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Chapter 3The 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
You must be connected to the internet to run this animation.
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
You must be connected to the internet to run this animation.
TransportInteractions Animations
• Transport Across the Plasma Membrane
You must be connected to the internet to run this animation.
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
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