4.2.1
Passive Movement Across the Membrane
Simple Diffusion
Simple Diffusion
Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration.


Diffusion
Diffusion
- Diffusion describes the passive movement of particles in fluids (liquids and gases).
- Molecules move randomly but tend to move into any space available until it is evenly distributed.
- So, the net movement of particles will be towards the area of lower concentration or down a concentration gradient.
- Substances such as CO2, O2 and urea can move in and out of cells across cell membranes via diffusion.


E.g. Perfume
E.g. Perfume
- When you spray perfume, the smell diffuses across the room from the area of high concentration (where you just sprayed it) to the area of low concentration (the other side of the room).
Facilitated Diffusion
Facilitated Diffusion
In facilitated diffusion, molecules diffuse through carrier proteins or channel proteins spanning across the membrane.


Large or polar molecules
Large or polar molecules
- Some materials are too large to move through the membrane.
- Other materials are polar molecules that are repelled by the hydrophobic part of the membrane.
- These materials move across the membrane by facilitated diffusion.


Facilitated diffusion
Facilitated diffusion
- Facilitated diffusion describes the net movement of particles down a concentration gradient (i.e. from a region of high concentration to a region of lower concentration).
- Facilitated diffusion is a passive process so it does NOT use energy.


Channel proteins
Channel proteins
- Facilitated diffusion takes place through channel proteins.
- Channel proteins allow polar compounds to avoid the non-polar central layer of the plasma membrane.
- The non-polar layer would slow or prevent entry into the cell.
- Aquaporins are channel proteins that allow water to pass through the membrane.


Carrier proteins
Carrier proteins
- Facilitated diffusion takes place through carrier proteins.
- When a substance binds to carrier proteins, there is a change in the shape of the carrier protein.
- This change in shape moves the substance from the outside of the cell to the inside.
Factors Affecting Diffusion
Factors Affecting Diffusion
The rate of diffusion can speed up or slow down based on the following factors:


Concentration gradient
Concentration gradient
- The concentration gradient is linked to the difference in concentration between two areas.
- The bigger the difference in concentration between two areas, the greater the concentration gradient and the faster the rate of diffusion.


Temperature
Temperature
- The higher the temperature, the faster the rate of diffusion.


Membrane surface area
Membrane surface area
- The larger the surface area of the membrane that a substance is diffusing through (e.g. the membrane around a cell), the faster the rate of diffusion.


Thickness of membrane
Thickness of membrane
- The thicker the exchange surface, the slower the rate of diffusion.


Channel or carrier proteins
Channel or carrier proteins
- The higher the concentration gradient, the faster the rate of facilitated diffusion until an equilibrium is reached and the rate will level off.
- The number of channel or carrier proteins affects the rate of facilitated diffusion. The more proteins, the faster the rate.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Unlock your full potential with Seneca Premium
Unlimited access to 10,000+ open-ended exam questions
Mini-mock exams based on your study history
Unlock 800+ premium courses & e-books