1. Unit 2A Human Form & Function
Cells, metabolism & regulation
Transport

2. Further information
Further information about this topic can be found in Our Human Species (3rd edtn) Chapter 3, section 5

3. The cell membrane
Copyright - National Institute of Standards and Technology
Drawing by Dana Burns

4. The cell membrane
The cell membrane (or plasma membrane) is the outer boundary of a cell.
The membrane is a phospholipid bi-layer.
Proteins and other molecules are embedded in the membrane.
The cell membrane is selectively permeable – i.e. it allows some substances to cross more easily than others.

5. The bi-layer TISSUE FLUID CYTOPLASM Hydrophilic phosphate
Membrane protein
TISSUE FLUID
Hydrophobic lipid tail
PHOSPHOLIPID
BI-LAYER
CYTOPLASM

6. Diffusion through the membrane
Because the cell membrane is fatty, most water soluble substances cannot diffuse through it.
Exceptions include oxygen & carbon dioxide.

7. Membrane proteins A variety of proteins are embedded in the bi-layer.
These serve various functions including the movement of substances in and out of the cell.

8. Membrane transport proteins
Membrane proteins that aid the movement of substances in and out of the cell include:
Channel proteins (ion channels) – open channels that allow simple diffusion.
Carrier proteins that allow facilitated diffusion (e.g. glucose) and active transport (specific membrane pumps).

9. Membrane transport Transport processes are either passive or active.
Passive processes require no cellular energy and include diffusion, osmosis & facilitated diffusion.
Active processes require ATP and include specific membrane pumps and phagocytosis/pinocytosis.

10. Passive processes Require no cellular energy (ATP).
Substances move from high concentration to low concentration.
E.g. – diffusion, facilitated diffusion & osmosis.

11. Diffusion
Diffusion is the tendency for particles to move from an area of higher concentration to an area of lower concentration.
Diffusion results from the random motion of atoms and molecules due to their kinetic energy.
Diffusion involves the movement of atoms & molecules in gases and liquids (specifically, solutes).

12. Diagram created by LadyofHats
Diffusion
Simple diffusion is the movement of particles from an area of high concentration to an area where their concentration is lower.
Diagram created by LadyofHats

13. Concentration gradient
When the concentration of a substance is different at two places, the substance will diffuse along the concentration gradient until the concentration of the two areas becomes equal.
High concentration
Diffusion gradient
Low concentration

14. Rate of diffusion slows Uniform concentration
Concentration at A
Concentration at B 1
Diffusion gradient
High concentration
Low concentration
Rate of diffusion slows 2
Equilibrium 3
Uniform concentration

15. Osmosis
This is the diffusion of water across a selectively permeable membrane.
Water molecules move from high concentration to low concentration.
This is a passive process (does not need ATP).

16. Osmosis Net water movement from high conc. To low conc.
Low water concentration (concentrated soln.)
High water concentration (dilute soln.)
Net water movement from high conc. To low conc.
Semipermeable membrane

17. Facilitated diffusion
Facilitated diffusion is a process of diffusion where molecules diffuse across cell membranes with the assistance of transport proteins.
Diffusion takes place from high concentration to low concentration and does not require ATP.
Examples: glucose & amino acids.

18. Simple diffusion (left) & facilitated diffusion
Diagram created by LadyofHats

19. Active transport Require energy (ATP).
Substances move from low concentration to high concentration (i.e. against the concentration gradient).
Example: membrane pumps.

20. Active transport (sodium pump)
Diagram created by LadyofHats

21. Endocytosis
Pinocytosis & phagocytosis are specific types of endocytosis.
Both processes involve cells absorbing large particles such as proteins (or even whole organisms, such as bacteria & viruses) from the outside by engulfing them with their cell membrane to form a vesicle (like a bubble) within the cytosol.

22. Endocytosis & exocytosis
Endocytosis = things entering the cell
Phagocytosis = cell eating
Pinocytosis = cell drinking
Exocytosis = things leaving the cell

23. Diagram created by LadyofHats
Endocytosis
Diagram created by LadyofHats

24. Diagram created by LadyofHats
Exocytosis
Diagram created by LadyofHats

25. Study Guide Read: Our Human Species Chapter 3, section 5 Complete:
Workbook Topic 4 - Transport