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This set of revision notes goes through Biology Paper 2 [as specified for the Pearson Edexcel course] summarising and explaining everything you need to kn...
GCSE Biology Paper 2 Notes
Photosynthesis
Photosynthesis:
o A process used by plants
and other organisms to
convert light energy into
chemical energy that,
through cellular
respiration, can later be
released to fuel the
organism's activities.
o Some of this chemical energy is stored in carbohydrate molecules, such as sugars and starches, which
are synthesized from carbon dioxide and water.
o Most plants, algae, and cyanobacteria perform photosynthesis; such organisms are called
photoautotrophs.
o Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's
atmosphere, and supplies most of the energy necessary for life on Earth.
o If there is no photosynthesis occurring [such as at night], then respiration will occur instead.
Producers: Plants are called producers because they produce their own food.
Biomass:
o The mass of living material at a particular stage in a food chain.
o Photosynthetic organisms are the main producers of food
and therefore biomass.
Limiting Factors: Light; CO2; Temperature; Chlorophyll amount in
leaves.
Hydrogen carbonate indicator: Can be used as a CO2 indicator.
Xylem cells:
Xylem vessels are dead cells which have no cytoplasm or cell
contents.
They are found in plant stems and form long tubes called xylem vessels.
They carry water and dissolved mineral irons from the roots to the leaves.
Adaptations:
o Thick walls containing lignin:
Provide support to the plant.
Allows xylem vessels to withstand the water pressure.
The cell walls being sealed with lignin causes the xylem vessels to die, allowing them to be
turned into xylem vessels.
o End walls between cells are broken down:
Forms long tubes for easy water [and dissolved mineral] flow.
o No internal structures:
Easier for water mineral flow.
o Pits in the walls:
Allow water and mineral ions to move out of the vessel.
Transpiration:
o The movement of water from the roots to the leaves and the loss of water by evaporation from the
leaves is called the transpiration stream:
Transpiration starts with water evaporation from cells inside the leaves [from palisade cells].
Water vapour diffuses through airspaces in the spongy mesophyll and out of the leaves
through the stomata. [It is constantly evaporating from the surface of the leaves].
Then water passes from the xylem into the leaves to replace the lost water [forces of attraction].
Water is drawn into root hair cells by osmosis and up the xylem vessels into the leaves.
The entire process is called the transpiration stream.
, o Benefits:
Brings water to the leaves, where it is needed for photosynthesis.
That stream transports dissolved mineral irons, like magnesium, which play important roles.
Evaporation from the leaves cools them down, especially in the heat.
o Transpiration rate:
Greater at:
Higher temperatures: faster evaporation & water molecules also have more energy.
Dry conditions [no humidity]: faster evaporation [increased concentration gradient].
Increased light intensity: more light allows for faster photosynthesis. Also, the
stomata open wider so more carbon dioxide can diffuse in for photosynthesis.
o Stomata cells:
Surrounded by two guard cells: water flows in during the day and leaves at night.
In a high light intensity [daytime], the guard cells swell and change shape: stomata open.
In the heat, the stomata close to reduce water loss by transpiration – prevents photosynthesis.
Phloem cells
Food substances [such as sucrose] are moved around the plant in the phloem sieve tubes through active
transport to where they are needed for processes such as:
o Growing parts of the plant for immediate use.
o Storage organs such as bulbs and tubers.
o Developing seeds.
Transport in the phloem is therefore both up and down the stem, and it is called translocation.
Phloem consist of living cells which are each adapted to carry out their own functions.
Adaptations:
o Sieve tube elements:
No nucleus and limited cytoplasm allow for a larger space to transport sucrose and other
nutrients.
o Companion cells:
As there is limited cytoplasm, there is not a lot of mitochondria inside the phloem cells.
So each phloem vessel cell has a companion cell connected by pores.
Companion cells contain lots of mitochondria which provide energy for active transport of
sucrose into / out of sieve tubes [via protein carrier molecules].
o Sieve plates:
Pores in the walls that allow sugar to move through the cell interior.
, Translocation:
o The movement of sugars produced during photosynthesis from the leaves to other parts of the plant.
o Flowing vessel pose enable the movement of cell sap [water and sugar] everywhere.
o The sugars are used for two things when they reached their destinations:
1. Directly for energy.
2. Stored for future energy.
o Phloem can transport substances up and down – it is an active process so requires mitochondria.
Translocation is the movement of the sugar sucrose through the plant.
Sucrose is formed from two glucose molecules bonded together.
Translocation takes place in two directions [bi directional] and takes place in phloem.
Sucrose is pumped via protein carrier molecules in companion cells.
Hormones
Hormones: Chemical messengers that target organs in the
body.
Production and transport:
o They are produced by endocrine glands and released
into the blood. Nerves and hormones both help you
to respond to changes in the environment and in your
body
o They travel around the body in the blood until they
reach their target organs.
o Each hormone causes its target organ[s] to respond, e.g. by releasing another chemical substance.
o Different hormones have different organs and cause different responses.
Hormones and nerves:
o Nerves and hormones both help you to respond to changes in the environment and in your body.
o Hormones usually
have a long lived
effect while
nerves have a
short term effect.
o Nerve impulses
work quickly
while hormones
take longer to
work.
Adrenaline
A hormone that is released from the adrenal glands into the blood in response to sudden stress.
It has many target organs, including the liver, the heart and blood vessels.
It brings about the ‘flight or flight’ response.
Effects:
o Increased heart rate – faster beats allow for more O2
and glucose [which are needed for aerobic respiration]
to reach the blood and muscles, where they are needed
for aerobic respiration.
o Constrict some blood vessels to make blood pressure
higher.
o Dilate other blood vessels to increase blood flow to
muscles.
o Causes the liver to convert glycogen to glucose, which is released into the blood for energy.
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