Zusammenfassung der Ressource
Plant Structures and their Functions
- CB6a
- All organisms need energy
- Plants and algae (a type of protist) can trap
energy transferred by light from the Sun
- This energy is then transferred to molecules of a sugar
called glucose in a process called photosynthesis
- Glucose and substances made from glucose
are stores of energy
- As glucose molecules are made, they are linked
together to form a polymer called starch
- This stays in the chloroplasts
until photosynthesis stops
- The starch is then broken down into simpler substances, which
are moved into the cytoplasm and used to make sucrose
- Sucrose is moved around the plant and may be used to make: starch (in a
storage organ such as potato), other molecules for the plant (such as
cellulose, lipids or proteins) or glucose for respiration (to release energy)
- When animals eat plants, they
get the energy from these stores
- The materials in an organism
are its biomass
- Plants and algae produce their own biomass and so
produce the food for almost all other life on Earth
- They are the producers in food chains
- Photosynthesis is a series of
chemical reactions, catalysed
(speeded up) by enzymes
- We can model the overall process using a word equation: carbon
dioxide + water --> glucose + oxygen
- Photosynthesis occurs in chloroplasts, which contain a green
substance called chlorophyll that traps energy transferred by
light
- Since energy enters from the surroundings, the products of
photosynthesis have more energy than the reactants and so this
is an endothermic reaction
- Leaves are often broad and flat,
giving them a large surface area
- The palisade cells near the top of the
leaf are packed with chloroplasts
- These adaptations allow a leaf
to absorb a great deal of light
- Carbon dioxide for photosynthesis comes
from the air
- Leaves contain microscopic pores called stomata
which allow carbon dioxide to diffuse into the leaf
- The stoma are opened and closed by
specialised guard cells
- In the light, water flows into pairs of guard cells making them rigid, this opens the stomata, at
night water flows out of the guard cells, they loose their rigidity and the stomata shuts
- Leaves are thin, carbon dioxide does not have far to diffuse
into the leaf before reaching the cells that need it
- Stomata also allow the oxygen produced by
photosynthesis to escape into the air, as wells as
water vapour
- The flow of different substances in and out of a leaf is an example of gas
exchange
- CB6b
- There are fewer molecules in each cubic centimetre of air at the
top of a mountain than at the bottom
- This reduced concentration of air molecules causes a lower
rate (speed) of photosynthesis in high mountains
compared with sea level
- Reactions in photosynthesis are
catalysed by enzymes that work better at
warmer temperatures
- High mountain areas are cold, which is another reason why
photosynthesis is slower at the top of a mountain
- A factor that prevents a
rate increasing is called a
limiting factor
- Limiting factors for photosynthesis
can be carbon dioxide concentration,
temperature and light intensity
- The maximum rate of photosynthesis is
controlled by the factor in shortest supply
- CB6c
- Some trees like buttress trees are adapted
to living in the rain forest because they
absorb water and dissolved mineral ions
from the soil like most roots, but they also
help stop the tree from falling and trap
leaves and other dead vegetation which
then provides more minerals for the tree
- The water absorbed by plant
roots is used for carrying
dissolved mineral ions, keeping
cells rigid (otherwise the plants
wilt), cooling the leaves (when it
evaporates) and photosynthesis
- The outer surfaces of many
roots are covered with root
hair cells, these are
extensions of the cell that
provide a large surface area
so water and mineral ions
can be absorbed quicker
- The cells also have thin cell walls so that the flow
of water into the cells is not slowed down
- A certain volume containing more molecules of a substance than
another identical volume has a greater concentration of the molecules
- If the two spaces are connected, there will be a concentration
gradient from higher concentration to lower concentration
- Particles constantly move in random directions and so
particles in a fluid spread down a concentration
gradient, this is diffusion
- Inside plant roots, the cell walls have an open structure allowing water particles to
diffuse towards the middle of the root (from where there are more of them to fewer)
- Osmosis is when solvent molecules (such as water) diffuse
through a semi-permeable membrane
- They diffuse from where there are more of them (a dilute solution of
solutes) to where there are fewer (a more concentrated solution)
- Cell membranes are semi-permeable and so water passes
into the cytoplasm of root hair cells by osmosis
- Mineral salts are naturally
occurring ionic compounds, plants
need ions from these compounds
to produce new substances
- For example, nitrate ions are needed to
make proteins
- The concentration of ions inside a root
hair cell is greater than in the soil
- Mineral ions cannot diffuse against
this concentration gradient
- So, proteins in the cell membrane
pump the ions into the cell, this is an
example of active transport
- CB6d
- The evaporation of water from leaves keeps
them cool and helps move water (and
dissolved mineral ions) up the plant
- The flow of water into a root, up the stem and
out of the leaves is called transpiration
- Xylem vessels form tiny continuous pipes
leading from a plant's roots up into it
leaves
- Inside the vessels is an unbroken chain of water, due to the
weak forces of attraction between water molecules
- Water is pulled up the xylem vessel in
the stem as water evaporates from the
xylem vessels in the leaves
- As the water vapour diffuses out of a leaf, more
water evaporates from the xylem inside the leaf
- The concentration of water vapour in the air spaces inside
a leaf is greater than outside it, so, water molecules diffuse
down the concentration gradient, out of the leaf
- A bigger difference between the
concentrations makes the gradient
steeper, which makes diffusion faster
- During their development, xylem
cells die and their top and
bottom cell walls disintegrate
- This creates long empty vessels (tubes)
through which water can move easily
- Xylem vessels are rigid because they
have thick side walls and rings of
hard lignin, and so water pressure
inside the vessels does not burst or
collapse them
- The rigid xylem vessels also helps to
support the plants
- Plants make sucrose from the glucose and
starch made by photosynthesis
- Sucrose is translocated (transported) in the
sieve tube of the phloem tissue