QUESTION ONE
Mitochondria carry out an essential cell process. The number of mitochondria can be
different, depending on the location and function of the cell in an organism.
Relate the high number of mitochondria in some cells to their function.
In your answer you should consider:
• the structure of mitochondria (you may use a labelled diagram to support your
answer)
• the process carried out by mitochondria
• the types of cells that would have the most mitochondria
• the reasons for higher numbers in some cells.
Mitochondria carry out aerobic respiration which is the release of energy (as
ATP) from glucose and oxygen. This energy is used by the cell to carry out
cellular functions. The mitochondria have a double membrane structure which
is folded into cristae which increases the surface area. This means that there is
more surface area for respiration reactions to take place.
Cells which require greater amounts of energy will have more mitochondria.
This may include cells involved in active transport, and movement such as
muscle or sperm cells.
QUESTION TWO
Changes in temperature can alter the rate of activity of enzymes, which in turn affects
the metabolic rate of an organism.
Discuss the reasons why enzyme activity is affected by temperature.
In your answer you should include:
• the structure of enzymes (you may use a labelled diagram to support your answer)
• the purpose of enzymes
• how enzymes work
• reasons for the different effects of temperature on enzymes.
Enzymes are biological catalyst which speed up reactions in cells. Enzymes
have a specific shape for a certain substrate. The lock and key model describes
how the active site of the enzyme fits a particular substrate (reacting chemical).
If the shape of the active site changes then it will no longer fit the substrate and
therefore no longer catalyse the reaction.
The rate of activity changes depending on temperature. At lower temperatures
there is less energy so there are less collisions between the active site and
substrate therefore fewer reactions. As the temperature increases more
collisions occur and therefore a greater amount of reactions. If the temperature
is too high the enzyme irreversibly changes structure and will no longer work.
QUESTION THREE
Organisms use a variety of cellular processes for the movement of materials,
including osmosis and active transport.
(a) Describe the processes of osmosis and active transport.
Osmosis:
Osmosis is the movement of water from an area of high water concentration to
an area of low water concentration through a semi-permeable membrane.
Active transport:
Active transport is the movement of substances from an area of low
concentration to an area of high concentration. It is against the concentration
gradient so requires energy.
(b) Explain why the two different processes are necessary, including an example of
where each may occur in an organism.
Osmosis occurs where water is needed to be transported with a concentration
gradient. This maybe the transport of water through root cells or absorption of
water into intestinal cells.
Active transport will occur when movement needs to occur against a
concentration gradient. This may include the re-absorption of glucose in the
kidney or the pumping of water out of unicells by the contractile vacuole.
(c) Compare and contrast the movement of water in and out of the cells of
unicellular organisms living in freshwater and those living in saltwater environments,
and discuss how unicellular organisms that live in freshwater environments regulate
this movement of water.
In fresh water the concentration of water outside the cell is higher than inside,
therefore water moves via osmosis (implication of) into the cell.
In salt water the concentration of water outside the cell is lower than the
concentration inside
the cell, therefore water moves out of the cell via osmosis.
In Salt water the concentration of water is lower outside the cell because there
are more dissolved
particles in the water.
Unicellular organisms in freshwater have a contractile vacuole to pump
(actively move) excess water out of the cell to prevent the cell membrane from
bursting.
QUESTION ONE
The endoplasmic reticulum takes up most of the space inside the cytoplasm. Many muscle
cells contain both smooth endoplasmic reticulum and rough endoplasmic reticulum.
Describe the main structural difference between these two types of endoplasmic reticulum.
Rough endoplasmic reticulum has ribosomes on the outside of the membrane
whereas smooth endoplasmic reticulum does not.
Explain the function of both rough and smooth endoplasmic reticulum, and why they are
both needed in muscle cells.
Rough endoplasmic reticulum is the sight of protein production and is needed in
muscle cells as the protein is a major structural component in muscle cells.
Smooth endoplasmic reticulum produces carbohydrates and lipids which are needed
for energy provision for muscle cells.
QUESTION TWO
Enzymes are found in both animals and plants, and have important roles in their
metabolism.
Explain the difference between the ‘lock and key’ and ‘induced fit’ models of enzyme
activity. You may use diagrams in your answer if you wish.
In the Lock and Key model active site of enzyme matches substrate shape model
In the Induced fit model the active site is flexible and changes shape slightly when
combined with substrate.
(c) Some of the factors that affect the rate of enzyme activity within cells include the amount
of substrate, the enzyme concentration, coenzymes, and enzyme poisons (inhibitors).
Discuss how any THREE of these factors can change the rate of activity within cells.
Higher Substrate concentration means more enzyme activity because there is an
increase in the number of reactions. The activity increases until available enzyme
molecules become ‘saturated’ and the number of reactions levels off .
Higher concentration of enzyme means a greater number of reactions and therefore
reaction rates increase. The actual enzyme activity does not increase (ie no change in
product per unit time per unit mass of enzyme).
Coenzyme are organic molecules that alter the shape of the active site. Enzymes
more effectively combine with the substrate(s) so without them the enzymes will not
work or work at a very low rate of activity.
Inhibitors alter or block the active site to prevent the enzyme-substrate complex
forming and therefore stop the reaction or only a few reactions may still occur.
QUESTION THREE
Euglena and Paramecium are single-celled organisms that live in areas of fresh water such
as lakes and ponds. Both of them have a number of organelles that enable them to function
and survive in their environments.
(a) The cell membrane that encloses each organism is semi-permeable.
A membrane needs to be semi-permeable so that cells can control their internal
environment.
The semi-permeable membrane allows the organism to effectively control the
passage of wanted and unwanted materials in/ out of the cell.
(b) Explain why unicellular organisms such as Euglena and Paramecium are restricted to
being microscopic in size.
When cells grow too large the movement of particles or diffusion is too slow. The cell
becomes inefficient when cell too big therefore cell size is limited.
QUESTION FOUR
Describe the purpose of photosynthesis.
Converts Light energy / (sunlight) into chemical energy in the form of glucose.
OR
Converts energy (from the sun) for use in cellular processes e.g. respiration.
(b) Explain how the structure AND location of chloroplasts affect the process of
photosynthesis.
Chloroplasts contain flat stacks of thylakoids which have an increased surface area
for the absorption of light. The stroma is a clear fluid which doesn’t block the light
and chloroplasts have thin membranes with a large surface area for absorption of
light.
Chloroplasts are located near the top of the leaf in palisade cells which means that
they are directly exposed to the light.