Membrane Structure & Function

Membrane Structure and Function A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer. The plasma membrane has a unique collection of proteins.

Membrane structure Biological membrane are sheet-like structures composed mainly of proteins and lipids. Membrane lipids are amphipathic and are also organized in a bilayer. While proteins are scattered across the membrane and perform most membrane functions. Membrane proteins are crucial both for membrane function and stability. Proteins that are connected to only one side of the lipid bilayer are called peripheral membrane proteins. Proteins that interact directly with the hydrophobic portion of the lipid bilayer are called integral membrane proteins.

Plasma membrane characteristics Separates the living cell from surroundings Selectively permeable Phospholipids are the most abundant lipids It’s amphipathic Fluid mosaic model environment

The protein in the plasma membrane 1.Not embedded in the bilayer 2.Bounded to the surface 3.Sometimes connects with the integral protein 1.In the hydrophobic core 2.It open spins the membrane Alpha Helices Alpha Helices

The proteins in the plasma membrane may provide a variety of major cell functions:

The protein in the plasma membrane

Signal transduction Enzymatic activity Transport

Intercellular joining Cell-cell recognition Attachment to the cytoskeleton and extracellular matrix

Functions of cell membrane

Selective permeability Cross easily Cross difficultly Hydrocarbons CO2 O2 Ions – nonpolar Water sugar

Passive/ Active transport A substance will diffuse from where it is more concentrated to where it is less concentrated. Until it reaches the dynamic equilibrium A substance will diffuse from where it is more concentrated to where it is less concentrated. Until it reaches the dynamic equilibrium It requires NO ENERGY

Orange juice Hypertonic Orange Water Orange juice Hypotonic Orange Water Orange juice Isotonic Orange = Water

Osmosis: Is a passive transport in which water diffuses across a selectively permeable membrane from the hypotonic solution to the hypertonic solution until the solutions become isotonic.

Cells in hypertonic environment will lose water. Cells in hypotonic environment will gain water. Cells must Osmo-regulate to maintain their internal environment. e.g. Paramecium have ( contractile vacuole ) that pumps the water out of the cell.

Facilitated diffusion is the movement of molecules ( like ions and polar molecules ) down its concentration gradient.

low concentrationhigh concentration ATP The active transportation is performed by specific proteins called transport protein and it is critical for a cell to maintain its internal concentrations of small molecules AGAINST CONCENTRATION GRADIENT

The sodium-potassium pump

1)- Small molecules: Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins. 2)- Large molecules: Large molecules, such as polysaccharides and proteins, cross the membrane by vesicles and transported by Exocytosis and endocytosis.

a transport vesicle budded from the Golgi apparatus and the membrane spills the contents to the outside cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane

VIDEO

Endocytosis:

Endocytosis includes

- Q1: Write whether each of the following statements is True (T) or False (F): 1-Active transport needs ATP to move substances down their concentration gradient. 2- Endocytosis and exocytosis move materials across the cell membrane by passive diffusion. 3- Diffusion of solutes across a membrane with the help of transport proteins, without the of energy is known as facilitated diffusion. 4-The Na + - K + pump works in order to pump3Na + out of the cell and 2K + into the cell. 5- The movement of water through a semi-permeable membrane is called osmosis.

Q 2: Choose the correct answer (one answer only): The ability of the cell to take up particular molecules and exclude others by its cellular membrane is called: a)- endocytosis b)- exocytosis c)- selective permeability d)- permeability Which of the following get in and out of the cell by endocytosis and exocytosis: a)- micromolecules b)- macromolecules c)- essential elements d)- CO 2 and H 2 O The process of cellular eating of small organisms or food particles is called: a)- receptor-mediated endocytosis b)- exocytosis c)- autophagy d)- phagocytosis

Transport protein molecules in the cell membrane play a role in: a). osmosis b). facilitated diffusion c). active transport d). both b and c Receptor-mediated endocytosis is: a). specific b). unspecific c).both a and b d). none of them Pinocytosis is a type of: a). endocytosis b). exocytosis c). passive diffusion d). facilitated diffusion. -The part of the membrane protein which is embedded within the plasma membrane of the cell is described as: a)- peripheral protein b)- integral protein c)- both a and b d)- none of the above -The movement of water through a semi-permeable membrane is called: a). osmosis b). diffusion c). active transport d). facilitated diffusion