Cell Membranes

Cells are surrounded by a membrane: the cell surface membrane or plasma membrane On an EM it appears as a double line The distance across the membrane is 7-8nm

WHAT DOES A MEMBRANE LOOK LIKE? The Fluid Mosaic Model

TRANSPORT ACROSS MEMBRANES

There are four main methods by which substances can move across a cell membrane: 1.Diffusion Osmosis 2.Active Transport 3.Endocytosis and Exocytosis Passive Transport

1: Diffusion Diffusion is the movement of molecules from a region where they are in high concentration to a region of low concentration. The difference in concentration is the concentration gradient

Diffusion is a PASSIVE process There is a net movement of molecules down the concentration gradient until equilibrium is reached Ions move by diffusion along electrochemical gradients

Rate of diffusion The rate of diffusion depends on: –Concentration gradient –Temperature –Size of molecule –Lipid solubility Extension: Look up Fick’s Law…

Lipid Bilayer concentration gradient Intrinsic globular proteins within the membrane function as carriers for the transport of certain molecules Carriers are specific for the molecules that they transport them across the membrane in the direction of the concentration gradient at a faster rate than occurs for simple passive diffusion Glucose molecules enter body cells from the bloodstream by facilitated diffusion Facilitated Diffusion

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Lipid Bilayer concentration gradient

Increasing external concentration of glucose Rate of uptake A Rate of uptake increases as the glucose concentration increases (i.e. the steepness of the concentration gradient increases) Rate of uptake reaches a maximum at glucose concentration A No further increase in the uptake rate despite the increasing glucose concentration When glucose concentrations exceed a certain value, there are insufficient glucose carriers within the membrane for the rate of uptake to increase any further LIMITING FACTORS AND FACILITATED DIFFUSION The rate at which molecules like glucose enter cells by facilitated diffusion is affected by factors other than the concentration gradient The availability of carriers in the membrane is the limiting factor

Summary of Passive Transport

2: Osmosis Osmosis is the diffusion of water molecules from an area of high concentration (of water) to an area of lower concentration across a semi-permeable membrane

Water Potential Osmosis can be quantified using water potential, so we can calculate which way water will move, and how fast. Water potential (ψ, the Greek letter psi, pronounced "sy") is simply the effective concentration of water. It is measured in units of pressure (Pa, or usually kPa) water always "falls" from a high to a low water potential 100% pure water has ψ = 0, which is the highest possible water potential, so all solutions have ψ < 0 you cannot get ψ > 0.

ψ ψ ψ

Cells and Osmosis. The concentration of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider: Isotonic solutiona solution of equal concentration to a cell Hypertonic solutiona solution of higher (salt) concentration than a cell Hypotonic solutiona solution of lower (salt) concentration than a cell

3: Active Transport Pumping of molecules across cell membrane using a protein pump UP a concentration Gradient protein pump ATP ADP + Pi active site

Most Active Transport systems are driven by metabolic energy derived from ATP Active Transport allows cells to uptake necessary ions and molecules and remove waste products These Mechanisms often referred to as Pumps Most common is Sodium Pump

Na + / K + Pump Present in all animal cells, most abundant and important of membrane pumps

Sodium-Potassium Pump important in controlling cell volume, reducing Na thereby reducing water uptake by osmosis The accumulation of K ions are used in cell metabolic processes eg protein synthesis Na-K pump can be linked to active uptake of organic molecules such as glucose and amino acids

4: Endocytosis & Exocytosis Endocytosis

Phagocytosis Eg. Amoeba White Blood Cells (neutrophils & monocytes) Pinocytosis Eg.Protozoans, white blood cells, cells in embryos, liver and kidneys.