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Membrane: Structure And Function

Paper Type: Free Essay Subject: Biology
Wordcount: 2283 words Published: 5th Jun 2018

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Chapter title: Membrane Structure and Function. The “ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible.”


The membranes that are found within cells (plus the plasma membrane surrounding cells) consist of phospholipids (and other lipids plus membrane proteins) arrayed by hydrophobic exclusion into two-dimensional fluids known as known as lipid bilayers


Phospholipids are amphipathic molecules meaning that they have both a hydrophobic and a hydrophilic end

Lipid bilayer

Phospholipids can exist as bilayers in aqueous solutions

The hydrophobic portion of the phospholipid is shielded in middle of these bilayers

The hydrophilic portion is exposed on both sides to water

Lipid bilayers are held together mainly by hydrophobic interactions (including hydrophobic exclusion)

Fluid mosaic model

The plasma membrane contains proteins, sugars, and other lipids in addition to the phospholipids

The model that describes the arrangement of these substances in and about lipid bilayers is called the fluid mosaic model

Basically, membrane proteins are suspended within a two-dimensional fluid that in turn is made up mostly of phospholipids


Cholesterol, a kind of steroid, is an amphipathic lipid that is found in lipid bilayers that serves as a temperature-stability buffer

At higher temperatures cholesterol serves to impede phospholipid fluidity

At lower temperatures cholesterol interferes with solidification of membranes (e.g., cholesterol functions similarly, in the latter case, to the effect of unsaturated fatty acids on lipid-bilayer fluidity)

Cholesterol is found particularly in animal cell membranes

Membrane proteins

Proteins are typically associated with cell membranes

Integral membrane proteins are typically hydrophobic where they interact with the hydrophobic portion of the membrane or hydrophilic where they interact with the hydrophilic portion of the membrane and overlying

Functions of membrane proteins

Functions of membrane proteins include:

  • Transport of substances across membranes
  • Enzymatic activity
  • cell communication
  • Cell-to-cell joining
  • Attachment to the cytoskeleton and extracellular matrix

Selective permeability

Lipid bilayers display selective permeability

In general, intact lipid bilayers are permeable to:

Hydrophobic molecules (including many gasses)

Small, not-ionized molecules 

Simultaneously, lipid bilyaers are NOT permeable to:

Larger, polar molecules (e.g., sugars)

Ions, regardless of size

Thus, lipid bilayers are selectively permeable barriers that allow the entry of small or hydrophobic molecules while blocking the entry of larger polar or even small charged substances

Transport across membranes

Movement across membranes is important, for instance as a means of removing wastes from a cell or bringing food into a cell

Categories of substance transport across membranes include:

  • Passive transport
  • Facilitated diffusion
  • Active transport (including cotransport)

Endocytosis, phagocytosis, and exocytosis, also considered below, technically are not mechanisms of movement of substances across lipid bilayers (though these do represent movements of substances into and out of cells; to be movement across the euakaryotic cell membrane, a substance must actually pass through an endomembrane lipid bilayer)

Note that in considering transport across membranes we will once again confront the concept of movement away from or towards equilibrium, i.e., endergonic and exergonic processes

There are three basic types of movement across membranes: simple diffusion, passive transport, and active transport:

Simple diffusion

Simple diffusion is the movement of substances across lipid bilayers without the aid of membrane proteins

This image (below) shows how substances move through membranes, regardless of net direction and concentration gradients:

This image (below) shows how substances net move through membranes in the direction of their concentrations gradients (i.e., with their concentration gradients)-note that regardless of how net movement is accomplished, all simple diffusion across membranes occurs in the manner illustrated above, i.e., it is a process that is driven by the random movement of molecules:

This figure (below) indicates the kinds of molecules that are capable of moving across membranes via simple diffusion:

Passive transport

Passive transport is the term used to describe the diffusion (as well as what is termed facilitated diffusion, below) of substances across lipid bilayers

Passive transport is a consequence of movement through the lipid bilayer (whether by diffusion through the membrane or with movement across facilitated by an integral membrane protein) a concentration gradient thereby contrasting with active transport

Down the concentration gradient

Diffusion is a random process that tends to result in the net movement of substances from areas of high concentration to areas of low concentration

This includes movement from one side of a permeable lipid bilayer to the other from the higher concentration side to the lower concentration side (i.e., passive transport)

Movement from high to low concentration areas is described as going “down its concentration gradient.”

The direction of movement of substances across lipid bilayers by passive transport is controlled by concentration gradients


Movement of water across selectively permeable membranes down the water concentration gradient is called osmosis

Note that this is movement toward equilibrium (exergonic process)

Tonicity (isotonic, hypertonic, hypotonic)

Picture a membrane separating two solutions, one side with a higher solute concentration than the other

The side with the higher solute concentration is said to be hypertonic

The side with the lower solute concentration is said to be hypotonic

(I keep track of the difference by recalling that a hypodermic syringe is so named because the tip of the needle is placed “beneath” the dermis, i.e., under the skin; a hypotonic solution has a solute concentration that is beneath, i.e., lower than that of the reference solution)

If both sides have the same solute concentration, they are said to be isotonic

Animal cells and tonicity

Normally animal cells are bathed in an isotonic solution

Placement of an animal cell in a hypertonic solution causes the cell to shrink (i.e., water is lost from the cell by osmosis)

Placement of an animal cell in a hypotonic solution causes it to take on water then burst (lyse, i.e., die) (water is gained by the cell, lost from the environment bathing the cell, both by osmosis)


Normally a plant cell exists in a hypotonic environment

The hypotonicity causes the plant cytoplasm to expand

However the plant cell does not lyse and this is due to the presence of its cell wall

This conditions is known as turgidity (i.e., the pressing of the plant plasma membrane up against its cell wall)

Plant cells prefer to display turgidity


A plant or bacterial cell placed in a hypertonic environment will show a shrinkage of its cytoplasm

This shrinkage is called plasmolysis

At the very least plasmolysis will inhibit growth

Often plasmolysis will lead to cell death

This is the principle upon which foods are preserved in highly osmotic solutions (e.g., salt or sugar); such solutions impede most microbial growth


Plant cells bathed in isotonic solutions will fail to display turgidity

Instead they display flaccidity

At a whole-organismal level, flaccidity is otherwise known as wilting

Transport proteins

Substances (e.g., sugars) that are not permeable through lipid bilayers may still cross via membrane-spanning transport proteins

Facilitated diffusion

Facilitated diffusion is the movement of a substance across a membrane via the employment of a transport protein, where net movement can only occur with the concentration gradient, is called facilitated diffusion

The key thing to keep in mind is that facilitated diffusion, in contrast to other mechanisms of transport-protein-mediated membrane crossing, does not require any input of energy beyond that necessary to place the protein in the membrane in the first place (i.e., facilitated diffusion is an exergonic process)

Passive versus active transport

Two general categories of transport across membranes exist:

Those that don’t require an input of energy (passive transport, simple diffusion, facilitated diffusion)

Those that do require an input of energy (active transport)

Passive Transport

Active Transport

Concentration gradient

With (Down)

Against (Up)

Without Integral Protein

Yes (Simple Diffusion)


With Integral Protein




Small or Hydrophobic Substances, Osmosis(by simple diffusion) or Not-Small or Charged Substances (by facilitated diffusion)

Cotransport, Proton Pump, Sodium-Potassium Pump

Active transport

Active transport is the movement of substances across membranes against their concentration gradients

Moving things against their concentration gradients requires an expenditure of energy (i.e., it is an endergonic process)

This energy can be in the form of ATP (e.g., sodium-potassium pump)

This energy can also be in the form of electrochemical gradients (i.e., cotransport)

Note that the movement of substances by active transport is in a direction that is away from equilibrium

Sodium-potassium pump

One means by which cells actively transport substances across membranes is via the sodium-potassium pump

The sodium-potassium pump is important especially in animal cells, and is the means by which the sodium-potassium electrochemical gradient is established by these cells

Proton pump

The sodium-potassium pump is the means by which animal cells generate membrane potentials

In bacteria, plants, and fungi, proton pumps play the same role

The proton pump is simply ATP-driven active transport in which the substance pumped across the membrane is a hydrogen ion


Much of the active transport accomplished by a cell isn’t directly powered by ATP

Instead, much active transport is powered by membrane potentials (i.e., electrochemical gradients)

Such electrochemical-gradient-driven active transport is called cotransport

In cotransport, one substance, such as a sugar, is driven up its concentration gradient while a second substance, e.g., sodium ions or protons, are allowed to fall down their electrochemical gradient; the energy gained from the latter is employed to power the former (i.e., energy coupling)


Endocytosis is a general category of mechanisms that move substances from outside of the cell to inside of the cell, but neither across a membrane (technically) nor into the cytoplasm (again, technically speaking)

Instead, substances are moved from outside of the cell and into the lumens of endomembrane system members

To enter the cytoplasm an endocytosed substance must still be moved across the membrane of the endomembrane system, e.g., following their digestion (typically hydrolysis) to smaller molecules

Examples include: phagocytosis, pinocytosis, and receptor-mediated endocytosis


Phagocytosis is the engulfing of extracellular particles is achieved by wrapping pseudopodia around the particles, thus internalizing the particles into vacuoles

Amoebas employ phagocytosis to “eat”

Most protozoa obtain their food by engulfing, i.e., via some form of endocytosis

The advantage of endocytosis as a mechanism of food gathering has to do with minimizing the volume within which digestive enzymes must work in order to digest food, i.e., the engulfed food particle

Cells in our own bodies, called phagocytes and macrophages employ phagocytosis to engulf (and then destroy) debris floating around our bodies as well as to engulf and destroy invading bacteria


Pinocytosis is the engulfing of liquid surrounding a cell

This is how developing ova obtain nutrients from their surrounding nurse cells (ova are very large cells so have surface-to-volume problems-pinocytosis solves the problem of nutrient acquisition by allowing nutrients to be obtained across many internal membranes rather than being limited to crossing the plasma membrane)

Receptor-mediated endocytosis

Receptor-mediated endocytosis involves the binding of extracellular substances to membrane-associated receptors, which in turn induces the formation of a vesicles


Exocytosis is more or less the mechanistic opposite of endocytosis


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