Sumaira Sarwar Khan SENIOR LECTURER

DEPARTMENT OF BIOCHEMISTRY

IM&DC

Cell Structure & Function

cell

Definition of Cell

A cell is the smallest unit that is capable of performing life functions.

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Cell Theory

• All living things are made up of cells. • Cells are the smallest working units of all living

things. • All cells come from preexisting cells through

cell division.

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Examples of Cells

Amoeba Proteus

Plant Stem

Red Blood Cell

Nerve Cell

Bacteria

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Two Types of Cells

•Prokaryotic•Eukaryotic

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Prokaryotic

• Do not have structures surrounded by membranes

• Few internal structures

• One-celled organisms, Bacteria

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Eukaryotic• Contain organelles surrounded by membranes• Most living organisms

Plant Animal

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Difference between prokaryotic and eukaryotic cell

• Prokaryotic• Single membrane• No well defined nucleus • No membrane bounded

organelles• Lysosome absent• RNA and Protein

synthesis in same compartment

• Eukaryotic• Bilayer membrane• Nucleus containing DNA

+ histones• Membrane bounded• Lysosome present• RNA in nucleous and

proteins in cytosol

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Cell PartsOrganelles

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• In the cell the biomolecules like nucleic acids, proteins, lipids and carbohydrates are organized into complex structures called the organelles

• These organelles are the functional structures of the cell

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Arrangements inside a typical eukaryotic cell

• cell

• protoplasm (organic+inorganic matter)

• cell inclusions cell organelles

storage granules membranous starch and Glycogen granules nonmembranous

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• Membranous non membranous

• Nucleus ribosomes• Mitochondria chromosomes• ER centrioles• Golgi apparatus• Lysosome

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“Typical” Animal Cell

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http://waynesword.palomar.edu/images/plant3.gif

“Typical” Plant Cell

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Surrounding the Cell

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Cell Membrane

• Outer membrane of cell that controls movement in and out of the cell

• Double layer • The plasma membrane

regulates what enters and exits the cell.

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PLASMA MEMBRANE

• Maintains cellular stability and integrity• Is a protective and selective barrier• Controls and directs cellular activity• The plasma membrane borders the entire cell

separating the internal environment from the external environment

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2007-2008

The Cell Membrane

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Overview• Cell membrane separates living cell from nonliving

surroundings– thin barrier = 8nm thick

• Controls traffic in & out of the cell– selectively permeable– allows some substances to cross more easily than

others• hydrophobic vs hydrophilic

• Made of phospholipids, proteins,carbohydrate

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Structure – Organization of the Plasma Membrane

• Lipids • Proteins• Carbohydrates

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Lipids in plasma membrane

1) Phospholipids (glycerol backbone)2) Sphingolipids (sphingosine base)3) Sterols (non-polar & only slightly water soluble4) glycolipids

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Phospholipids

• A special type of lipid• 2 fatty acid chains

making a hydrophobic tail.

• Chains have no charge and are non-polar / insoluble.

• Phosphate group is hydrophilic = has a charge so is soluble.

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Phospholipids

Fatty acid

Phosphate

• Fatty acid tails– hydrophobic

• Phosphate group head – hydrophilic

• Arranged as a bilayer

Phospholipid bilayer

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

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Cholesterol

Cholestrol helps to keep the lipid layer free to move.

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Membrane fat composition varies• Fat composition affects flexibility– membrane must be fluid & flexible– % unsaturated fatty acids in phospholipids• keep membrane less viscous

– cholesterol in membrane

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More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed

that membrane proteins are inserted into the phospholipid bilayer

It’s like a fluid…It’s like a mosaic…It’s the Fluid Mosaic Model!

Fluid-mosaic Model of Plasma Membrane Structure

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Membrane Proteins• Proteins determine membrane’s specific functions– cell membrane & organelle membranes each have unique

collections of proteins

• Membrane proteins:– peripheral proteins

• loosely bound to surface of membrane• cell surface identity marker (antigens)

– integral proteins • penetrate lipid bilayer, usually across whole membrane • transmembrane protein• transport proteins

– channels, permeases (pumps)cell

Proteins domains anchor molecule• Within membrane– nonpolar amino acids • hydrophobic • anchors protein

into membrane

• On outer surfaces of membrane– polar amino acids • hydrophilic• extend into extracellular

fluid & into cytosol

Polar areasof protein

Nonpolar areas of protein

Mosaic• Combination of proteins

makes membrane unique• Membrane proteins may

be fluid or anchored• Proteins may penetrate the bilayer fully (integral) or

reside on the surfaces of membranes (peripheral)• Integral proteins typically have hydrophobic regions

that span the bilayer as a result of nonpolar amino acids arranged as helices

• Anchored proteins strengthen membranes

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Membrane is a nexus of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

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Types of Membrane Proteins

– Channel Proteins– Carrier Proteins– Cell Recognition Proteins– Receptor Proteins– Enzymatic

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Many Functions of Membrane ProteinsOutside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

Membrane carbohydrates • Play a key role in cell-cell recognition– ability of a cell to distinguish one cell from

another• antigens

– important in organ & tissue development

– basis for rejection of foreign cells by immune system

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Carbohydrates diversify membranes

• Membrane carbohydrates are only found on the outside (external) face of membranes

• Attach to lipids or protein (glycolipid/ glycoprotein)

• Enable cells to distinguish/ recognize one another

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Carbohydrates present

• The carbohydrate structure is called the glycocalyx

• These contain• galactose • Mannose• Fucose• N-acetyl glucose amine• N-acetyl neuraminic acid

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Any Questions??

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Movement across the Cell Membrane

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Fluidity• Membranes are maintained by

hydrophobic interactions of the phospholipids resulting in the alignment of the polar phosphate regions toward the aqueous environment and the nonpolar regions’ hydrocarbon chains toward each other.

• Membranes are in motion with fast drifting lipids and slower drifting proteins

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•Membrane fluidity may be influenced by presence/absence of unsaturated FA chains and Cholesterol•Fluidity of membranes is important for proper function

Permeability of the Plasma Membrane

1. Diffusion– Diffusion is the movement of molecules from an

area of higher to lower concentration– Oxygen and Carbon Dioxide are two gases that

can diffuse through the plasma membrane

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Diffusion• Move from HIGH to LOW concentration– “passive transport”– no energy needed

diffusion osmosis

movement of water

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Diffusion across cell membrane• Cell membrane is the boundary between

inside & outside…– separates cell from its environment

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2

H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

Diffusion through phospholipid bilayer• What molecules can get through directly?– fats & other lipids

inside cell

outside cell

lipid

salt

aa H2Osugar

NH3

What molecules can NOT get through directly?

polar molecules H2O

ions salts, ammonia

large molecules starches, proteins

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Channels through cell membrane• Membrane becomes semi-permeable with

protein channels – specific channels allow specific material across

cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

Facilitated Diffusion

• Diffusion through protein channels– channels move specific molecules across

cell membrane– no energy needed

“The Bouncer”“The Bouncer”

open channel = fast transport

facilitated = with help

high

low

Active Transport

“The Doorman”“The Doorman”

conformational change

• Cells may need to move molecules against concentration gradient– shape change transports solute from

one side of membrane to other – protein “pump”– “costs” energy = ATP

ATP

low

high

Getting through cell membrane• Passive Transport– Simple diffusion

• diffusion of nonpolar, hydrophobic molecules– lipids– high low concentration gradient

– Facilitated transport• diffusion of polar, hydrophilic molecules• through a protein channel

– high low concentration gradient

• Active transport– diffusion against concentration gradient

• low high– uses a protein pump– requires ATP

ATP

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Permeability of the Plasma Membrane

-Transport by Carrier Proteins – Carrier proteins combine with a molecule or ion

to be transported across the membrane.

– Carrier proteins are required for:• Facilitated Transport• Active Transport

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Facilitated Transport

• Small molecules that are not lipid-soluble• Molecules combine with carrier proteins• Molecules follow the concentration gradient• Energy is not required

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Permeability of the Plasma Membrane

-Active Transport– Small molecules– Molecules combine with carrier proteins– Molecules move against the concentration

gradient– Energy is required

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The Sodium-Potassium Pump

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Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP

Permeability of the Plasma Membrane

3-Osmosis– Osmosis is the diffusion of water across a

differentially permeable membrane.– Osmotic pressure is the pressure that develops

in a system due to osmosis.

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Isotonic:the solute concentration is equal inside and outside

of a cell

Hypotonic: a solution has a lower solute concentration than the inside of a cell

Hypertonic: a solution has a higher solute concentration than the inside

of a cell

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How about large molecules?• Moving large molecules into & out of cell– through vesicles & vacuoles– endocytosis• phagocytosis = “cellular eating”• pinocytosis = “cellular drinking”

– exocytosis

exocytosiscell

– 4-Endocytosis –

Vesicles form as a way to transport molecules into a cell

• Phagocytosis: Large,particulate matter

• Pinocytosis: Liquids and small particles dissolved in liquid

• Receptor Mediated Endocytosis: A type of pinocytosis that involves a coated pit

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Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specificprocess

triggered bymolecular signal

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