Cell Transport

Table of Contents:
Intro 00:00
Importance of Cell Membrane for Homeostasis 0:41
Cell Membrane Structure 1:07
Simple Diffusion 1:45
What does it mean to "go with the concentration gradient?" 2:20
Facilitated Diffusion 2:38
Active Transport 4:53 (including endocytosis 5:36 exocytosis 6:36)

Transport types covered include simple diffusion, facilitated diffusion, endocytosis, and exocytosis.

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0:40 To keep a stable environment or homeostasis, cells must have some control over what goes in and what goes out of them. A very important structure that all cells contain is the cell membrane which helps regulate homeostasis by controlling what goes in and out.
1:14 The cell membrane has signaling abilities. It's made of a phospholipid bilayer (two layers of lipids). These lipids have a polar hydrophilic water-loving (polar molecules have unequal sharing of electrons among atoms) head and a hydrophobic water disliking tail that's nonpolar (nonpolar molecules have equal sharing of electrons among atoms) made of two fatty acid tails.
1:38 Some molecules have no problem going through the cell membrane and go directly through the phospholipid bilayer. Small nonpolar molecules such as oxygen, carbon dioxide gas, steroid hormones and fatty acids. This process is known as simple diffusion. Simple diffusion is a passive transport that doesn't require added energy, it uses kinetic (movement) energy. In simple diffusion, molecules go with/along the concentration gradient (from high concentration to low concentration).
2:33 Some proteins in the membrane are known as transport proteins (channels or pumps). They help transport molecules across the membrane that are too large to directly go through to membrane or are polar.
3:08 This process is known as facilitated diffusion. It's still a form of diffusion and still moves molecules from high concentration to low concentration via passive transport. The proteins are facilitating/helping things pass. Carrier proteins or water filled channel proteins are used to move molecules such as glucose, other sugars, some amino acids and charged ions passively across the membrane.
3:38 In osmosis, for water to travel at a fast rate across the membrane, it passes through protein channels called aquaporins.
4:11 Transport from low concentration to high concentration will require energy from adenosine triphosphate (ATP). For example, the cells lining your gut, they need to take in glucose but what if the concentration of glucose is higher than the glucose concentration in the environment? The cells still need to take glucose in but it's going to be forced against the regular gradient flow. ATP can power active transport and force molecules to go against their concentration gradient. One way ATP can do that is by energizing the transport protein itself (sodium potassium pump).
5:14 During active transport, there's other times a cell needs to exert energy for transport (known as bulk transport). If a cell needs a very large molecule (for ex a polysaccharide - a large carbohydrate), you may need the cell to fuse with the molecule in order to bring it inside. This is called endocytosis (endo means in), often this fusing of substances within the cell membrane will form vesicles that can be taken into the cell.
Phagocytosis (cell eating)- a process where a cell binds to the item it wants to engulf on the cell surface and draws the item inward while engulfing around it. Often happens when the cell wants to destroy something, like a virus or an infected cell. Immune system cells (macrophages) use phagocytosis to engulf pathogens.
Receptor-mediated endocytosis - A process where receptor proteins on the cell surface are used to capture or bind to a specific target molecule.
Pinocytosis (cell drinking) - A process in which a cell takes in small amounts of extracellular fluid.
6:38 - Exocytosis (exo means exit) can be used to get rid of cell waste. It is also used to get valuable materials out that a cell has made. These materials are transported from the inside to the outside of the cell in membrane-bound vesicles that fuse with the plasma membrane.

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