Movement Across the Cell Membrane

Unit 1 – Matter and Energy for Life

Functions of the Cell Membrane

• Primary function of the cell membrane is to maintain homeostasis– The conditions of the

cell must remain more or less constant under many different conditions in order for the cell to carry out its life function

– Balance

Functions of the Cell Membrane

• Transport material into the cell

• Transport manufactured products and wastes out of the cell

• Prevent the entry of unwanted material (protection)

• Prevent the escape of matter needed to perform cellular functions

Structure of the Cell Membrane• The cell membrane is made of a

phospholipid bi-layer• Each phospholipid is made of a

“tail” containing two fatty acids and a phosphate “head”

• The phosphorous “head” has an attraction to water, it is hydrophilic

• The fatty acid “tail” is not attracted to water, it is hydrophobic

Structure of the Cell Membrane• There are other molecules that make up

the cell membrane1. Carrier and Channel Proteins that help move

material in and out of the cell2. Cholesterols which allows the cell membrane

to operate in a wide range of temperatures• High temperatures it helps maintain rigidity• Lower temperatures it keeps the membrane flexible

and prevents it from freezing

Fluid-Mosaic Model• Describes the idea that the cell

membrane has a fluid-like consistency

• This allows each phospholipid and all membrane components to move freely as if they were floating

• This allows the membrane to change shape without necessarily damaging it

• Watch this…– http://www.youtube.com/watch?v

=Qqsf_UJcfBc

Exchange of Material Through the Cell Membrane

Movement Across the Cell Membrane

• There are a number of methods used by the cell membrane in maintaining homeostasis

• This is due to the fact the cell membrane is selectively permeable– It allows certain substances

to pass (Oxygen) but prevents others

– Must also get rid of waste products (Carbon Dioxide)

How do materials enter and leave a cell?

Passive TransportTransport Across the Membrane #1

Passive Transport• Process of living cells whereby

small (non-polar) molecules pass through the cell membrane from a region of high concentration to one of low concentration

• No energy is needed!• Examples of Passive Transport

include:1. Diffusion2. Osmosis3. Facilitated Diffusion

Diffusion• No energy, passive transport!• The movement of molecules from

a region of high concentration to a region of lower concentration– They follow the concentration

gradient• Many molecules (especially small

uncharged ones) can move easily through the cell membrane– Oxygen– Carbon Dioxide

Diffusion

• Permeability is the ability to allow substances to pass through

• Selective Permeability is a material that will allow some substances to pass but not others– http://www.youtube.com/wat

ch?v=RKqctivXExQ• Factors determining whether

or not a molecule will pass though a membrane1. Size2. Charge3. Solubility in Phospholipids

Osmosis

• Is the diffusion of water across a cell membrane

• It is important that water balance within a cell be kept

• Movement of water molecules from an area of high concentration to an area of lower concentration– Occurs until water balances

on both sides of the membrane

• Remember, passive transport!

• http://www.youtube.com/watch?v=sdiJtDRJQEc

Osmosis

High H20 PotentialLow Solute Concentration

Low H20 PotentialHigh Solute Concentration

Osmotic Pressure

• The pressure caused by the movement of water across a membrane

• In plants this is called Turgor Pressure– Responsible for stem

support in non-woody plants

• Three conditions that exist for osmosis1. Isotonic2. Hypotonic3. Hypertonic

Isotonic Solution

• Concentration of water inside the cell equals the concentration outside the cell

• Both solution are in equilibrium with each other

• No change to the cell

Hypotonic Solutions

• Concentration of water outside the cell is greater than that inside the cell

• More water will move into the cell causing it to swell

• Animal cells may burst, plant cells swell and become rigid

Hypertonic Solutions

• When the concentration of water is greater inside the cell than outside

• More water will move out of the cell than enter it

• In both plant and animal cells the cytoplasm shrinks

Osmosis in Plant Cells

• Plant cell placed in a Hypotonic Solution water will enter the cell, causing it to swell– Cell will not burst, cell wall resists the osmotic

pressure– Plant cells that are swollen are said to be turgid

• Plant cell placed in a Hypertonic Solution, water leaves the cells, cytoplasm shrinks– Turgor Pressure drops, process called Plasmolysis– Plant wilts

• What about animal cells? What do you think will happen?

Osmosis in Plant and Animal Cells

• The presence or absence of the cell wall plays a huge role in determining the effects osmosis will have on plant and animal cells

Facilitated Diffusion

• Not all molecules that must enter the cell are small enough to pass through easily– Ex: Glucose

• Carrier proteins within the cell membrane recognize and bind with only one type of particle

• They facilitate the movement of particles through the cell membrane by changing shape

• Still an example of passive transport, no energy needed!

Active TransportTransport Across the Membrane #2

Active Transport• Passive transport alone cannot

tend to all of the functions necessary of the cell membrane

• The cell will ultimately have to expel energy to transport some substances

• Active transport uses cellular energy and transports substances from areas of low concentration to high concentration– Across the concentration

gradient• Examples of Active Transport:

• Use of Carrier Proteins• Use of Vesicles

Use of Carrier Proteins

• Carrier proteins can be used both passively and actively

• When used in active transport, remember that substances are being moved from areas of low concentration to high concentration– Across the

concentration gradient

Carrier Proteins – Active Transport

• Carrier proteins within the cell membrane receive an energy boost that permits them to aid in active transport

• Proteins capture solutes and move them either into or out of the cell

• Example– Sodium-Potassium Pump

moves three Na+ ions outside of the cell for every two K+ ions inside the cell

Use of Vesicles

• Some macromolecules that the cell must take in or expel are too big or too polar

• Cell membrane can fold in on itself to create a vesicle

• Cell uses these vesicles to “swallow” or expel various materials

• Cells engulf large particles by extending their cytoplasm around the particle

• Examples:1. Endocytosis2. Exocytosis

Endocytosis

• The process where the cell membrane folds into itself and pinches off forming a vesicle

• Materials too large to pass via diffusion or carrier proteins can be brought into the cell

• Examples– Viruses and bacteria

Endocytosis

• There are three main types of endocytosis:1. Pinocytosis2. Phagocytosis3. Receptor-Assisted

Endocytosis

Pinocytosis• Also known as “Cell Drinking”• Process which involves the cell

taking in small droplets of extracellular fluid along with dissolved materials

• Cell membrane surrounds the droplets and pinches off inside the cell once it’s inside

• Process occurs in nearly all cell types

Phagocytosis

• Also known as “Cell Eating”• Similar to pinocytosis,

involves the intake of a large droplet of extracellular fluid

• However, fluid would include larger more specific particles such as bacteria or organic matter

• Process occurs only in:– Specialized cells

• Ex: White Blood Cells– Single-celled amoeba

Receptor-Assisted Endocytosis

• Involves the intake of molecules that attach to membrane receptors specific to certain molecules

• Cholesterol is required by all cells and is brought into the cell via this process

• Each cholesterol has its own tag, a marker that will attach to a membrane receptor

• Tag and receptor trigger endocytosis

Endocytosis

http://www.maxanim.com/physiology/Endocytosis%20and%20Exocytosis/ee4.swf

Exocytosis

• The transport of molecules out of the cell by means of a vesicle

• The reverse process of endocytosis

• Vesicles fuse with the cell membrane, contents are secreted into extracellular fluid

Membrane Surface Area and Cell Size

• Everything the cell needs in order to survive must diffuse through the cell membrane

• At the same time, wastes must pass out of the cell • Diffusion is very efficient over small distances and

very poor over larger distances• If cells were larger than they are, it would take

much longer for nutrients to diffuse from the surface of the cell, making the cell function improperly

• To function best, a cell needs a large surface area and a small volume