1. Cell Membranes Recall that the smallest unit of life is the cell. All cells are surrounded by a plasma membrane that is vital in various aspects of the cell’s biology. General functions: 1. Selective isolation of cell’s contents from the external environment. 2. Regulation of material that moves in and out of the cell. 3. Communication with other cells.

2. Cell Membranes membrane structure Cellular membranes are composed of phospholipids that have hydrophilic heads and hydrophobic tails. How do you think these phospholipids would line up in a water environment? And Why? They form a bilayer. Note how the hydrophobic tails are all Lined up together towards the inside, Whereas the hydrophilic heads are lined Up towards the outside watery environment. Most biological molecules are water soluble (i.e. polar) and can not pass this membrane. The cell has effectively isolated the inside from the outside!!

3. Cell Membranes membrane structure The cellular membrane (or plasma membrane) is more complex than that… Membranes contain varying amounts of cholesterol… They regulate the fluidity and flexibility of the membrane… (remember that the membrane is “fluid”) There are also a lot of proteins and glycoproteins (proteins with sugar moieties) throughout the membrane. We will discuss their function separately.

4. Cell Membranes membrane proteins Membrane protein types and functions: 1. Transport proteins - these proteins are involved in regulating transport of various substances in and out of the cell. 2. Receptor proteins – these proteins recognize “signals” or “messages” from their environment and can induce biological responses within the cells. 3. Recognition proteins - these proteins allow the cell to be “recognized” by other cells. In multicellular organisms, these proteins are involved in self recognition. Recognition proteins are often glycoproteins.

5. Cell Membranes Transport or material across plasma membranes 1. Simple diffusion: The net movement of molecules down a concentration gradient. The greater the gradient, the faster the movement (until the gradient is eliminated). Note that diffusion is a very slow process. Simple diffusion is involved in transport of water, dissolved gases, and small, Lipid soluble substances (why do you think that is?)

6. Cell Membranes Transport or material across plasma membranes 2. Facilitated diffusion: Diffusion of molecules through a channel or carrier protein. Why would you think there would be a need for “channels” or a “carrier” proteins?

7. Cell Membranes Transport or material across plasma membranes 2. Facilitated diffusion: (cont.) Diffusion of molecules through a channel or carrier protein. Why would you think there would be a need for “channels” or a “carrier” proteins?

8. Cell Membranes Transport or material across plasma membranes 3. Osmosis: Diffusion of water across a differentially permeable membrane - this means a membrane that allows water to pass through freely, but not various dissolved molecules. (e. g. human blood cells)

9. Cell Membranes Transport or material across plasma membranes Vocabulary review for osmosis: Isotonic solution - concentrations of solutes is same inside and outside the membrane. Which direction do you think water will flow? Hypertonic solution – concentration of solutes is higher outside the membrane. Which direction will water flow? Hypotonic solution – concentration of solutes is lower outside the membrane. Which direction will water flow?

10. Cell Membranes Transport or material across plasma membranes Cells however sometimes need to transfer molecules from areas of low concentration to areas of higher concentration. This processes are energy-requiring transport systems (i.e. active transport). In case of simple diffusion, facilitated diffusion, and osmosis, substances were moving from areas of high concentration to low concentration. There was no need for energy (i.e. transport was passive).

11. Cell Membranes Transport or material across plasma membranes Active transport – uses ATP as a source of energy to transfer material (in this example Calcium ions) up a concentration gradient. In this process, the ATP is broken down to ADP.

12. Cell Membranes Transport or material across plasma membranes Cells can also use energy to actively engulf extracellular fluid or small particles. This process is referred to as endocytosis, and there are three general forms:

13. Cell Membranes Transport or material across plasma membranes Review of three types of endocytosis: Pinocytosis – pinching off of a small amount of the fluid around the cell. Receptor-mediated endocytosis – this is the process where the cell selectively bind to and engulf material found outside the cell. Phagocytosis – process by which cells pick up large particles, including whole microorganisms.

14. Cell Membranes Transport or material across plasma membranes Review of different forms of transport:

15. Cell Membranes Transport or material across plasma membranes Note that both active and passive transport can be for material being transferred both into as well as out of the cells. In case of endocytotic processes, the release of material out of the cell is referred to as exocytosis.

16. Cell Membranes In case of plants, cells secrete one or two additional primarily cellulose cell walls around their plasma membrane.

17. Cell Structure and Function Next lecture…