CELL MEMBRANE: STRUCTURE

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The cell membrane is composed of lipids, proteins, and carbohydrates. They do jobs for the cell membrane that are separate from the main jobs that we have already discussed. The cell membrane is not solid, but has the consistency of oil. Its components are constantly moving. We sometimes refer to the composition of the cell membrane as the fluid mosaic model.

LIPIDS: THE MAIN COMPONENT OF THE CELL MEMBRANE. The main component of the cell membrane is a phospholipid bilayer, which is two layers of phospholipids, with the hydrophilic heads (loves water) faced outward and the hydrophobic tails (fears water) faced inward.

Bilayer = two layers Hydrophilic = water-loving Hydrophobic = water-fearing Some molecules can pass directly through the phospholipid bilayer. Other molecules must travel through proteins.

CARBOHYDRATES Carbohydrates are usually attached to the outside of the cell membrane and act as “ID cards” for the cell so that cells can recognize and identify one another.

PROTEINS: HELP WITH TRANSPORT Channel Proteins: Form passageways for molecules, such as water, to pass through the cell membrane quickly. Carrier Proteins: Binds with a substance on one side of the cell membrane, changes shape, and moves it to the opposite side of the cell membrane. - Protein Pumps: Move molecules against their concentration gradient (from low to high concentration)

PASSIVE TRANSPORTACTIVE TRANSPORT DOES NOT REQUIRE ENERGY DOES REQUIRE ENERGY MOVES THINGS ALONG THEIR CONCENTRATION GRADIENT (FROM HIGH TO LOW CONCENTRATION) MOVES THINGS AGAINST THEIR CONCENTRATION GRADIENT (FROM LOW TO HIGH CONCENTRATION) EXAMPLES: 1.DIFFUSION 2. OSMOSIS 3. FACILITATED DIFFUSION EXAMPLES: 1.PROTEIN PUMPS 2.ENDOCYTOSIS 3.EXOCYTOSIS

PASSIVE TRANSPORT 1. DIFFUSION- “Spreading out” of a substance. The movement of substances from an area of high concentration to an area of low concentration. 2. OSMOSIS- A type of diffusion, osmosis refers to the movement of water across the cell membrane. Water uses aquaporins, a type of channel protein, for movement. 3. FACILIATED DIFFUSION- Any type of diffusion, like osmosis, that occurs with the help of a channel or carrier protein.

ATP In Biology, when we say that a process “requires energy,” what the process really needs is a molecule called ATP. In most organisms, the major molecule that provides energy is ATP (adenosine triphosphate).

ACTIVE TRANSPORT 1. PROTEIN PUMPS- like facilitated diffusion this is the movement of molecules using proteins, but in this case, the cell membrane is moving molecules against their concentration gradient (from low to high concentration.)

ACTIVE TRANSPORT 2. ENDOCYTOSIS: the movement of large amounts of a substance into the cell using vesicles. 3. EXOCYTOSIS: the movement of large amounts of a substance out of the cell using vesicles. Remember: vesicles are small containers that are themselves surrounded by a lipid bilayer and can pinch off or fuse with the cell membrane.