusion rapidly falls however because the transport proteins denature (H-bonds break causing shape change) meaning that no transport can occur

44
Q

What is active transport ?

A

Active transport is the movement of molecules from an area of low concentration to an area of high concentration (against the concentration gradient). It therefore requires the hydrolysis of a molecule of ATP to provide energy and a carrier protein

45
Q

How does active transport occur (refer to transport proteins) ?

A

A molecule binds to the transport protein (usually carrier protein).

A molecule of ATP is hydrolysed, providing energy for a conformational (shape) change in the protein (by temporary bonds forming in the tertiary structure)

This allows the molecule to move across the membrane to be released to the other side.

The protein returns to its original shape

46
Q

What factors affect the rate of active transport ?

A

The number of carrier proteins in the cell membrane
The availability of ATP

47
Q

How does the number of carrier proteins affect the rate of active transport ?

A

As the number of carrier proteins increases, the rate of active transport increases as there are more proteins to transport molecules in a given time

48
Q

How does the availability of ATP affect the rate of active transport ?

A

If there is not enough ATP, then there is not a source of energy that allows for the carrier protein to undergo a conformational change. This would therefore decrease the rate of active transport

49
Q

What is cotransport ?

A

The movement of two molecules simultaneously through a transport protein

50
Q

Why is active transport needed in the mammalian ileum of the small intestine needed ?

A

This is because glucose is essential in the body for respiration, so all of the glucose from digestion must be absorbed

51
Q

Describe the three transports in the mammalian ileum to allow the transport of glucose into the blood ?

A

Na + ions are actively transported out of epithelial cells through the Na+/K+ pump (antiport), meaning there is now a low Na+ concentration in the epithelial cell

The low concentration of Na+ ions in epithelial cells causes the cotransport of Na+ from the small intestine into the epithelial cell. The Na+/glucose pump also causes the movement of glucose into the epithelial cell (symport)

This means there is a high concentration of glucose in the epithelial cell, so the glucose can diffuse into the blood through a channel protein.

52
Q

What are the two types of cotransport ?

A

Antiport - When the two molecules being transported are moved in opposite directions

Symport - When the two molecules being transported are moved in the same direction

53
Q

Give an example of a problem that deformations in transport proteins can cause

A

Cystic fibrosis - a change in the structure of channel proteins causes Cl- ions and H2O molecules to not be able to undergo cotransport. This makes mucus very concentrated and sticky

54
Q

Give two examples of active transport ?

A

Root hair cells transporting mineral ions in the soil

The transport of glucose at the end of the small intestine (mammalian ileum)

55
Q

Describe exocytosis and give an example ?

A

A double membrane vacuole forms around a large molecule. The vacuole fuses with the cell membrane allowing the vacuole and its contents to leave the cell

e.g. golgi apparatus forming vesicles to transport cells

56
Q

Describe endocytosis and give an example ?

A

A double membrane vacuole forms around a molecule. The vacuole fuses with the cell membrane and allows the vacuole and its contents to move into the cell

e.g. phagocytosis in white blood cells

Biology A Level (3 decks)

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