THE CELL MEMBRANE The Cell Membrane The cell
THE CELL MEMBRANE
The Cell Membrane • The cell membrane is incredibly important for the transport of materials in all cells. • Protists carry out feeding, respiration and excretion through the cell membrane. • Animal, plant and fungus cells also rely on the cell membrane for the exchange of gases and nutrients. • Cell membranes of all organisms have a similar structure.
Fluid Mosaic Model • “Two layers of phospholipids, with proteins embedded like mosaic tiles. • Carbohydrates are attached to the proteins, extending to the exterior. • Proteins and lipids move fluidly along the cell membrane. • Proteins are used as transport facilitators, receptors for messenger molecules, and enzymes.
The Fluid Mosaic Model
The Cell Membrane
Another Cell Membrane
Phospholipid bilayer
https: //www. youtube. com/watch? v=y 31 Dl. J 6 u Gg. E
Permeability of the cell membrane • Cell membranes are called “selectively permeable”. • This means that some substances pass through easily (e. g. water, glucose, carbon dioxide, fats, alcohols, oxygen). • Large molecules cannot pass through (starches, proteins). • Some ions cannot pass through. • Permeability changes from cell to cell, and even from moment to moment • https: //www. youtube. com/watch? v=d. PKv. Hr. D 1 e. S 4
Passive Transport • No cellular energy is required • Diffusion and osmosis are examples
Diffusion • Molecules move from an area of high concentration to an area of low concentration • This occurs because molecules are in constant motion • The difference in concentration between a region of high and low concentration is called a concentration gradient. • Once the molecules are evenly distributed, no further diffusion takes place.
What does diffusion look like?
Facilitated Diffusion • Still requires a concentration gradient • Transport proteins in the cell membrane aid substances to diffuse more quickly than they normally would.
Osmosis • Diffusion of water • Also depends on concentration gradients. • We usually discuss it in terms of the concentration of what is DISSOLVED in the water. • Water moves towards a higher concentration of solute.
Isotonic Solution • Has the same concentration of dissolved substances as the cell placed in it. • The concentration gradient is thus zero. • There is no net movement of water into or out of the cell.
Isotonic Solution
Hypotonic Solution • Solution contains a lower concentration of dissolved substances than the cell placed in it. • This means there is a higher WATER concentration outside the cell. • Therefore, there is a net movement of water INTO the cell. • This can cause animal cells to swell and burst, and a plant cell to become turgid.
Hypotonic Solution
Hypertonic Solution • Solution contains a higher concentration of dissolved substances than the cell placed in it. • This means there is a lower WATER concentration outside the cell. • Therefore, there is a net movement of water OUT OF the cell. • This can cause animal cells to shrivel and shrink.
Hypertonic Solution
Osmosis: All three scenarios
Osmosis in action
Active Transport • Movement of materials against a concentration gradient. • Requires energy • Can occur like facilitated diffusion, with a transport protein moving molecules across.
Active Transport
Endocytosis and Exocytosis • Involve vesicles. • In endocytosis, materials become inclosed in an inpocketing of the cell membrane. • In exocytosis, a vesicle or vacuole carrying materials fuses with the cell membrane and its contents are ejected.
Endocytosis and Exocytosis
Endo and Exocytosis again
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