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4.2.13 Microtubules
- slender unbranched tubes about 20 nm in diameter &
several um in length
Functions:
1 act as an internal skeleton (cytoskeleton) for the cells &
determine their shapes
2 aid in transport within cells by providing routes for
materials to move
3 form a framework along which the cell wall is laid down
4 major component of cilia and flagella for movement
5 form centrioles of the spindle during cell division
4.2.14 Cilia and flagella
- cilia are shorter & more numerous than flagella
- both about 0.2 um in diameter;
cilia about 10 um long, flagella about 100 um
Functions:
1 To move an entire organism,
e.g. cilia of parameucium and flagellum of bacteria
2 To move materials within an organism,
e.g. cilia within the respiratory tract, oviduct
Flagellum (EM) (x92000 app.)
4.2.15 Centrioles
- same basic structure as basal bodies of cilia
Centrosome: a distinct region of the cytoplasm containing
the two centrioles
During cell division they migrate to opposite poles of the
cell and make the microtubules of the spindle.
* Higher plants (flowering plants) do not have centrioles, but
they form spindle for cell division
4.2.16 Microfilaments
- very thin strands about 6 nm in diameter
- with actin & myosin proteins (similar to muscles), thus
probably play a role in movement within cells/cell as a
whole
4.2.17 Microvilli
- finger-like projection about 0.6 um in length on the
membranes of cells
brush border:
- actin filaments within the microvilli allow them to
contract, with a larger surface area created, microvilli
facilitate absorption,
e.g. kidney tubules and intestinal epithelium
Microvilli (EM)
(x23000 app.)
Cotton plants
Cellulose microfibril
4.2.18 Cellulose and cell wall
-glucose  cellulose chain
 micelle
 microfibril
 fibril embedded in polysaccharide matrix
Functions:
1 To provide support for young seedlings &
herbaceous plants
2 To give mechanical strength to the plant as a
whole. Strength may be increased by ligninfication.
3 Freely permeable to water, thus allow movement
of water in the plant, e.g. in the cortex of roots
4 Lignin helps to keep water within the xylem;
Cutin (epidermis of leaves) prevents water loss;
Suberin (in root endodermis) prevents movement
of water across them and controls water in by
passage cells.
(a) protein synthesis - G: endoplasmic reticulum;
(b) photosynthesis - B: chloroplast
(c) electron transport (respiratory chain) - A: mitochondrion (or B)
(d) protein/carbohydrate complex formation - H: golgi apparatus
(e) generation of ATP - A: mitochondrion (or B)
(f) ribosome production - F: nucleolus
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4.3 Movement in and out of cells
1 diffusion,
2 osmosis,
3 active transport,
4 phagocytosis, and
5 pinocytosis
4.3.1 Diffusion - the process by which a substance
moves from a region of high concentration of that
substance to a region of low concentration of the
same substance.
Rate of diffusion depends upon:
1. The concentration gradient
2. The distance over which diffusion takes place
3. The area over which diffusion takes place
4. The nature of any structure across which diffusion
occurs
5. The size & nature of the diffusing molecule
Facilitated diffusion is a special form of
diffusion which allows more rapid exchange.
It involves channels (or carriers) within a
membrane to make diffusion easier. It is
passive, not involving energy.
Osmosis - the passage of water from a region of higher
water potential to a region of lower water potential
through a selectively permeable membrane.
osmotic pressure (old term – a +ve value): pressure
applied to the system to prevent osmosis to occur
water potential: the difference between the chemical
potential of water in the system and the chemical
potential of pure water under standard temperature
(25°C) and pressure (1 atmosphere).
- water potential of pure water is 0.
- water will diffuse from a region of higher water potential
to a region of lower water potential
hypotonic solution: a dilute solution with higher water
potential than the cell sap
hypertonic solution: a concentrated solution with lower
water potential than the cell sap
pressure potential
100
0
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4.3.4 Active transport
4.3.4 Active transport
- energy is spent to drive the transport of molecules
across membranes because molecules are transported
against a concentration gradient, e.g. from a region of
lower concentration to a region of higher concentration.
- Mechanism: through proteins that span the membrane
Characteristics of cells & tissues carrying active transport:
1 presence of numerous mitochondria (why?)
2 high concentration of ATP
3 high respiratory rate
- any factors that affect respiratory rate will affect active
transport,
e.g. temperature, oxygen concentration and cyanide
4.4.3.5 Phagocytosis - the process by which the cell can
obtain particles which are too large to be absorbed
by diffusion or active transport
cell invagination to contain particle
 vacuole
 lysosomes fuse with vacuole
 enzymes digest particle
 useful materials absorbed
examples: phagocytes (WBCs) and amoeba for feeding
4.3.6 Pinocytosis - cell drinking
- similar to phagocytosis but vacuoles (pinocytic or
micropinocytic vesicles) are smaller
- for intake of liquids
endocytosis: both pinocytosis and phagocytosis are
methods by which materials are taken into the cell in
bulk
exocytosis: materials are removed from cells in bulk
The enzymes contained within lysosomes are synthesized on rough
ER and transported to the Golgi apparatus.
Golgi vesicles containing the processed enzymes later bud off are
called primary lysosomes.
Functions of lysosomes:
1 Digestion of material taken in by endocytosis
2 Autophagy: a process to remove unwanted structures
within the cell
3 Release of enzymes outside the cell (Exocytosis), e.g.
osteoclasts (remodelling of bones)
4 Autolysis: self-destruction of the cell
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