Cell Structure and Function Part 1:Cell intro and the plasma membrane

Cell Factoids (not on test)• 75-100 trillion per body

– 75,000,000,000,000

• 200+ different types– Each is structurally and functionally different

• 7µm – 120µm in size– 7/10,000th – 12/1000th of a cm

– 7/125,000 -120/125,000ths of an inch

Cell Theory (for AP150)

• All known living things are made up of one or more cells

• Cells are the fundamental structural and functional unit of the body.

– Cells are responsible for the fundamental structure of the human body

– Cells are responsible for the fundamental functions of the human body

• The structure (and function) of higher levels of organization (e.g., tissues, organs) reflects the activities and structures of cells

• The activity of an organism/the whole body depends on the total activity of independent cells.

• Cells contain DNA which is passed from cell to cell during division

• Energy processing and most chemical reactions occurs in cells

• Cells only come from other, pre-existing cells.

Things Cells Do:

• Obtain nutrients and O2 from its environment

• Perform chemical reactions and process nutrients to release energy (metabolism)

• Eliminate cell waste

• Regulate their internal environment

• Move (external or internal)

• Sensitive to and responds to surroundings

• Grow

• Reproduce

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A Prototypical/Generic Cell

The Generic CellThe major parts of the cell include• Plasma membrane — the outer boundary of the cell• Cytoplasm — within PM, performs most cell activities• Nucleus— contains protects DNA

Plasma Membrane

Cytoplasm

Nucleus

Parts/Components of cell

Cell = Plasma Membrane + Cytoplasm + Nucleus

Cytosol + organelles + inclusions

Body/Fluid Compartments

• intracellular v. extracellular compartments– Intracellular = inside cells– Extracellular = outside cells– Plasma Membrane separates

1-45

A.

B.

intracellular

extracellular

Body/Fluid Compartments

• Extracellular :– blood plasma – interstitial fluid or tissue fluid

1-45

• The extracellular and intracellular environments can be very different

• Example is extracellular v. intracellular Na+ and K+ concentration

Functions of the Plasma Membrane (PM)those from text in blue

• Forms a physical barrier (that separates):– separates inside of cell from outside (forms a compartment).– Selectively Permeable allows some things through but not others

• regulation of movement– Actively regulates or influences what can enter/exit cell

• Connection (Attachment)– connects cells to other cells and/or surrounding structures– connects to internal cell parts

• Communication regulation/coordination– allows cell-cell communication/signalling for coordination of activity

• Chemical reactions – chemical reactions take place on the PM

• Cell recognition– The PM “labels”/identifies the cell

*italicized words represent the four functions the book describes, I have elaborated and reworded

Reference Slides:• The plasma membrane creates a boundary between the

cells internal environment and its external environment.– It makes the cell a compartment that is separated from other

areas/compartments of the body.

• It creates a selectively permeability barrier that some substances can pass through and others can’t– Because of this:

• There is a difference in the composition of the intracellular (inside cell) and extracellular (outside cell) environment.

• The cells internal environment can be regulated

PM Structure:Composition of the PM

• Three types of molecules make up the PM1. Lipids—about 45% of PM (by mass/weight), more by surface area

• 5-10% of lipids have carbohydrates attached2. Proteins—about 55% of PM (by mass)

• Includes glycoproteins

The PM is mostly Lipids and Proteins and these two molecules exist in relatively equal proportions.

* Carbs make up about 3%of overall PM by weight/mass

Membrane Lipids2 primary types of Membrane Lipids

1. Phospholipids—Most abundant (~70-75%), • One factor that creates selective permeability-- prevents the movement of most substances across the PM-- Especially fluids: Prevents intracellular fluids from escaping and

extracellular fluids from entering

2. Cholesterol—less abundant (~20%), effects membrane fluidity/ stabilizes at high temperatures (i.e., prevents it from becoming too fluid)keeps membrane from being too rigid

3. Glycolipids—5-10% of lipidsthe sugar portion located on cells exterior and helps form glycocalyx

Phospholipids have 2 regions• Head—hydrophilic, attracted to water• Tails—hydrophobic, repelled by water

Phospholipid

Head

Tails

Extracellular (outside cell)

Intracellular (inside cell)

Lipid bilayer

CholesterolSpans hydrophobic and hydrophilic regions. Doesn’t pass through both sides of PM

Non-polar tail region: Impermeable to ions and polar moleculesExcept Water

Functional Consequences of phospholipids

• Because the center of the phospholipid bilayer is hydrophobic (and non-polar):

1. Most polar/water soluble substances and ions cannot pass through the lipid portion of membrane.

2. Only non-polar (lipid soluble) can substances can pass directly through the lipids of the membrane.

Thus: the lipid bilayer creates selective permeability and influences what can pass into and out of a cell (contributing to the difference between the intracellular and extracellular environments).

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Two different ways to describe membrane proteins

• structural classifications– Integral proteins

• deeply embedded• extend from both inner and

outer surfaces

– Peripheral proteins • only attached to a single

side of PM

– Glycoproteins• Sugar+protein• 90% of membrane carbs• Glycocalyx

• Functional Classifications– Transport

• Into/out of cell

– Connection• Intercellular• Intracellular—to cytoskeleton

– Enzymes• Chemical rxns

– Recognition– Receptors

• (signal transduction)

structure

Transport Proteins• Move substances that cannot pass through phosolipid bilayer

– Ions

– Polar molecules

Examples:

Transport Proteins• Allow ions and polar molecules to pass through membrane.

– Selective permeability

proteins

CELL

CBA

K+K+K+

K+K+

K+

K+K+

K+K+

K+K+

Na+

Na+

Na+

Na+

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Transport Protein Composition

• Types of transport proteins influences permeability

Ion Channels• Protein based tubes• Allow ions to pass through membranes• Can be ion and direction specific• Types

A) Non-gated/leak channels: always open

B) Gated: open and close under specific conditions

A B

•Temperature•Ligands/chemical•Voltage•Mechanical distortion

Ligand Gated Channels

Closed open

CarriersA. Carriers (facilitated diffusion)

B. Ions pumps (also a type of carrier)

ATP

ATP

Ion pump

Characteristics of Carriers

• Can transport ions and polar molecules

• Specific

• Due to shape

• Can be Direction specific

• Some require/use ATP

• Can be activated and deactivated

or channel

Transport Proteins• Channel and carrier proteins are specific to

certain substances (i.e., different molecules move through different transport proteins).

• Which types of transport proteins and how many of each type is a very important aspect of what is able to move into and out of a cell.

• The transport proteins of individuals cells are the major influence on what is able to move into and out of different types of cells under different conditions.

Attachment Proteins

• Holds/attaches the PM to surrounding structures• E.g., Hold a cell to an extracellular structure• E.g., Connects plasma membrane to cytoskeleton

Cell A

Cell B

Connection/Anchoring/Attachment• Often contain a carbohydrate component (glycoproteins)• Cell to Cell connections• Cell to extracellular material• Cell to intracellular material

Recognition/Marker Proteins• Identify the cell

– E.g., “self”, to prevent immune response on cell

• Often have carb component (glycoproteins)– Glycoproteins

– Part of glycocalyx

I’m a liver cell I’m a kidney cell I’m a bone cell

Cell A Cell B Cell C

Marker Proteins can indicate cell type

Receptor Proteins• Binding sites for specific chemical messengers/signals (i.e., ligands):

– Typically polar messengers/signallers

• Specificity based on:– Shape

• Enables cellular communication/coordination and responsiveness to environment

Possible Actions of Receptor Proteins

• Opening/activating transport proteins

• Activate enzymes

• Activate DNA/proteins synthesis

• Activate movement of vesicles to the PM

Alters activity/behavior of receiving cell

I’ll stop secreting

Receptors and endocrine signaling

ADCB

Now, I’ll uptake nutrients

I’ll work faster

OK, I’ll start making proteins

Nervous & endocrine system signaling, both rely on membrane receptors to work

• Nervous system: receptors for neurotransmitters on post-synaptic cell

• Endocrine system: hormone receptors on target cells

Enzymes

• Perform chemical reaction on PM surface• Breakdown molecules• Create molecules

• Extracellular• intracellular

Membrane Carbohydrates: make the glycocalyx

• outermost/most external part of the cell’s membrane.

• Made of carbs bound to lipids (glycolipids) and proteins (glycoproteins)

• Functions in:

– cell identification (its type and in self/non-self)

– attachment of the cell to other cells/structures

Relationship between membrane components and cell function

Membrane Component Membrane Function it Enables

Phospholipids

Proteins

Carbohydrates •