(a,b) the roles of membranes within cells and at the surface of cells (b) the fluid mosaic model of membrane structure and the roles of its components

What are the functions of membranes at the cell surface (plasma membranes)?

• Plasma membranes 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
• Plasma membranes also allow recognition by other cells (e.g. the cells of the immune system)
• They also allow cell communication (sometimes called cell signalling)

How do membranes within cells act as barriers and compartments?

• 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

How do membranes keep substances organised within organelles?

• The substances needed for respiration (like enzymes) are kept together inside a mitochondrion
• This is done by the mitochondrion’s outer membrane

How do membranes transport substances within cells?

• Membranes can form vesicles to transport substances between different areas of the cell
• Proteins are transported in vesicles from the rough endoplasmic reticulum to the Golgi apparatus during protein synthesis

How are membranes within cells partially permeable?

• Membranes within cells are also partially permeable
• They can control which substances enter and leave the organelle
• RNA leaves the nucleus via the nuclear membrane (also called the nuclear envelope)
• DNA is too large to pass through the partially permeable membrane, so it remains in the nucleus

What are membranes within organelles?

• You can also get membranes within organelles
• These act as barriers between the membrane contents and the rest of the organelle
• Thylakoid membranes in chloroplasts keep the components needed for the light-dependent reactions of photosynthesis together

How do membranes increase efficiency of chemical reactions?

• Membranes within cells can be the site of chemical reactions
• The membranes of some organelles are folded
• This increases their surface area
• This makes chemical reactions more efficient

How does the mitochondrion membrane increase efficiency?

• The inner membrane of a mitochondrion contains enzymes needed for respiration
• It has a large surface area
• This increases the number of enzymes present
• This makes respiration more efficient

What is the fluid mosaic model?

• The structure of all membranes is basically the same
• They’re all composed of lipids (mainly phospholipids), proteins and carbohydrates
• Carbohydrates are usually attached to proteins or lipids
• In 1972, the fluid mosaic model was suggested
• It describes the arrangement of molecules in the membrane

How are phospholipids arranged in the fluid mosaic model?

• Phospholipid molecules form a continuous, double layer (bilayer)
• The bilayer is ‘fluid’ because the phospholipids are constantly moving

How are proteins and other molecules arranged in the membrane?

• Protein molecules are scattered through the bilayer, like tiles in a mosaic
• Some proteins have a carbohydrate attached — these are called glycoproteins
• Some lipids also have a carbohydrate attached — these are called glycolipids
• Cholesterol molecules are also present within the bilayer
• The phospholipid bilayer is approximately 7 nm thick

What are phospholipids?

• Phospholipid molecules form a barrier to dissolved (water-soluble) substances
• They have a ‘head’ and a ‘tail’
• The head is hydrophilic — it attracts water
• The tail is hydrophobic — it repels water

How do phospholipids form a bilayer?

• The molecules automatically arrange themselves into a bilayer
• The heads face out towards the water on either side of the membrane

How does the phospholipid bilayer act as a barrier?

• The centre of the bilayer is hydrophobic
• The membrane doesn’t allow water-soluble substances (like ions and polar molecules) to diffuse through it
• It acts as a barrier to these dissolved substances

Which substances can pass through the phospholipid bilayer?

• Fat-soluble substances dissolve in the bilayer
• They pass directly through the membrane

What is meant by polar and non-polar molecules?

• A polar molecule has one end with a slightly positive charge and one end with a slightly negative charge
• These charges are weaker than full ionic charges
• They help polar molecules dissolve in water
• Non-polar substances have no charges

Why can water pass through the membrane?

• Water is a polar molecule
• It can diffuse through the membrane by osmosis
• This is because it is very small

What is the role of cholesterol in membranes?

• Cholesterol gives the membrane stability
• It is present in all cell membranes except bacterial membranes

How does cholesterol affect membrane structure?

• Cholesterol molecules fit between phospholipids
• They bind to the hydrophobic tails
• This causes phospholipids to pack more closely together
• This makes the membrane less fluid and more rigid

How does cholesterol act as a barrier?

• Cholesterol has hydrophobic regions
• It creates a further barrier to polar substances moving through the membrane

What is the role of proteins in membranes?

• Proteins control what enters and leaves the cell

What are channel and carrier proteins?

• Some proteins form channels in the membrane
• These allow small, charged particles through
• Other proteins (carrier proteins) transport larger molecules and charged particles
• This occurs by active transport and facilitated diffusion

What is the role of proteins in cell signalling?

• Proteins act as receptors for molecules (e.g. hormones)
• When a molecule binds, a chemical reaction is triggered inside the cell

What are glycolipids and glycoproteins?

• Glycolipids are carbohydrates attached to lipids
• Glycoproteins are carbohydrates attached to proteins

What are the functions of glycolipids and glycoproteins?

• They stabilise the membrane by forming hydrogen bonds with surrounding water molecules
• They act as receptors for messenger molecules in cell signalling
• They are sites where drugs, hormones and antibodies bind
• They are antigens involved in self-recognition and the immune response

What is a hydrogen bond?

• A hydrogen bond is a weak bond
• It forms between a slightly positively-charged hydrogen atom
• And a slightly negatively-charged atom or group in another molecule (e.g. oxygen)