Fishew1 Cell Physiology

8/10/2019 Fishew1 Cell Physiology

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Animal Cell Physiology


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THE PLASMA MEMBRANE

OBJECTIVES

Distinguish between cytoplasm and cytosol.

Explain the concept of selective permeability.

Define the electrochemical gradient and describe its components


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Structure of the Plasma Membrane

The Lipid Bilayer

The basic structural framework of the plasma membrane is the lipid bilayer, two bac

layers made up of three types of lipid moleculesphospholipids, cholesterol, and g

About 75% of the membrane lipids are phospholipids, lipids that contain phosphoru

Present in smaller amounts are cholesterol(about 20%), a steroid with an attached O

group, and various glycolipids(about 5%), lipids with attached carbohydrate groups.

The bilayer arrangement occurs because the lipids are amphipathic molecules, whic

they have both polar and nonpolar parts. In phospholipids, the polar part is the phocontaining head, which is hydrophilic.

The nonpolar parts are the two long fatty acid tails, which are hydrophobic hydroc


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Arrangement of Membrane Proteins

Membrane proteins are classified as integral or peripheral according to whether they are firmthe membrane (Figure 3.2).

Integral proteinsextend into or through the lipid bilayer among the fatty acid tails and are firm

it.

Most integral proteins are transmembrane proteins,which means that they span the entire li

protrude into both the cytosol and extracellular fluid.

A few integral proteins are tightly attached to one side of the bilayer by covalent bonding to f

membrane lipids, integral membrane proteins are amphipathic.

Their hydrophilic regions protrude into either the watery extracllular fluid or the cytosol, and

regions extend among the fatty acid tails.


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As their name implies, peripheral proteins are not as firmly embedded in the membrane.

They are attached to the polar heads of membrane lipids or to integral proteins at the inn

or outer surface of the membrane.

Many integral proteins are glycoproteins,proteins with carbohydrate groups attached to t

that protrude into the extracellular fluid.

The carbohydrates are oligosaccharides (oligo-few; -saccharides sugars), chains of 2 to 60

monosaccharides that may be straight or branched.

The carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat

called the glycocalyx.

The pattern of carbohydrates in the glycocalyx varies from one cell to another. Therefore,

glycocalyx acts like a molecular signature that enables cells to recognize one another.


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Functions of Membrane Proteins

Generally, the types of lipids in cellular membranes vary only slightly.

In contrast, the membranes of different cells and various

intracellular organelles have remarkably different assortments of

proteins that determine many of the membranes functions


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Some integral proteins form ion channels, pores or holes that specific ions, such

ions (K), can flow through to get into or out of the cell. Most ion channels are seallow only a single type of ion to pass through.

Other integral proteins act as carriers,selectively moving a polar substance or io

side of the membrane to the other. Carriers are also known as transporters.

Integral proteins called receptorsserve as cellular recognition sites. Each type of

recognizes and binds a specific type of molecule. For instance, insulin receptors hormone insulin. A specific molecule that binds to a receptor is called a ligand o

receptor.


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Some integral proteins are enzymes that catalyze specific chemical reactions at the ins

surface of the cell.

Integral proteins may also serve as linkers that anchor proteins in the plasma membran

cells to one another or to protein filaments inside and outside the cell. Peripheral prote

enzymes and linkers.

Membrane glycoproteins and glycolipids often serve as cellidentity markers. They may

(1) recognize other cells of the same kind during tissue formation or (2) recognize and r

potentially dangerous foreign cells.

The ABO blood type markers are one example of cell-identity markers. When you receiv

transfusion, the blood type must be compatible with your own, or red blood cells may c


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Membrane Permeability

The term permeablemeans that a structure permits the passage

of substances through it, while impermeable means that a structure d

not permit the passage of substances through it.

The permeability of the plasma membrane to different substances

varies.

Plasma membranes permit some substances to pass more readily thothers.

This property of membranes is termed selective permeability


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The lipid bilayer portion of the membrane is permeable to nonpolar, un

molecules, such as oxygen, carbon dioxide, and steroids, but is imperme

large, uncharged polar molecules such as glucose.

It is also slightly permeable to small, uncharged polar molecules such as

a waste product from the breakdown of amino acids.

The slight permeability to water and urea is an unexpected property sin

molecules.


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These two small molecules are thought to pass through the lipid

bilayer in the following way.

As the fatty acid tails of membrane phospholipids and glycolipids

randomly move about, small gaps briefly appear in the

hydrophobic environment of the membranes interior.

Water and urea molecules are small enough to move from onegap to another until they have crossed the membrane


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3.3TRANSPORT ACROSS

THE PLASMA MEMBRANE

OBJECTIVE

Describe the processes that transport substances

across the

plasma membrane

Substances generally move across cellular membranes via

transport processes that can be classified as passive or active, depending

they require cellular energy. In passive processes, a substance moves down its concentration or elec

to cross the membrane using only its own kinetic energy (energy of mot

Kinetic energy is intrinsic to the particles that are moving.

There is no input of energy from the cell.

An example is simple diffusion.


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In active processes,cellular energy is used to drive the substance uph

against its concentration or electrical gradient.

The cellular energy used is usually in the form of adenosine triph

(ATP).

An example is active transport.

Another way that some substances may enter and leave cells is a

process in which tiny, spherical membrane sacs referred to as ve

used.

Examples include endocytosis, in which vesicles detach from the

membrane while bringing materials into a cell, and exocytosis, th

merging of vesicles with the plasma membrane to release mater

the cell


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Simple Diffusion

Simple diffusionis a passive process in which substances move f

the lipid bilayer of the plasma membranes of cells without the membrane transport proteins

Facilitated Diffusion

Solutes that are too polar or highly charged to move through

by simple diffusion can cross the plasma membrane by a pascalled facilitated diffusion.

In this process, an integral membrane protein assists a speci

across the membrane.

The integral membrane protein can be either a membrane c

carrier


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Osmosis

Osmosis is a type of diffusion in which there is net movement of a solvent

selectively permeable membrane.

During osmosis, water molecules pass through a plasma membrane in two

(1) by moving between neighboring phospholipid molecules in the lipid bi

simple diffusion, as previously described, and

(2) by moving through aquaporins integral membrane proteins that functi

channels.


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Active Processes

Active Transport

Active transport is considered an active process because energy is requ

carrier proteins to move solutes across the membrane against a concen

gradient.

Two sources of cellular energy can be used to drive active transport:

(1) Energy obtained from hydrolysis of adenosine triphosphate (ATP) is

source in primary active transport;

(2) energy stored in an ionic concentration gradient is the source in secoactive transport.


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PRIMARY ACTIVE TRANSPORT

In primary active transport, energy derived from hydrolysis of ATP ch

the shape of a carrier protein, which pumps a substance across a p

membrane against its concentration gradient.


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