CELL MEMBRANE, CYTOSKELETON

1
CELL MEMBRANE, CYTOSKELETON CELL-CELL
INTERACTIONS

• Chapter 05

Cells are the most basic units of life because
they exhibit all of the characteristics of
life. This is a picture of a human white blood
cells (T lymphocyte). Its inner skeleton and
surface features enable it to move in the body
and to recognize foreign cell surfaces.
2

• A. Cell Membrane Structure
• Cell membrane is a phospholipid bilayer embedded
  with mobile proteins.

Phosphate head of phospholipid is
hydrophilic. Fatty acid tails are hydrophobic.
Proteins
Hydrophilic - water loving Hydrophobic - water
fearing
3

• Types of membrane proteins
• Transport proteins - move substances across
  membrane
• Cell surface proteins - establish self
• Cellular adhesion molecules (CAMs) - enable cells
  to stick to each other
• Receptor proteins - receive transmit messages
  into a cell

4

• Additional molecules may be associated with
  proteins phospholipids
• cholesterol
• sugar molecules
• glycoproteins
• glycolipids

Glycolipids glycoproteins function in cell
recognition. Note plant cells do not have
carbohydrates extending from the
protein-phospholipid bilayer.
5

• B. Movement Across Membranes
• Cell membranes are selectively permeable.
• 1. Simple diffusion (passive)
• Substance moves across phospholipids from an area
  of high to an area of low concentration, without
  using energy.
• Substance moves down its concentration
  gradient
• Ex. O2, CO2

Selectively permeable - allow some substances to
pass through membrane, while preventing others.
6

• Simple Diffusion

Cell membrane
Transport protein
Diffusion continues until dynamic equilibrium is
reached.
Notice that transport proteins are not involved
in simple diffusion. Dynamic equilibrium - point
of equal movement back forth across membrane
no NET movement of molecules.
7

• Movement of water across membranes by simple
  diffusion is called osmosis.
• Water is driven to move because the membrane is
  impermeable to solute(s).

8

• Tonicity
• Refers to differences in solute concentration on
  either side of a semipermeable membrane.
• Isotonic - both solutions have the same solute
  concentrations.
• Hypotonic - solution with the lower solute
  concentration.
• Hypertonic - solution with the higher solute
  concentration.

Solutions are composed of solutes solvents.
solute - substance dissolved in a solution.
solvent - the dissolving agent. Most versatile
dissolving agent is water.
9

• What is effect of immersing an animal cell in a
  hypertonic or hypotonic solution?

10

• Plasma is normally isotonic to cytoplasm of RBC.
  Cell
• is in dynamic equilibrium with plasma maintains
  its shape.
• If RBC is placed in a hypertonic solution, solute
  
• concentration is greater in solution than inside
  the cell.
• Since RBC membrane is impermeable to solutes,
  water is
• driven to move. Thus, water tends to leave the
  cell to dilute
• the outside solute. The cell shrinks.
• If RBC is placed in a hypotonic solution, solute
  concentration
• is greater in cytoplasm of RBC. Here again,
  water is driven
• to move because the RBC membrane is impermeable
  to solutes.
• Thus, water tends to enter the cell. The cell
  swells may even
• burst.
• Note Some single-celled protists (paramecium)
  live in fresh
• water. They use structures called contractile
  vacuoles to rid
• themselves of excess water that is continuously
  diffusing
• inward.

11

• What is effect of immersing a plant cell in a
  hypertonic or hypotonic solution?

Cell immersed in hypotonic solution
Cell immersed in hypertonic solution
12
If plant cell is placed in a hypertonic solution,
solute concentration is greater in solution than
inside the cell. Since plant cell membrane is
impermeable to solutes, water is driven to move.
Thus, water tends to leave the cell to dilute
the outside solute. The cell shrinks pulls
away from the cell wall. Plant wilts. If plant
cell is placed in a hypotonic solution (left),
solute concentration is greater in cytoplasm of
plant cell. Here again, water is driven to move
because the plant cell membrane is impermeable
to solutes. Thus, water tends to enter the
cell. The cell swells, but will not rupture
because of the surrounding cell wall. Plant
stands erect.
13

• 2. Facilitated Diffusion (passive)
• Substance moves through a transport protein from
  an area of high to an area of low concentration,
  without using energy.
• Substance moves down its concentration
  gradient
• Ex. glucose

14

• Facilitated Diffusion

Cell membrane
Transport protein
15

• Cell Environment
• 1 sucrose 3 sucrose
• 1 glucose 2 glucose
• 1 fructose 1 fructose
• 97 water 94 water

Assume cell membrane is permeable to water,
glucose fructose, but impermeable to
sucrose. In which direction will sucrose,
glucose, fructose water move?
16
Key to figuring this problem out is to remember
that in diffusion, substances always move from a
higher concentration to a lower
concentration. Sucrose would diffuse into the
cell if it could however, the membrane is
impermeable to sucrose. Glucose diffuses into
the cell until dynamic equilibrium is
reached. Fructose is at dynamic equilibrium, so
would experience no NET diffusion. Water
diffuses out of the cell until dynamic
equilibrium is reached. Water diffuses outward
because the solute concentration is greater
outside the cell than in the surrounding
environment.
17

• 3. Active Transport (active)
• Substance moves through a transport protein from
  an area of low to an area of high concentration
  requires energy.
• Substance moves against its concentration
  gradient
• Ex. ions (Na, K, Cl-)

18

• Active transport

Cell membrane
Transport protein
ATP
19

• Active transport of Na K through the
  sodium-potassium pump (transport protein).

20
Cells must contain high concentrations of
potassium ions low concentrations of sodium
ions to function. The only way cells can
maintain these concentrations is by activity of
sodium-potassium pumps in the cell membrane.
Note 3 sodium ions are pumped outward
for every 2 potassium ions
pumped inward.
21

• 4. Cotransport
• The active transport of one substance is coupled
  to the passive transport of another.
• Ex. sucrose (plant cells)

22
Example of cotransport sucrose loading
Energy is used to actively transport protons
to the outside of the cell, creating a
concentration gradient. Protons passively flow
back through the cell membrane through a
symporter, which couples the movement of sucrose
with the movement of protons.
23

• 5. Exocytosis, Endocytosis Transcytosis
• Movement of large particles across membranes with
  the help of vesicles.
• Exocytosis - vesicles move particles out of the
  cell.

Ex. release of enzymes from head of sperm
neurotransmitter release
24
Exocytosis vesicle fuses with cell membrane,
expelling contents to the outside of the
cell. Acrosomal enzymes are found in the head
of a sperm. They are released by exocytosis
when the sperm encounters an egg. Nerve cells
release neurotransmitters by exocytosis.
25

• Endocytosis - vesicles move particles into the
  cell.
• Three types of endocytosis

Receptor-mediated endocytosis
Pinocytosis
Phagocytosis
26
Endocytosis part of cell membrane surrounds
substance pinches off forming a
vesicle. Pinocytosis cell drinking vesicle
brings water containing substances into the
cell. Phagocytosis cell eating vesicle
brings large clumps of nutrients into the cell.
WBCs phagocitize bacteria Receptor-mediated
endocytosis substance must bind to a receptor
protein on cell membrane before it can be
brought into the cell. liver cells envelop
cholesterol by receptor-mediated endocytosis
27

• Transcytosis - combines endocytosis exocytosis.
• Vesicles rapidly transport particles through
  cells.
• Ex. transport of nutrient monomers through cells
  lining digestive tract into the bloodstream

28

• C. Cytoskeleton
• The structural framework of a cell.
• 1. Microtubules - hollow, thick elements made of
  the protein tubulin.

• Functions
• move chromosomes apart during cell division
• form cilia flagella

29
Cilia flagella have a 9 2 arrangement of
microtubules. Cilia are short, numerous
structures that function to move cells
(paramecium) or move materials past cells
(ciliated cells line the upper respiratory
tract). Flagella are long, whip-like structures
that function to propel cells. Human sperm have
only a single flagellum.
30

• 2. Microfilaments - long, thin elements made of
  the protein actin.

• Functions
• connect cells to each other
• move vesicles organelles within cytoplasm
• help cells move

31

• 3. Intermediate filaments - elements with
  diameters in between that of microtubules
  microfilaments.
• Made of various proteins (ie. keratin)

• Functions
• maintain cell shape
• connect cells to each other to underlying
  tissue (skin cells)

Abundant in skin nerve cells.
32

• D. Intercellular Junctions
• Structures that connect cells of multicellular
  organisms to form tissues.
• 1. Animal cell Connections

Tight Junctions - cell membranes of adjacent
cells are fused, creating a tight seal. Ex. cells
lining small intestine cells lining capillaries
in brain
33

• Desmosomes - intermediate filaments weld cell
  membranes of adjacent cells together in isolated
  spots.
• Ex. skin cells

Gap Junctions - channels that link the cytoplasm
of adjacent cells, allowing exchange of
materials. Ex. heart muscle cells
34

• 2. Plant Cell Connections
• Plasmodesmata - channels that link the cytoplasm
  of adjacent plant cells, allowing the exchange of
  cytoplasm organelles.
• Ex. cells conducting water nutrients

35

• E. Cell-Cell Interactions
• 1. Cell Adhesion
• Process that uses membrane proteins called
  cellular adhesion molecules (CAMs) to direct the
  migration of cells.
• Various CAMs function in sequence to
• guide WBCs to injury sites
• guide embryonic cells to help form placenta
• establish nerve connections involved in learning
  memory

36
CAMs directing WBCs to injury sites.
37

• 2. Signal Transduction
• Process by which cells receive, amplify,
  respond to outside stimuli.

Outside stimulus (first messenger) is received
by a receptor protein in the cells membrane.
This triggers a series of chemical reactions on
the cells surface. Eventually, a second
messenger is activated which triggers the cells
response.