5.1 the nature of the plasma membrane

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Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.

5.1 The Nature of the Plasma Membrane


The Plasma Membrane• Four principal components in animals

– Phospholipid bilayer– Molecules of cholesterol interspersed within the

bilayer.– Membrane proteins

• embedded within– integral or transmembrane

• lie on the surface– peripheral

– Glycocalyx• short carbohydrate chains on the cell surface• function in cell adhesion• binding sites on proteins.


The Plasma Membrane

Figure 5.1

glycocalyxproteins

integralprotein

peripheralprotein

cytoskeleton

cholesterol

Phospholipid bilayer:a double layer of phospholipid molecules whose hydrophilic “heads” face outward, and whose hydrophobic “tails” point inward, toward each other.

Glycocalyx: sugar chains that attach to proteins and phospholipids, serving as protein binding sites and as cell lubrication and adhesion molecules.

Cholesterol molecules that act as a patching substance and that help the cell maintain an optimal level of fluidity.

Proteins, which are integral, meaning bound to the hydrophobic interior of the membrane, or peripheral, meaning not bound in this way.

phospholipids

cell interior

cell exterior


Phospholipid Bilayer• Composed of two fatty acid chains linked to a

charged phosphate group.– fatty acid chains (tails)

• hydrophobic– non-polar– cannot form hydrogen bonds with water– repel polar (hydrophilic) molecules– allow non-polar (hydrophobic) molecules to pass through

– phosphate group (head)• hydrophilic

– polar– can form hydrogen bonds with water

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The Phospholipid Bilayer

Figure 5.2

polarhead

nonpolartails

wateryextracellularfluid

waterycytosol

hydrophilic

hydrophobic

hydrophobic moleculespass through freely

hydrophilic moleculesdo not pass

through freely

hydrophilic

(a) (b)Phospholipid molecule Phospholipid bilayer

–


Phospholipid Bilayer

• Spontaneously arrange themselves into bilayers– two layers of phospholipids

• fatty acid “tails” of each layer point inward (avoidingwater)

• phosphate “heads” point outward (hydrogen bondingwith it).

– due to watery (aqueous) environment on either sideof the membrane.



• Cholesterol molecules– interspersed between phospholipid molecules in

the plasma membrane– perform two functions:

1. They act as a patching material that helps keepsome small molecules from moving through the membrane.

2. They keep the membrane at an optimal levelof fluidity.



• Plasma membrane proteins– integral

• bound to the hydrophobic interior of the phospholipidbilayer.

– peripheral• lie on either side of the membrane but are not bound to

its hydrophobic interior• often bound to other integral proteins

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Membrane Protein Functions• In animal cells, membrane protein molecules

perform four functions:1. structural support

• Connect to cytoskeleton2. cell identification

• serve as external recognition proteins thatinteract with immune system cells

3. Communication• serve as external receptors for signaling

molecules4. Transport

• provide channels for the movement ofcompounds into and out of the cell


The Plasma Membrane

Figure 5.3

(a) (b) (c) (d)Structural support Recognition Communication Transport

Membrane proteins can provide structural support, often when attached to parts of the cell’s scaffolding or “cytoskeleton.”

Binding sites on some proteins can serve to identify the cell to other cells, such as those of the immune system.

Receptor proteins, protruding out from the plasma membrane, can be the point of contact for signals sent to the cell via traveling molecules, such as hormones.

Proteins can serve as channels through which materials can pass in and out ofthe cell.


Plasma Membrane

• Described by a conceptualization called thefluid-mosaic model– views the membrane as a fluid, phospholipid

bilayer that has a mosaic of proteins either fixedwithin it or capable of moving laterally across it.


Plasma Membrane

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Fluid Mosaic Model

Fluid Mosaic Model


5.2 Diffusion, Gradients, and Osmosis


Diffusion, Gradients, and Osmosis

• Diffusion– movement of molecules or ions from a region of

their higher concentration to a region of lowerconcentration.



• Concentration gradient– defines the difference between the highest and

lowest concentrations of a solute within a givenmedium.

– compounds move from higher to lowerconcentrations

• down their concentration gradients• due to Brownian movement

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Figure 5.4

water molecules

dye molecules

(a) (b) (c)Dye is dropped in Diffusion begins Dye is evenly distributed



• Moving against concentration gradients– lower to a higher concentration– requires energy



• A semipermeable membrane is one that allowssome compounds to pass through freely whileblocking the passage of others.



• Osmosis– net movement of water

across a semipermeablemembrane from an areaof lower solute concentrationto an area of higher soluteconcentration.

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• Because the plasma membrane is asemipermeable membrane, osmosis operates inconnection with it.

• Osmosis is a major force in living things; it isresponsible for much of the movement of fluidsinto and out of cells.



Figure 5.5

(a) An aqueous solution divided by a semipermeable membrane has a solute—in this case, salt—poured into its right chamber.

solute

solvent

semipermeable membrane

pure water

osmosis

water bound tosalt ions

(b) As a result, though water continues to flow in both directions through the membrane, there is a net movement of water toward the side with the greater concentration of solutes in it.

(c) Why does this occur? Water molecules that are bonded to the sodium (Na+) and chloride (Cl–) ions that make up salt are not free to pass through the membrane to the left chamber of the container.


Osmotic Imbalances• Osmotic imbalances

– condition where a solute concentration gradient ispresent on opposite sides of a cell membrane

– effects cell shape• animal cells

– Lysis - break from taking in too much water– Crenation - shriveling from losing water

• plant cells– Turgor pressure - central vacuole swells, exerting pressure on

cell wall» No lysis occurs

– Plasmolysis - central vacuole shrinks, when too much water islost pulls cell membrane away from cell wall


Solute Concentration

• Cells– gain or lose water relative to their surroundings

• due to solute concentration inside the cell as opposed tooutside it

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• Solutions– Hypertonic

• A solution that has a higher concentration of solutes init than does the cell’s cytosol

• A cell will lose water to a surrounding hypertonicsolution.

– Hypotonic• A solution that has a lower concentration of solutes in it

than does the cell’s cytosol• A cell will gain water from a surrounding hypotonic

solution



• Isotonic– Equal solute concentration inside and outside the

cell– Water flow is balanced between the cell and its

surroundings



Figure 5.6

Balanced watermovement

Net movement ofwater into cell

Animal cell:plasmamembrane

plasma membrane

H2O

H2O

Plant cell:

cell wall

wilted turgid

(a) (b) (c)Hypertonic surroundings

Isotonic surroundings

Hypotonic surroundings

Net movement ofwater out of cell

H2O

H2O

H2O

H2O


Plasma Membranes and Diffusion

Diffusion

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5.3 Moving Smaller Substances In and Out


Moving Smaller Substances In and Out

• Some compounds are able to cross the plasmamembrane strictly through diffusion; othersrequire diffusion and special protein channels;still others require protein channels and theexpenditure of cellular energy.


Transport Through the PlasmaMembrane

• Two types of transport– Active transport

• movement of molecules or ions across a cell membranethat requires the expenditure of energy.

– Passive transport• movement of molecules or ions across a cell membrane

that does not require the expenditure of energy.


Types of Passive Transport

• Two types of passive transport– simple diffusion

• As discussed previously– facilitated diffusion

• Requires a membrane protein channel

• For either form of transport to bring about a netmovement of materials into or out of a cell, aconcentration gradient must exist.

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Facilitated Diffusion

• Transport proteins– function as channels

• For larger hydrophilic substances—substances that,because of their size and electrical charge, cannot diffusethrough the hydrophobic portion of the plasmamembrane.


Facilitated Diffusion

Figure 5.8

inside cell

outsidecell

plasmamembrane

glucose

The transport pro-tein has a binding site for glucose that is open to the outside of the cell.

Glucose binds to the binding site.

This binding causes the protein to change shape, exposing glucose to the inside of the cell.

Glucose passes into the cell and the protein returnsto its original shape.

1. 2. 3. 4.


Active Transport

• Cells cannot rely solely on passive transport tomove substances across the plasma membrane.

• A cell may need to maintain a greaterconcentration of a given substance on one sideof its membrane.– Yet, passive transport equalizes concentrations of

substances on both sides of the plasma membrane.


Active Transport

• Active transport– Uses energy and protein channels– Chemical pumps

• Moves compounds across the plasma membrane againsttheir concentration gradients.

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Active Transport

• One example of such transport is the pumpingof glucose into cells that line the smallintestines.


Transport Through the PlasmaMembrane

Figure 5.7

Passive transportActive transport

simple diffusion facilitated diffusion

Materials move down their concentration gradient through the phospholipid bilayer.

The passage of materials is aided both by a concentration gradient and by a transport protein.

Molecules again move through a transport protein, but now energy must be expended to move them against their concentration gradient.

ATP


5.4 Getting the Big Stuff In and Out


Getting the Big Stuff In and Out

• Two processes for moving larger materials– endocytosis

• Brings material in– Exocytosis

• Sends material out

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Exocytosis and Endocytosis

• Vesicles– Used by endo- and exocytosis– membrane-lined enclosures that alternately bud off

from membranes or fuse with them.


Exocytosis

• Exocytosis– transport vesicle moves from the interior of the cell

to the plasma membrane and fuses with it, at whichpoint the contents of the vesicle are released to theenvironment outside the cell.


Exocytosis

Figure 5.9

(a) (b)Exocytosis Micrograph of exocytosis

extracellular fluidprotein

cytosoltransport vesicle


Endocytosis

• Two types of endocytosis– Pinocytosis

Uptake of fluids– Cellular drinking

– Phagocytosis• Uptake of solids

– Cellular eating

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Endocytosis

• Pinocytosis– Cell drinking

• movement of moderate-sized molecules into a cell bymeans of the creation of transport vesicles producedthrough an infolding or “invagination” of a portion ofthe plasma membrane.

• vesicles will bud off from the plasma membrane andtravel deeper into the cell


Endocytosis

• Phagocytosis– when certain cells use pseudopodia or “false

feet” to surround and engulf whole cells,fragments of them, or other large organicmaterials.


Endocytosis

Figure 5.10

receptors capturedmolecules

coatedpit

vesicle

vesicle

bacterium(or food particles)

pseudopodium

(a) Pinocytosis

(b) PhagocytosisFormation of a pinocytosis vesicle.

A human macrophage (colored blue) uses phagocytosis to ingest an invading yeast cell.

5.1 the nature of the plasma membrane

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