Notes

Plasma Membrane and Cell Transport

Madhu R Biology 9th grade English 5.0Average rating 5.0 out of 5 (1 rating) 3 pages

Content of the notes (3)

Page 1

Plasma Membrane

Cell membranes are composed of two

phospholipid layers.

The

cell membrane ,

or the plasma membrane, forms a boundary between a

cell and the outside environment and controls the passage of materials into

and out of a cell. The cell membrane consists of a double layer of phospholipids interspersed with a variety of

other molecules. A

phospholipid is a molecule

composed of three basic parts:

•

a charged phosphate group

•

glycerol

•

two fatty acid chains

Together, the glycerol and the phosphate group form the “head” of a

phospholipid; the fatty acids form the

“tail.” Because the head bears a charge,

it is polar. Recall that water molecules

are also polar. Therefore, the polar

head of the phospholipid forms hydrogen bonds with water molecules. in

contrast, the fatty acid tails are non-polar and cannot form hydrogen bonds

with water. As a result, the non-polar tails are attracted to each other and

repelled by water.

Because the membrane touches the cytoplasm inside the cell and the

watery fluid outside the cell, the properties of polar heads and non-polar tails

cause the phospholipids to arrange themselves in layers, like a sandwich.

The polar heads are like the bread. They form the outer surfaces of the

membrane, where they interact with the watery environment both outside

and inside a cell. The non-polar tails are like the filling. They are sandwiched

between the layers of polar heads, where they are protected from the

watery environment.

Molecules are embedded within the phospholipid layers.

They give the membrane properties and characteristics it would not otherwise

have. These molecules serve diverse functions. Here are a few examples:

•

Cholesterol molecules strengthen the cell membrane.

•

Some proteins extend through one or both phospholipid layers and help

materials cross the membrane. Other proteins are key components of the

cytoskeleton. Different cell types have different membrane proteins.

•

Carbohydrates attached to membrane proteins serve as identification tags,

enabling cells to distinguish one type of cell from another.

Fluid Mosaic Model

Scientists have developed the fluid mosaic model , which describes the

arrangement of the molecules that make up a cell membrane. This model of

cell membrane structure takes its name from two characteristics. First, the cell

membrane is flexible, not rigid. The phospholipids in each layer can move

from side to side and slide past each other. As a result, the membrane behaves

like a fluid, similar to a film of oil on the surface of water. However, proteins

embedded in the membrane do not flip vertically. If one part of a protein is

outside the membrane, it will stay outside the membrane. Second, the variety

of molecules studding the membrane is similar to the arrangement of colorful

tiles with different textures and patterns that make up a mosaic.

Selective Permeability

The cell membrane has the property of selective permeability , which means it allows

some, but not all, materials to cross. The

terms semipermeable and selectively permeable

also refer to this property. As an example,

outdoor clothing is often made of semipermeable fabric. The material is waterproof yet

breathable. Molecules of water vapor from

sweat are small enough to exit the fabric, but

water droplets are too large to enter.

Selective permeability enables a cell to maintain homeostasis in spite of

unpredictable, changing conditions outside the cell. Because a cell needs to

maintain certain conditions to carry out its functions, it must control the

import and export of certain molecules and ions. Thus, even if ion concentrations change drastically outside a cell, these ions won’t necessarily interfere

with vital chemical reactions inside a cell.

Molecules cross the membrane in several ways. Some of these methods

require the cell to expend energy; others do not. How a particular molecule

crosses the membrane depends on the molecule’s size, polarity, and concentration inside versus outside the cell. In general, small non-polar molecules easily

pass through the cell membrane, small polar molecules are transported via

proteins, and large molecules are moved in vesicles.

Chemical signals are transmitted across the

cell membrane.

Recall that cell membranes may secrete molecules and may contain identifying

molecules, such as carbohydrates. All these molecules can act as signals to

communicate with other cells. How are these signals recognized?

A receptor is a protein that detects a signal molecule and performs an

action in response. It recognizes and binds to only certain molecules, ensuring

that the right cell gets the right signal at the right time. The molecule a receptor binds to is called a ligand. When a receptor and a ligand bind, they change

shape. This change is critical because it affects how a receptor interacts with

other molecules. Two major types of receptors are present in your cells.

Intracellular Receptor

A molecule may cross the cell membrane and bind to an intracellular receptor. The word intracellular means “within, or inside, a cell.”

Molecules that cross the membrane are generally non-polar and may be relatively small. Many hormones fit within this category. For example, aldosterone

can cross most cell membranes. However, it produces an effect only in cells

that have the right type of receptor, such as kidney cells. When aldosterone

enters a kidney cell, it binds to an intracellular receptor. The receptor-ligand complex enters the nucleus, interacts with the DNA, and turns on certain

genes. As a result, specific proteins are made that help the kidneys absorb

sodium ions and retain water, both of which are important for maintaining

normal blood pressure.

Membrane Receptor

A molecule that cannot cross the membrane may bind to a receptor in the cell

membrane.The receptor then sends the message to the cell

interior. Although the receptor binds to a signal molecule outside the cell, the

entire receptor changes shape—even the part inside the cell. As a result, it causes

molecules inside the cell to respond. These molecules, in turn, start a complicated chain of events inside the cell that tells the cell what to do. For instance,

band 3 protein is a membrane receptor in red blood cells. When activated, it

triggers processes that carry carbon dioxide from body tissues to the lungs.

Plasma Membrane and Cell Transport

Page 2

Diffusion and Osmosis

Diffusion and osmosis are types of passive transport.

Cells almost continually import and export substances. If they had to expend

energy to move every molecule, cells would require an enormous amount of

energy to stay alive. Fortunately, some molecules enter and exit a cell without

requiring the cell to work. Passive transport is the movement of molecules across a cell membrane without energy input from the cell.

It may also be described as the diffusion of molecules across a membrane.

Diffusion

Diffusion is the movement of molecules in a fluid or gas from a region of

higher concentration to a region of lower concentration. It results from the

natural motion of particles, which causes molecules to collide and scatter.

Concentration is the number of molecules of a substance in a given volume,

and it can vary from one region to another. A concentration gradient is the

difference in the concentration of a substance from one location to another.

Molecules diffuse down their concentration gradient—that is, from a region

of higher concentration to a region of lower concentration.

In the tie-dye example, dye molecules are initially at a high concentration

in the area where they are added to the water. Random movements of the dye

and water molecules cause them to bump into each other and mix. Thus, the

dye molecules move from an area of higher concentration to an area of lower

concentration. Eventually, they are evenly spread throughout the solution.

This means the molecules have reached a dynamic equilibrium. The concentration of dye molecules is the same throughout the solution (equilibrium),

but the molecules continue to move (dynamic).

In cells, diffusion plays an important role in moving substances across the

membrane. Small lipids and other non-polar molecules, such as carbon dioxide

and oxygen, easily diffuse across the membrane. For example, most of your

cells continually consume oxygen, which means that the oxygen concentration

is almost always higher outside a cell than it is inside a cell. As a result, oxygen

generally diffuses into a cell without the cell’s expending any energy.

Osmosis

Water molecules, of course, also diffuse. They move across a semipermeable

membrane from an area of higher water concentration to an area of lower

water concentration. This process is called

osmosis.

It is important to recognize that the higher the concentration of dissolved particles in a solution, the

lower the concentration of water molecules in the same solution. So, if you put

1 teaspoon of salt in a cup of water and 10 teaspoons of salt in a different cup

of water, the first cup would have the higher water concentration.

A solution may be described as isotonic, hypertonic, or hypotonic relative

to another solution. Note that these terms are comparisons; they require a

point of reference, as shown in FIGURE 4.3. For example, you may be taller than

your coach or taller than you were two years ago, but you are never just taller.

Likewise, a solution may be described as isotonic only in comparison with

another solution. To describe it as isotonic by itself would be meaningless.

A solution is

isotonic

to a cell if it has the same concentration of dis-

solved particles as the cell. Water molecules move into and out of the cell

at an equal rate, so the cell’s size remains constant.

hypertonic

solution has a higher concentration of dissolved particles

than a cell. This means water concentration is higher inside the cell than

outside. Thus, water flows out of the cell, causing it to shrivel or even die.

hypotonic

solution has a lower concentration of dissolved particles

than a cell. This means water molecules are more concentrated outside

the cell than inside. Water diffuses into the cell

If too much water enters

a cell, the cell membrane could potentially expand until it bursts.

Some animals and single-celled organisms can survive in hypotonic

environments. Their cells have adaptations for removing excess water. In

plants, the rigid cell wall prevents the membrane from expanding too much.

Remember from Section 2 that pressure exerted on the cell wall by fluid

inside the central vacuole provides structural support for each cell and for the

plant as a whole.

Some molecules diffuse through

transport proteins.

Some molecules cannot easily diffuse across a membrane. They may cross

more easily through transport protein openings formed by proteins that

pierce the cell membrane. Facilitated diffusion is the diffusion of molecules

across a membrane through transport proteins. The word facilitate means “to

make easier.” Transport proteins make it easier for molecules to enter or exit a

cell. But the process is still a form of passive transport. The molecules move

down a concentration gradient, requiring no energy expenditure by the cell.

There are many types of transport proteins. Most types allow only certain

ions or molecules to pass. Some transport proteins are

simple channels, or tunnels, through which particles such as ions can pass.

Others act more like enzymes. When bound, the protein changes shape,

allowing the molecule to travel the rest of the way into the cell.

Plasma Membrane and Cell Transport

Page 3

Active Transport, Endocytosis, and Exocytosis

Proteins can transport materials against a concentration gradient.

You just learned that some transport

proteins let materials diffuse into

and out of a cell down a concentration

gradient. Many other transport

proteins, often called pumps, move

materials against a concentration

gradient.

Active transport drives

molecules across a membrane from

a region of lower concentration to a

region of higher concentration.

This process uses

transport proteins powered by chemical energy. Cells use active transport to

get needed molecules regardless of the

concentration gradient and to maintain homeostasis.

Before we discuss active transport proteins, let’s look at transport proteins

in general. All transport proteins span the membrane, and most change shape

when they bind to a target molecule or molecules. Some transport proteins

bind to only one type of molecule. Others bind to two different types. Some

proteins that bind to two types of molecules move both types in the same

direction. Others move the molecules in opposite directions.

The key feature of active transport proteins is that they can use chemical

energy to move a substance against its concentration gradient. Most use

energy from a molecule called ATP, either directly or indirectly. For example,

nerve cells, or neurons, need to have a higher concentration of potassium ions

and a lower concentration of sodium ions than the fluid outside the cell. The

sodium-potassium pump uses energy directly from the breakdown of ATP. It

pumps three sodium ions out of the cell for every two potassium ions it

pumps in. The proton pump, another transport protein, uses energy from the

breakdown of ATP to move hydrogen ions (or protons) out of the cell. This action forms a concentration gradient of hydrogen ions (H

), which makes the

fluid outside the cell more positively charged than the fluid inside. In fact, this

gradient is a form of stored energy that is used to power other active transport

proteins. In plant cells, this gradient causes yet another protein to transport

sucrose into the cell—an example of indirect active transport.

Endocytosis and exocytosis transport materials

across the membrane in vesicles.

A cell may also use energy to move a large substance or a large amount of a

substance in vesicles. Transport in vesicles lets substances enter or exit a cell

without crossing through the membrane.

Endocytosis

Endocytosis is the process of taking liquids or fairly

large molecules into a cell by engulfing them in a membrane. In this process,

the cell membrane makes a pocket around a substance. The pocket breaks

off inside the cell and forms a vesicle, which then fuses with a lysosome or a

similar type of vesicle. Lysosomal enzymes break down the vesicle membrane

and its contents (if necessary), which are then released into the cell.