Paper 1 AQA A+ level Biology
Solutions
Explain why plasma membranes are said to have a fluid mosaic structure. - ANS-1.
Fluid because the phospholipids are constantly moving. 2. Mosaic because the proteins
are scattered throughout the membrane-like tiles in a mosaic.
What effect does cholesterol have on the cell membrane? - ANS-1.Cholesterol
molecules fit between the phospholipids and bind to the hydrophobic tails of the
phospholipids. 2.Causes them to pack together more closely, restricting the movement
of other molecules in the membrane. 3.Cholesterol makes the membrane less fluid and
more rigid. 4.Cholesterol helps to maintain the shape of animal cells, which is important
because animal cells don't have cell walls (especially important for cells which are not
supported by other cells e.g. red blood cells).
State 3 functions of glycolipids in the cell membrane. - ANS-1. Act as recognition sites.
2. Maintain the stability of the membrane. 3. Help cells attach to one another and so
form tissues.
State 3 functions of glycoproteins in the cell membrane. - ANS-1.Act as recognition
sites.2.Help cells to attach to one another so form tissues.3.Allow cells to recognise one
another e.g. lymphocytes can recognise an organisms's own cells.
Describe the functions of membranes within cells. - ANS-1.Control the entry and exit of
materials in membrane-bound organelles such as mitochondria and
chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic reactions
can take place.3.Provide an internal transport system e.g. endoplasmic
reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide
surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough
ER.
Suggest reasons why most molecules do not diffuse freely through the cell membrane. -
ANS-1.They're not lipid soluble.2.Too large to pass through protein channels.3.Have the
same charge as the charge on the protein channels, so they are repelled even if they
are small enough to pass through the channel.1
State two factors which affect the permeability of cell membranes -
, ANS-.1.Temperature2.Solvent concentration
How can substances move across a cell surface membrane? - ANS-1.Simple diffusion
(passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active transport
(active)5.Co-transport (active)
Describe how substances can move across a cell surface membrane. -
ANS-1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a
concentration gradient.2.Large/polar molecules - pass through channel proteins/carrier
proteins, by facilitated diffusion, down a concentration gradient.3.Water - moves by
osmosis from an area of high water potential to an area of lower water potential.4.Active
transport - moves substances against a concentration gradient, involves protein carriers
and requires ATP.5.Glucose/amino acids can be co-transported across a cell membrane
alongside sodium ions.* DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS'
OR 'ALONG' A CONCENTRATION GRADIENT - MAKE SURE YOU SAY 'UP' OR
'DOWN' OR 'AGAINST'
Define diffusion. - ANS-The net movement of molecules or ions from an area of higher
concentration to an area of lower concentration until evenly distributed.Define
osmosis.The movement of water from a region of higher water potential to a region of
lower water potential across a partially permeable membrane.
Define active transport. - ANS-The movement of molecules from an area of lower
concentration to an area of higher concentration using ATP and carrier proteins.
State factors which affect the rate of diffusion. - ANS-1.Concentration gradient2.Area
over which diffusion takes place3.Thickness of exchange surface4.Temperature5.Size
of molecules (smaller the molecule, the faster the rate of diffusion).2
What is meant by the term simple diffusion - ANS-.The diffusion of molecules directly
through a cell membrane.(most molecules don't easily pass across the cell surface
membrane - those that do are small, non-polar molecules).
What is Ficks Law? - ANS-The rate of diffusion is proportional to : (Surface Area x
Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The
higher the concentration gradient, the faster the rate of diffusion.3.The thinner the
exchange surface, the faster the rate of diffusion.
What is meant by the term passive process. - ANS-No energy is needed for it to
happen.
Why are oxygen and carbon dioxide able to diffuse across cell membranes? -
Solutions
Explain why plasma membranes are said to have a fluid mosaic structure. - ANS-1.
Fluid because the phospholipids are constantly moving. 2. Mosaic because the proteins
are scattered throughout the membrane-like tiles in a mosaic.
What effect does cholesterol have on the cell membrane? - ANS-1.Cholesterol
molecules fit between the phospholipids and bind to the hydrophobic tails of the
phospholipids. 2.Causes them to pack together more closely, restricting the movement
of other molecules in the membrane. 3.Cholesterol makes the membrane less fluid and
more rigid. 4.Cholesterol helps to maintain the shape of animal cells, which is important
because animal cells don't have cell walls (especially important for cells which are not
supported by other cells e.g. red blood cells).
State 3 functions of glycolipids in the cell membrane. - ANS-1. Act as recognition sites.
2. Maintain the stability of the membrane. 3. Help cells attach to one another and so
form tissues.
State 3 functions of glycoproteins in the cell membrane. - ANS-1.Act as recognition
sites.2.Help cells to attach to one another so form tissues.3.Allow cells to recognise one
another e.g. lymphocytes can recognise an organisms's own cells.
Describe the functions of membranes within cells. - ANS-1.Control the entry and exit of
materials in membrane-bound organelles such as mitochondria and
chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic reactions
can take place.3.Provide an internal transport system e.g. endoplasmic
reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide
surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough
ER.
Suggest reasons why most molecules do not diffuse freely through the cell membrane. -
ANS-1.They're not lipid soluble.2.Too large to pass through protein channels.3.Have the
same charge as the charge on the protein channels, so they are repelled even if they
are small enough to pass through the channel.1
State two factors which affect the permeability of cell membranes -
, ANS-.1.Temperature2.Solvent concentration
How can substances move across a cell surface membrane? - ANS-1.Simple diffusion
(passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active transport
(active)5.Co-transport (active)
Describe how substances can move across a cell surface membrane. -
ANS-1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a
concentration gradient.2.Large/polar molecules - pass through channel proteins/carrier
proteins, by facilitated diffusion, down a concentration gradient.3.Water - moves by
osmosis from an area of high water potential to an area of lower water potential.4.Active
transport - moves substances against a concentration gradient, involves protein carriers
and requires ATP.5.Glucose/amino acids can be co-transported across a cell membrane
alongside sodium ions.* DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS'
OR 'ALONG' A CONCENTRATION GRADIENT - MAKE SURE YOU SAY 'UP' OR
'DOWN' OR 'AGAINST'
Define diffusion. - ANS-The net movement of molecules or ions from an area of higher
concentration to an area of lower concentration until evenly distributed.Define
osmosis.The movement of water from a region of higher water potential to a region of
lower water potential across a partially permeable membrane.
Define active transport. - ANS-The movement of molecules from an area of lower
concentration to an area of higher concentration using ATP and carrier proteins.
State factors which affect the rate of diffusion. - ANS-1.Concentration gradient2.Area
over which diffusion takes place3.Thickness of exchange surface4.Temperature5.Size
of molecules (smaller the molecule, the faster the rate of diffusion).2
What is meant by the term simple diffusion - ANS-.The diffusion of molecules directly
through a cell membrane.(most molecules don't easily pass across the cell surface
membrane - those that do are small, non-polar molecules).
What is Ficks Law? - ANS-The rate of diffusion is proportional to : (Surface Area x
Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The
higher the concentration gradient, the faster the rate of diffusion.3.The thinner the
exchange surface, the faster the rate of diffusion.
What is meant by the term passive process. - ANS-No energy is needed for it to
happen.
Why are oxygen and carbon dioxide able to diffuse across cell membranes? -
