Cell Membranes and the Movement of Substances

Question 1

Phospholipids

Answer 1

THREE components:
- Two fatty acids
- Glycerol
- Phosphate group

Contain both a hydrophobic and hydrophilic region:
- Fatty acids ~ HYDROPHOBIC (non-polar)
- Phosphate group ~ HYDROPHILIC (polar)

Question 2

Phospholipid bilayer

Answer 2

When phospholipids are placed in water:
- Hydrophilic part positions itself so it can interact with water
- Hydrophobic parts are buried in the centre, away from any water.

This is known as the phospholipid bilayer and is the BASIS of all cell membranes.

Includes:
- Cell-surface membranes
- Membranes that surround ORGANELLES e.g lysosomes & mitochondria.

Question 3

Function of phospholipids

Answer 3

Act as a BARRIER to most substances, helping control what enters and exits the cell.

• What substances can pass through depends on several factors which determine their ability to diffuse across.

SMALL & NON-POLAR:
- Oxygen and carbon dioxide
- Rapidly diffuse across a membrane

SMALL & POLAR:
- Water and urea
- Diffuse across , but much more slowly , usually transported by channel proteins in facilitated diffusion.

CHARGE PARTICLES:
- Ions
- Cannot pass through the phospholipid
- Need to pass through a channel or carrier protein

LIPID SOLUBLE MOLECULES:
- Some vitamins
- Dissolve in the bilayer and pass through.

Question 4

FUNCTIONS of the cell surface membrane

Answer 4

COMPARTMENTALISATION ~ Separates the internal contents of the cell from its external environment.

Also SEPARATES:
- the contents of an organelle from the cytoplasm.
- one part of an organelle from another part.

• Site of CHEMICAL REACTIONS

CELL SIGNALLING ~ communication between cells.

Question 5

Cholesterol

Answer 5

STRUCTURE:
- Polar hydrophobic group at one end
- The rest of the molecule is non-polar and
hydrophobic.

INCREASES STRENGTH & STABILITY:
- The hydrophilic group attracts the polar head groups on the phospholipid molecules.
- The hydrophobic part attracts the non-polar fatty acids in the phospholipids.

REDUCES SIDEWAYS MOVEMENT OF PHOSPHOLIPIDS:
- Helps to control the fluidity of the membrane.
- Prevents the membrane from becoming:
- Too FLUID. under warm conditions
- Too RIGID under cool conditions

Question 6

Why is the structure of the cell surface membrane called the :
FLUID MOSAIC MODEL

Answer 6

FLUID:
- The phospholipid molecules can move around within each layer.
- This means the membrane is flexible and can change shape.

MOSAIC:
- The membrane is studded with protein molecules.
- The arrangement of these proteins varies just like the tiles in a mosaic.

Question 7

Integral / intrinsic membrane proteins

Answer 7

Fully EMBEDDED in the membrane from one side to the other.

Because they pass right through the lipid bilayer:
-they have hydrophobic amino acids on the outside surface of the protein.
- These can INTERACT with the hydrophobic fatty acid tails in the phospholipid bilayer

Examples:
- protein channels
- carrier proteins

Question 8

Protein channels

Answer 8

• Contain a CHANNEL running through the centre.
• This is lined with HYDROPHILIC amino acids and is filled with water molecules.
• Allows WATER-SOLUBLE molecules and IONS to diffuse through

Question 9

Carrier proteins

Answer 9

• Can change SHAPE or POSITION
• Transfer IONS or MOLECULES from one side of the membrane to another

Question 10

Extrinsic proteins

Answer 10

• DO NOT span the membrane.
• Found on one side of the membrane OR the other.
• Can be attached to INTRINSIC proteins.

ROLES:
- Structural role
- Enzymes
- Receptors for other molecules such as
hormones

Question 11

Glycoproteins

Answer 11

A PROTEIN attached to a CARBOHYDRATE molecule.

• Allow cells to ATTACH to each other to form TISSUES e.g nervous tissue
• Play a role in the IMMUNE SYSTEM ~ presenting antigens to T cells.
• Receptors for hormones

Question 12

Glycolipids

Answer 12

CARBOHYDRATE attached to a PHOSPHOLIPID molecule.

• The glycolipids on the surface of one cell can be RECOGNISED by another cell.
• This can determine where cells come into CONTACT.
• Act as ANTIGENS e.g determining blood group.

Question 13

Describe the FUNCTIONS of GLYCOPROTEINS in the cell surface membrane (5)

Answer 13

• Help stabilise the membrane by forming HYDROGEN BONDS with the surrounding water molecules.
• Act as RECEPTORS for messenger molecules in CELL SIGNALLING.
• Act as a site for drugs, hormones and antibodies to BIND.
• Act as ANTIGENS for CELL RECOGNITION and IMMUNE response.

Question 14

Diffusion

Answer 14

The NET (or overall) movement of particles from a region of HIGHER concentration to a region of LOWER concentration ( down the concentration gradient)

PASSIVE PROCESS ~ does not require metabolic energy ( energy released by respiration).

Question 15

The ACTION of diffusion in and out of a cell using CARBON DIOXIDE as an example.

Answer 15

• HIGH amount of CO2 particles inside the cell and LOW amounts outside the cell.
• This means we have a CONCENTRATION GRADIENT for CO2.
• The CO2 molecules will diffuse from the region with a higher concentration ( inside) to the region with a lower concentration (outside).
• Eventually , the concentration of CO2 will be the SAME both inside and outside.
• This means EQUILBRIUM is reached and there is now now NO NET movement of particles so diffusion has STOPPED.

Question 16

Factors that influence the RATE of diffusion in and out of cells

Answer 16

CONCENTRATION GRADIENT:
- The greater the concentration gradient ,
the greater the rate of diffusion.

PARTICLE CHARGE:
- the cell membrane contains a hydrophobic core.
- therefore, charge particles such as ions WILL NOT be able to diffuse through the membrane
- WATER as an exception although polar as it is very small.
- uncharged molecules such as oxygen CAN diffuse rapidly across the membrane

PARTICLE SIZE:
- Generally, smaller particles will diffuse faster than larger ones.

TEMPERATURE:
- In warmer conditions , particles have more KINETIC ENERGY and diffuse faster than in colder conditions.
- not an issue in MAMMALS & BIRDS as they maintain a constant body temperature.

SURFACE AREA:
- Diffusion takes place more rapidly of the membrane has a LARGER SA compared to a smaller SA.

DISTANCE:
- The GREATER the diffusion distance, the SLOWER the rate of diffusion.
- Why membranes are extremely THIN.

CHANNELS / CARRIER PROTEINS:
- Facilitated diffusion.