Cell Membranes

Question 1

Membranes Control What?

Answer 1

Membranes Control What Passes Through Them

Question 2

What are the function of membranes?

Answer 2

Membranes at the surface of cells (PLASMA membranes)
1) They are a barrier between the cell and its environment, controlling which substances
enter and leave the cell. They’re partially permeable — they let some molecules
through but not others. Substances can move across the plasma membrane by
diffusion, osmosis or active transport (see pages 54-59).
2) They allow recognition by other cells, e.g. the cells of the immune system
3) They allow cell communication (sometimes called cell signalling)

Question 3

Membranes with in cells?

Answer 3

Membranes within cells
1) The membranes around organelles divide the cell into different compartments — they act as a
barrier between the organelle and the cytoplasm. This makes different functions more efficient,
e.g. the substances needed for respiration (like enzymes) are kept together inside mitochondria.
2) They can form vesicles to transport substances between different areas of the cell (see p. 57).
3) They control which substances enter and leave the organelle, e.g. RNA (see p. 38) leaves the
nucleus via the nuclear membrane. They are also partially permeable.
4) You can also get membranes within organelles — these act as barriers between the membrane
contents and the rest of the organelle, e.g. thylakoid membranes in chloroplasts (see p. 12).
5) Membranes within cells can be the site of chemical reactions, e.g. the inner membrane of a
mitochondrion contains enzymes needed for respiration.

Question 4

Structure of membrane?

Answer 4

The structure of all membranes is basically the same. They’re composed of lipids
(mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids).

Question 5

Fluid mosaic model?

Answer 5

1) In 1972, the fluid mosaic model was suggested to describe the
arrangement of molecules in the membrane.
2) In the model, phospholipid molecules form a continuous, double layer (bilayer).
3) This bilayer is ‘fluid’ because the phospholipids are constantly moving.
4) Cholesterol molecules are present within the bilayer
5) Protein molecules are scattered through the bilayer, like tiles in a mosaic.
6) Some proteins have a polysaccharide (carbohydrate) chain attached — these are called glycoproteins.
7) Some lipids also have a polysaccharide chain attached these are called glycolipids.

Question 6

What are the Components of cell membranes?

Answer 6

Phospholipids Form a Barrier to Dissolved Substances
Cholesterol Gives the Membrane Stability
Proteins Control What Enters and Leaves the Cell
Glycolipids and Glycoproteins act as Receptors for Messenger Molecules

Question 7

What does Phospholipids Form a Barrier to Dissolved Substances mean?

Answer 7

Phospholipids Form a Barrier to Dissolved Substances
1) Phospholipid molecules have a ‘head’ and a ‘tail’.
2) The head is hydrophilic — it attracts water.
3) The tail is hydrophobic — it repels water.
4) The molecules automatically arrange themselves into a bilayer —
the heads face out towards the water on either side of the membrane.
5) The centre of the bilayer is hydrophobic so the membrane
doesn’t allow water-soluble substances (like ions) through it, it acts as a barrier to these dissolved substances.
(But fat-soluble substances, e.g. fat-soluble vitamins, can dissolve in the
bilayer and pass directly through the membrane.)

Question 8

Cholesterol Gives the Membrane Stability Meaning?

Answer 8

Cholesterol Gives the Membrane Stability
1) Cholesterol is a type of lipid.
2) It’s present in all cell membranes (except bacterial cell membranes).
3) Cholesterol molecules fit between the phospholipids. They bind to the
hydrophobic tails of the phospholipids, causing them to pack more
closely together. This makes the membrane less fluid and more rigid.
4) At lower temperatures, cholesterol prevents phospholipids from packing
too close together and so increases membrane fluidity.

Question 9

Proteins Control What Enters and Leaves the Cell Meaning?

Answer 9

Proteins Control What Enters and Leaves the Cell
1) Some proteins form channels in the membrane (see p. 56) — these allow small or charged particles through.
2) Other proteins (called carrier proteins) transport molecules and ions across the membrane
by active transport and facilitated diffusion (see p. 56).
3) Proteins also act as receptors for molecules (e.g. hormones) in cell signalling (see next page).
When a molecule binds to the protein, a chemical reaction is triggered inside the cell.

Question 10

Glycolipids and Glycoproteins act as Receptors for Messenger Molecules Meaning?

Answer 10

Glycolipids and Glycoproteins act as Receptors for Messenger Molecules
1) Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with
surrounding water molecules.
2) They’re also sites where drugs, hormones and antibodies bind.
3) They act as receptors for cell signalling (see next page).
4) They’re also antigens — cell surface molecules involved in the immune response

Question 11

What is cell signalling?

Answer 11

How cells communicate with eachother

Question 12

Why do cells need to comunicate?

Answer 12

Cells need to communicate with each other to control processes inside the body and to respond to changes in the environment.

Question 13

How do cells communicate with each other?

Answer 13

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.

Question 14

How do cell membrane receptors play an important role in cell signalling?

Answer 14

1) Proteins in the cell membrane act as receptors for messenger molecules.
These are called ‘membrane-bound receptors’.
2) Receptor proteins have specific shapes — only messenger molecules with a complementary shape can bind to them.
3) Different cells have different types of receptors — they respond to different messenger molecules.
4) A cell that responds to a particular messenger molecule is called a target cell.
The diagram below shows how messenger molecules bind to target cells.

Question 15

Drugs Also Bind to?

Answer 15

Cell Membrane Receptors

Question 16

How do drugs bind to Cell Membrane Receptors?

Answer 16

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

Question 17

The permeability of cell membranes is affected by?

You can investigate how these things affect permeability by doing an experiment using?

Answer 17

The permeability of cell membranes is affected by different conditions, e.g. temperature, solvent type and
solvent concentration. You can investigate how these things affect permeability by doing an experiment
using beetroot. Beetroot cells contain a coloured pigment that leaks out — the higher the permeability of the membrane, the more pigment leaks out of the cell.

Question 18

Here’s how you could investigate how temperature affects beetroot membrane permeability:

Answer 18

1) Cut five equal sized pieces of beetroot and rinse them to remove any pigment released during cutting.
2) Place the five pieces in five different test tubes, each with 5cm3 of water.
3) Place each test tube in a water bath at a different temperature, e.g. 10 °C, 20 °C, 30 °C, 40 °C, 50 °C,
for the same length of time.
4) Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
5) Now you need to use a colorimeter — a machine that passes light through the liquid and measures
how much of that light is absorbed (see p. 31). The higher the permeability of the membrane, the
more pigment is released, so the higher the absorbance of the liquid.

Question 19

Increasing the Temperature Increases Membrane Permeability results?
temp below 0

Answer 19

Temperatures below 0 °C
The phospholipids don’t have much energy, so they can’t move very
much. They’re packed closely together and the membrane is rigid.
But channel proteins and carrier proteins in the membrane deform,
increasing the permeability of the membrane. Ice crystals may form
and pierce the membrane making it highly permeable when it thaws.

Question 20

Increasing the Temperature Increases Membrane Permeability results?
Temperatures between 0 and 45 °C

Answer 20

Temperatures between 0 and 45 °C
The phospholipids can move around and aren’t packed as tightly
together — the membrane is partially permeable. As the temperature
increases the phospholipids move more because they have more
energy — this increases the permeability of the membrane.

Question 21

Increasing the Temperature Increases Membrane Permeability results?
Temperatures above 45 °c

Answer 21

Temperatures above 45 °C

The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable.
Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins
deform so they can’t control what enters or leaves the cell — this increases the permeability of the membrane.

Question 22

How doesnChanging the Solvent Affects Membrane Permeability?

Answer 22

Changing the Solvent Affects Membrane Permeability
Different solvents and their concentration can affect the permeability of cell membranes.
1) Surrounding cells in a solvent (such as ethanol) increases the permeability of their cell membranes.
2) This is because solvents dissolve the lipids in a cell membrane,
so the membrane loses its structure.
3) Some solvents increase cell permeability more than others,
e.g. ethanol increases cell permeability more than methanol.
4) You could investigate the effects of different solvents by doing
an experiment using beetroot like the one on the previous page.
5) Increasing the concentration of the solvent will also increase
membrane permeability. For example, this graph shows the
effect of alcohol concentration on membrane permeability.

Question 23

What is diffusion?

Answer 23

Diffusion is the Passive Movement of Particles
1) Diffusion is the net movement of particles (molecules or ions) from
an area of higher concentration to an area of lower concentration.
2) Molecules will diffuse both ways, but the net movement will be to
the area of lower concentration. This continues until particles are
evenly distributed throughout the liquid or gas.
3) The concentration gradient is the path from an area of higher
concentration to an area of lower concentration. Particles diffuse
down a concentration gradient.
4) Diffusion is a passive process — no energy is needed for it to happen.

Question 24

What can diffuse through cell membranes?

Answer 24

• Small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids.
•Water is also small enough to fit between phospholipids, so it’s able
to diffuse across plasma membranes even though it’s polar. The diffusion of water molecules like this is called osmosis

Question 25

What does the rate of diffusion depend on?

Answer 25

The Rate of Diffusion Depends on Several Factors
1) The concentration gradient — the it is, the the rate of diffusion.
2) The thickness of the exchange surface — the thinner the exchange surface (i.e. the the the particles have to travel), the the rate of diffusion.
3) The surface area — the larger the surface area (e.g. of a cell membrane), the the rate of diffusion.
4) The temperature — the warmer it is, the faster the rate of diffusion
because the particles have so they move faster.

Question 26

What is phenolphthalein?

Answer 26

Phenolphthalein is a pH indicator — it’s pink in alkaline solutions and colourless in acidic solutions.
You can use it to investigate diffusion in agar jelly:

Question 27

How to investigate diffusion in cells?

Answer 27

1) First, make up some agar jelly with
phenolphthalein and dilute sodium hydroxide.
This will make the jelly a lovely 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) 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.

Question 28

How to investigate the rate of diffusion- surface area?

Answer 28

SURFACE AREA — Cut the agar jelly into
different sized cubes and work out their surface
area to volume ratio (see p. 70). Time how long it
takes each cube to go colourless when placed in
the same concentration of hydrochloric acid. You
would expect the cubes with the largest surface
area to volume ratio to go colourless fastest.

Question 29

How to investigate the rate of diffusion- conc gradient?

Answer 29

CONCENTRATION GRADIENT — 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 fastest.

Question 30

How to investigate the rate of diffusion- temp?

Answer 30

TEMPERATURE — 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.

Question 31

What is facilitated diffusion?

Answer 31

Facilitated Diffusion uses Carrier Proteins and Channel Proteins
1) Some larger molecules (e.g. amino acids, glucose), ions and polar molecules don’t diffuse
directly through the phospholipid bilayer of the cell membrane.
2) Instead they diffuse through carrier proteins or channel proteins in the cell membrane —
this is called facilitated diffusion.
3) Like diffusion, facilitated diffusion moves particles down a concentration gradient,
from a higher to a lower concentration.
4) It’s also a passive process — it doesn’t use energy.

Question 32

What do carrier proteins do?

Answer 32

Carrier proteins move large molecules into or
out of the cell, down their concentration gradient.
Different carrier proteins facilitate the diffusion
of different molecules.
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.

Question 33

What do channel proteins do?

Answer 33

Channel proteins form pores in the
membrane for charged particles to diffuse
through (down their concentration gradient).
Different channel proteins facilitate the
diffusion of different charged particles.

Question 34

What is active transport?

Answer 34

Active Transport Moves Substances Against a Concentration Gradient
Active transport uses energy to move molecules and ions across plasma membranes,
against a concentration gradient. This process involves carrier proteins.
1) The process is pretty similar to facilitated
diffusion — 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) The only difference is that energy is
used (from ATP — a common source
of energy used in the cell), to move the
solute against its concentration gradient.
3) The diagram on the right shows the active
transport of calcium ions (Ca2+).

Question 35

How do cells Take in Substances by Endocytosis?

Answer 35

1) Some molecules are way too large to be taken into a cell by
carrier proteins, e.g. proteins, lipids and some carbohydrates.
2) Instead a cell can surround a substance with a section of its
plasma membrane.
3) The membrane then pinches off to form a vesicle inside the
cell containing the ingested substance — this is endocytosis.
4) Some cells also take in much larger objects by endocytosis —
for example, some white blood cells (mainly phagocytes, see p. 102)
use endocytosis to take in things like microorganisms and dead cells
so that they can destroy them.
5) Like active transport, (see previous page), this process also
uses ATP for energy.

Question 36

How do cells Secrete Substances by Exocytosis?

Answer 36

Cells can Secrete Substances by Exocytosis
1) Some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) need to
be released from the cell — this is done by exocytosis.
2) Vesicles containing these substances pinch off from the sacs of the Golgi apparatus (see p. 12)
and move towards the plasma membrane.
3) The vesicles fuse with the plasma membrane and release their contents outside the cell.
4) Some substances (like membrane proteins) aren’t released outside the cell — instead they are
inserted straight into the plasma membrane.
5) Exocytosis uses ATP as an energy source.

Question 37

What is osmosis?

Answer 37

Osmosis is the Diffusion of Water Molecules
1) Osmosis is the diffusion of water molecules
across a partially permeable membrane down
a water potential gradient. This means water
molecules move from an area of higher water
potential (i.e. higher concentration of water
molecules) to an area of lower water potential
(i.e. lower concentration of water molecules).
2) Water potential is the potential (likelihood)
of water molecules to diffuse out of or into
a solution.
3) Pure water has the highest water potential.
All solutions have a lower water potential
than pure water.

Question 38

How are animal cells affected by the water potential and plants?

Answer 38

diagram so on other flashcard or poster

Question 39

How to investigate water potential?

Answer 39

You can do a simple experiment, using potato cylinders, to find out the water potential of plant tissue.
This experiment involves putting potato cylinders into different concentrations of sucrose solution —
remember, the higher the sucrose concentration, the lower the water potential.
1) Prepare sucrose solutions of the following concentrations:
0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M.
2) Use a cork borer or chip maker to cut
potatoes into the same sized pieces.
(They need to be about 1 cm in diameter.) 0.0 M 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M
3) Divide the chips into groups of three and use a
mass balance to measure the mass of each group.
4) Place one group in each solution.
5) Leave the chips in the solution for as long as
possible (making sure that they all get the same
amount of time). Try to leave them for at
least 20 minutes.
6) Remove the chips and pat dry gently with a
paper towel.
7) Weigh each group again and record your results.
8) Calculate the % change in mass for each group.
9) Plot your results on a graph

Question 40

Similar experiment with eggs

Answer 40

You can do a Similar Experiment with Eggs
You can carry out a similar experiment using chickens’ eggs that have had
their shells dissolved. The remaining membrane is partially permeable,
so it’s a good model for showing the effects of osmosis in animal tissue.