Chapter 3

The Cellular Level of

Organization

Lecture slides prepared by Curtis DeFriez, Weber State University

• A cell is the basic unit of all living things. – Prokaryotic cells are simple cells that are Pro

(“before”) karyon (“nucleus”) – they have no nucleus.• Most are unicellular bacteria.

– Eukaryotic cells are complex cells with a nucleus and subcellular structures (organelles).• All fungi, plants, and animals are eukaryotes.

The Cell

• All eukaryotic cells are composed of three main parts:

1. Plasma membrane or “plasmalemma”

2. Cytoplasm - a gelatin-like substance, plus structural

fibers and organelles (but

not the nucleus)

3. Nucleus - contains the

genetic library of the cell

A Generalized Cell

1. The plasma membrane forms the cell’s outer boundary and separates the cell’s internal environment from the outside environment.– It is a selectively permeable barrier,

allowing the passage of some things and not others.

– It plays a role in cellular communication.

A Generalized Cell

2. The cytoplasm contains all the cellular contents between the plasma membrane and the nucleus. – The cytosol is the fluid portion (mostly water).– Organelles are subcellular

structures embedded in the cytosol, having characteristic shapes and specific functions.

A Generalized Cell

3. The nucleus is a large organelle that contains DNA in molecules called chromosomes.– Each chromosome consists of

a single molecule of DNA and associated packaging proteins.

– A chromosome contains thousands of hereditary units called genes.

A Generalized Cell

A Generalized Cell Fig. 3.1 shows a generalized body cell labeled

with the plasma membrane, cytoplasm (and organelles) and nucleus.

• The plasma membrane is much more than just a

“fence” – it is a flexible yet sturdy, “intelligent”

semipermeable regulator that:

– Covers and protects the cell

– Controls what goes in and comes out

– Links to other cells

– Flies certain “flags” to tell

other cells “who” it is

The Plasma Membrane

The Plasma Membrane• The Fluid Mosaic Model describes the

arrangement of molecules within the membrane: They resemble a sea of phospholipids with protein “icebergs” floating in it.– The lipids act as a barrier to certain polar substances.– The proteins act as

“gatekeepers”, allowing passage of specificmolecules and ions.

The Plasma MembraneInteractions Animation

• Membrane Functions

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The Plasma Membrane• The structure of the membrane

– Phospholipids form a lipid bilayer - cholesterol and

glycolipids (sugar-lipids) also contribute.

– Integral proteins - extend into or through the bilayer.

• Transmembrane proteins (most integral proteins) span the

entire lipid bilayer.

• Peripheral proteins attach to the inner or outer surface but

do not extend through the membrane.

The Plasma Membrane

The Plasma Membrane• Glycoproteins are membrane proteins with a

carbohydrate group attached that protrude into

the extracellular fluid.

• The Glycocalyx is the entire

“sugary coating” surrounding

the membrane (made up of

the carbohydrate portions

of the glycolipids and

glycoproteins).

The Plasma Membrane• The Functions of the membrane– Some integral proteins are ion channels.– Transporters - selectively move substances through

the membrane.– Receptors - for cellular recognition; a ligand is a

molecule that binds with a receptor.– Enzymes - catalyze chemical reactions– Others act as cell-identity markers.

The Plasma MembraneExamples of different membrane proteins

include

Ion channels

Carriers

Receptors

The Plasma MembraneExamples of different membrane proteins

include

Enzymes

Linkers

Cell identity markers

• Because of the distribution of lipids and the proteins embedded in it, the membrane allows some substances across but not others; this is called Selective permeability – Rule of thumb: small, neutrally-charged, lipid-

soluble substances can freely pass. Water is a special case - it is highly polar, yet still freely

permeable.

The Plasma Membrane

courtesy of Dr. Jim Hutchins

Membrane Permeability• For those substances that are needed by the cell

but for which the membrane is impenetrable (impermeable), transmembrane proteins act as channels and transporters. – They assist the entrance of certain substances that

either can’t pass at all (glucose) or for which the cell needs to hasten passage (ions).

Transport Processes• Passive processes involve substances moving

across the cell membranes without the input of any energy - they are said to move “with” or “down” their concentration gradient ([gradient] , where [ ] indicates “concentration”).

• Active processes involve the use of energy, primarily from the breakdown of ATP, to move a substance against its [gradient].

Transport Processes• Passive processes – Diffusion of solutes– Diffusion of water (called osmosis)– Facilitated diffusion (requires a specific channel or a

carrier molecule, but no energy is used)• Active processes – Various types of transporters are used, and energy is

required.

Passive Transport Processes• Diffusion is the passive spread of particles through random

motion, from areas of high concentration to areas of low

concentration.

– It is affected by the amount of

substance and the steepness of

the concentration gradient.

– Temperature

– Surface area

– Diffusion distance

Passive Transport Processes• Channel-Mediated Facilitated Diffusion • Carrier-Mediated Facilitated Diffusion

Passive Transport Processes• An example of Channel-Mediated Facilitated

Diffusion is the passage of potassium ions

through a gated K+ Channel

• An example of Carrier-Mediated Facilitated

Diffusion is the passage of glucose across the

cell membrane.

Passive Transport Processes• Osmosis is the net movement of water through a

selectively permeable membrane from an area of high water concentration to one of lower water concentration.

• Water can pass through plasma membrane in 2 ways:– through lipid bilayer by simple diffusion– through aquaporins (integral membrane proteins)

Passive Transport Processes

In the third tube, the force generated by the

movement of water from the left to the right

side is called osmotic pressure.

Passive Transport ProcessesIn the body, tonicity refers to the concentration

of salt solutions in the blood and elsewhere.

Since semipermeable membranes separate these

fluid compartments, osmosis of water is free to

occur between

any fluid space and another.

The effect of changing tonicity

is demonstrated in this graphic,

as water moves in and out

of red blood cells.

Active Transport ProcessesSolutes can also be actively transported across a

plasma membrane against their concentration

gradient ([low] to [high]) by using energy (usually

in the form of ATP).

The sodium-potassium pump is found in all

cells.

Secondary Active Transport Mechanisms• Antiporters carry two substances across the membrane in

opposite directions.

• Symporters carry two substances across the membrane in the same direction.

Transport in Vesicles

Vesicle - a small spherical sac formed by budding off from a membrane

Endocytosis - materials move into a cell in a vesicle formed from the

plasma membrane

three types: receptor-mediated endocytosis

phagocytosis

bulk-phase endocytosis (pinocytosis)

Exocytosis - vesicles fuse with the plasma membrane, releasing their

contents into the extracellular fluid

Transcytosis - a combination of endocytosis and exocytosis

Receptor-Mediated

Endocytosis

Phagocytosis

Bulk-phase Endocytosis

Transport• Transport Across the Plasma Membrane

Interactions Animation

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TransportInteractions Animations

• Transport Across the Plasma Membrane

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Cytoplasm - 2 Components

1. Cytosol - intracellular fluid, surrounding the organelles

- The site of many chemical reactions

- Energy is usually released by these reactions.

- Reactions provide the building blocks for cell maintenance,

structure, function and growth.

2. Organelles

- Specialized structures within the cell

• Network of protein filaments throughout the cytosol

• Provides structural support for the cell

The Cytoskeleton

• Types– Microfilaments

– Intermediate

filaments

– Microtubules

The Cytoskeleton

OrganellesCentrosome - located near the nucleus, consists

of two centrioles and pericentriolar material

Organelles

• Cilia - short, hair-like projections from the cell surface, move fluids along a cell surface

• Flagella - longer than cilia, move an entire cell; only example is the sperm cell’s tail

Organelles