Chapter 4 Membrane Structure & Function · 2016-09-25 · 9/25/16 1 Chapter 4 Membrane Structure &...

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Chapter 4Chapter 4Chapter 4Chapter 4

Membrane Structure Membrane Structure Membrane Structure Membrane Structure & Function& Function& Function& Function

Plasma Membrane Structure & FunctionPlasma Membrane Structure & FunctionPlasma Membrane Structure & FunctionPlasma Membrane Structure & Function

• The plasma membrane separates the internal environment of the cell from its

external environment.

• It regulates the entrance & exit of

molecules into and out of the cell.

• The steady internal environment maintained is called homeostasis.


• The plasma membrane is a phospholipid bilayer

embedded with proteins.

• Polar heads are

– Face the outer & inner layer of the cell

• Nonpolar tails are

– Face away from the water; face each other

• Cholesterol located between the phospholipids & proteins to control stability

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• 2 major types of proteins in the cell membrane are...

– Integral proteins - span the width of the membrane

– Peripheral proteins -


• Both phospholipids & proteins can have

attached carbohydrate chains.

• Glycolipids are lipids attached to carbohydrates.

• Glycoproteins are proteins attached to

carbohydrates.

• SEE PAGE 64

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Functions of Membrane ProteinsFunctions of Membrane ProteinsFunctions of Membrane ProteinsFunctions of Membrane Proteins

• Channel proteins are involved in the

passage of solutes through the membrane.

– Substances move across the membrane.

– Some may contain a gate that must be

opened in response to a signal.

• Carrier proteins allow the passage of a

solute by combining with it and helping it

to move across the membrane.

Functions of Membrane ProteinsFunctions of Membrane ProteinsFunctions of Membrane ProteinsFunctions of Membrane Proteins

• Cell recognition proteins(glycoproteins)

– Helps cells recognize self & when it is being invaded by pathogens.

• Receptor proteins have a shape that allows a specific molecule to bind.

– The binding causes the receptor to change shape to initiate a cellular response.

• Enzymatic proteins carry out metabolic

reactions

5 Membrane Proteins 5 Membrane Proteins 5 Membrane Proteins 5 Membrane Proteins

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Plasma Membrane PermeabilityPlasma Membrane PermeabilityPlasma Membrane PermeabilityPlasma Membrane Permeability

• The plasma membrane regulates the passage of molecules into & out of the cell

because it is selectively permeable.

• Which molecules can freely cross the

membrane & which require carrier proteins depends on

– Nature of molecule –


• Small, uncharged molecules freely

cross membrane

– Ex. CO2, O2, glycerol, and alcohol

– Slip in between the hydrophilic heads and

pass through hydrophobic tails

– Driven by the concentration gradient


• Concentration gradient

– More of a substance on one side of the membrane

– Going “down” a concentration gradient

• From an area of

• Passive transport; no energy required

– Going “up” a concentration gradient

• From an area of

• Active transport; requires energy

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• Water, which is polar, would NOT be

expected to readily cross the

membrane.

– Aquaporins are special channels that


• Large molecules, ions, & charged molecules are

unable to cross the membrane, but can cross the membrane via

– Channel proteins forming a pore through the membrane

– Carrier proteins that are specific for a substance they transport

– Vesicle formation in endocytosis or exocytosis

Diffusion & OsmosisDiffusion & OsmosisDiffusion & OsmosisDiffusion & Osmosis

• Diffusion - movement of molecules from an

area of higher to lower concentration

– Down a concentration gradient; passive

– Occurs until equilibrium is reached

■ EX. dye placed in water - the dye & water molecules move about until equilibrium occurs

– All solution contains a solute (dissolved solid) and a solvent (substance dissolving others)

– Water is the main solvent in the body

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Diffusion Diffusion Diffusion Diffusion

• Several factors influence rate of diffusion

– Temperature

• As temp.

– Pressure

• As pressure ↑,

– Electrical currents

• Electrical charges may either attract or repel diffusion of molecules

– Molecular size

• Small molecules diffusion faster than large one

OsmosisOsmosisOsmosisOsmosis

• Diffusion of water across a selectively

permeable membrane

– Diffusion occurs from higher to lower conc.

● Osmotic pressure is the pressure that

develops in a system due to osmosis.• The greater the possible osmotic pressure, the

more likely it is that water will diffuse in that direction.

3 Cellular Environments3 Cellular Environments3 Cellular Environments3 Cellular Environments

• Isotonic: the solute conc. is equal inside and outside of a cell

• Hypotonic: a solution has a lower solute conc. than the inside of a cell

• Hypertonic: a solution has a higher solute conc. than the inside of a cell

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3 Cellular Environments3 Cellular Environments3 Cellular Environments3 Cellular Environments• Isotonic

– No net gain or loss of water; cells remain normal

– 0.9% NaCl

• Hypotonic

– Cell gains water

– Cytolysis or hemolysis in animal cells;

– Turgor pressure

• Hypertonic

– Cell loses water

– Crenation; shrinks in animal cells

– Plasmolysis;

Transport by Carrier ProteinsTransport by Carrier ProteinsTransport by Carrier ProteinsTransport by Carrier Proteins

• The plasma membrane impedes the passage of all but a few substances.

• Substances enter or exit cells because of carrier proteins.

• Carrier proteins are specific.

– Combine with a molecule to be transported across the membrane

– Change shape to move molecules across membranes

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Transport by Carrier ProteinsTransport by Carrier ProteinsTransport by Carrier ProteinsTransport by Carrier Proteins

• Carrier proteins are required for

– Facilitated transport

– Active transport

Facilitated TransportFacilitated TransportFacilitated TransportFacilitated Transport

● Facilitated transport is the passive transport

of molecules or ions across a membrane via

specific carrier proteins. Explains the passage of molecules such as glucose or amino acids.

– Neither molecule is lipid-soluble

– Reversible combination and transport occurs

– Passive; ATP is not required because molecules are move down their conc. gradient.

Active TransporActive TransporActive TransporActive Transport• Active Transport

– Molecules combine with carrier proteins

• Often called pumps

– Molecules move against the concentration

gradient

– Energy(ATP) and carrier proteins are

required.

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Active TransporActive TransporActive TransporActive Transport• Proteins in active transport are referred to

as pumps.

• Na+/K+ pump is important for nerve and muscle cells – it moves Na+ out and K+

into cells.

• The carrier changes shape after phosphate attaches, and then again after

it detaches.

Bulk TransporBulk TransporBulk TransporBulk Transport• Macromolecules are transported in or out

of cells by vesicle formation– Macromolecules are too large to be

transported by carrier proteins

– Energy is required to form vesicles.

– 2 type of Bulk Transport

• Exocytosis – exit

• Endocytosis – enter

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ExocytosisExocytosisExocytosisExocytosis

• The vesicle fuses with plasma membrane & becomes part of the membrane as secretion

occurs• Examples

– Pancreatic cells secrete insulin

– Anterior pituitary cells secrete, growth

hormone, prolactin, & others

EndocytosisEndocytosisEndocytosisEndocytosis

• Cells take in substances by vesicle formation.– Part of the plasma membrane invaginates to envelop

the substance.

– The membrane then pinches off to form an intracellular vesicle.

• 3 types of endocytosis– Phagocytosis– Pinocytosis– Receptor-mediated

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Types of EndocytosisTypes of EndocytosisTypes of EndocytosisTypes of Endocytosis

• Phagocytosis: Cell surrounds & engulfs food

molecules, viruses or damaged cells

– Amoeba & macrophages

• Pinocytosis: Cells take in liquids & small particles dissolved in liquid

– Certain blood cells or plant root cells

• Receptor Mediated Endocytosis: involves a

Modifications of Cell Surfaces

• Cells live & interact with surrounding

cells

• Plants, prokaryotes, & fungi are surrounded by cell walls.

• Animals have more varied extracellular environments that can

change.

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Cell Surfaces in Animals

• Animal cells have 2 different types of cell surfaces

– Extracellular matrix outside of cells

– Junctions that occur between cells

• Both can contribute to cell-to-cell

communication

Extracellular Matrix• A mesh of proteins & polysaccharides

located just outside the cell that provides

support & allows communication

• Common structural proteins in ECM

– Collagen resists some stretching

– Elastin provide resilience to ECM

– Fibronectin is an adhesive protein that links integrin (integral protein)

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Junctions Between Cells• Adhesion Junctions

– Attaches adjacent cells. Desmosomes are 1 type of adhesion junction. There are intercellular filaments between cells (RIVET)

• Tight Junctions– Connects cell membrane between cells; (ZIPPER-

LIKE)

• Gap Junctions– membrane channels between cells allows for cell

communication & flow of ions between cells

Plant Cell Walls

• All plants have a cell wall made of

• Cytoplasm connects all the cells within a plant

& allows exchange of small solutes & water.

• Plasmodesmata are narrow channels that

penetrate the cell wall to connect adjacent

cells.

• Both can contribute to cell-to-cell

communication

Chapter 4 Membrane Structure & Function · 2016-09-25 · 9/25/16 1 Chapter 4 Membrane Structure &...

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