Transcript Substances enter and leave cells through the cell membrane

Substances enter and leave
cells through the cell membrane
WJEC Module 2
Questions you’ll be answer by the
end!
 How do substances enter and leave cells?
 What is diffusion?
 What is osmosis?
 Why does some transport need energy?
Learning Outcomes
 know that diffusion is the movement of
substances down a concentration gradient.
The process does not require energy and only
certain substances pass through the cell
membrane in this way
Diffusion
 Diffusion is
 The movement of particles in a gas or any
dissolved substance from a region of higher
concentration to a region of lower
concentration
 In cells, diffusion occurs across the cell
membrane. The cell membrane can be
described as partially permeable. It will only
allow some substances to pass through it.
Learning outcome
 know that osmosis is the diffusion of water
through a selectively permeable membrane,
from a region of high-water concentration to
region of low-water concentration.
Osmosis
 Osmosis is the diffusion of water, through a
partially permeable membrane, from a
region of high water concentration to a
region of low water concentration.
Learning outcome
 carry out experimental work using living plant
material and visking tubing as non-living
material and be able to interpret
experimental results in terms of membrane
pore and particle size: the pore size is large
enough to allow water molecules through but
restricts the movement of solute molecules.
Osmosis Practical
 Activity 1
 Changing the rate of osmosis
 Two students carried out an investigation using
cylinders of potato. They cut out 5 potato cylinders
and measured the mass of each one. They made up
five different glucose solutions with a different
concentration and left a potato cylinder in each
solution for 24 hours.
 After 24 hours they dried each potato cylinder and
measured its mass again.
 Look at the table of results
 Answer the questions that follow
Osmosis in potato
Solution
Water
0.1
0.2
0.5
1.0
Mass before (g) Mass after (g) Change in mass (g) %age change
in mass
Osmosis practical
 A model Cell
 Visking tubing is partially permeable.
 Cut 2 pieces of Visking tubing to 12cm long
 Tie a knot in one end
 Fill “cell A” with sugar solution
 Fill “cell B” with water
 Tie the other end of each piece of visking tubing
 Weigh each “cell”
 Put “cell A” into a boiling tube of water
 Put “cell B” into a boiling tube of sugar solution
 After 30 minutes reweigh each “cell”
Conclusions
 Describe and explain your results for each
cell
 Cell A increases in mass because water has
moved into the cell by osmosis, from a high
water concentration in the boiling tube to a
low water concentration inside the visking
tubing.
 Cell B decreases in mass because water has
moved by osmosis.......
Osmosis Practical
 Osmosis in potato cells
 Cut three potato cylinders to the same length
 Measure their mass and write it down
 Set up the following test tubes
 Test tube A – distilled water
 Test tube B – weak sugar solution
 Test tube C – strong sugar solution
 Place one potato cylinder in each test-tube
and leave it for 30 minutes
 Re-weigh each chip
conclusions
 Calculate the %age increase in mass for




each potato chip
Which chip has increased in size? Why?
Which chip has decreased in mass? Why?
Describe the “feel” of each of the potato
cylinders
Explain why the potato cylinder in A feels
different to the potato cylinder in C.
Learning outcome
 understand active transport as an energy
requiring process whereby substances can
enter cells against a concentration gradient.
Active Transport
 Active transport is the movement of particles
across a membrane up a concentration
gradient.
 Particles move from where they are in low
concentration to where they are in high
concentration.
 This requires energy from respiration.
Comparing diffusion and active
transport
Diffusion
 Substances move
from a region of
higher concentration
to a region of lower
concentration
 This relies on the
random movement of
molecules
 Diffusion is like
freewheeling down a
steep hill
Active Transport
 Substances are moved
from a region of lower
concentration to a
region of higher
concentration
 This process required
energy
 Active transport is like
cycling up a steep hill,
you use lots of energy