Chapter 3 - Cells: Plasma Membrane

Prefix for “the cell”

Prefix for “the cell”

Cyto-

Suffix for “the cell”

-cyte

Cytoplasm

• internal cell like component of the cells in which the organelles are suspended

Osteocyte

Bone cell, cell only found in bone tissue

Cell

• smallest living unit of life capable of carrying out multiple functions on its own

• different types of cells have different functions in the body

• loss of homeostasis in cells often leads to disease

• All cells can produce ATP

What allows cells to have different functions?

• Cells in different parts of the body have different functions

• Different microscopic anatomy

• Different particular organelles

  • ex: Lining of stomach cells have organelles that produce hydrochloric acid

Every cell has 3 basic parts

• Plasma membrane

  • outermost boundary

  • selectively permeable

    • some materials are locked, some locked in, some can pass

• Cytoplasm

  • intracellular fluid

    • fluid part is cytosol

• Nucleus

  • conotains all of the genetic info

    • codes for proteins

    • controls cellular activities

  • not all cells have nucleus

    • e.x: red blood cells and thrombocytes (platelets) do not

      • short life span

fluid mosaic model

• Describes the general structure of the plasma membranes

• Fluid portion: plasma membranes

  • So thin almost fluid in nature

  • Ver flexible

• Mosaic portion: Proteins and other molecule randomly dispersed in it

  • They are more or less randomly arranged

  • Some mobile, others anchored

Plasma membrane

• Consists of a phospholipid bilayer with proteins randomly dispersed in it

  • Hydrophobic tail point inward

  • Hydrophilic phosphate head point outword

• separates the intracellular fluid (ICF) from the extracellular fluid/interstitial fluid (ECF)

Chemical components of cell membranes

1. lipids

   1. phospholipids

   2. cholesterol

2. Proteins

   1. integral proteins

   2. peripheral proteins

3. Carbohydrates

Phospholipids (composition of cell membrane)

• 1 of the types of lipids that cell membranes is composed of

• forms basic structure of membrane

• Polar phosphate head

  • hydrophilic portions contact intracellular or extracellular fluid

• Nonpolar Fatty acid tails

  • 2

  • hydrophobic portions that face the inside of the membrane

• Aggregation

  • leads to ability of cells to reseal when torn/damaged

  • limited function, only in small portions

Cholesterol (composition of cell membrane)

• 1 of the types of lipids that cell membranes is composed of

• inserted in membrane

• “stiffens” the membrane

  • still flexible but not as easily damaged

  • increases membrane stability

Proteins (composition of cell membrane)

• functionally important

• constitute most of the cells specialized membrane functions

• can be classified by location or function

Proteins in the cell membrane classification by location

1. Integral proteins

   • embedded in the plasma membrane

   • proteins that span the entire width of the membrane have a name: Transmembrane proteins

   • Functions:

     • Transport, carriers, enzymes, receptors, cell-cell recognition, etc

2. Peripheral proteins

   • loosely attached to integral protein

   • are not found ‘in’ the lipid bilayer

   • Functions:

     • Enzymes, motor proteins, cell-cell attachment

Proteins in the cell membrane classification by function

1. Transport proteins

2. Receptor proteins

3. Enzymes

4. Cell-cell recognition proteins

5. Attachment proteins

6. Intercellular junctions

Transport proteins

• move substance in and/or out of cells

  • or around inside the cell

• Other types

  • carrier protein

  • motor protein

• Some proteins (left) form channels through which a particular solute can be selectively moved

  • Channel proteins

  • usually water filled

  • still selective

• Other proteins (right) actively pump substances across the membrane surface by using ATP

  • Pumps

  • require energy inputs

    • Molecule may be huge

    • pump may need to change shape

Receptor proteins

• Steps:

  1. bind to some type of chemical messenger (ex.: hormone)

  2. when bind the proteins changes shape

  3. change of shape will lead to chain of events inside cell

• can relay messages to cell interior when protein is bound to/exposed to certain chemical messengers

• varying specificity

  • some only bind to one to of message

  • other bind to multiple types

  • some need multiple types to be bound to change shape

• not always stimulatory, some slow down or stop activity

Enzymes

• Proteins that catalyze chemical reactions

  • Lower required activation energy

• varying specificity

• some act alone, others may act as a “team” to catalyze sequential steps

Cell-cell recognition proteins

• allow body cells to recognize one another

• every individual person has their own glycoprotein

• must useful in immune systems

Attachment proteins

• help to hold amount protein in place

  • maintains cell shape

• can be inside or outside cell

Intercellular junctions

• some proteins are used to link cells together

• length of time to link cells varies

  • some milliseconds to seconds

  • some permanent

    • hold tissue together

Carbohydrates (composition of cell membrane)

• are not big in membrane

  • relatively not alot of them

• extracellular surface is dotted with short-branching carbohydrates

• can be attached to membrane lipids or proteins

  • glycoproteins

  • glycolipids

  • carbohydrate portion always faces extracellular fluid

• Both produce gylcocalyx

  • Different cell types have different arrangements

    • allows for identification of cell types by other body cells

  • Can also be used by immune cells to identify “self” cells from “non-self” cells

  • Can also contribute to structure/shape of cell membrane

Cell junctions

• a class of proteins that provide contact or adhesion between two or more cells

  • Can be permanent or temporary

• 3 types

  • Tight junction

  • desmosomes

  • gap junctions

Tight junction

• proteins in cell membranes of neighboring cells fuse together

  • riveting cells together

• normally permanent

• junction in impermeable

  • prevents passage of materials between cells

  • e.x: inner-lining of the stomach cells are joined by tight junctions, prevent gastric juice from “leaking out”

Desmosomes

• anchoring junctions from one cell to another that prevents separation

  • 2 neighboring cells stick together

• Most likely to find in parts of the body that’s frequently pulled or stretched

  • e.x: skin

• are permittable

• components

  • cadherins

    • protein filaments extend from cell surface and link to filaments on other cell surface

    • cellular velcro

  • Plaque

    • holds cadherins in place from inside the cell

  • Keratin filaments

    • hold plaque in place to prevent excessive movement/shifting

Gap junctions

• communication junctions

• intercellular channels between two cells

• line up with one on neighboring cells exactly

  • make 1 straight told

• always cells to communicate with each other

  • exchange ions

  • pass materials

• different type of gap junction proteins

  • selective

Membrane transport

• Important for

  • gaining the materials cell needs

    • oxgyen

    • nutrients

  • depositing waste

    • CO₂

    • _{nitrogenous waste}

  • release products

• 2 types

  • Passive

  • Active

Passive Membrane Transport

• Movement of molecules across the membrane down their concentration gradient with no ATP required

• Diffusion

What is the driving force of diffusion

• Kinetic energy of molecules

• In areas of high molecule concentration molecules collide & bounce off one another more frequently increasing the rate of diffusion

Diffusion speed determined by what factors

1. Concentration: greater concentration difference between two areas

   leads to faster diffusion

2. Molecular size: smaller molecules diffuse faster

3. Temperature: higher temperatures results in faster diffusion rates

What are the types of passive diffusion?

1. simple diffusion

2. facilitated diffusion

   1. carrier-mediated

   2. channel-mediated

3. osmosis

simple diffusion

• diffusion of substance directly through the lipid bilayer

• Most molecules diffusing are small in size & nonpolar

• ex: most gases, steroid hormones, fatty acids

Facilitated diffusion

• diffusion of molecules through the membrane with the use of a protein

• 2 types

  • carrier-mediated

  • channel-mediated

Carrier mediated facilitated diffusion

• transmembrane proteins used to carry large molecules through the membrane

• Protein changes shape while moving substance

• Limits: the cell can only move substances as fast as proteins become available to move them

Channel mediated facilitated diffusion

• transmembrane proteins form water-filled channels through which molecules can pass

• Selective size of channel determines what substance can/cannot pass through

• Proteins can form leaky or gated channels

  • These channels are always open, allowing ions and substances to pass through.

  • These channels open and close in response to specific signals, such as changes in membrane potential, mechanical stress, or the binding of a molecule.

Osmosis

• diffusion of water through a selectively permeable membrane

• Movement of water across a semipermeable membrane from a less concentrated solution into a more concentrated solution until concentration is equal on both sides of the membrane

• Can occur without proteins or with the use of aquaporin proteins

Osmolarity

• total concentration of all solute particles in a solution

• A solution with a high osmolarity will have a greater number of solute particles than a solution with low osmolarity

• One solute particle displaces one water molecule

• Imbalances in osmosis cause body cells to swell or shrink (depending on total water volume inside cell)

Water moves by osmosis until

• hydrostatic pressure (the pressure of water pushing on the inner cell wall) is equal to osmotic pressure (tendency of water to move into a cell by osmosis)

• When the two pressures are equal there is no net movement of water observed

What will happen?

What will happen?

Tonicity

• ability of a solution to change the shape of a cell by altering the cells internal water volume

• Water will follow solutes a change in solute concentration on either side of a membrane will also cause a change in water concentration

• Tonicity always refers to the solution that a cell is submerged in!!!!

Isotonic solutions

• have the same concentration of nonpenetrating solutes as those found inside the cell

• No net loss or gain of water observed

• ex: 0.9% NaCl solutions, extracellular fluid

Hypertonic solutions

• have a higher concentration of solutes than inside the cell

• Water moves out of the cell

• Cell will “shrivel up” or crenate

• Ex: 10% NaCl solutions

Hypotonic solutions

• have a lower concentration of solutes than inside the cell

• Water moves into the cell

• Cell will ”swell up” until they burst (lyse)

• Ex: distilled water

Active Forms of Membrane Transport

• Movement of molecules across the plasma membrane that requires energy input (use of ATP)

• Active transport requires transport proteins

Types of active forms of membrane transport

1. Active transport

2. Vesicular transport

Why use energy for active transport

Molecules may be too big, too charged, insoluble in lipid membrane, or moving against their concentration gradient

Active Transport

use of a transmembrane protein and ATP to move molecules across the plasma membrane against their concentration gradient

What are the types of active transport?

1. Primary active transport

2. Secondary active transport

Primary Active Transport

• energy required to do work comes directly from ATP hydrolysis by transport proteins called pumps

• Hydrolysis of ATP leads to transfer of phosphate group from ATP to the pump

• Phosphorylation of pump leads to a change in protein shape allows protein to move molecule across the membrane

• Important example: Sodium-potassium (Na+ - K+) pump

  • Uses enzyme Na+ - K+ ATPase enzyme

What is Na+ - K+ ATPase?

• Pumps Na+ and K+ against their gradients & in opposite directions across the membrane

• For each ATP molecule, ATPase moves 3 Na+ ions out of the cell, 2 K+ ions in to the cell

• Importance: ATPase pumps maintain electrochemical gradient necessary for function of muscle and nervous tissue

Secondary active transport

• indirectly uses energy stored in concentration gradients of ions

  created by primary active transport

• Example: Moving Na+ out of cell creates concentration gradient

• Cotransport protein pumps Na+ back into the cell, and carries another molecule with it

Symporter

• movement of two transported substances in the same direction

• active transport systems

Antiporter

• movement of two transported substances in the opposite direction

• active transport systems

Uniporter

• movement of one substance

• active transport systems

Vesicular Transport

• movement of fluids with large particles & macromolecules inside membranous sacs called vesicles

Functions of vesicular transport

Functions:

1. Endocytosis—movement of a substance into the cell

2. Exocytosis—movement of a substance out of the cell

3. Transcytosis—movement of substances into, across, then out of a cell

4. Vesicular trafficking—movement of a substance from one area of the cell to another

Endocytosis

• Vesicular transport used to bring substance into the cell from the ECF

• Begins with formation of infolding of plasma membrane

Types of endocytosis

1. Phagocytosis

2. Pinocytosis

3. Receptor-mediated endocytosis

Phagocytosis

• cell engulfs large and/or solid material

• Forms vesicle called a phagosome

• Pseudopod formation involves receptors formation is specific

• Phagosome usually fuses with lysosome, where contents are digested

Pinocytosis

• cell brings in a small volume of extracellular fluid containing small solute particles

• No receptor use needed, endocytosis is not a specific process

Receptor-mediated endocytosis

• allows endocytosis of specific substances to occur

• Extracellular substances bind specific receptor proteins

• Importance: Substances can be specifically concentrated in vesicles & brought into cell

• Fate of contents:

  • Substance can be distributed through the cell

  • Vesicle can fuse with lysozyme for digestion of concentrated substance

Exocytosis

• Vesicular transport used to remove substances from cell to the ECF

• Secretory vesicle created around the substance to be removed

• Secretory vesicle travels to plasma membrane, fuses with it, and dumps contents out of the cell

• Functions: hormone secretion, neurotransmitter release, mucus secretion, waste removal, etc. etc.

Membrane Potential

• Selective permeability of plasma membrane generates a membrane potential (voltage) across the membrane

• Voltage electrical potential energy resulting from separation of oppositely charged particles (ions)

• All cells have a resting membrane potential voltage difference across cell membrane when cell is at res

Average resting membrane potential of cells

-70mV

Are cells electrically polarized?

• Yes!

  • Negatively charged inside

  • Positively charged outside

How is the resting membrane potential created?

• Creation of membrane potential involves an ion imbalance on either side of the plasma membrane

• Ion concentrations of Na+ and K+ are different on either side of the membrane

  • Na+ concentration is higher outside the cell

  • K+ concentration is higher inside the cell

How does potassium effect resting membrane potential

• Potassium ions (K+) have a pivotal role in creating the resting membrane potential

• Plasma membranes are more permeable to K+ than to Na+

• K+ “leaks” out of cell, proteins remain inside the cell

• The more K+ that leaves, the more (-) charged the inside of the cell becomes

• Some K+ ions will enter the cellprevents inside of cell from becoming too negative

How is the resting membrane potential maintained?

• Active transport maintains electrochemical gradients to keep the cell in a steady state

  • Electro=charged

  • Chemical=ion concentration

Can cells respond to both extracellular chemicals (hormones, neurotransmitter)? What about to other surrounding cells?

• Yes, cells can respond to both extracellular chemicals (hormones, neurotransmitter) and to other surrounding cells

• These interactions are used to maintain homeostatic balance in the body

• Plasma membrane receptors are important for allowing a cell to interact with its environment

Plasma Membrane Receptors

• Integral proteins at membrane surface serve as binding sites

Main functions plasma membrane receptors:

1. contact signaling

2. chemical signaling

Contact signaling (plasma membrane)

• cellular recognition by physical contact between cells

• Importance: normal cellular development and immunity rely on contact signaling

Chemical signaling (plasma membrane)

• when a chemical messenger (called a ligand) binds a specific receptor and initiates a response

• Overall process: ligand binds to receptorreceptor structure

  changes cell proteins are altered

• The specific response is linked to the cell’s internal machinery (its structure & function), not the ligand itself

• Example: G protein-coupled receptors