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photosynthesis
What is it?
Photosynthesis is the chemical change which
happens in the leaves of green plants. It is
the first step towards making food - not just
for plants but ultimately every animal on the
planet.
During this reaction, carbon dioxide and
water are converted into glucose and
oxygen. The reaction requires light energy,
which is absorbed by a green substance
called chlorophyll.
Photosynthesis takes place in leaf cells.
These contain chloroplasts, which are tiny
objects containing chlorophyll.
Here is the equation for photosynthesis:
carbon dioxide + water (+ light energy) glucose + oxygen
'Light energy' is shown in brackets because it is not a substance. You will also see the
equation written like this:
Plants absorb water through their roots, and carbon dioxide through their leaves. Some
glucose is used for respiration, while some is converted into insoluble starch for storage.
The stored starch can later be turned back into glucose and used in respiration. Oxygen is
released as a by-product of photosynthesis.…read more

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Limiting Factors
Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.
Light intensity
Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and carbon dioxide. Increasing the
light intensity will boost the speed of photosynthesis.
Carbon dioxide concentration
Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot
photosynthesize if there is insufficient carbon dioxide.
Temperature
If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot.
If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.
In practice, any one of these factors could limit the rate of photosynthesis.…read more

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d What is digestion ?
Digestion is the breakdown of large molecules into smaller, soluble
i
molecules that can be absorbed into the body. Digestion happens inside
the gut, and relies on enzymes.
g
e
s
t
i Enzymes Amylase, Proteases, Lipases
o
The enzymes involved in respiration,
Amylase catalyzes the breakdown of starch into sugars in the
photosynthesis and protein synthesis work
mouth and small intestine.
inside cells. Other enzymes are produced by
Proteases catalyze the breakdown of proteins into amino acids
specialized cells and released from them; the
in the stomach and small intestine.
n
digestive enzymes are like this. They pass out
Lipases catalyze the breakdown of fats and oils into fatty acids
into the gut, where they catalyze the breakdown
and glycerol in the small intestine
of food molecules…read more

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What is mitosis?
M Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It produces two new cells that are
identical to each other, and to the parent cell. The process of growth and division is called the cell cycle.
The cycle starts as the number of organelles - the different parts of the cell - increases. This is to ensure that each of the
i
two new cells receives copies of all the organelles.
Before a cell divides, its chromosomes are copied exactly. The DNA molecule
is made of two strands. As each of the two strands separate, new strands are
made alongside each of them, thereby making two new copies.
t
o
s
i
s…read more

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M Meiosis is a different kind of cell division. It is used to produce male and female gametes. A human body cell contains 46
chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many chromosomes (23).
This is why meiosis is sometimes called reduction division.
e
At fertilization, the nuclei of the sperm and an egg join to form the
zygote. The zygote contains 23 pairs of chromosomes - 23 single
chromosomes from the sperm, and 23 single chromosomes from
the egg, thereby creating the correct number of 46 chromosomes
for all body cells. It also means the zygote contains a complete
i
set of chromosomes from each parent.
o
s
i
s
Stages of Meiosis…read more

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