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THE CELL MEMBRANE
Transcript of THE CELL MEMBRANE
SURFACE AREA TO VOLUME RATIO
• When an object (e.g. a cell) is small, it has a large surface area in comparison to its volume.
• In this case, diffusion will be an effective way to transport materials (e.g. gases) into the cell.
• As an object becomes larger, its surface area compared to its volume is smaller. Diffusion is no longer an effective way to transport materials to the inside. Therefore, there is a physical limit for the size of a cell, with the effectiveness of diffusion being the controlling factor.
• Bottom line: SMALLER IS BETTER FOR DIFFUSION
• Imagine cells are cube shaped and can grow several cm in size. Cells do not actually grow to this size!
Calculate volume, surface area and ratio!
PLANT CELL WALL
• Plants have cell walls in addition to cell membranes. The cell wall lies
outside the cell membrane
• (Bacteria have cell walls too, but bacterial cell walls are NOT the same in composition as plant cell walls).
• Thickness of cell wall varies with cell function
• Primary cell wall is outermost layer, composed of threadlike cellulose
FUNCTION & SIGNIFICANCE
• Separates internal environment of cell from exterior
• The cell membrane is the gateway into the cell
• Must allow needed things such as nutrients into the cell without letting them escape. In the same way, it must allow wastes to leave the cell.
FLUID MOSAIC MODEL
A wide variety of molecules and substances must pass through the cell membrane -- large, small, hydrophobic, hydrophilic. Molecules of the same size must be sorted out, and the cell must also be able to get large amounts of molecules in and out when necessary.
How can the cell membrane accomplish this?
• The answer lies in its structure and the ratio of membrane to cytoplasm.
Cube Surface Volume SA:V
Size Area(cm ) (cm )
1cm 6 1 6:1
2cm 24 8 3:1
3cm 54 27 2:1
4cm 96 64 3:2
5cm 150 125 6:5
• Double layer of phospholipid
• Consistency of light machine oil (~fluid)
• Proteins wholly or partly embedded in phospholipid bilayer * forms mosaic pattern.
• Carbohydrates strung together in chains are attached to proteins ("
") or lipids ("
") of membrane.
• Woody plants also have a secondary cell wall which forms inside the primary wall. Composed of alternating layers of cellulose microfibrils reinforced with lignin (which adds strength). Function is support of large plants. Wood is made largely of secondary cell wall material.
• Plant cell wall is freely permeable (anything small enough to fit through openings in cellulose microfibrils will get through).
• Plant cell therefore relies instead on its cell membrane to regulate what gets in and out.
• The cell membrane can discriminate between different molecules (even those of the same size) so we say it is SELECTIVELY PERMEABLE.
• Sticky substance called middle lamella binds cells together
• Function as identification markers for cell recognition (helps immune system identify which cells belong to body and which are invaders).
• Cellulose of plant cell walls used by humans: cotton, rayon, flax, hemp, wood, paper (paper has lignin removed to prevent yellowing). Lignin used in manufacture of rubber, plastics, pigments, adhesives.
1. Remove notes, textbooks, phones
2. Get into groups 2-3
3. After the question is read you have 1minute to deliberate it
4. One member of the group will raise their hand when they come to a consensus on the answer to the question
What are the benefits of the cell membrane being selectively permeable?
What are the benefits of having a "mosaic" plasma membrane? Provide an example to prove your point
What are the limitations regarding cell size?
What is wood composed of?