Cell Membranes & Movement Across Them 2006-2007

Cell (plasma) membrane separate cell from its environment cell membrane is the boundary IN food sugars proteins fats salts O2 H2O OUT waste ammonia salts CO2 H2O products cell needs materials in & products or waste out

Building a membrane How do you build a barrier that keeps the watery contents of the cell separate from the watery environment? What substance do you know that doesn’t mix with water?  FATS   LIPIDS  oil & water don’t mix!!

Lipids of cell membrane Membrane is made of phospholipids phospholipid bilayer phosphate “attracted to water” inside cell outside cell lipid “repelled by water”

Semi-permeable membrane Need to allow passage through the membrane But need to control what gets in or out membrane needs to be semi-permeable So what needs to get across the membrane? sugar aa lipid H2O salt NH3 So how do you build a semi-permeable membrane?

Phospholipid bilayer What molecules can get through directly? *small molecules fats & other lipids can slip directly through the phospholipid cell membrane, but… what about other stuff? lipid inside cell outside cell salt waste sugar aa H2O

Permeable cell membrane Need to allow more material through membrane needs to be permeable to… all materials a cell needs to bring in all waste a cell needs excrete out all products a cell needs to export out inside cell aa H2O sugar lipid “holes”, or channels, in cell membrane allow material in & out salt waste outside cell

Semi-permeable cell membrane But the cell still needs control membrane needs to be semi-permeable specific channels allow specific material in & out inside cell H2O aa sugar salt waste outside cell

How do you build a semi-permeable cell membrane? channels are made of proteins proteins both “like” water & “like” lipids bi-lipid membrane protein channels in bi-lipid membrane

Protein channels Proteins act as doors in the membrane channels to move specific molecules through cell membrane Donuts! Each transport protein is specific as to the substances that it will translocate (move). For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer. Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane. These channel proteins allow fast transport. For example, water channel proteins, aquaprorins, facilitate massive amounts of diffusion.

Most cell membranes are composed principally of DNA and ATP proteins and lipids chitin and starch nucleotides and amino acids

Most cell membranes are composed principally of DNA and ATP proteins and lipids chitin and starch nucleotides and amino acids

Movement through the channel Why do molecules move through membrane if you give them a channel? ? ?

Molecules move from high to low Diffusion move from HIGH to LOW concentration Movement from high concentration of that substance to low concentration of that substance.

Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed

Simple Diffusion Move from HIGH to LOW low  high fat fat fat Which way will fat move? inside cell fat fat fat low high  fat outside cell fat fat fat fat fat fat fat

Diffusion through a channel Move from HIGH to LOW sugar sugar sugar sugar inside cell sugar sugar low Which way will sugar move?  high outside cell sugar sugar sugar sugar sugar sugar sugar

Diffusion Move from HIGH to LOW concentration through membrane simple diffusion no energy needed through a protein channel facilitated diffusion (with help) high low

Active transport Cells may need molecules to move against concentration difference need to pump “uphill” from LOW to HIGH using ATP protein pump requires energy ATP Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. ATP

Transport summary diffusion facilitated diffusion ATP active transport

Osmosis Movement of Water Across Cell Membrane 2006-2007

Osmosis Osmosis diffusion of water from HIGH concentration of water to LOW concentration of water across a semi-permeable membrane

Normally, in the process of osmosis, the net flow of water molecules into or out of the cell depends upon differences in the 1. concentration of water molecules inside and outside the cell 2. concentration of enzymes on either side of the cell membrane 3. rate of molecular motion on either side of the cell membrane 4. rate of movement of insoluble molecules inside the cell

Keeping water balance Cell survival depends on balancing water uptake & water loss freshwater balanced saltwater

Managing water balance Balanced conditions no difference between cell & environment example: blood problem: none water flows across membrane equally, in both directions volume of cell doesn’t change balanced

Managing water balance Freshwater a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell freshwater

Controlling water Contractile vacuole in Paramecium

Managing water balance Saltwater a cell in salt water example: shellfish problem: lose water plasmolysis in plants shrinking cell solution: take up water saltwater