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BSC 2010 - Exam I Lectures and Text Pages. I. Intro to Biology (2-29) II. Chemistry of Life Chemistry review (30-46) Water (47-57) Carbon (58-67) Macromolecules (68-91) III. Cells and Membranes Cell structure (92-123) Membranes (124-140) IV. Introductory Biochemistry

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bsc 2010 exam i lectures and text pages
BSC 2010 - Exam I Lectures and Text Pages
  • I. Intro to Biology (2-29)
  • II. Chemistry of Life
    • Chemistry review (30-46)
    • Water (47-57)
    • Carbon (58-67)
    • Macromolecules (68-91)
  • III. Cells and Membranes
    • Cell structure (92-123)
    • Membranes (124-140)
  • IV. Introductory Biochemistry
    • Energy and Metabolism (141-159)
    • Cellular Respiration (160-180)
    • Photosynthesis (181-200)
ways of crossing the plasma membrane
Ways of Crossing the Plasma Membrane
  • Passive Transport vs. Active Transport
  • Passive transport: pathways that do not involve energy expenditure from the cell (in the form of ATP)
    • Diffusion : Molecules naturally move from an area of higher concentration to one of lower concentration until equilibrium is reached. Rate can be affected by temperature, molecule size, and charge.
passive transport diffusion

(a)

Diffusion of one solute. The membrane

has pores large enough for molecules

of dye to pass through. Random

movement of dye molecules will cause

some to pass through the pores; this

will happen more often on the side

with more molecules. The dye diffuses

from where it is more concentrated

to where it is less concentrated

(called diffusing down a concentration

gradient). This leads to a dynamic

equilibrium: The solute molecules

continue to cross the membrane,

but at equal rates in both directions.

Molecules of dye

Membrane (cross section)

Equilibrium

Net diffusion

Net diffusion

Figure 7.11 A

PASSIVE TRANSPORT - Diffusion
  • Is the tendency for molecules of any substance to spread out evenly into the available space
diffusion

(b)

Diffusion of two solutes. Solutions of

two different dyes are separated by a

membrane that is permeable to both.

Each dye diffuses down its own concen-

tration gradient. There will be a net

diffusion of the purple dye toward the

left, even though the total solute

concentration was initially greater on

the left side.

Equilibrium

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Net diffusion

Figure 7.11 B

Diffusion
  • Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
effects of osmosis on water balance
Effects of Osmosis on Water Balance
  • Osmosis :  the diffusionof water through a semi-permeable membrane. Water moves from an area of higher water concentration to an area of lower water concentration to reach equilibrium on both sides of the membrane.
osmosis

Lower

concentration

of solute (sugar)

Higher

concentration

of sugar

Same concentration

of sugar

Selectively

permeable mem-

brane: sugar mole-

cules cannot pass

through pores, but

water molecules can

Water molecules

cluster around

sugar molecules

More free water

molecules (higher

concentration)

Fewer free water

molecules (lower

concentration)

Osmosis

Water moves from an area of higher

free water concentration to an area

of lower free water concentration

Figure 7.12

Osmosis
  • Is affected by the concentration gradient of dissolved substances
water balance of cells without walls
Water Balance of Cells Without Walls
  • Tonicity
    • Is the ability of a solution to cause a cell to gain or lose water.
    • Has a great impact on cells without walls.
    • Is always a comparison of relative solute concentrations between two solutions.
3 categories of relative concentration tonicity
3 Categories of Relative Concentration (Tonicity)
  • When comparing two solutions (A and B), solution A is …

* Hypotonic  to solution B if solution A has a lower concentration of solutes than B.

* Hypertonic to solution B if solution A has a higher concentration of solutes than B.

* Isotonic to solution B if the concentration of solutes is equal.

3 categories of relative concentration tonicity9
3 Categories of Relative Concentration (Tonicity)
  • Example:  A cell with a concentration of sodium at 1 g/L is placed in beaker of water with a sodium concentration of 10 g/L. How do we describe this?

---We can say that the cell is hypotonic to the water in the beaker OR we can say that the water is hypertonic to the cell.

Most cells are bathed in an isotonic solution, so there is no net osmosis occurring.

  • Be sure you understand these terms and read this section of your text.
isotonicity
Isotonicity
  • If a solution is isotonic to the cell.
    • The concentration of solutes is the same as it is inside the cell.
    • Therefore, the concentration of water is the same between the two solutions.
    • There will be no net movement of water.
hypertonicity
Hypertonicity
  • If a solution is hypertonic to the cell
    • The concentration of solutes is greater in the external solution than it is inside the cell.
    • Therefore the concentration of water is greater inside the cell than outside.
    • The cell will lose water to the external solution.
hypotonicity
Hypotonicity
  • If a solution is hypotonic
    • The concentration of solutes in the external solution is less than it is inside the cell.
    • Therefore, the concentration of water is greater outside the cell.
    • The cell will gain water from the external solution.
water balance in cells without walls

Hypotonic solution

Hypertonic solution

Isotonic solution

(a)

Animal cell. An

animal cell fares best

in an isotonic environ-

ment unless it has

special adaptations to

offset the osmotic

uptake or loss of

water.

H2O

H2O

H2O

Normal

Shriveled

Lysed

Figure 7.13

Water balance in cells without walls
  • Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments
    • Must have special adaptations for osmoregulation

H2O

water balance of cells with walls
Water Balance of Cells with Walls
  • Cell walls
    • Help maintain water balance
turgidity
Turgidity
  • If a plant cell is turgid
    • It is in a hypotonic environment
    • It is very firm, a healthy state in most plants
flaccidity
Flaccidity
  • If a plant cell is flaccid
    • It is in an isotonic or hypertonic environment
  • In a hypertonic environment, the cell may even become separated from the cell wall.
water balance in cells with walls

(b)

Plant cell. Plant cells

are turgid (firm) and

generally healthiest in

a hypotonic environ-

ment, where the

uptake of water is

eventually balanced

by the elastic wall

pushing back on the

cell.

H2O

H2O

H2O

H2O

Turgid (normal)

Flaccid

Plasmolyzed

Figure 7.13

Water balance in cells with walls

Hyptotonic Solution Isotonic Solution Hypertonic Solution

facilitated diffusion passive transport aided by proteins
Facilitated Diffusion: Passive Transport Aided by Proteins
  • In facilitated diffusion
    • Transport proteins speed the movement of molecules across the plasma membrane
  • Facilitated diffusion: Normal diffusion occurs, but through a protein channel in the membrane, not through the phospholipid bilayer. This also takes no added energy from the cell.
facilitated diffusion

EXTRACELLULAR

FLUID

Channel protein

Solute

CYTOPLASM

(a)

A channel protein (purple) has a channel through which

water molecules or a specific solute can pass.

Figure 7.15

Facilitated Diffusion
  • Channel proteins
    • Provide corridors that allow a specific molecule or ion to cross the membrane
carrier proteins

Solute

Carrier protein

(b)

A carrier protein alternates between two conformations, moving a

solute across the membrane as the shape of the protein changes.

The protein can transport the solute in either direction, with the net

movement being down the concentration gradient of the solute.

Figure 7.15

Carrier proteins
  • Carrier proteins
    • Undergo a subtle change in shape that translocates the solute-binding site across the membrane
active transport
ACTIVE TRANSPORT
  • Active transportuses energy (ATP) to move solutes against their gradients
  • Uses carrier proteins to help molecules across membrane

1. usually pumps one thing out of cell while pumping another into the cell. e.g. Na-K pump in nerve cells - pumps Na+ out and K+ in.

2. used to keep the concentration of a certain substance higher inside the cell than outside.  Very important in keeping trace element levels high enough in the cell though they may be low outside.

the sodium potassium pump

6

2

1

5

3

4

[Na+] high

[K+] low

Na+

Na+

Na+

Na+

Na+

ATP

[Na+] low

[K+] high

P

Na+

ADP

CYTOPLASM

Na+ binding stimulates

phosphorylation by ATP.

Cytoplasmic Na+ binds to

the sodium-potassium pump.

Na+

Phosphorylation causes the

protein to change its conformation, expelling Na+ to the outside.

Na+

Na+

K+

P

K+

K+ is released and Na+

sites are receptive again;

the cycle repeats.

K+

K+

K+

K+

Loss of the phosphate

restores the protein’s

original conformation.

Extracellular K+ binds to the

protein, triggering release of the

Phosphate group.

The sodium-potassium pump
  • The sodium-potassium pump
    • Is one type of active transport system

EXTRACELLULAR

FLUID

P

P i

Figure 7.16

maintenance of membrane potential by ion pumps
Maintenance of Membrane Potential by Ion Pumps
  • Membrane potential
    • Is the voltage difference across a membrane
  • An electrochemical gradient
    • Is caused by the difference in concentration of ions across a membrane
electrogenic pumps

EXTRACELLULAR

FLUID

+

ATP

+

H+

H+

Proton pump

H+

+

H+

H+

+

CYTOPLASM

+

H+

+

Electrogenic pumps
  • An electrogenic pump
    • Is a transport protein that generates the voltage across a membrane

Figure 7.18

slide25

+

H+

ATP

H+

+

H+

Proton pump

H+

+

H+

+

H+

Diffusion

of H+

Sucrose-H+

cotransporter

H+

+

Sucrose

+

Figure 7.19

Cotransport: Coupled Transport by a Membrane Protein

  • Cotransport:
    • active transport driven by a concentration gradient
    • occurs when active transport of a specific solute indirectly drives the active transport of another solute
bulk transport
Bulk Transport
  • Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
  • Large proteins and polysaccharides
    • Cross the membrane in vesicles
exocytosis and endocytosis
Exocytosis and Endocytosis
  • In exocytosis, transport vesicles migrate to the plasma membrane and fuse with it, becoming part of the plasma membrane. Then, vesicle contents are released to the outside of the cell = secretion
  • In endocytosis, the cell takes in macromolecules. New vesicles bud inward from the plasma membrane
three types of endocytosis

EXTRACELLULAR

FLUID

1 µm

CYTOPLASM

Pseudopodium

Pseudopodium

of amoeba

“Food” or

other particle

Bacterium

Food

vacuole

Food vacuole

An amoeba engulfing a bacterium via

phagocytosis (TEM).

In pinocytosis (cell drinking),

the cell “gulps” droplets of

extracellular fluid into tiny

vesicles. It is not the fluid

itself that is needed by the

cell, but the molecules

dissolved in the droplet.

Because any and all

included solutes are taken

into the cell, pinocytosisis nonspecific in the substances it transports.

PINOCYTOSIS

0.5 µm

Plasma

membrane

Pinocytosis vesicles

forming (arrows) in

a cell lining a small

blood vessel (TEM).

Vesicle

Three Types of Endocytosis
  • Phagocytosis, pinocytosis, receptor-mediated endocytosis

PHAGOCYTOSIS

In phagocytosis (cell eating),

a cell engulfs a particle by

wrapping pseudopodia

around it and packaging

it within a membrane-

enclosed sac large

enough to be classified

as a vacuole. The particle

is digested after the vacuole

fuses with a lysosome

containing hydrolytic enzymes.

Figure 7.20

slide29

Receptor-mediated endocytosis enables the

cell to acquire bulk quantities of specific

substances, even though those substances

may not be very concentrated in the

extracellular fluid. Embedded in the

membrane are proteins with

specific receptor sites exposed to

the extracellular fluid. The receptor

proteins are usually already clustered

in regions of the membrane called coated

pits, which are lined on their cytoplasmic

side by a fuzzy layer of coat proteins.

Extracellular substances (ligands) bind

to these receptors. When binding occurs,

the coated pit forms a vesicle containing the

ligand molecules. Notice that there are

relatively more bound molecules (purple)

inside the vesicle, other molecules

(green) are also present. After this ingested

material is liberated from the vesicle, the

receptors are recycled to the plasma

membrane by the same vesicle.

RECEPTOR-MEDIATED ENDOCYTOSIS

Coat protein

Receptor

Coated

vesicle

Coated

pit

Ligand

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis

(TEMs).

Coat

protein

Plasma

membrane

0.25 µm

review passive and active transport compared

Passive transport. Substances diffuse spontaneously

down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell.

The rate of diffusion can be greatly increased by transport

proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

ATP

Diffusion. Hydrophobic

molecules and (at a slow

rate) very small uncharged

polar molecules can diffuse through the lipid bilayer.

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,

either channel or carrier proteins.

Review: Passive and active transport compared
  • Review: Passive and active transport compared

Figure 7.17