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A level Bio OCR A > 2.1.5 Biological membranes > Flashcards

2.1.5 Biological membranes Flashcards

1
Q

Role of membranes at surface of cell (plasma membranes)

A

barrier between the cell and its environment
- control which substances enter and leave the cell

partially permeable, let some molecules through but not others
- can more across membrane by diffusion, osmosis or active transport

cell recognition eg. the cells of the immune system

cell communication (cell signalling)

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2
Q

Role of membranes within cells

A

compartmentalise

  • membranes around organelles divide them into different compartments
  • this makes functions more efficient
    eg. lysosomes contain digestive enzymes within their membrane

can form vesicles
- to transport substances between different areas of the cell

control which substances enter and leave the organelle
- partially permeable

membranes within organelles

  • act as barriers between membrane and contents of the rest of the organelle
    eg. thylakoid membrane in chloroplasts

site of chemical reactions
eg. inner membrane of a mitochondrion contains enzymes needed for respiration

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3
Q

Describe the fluid mosaic model of membrane structure

A
  1. phospholipid molecules from a continuous double layer (bilayer)
  2. this bilayer is fluid as phospholipids constantly moving
  3. cholesterol molecules are present within the bilayer
  4. protein molecules are scattered through the bilayer
    - glycoproteins have a polysaccharide chain attached
  5. glycolipid molecules - lipids with a polysaccharide chain attached
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4
Q

Roles of components in membrane - phospholipids

A

Have a head and a tail

  • head is hydrophilic it attracts water
  • tail is hydrophobic it repels water

the molecules automatically arrange themselves in a bilayer (heads face outwards)

the centre of the bilayer is hydrophobic

  • doesn’t allow water soluble substances (ions) though it, its acts as a barrier
  • fat soluble substances can dissolve in the bilayer and pass directly through the membrane
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5
Q

Roles of components in membrane - cholesterol

A

a type of lipid

in all cell membranes (except bacteria)

fit between phospholipids

  • they bind to the hydrophobic tails of the phospholipids
  • this causes them to pack more closely together

this makes the membrane less fluid and more rigid

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6
Q

Roles of components in membrane - intrinsic proteins

A

control what enters and leaves the cell

channel proteins
- allow small or charged particles through (diffusion)

carrier proteins and transport molecules

  • active transport
  • facilitated diffusion
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7
Q

Roles of components in membrane - extrinsic proteins

A

glycolipids and glycoproteins

stabalise membrane by forming hydrogen bonds with surrounding water molecules

the sites where drugs hormones and antibodies bind

act as receptors for cell signaling

antigens - cell surface molecules involves in the immune system

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8
Q

how do cells communicate with each other?

A

by cell signalling

one cell releases a messenger molecule (eg. a hormone)

this molecule travels (eg. in the blood) to another cell

the messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

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9
Q

Why do cells need to communicate with each other?

A

to control processes inside the body and to respond to changes in the environment

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10
Q

How are plasma membranes involved in cell signalling?

A

proteins in the membrane act as receptors for messenger molecules

receptor proteins have specific shapes, only a messenger molecule with a complementary shape can bind to them (target cells)

different cells have different types of receptors, they respond to different messenger molecules

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11
Q

How do drugs work?

A

by binding to receptors in cell membranes

they either trigger a response or block the receptor and prevent it from working

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12
Q

Factors affecting membrane permeability

A

temperature

solvents

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13
Q

How does temperature affect membrane affect solubility? - below 0degrees

A

phospholipids dont not have much energy so cannot move very much

packed closely together and the membrane is rigid

channel proteins and carrier proteins deform - INCREASING PERMEABILITY

ice crystals may form
- this pierces the membrane making it highly permeable when it thaws

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14
Q

How does temperature affect membrane affect solubility? - between 0 and 45 degrees

A

the phospholipids can move around and arent as tightly packed
- the membrane is partially permeable

as temp. increases the phospholipids move more (have more kinetic energy)
- this INCREASES PERMEABILITY of the membrane

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15
Q

How does temperature affect membrane affect solubility? - above 45degrees

A

the phospholipid bilayer starts to melt (break down)
- membrane MORE PERMEABLE

water inside the cell expands putting pressure on the membrane

channel and carrier proteins deform (denature) so cannot control when enters or leaves the cell - INCREASING PERMEABILITY

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16
Q

How does changing the solvent affect membrane permeability?

A

surrounding cells in a solvent INCREASES PERMEABILITY of the membrane

  • solvents dissolve the lipids in the membrane so it loses its structure
  • some solvents increase permeability more than others
  • INCREASING SOLVENT CONC. INCREASES PERMEABILITY
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17
Q

Experiment to measure permeability of the membrane

A

using beetroot
1. cut equal-sized pieces of beetroot and rinse them (removed pigment released during cutting)

  1. place in five different test tubes
    - temp –> same vol. of water and put in water baths for the same length of time
    - solvent –> same vol. of different concs. same length of time (poss. same temp. water bath)
  2. remove pieces of beetroot from the tubes leaving just the coloured liquid
  3. use a colorimeter to measure absorbance
    - the higher the permeability of the membrane the more pigment is released so the higher the absorbance of the liquid
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18
Q

Transport across membranes - passive movement

A

doesn’t require ATP (energy from metabolic processes)

  • diffusion
  • facilitated diffusion
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19
Q

Transport across membranes - active transport?

A

requires ATP (from metabolic processes)

  • active transport
20
Q

What is diffusion?

A

the net movements of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. It is a passive process

molecules diffuse both ways (net movement high –> low) this continues until particles are evenly distributed throughout the gas or liquid (equilibrium)

21
Q

What molecules diffuse through membranes, why?

A

small, non-polar molecules

  • oxygen
  • carbon dioxide

can fit through spaces in the phospholipid bilayer

22
Q

Factors that affect the rate of diffusion

A

concentration gradient

diffusion distance

surface area

temperature

23
Q

how does the concentration gradient affect the rate of diffusion?

A

the higher it is, the faster the rate of diffusion

24
Q

how does the thickness of exchange surface affect the rate of diffusion?

A

the thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion

25
Q

how does the surface area affect the rate of diffusion?

A

the larger the surface area (eg. of a cell membrane) the faster the rate of diffusion

26
Q

how does temperature affect the rate of diffusion?

A

the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster

27
Q

Investigating diffusion in model cells - agar jelly

A
  1. make up some agar jelly with phenolphthalein (pink in alkaline, colourless in acidic) and dilute sodium hydroxide
    - this will make the jelly a shade of pink
  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 the beaker of acid
  3. of you leave the ecubes for a while they’ll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide
28
Q

Investigating diffusion in model cells (agar jelly) - surface area

A

cut the agar jelly into different sized cubes and work out their surface area to volume ratio

time how long it takes each cube to go colourless when placed into THE SAME CONC. of hydrochloric acid

expect the cubes with the highest surface area to vol. ratio to go colourless fastest

29
Q

Investigating diffusion in model cells (agar jelly) - conc. gradient

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

expect the cubes in the highest conc. of hydrochloric acid to go colourless

30
Q

Investigating diffusion in model cells (agar jelly) - temperature

A

prepare several boiling tubes conatining the SAME CONCENTRATION of hydrochloric acid and put tubes into water baths of VARYING TEMPERATURES

put an EQUAL SIZED tube of agar jelly into each boling tube and time how long it takes for each cube to go colourless

expect the cubes in the highest temperature to go colourless fast

31
Q

What is facilitated diffusion?

A

some larger molecules (amino acids, glucose), ions and polar molecules don’t diffuse directly through phospholipid bilayer of the cell membrane

instead they diffuse through CARRIER OR CHANNEL PROTIENS in the cell membrane

particles move down a concentration gradient (high —> low)

PASSIVE PROCESS

32
Q

Facilitated diffusion - carrier proteins

A

move large molecules into or out of the cell down their concentration gradient

different carrier protiens facilitate the diffusion of different molecules

  1. first a large molecule attaches to a carrier protein in the membrane
  2. then the eprotien changes shape
  3. this releases the molecule on the opposite side of the membrane
33
Q

Facilitated diffusion - channel proteins

A

form pores in the membrane for charged particles to diffuse through down their concentration gradient

different channel protiens facilitate the diffusion of different charged particles

selectively permeable

34
Q

what is active transport?

A

active transport uses energy to move molecules and ions across plasma membranes. against a concentration gradient. This process involves carrier protiens

  1. a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
  2. ATP is needed to move the solute against the conc. gradient
35
Q

What is endocytosis?

A

taking in substances

some molecules too large to be taken into a cell by carrier protiens (protiens, lipids, carbohydrates)

a cell can surround a substance with a section of its plasma membrane

the membrane then pinches off to form a vesicel inside the cell containing the ingested substance

requires ATP (energy)

36
Q

example of endocytosis

A

some white blood cells (phagocytes) use endocutosis to take in things like microorganisms and dead cells so they can destroy them

37
Q

what is exocytosis?x

A

secreting substances

some substances produced by the cell (eg. digestive enzymes, hormones, lipids) need to be released from the cell

vesicles containing these substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane

the vesicles fuse with the plasma membrane and release their contents outside the cell

some substances (like membrane protiens) aren’t released outside the cell - instead they are inserted straight into the plasma membrane

requires ATP

38
Q

What is osmosis?

A

the movement of water molecules across a partially permeable membrane down a water potential gradient (from a high water conc. —> low water conc.)

39
Q

What is the water potential of pure water?

A

0

all solutions have a lower water potential than pure water

40
Q

How are cells affected by the water potential of the surrounding solution?

A

how much water moves in or out of the cell depends on the water potential of the surrounding solution

41
Q

animal cells affected by water potential? - hypotonic solution

A

when a animal cell is in a solution with a higher water potential

net movement of water into cell, cell bursts

42
Q

animal cells affected by water potential? - isotonic solution

A

when an animal cell is in a solution with the same water potential

water molecules pass into and out of the cell in equal amounts

the cell stays the same

43
Q

animal cells affected by water potential? - hypertonic solution

A

when an animal cell is in a solution with a lower water potential

net movement of water out of cell, cell shrinks

44
Q

plant cells affected by water potential? - hypotonic solution

A

when a plant cell in a solution with a higher water potential

the net movement of water is into the cell.

the vacuole swells

the vacuole and cytoplasm push against the cell wall

the cell becomes turgid (swollen)

45
Q

plant cells affected by water potential? - isotonic solution

A

when a plant cell is in a solution with the same water potential

water molecules move into and out of the cell in equal amounts

the cell stays the same

46
Q

plant cells affected by water potential? - hypertonic solution

A

when a plant cell is in a solution with a lower water potential

net movement of water is out of the cell

the cell becomes flaccid (limp)

the cytoplasm and the membrane pulls away from the cell wall
- plasmolysis

47
Q

experiments investigating water potential

A

using potato cylinders in different concentrations of sucrose solution to find the water potential of plant tissue

  1. prepare sucrose solutions, - 0.0M to 1.0M
  2. use a cork borer or chip maker to cut potatoes into the same size (1cm in diameter)
  3. divide the chips into groups of 3 and measure the mass of each using a mass balance
  4. place one group in each solution
  5. leave the chip for as long as possible - 20mins (same amount of time)
  6. remove the chips and pat dry with a paper towel
  7. weigh each group again and record your results
  8. calculate % change in mass for each group
  9. plot results on a graph

A level Bio OCR A (12 decks)

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