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

1 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.

2 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)

3 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

4 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 Normal plant central vacuoles.

5 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 and outside of cell

3) Communicate with other cells

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

6 The Fluid Mosaic Model of Cellular Membranes:

Phospholipid Bilayer: Double layer of phospholipids

• Hydrophilic ends form outer border • Hydrophobic tails form inner layer

• Lipid tails of phospholipids are unsaturated (C = C)

Cell Membrane Proteins: 1) Transport Proteins: • Regulate movement of hydrophilic molecules through membrane 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 outside environment binds to protein 3) Recognition Proteins: • Allow cells to recognize / attach to one another • Glycoproteins: Proteins with attached carbohydrate groups

7 How are Substances Transported Across Membranes? Answer: Concentration Gradients

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

Gradient = Difference between two regions of space such that molecules move from one region to the 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 phospholipid

bilayer (e.g., CO 2, H 2O, O 2)

Rate depends on: 1) Molecule size 2) Concentration gradient 3) Lipid solubility

(Figure 4.3a)

8 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 or carrier proteins (e.g. ions, amino acids, sugars)

Channel Proteins (Figure 4.3b)

Carrier Proteins (Figure 4.3c)

Channel and Carrier proteins

Types of Movement Across Membranes: 1) Passive Transport A) Simple Diffusion • Requires no energy B) Facilitated Diffusion • 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

9 Osmosis:

water

In which direction will there be a net osmotic movement of water?
out of the cell
into the cell
neither

Permeable 0.02 M glucose 0.05 M glucose only to water 0.01 M sucrose

In which direction will there be a net osmotic movement of water?
out of the cell
into the cell
neither

Permeable 0.02 M glucose 0.05 M glucose only to water 0.03 M sucrose

10 Osmosis and Living Cells:

Isotonic Solution: • Outside of cell has same [solute] as inside of cell

Iso = same.

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

(no net water movement)

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

• Hyper = higher

(net water movement out of 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 Hypotonic Solution: • Inside of cell has higher [solute] than outside of cell

Hypo = lower Tonicity is relative to the inside of the cell

(net water movement into cell)

11 Water balance in cells without walls Solution is:

hypotonic isotonic hypertonic

lyse normal shrivel

Water balance in cells with walls

hypotonic 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

12 (Figure 4.7) Types of Movement Across Membranes: 1) Passive Transport 2) Active Transport 3) Endocytosis • Movement of large particles into cells (vesicle formation) 1) Pinocytosis (“cell drinking”) • Uptake of fluid droplets 2) Receptor-mediated Endocytosis • Uptake of specific molecules via coated pits 3) Phagocytosis (“cell eating”) • Uptake of large particles (e.g. bacteria)

Types of Movement Across Membranes: 1) Passive Transport 2) Active Transport 3) Endocytosis 4) Exocytosis • Movement of large particles out of cells (e.g., hormones)

(Figure 4.9)

Exocytosis

13 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 to pass between cells (animals)

(Figure 4.11a)

14 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 Cell Walls: • Found in bacteria, plants, fungi, & some protists • Composed of carbohydrates (e.g. cellulose, chitin) , proteins, or inorganic molecules (e.g. silica) • Produced by the cell it protects/supports

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!

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