cell membranes

Question 1

phospholipids

Answer 1

-hydrophillic heads point to outside of the cell surface membrane and are attracted by water on both sides
-hydrophobic tails point into the centre of the cell membrane, repelled by water on both sides

Question 2

functions of phospholipids

Answer 2

-allow lipid soluble molecules to enter and leave the cell
-prevent water soluble substances entering and leaving the cell
-make the membranes flexible and self- sealing

Question 3

proteins within the membrane

Answer 3

-some proteins occur on the surface of the bilayer and never extend completely across it, exsintric proteins. They act to give mechanical support, or in conjunction with glycolipids, as cell receptors for molecules such as hormones
- other proteins, insintric proteins, completely span the phospholipid bilayer. Some are protein channels, others are carrier proteins

Question 4

functions of proteins in the membrane

Answer 4

-provide structural support
-act as a channel transporting water soluble substances across the membrane
-allow active transport across the membrane through carrier proteins
- form cell-surface receptors for identifying cells
-help cells adhere together
-act as receptors eg hormones

Question 5

cholesterol

Answer 5

-occur within the phospholipid bilayer. These molecules are very hydrophobic and play an important role in preventing the loss of water and dissolved ions.. They also pull together the fatty acid tails of phospholipid molecules, limiting their movement and that of other molecules but without making the whole membrane too rigid
main functions:
-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 6

glycolipids

Answer 6

• made of up carbohydrate bonded to a lipid. The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals
  -functions:
  -act as recognition sites
  -help maintain membrane stability
• help cells attach to one another to form tissues

Question 7

glycoproteins

Answer 7

-act as recognition sites
-help cells attach to one another to form tissues
-allow cells to recognise one another, eg lymphocytes can recognise an organisms own cells

Question 8

functions of membranes within cells

Answer 8

-control the entry and exit of materials in discrete organelles eg mitrocondria and chloroplasts
-seperate organelles from cytoplasm so specific metabolic reactions can take place within them
-provide internal transport system eg endoplasmic reticulum
-isolate enzymes that may damage the cell eg lysosomes
-provide surfaces on which reactions can occur eg protein synthesis using ribosomes on RER

Question 9

most molecules dont diffuse freely across cell surface membrane

Answer 9

-not soluble in lipids
-too large to pass through channels in membrane
- of the same charge as charge on protein channels so are repelled
-electrically charged (polar) so have difficulty passing through non-polar hydrophobic tails in bilayer

Question 10

fluid- mosaic model

Answer 10

fluid- individual phospholipid molecules can move relative to one another. Gives membrane flexible structure that is constantly changing shape
mosaic- proteins are embedded in phospholipids vary in shape, size and pattern