• No results found

cell membranes chapter 7

N/A
N/A
Protected

Academic year: 2020

Share "cell membranes chapter 7"

Copied!
53
0
0

Loading.... (view fulltext now)

Full text

(1)
(2)

Plasma Membrane

Plasma Membrane

Is the boundaryboundary that separates

the living cell from its nonliving surroundings

Selectively Permeable

Selectively Permeable (chooses what may cross the membrane) Fluid mosaic

Fluid mosaic of lipids and proteins

Lipid bilayerbilayer

Contains embedded

(3)
(4)

Phospholipids

Phospholipids

Are the most abundantmost abundant lipid in the plasma membrane

Are amphipathicamphipathic, containing both hydrophilic (head) and hydrophobic regions (tails)

Head

Head composed of phosphate group attached to one carbon of glycerol is hydrophilichydrophilic

(5)

Hydrophilic head

Hydrophobic Hydrophobic

tail tail

WATER

WATER

(6)

Singer and Nicolson

Singer and Nicolson

In 19721972, Singer and Nicolson, Proposed that membrane proteinsmembrane proteins are dispersed and individually inserted into the inserted into the

phospholipid bilayer

phospholipid bilayer of the plasma of the plasma

membrane

membrane

Phospholipid bilayer

Hydrophilic region of protein

(7)

Fluid Mosaic Model

Fluid Mosaic Model

A membrane is a fluid structurefluid structure with a “mosaic” of various proteins

embedded in it when viewed from the top

Phospholipids

Phospholipids can move laterallylaterally a small amount and can “flex” their tails

Membrane proteins

Membrane proteins also move side to side or lateralllaterally making the

(8)

Freeze-fracture

Freeze-fracture studies of the plasma

membrane support the fluid mosaic model of membrane structure

A cell is frozen and fractured with a knife. The fracture plane fracture plane

often follows the hydrophobic interior of a membrane

often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layerstwo separated layers. The

membrane proteins go wholly with one of the layers

(9)

The Fluidity of Membranes

Phospholipids in the plasma membrane Can move within the bilayer two ways

Lateral movement

(10)

The type of hydrocarbon tailstype of hydrocarbon tails in

phospholipids Affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbon

tails with kinks Saturated hydro-Carbon tails

(11)

The Fluidity of Membranes

The steroid cholesterolsteroid cholesterol Has different effects on membrane fluidity at

different temperatures

Figure 7.5

(12)

Membrane Proteins and Their Functions

A membrane is a collage of different collage of different

proteins embedded

proteins embedded in the fluid matrix

of the lipid bilayer

(13)

Types of Membrane Proteins

Integral proteins

Integral proteins

Penetrate the hydrophobic core of the lipid bilayer

Are often transmembrane proteinstransmembrane proteins, completely spanning the membrane

(14)

Types of Membrane Proteins

Peripheral proteins

Peripheral proteins

(15)

Six Major Functions of Membrane Proteins

Figure 7.9

Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the

membrane that is selective for a particular solute.

(right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to

substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a

metabolic pathway.

Signal transduction. A membrane protein may have 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

conformational change in the protein (receptor) that relays the message to the inside of the cell.

(16)

Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells.

Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions

Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the

cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (d) (e) (f) Glyco-protein

(17)

The Role of Membrane Carbohydrates

in Cell-Cell Recognition

Cell-cell recognition

Cell-cell recognition

Is a cell’s ability to distinguish one I

type of neighboring cell from another

Membrane carbohydrates

Membrane carbohydrates

(18)

Synthesis and Sidedness of Membranes

Membranes have distinct inside and distinct inside and

outside faces

outside faces

This affects the movementaffects the movement of proteins synthesized

proteins synthesized in the endomembrane system

endomembrane system (Golgi and (Golgi and

ER)

(19)

Synthesis and Sidedness of Membranes

Membrane proteins and lipids

Membrane proteins and lipids are made in

the ER ER and Golgi apparatusand Golgi apparatus

(20)

Membrane Permeability

Membrane structurestructure results in selective permeability

selective permeability

A cell must exchange materials cell must exchange materials

with its surroundings

with its surroundings, a process controlled by the plasma

(21)

Permeability of the Lipid Bilayer

Hydrophobic molecules

Hydrophobic molecules

Are lipid solublelipid soluble and can pass through the membrane rapidlyrapidly

Polar molecules

Polar molecules

Do NOT cross the membrane rapidly

(22)

Transport Proteins

Transport proteins

Transport proteins

Allow passage of hydrophilic hydrophilic

substances

(23)

Passive Transport

Passive transport

Passive transport is diffusion of a substance across a membrane

with no energyno energy investment

CO

CO22, H, H22O, and OO, and O22 easily diffuse across plasma membranes

Diffusion of water is known as Osmosis

(24)

Simple Diffusion

Diffusion

Diffusion

Is the tendency for molecules of any substance to spread out evenlyspread out evenly into the available space

Move from high to low concentrationhigh to low concentration

Down

(25)

Effects of Osmosis on Water Balance

Osmosis

Osmosis

Is the movement of water across a semipermeable membrane

Is affected affected by the by the

concentration gradient of

concentration gradient of

dissolved substances

dissolved substances called the called the

solution’s

(26)

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 impact on cells

without walls

(27)
(28)

Isotonic Solutions

If a solution is isotonicisotonic

The concentration of solutesconcentration of solutes is the same

same as it is inside the cell

There will be NO NETNO NET movement of

(29)

Hypertonic Solution

If a solution is hypertonichypertonic

The concentration of solutesconcentration of solutes is greatergreater

than it is inside the cell

(30)

Hypotonic Solutions

If a solution is hypotonichypotonic

The concentration of solutesconcentration of solutes is lesless than it is inside the cell

(31)

Water Balance in Cells Without

Walls

Animal cell. An animal cell fares best in an isotonic isotonic

(32)

Water Balance of Cells with Walls

Cell Walls

Cell Walls

Help maintain water balance

Turgor pressure

Turgor pressure

Is the pressure of water inside a plant cell pushing outward against the cell membrane

If a plant cell is turgidturgid

It is in a hypotonichypotonic environment

It is very firm, firm, a healthy state in most a healthy state in most plants

plants

If a plant cell is flaccidflaccid

(33)

Water Balance in Cells with Walls

Plant cell. Plant cells are turgid (firmturgid (firm) and generally

(34)

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?

How will the water molecules move?

(35)

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?

How will the water molecules move?

There will be a net movement of water from

Solution B to Solution A until both solutions have

(36)

How Will Water Move Across Semi-Permeable Membrane?

Solution A has 100 molecules of glucose per ml

Solution B has 100 molecules of NaCl per ml

How will the water molecules

How will the water molecules

move?

move?

(37)

Facilitated Diffusion

Facilitated diffusion

Facilitated diffusion

Is a type of PassivePassive Transport Aided by Proteins

Proteins

In facilitated diffusion Transport proteins

Transport proteins speed the movement of

(38)

Facilitated Diffusion & Proteins

Channel proteins

Channel proteins

Provide corridors that allow a specific molecule or ion to cross the membrane

EXTRACELLULAR FLUID

Channel protein

Solute

CYTOPLASM

(39)

Facilitated Diffusion & Proteins

Carrier proteins

Carrier proteins

Undergo a subtle change in shape that translocates the solute-binding site across the membrane

A carrier proteincarrier protein alternates between two conformationsalternates between two conformations, moving a solute

across the membrane as the shape of the protein changes. The protein can transport the solute in either direction

can transport the solute in either direction, with the net movement being down the concentration gradient

(40)

Active Transport

Active transport Uses energy

Uses energy to move solutes againstagainst their concentration gradients

(41)

The sodium-potassium pumpsodium-potassium pump

Is one type of active transport system

Active Transport

P P i

EXTRACELLULAR

FLUID Na+ binding stimulates

phosphorylation by ATP.

2

Na+ Cytoplasmic Na+ binds to

the sodium-potassium pump.1

K+ is released and Na+

sites are receptive again; the cycle repeats.

3

Phosphorylation causes the protein to change its

conformation, expelling Na+ to

the outside.

4

Extracellular K+ binds to the

protein, triggering release of the Phosphate group.

6

Loss of the phosphate restores the protein’s original conformation.

5

CYTOPLASM

[Na+] low

[K+] high

Na+ Na+ Na+ Na+ Na+ P ATP Na+ Na+ Na+ P ADP K+ K+ K+

K+ K+

K+

[Na+] high

(42)

Comparison of Passive & Active Transport

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged

polar molecules can diffuse through the lipid

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,

either channel or carrier proteins.

(43)

Maintenance of Membrane Potential by Ion Pumps

Membrane potential

Membrane potential

Is the voltage difference across a membrane

An electrochemical gradientelectrochemical gradient

Is caused by the concentration electrical gradient of ions across a membrane

An electrogenic pumpelectrogenic pump

(44)

Proton Pump

EXTRACELLULAR FLUID + H+ H+ H+ H+ H+

H+ Proton pump

(45)

Cotransport

Cotransport

Cotransport

Occurs when active transportactive transport of a

specific solute indirectly drives the active transport of another solute

Involves transport by a membrane membrane

protein

protein

(46)

Example of Cotransport

Cotransport: active transport driven by driven by

a concentration gradient

(47)

Bulk Transport

Bulk transport across the plasma membrane occurs by exocytosis exocytosis

and endocytosis

and endocytosis

Large proteins

(48)

Exocytosis & Endocytosis

In exocytosisexocytosis

Transport vesicles

Transport vesicles migrate to the

plasma membrane, fuse with it, and release their contents

In endocytosisendocytosis

The cell takes in macromolecules by

forming new vesicles from the plasma

forming new vesicles from the plasma

membrane

(49)
(50)
(51)

In phagocytosisphagocytosis, a cell

engulfs a particle by

engulfs a particle by

Wrapping pseudopodia

Wrapping pseudopodia

around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole vacuole. The

particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.

Three Types of Endocytosis

PHAGOCYTOSIS

In pinocytosispinocytosis, the cell

“gulps” droplets of gulps” droplets of extracellular fluid

extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all

(52)

RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Ligand Coat protein Coated pit Coated vesicle

A coated pit and a coated vesicle formed during receptor-mediated endocytosis (TEMs). Plasma membrane Coat protein

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific

substances, even though those substances may not be very concentrated in the

extracellular fluid. Embedded in the membrane are proteins with

specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, other molecules

(53)

References

Related documents

One reason that patients may not get treated appropriately for stroke is lack of access to hospitals that have specialized stroke centers.. In rural areas, this is a

We aim to elucidate (1) how foliar stomatal structural and physio- logical characteristics (e.g., stomatal density, size and ψ TLP) of Moso bamboo respond to manipulated drought;

International Research Journal of Engineering and Technology (IRJET) e ISSN 2395 0056 Volume 02 Issue 03 | June 2015 www irjet net p ISSN 2395 0072 ? 2015, IRJET NET All Rights

This study aims at studying the impact of soil inter-space bulk density and moisture contents on vitamin A content of stored oranges in passive evaporative

Finally Jones [ 7 , 8 ] studied basis property from the point of view exchange properties, then semigroups are determined for regular and periodic semigroup.. The main method of

Because Wnt10b signaling also plays a dominant role in the telogen-anagen transition of hair follicles, we analyzed the expression of Wnt10b in AdBMP6- or AdGFP- treated ana- gen

Artificial neural network (ANN) based fault loca- tion approach was presented in reference [14]. However tedious training is demanded for the glide slope to achieve acceptable

With ARM7 as the core, the new intelligent mobile vehicle checking system integrated a lot of hardware modules such as video capture, GPS positioning and wireless transmission,