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A-Level Biology > 5 Plasma membranes > Flashcards

5 Plasma membranes Flashcards

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1
Q

What are membranes formed from?

A

Membranes are formed from a phospholipid bilayer.

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2
Q

Who proposed the fluid-mosaic model and when?

A

American scientists Singer and Nicolson

In 1972

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3
Q

Why is it called the fluid mosaic model?

A

The bilayer is ‘fluid’ because the phospholipids are constantly moving and protein molecules are scattered through the bilayer, like tiles in a mosaic.

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4
Q

What are the different components of a cell membrane?

A

Phospholipids, cholesterol, proteins, glycolipids and glycoproteins.

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5
Q

What is an intrinsic protein?

A
  • A protein molecule spanning the phospholipid layer.
  • It has amino acids with hydrophobic R-groups on its external surface, which interacts with the hydrophobic core of the membrane, keeping it in place.
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6
Q

What are some examples of intrinsic proteins?

A
  • Channel proteins & carrier proteins.

- They are both involved in transport across the membrane.

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7
Q

What do channel proteins do?

A
  • Channel proteins provide a hydrophilic channel that allows the passive movement. of polar molecules and ions down a concentration gradient through membranes.
  • They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
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8
Q

What do carrier proteins do?

A
  • Carrier proteins have an important role in both passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells.
  • This often involves the shape of the protein changing.
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9
Q

What is a glycoprotein?

A
  • A glycoprotein is an intrinsic proteins.
  • recognition of cells as self and foreign for non-self
  • cell signalling ; communication between cells
  • play a role in cell adhesion
  • acts as antigen
  • receptor/ binding site for hormone
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10
Q

What are glycolipids?

A
  • Lipids with attached carbohydrate chains.

- It acts as a recognition site e.g for cholera toxins

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11
Q

What is an extrinsic protein?

A
  • A protein molecule lying on the surface.
  • It is present in one side of the bilayer.
  • It normally has hydrophilic R-groups on its outer surface and interacts with the polar heads of the phospholipids or with intrinsic proteins.
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12
Q

What is cholesterol?

A
  • Cholesterol is a lipid with a hydrophilic end and a hydrophobic end.
  • It regulates the fluidity of the membrane.
  • Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together.
  • Cholesterol adds stability to the membrane.
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13
Q

How does temperature affect membrane structure?

A
  • When temperature is increased, the phospholipids will have more kinetic energy and will move more.
  • This makes a membrane more fluid and it begins to lose its structure.
  • If temperature continues to increase, the cells will eventually begin to break down.
  • This loss of structure increases the permeability of the membrane, making it easier for particles to cross it.
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14
Q

How do solvents affect membrane temperature?

A
  • Organic solvents such as alcohol are less polar than water.
  • Organic solvents will dissolve membranes, disrupting cells.
  • When the membrane is disrupted it becomes more fluid and more permeable.
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15
Q

How do cells communicate with each other?

A
  • Cells communicate with each other using messenger molecules.
    1) One cell releases a messenger molecule (e.g a hormone)
    2) This molecule travels (e.g in the blood) to another cell.
    3) The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane.
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16
Q

What are membrane-bound receptors?

A
  • Proteins in the cell membrane which act as receptors for messenger molecules.
  • Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to.
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17
Q

What are target cells?

A
  • A cell that responds to a particular messenger molecule.
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18
Q

Give an example of a messenger molecule binding to a receptor on a target cell.

A

Glucagon.

  • Glucagon is a hormone that’s released when there isn’t enough glucose in the blood.
  • It binds to receptors on liver cells, causing liver cells to break down stores of glycogen to glucose.
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19
Q

What can also bind to cell membrane receptors?

A

Drugs.

  • Many drugs work by binding to receptors in cell membranes.
  • They either trigger a response in the cell, or block the receptor and prevent it from working.
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20
Q

What is an example of a drug?

A

Antihistamines.

  • Cell damage causes the release of histamine.
  • Histamine binds to receptors on the surface of other cells and causes inflammation.
  • Antihistamines work by blocking histamine receptors on cell surfaces.
  • This prevents histamine from binding to the cell and stops inflammation.
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21
Q

How can you investigate the permeability of the cell membrane?

A
  • You could investigate how temperature affects beetroot membrane permeability.
  • The higher the permeability of the membrane, the more pigment leaks out of the cell.
    1) Cut five equal sizes pieces of beetroot and rinse them with distilled water to remove any pigment released during the cutting.
    2) Place the five pieces in five different test tubes, each with 5cm^3 of water.
    3) Place each test tube in a water bath at a different temperature e.g 10, 20, 30, 40, 50 degrees for the same length of time. (15 mins)
    4) Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
    5) Use a colorimeter and measure the absorbance. The higher the permeability of the membrane, the more pigment in released, so the higher the absorbance of the liquid.
22
Q

What is diffusion?

A
  • Diffusion is the net movement of particles from a region of higher concentration to a region of a lower concentration.
  • It is a passive process- no energy is required for it to happen.
23
Q

What diffuses through the cell membranes?

A

Small, non-polar molecules.
- Small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids.

24
Q

What factors affect the rate of diffusion?

A
  • The concentration gradient: The higher the concentration, the faster the diffusion.
  • The thickness of the exchange surface: The thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion.
  • The surface area: The larger the surface area, the faster the rate of diffusion.
  • The temperature: The higher the temperature, the faster the rate of diffusion because the particles have more kinetic energy so they move faster.
25
Q

How can you investigate diffusion in model cells?

A
  • You can investigate factors that affect the rate of diffusion.
    1) First, make up some agar jelly with phenolphthalein and dilute sodium hydroxide.
    2) Then fill a beaker with some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly and put them in a beaker of acid.
    3) If you leave the cubes for a while they’ll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide,
26
Q

How can you investigate surface area as a factor which affects the rate of diffusion?

A
  • Cut the agar jelly into different sized cubes e.g 0.50.50.5, 111, 222 and work out their surface area to volume ratio.
  • Take out the cubes at the same, and measure the distance the acid travels in each cube.
  • You would expect the cubes with the largest surface area to volume ratio to have the largest distance travelled by acid.
27
Q

How can you investigate concentration gradient as a factor affecting the rate of diffusion?

A
  • Prepare test tubes containing different concentrations of hydrochloric acid.
  • Put an equal sized cube of the agar jelly in each test tube and time how long it takes each one to turn colourless.
  • You would expect the cubes in the highest concentration of hydrochloric acid to go colourless fast.
28
Q

How can you investigate temperature as a factor affecting the rate of diffusion?

A
  • Prepare several boiling tubes containing the same concentration of hydrochloric acid and put the tubes into water baths of varying temperatures.
  • Put an equal-sized cube of the agar jelly into each boiling tube and time how long it takes each cube to go colourless.
  • You would expect the cubes in the highest temperature to go colourless fastest.
29
Q

What is facilitated diffusion?

A
  • Facilated diffusion is diffusion across a membrane through protein channels.
  • Larger molecules (e.g amino acids, glucose), ions and polar molecules diffuse via facilitated diffusion.
  • It is a passive process.
30
Q

What are the roles of carrier proteins in diffusion?

A
  • Carrier proteins move large molecules into or out of cell, down their concentration gradient.
  • Different carrier proteins facilitate the diffusion of different proteins.
    1) First, a large molecule attaches to a carrier protein in the membrane.
    2) Then, the protein changes shape.
    3) This releases the molecule on the opposite side of the membrane.
31
Q

What are the roles of channel proteins in diffusion?

A
  • Channel proteins form pores in the membrane for charged particles to diffuse through.
  • Different channel proteins facilitate the diffusion of different charged particles.
32
Q

What is simple diffusion?

A
  • Diffusion in the absence of a barrier or membrane.
33
Q

What is active transport?

A
  • Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.
  • This process requires energy as particles are being moved up a concentration gradient, in the opposite direction to diffusion.
  • Carrier proteins are also needed.
34
Q

What is the general process of active transport?

A

1) The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell.
2) On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
3) Binding of the phosphate molecule to the carrier protein causes the protein to change shape- opening up to the inside of the cell.
4) The molecule or ion is released to the inside of the cell.
5) The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP.
6) The carrier protein returns to its original shape.
- It is a selective process.

35
Q

What is bulk transport?

A
  • A form of active transport where large molecules or whole bacterial cells are moved into or out of a cell by endocytosis or exocytosis.
36
Q

What is endocytosis?

A
  • The bulk transport of material (e.g proteins, lipids and some carbohydrates) into cells.
37
Q

What are the two types of endocytosis?

A
  • Phagocytosis (for solids)

- Pinocytosis (for liquids)

38
Q

What is the process of endocytosis?

A

1) The cell surface membrane first bends inwards when it comes into contact with the material to be transported.
2) The membrane enfolds the material until eventually the membrane fuses, forming a vesicle.
3) The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.
E.g vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes.

39
Q

What is exocytosis?

A
  • The bulk transport of materials out of the cell.

- Substances produced by the cell (e.g digestive enzymes, hormones, lipids) need to be released from the cell.

40
Q

What is the process of exocytosis?

A
  • Vesicles containing these substances pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane.
  • The vesicles fuse with the cell surface membrane and the contents of the vesicle are then released outside of the cell.
41
Q

Is ATP required for bulk transport?

A

Yes, energy in the form of ATP is required for the movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of the cell membranes as vesicles form or as they meet the cell-surface membrane.

42
Q

What is osmosis?

A

Osmosis is the net movement of water molecules from a high water potential to a low water potential.

43
Q

What is water potential ?

A

It is the measure of the quantity of water compared to solutes.

44
Q

How is water potential measured?

A

It is measured as the pressure created by the water molecules in kilopascals (kPa).

45
Q

What is hydrostatic pressure?

A

The pressure created by water in a closed system.

46
Q

What happens when an animal cell is placed in a solution with a higher water potential than that of the cytoplasm?

A
  • Water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell.
  • The cell-surface membrane cannot stretch much and cannot withstand the increased pressure.
  • It will break and the cell will burst, an event called cytolysis.
47
Q

What happens when an animal cell is placed in a solution that has a lower water potential than the cytoplasm?

A
  • It will lose water to the solution by osmosis down the water potential gradient.
  • This will cause a reduction in the volume of the cell and the cell crenates.
48
Q

What happens when water enters a plant cell by osmosis?

A
  • The increased hydrostatic pressure pushes the cell surface membrane against the rigid cell walls.
  • This pressure against the cell wall is called turgor.
49
Q

What happens when water leaves a plant cell by osmosis?

A
  • There is a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall.
  • The cell is said to be plasmolysed.
50
Q

what is cell signalling ?

A

communication between cells to trigger a response inside the cell

51
Q

functions of phospholipid bilayer :

A
  • cell compartmentalisation

- to act as a barrier, selects what leaves and enters cell

A-Level Biology (41 decks)

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