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Movement of Substances through a Cell Membrane

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Movement of Substances through a Cell Membrane GCSE Additional Science Chapter 3 BIOLOGY 2 How do Substances Enter and Leave Cells? Molecules move quickly, at random ... – PowerPoint PPT presentation

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Title: Movement of Substances through a Cell Membrane


1
Movement of Substances through a Cell Membrane
  • BIOLOGY 2

GCSE Additional Science
Chapter 3
2
How do Substances Enter and Leave Cells?
  • Molecules move quickly, at random, through a
    liquid this is called DIFFUSION. Substances
    move from a region of high to low concentration.

GCSE Additional Science
Chapter 3
3
  • Here the pink molecules will move from the left
    to the right until there are the same number of
    pink molecules on both sides. The orange
    molecules move in the opposite direction. This is
    how substances such as oxygen and carbon dioxide
    enter and leave cells.

Flow
GCSE Additional Science
Chapter 3
Flow
4
The Following Factors Affect the Rate of
Diffusion
  1. Concentration the larger the difference between
    the concentration of molecules, the faster they
    will diffuse.
  2. Temperature the higher the temperature, the
    faster diffusion of the molecules.
  3. Pressure the higher the pressure on the
    molecules, the faster the movement (from the
    region of higher pressure to the region of lower
    pressure)

GCSE Additional Science
Chapter 3
5
The Effect of the Cell Membrane on Diffusion
  • If a cell membrane was totally permeable (allows
    everything in and out), the cell would die in no
    time. Even though useful substances would enter
    the cell easily, the cells content would diffuse
    out. In order to survive therefore, the cell
    membrane has to be selectively permeable (allows
    only some substances to enter and leave)

GCSE Additional Science
Chapter 3
6
Osmosis
Higher
  • Here we can see the water diffusing through a
    selectively permeable membrane, from a region of
    high concentration to a region of lower water
    concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
7
Osmosis
Higher
  • Here we can see the water diffusing through a
    selectively permeable membrane, from a region of
    high concentration to a region of lower water
    concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
8
Osmosis
Higher
  • Here we can see the water diffusing through a
    selectively permeable membrane, from a region of
    high concentration to a region of lower water
    concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
9
Practical Work
  • We can use a special plastic called Visking
    tubing to demonstrate osmosis.
  • Here we can see that the water molecules can
    move through the tubing but the sucrose molecules
    are prevented. The reason for this is to do with
    the size of the molecules. The sucrose molecule
    is larger than the water and cannot fit through
    the tiny holes in the wall of the tubing. Water
    can move into the sucrose solution, but the
    sucrose cannot go into the water.
  • After about half an hour, the solutions level
    in the tube will have risen, and the water level
    will have dropped.

capillary tube
tight knot
GCSE Additional Science
Chapter 3
sucrose solution
Visking tube
water
10
Practical Work
  • Potatoes can be used to demonstrate osmosis in
    living tissue.
  • Potato cylinders of the same length and diameter
    are each placed in different concentrations of
    sucrose solutions for 20 minutes.
  • The length and mass of each cylinder should
    then be re-measured. The potatoes that have been
    in strong sucrose solutions will have reduced in
    length and mass, whilst the ones placed in weak
    sucrose solutions will have gained length and
    mass.
  • Cylinders showing no change will have cells with
    cytoplasm of equal strength to the sucrose
    solution that they were placed in.

Distilled Water
0.1M
0.2M
1M
0.5M
Labelled Petri dishes
5mm
GCSE Additional Science
Chapter 3
50mm
Potato cylinder
11
The Importance of Osmosis to Cells
Higher
  • If a red blood cell was in a solution containing
    less water than itself (hypertonic), then the
    cell would lose water. It would shrivel and die.

GCSE Additional Science
Chapter 3
12
Homeostasis
Higher
  • It is very important therefore to keep the blood
    constant so that the cells in it do not gain or
    lose too much water.
  • The term for keeping these internal conditions
    constant is Homeostasis.

GCSE Additional Science
Chapter 3
13
The importance of Osmosis to Cells
Higher
  • If a red blood cell was in a solution that had
    more water in it than was in the cell
    (hypotonic), then it would gain water. It would
    expand, burst and die.

GCSE Additional Science
Chapter 3
14
Active Transport
Higher
  • In many cases substances are moved into or out
    of cells against a diffusion gradient i.e.
    opposite to the movement of the molecules.
  • Energy is needed for this process, and so its
    called active transport.

GCSE Additional Science
Chapter 3
15
Examples of Active Transport
Higher
  • In the body we see active transport in the small
    intestine, when digested food is absorbed into
    the blood in the opposite direction to diffusion.
  • In a plant, rare minerals from the soil are
    pulled into the roots opposite to the direction
    of diffusion.

GCSE Additional Science
Chapter 3
16
Summary
  • Diffusion is the movement of particles down a
    concentration gradient.
  • Osmosis is the diffusion of water molecules
    through a selectively permeable membrane.
  • Active transport is the movement of molecules
    against a concentration gradient this requires
    energy.

GCSE Additional Science
Chapter 3
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