1. Membrane Function Bulk Transport Cell Signaling Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

2. Membrane Function
Working cells must control the flow of materials to and from the environment.
A cell membrane or wrapper made of phospholipids is relatively impermeable
Proteins embedded in the membrane provide for selective permeability and transport of materials into and out of the cell, among other things
Membrane Selectivity

3. Cell or Plasma Membrane
Phospholipid
bilayer

4. Extracellular fluid (watery environment) Cholesterol Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outward-
facing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Inward-facing
layer of
phospholipids
Bimolecular
lipid layer
containing
proteins
Peripheral
proteins
Nonpolar
tail of
phospholipid
molecule
Cytoplasm
(watery environment)
Figure 3.3

5. Functions of Membrane Proteins
Figure 5.11

6. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

7. Passive Transport: Diffusion Across Membranes
Molecules contain heat energy.
They vibrate and wander randomly.
Diffusion is movement of molecules from regions of high concentration to low concentration
Molecules tend to spread into the available space.
Diffusion Across A Membrane by passive transport
Simple diffusion of small or hydrophobic molecules
Facilitated diffusion of larger and polar molecules
Diffusion

8. Examples of Passive Transport Across a Phospholipid Bilayer

9. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

10. Osmosis and Water Balance in Cells
Osmosis is the facilitated transport of water across a selectively permeable membrane.
Water moves across a membrane from high concentration (high purity) to low concentration (low purity)

11. Terms Used to Compare the Purity of Water Solutions
A hypertonic solution
Has a higher concentration of dissolved substances (solute) like salts, proteins, etc.
5% salt
1% salt
A hypotonic solution
Has a lower concentration of dissolved substances (solute)
1% salt
5% salt
An isotonic solution
Has an equal concentration of dissolved substances (solute).
1% salt
1% salt

12. Cells Must Regulate Salt and Water Balance
(a) Isotonic solutions
(b) Hypertonic solutions
(c) Hypotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Turgid Elodea
Plasmolysis

13. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

14. Active Transport: The Pumping of Molecules Across Membranes
Active transport is the movement of molecules from low concentration to high concentration, against a concentration gradient
Active transport requires energy to pump molecules across a membrane, into more a more crowded space
Concentration gradient
Active Transport

15. The Sodium-Potassium Pump Creates a Gradient Using ATP
Extracellular fluid
Na+
Na+-K+ pump
ATP-binding site K+
Na+ bound
Cytoplasm 1
Cytoplasmic Na+ binds to pump protein. P
ATP
K+ released
ADP
K+ is released from the pump protein and Na+ sites are ready to bind Na+ again. The cycle repeats. 6
Binding of Na+ promotes phosphorylation of the protein by ATP. 2
Na+ released
K+ bound P Pi K+ 5
K+ binding triggers release of the phosphate. Pump protein returns to its original conformation.
Phosphorylation causes the protein to change shape, expelling Na+ to the outside. 3 P 4
Extracellular K+ binds to pump protein.

16. Active Transport Can Create Electrical AND Chemical Gradients
Both chemical and electrical gradients represent stored energy, and can be used to perform work (e.g. pumping something through the membrane by co-transport, or making ATP)

17. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

18. Bulk Transport: Exocytosis and Endocytosis
Dumping molecules out of the cell (export)
Exocytosis and Endocytosis Introduction

19. Bulk Transport: Exocytosis and Endocytosis
Bringing molecules into the cell (import)
Includes pinocytosis, phagocytosis, and receptor-mediated endocytosis
Pinocytosis: cell “drinking”
Phagocytosis: cell “eating”

20. Receptor-mediated endocytosis

21. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling

22. The Role of Membranes in Cell Signaling
Cellular communication
Begins with the reception of an extracellular signal.
The signal is transduced or passed across the membrane
A response is elicited from the transduced signal

23. The Working Cell: Membrane Function
Passive Transport
Osmosis and Water Balance
Active Transport
Bulk Transport
Exo- and Endocytosis
Cell Signaling