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Chapter 4:

Cell Structure and Function

Cytoskeleton

• The cytoskeleton is a network

of fibers that organizes

structures and activities in the

cell.

�Microtubules (the largest)

�Intermediate fibers

�Microfilaments (the smallest)

�Think tube vs filaments – tubes are

always larger

Provide shape and support for the cell

MTs MFs IFs

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Other functions of the cytoskeleton

• Provide movement

�if the organism has flagellum or

cilia

• Intracellular movement

�Transports organelles, proteins

and other cell components along

microtubule “rails”.

Nucleus

• Only present in eukaryotic cells

• Location of the organism’s genome

�DNA

• Nuclear envelope

�Membrane that surrounds the nucleus

�Semipermeable

� does not allow DNA out, but messenger RNA does leave the nucleus.

Nucleolus

• Darkly stained regions within

the nucleus

• Ribosome assembly takes

place in this organelle.

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Ribosomes

• Ribosomes

�Protein factories

�Synthesizes most proteins.

• Two types

�Free ribosomes

�Found in all cells

�Ribosomes attached to the

endoplasmic reticulum (ER)

Endoplasmic reticulum (ER)

• Rough endoplasmic reticulum�Place where most

ribosomes attach

�Protein synthesis & transportation

• Smooth endoplasmic reticulum�Lipid production, including

steroids.

�In liver cells, it detoxify drugs and alcohol

�Also in liver cells, it breaks down

Golgi apparatus

• The warehouse of the cell

�Stores, sorts, and transports

proteins and other molecules

within the cell

�Directs transportation of

proteins and other molecules

to the outside of the cell.

�Also synthesizes cellulose

and pectin (plants)

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Lysosomes

• Made by the golgi

apparatus

• Highly acidic

environments

�Due to digestive enzymes.

• Digest waste particles &

malfunctioning

organelles.

Vacuoles

• Food vacuoles

�Transport food from cell

membrane to other parts of

the cell.

�This process is known as

phagocytosis.

Vacuoles

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Contractile vacuoles

• Freshwater protists need these vacuoles to pump excess

water out of the cell.

�Otherwise the cell continues to expand until it explodes.

Paramecium

Central vacuoles

• Found in plant cells

• Storage organelle

�Water

�May store proteins and carbohydrates

�Provides turgor, giving plants strength

�Wilted plants have little turgor due to lack of water

Plant turgor

Plant needs water to fill

central vacuoles.

Normal plant

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Mitochondria & chloroplasts

• Mitochondria are the

batteries of the cell.

�Produces a lot of ATP, the

energy currency of most

metabolic functions.

• Chloroplasts

�Found only in plants and

some protists

�Site of photosynthesis.

Chapter 5:

Membrane Structure and Function

Plasma Membrane: Thin barrier separating inside of cell(cytoplasm) from outside environment

Function:

1) Isolate cell’s contents from outside environment

2) Regulate exchange of substances between inside andoutside of cell

3) Communicate with other cells

Note: Membranes also exist within cells forming various compartments where different biochemical processes occur

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The Fluid Mosaic Model of Cellular Membranes:

Phospholipid Bilayer: Double layer of phospholipids

• Lipid tails of phospholipidsare unsaturated (C = C)

• Hydrophilic ends form outer border

• Hydrophobic tails form inner layer

Cell Membrane Proteins:1) Transport Proteins:

• Regulate movement of hydrophilic molecules throughmembrane

A) Channel Proteins (form pores; e.g., Na+ channels)

B) Carrier Proteins (binding sites; e.g., glucose transporter)

2) Receptor Proteins:

• Trigger cell activity when molecule from outsideenvironment binds to protein

3) Recognition Proteins:

• Allow cells to recognize / attach to one another

• Glycoproteins: Proteins with attached carbohydrategroups

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How are Substances Transported Across Membranes?

Answer: Concentration Gradients

Concentration = Number of molecules in a given unit ofvolume (e.g., grams / liter; moles / liter)

Gradient = Difference between two regions of spacesuch that molecules move from one region tothe other

Diffusion: Movement of molecules from an area of high concentration to an area of low concentration

• Greater the concentration gradient, the faster diffusion occurs

• Diffusion will continue until gradient eliminated (dynamic equilibrium)

• Diffusion cannot move molecules rapidly over long distances

Types of Movement Across Membranes (Table 4.1):

1) Passive Transport

• Requires no energy

• Substances move down concentration gradients

A) Simple Diffusion

• Small molecules pass directly through the phospholipidbilayer (e.g., CO2, H2O, O2)

(Figure 4.3a)

Rate depends on:

1) Molecule size

2) Concentration gradient

3) Lipid solubility

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Types of Movement Across Membranes:

1) Passive Transport

• Requires no energy

• Substances move down concentration gradients

B) Facilitated Diffusion

• Molecules need assistance of channel proteins orcarrier proteins (e.g. ions, amino acids, sugars)

(Figure 4.3b)Channel Proteins

(Figure 4.3c)Carrier Proteins

Channel and Carrier proteins

Types of Movement Across Membranes:

1) Passive Transport

• Requires no energy

• Substances move down concentration gradients

C) Osmosis

• Movement of water from an area of high [water] to area of low [water] across semi-permeable membrane

water

A) Simple Diffusion

B) Facilitated Diffusion

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Osmosis:

water

In which direction will there be a net osmotic

movement of water?

� out of the cell

� into the cell

� neither

0.05 M glucose0.02 M glucose

0.01 M sucrose

Permeable

only to

water

In which direction will there be a net osmotic

movement of water?

� out of the cell

� into the cell

� neither

0.05 M glucose0.02 M glucose

0.03 M sucrose

Permeable

only to

water

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Osmosis and Living Cells:

Isotonic Solution:

• Outside of cell has same [solute] as inside of cell

(no net water movement)

Iso = same.

Isotonic = Same solute concentration inside and outside of the cell.

Osmosis and Living Cells:

Isotonic Solution:

• Outside of cell has same [solute] as inside of cell

Hypertonic Solution:

• Outside of cell has higher [solute] than inside of cell

(net water movement

out of cell)

• Hyper = higher

Osmosis and Living Cells:

Isotonic Solution:

• Outside of cell has same [solute] as inside of cell

Hypertonic Solution:

• Outside of cell has higher [solute] than inside of cell

(net water movement into cell)

Hypotonic Solution:

• Inside of cell has higher [solute] than outside of cell

Tonicity is relative to the inside of the cell

Hypo = lower

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Water balance in cells without walls

Solution is:

hypotonic

lyse

hypertonic

shrivel

isotonic

normal

hypotonic

Water balance in cells with walls

isotonic hypertonic

turgid flaccid shriveled

Types of Movement Across Membranes:

1) Passive Transport

2) Active Transport (aka pumps)

• Requires energy (ATP)

• Substances move against concentration gradients

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3) Endocytosis

• Movement of large particles into cells (vesicle formation)

1) Pinocytosis (“cell drinking”)

• Uptake of fluid droplets

Types of Movement Across Membranes:

1) Passive Transport

2) Active Transport

3) Phagocytosis (“cell eating”)

• Uptake of large particles (e.g. bacteria)

2) Receptor-mediated Endocytosis

• Uptake of specific molecules via coated pits

(Figure 4.7)

4) Exocytosis

• Movement of large particles out of cells (e.g., hormones)

3) Endocytosis

Types of Movement Across Membranes:

1) Passive Transport

2) Active Transport

(Figure 4.9)

Exocytosis

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How are Cell Surfaces Specialized?

Answer: Junctions allow cells to connect and communicate

1) Connection Junctions:

A) Desmosomes: Hold cells together via protein filaments

(Figure 4.10a)

How are Cell Surfaces Specialized?

Answer: Junctions allow cells to connect and communicate

1) Connection Junctions:

B) Tight Junctions: Protein “seals” prevent leakage (cell →cell)

(Figure 4.10b)

How are Cell Surfaces Specialized?

Answer: Junctions allow cells to connect and communicate

2) Communication Junctions:

A) Gap Junctions: Protein channels allowing for signals topass between cells (animals)

(Figure 4.11a)

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How are Cell Surfaces Specialized?

Answer: Junctions allow cells to connect and communicate

2) Communication Junctions:

A) Plasmodesmata: Cytoplasmic bridges allowing for signals to pass between cells (plants)

(Figure 4.11b)

How are Cell Surfaces Specialized?

Answer: Cell walls offer support and protection

• Composed of carbohydrates (e.g. cellulose, chitin), proteins, or inorganic molecules (e.g. silica)

• Produced by the cell it protects/supports

Cell Walls:

• Found in bacteria, plants, fungi, & some protists

Exam on Thursday!

• Bring scantrons and 2 pencils in case one breaks

• Covers Chapters 1 – 5.

• Will be multiple choice, short answer, and 1 or 2

essay questions.

• Study Chapter 3 particularly carefully, it’s the

biggest chapter!