1. Biology I for Non-Majors
Cell Membranes

2. Cell Membranes are a Fluid Mosaic
Defines the boundary of the cell
Determines the nature of its contact with the environment
Dynamic and constantly in flux
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3. Major Components of Cell Membranes
Made up primarily of a bilayer of phospholipids with:
Embedded proteins
Carbohydrates
Glycolipids
Glycoproteins
Cholesterol (in animal cells)

4. Passive Transport
The most direct forms of membrane transport are passive
Naturally occurring phenomenon
Does not require the cell to exert any of its energy to accomplish the movement
Substances move from an area of higher concentration to an area of lower concentration
In other words, down a concentration gradient
Remember membranes have hydrophilic and hydrophobic regions
This helps the movement of some materials through the membrane and hinders the movement of others
Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane

5. Types of Passive Transport
Diffusion: single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space
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6. Types of Passive Transport
Facilitated transport: materials diffuse across the plasma membrane with the help of membrane proteins
Still requires a concentration gradient
Transport proteins are integral proteins involved in facilitated transport
Channels are specific for the substance that is being transported
Carrier proteins binds a substance and move the bound molecule from the outside of the cell to its interior
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7. Types of Passive Transport
Osmosis: a special case of diffusion
Only material capable of moving through a membrane will diffuse through it
If a solute is too large to fit, water will move instead down a concentration gradient

8. Types of Passive Transport
Osmosis: a special case of diffusion
Only material capable of moving through a membrane will diffuse through it
If a solute is too large to fit, water will move instead down a concentration gradient

9. Tonicity
Describes how an extracellular solution can change the volume of a cell by affecting osmosis
Hypotonic means that the extracellular fluid has a lower concentration of solutes
Hypertonic refers to the extracellular fluid having a higher osmolarity than the cell’s cytoplasm
Isotonic means the extracellular fluid has the same osmolarity as the cell
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10. Active Transport
Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP)
Usually when a substance is moving against its concentration gradient (from an area of lower concentration into an area of higher concentration)
Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane
The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient

11. Active Transport Requires Energy and Pumps
To move substances against a concentration or electrochemical gradient, the cell must use energy
Active transport mechanisms, collectively called pumps, work against electrochemical gradients
There are 3 types of pumps
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12. Types of Active Transport
Primary Active Transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP
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13. Types of Active Transport
Secondary Active Transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP
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14. Active Transport for Large Molecules
Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell
There are variations of endocytosis, but all share a common characteristic:
Plasma membrane of the cell invaginates, forming a pocket around the target particle
The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane

15. Phagocytosis
The process by which large particles, such as cells or relatively large particles, are taken in by a cell
“Cell eating”
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16. Pinocytosis
A process that takes in molecules, including water, which the cell needs from the extracellular fluid
Results in a much smaller vesicle than does phagocytosis
“Cell drinking”
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17. Receptor-Mediated Endocytosis
A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances
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18. Exocytosis
The reverse process of moving material into a cell is the process of exocytosis
Purpose is to expel material from the cell into the extracellular fluid
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19. Cellular Transport

20. Quick Review Why are cell membranes said to be fluid mosaics?
List the major components of membranes and their roles
What are the types of passive transport?
Can you describe which direction water will move in solutions of different tonicity?
Define the three types of pumps used in active transport.
What are the types of active transport?
Define three types of endocytosis
What are the steps of exocytosis?