Bio. 12 Chapter 4 Membrane Structure and Function

Overview 4.1 – Plasma Membrane Structure & Function Structure and function of the PM Major functions of proteins 4.2- Permeability of the PM Types of transport and their requirements Diffusion vs. osmosis Facilitated vs active transport Exocytosis vs Endocytosis

4.1 Plasma Membrane What is the plasma membrane? -The boundary that separates the internal environment from the external environment -Regulates the entrance and exit of molecules

Phospholipid Bilayer The plasma membrane is composed mainly of phospholipids There are two layers: Hydrophilic (water loving) polar heads of the phospholipids face the outside and inside of the cell Hydrophobic (water hating) non-polar tails face each other Hydrophilic head Hydrophobic tail

Fluid Mosaic Model What does Fluid Mosaic Model mean? - Fluid= consistency of light oil - Mosaic= contains embedded proteins scattered throughout

The Fluidity of Membranes Cholesterol (another type of lipid) Has different effects on membrane fluidity at different temperatures Figure 7.5 Cholesterol

Attached to the membrane Carbohydrate (sugar) chains can be found attached to phospholipids and proteins on the outer surface of the membrane on phospholipids = glycolipid on proteins = glycoprotein

Two Types of Membrane Proteins 1. Integral proteins -Penetrate the hydrophobic core of the lipid bilayer -Are often transmembrane proteins, completely spanning the membrane -Can move around latterally

Two Types of Membrane Proteins 2. Peripheral proteins Are attached to one side of the membrane

Label the following in your notes

Five Major Functions of Membrane Proteins

1. Channel Proteins Provides a channel for molecules to pass through passively (energy is not required) EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

2. Carrier Protein Combine with a substance and help it pass through the membrane. Energy is required (active transport)

3. Cell Recognition Protein Some glycoproteins serve as identification tags that are specifically recognized by other cells Glyco- protein

4. Receptor Protein Has a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a change in shape of the protein (receptor) relaying the message to the inside of the cell

5. Enzymatic Protein Protein is an enzyme with an active site Speeds up a metabolic reaction In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

4.2 Membrane Permeability Differentially permeable (selectively permeable) = certain substances can move across the membrane while others cannot Some substances can pass freely though the membrane (passive transport) and others require energy (active transport)

Part 1:Passive Transport A) Diffusion -Moves down the concentration gradient (goes from high to low concentration) -H2O and small, noncharged molecules(CO2, O2, glycerol, alcohol) easily diffuse across plasma membranes -Diffusion of water is known as Osmosis

A Look at Diffusion Diffusion= the tendency for molecules of any substance to spread out evenly into the available space

Some material can’t diffuse through the membrane… instead they need Transport Proteins Allow passage of hydrophilic substances across the membrane Needed for facilitated and active transport

Passive Transport: B) Facilitated Transport Channel or carrier proteins and a concentration gradient are needed (no energy required) Some sugars and amino acids Transport proteins speed the movement of molecules across the plasma membrane

Effects of Osmosis on Water Balance Is the movement of water across a semipermeable membrane Is affected by the concentration gradient of dissolved substances called the solution’s tonicity

Water Balance of Cells Without Walls Tonicity Is the ability of a solution to cause a cell to gain or lose water Has a great impact on cells without walls

Three States of Tonicity

Isotonic Solutions If a solution is isotonic The concentration of solutes is the same as it is inside the cell There will be NO NET movement of WATER

Hypertonic Solution If a solution is hypertonic The concentration of solutes is greater than it is inside the cell The cell will lose water (PLASMOLYSIS)

Hypotonic Solutions If a solution is hypotonic The concentration of solutes is less than it is inside the cell The cell will gain water

Water Balance in Cells without Walls Animal cell. An animal cell fares best in an isotonic environment

Water Balance of Cells with Walls Cell Walls Help maintain water balance Turgor pressure Is the pressure of water inside a plant cell pushing outward against the cell membrane If a plant cell is turgid It is in a hypotonic environment It is very firm, a healthy state in most plants If a plant cell is flaccid It is in an isotonic or hypertonic environment

Water Balance in Cells with Walls Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment

How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of fructose per ml How will the water molecules move? There will be no net movement of water since the concentration of solute in each solution is equal

How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 75 molecules of fructose per ml How will the water molecules move? There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

Part 2:Active Transport Uses energy to move solutes against their concentration gradients Requires energy, usually in the form of ATP

Active Transport The sodium-potassium pump Is one type of active transport system P P i

Comparison of Passive & Active Transport Passive transport. Substances diffuse down their concentration gradients no energy required The rate of diffusion can be greatly increased by transport proteins Active transport. Against concentration gradient Energy for this work is usually supplied by ATP Sodium-potassium pump. Diffusion Facilitated diffusion ATP

Cotransport Cotransport Occurs when active transport of a specific solute indirectly drives the active transport of another solute Involves transport by a membrane protein Driven by a concentration gradient

Example of Cotransport Cotransport: active transport driven by a concentration gradient

Bulk Transport Bulk transport across the plasma membrane occurs by: 1.Exocytosis 2.Endocytosis a) phagocytosis b) pinocytosis c) receptor-mediated endocytosis

1. Exocytosis In exocytosis Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

Exocytosis

2. Endocytosis In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane

Endocytosis

Three Types of Endocytosis Phagocytosis -large particles like food is taken into the cell -“cell eating” - Seen in ameobas and human white blood cells

Three Types of Endocytosis 2. Pinocytosis Liquid or very small particles are brought into the cell “cell drinking” Seen in blood cells and plant roots

Three Types of Endocytosis 3. Receptor-mediated endocytosis Specific type of pinocytosis which uses receptor proteins shaped for specific molecules Selective and more efficient Seen in genetic disorders

Questions?