Chap 4: Cell Membranes

Question 1

Describe 3 functions of PHOSPHOLIPIDS in the CELL MEMBRANE

Answer 1

1. allow lipid-soluble substances to enter and leave the cell
2. prevent water-soluble substances entering and leaving the cell
3. make the membrane flexible and self-sealing.

Question 2

Describe the phospholipid bilayer

Answer 2

• The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
• The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.

Question 3

What are CHANNEL proteins, and what do they do?

Answer 3

-Protein channels form water-filled tubes

• to allow water-soluble ions to diffuse across the membrane.

(NOTE: channel and carrier proteins completely span the phospholipid bilayer from one side to the other)

Question 4

What are CARRIER proteins, and what do they do?

Answer 4

Carrier proteins bind to ions or molecules like glucose and amino acids,

then change shape in order to move these molecules across the membrane.

Question 5

Some proteins occur in the surface of the bilayer and never extend completely across it.

What is their purpose?

Answer 5

They act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones.

Question 6

What are the functions of the proteins in the membrane? (5)

Answer 6

1. provide structural support
2. act as channels transporting water-soluble substances across the membrane
3. allow active transport across the membrane through carrier proteins
4. help cells adhere together
5. act as receptors, for example for hormones.
6. form cell-surface receptors for identifying cells

Question 7

Describe cholesterol

Answer 7

• occur within the phospholipid bilayer of the cell-surface membrane.

-add strength to the membranes.

• very hydrophobic and therefore prevent loss of water and dissolved ions from the cell.

-pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.

Question 8

What are the functions of cholesterol?

Answer 8

• reduce lateral movement of other molecules including phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell.

Question 9

What are the functions of glycolipids in the membrane?

Answer 9

1. act as recognition sites
2. help maintain the stability of the membrane
3. help cells to attach to one another and so form tissues.

Question 10

What are the functions of glycoproteins in the membrane?

Answer 10

• act as recognition sites
• help cells to attach to one another and so form tissues
• allows cells to recognise one another, for example lymphocytes can recognise an organism’s own cells.

Question 11

Why aren’t most molecules able to diffuse through freely in the cell membrane?

Answer 11

• not soluble in lipids and therefore cannot pass through the phospholipid layer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so, even if small enough to pass through, they are repelled
• polar molecules (electrically charged) have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

Question 12

Why is the fluid mosaic described as ‘fluid’ and ‘mosaic’?

Answer 12

Fluid: bc individual phospholipid molecules can move relative to one another. This gives a flexible structure that is constantly changing in shape.

Mosaic: bc the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as a mosaic.

Question 13

What are the functions of membranes WITHIN cells?

Answer 13

• control the entry and exit of materials in e.g. mitochondria and chloroplasts
• separate organelles from cytoplasm so that specific metabolic reactions can take place within them
• provide an internal transport system, e.g., ER
• isolate enzymes that might damage the cell, e.g., lysosomes
• provide surfaces on which reactions can occur, e.g., protein synthesis using ribosomes on RER

Question 14

What type of transport is diffusion?

(Active or passive?)

Answer 14

Passive transport

Question 15

What type of molecules can diffuse through the cell membrane?

Answer 15

• small
• lipid soluble
• non polar (e.g. CO2, O2)

molecules

Question 16

How do non polar (e.g. CO2, O2) molecules diffuse through the cell membrane?

(what process?)

Answer 16

simple diffusion —> through the phospholipid bilayer

Question 17

Why are charged ions and non polar molecules not able to diffuse as easily?

Answer 17

Bc of the hydrophobic nature of the fatty acid tails of the phospholipids in the membrane.

Question 18

So how are charged ions and non polar molecules able to diffuse through?

Answer 18

• Facilitated diffusion
• By using transmembrane channels and carriers that span the membrane

Question 19

Define DIFFUSION

Answer 19

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

Question 20

How would you describe the movement of particles in general?

Answer 20

• all particles are constantly in motion due to KE they possess
• this motion is random, no set pattern to the way the particles move around
• particles are constantly bouncing off one another as well as off other objects

Question 21

What is facilitated diffusion?

Answer 21

• passive

-relies only on the KE of the diffusing molecules.

• no external input of ATP from respiration.
• occurs down a concentration gradient, but it differs from simple diffusion in that it occurs at specific points on the plasma membrane where there are special protein molecules.
• Two types of protein are involved - protein channels and carrier proteins.

Question 22

What are channel proteins?

Answer 22

• Formed from water-filled hydrophilic channels across the membrane.
• Allow specific water-soluble ions to pass through.
• Very selective, each opening in the presence of a specific ion.
• If particular ion is not present, the channel remains closed = there is control over the entry and exit of ions.
• The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.

Question 23

What are CARRIER proteins?

Answer 23

When a molecule (e.g. glucose) that is specific to the protein is present, it binds with the protein.

= change in shape in such a way that the molecule is released to the inside of the membrane

• No external energy is needed for this.
• molecules move from a region where they are highly concentrated to one of lower concentration, using only the KE of the molecules themselves.

Question 24

What is diffusion proportional too??

Answer 24

Diffusion is proportional to the DIFFERENCE in concentration between two regions (the concentration gradient).

Question 25

Where does diffusion takes place?

Answer 25

ONLY fluids

Question 26

Describe characteristics of Faciliated diffusion?

Answer 26

• limited by no. of proteins
• more specific: assures transport proteins–> 1. carrier charge specific 2. channel charge specific
• ions / water soluble / larger molecules / non polar molecules able to diffuse through (EG glucose, amino acids)