1
Think about…
3.1 Cell membrane
3.2 Movement of substances across
membranes
Recall ‘Think about…’
Summary concept map
2
Let me show you
how to make it!
spring onion flower
3
1
Cut several vertical slits at one end.
4
2
Place it in water for 1 minute.
5
3
Cut tips curl outwards like a flower!
6
1
Why do the cut
tips of spring onion
curl outwards after
placing in water
7
2
The cut tips will
curl inwards if the
spring onion is put in
sucrose solution.
Why
8
3.1 Cell membrane
cell membrane
9
3.1 Cell membrane
cell membrane
outside
cell
inside
cell
• separates the cell contents from the
outside environment
10
3.1 Cell membrane

outside
cell
inside
cell
• control the movement of substances
into and out of cells
11
3.1 Cell membrane
What is the
structure of the
cell membrane?
12
3.1 Cell membrane
Structure of cell membrane
• explained by
fluid mosaic model (流動鑲嵌模型)
3D Model
13
3.1 Cell membrane
Fluid mosaic model
phospholipid bilayer
(磷脂雙層)
protein
molecules
14
3.1 Cell membrane
Phospholipids
lipid molecule =
1 glycerol molecule
+
3 fatty acid molecules
15
3.1 Cell membrane
Phospholipids
phospholipid molecule:
1 fatty acid molecule
replaced by
phosphate group
16
3.1 Cell membrane
Phospholipids
water-loving ‘head’
17
3.1 Cell membrane
Phospholipids
water-repelling ‘tail’
18
3.1 Cell membrane
Phospholipids
outside cell
phospholipid
bilayer
inside cell
19
3.1 Cell membrane
Phospholipids
outside cell
phospholipid
molecules
arranged
tail-to-tail
inside cell
20
3.1 Cell membrane
Phospholipids
outside cell
water-loving
‘heads’ face
water-based
environment
inside cell
21
3.1 Cell membrane
Fluid mosaic model
phospholipid bilayer
(磷脂雙層)
protein
molecules
22
3.1 Cell membrane
Proteins
• protein arrangement:
embed halfway through
bilayer
penetrate
through
bilayer
23
3.1 Cell membrane
Proteins
What are their functions?
24
3.1 Cell membrane
Proteins
i) Act as channels
ions
ions
water soluble
substances
water soluble
substances
25
3.1 Cell membrane
Proteins
ii) Act as carriers
energy
active
transport
26
3.1 Cell membrane
Proteins
iii) Act as chemical receptors
hormones
turn on
cell
activities
27
3.1 Cell membrane
Proteins
iv) Act as enzymes
enzymes
speed up
reactions
28
3.1 Cell membrane
Proteins
v) For recognition (識別)
glycoprotein
(糖蛋白)
carbohydrate
molecule
different
among cells
29
3.1 Cell membrane
Fluid mosaic model
Why called fluid mosaic model?
30
3.1 Cell membrane
Fluid mosaic model
some protein molecules
phospholipid bilayer
can move laterally
31
3.1 Cell membrane
Fluid mosaic model
protein
molecules
interspersed in a mosaic pattern
32
3.1 Cell membrane
Fluid mosaic model
• cell membrane
- differentially permeable
- flexible
- supports the cell contents
How can the model explain
these properties and
function?
33
3.1 Cell membrane
1 Differential permeability
• lipid-soluble substances, simple and
small molecules:
outside cell
inside phospholipid bilayer:
through
bilayer
water-repelling / fat-loving
inside cell
34
3.1 Cell membrane
1 Differential permeability
• water molecules, certain ions, watersoluble substances:
outside cell
through channel
phospholipid bilayer:
proteins or
impermeable to them
carrier proteins
inside cell
35
3.1 Cell membrane
1 Differential permeability
• large molecules:
outside cell
cannot pass
through
inside cell

36
3.1 Cell membrane
2 Flexibility
fluid nature of phospholipid bilayer
membrane can change
shape or seal itself
37
3.1 Cell membrane
2 Flexibility
cell division
fluid nature of phospholipid bilayer
membrane can change
shape or seal itself
38
3.1 Cell membrane
3 Supporting role
• interspersed protein molecules give
strength
39
3.1 Cell membrane
3 Supporting role
• interspersed protein molecules give
strength
support
cell contents
40
3.1 Cell membrane
What is the membrane
structure according to
the fluid mosaic model?
How is it related to its
properties and functions?
41
3.1 Cell membrane
Structure
Properties & functions
- Phospholipid - Cell membrane is
bilayer
differentially
permeable
- Proteins act
as channels
or carriers
42
3.1 Cell membrane
Structure
Properties & functions
- Proteins are - Cell membrane is
interspersed strong enough to
in bilayer
support cell contents
43
3.1 Cell membrane
Structure
- Fluid in
nature
Properties & functions
- Cell membrane is
flexible so that it can
change its shape and
seal itself
44
3.2 Movement of substances
across membrane
diffusion
1
osmosis
2
active transport 3
4 phagocytosis
45
3.2 Movement of substances across membrane
Diffusion (擴散)
3D Animation
• all substances are made up of particles
cannot
move from
one place
to another
solid
can move more freely
liquid
gas
46
3.2 Movement of substances across membrane
Diffusion (擴散)
In liquid or gas
no difference in
concentration
• no concentration gradient
• particles move randomly in all directions
47
3.2 Movement of substances across membrane
Diffusion (擴散)
In liquid or gas
higher
concentration
lower
concentration
• concentration gradient exists
48
3.2 Movement of substances across membrane
Diffusion (擴散)
In liquid or gas
until evenly
distributed
net movement
• net movement of particles from
higher to lower concentration
49
3.2 Movement of substances across membrane
Diffusion (擴散)
In liquid or gas
until evenly
distributed
net movement
• no energy is required
• passive process
50
3.2 Movement of substances across membrane
Diffusion (擴散)
Factors affecting the rate of
diffusion
i) Difference in concentration
concentration gradient
higher rate
steeper
lower rate
less steep
51
3.2 Movement of substances across membrane
Diffusion (擴散)
Factors affecting the rate of
diffusion
ii) Temperature
higher rate
higher
temperature
lower rate
lower
temperature
52
3.2 Movement of substances across membrane
Diffusion (擴散)
Factors affecting the rate of
diffusion
iii) Size and nature of particles
lower rate
larger
higher rate
smaller
53
3.2 Movement of substances across membrane
Diffusion (擴散)
Factors affecting the rate of
diffusion
iii) Size and nature of particles
lower rate
water-soluble
higher rate
lipid-soluble
54
3.2 Movement of substances across membrane
Diffusion (擴散)
Importance
1 Transport of substances
small & lipid-soluble nutrients
oxygen
simple
waste
ions
2 Distribution of substances
55
3.2 Movement of substances
across membrane
diffusion
1
osmosis
2
active transport 3
4 phagocytosis
56
3.2 Movement of substances across membrane
Osmosis (滲透)
3D Animation
• diffusion of water molecules across a
differentially permeable membrane
Let me explain in terms of
water potential (水勢)
57
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
• describe tendency of water molecules
to move from one place to the other
58
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
water molecule
Ψ=0
Solution
solute
Ψ = negative
• solutes lower the water potential
59
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
Solution
The higher the
concentration of
a
solution,
the
water molecule
solute
lower its water
potential.
Ψ=0
Ψ = negative
• solutes lower the water potential
60
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
Solution
differentially
permeable
membrane
61
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water

Solution
solutes are
too large to
pass through
62
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
Solution
water
molecules
can pass
through
63
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
Solution
more water
molecules
move to the
right
higher Ψ
lower Ψ
64
3.2 Movement of substances across membrane
Osmosis (滲透)
Water potential Ψ
Pure water
Solution
net movement
higher Ψ
lower Ψ
65
3.2 Movement of substances across membrane
Osmosis (滲透)
• diffusion of water molecules across a
differentially permeable membrane
net movement of water molecules
from a region of higher water
potential to a region of lower
water potential
66
3.2 Movement of substances across membrane
3.1
Video
Demonstration of osmosis using
dialysis tubing
1 Wet a dialysis tubing with tap water.
Tie a knot at one end.
2 Fill the tubing with 20% sucrose
solution.
3 Tie the other end to a capillary
tube. Rinse with distilled water.
67
3.2 Movement of substances across membrane
3.1
4 Immerse the tubing in water.
Mark the initial liquid level.
liquid level
68
3.2 Movement of substances across membrane
3.1
5 Set up a control by filling
another tubing with
distilled water instead of
sucrose solution.
distilled water
69
3.2 Movement of substances across membrane
3.1
6 Note any changes in the liquid levels
after 30 minutes.
experimental
set-up
control
set-up
70
3.2 Movement of substances across membrane
3.1
Results and discussion
• The liquid level in the experimental
set-up rises.
• The liquid level in the control set-up
falls until it reaches the liquid level of
the water in the beaker.
71
3.2 Movement of substances across membrane
3.1
Results and discussion
• When sucrose solution is separated from
distilled water by the differentially
permeable dialysis tubing, there is a net
movement of water molecules from
distilled water to the sucrose solution.
72
3.2 Movement of substances across membrane
3.2
Video
Demonstration of osmosis using
living animal tissue
1 Cover the mouth of a thistle funnel with a
piece of living animal tissue. Tie the tissue
tightly with a thread.
2 Invert the funnel and fill it with
concentrated sucrose solution.
73
3.2 Movement of substances across membrane
3.2
3 Immerse the funnel in a beaker of water.
(Set-up A). Mark the initial liquid level.
liquid level
concentrated
sucrose
solution
animal tissue
distilled water
74
3.2 Movement of substances across membrane
3.2
4 Prepare a similar set-up
(set-up B) by filling the
thistle funnel with
distilled water instead of
sucrose solution.
distilled water
75
3.2 Movement of substances across membrane
3.2
5 Observe any changes in the liquid levels
after 30 minutes.
set-up A
set-up B
76
3.2 Movement of substances across membrane
3.2
Results and discussion
• In set-up A, the liquid level rises.
- Distilled water has a higher water potential
than concentrated sucrose solution.
- There is a net movement of water
molecules from the outside into the thistle
funnel through the differentially
permeable animal tissue by osmosis.
77
3.2 Movement of substances across membrane
3.2
Results and discussion
• In set-up B, the liquid level gradually falls.
- It serves as a control.
- When the liquid levels in the thistle funnel
and the beaker become the same, no
osmosis takes place.
78
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
animal cells
plant cells
• gain or lose water by osmosis,
depending on the surrounding solution
79
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic solution with Ψ higher
(低滲的) solution than cytoplasm
80
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic cell swells
(低滲的) solution cell finally bursts
animal
cell
lower Ψ
higher Ψ
water enters
by osmosis
continuously
81
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic cell swells
(低滲的) solution cell finally bursts
animal
cell
82
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic
(低滲的) solution
lower Ψ
plant cell
higher Ψ
water enters by
osmosis
83
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic
(低滲的) solution
plant cell
restricted by rigid
cell wall, water
stops entering 84
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypotonic cell finally becomes
(低滲的) solution turgid (膨脹)
plant cell
85
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypertonic solution with Ψ lower
(高滲的) solution than cytoplasm
86
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypertonic cell shrinks (萎縮) &
(高滲的) solution becomes wrinkled (皺褶)
animal
cell
higher Ψ
lower Ψ
water leaves
by osmosis
87
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In hypertonic cell shrinks (萎縮) &
(高滲的) solution becomes wrinkled (皺褶)
animal
cell
88
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
plasmolysis (質壁分離)
In hypertonic
occurs & cell becomes
(高滲的) solution
flaccid (軟縮)
plant cell
higher Ψ
lower Ψ
water leaves
by osmosis
89
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
plasmolysis (質壁分離)
In hypertonic
occurs & cell becomes
(高滲的) solution
flaccid (軟縮)
plant cell
90
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In isotonic
solution with Ψ same as
(等滲的) solution cytoplasm
91
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
In isotonic
cell volume remains the
(等滲的) solution same
no net
water
movement
animal cell
plant cell
92
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
When a cell is put in
an isotonic solution,
there is no water
movement across
its membrane.
93
3.2 Movement of substances across membrane
Osmosis (滲透)
Cells and osmosis
Water moves into and out of
the cell across the cell
membrane all the time, but
with no NET water movement
across the membrane.
94
3.2 Movement of substances across membrane
Osmosis (滲透)
Importance
• for entry and exit of water into and
out of the cells
animal cell
plant cell
95
3.2 Movement of substances across membrane
Osmosis (滲透)
Importance
• for movement of water from one cell
to another in plants
H2O
96
3.2 Movement of substances across membrane
Osmosis (滲透)
Importance
• for transporting water through
living tissues
H2O
H2O
blood
97
3.2 Movement of substances across membrane
3.3
Study of osmosis in red blood cells
1 Examine prepared slides
of red blood cells in
different concentration
of NaCl solutions under
the microscope.
red blood cells
98
3.2 Movement of substances across membrane
3.3
2 Note the appearance of red blood cells.
Suggest which concentration of NaCl
solution is hypertonic, hypotonic and
isotonic to the cells.
99
3.2 Movement of substances across membrane
3.3
Red blood cells in NaCl solutions (×400)
0%
0.45%
1.35%
0.9%
1.8%
100
3.2 Movement of substances across membrane
3.3
Results and discussion
In 0.9% NaCl solution
• red blood cells
appear normal
isotonic
0.9%
101
3.2 Movement of substances across membrane
3.3
Results and discussion
In 0% and 0.45% NaCl solution
hypotonic
0%
0.45%
• red blood cells swell
102
3.2 Movement of substances across membrane
3.3
Results and discussion
In 1.35% and 1.8% NaCl solution
hypertonic
1.35%
1.8%
• red blood cells shrink and become wrinkled
103
3.2 Movement of substances across membrane
3.4
Video
Study of osmosis in living plant cells
1 Peel off the epidermis of a Zebrina (水竹草)
leaf. Cut a small piece and lay it flat on a
slide.
104
3.2 Movement of substances across membrane
3.4
2 Add a drop of concentrated sucrose solution
and put a cover slip over the epidermis.
Wait for 3 minutes.
3 Observe the epidermis
under a microscope.
105
3.2 Movement of substances across membrane
3.4
4 Slowly replace the concentrated sucrose
solution with distilled water. Observe with
a microscope.
add distilled
water slowly
draw distilled
water slowly
by tissue
paper
106
3.2 Movement of substances across membrane
3.4
Results and discussion
In concentrated sucrose solution
• cells lose water by
osmosis and become
plasmolysed
Zebrina epidermal cells (×100)
107
3.2 Movement of substances across membrane
3.4
Results and discussion
In distilled water
• cells gain water by
osmosis and become
turgid
Zebrina epidermal cells (×100)
108
3.2 Movement of substances across membrane
3.4
Results and discussion
• plasmolysis is usually reversible
plasmolysed
turgid
109
3.2 Movement of substances across membrane
3.5
Video
Study of osmosis in living plant tissue
1 Add distilled water, 10% and 20% sucrose
solution into 3 beakers respectively.
110
3.2 Movement of substances across membrane
3.5
2 Use a cork borer to make 9 strips from a
potato. Cut each of them to 5 cm long.
cork borer
potato
111
3.2 Movement of substances across membrane
3.5
3 Blot the potato strips with tissue paper.
Weigh the strips with an electronic balance.
Record the initial weight.
potato strip
112
3.2 Movement of substances across membrane
3.5
4 Put 3 potato strips into each beaker. Cover
the beakers and leave them for 1 hour.
plastic food wrap
distilled
water
10% sucrose 20% sucrose
solution
solution
113
3.2 Movement of substances across membrane
3.5
5 Remove the potato strips and blot them
with tissue paper. Weigh the strips
immediately. Record the final weight of
the strips.
6 Calculate the average values
of the percentage change in
the weights of the strips in
each beaker.
114
3.2 Movement of substances across membrane
3.5
Results and discussion
In distilled water
• potato strips become heavier
net movement of water into the cells by
osmosis
distilled water is hypotonic to potato tissue
115
3.2 Movement of substances across membrane
3.5
Results and discussion
In 10% sucrose solution
• weight changes slightly
10% sucrose solution is nearly
isotonic to potato tissue
10%
116
3.2 Movement of substances across membrane
3.5
Results and discussion
In 20% sucrose solution
• potato strips become lighter
net movement of water out of
the cells by osmosis
20% sucrose solution is
hypertonic to potato tissue
20%
117
3.2 Movement of substances
across membrane
diffusion
1
osmosis
2
active transport3
4 phagocytosis
118
3.2 Movement of substances across membrane
Active transport (主動轉運)
3D Animation
Can move against
higher concentration lower
gradient using
concentration
concentration
ENERGY!
119
3.2 Movement of substances across membrane
Active transport (主動轉運)
net movement
• Usually from lower to higher
concentration
120
3.2 Movement of substances across membrane
Active transport (主動轉運)
net movement
• carried out by carrier proteins in
the membrane
121
3.2 Movement of substances across membrane
Active transport (主動轉運)
• carried out by carrier proteins in
the membrane
122
3.2 Movement of substances across membrane
Active transport (主動轉運)
outside cell
carrier
lower
concentration protein
inside cell
higher
concentration
123
3.2 Movement of substances across membrane
Active transport (主動轉運)
outside cell
inside cell
substances combine
with carrier protein
lower
concentration
higher
concentration
124
3.2 Movement of substances across membrane
Active transport (主動轉運)
outside cell change shape
inside cell
substances
released into cell
energy
lower
concentration
higher
concentration
125
3.2 Movement of substances across membrane
Active transport (主動轉運)
• active process
• energy obtained from respiration
• only in living cells
respiration
energy
mitochondrion
126
3.2 Movement of substances across membrane
Active transport (主動轉運)


• no respiration, no active transport
active
cyanide
lack of oxygen
transport
respiration

energy
mitochondrion
127
3.2 Movement of substances across membrane
Active transport (主動轉運)
Importance
• mineral absorption
soil: lower mineral
concentration
root: higher mineral
concentration
128
3.2 Movement of substances across membrane
Active transport (主動轉運)
Importance
• mineral absorption
soil: lower mineral
concentration
against
concentration
gradient
root: higher mineral
concentration
129
3.2 Movement of substances across membrane
Active transport (主動轉運)
Importance
• glucose absorption active transport
lumen:
higher glucose
concentration
blood:
lower glucose
concentration
villus of
small intestine
130
3.2 Movement of substances across membrane
Active transport (主動轉運)
Importance
• glucose absorption
lumen:
higher glucose
concentration along
blood:
lower glucose
concentration
concentration
gradient at a higher speed
131
3.2 Movement of substances
across membrane
diffusion
1
osmosis
2
active transport 3
4 phagocytosis
132
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
‘eat’
133
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
‘cell’
134
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
• ‘cell-eating’
• single-celled organisms and certain
white blood cells engulf large particles
particle
white
blood cell
• energy is required
135
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
• start with infolding of membrane or
formation of pseudopodia (偽足)
Animation
particle
outside cell
digested products
pit
diffuse to
cytoplasm
vacuole
enzyme
inside cell136
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
Importance
• nutrition of some single-celled
organisms
alga
Ameoba
137
3.2 Movement of substances across membrane
Phagocytosis (吞噬)
Importance
• body defence against diseases
harmful
microorganism
white
blood cell
138
3.2 Movement of substances across membrane
3.6
Video
Examination of phagocytosis in Amoeba
Examine the process of phagocytosis in
Amoeba with a video-imaging device.
Note the following:
1 the formation of pseudopodia
2 how the cell engulfs the particle
3 the type of particle the cell engulfs
139
3.2 Movement of substances across membrane
3.6
Amoeba
alga
Amoeba engulfing food particle by
phagocytosis (×100)
140
3.2 Movement of substances across membrane
3.6
• The Amoeba start to surround the alga.
141
3.2 Movement of substances across membrane
3.6
• The alga is engulfed by the Amoeba.
142
3.2 Movement of substances across membrane
1
Let’s compare diffusion,
osmosis, active transport and
phagocytosis!!!
143
3.2 Movement of substances across membrane
1a Net movement of particles:
Diffusion
Osmosis
From high to low From high Ψ to low
concentration
Ψ
Active transport
Phagocytosis
Usually from low to
Into the cell
high concentration
144
3.2 Movement of substances across membrane
1b Membrane needed?
Diffusion
No
Active transport
Living cell
membrane
Osmosis
Differentially
permeable membrane
Phagocytosis
Living cell membrane
145
3.2 Movement of substances across membrane
1c Energy needed?
Diffusion
No
Osmosis
No
Active transport
Yes
Phagocytosis
Yes
146
3.2 Movement of substances across membrane
2
What happens to a cell if it is put
in a hypotonic, a hypertonic or an
isotonic solution respectively?
147
3.2 Movement of substances across membrane
solution
surrounding
the cell
isotonic solution
hypotonic solution
hypertonic solution
net movement
of water
148
3.2 Movement of substances across membrane
3
What happens to a cell if it is an
animal cell and a plant cell
respectively?
149
3.2 Movement of substances across membrane
Hypotonic Hypertonic
solution
solution
Animal Swells
Shrinks
cell
and may and
burst
becomes
wrinkled
Isotonic
Solution
Remains
normal
in water
content
150
3.2 Movement of substances across membrane
Hypotonic
solution
Plant
cell
Hypertonic
solution
Isotonic
solution
Becomes Vacuole
Remains
shrinks ;
turgid
normal
plasmolysis in water
occurs;
content
becomes
flaccid
151
1
Why do the cut tips of spring onion
curl outwards after placing in water?
• Water is hypotonic to the cells.
Water enters cells by osmosis.
Cells become turgid & increase in size.
152
• The cells at the outer layer are
covered with a waxy layer.
It limits the increase in cell size.
The cells at the inner layer expand
more quickly.
153
The cut tips curl outwards
outer waxy
layer
inner layer of cells
expand more quickly
154
2
The cut tips will curl inwards if the
spring onion is put in sucrose solution.
Why?
• Sucrose solution is hypertonic to the
cells.
Cells lose water and lose turgidity.
155
The cut tips curl inwards.
outer waxy
layer
inner layer of cells lose water and
become flaccid more quickly
156
Cell membrane
its structure can be
explained by
fluid mosaic model
states that the cell membrane
is made up of
a phospholipid
proteins
embedded
bilayer
with
157
proteins
including
channel
proteins
carrier
proteins
158
Cell membrane
allows substances
to move across it by
osmosis
diffusion
phagocytosis
active
transport
159
diffusion
is the net movement
of particles
down a
concentration gradient
160
osmosis
is the net movement
of water molecules
down a water
potential gradient
161
active transport
is the movement
of particles
usually against a
concentration gradient
process requires
energy
162
phagocytosis
in which
large particles
are engulfed
by forming
pits or
pseudopodia
163