Cell Membrane

Chapter Outline

• 1) Plasma Membrane Structure and Function

• 2) Permeability of the Plasma Membrane

• 3) Diffusion and Osmosis

• 4) Transport by Carrier Proteins

• 5) Exocytosis and Endocytosis

1) Plasma Membrane Structure and Function

• Fluid-Mosaic Model of Membrane

• 1) Phospholipid bilayer fluid

• 2) Proteins on/in membrane mosaic

• 3) Cholesterol in animal cells and related steroid in plant cells strengthen membrane and regulate fluidity

• 4) Glycoproteins and glycolipids cellular interaction/ communication

Types of Membrane Proteins

• Integral (or Transmembranal) proteins have hydrophilic ends and a hydrophobic midsection

• Peripheral proteins which are hydrophilic

Functions of Membrane Proteins

• 1) Channel Protein

• 2) Carrier Protein

• 3) Cell Recognition Protein

• 4) Receptor Protein

• 5) Enzymatic Protein

Channel Protein

• Allow a specific molecule or ion to cross the plasma membrane freely through water filled pores

• No change in protein shape

• Most plasma membranes contain specific channel proteins for common ions such as Na+, K+, Cl-

A Simpson’s Moment

• Tetrodotoxin is a toxin that works by inhibiting a specific Na+ channel protein.

• This toxin is made by the pufferfish.

Carrier/ Transporter Protein

• Selectively combine and interact with a specific molecule so that it can cross the plasma membrane

• Change in shape of protein

• Amino acids, needed for synthesis of new proteins, enter the cell via carrier proteins

• Na-K pump in nerve cells

Cell Recognition Protein

• Proteins that have attached carbohydrate chains

• Carbohydrate chains can vary in number of sugars, types of sugars, sequence of sugars and branching of carbohydrate chains

• Diversity of glycoprotein cell-to-cell recognition, adhesion between cells, reception of specific molecules

Cell Recognition Protein

• Major histocompatability (MHC) proteins distinguish your body cells from other person’s body cells

• Problem for organ transplant

Receptor Protein

• Has a specific shape that only a specific molecule will bind to

• The binding causes a change in the shape of the protein cellular response

• Anti-diuretic hormone (ADH) attaches to receptors in kidney change in permeability of membrane to water

Enzymatic Protein

• Catalyze a specific reaction

• Epithelial cells lining small intestine manufacture lactase

2) Permeability of Plasma Membrane

• Membrane is selectively permeable some molecules can cross membrane, while other molecules can not

• Molecules that can cross the membrane either cross by passive transport or active transport

Passive Transport

• Does not use chemical energy (ATP)• Molecules move along a concentration

gradient : areas of high concentration areas of low concentration

• Diffusion : No channel or carrier protein• e.g. lipid soluble molecules, gases, water • Facilitated diffusion: Channel or carrier

protein• e.g. glucose, amino acids

Active Transport

• Chemical energy (ATP) is used by cell to move molecules

• Carrier protein moves molecules across membrane against concentration gradient: areas of low concentration areas of high concentration

• e.g. sucrose, some amino acids, ions

3) Diffusion and Osmosis

• Diffusion: molecules diffuse down their concentration gradient

• Diffusion is a physical phenomenon

• Osmosis: the diffusion of water across a selectively permeable membrane due to concentration differences

• Osmosis is a biological phenomenon

Higher WaterConcentration

Lower WaterConcentration

Osmosis Jones

Osmosis in Cells

• 1) Isotonic Solution

• 2) Hypotonic Solution

• 3) Hypertonic Solution

Isotonic Solution

• If the concentration of solute (salt) is equal on both sides, the water will move back and forth but it won't have any result on the overall amount of water on either side.

• "ISO" means the same

Hypotonic Solution

• The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell

• Water will move into the cell to dilute the solute (salt) concentration until an isotonic balance is reached.

• The cell will gain water, grow larger and potentially burst in a process called lysis

Hypertonic Solution

• The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell

• Water will move out of the cell to dilute the solute (salt) concentration until an isotonic balance is reached.

• The cell will lose water and shrink in a process called crenation

AnimalCell

PlantCell

Osmosis and Diffusion

• Water always moves by diffusion across a selectively permeable membrane from a hypotonic solution (higher water potential) to a hypertonic solution (lower water potential)

4) Transport by Carrier Proteins

• Carrier proteins in the membrane combine with specific molecules, which are then transported across the membrane

• Carrier proteins are required for both facilitated diffusion and active transport

Facilitated Diffusion

• Facilitated diffusion is passive transport

• The cell does not use energy to move molecules across the membrane

• Molecules move through a carrier protein from a region of higher concentration to a region of lower concentration

Active Transport

• The cell uses energy (ATP) to move molecules across the membrane against a concentration gradient

• Molecules move through a carrier protein from a region of lower concentration to a region of higher concentration

• The Na-K pump is an example of active transport

Na-K Pump

• 1) ATP binds to an active site on the protein forming the Na-K pump, thus providing energy for it.

• 2) 3 sodium ions (red balls) from the cytoplasm bind to lock and key sites on the Na-K pump.

• 3) The energized protein of the Na-K pump changes shape, releasing the 3 sodium ions to the extracellular environment

Na+ ion

Na-K Pump

• 4) Two potassium ions (from the extracellular environment) bind to lock and key sites on the protein of the Na-K pump.

• 5) The protein of the Na-K pump changes shape as the phosphate group leaves the protein's active site.

• 6) Potassium ions are released into the cytoplasm.

5) Exocytosis and Endocytosis

• Macromolecules such as proteins and complex carbohydrates are too large to be moved into or out of the cell through a carrier protein

• Macromolecules are moved into or out of the cell by vesicle formation

• Vesicle formation requires energy (ATP)

Exocytosis

• To export materials out of the cell:

• 1) a vesicle forms around the molecules,

• 2) the vesicle moves towards the cell membrane,

• 3) the vesicle fuses with the cell membrane, and

• 4) the molecules are released into the external environment

Exocytosis

Endocytosis

• Molecules are taken into the cell by vesicle formation:

• 1) the molecules are engulfed by the cell membrane

• 2) a vesicle forms around the molecules, and

• 3) the vesicle moves into the cytoplasm

Endocytosis

Forms of Endocytosis

• 1) Phagocytosis: endocytosis of a food particle or another cell

• 2) Pinocytosis: endocytosis of a liquid or very small particle

• 3) Receptor-mediated Endocytosis: receptor proteins bind to a specific molecules such as vitamin or hormone, which are then brought in by endocytosis