Introduction to CellsIntroduction to Cells&&

The Cell MembraneThe Cell Membrane

Chapter 4

OverviewOverview

• Cell structure & function

• Cell membrane structure & function

• How cells interact

Cell TheoryCell Theory

1. Organisms consist of 1 or more cells.

2. Cell = smallest unit of life

3. Continuity of life comes from growth & division of cells

What is a Cell?What is a Cell?

Made of C, H, O, N + trace elements

Has all the properties of life:– Metabolism

– Responsiveness– Growth

– Reproduction

Cells differ in size, structure, & function

Cell FunctionCell Function

Performs all vital physiological functions

Maintains homeostasis via processes happening within

Building blocks of all organisms

Generalized Cell StructureGeneralized Cell Structure

Plasma membrane

Region of DNA

Cytoplasm

Plasma MembranePlasma Membrane

Lipid bilayer– Separates internal & external environments

Semi-permeable– Controls passage of substances in/out of cell

Region of DNARegion of DNA

Cellular control centre

Holds genetic information

Eukaryotic cells:– Inside membrane-bound

nucleus

Prokaryotic cells:– Free-floating in cytoplasm

CytoplasmCytoplasm

“Guts” of cell

Semi-fluid matrix– Colloidal properties

Contains all structural components

(organelles, etc.)

Why Are Cells So Small?Why Are Cells So Small?

Surface area:volume ratio

Volume increases with cube of diameterV = 4/3πr3

SA increases with square of diameterSA = 4πr2

Diameter (cm)

0.5 1.0 1.5

SA (cm2) 0.79 3.14 7.07

Volume (cm3)

0.06 0.52 1.77

SA:volume ratio

13:1 6:1 4:1

If cells were big:

– Plasma membrane would have to work very hard to service all of cytoplasm

– Materials would have a harder time moving through the cytoplasm

– Movement of substances across membrane would not be fast enough to maintain cell activity

Cells that aren’t tiny are usually long & thin or have folds to increase SA

Cell (Plasma) MembraneCell (Plasma) Membrane

Semi-permeable barrier between interior of cell & external environment

Fluid-mosaic model:

Lipid bilayer(prevents movement of water-soluble substances)

Dynamic pattern of proteins (some able to move through membrane)

(involved in membrane function)

Lipid BilayerLipid Bilayer

Consists of:

– Phospholipids

– Glycolipids

– Cholesterol

– Lipid Rafts

Membrane Lipids: PhospholipidsMembrane Lipids: Phospholipids

Hydrophilic (polar) head

Hydrophobic (non-polar) fatty acid tails(unsaturated = kinks in tails ↑ membrane fluidity)

Biological membranes naturally form closed, spherical structures that can

reseal quickly if torn

Membrane Lipids: GlycolipidsMembrane Lipids: Glycolipids

5% of membrane lipids

Phospholipids with sugar groups

Found only on outer membrane surface(cell signalling & recognition)

Membrane Lipids: CholesterolMembrane Lipids: Cholesterol

20% of membrane lipids

Wedges between phospholipid tails

Has both hydrophobic & hydrophilic regions

Maintains integrity of membrane by keeping it firm & impermeable to some water-soluble molecules

Increases fluidity of membrane by ensuring that fatty acid chains don’t

crystallize

Membrane Lipids: Lipid RaftsMembrane Lipids: Lipid Rafts20% of membrane lipids

Found only on outer membrane surface

Variety of tightly packed saturated lipids(= stable, less fluid)

Used as concentrating platforms for cell signalling molecules

Overview of Membrane ProteinsOverview of Membrane Proteins50% of membrane mass

Carry out most membrane functions

• Integral proteins

• Peripheral proteins

Overview: Integral ProteinsOverview: Integral Proteins

Most are transmembranal

Have both hydrophobic & hydrophilic regions

Used mainly for transport(channels, carriers, receptors)

Overview: Peripheral ProteinsOverview: Peripheral ProteinsUsually located at one membrane surface

Attach loosely to integral proteins or membrane lipids

Functions include: structural support for cell, enzymatic action, joining cells, changing cell

shape

Major Membrane ProteinsMajor Membrane Proteins

Receptor proteins

Recognition proteins

Adhesion proteins

Communication proteins

Transport proteins(passive & active transporters)

1. Receptor Proteins1. Receptor Proteins

Binding sites for hormones, etc.

Allow changes in cell activities(protein synthesis, cell division, etc.)

Different cells have different receptor proteins

2. Recognition Proteins2. Recognition Proteins

In multicellular organisms

Identify cells as foreign or self

Used in tissue defense, cell adhesion, etc.

3. Adhesion Proteins3. Adhesion Proteins

In multicellular organisms

Allow cells of same type to find & stick to each other or to other substances

(e.g. proteins in EC matrix)

4. Communication Proteins4. Communication Proteins

Multicellular organisms

Form channels between cytoplasm of 2 cells

Allow chemical & electrical signals to flow between

cells

5. Transport Proteins5. Transport Proteins

Have interior channels

Solute enters channel & binds weakly to protein

Protein changes shape

Channel closes behind solute & opens in front of solute

Solute is released on other side

Protein regains normal shape

5a. Passive Transport Proteins5a. Passive Transport Proteins

Move solutes & water across membrane down concentration

gradients

No energy input required

One-way or bi-directional

Some are ion-selective channels:(have gates that open/close depending

on molecular, chemical, etc. signal)

5b. Active Transport Proteins5b. Active Transport ProteinsPump solutes across membrane against

concentration gradients

Require energy input

One-way or bi-directional

Some are co-transporters:(allow passive transport of some solutes while pumping

others in the opposite direction)

So What is a Concentration Gradient?So What is a Concentration Gradient?

Different in concentration of ions or molecules between 2 adjacent areas

With no energy input, molecules move down gradient from [high] to [low]

DiffusionDiffusion

Net movement of ions/molecules down concentration gradient

Each ion/molecule has its own gradient

Factors Affecting Diffusion RateFactors Affecting Diffusion Rate1. Steepness of concentration gradient

2. Temperature

3. Size of ions/molecules

4. Electric gradients

5. Pressure gradients

So … the cell membrane is semi-permeable

= allows passage of some substances but not of others

What controls what, how much, & when substances cross the membrane?

Membrane is mostly non-polar

= allows small, non-polar molecules to cross (e.g. O2, CO2, etc.)

= impermeable to ions & large, polar molecules (e.g. glucose, Na+, K+, etc.)

Water (although polar) can slip through gaps caused by kinks in tails or can use

aquaporin transporters

Movement MechanismsMovement Mechanisms

Passive transport – Simple diffusion

– Facilitated diffusion

Active transport

Exocytosis

Endocytosis

1. Passive Transport1. Passive Transport

Net movement down concentration gradient

No ATP energy required

1a. Simple Diffusion1a. Simple Diffusion

Direct diffusion through membrane

Non-polar, lipid-soluble, & small molecules

e.g. O2, CO2, fat-soluble vitamins

e.g. [O2] in blood is higher than in cell, so continuously diffuses in

1b. Facilitated Diffusion1b. Facilitated Diffusion

Substances transported passively via channels/carriers (proteins)

Some channels are always open; others open & close on cue

Transport limited by # & activity of channels/carriers

2. Active Transport2. Active Transport

Pumps solutes against concentration gradient

ATP energy required

Substrate-specific transporters(activated by phosphate group from ATP)

e.g. sodium-potassium pumpe.g. sodium-potassium pump

• Uses carrier enzyme Na+K+ATPase

• [K+] ↑ inside cell, [Na+] ↑ outside cell

• Both seep through leakage channels down concentration gradients

• Na+K+ pump drives Na+ out & pumps K+ in

Now we know how ions & molecules cross the selectively permeable plasma

membrane.

What about water?

Osmoregulation

= control of water balance

Prevents excessive uptake / loss of water

e.g. fish have gills & kidneys

OsmosisOsmosis

Diffusion of water across semi-permeable membrane from [high] to [low]

How can water have a concentration?

[H2O] ↑ as [dissolved solute ] ↓

In other words, the more dilute a solution is, the more concentrated the water is

TonicityTonicity

Relative solute concentrations of two fluids e.g. on opposite sides of a membrane

Determines the direction & how much water movement will occur across a membrane

When comparing two fluids:

Hypotonic solution:= the one with fewer solutes

Hypertonic solution:= the one with more solutes

Isotonic solutions:= have the same concentrations

Water diffuses from hypotonic fluids to hypertonic fluids

a. Hypotonic Animal Cella. Hypotonic Animal Cell

b. Hypertonic Animal Cellb. Hypertonic Animal Cell

c. Isotonic Animal Cellc. Isotonic Animal Cell

So Why Don’t Animal Cells Burst Easily?So Why Don’t Animal Cells Burst Easily?

As H2O enters cell via osmosis, solutes are transported out

= osmoregulation

Hydrostatic pressure against the membrane also controls osmosis

Note: there is a point where cells will lyse (burst)

Because of their rigid cell walls, plant cells face a different scenario

Plant Cells & Osmoregulation

Isotonic solution

= becomes flaccid & wilts

Hypertonic solutionHypertonic solution

= shrivels= shrivels

= = plasmolysisplasmolysis

(plasma membrane (plasma membrane separates from cell wall)separates from cell wall)

Become turgid

Net inflow of water

Cell wall expands but does not burst

Plant cells are happiest in hypotonic Plant cells are happiest in hypotonic environmentsenvironments

Other Transport MechanismsOther Transport Mechanisms

Large particles & other substances can be moved between the external environment, the plasma membrane, & the interior of the

cell via:

• Exocytosis

• Endocytosis

Remember:

Membranes are self-sealing

Hydrophobic interactions between phospholipid tails & water molecules

creates spherical structures

1. Exocytosis1. Exocytosis

Vesicle w/i cytoplasm moves to cell membrane

Fuses with membrane

Releases contents to exterior of cell

2. Endocytosis2. Endocytosis

Outer membrane inpouches around particle outside of cell

Contents released to cell interior(can then be moved to or stored in organelles)

3 types:– Receptor-mediated endocytosis

– Phagocytosis– Bulk-phase endocytosis

2a. Receptor-Mediated Endocytosis2a. Receptor-Mediated Endocytosis

Substance (hormone, vitamin, mineral, etc.) binds to receptors on membrane

Pit forms beneath receptor

Pit sinks into cytoplasm, forming vesicle

2b. Phagocytosis2b. Phagocytosis

“Cell eating”

e.g. amoebas, macrophages, etc.

Pseudopods extend around substance, forming vesicle

Vesicle moves into cytoplasm & fuses with lysosome, which digests contents of vesicle

2c. Bulk-Phase Endocytosis2c. Bulk-Phase Endocytosis

Non-selective process

Vesicle forms around ECF & carries it into cytoplasm

Doesn’t the continuous formation of vesicles drastically change the surface area of the

cell membrane?

Endocytosis & exocytosis occur at rates that maintain the total SA of the plasma

membrane

Losses via endocytosis ≈ replacements via exocytosis

Cell JunctionsCell Junctions

Some cells are free-floating

Most knit together

Molecular structures allow communication b/w cells:Plant cells: plasmodesmata

Animal cells: tight junctions, desmosomes, gap junctions

a. Plasmodesmataa. Plasmodesmata

Channels

Connect cytoplasms of 2 adjacent cells

Allow rapid exchange of materials

b. Tight Junctionb. Tight Junction

Integral proteins of adjacent cells fuse

Impermeable to leakage between cells

Join cells of most body tissuese.g. stomach lining

c. Desmosomesc. Desmosomes

a.k.a. adhering junction

Zipper-like seal

Binds cells together internally & externally by protein filaments

Distributes tension evenly to ↓ risk of tearing

e.g. skin, heart muscle, neck of uterus

d. Gap Junctionsd. Gap Junctions

Channels that connect cytoplasms of 2 adjacent cells

Allow rapid exchange of materials

Occur in electrically-excitable tissues

e.g. heart muscle, smooth muscle

(allows synchronicity of electrical activity & contraction)

Cell-Environment InteractionsCell-Environment Interactions

Membrane receptors

Integral proteins & glycoproteins used as binding sites

– Contact signalling– Chemical signalling

a. Contact Signallinga. Contact Signalling

Physical contact between cells

Glycocalyx= glycoproteins with branching

sugar sidechains

= unique to each cell type

Aids in cell recognition

b. Chemical Signallingb. Chemical Signalling

Ligands= signalling chemicals that bind to membrane

receptors(include neurotransmitters, hormones, etc.)

Receptors sense molecules outside cell & activate signal transduction pathways that

lead to cellular responses

Many receptors involved in diseases but also used as targets for drugs

e.g. G-protein linked receptorse.g. G-protein linked receptors• Ligand (1st messenger) binds to

receptor

• Receptor activates G-protein

• G-protein stimulates effector protein (enzyme)

• Effector protein produces 2nd messenger inside cell

• 2nd messenger activates kinase enzymes

• Kinases activate other enzymes → various cellular responses