Movement of Substances In and Out of Cells

Cells need and take in many substances for their metabolism and optimum functioning…Examples?? As well as this, cells make wastes or products that need to be transported out of the cell…Examples?? Substances can move into and out of cells in a variety of ways. Passive forms of movement require no energy. Whereas active forms of movement require energy.

The cell (plasma) membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell. It separates the solution inside the cell (cytosol/intracellular fluid), from the solution outside the cell (extracellular fluid). As stated previously, the cell membrane is made up of fat and some protein molecules.

Substances move into and out of a cell depending on… Fat solubility – if substances dissolve in fat, they move through easier. Size – large molecules cannot pass through the membrane. Membrane channels and transporters – these are proteins in the membrane that assist or ferry substances that cannot normally cross (they are selective).

Passive Processes Simple Diffusion

Diffusion In a liquid/gas, molecules are always in motion due to their kinetic energy. Diffusion is the movement of a substance from a region of high concentration to a region of low concentration until equilibrium of the substance is achieved. This is a passive process that does not require energy.

During diffusion, substances move down a concentration gradient, going from high to low.

What factors could affect the rate (speed) of diffusion?

Higher temp – diffuse faster Temperature Larger surface – diffuse faster Surface area Higher gradient – diffuse faster Concentration gradient Smaller particles – diffuse faster Size of particles Solid – slowest; Liquid – faster; Gas - fastest Diffusion medium

Examples of substances that move through the cell membrane via diffusion include: Fats Fat soluble vitamins Oxygen Carbon dioxide

Facilitated Diffusion What does to facilitate mean? In facilitated diffusion, substances that are too large (or fat insoluble) enter the cell via protein channels inserted into the membrane (they act like gateways). Substances still move down their concentration gradient. Glucose is moved into the cell in this way.

Osmosis Another form of passive movement is osmosis. Osmosis only involves the movement of water through a selectively permeable membrane. Water molecules move from a weak solution (lots of water) to a strong solution (less water), until the water molecules are evenly distributed. Again, movement is down a concentration gradient.

Which side of the beaker contains… The strongest solution? The weakest solution? Will water move from A to B or from B to A? AB

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Active Processes Cells Use Energy

Active Transport Active transport is the movement of substances in or out of a cell using energy and involves substances that either: Cannot normally cross the cell membrane or, Move against their concentration gradient. Active transport uses proteins in the cell membrane to effectively pump substances into or out of the cell. The cell has to use energy in the form of ATP (made during cellular respiration).

Bulk Transport This is where substances enter and leave cells if they cannot do so in any other way. The main forms of bulk transport are: Exocytosis – moves substances out of cells and, Endocytosis – move substances into cells. Both processes require energy.

ExocytosisEndocytosis Substances to be removed are packaged into vesicles. The vesicles move to and merge with the cell membrane. The contents of the vesicle are expelled from the cell. Part of the cell membrane surrounds the substance to be taken in. A vesicle forms, which detaches from the cell membrane and moves into the cell.

Summary Passive MovementActive Movement Needs no energy. Substances move down a concentration gradient. Involves: Diffusion Facilitated diffusion Osmosis Needs energy. Substances move up a concentration gradient or are too large to pass via other means. Involves: Active transport Bulk transport

Questions 1. What does osmosis only involve the movement of? 2. If a process is passive, what doesn’t it use? 3. Besides water, name 2 other substances that move passively… 4. In simple diffusion, do substances move across the cell membrane or via protein channels? 5. In passive transport, do substances move up/down a concentration gradient? 6. In active transport, do substances move up/down a concentration gradient? 7. What is stored/chemical energy in a cell known as? 8. Which process produces it?

9. When would a cell use bulk transport? 10. What bulk transport process takes substances in? 11. What bulk transport process ejects substances? 12. What is facilitated diffusion? 13. In terms of a weak solution, does it have more water and less solute or less water and more solute? 14. Why is a membrane said to be selectively permeable? 15. In osmosis, water moves from… a strong solution to a weak solution or a weak solution to a strong solution? 16. In terms of temperature, when is diffusion faster and why? 17. Why can vitamins A,D, E and K pass freely into a cell? 18. What waste product of respiration diffuses out of cells? 19. What cellular organelle packages proteins? 20. During passive forms of movement, when would there be no more net movement of substances across a membrane?

Recap