2007 Cell biology
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Cell Biology 細胞生物學
Cell Theory 細胞學說
The basic unit of structure (結構) and function (功能) in living organisms is the cell (細胞). This
concept, known as the cell theory (細胞學說), evolved gradually during the nineteenth century as a
result of microscopy (顯微學)
1.
2.
3.
4.
All living organisms are composed of cells. Therefore cell is the basic unit of life.
All cells arising from pre-existing cells by means of cell division.
All metabolic reactions take place in cells.
All cells contain hereditary materials which can be duplicated and passed to offspring.
Cytology (細胞學)
The study of cells by microscopy.
Cell Structure (細胞結構)
- cell basically consists of a mass of protoplasm (一團原生質)
- protoplasm surrounded by a plasma membrane (質膜) (also a cell wall 細胞壁 in plant cells 植物細
胞)
cell 細胞
plasma membrane 質膜 (cell wall 細胞壁)
cytoplasm 細胞質 (with organelles 細胞器)
protoplasm 原生質
nucleus 細胞核
- Ultrastructure of a cell (viewed under Electron Microscope 電子顯微鏡, EM)
I.
Cell (plasma) membrane 細胞 (質) 膜
II. Cytoplasm 細胞質
III. Nucleus 核 -
1.
nuclear membrane 核膜
2.
nucleoplasm (karyoplasms)
3.
4.
nucleolus 核仁
chromatin 染色質 (chromosomes 染色體)
IV. Membranous organelles 有膜細胞器
1. Endoplasmic Reticulum 內質網
2. Golgi body 高爾基體
3. Lysosomes
/ 溶菌體
4. Vacuoles 液泡
5. Mitochondrion 粒線體
6. Chloroplast 葉綠體 / Plastids 質體 (in plants 植物中)
V. Non-membranous organelles 非膜細胞器
1. Ribosomes 核糖體
2. Microtubules 微管 / 微質管
3. Centrioles 中心粒
VI. Cell Wall 細胞壁 (in plants)
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2007 Cell biology
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I. Plasma Membrane 質膜
-
structure is invisible 不可見 with light microscope 光學顯微鏡
 only a transparent membrane (透明膜) can be seen
- Unit Membrane Hypothesis 單位膜假說:
-
about 7.5 nm wide
- trilayer 三層 (dense-light-dense深-淺-深) structure (Trilaminar structure) is observed under
electronic microscope 電子顯微鏡
- all biological membranes 生物膜 (include plasma membrane 質膜, nuclear membrane 核膜,
membrane of mitochondria 粒線體膜 ...etc.) shared basic structure
- consist of protein 蛋白質 and lipid layers 脂肪層
- Fluid Mosaic Model 液態鑲嵌模型 of plasma membrane (1972, Nicolson)
http://cd.ed.gov.hk/sci/biology/resources/L&t2/model_making/model_making_C_1.pdf
http://cd.ed.gov.hk/sci/biology/resources/L&t2/model_making/model_making_E_1.pdf
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- mainly phospholipid 磷 脂 with polar 極 性 (hydrophilic 親 水 ) head and non-polar 非 極 性
(hydrophobic 惡水的) tail to form a lipid bilayer 脂雙層
- hydrophobic tails 惡水尾巴 inwardly directed in the lipid bilayer
- proteins are thought to float 浮於 and embedded in a fluid manner 流體形式 in the lipid matrix
脂肪基質
- the membrane can be made less fluid 減低流動性 by the cholesterol 膽固醇 present in the
phospholipid bilayer
- some protein molecules are thought to be partially penetrate (部份地穿越) the lipid bilayer (脂
雙層) while others may completely penetrate (完全透過) it
--> hence both sides of the membrane are asymmetric 不對稱
- membrane structure are dynamic (動態的) in nature (proteins are able to perform translational
movement within the overall bilayer) and transport system 運輸系統across the membrane (橫
越膜) may be present (Many proteins are carrier proteins) for exchange of materials (物質交換)
between both sides of the membrane
- membrane may grow (生長), extend (伸展) and invaginate (摺疊) in or out
http://cd.ed.gov.hk/sci/biology/resources/L&t2/practical/Practical(C)-9.pdf
http://cd.ed.gov.hk/sci/biology/resources/L&t2/practical/Practical-9.pdf
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Further detailed structure of the plasma membrane
1.
Pores
 Plasma membranes are perforated by pores which are hydrophilic channel through which ions (ion channel)
and some polar molecules can pass.
2.
Glycoprotein
 Plasma membranes contains not only lipid and protein but also carbohydrate.
 The carbohydrate component in form of short polysaccharide chains which project from the outer side of the
membrane.
 Some of the polysaccharides are attached to the phospholipids but the majority are attached to the proteins (to
form glycoproteins).
 The composition and branching pattern of these surface polysaccharides vary from one type of cell to another
and are related to its function
 help to recognise similar cells and adhere these cells together by cell-to-cell contact.
 help some specific hormones and foreign substances (antigen) to recognise particular type of cells for
action.
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Properties (特性) and Functions (功能) of Plasma Membrane
Permeability (透性)
- selective permeable 選 透 (or called semi-permeable, or differential permeable), via simple
diffusion (簡單擴散)
---> control the entrance and exit of molecules and ions, as well as the free mixing of molecules
on the two sides of the membrane (i.e. homeostasis of cell 細胞平衡)
1. Simple Diffusion (簡單擴散)
- along the concentration gradients 順著濃度梯度
- energy not required 毋需能量
Water
水
almost all natural membranes are freely permeable
to water
water-soluble neutral substances
水溶性中性物質
can pass through membrane freely
fat-soluble neutral substances
脂溶性中性物質
pass more quickly than water-soluble substances
electrolytes / charged molecules
電解質 / 具電荷分子
the higher charge the slower it penetrate the
membrane
large molecules
大分子
too large molecules may be unable to pass through
natural membranes (except through endocytosis胞
吞作用)
* movement of water through semi-permeable membrane, e.g. plasma membrane from region of
high water potential to region of low water potential is called osmosis.
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2. Carrier-mediated transport
- solutes are bound to a corresponding specific protein, the carrier, which undergoes conformational
change and brings the solute into the cell
i. Facilitated Transport (Facilitated Diffusion)
-
transport of large or charged molecules, e.g. glucose and amino acids
along concentration gradients
energy not required
carrier protein involved
binding sites of carrier are accessible to solute molecules in both extra- and intra-cellular fluids
--> permits transport in either directions across membrane
- rate of transport greater than simple diffusion
- driven force is electrochemical gradient (= charge gradient + concentration gradient) across
membrane
ii. Active Transport
- transport of large and charged molecules against electrochemical gradients
- energy required
- carrier required for one direction transport across membrane
3. Endocytosis
- both are active processes (energy required)
- involve invagination of cell membrane
- for bulk transport of materials into cell
i. Phagocytosis (Cell Eating)
- take up of solid materials forming 'phagocytic vacuole' in cell
- special cells called phagocytes (or phagocytic cells)
- e.g. white blood cells
ii. Pinocytosis (Cell Drinking)
- take up of liquid materials forming vesicles in cell
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4. Exocytosis
- reverse process of endocytosis
- remove materials from cells
- e.g. neurotransmitters secretion
Function of Plasma Membrane
1. serve as a boundary for compartmentization (compartmentation)
- allow the contents of cell and membranous organelles be separated from their environment
 - independent reactions can be carried out in cell and
- cells and organelles with conditions (e.g. pH, conc. of subtances…etc.) different from
their environment at the same time
2. Permeability barrier
- restrict and regulate the flow of substances into or out of the cells
- prevents free mixing of molecules on both sides of the membrane
3. Signal transduction
- presence of specific receptors to recognize specific stimuli or molecules
Receptors for many hormones and neurotransmitters and for sensory input such as light are membrane proteins,
which recognize specific stimuli outside a cell and transduce them into appropriate response inside the cell.
also for contact inhibition
e.g. rejection of graft in RBC transfusion and organ transplantation
4. Membrane potentials
- potential difference on both sides of a membrane
- important in many physiological functions (e.g. impulse transmission)
5. Cell Wall synthesis
- in plants cells
- plasma membrane coordinate the synthesis and assembly of cell wall materials
6. Allow coordination of reactions in a metabolic pathway (Membrane bound enzymes)
- enzymes associated or attached on internal membranes for various cellular activities.
- ATPase (a protein) catalyses the hydrolysis of ATP to liberate energy for active transport is
attached on the membrane surface.)
7. Allow formation of Pseudopodia
- for movement in amoeboid cells
Microvillus -- A Modification of Plasma Membrane
- finger-like extensions of plasma membrane of some animal cells
- only visible under EM
- as a fuzzy line (the brush border or striated border) may be shown under LM, if densely packed
- e.g. cells of intestine epithelium
---> increase surface area for absorption and secretion
(may move by alternation contraction and relaxation of the bundles of actin and myosin filaments,
such movement probably aids the absorption process)
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II. Cytoplasm
The cytoplasm is that living matter in a cell which lies between the plasma membrane and the nucleus.
(Throughout the cytoplasm many kinds of non-living inclusions and various organelles are suspended).
- is a complicated colloid system contains:
-- small molecules and ions (e.g. ATP, Na+, K+...etc)
-- macromolecules
e.g. lipids, sugars, amino acids, storage granules (e.g. oil droplets, glycogen granules),
proteins, lipoproteins, and RNAs
-- water
- is viscous:
periphery : ectoplasm (or cytogel / plasmagel) is more viscous
interior : endoplasm (or cytosol / plasmasol) is a soluble aqueous material
- moves actively to "stir" (i.e. cytoplasmic streaming)
- contains a number of enzymes which are responsible for many metabolic pathways
Functions:
1. it holds the organelles and essential materials
2. it is the major site of many important biochemical processes.
3. it stores food in some cells (e.g. egg cells)
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III Nucleus
- the shape may related to that of the cell or completely irregular
- almost all eukaryotic cells are mono-nucleate, but bi-nucleate cells ( some liver and cartilage cells ) ,
poly-nucleate / multi-nucleate cells (skeletal muscle cells) and anucleate cells (e.g. mature human erythrocytes and
blood platelets) also exist.
Nuclear envelope
- composed of two membranes (double membrane)
- with nuclear pores for the passage of large molecules, e.g. RNA
- serves as physical barrier for restricting the material exchange between the nucleus and
cytoplasm.
Contents:
1. Nucleoplasm
the ground matrix in nucleus
2. Chromatins
- hereditary material DNA molecules associated with proteins to form invisible chromatins
- chromatins will coil into dense visible chromosomes (see the notes “Genetics”) during
mitosis and meiosis
3. Nucleolus
- manufacture of ribosome
Types of cells according to the form of nucleus
1.
Eukaryotic cells
- Cell in which the nucleus is enclosed in a definite membrane and has other
membranous structures such as mitochondria, chloroplasts, endoplasmic
reticulum etc.
e.g. all higher plant cells, all animal cells, algae, fungi, protozoan.
2.
Prokaryotic cells
- Cell in which genetic material is not confined within a nuclear membrane
and is absent of internal membranous organelles (inwardly folding of
plasma membrane in cytoplasm may be found to be related to respiration
or photosynthesis)
e.g. bacterium, blue-green algae.
Functions of nucleus:
1. to contain the genetic material of a cell in the form of chromosome
2. to carry the instructions for the synthesis of proteins in the nuclear DNA
3. to act as a control center for the activities of a cell
4. to be involved in the production of ribosomes
5. Nuclear division is the basis of cell replication and hence reproduction.
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IV. Membranous organelles
1. Endoplasmic Reticulum (ER)
- complex network of membrane extending throughout the cytoplasm
- these channels often appear to be in folding which is continuous with the outer nuclear membrane
(it is actually an extension of the outer nuclear membrane but without pores)
- flattened membranous sacs are called cisternae (singular: cisterna)
- types of ER
i. Rough ER (Granular ER)
- with numerous granules (ribosomes) attached
--->
to transport the proteins synthesized from ribosomes
 especially abundant in cells active in protein synthesis
e.g. secretory cells.
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ii. Smooth ER
- lack of ribosomes
---> concerned with synthesis and transport of lipids and steroids
 rich in steroid-secreting cells such as the interstitial cells of testis for the synthesis of male sex hormones.
- It also gives rise to Golgi bodies.
- functions of ER
i. biosynthesis
- smooth ER may assemble lipids and steroids carbohydrates?
- with ribosomes attached, the surface of rough ER isthe place where secretory and
membrane proteins are synthesized
ii. transportation
since the network extends throughout the cytoplasm, wastes and nutrients are transported
intracellularly
iii. support
extends throughout the cytoplasm to provide a supplementary mechanical support and forms
a sort of "cytoskeleton" to maintain the cell shape
iv. increase surface area
provides a lot of surface area for biochemical reactions (which largely depend on the
enzymes attached on membrane such as those concerned with protein and steroid synthesis)
to occur
v. storage
the "sarcoplasmic reticulum" in striated muscle cells stores calcium ions which involves in
muscle contraction
vi. detoxification
in liver cells, both rough and smooth ER are involved in detoxification of various drugs
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2. Golgi Body (or Golgi apparatus, Golgi complex)
- is a secretory organelle
- has been suggested to be modified from the ER
- structure of Golgi Body -- can be divided into 3 level of organization
i. Cisternae
- is a fluid-filled sac or cavity bound by smooth-surfaced membrane
- consists of a central plate-like flattened region
- is the basic functional unit of the Golgi Apparatus
- formed from vesicles pinched off the ER
ii. Golgi vesicles
- are formed (pinched off) from the edges of the cisternae
- usually are secretory vesicles
iii. Golgi apparatus / Dictyosome
- is a stack of cisternae
- in plant cells : a number of separate stacks (dictyosomes)
in animal cells : a single larger dictyosome is more often
- golgi apparatus are polarized in structure which consists of both convex and concave faces::
i. Forming face (outer face / convex face / cis- face)
- the side facing ER
- constantly formation of cisternae by fusion of vesicles
- the vesicles are probably derived from buds of the smooth ER
ii. Maturing face (inner face / concave face / trans- face)
- the face where cisternae break up into vesicles (secretory vesicles)
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- all proteins produced by ER are passed through golgi apparatus in a strict sequence :
proteins pass first through the cis-golgi network
Proteins and lipids undergoing transport are modified and added with
labels in the stack of cisternae
Proteins and lipids are sorted and packaged as vesicles in the trans-face
and are sent to their final destinations
(Here the proteins and lipids are sorted and sent to their final
destinations . in general, the golgi acts as the cell’s post office ,
receiving, sorting and delivering proteins and lipids. )
Secretion of proteins
1. The Golgi apparatus processes the newly synthesised protein (e.g. by adding a carbohydrate component) as
glycoprotein.
2. Small fluid-filled vesicles, containing the finished product (e.g. digestive enzyme, hormone etc.) become pinched
off the ends of the Golgi apparatus.
3. These vesicles move towards and finally fuse with the cell membrane. The contents of the vesicles are then
discharged to the outside.
Secretion :
the release of intracellular products from the cell to the outside and the products would be used
elsewhere.
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- function of Golgi apparatus:
i. storage, chemical modification, concentration and packaging of secretory vesicles
- e.g. secretory digestive enzymes form pancreas
materials that manufactured elsewhere in the cell move along the ER into cisternae of Golgi apparatus, via
the forming face, where they are packaged into small bubble-like secretory vesicles (e.g. secretory digestive
enzymes from acinar cells of pancreas into pancreatic duct, along which they pass to the duodenum---pathway have been confirmed by using radioactively labeled amino acids and following their incorporation
into protein and subsequent passage through different cell organelles. Fig 7.20 P. 204 Biological Science
2nd ed.) by pushed to the end of the organelle and pinched off.
- dilute secretion is firstly concentrated in the Golgi apparatus before discharged.
- producing glycoproteins (glycosylation)
e.g. mucin - (an important glycoprotein secreted by the golgi apparatus which form mucus in solution)
- is secreted by goblet cells of the respiratory and intestinal epithelia.
many cell secretions are in the form of glycoproteins and, although considerable glycosylation (adding
carbohydrates) takes place in ER, the finishing touch is a Golgi function.
ii. sometimes involved in lipid transport
e.g. intracellular transport of resynthsized lipids across epithelial cells of small intestinal
villi into lacteal.
when digested lipids are absorbed as fatty acids and glycerol in the small intestine, they are
resynthesized to lipids in the SER, coated in protein and then transported through the Golgi apparatus to
the plasma membrane where they leave the cell, mainly to enter the lymphatic system.
iii. lysosomes formation
in addition to the secretion of proteins, glycoprotein, carbohydrates and lipids, Golgi apparatus is the site for
the lysosome formation
iv. Secretion of carbohydrates
e.g polysaccharides of the cell wall matrix rather than cellulose
v.membrane differentiation
new membrane is synthesized at the ER, transferred to the Golgi apparatus where
modifications occur. Then added to the plasma membrane by fusions of Golgi vesicles
during exocytosis.
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3. Lysosomes
- lysis = 'splitting'; soma = 'body'
- tiny single membrane bounded vesicles containing digestive enzymes (lysozymes)
- located in most cells
proteins synthesis on rough ER
transported to Golgi apparatus
bud off of Golgi vesicles (Primary lysosomes) with processed enzymes
-
-
become Secondary lysosomes
(fuse with the vesicles formed by endocytosis or other intracellular organelles)
or
be released
secondary lysosomes
---> decomposition of endocytic nutrients, intracellular organelles, or even the entire cells
Two types of secondary lysosomes can be identified :
i. Heterophagic Vacuoles
They are formed by the fusion of primary lysosomes and endocytic vesicles which contain extracellular substances brought into
the cell by endocytosis.
ii. Autophagic Vacuoles
They are formed by the primary lysosomes and the cell's own organelles i.e. old mitochondria. fragments of ER etc. This kind of
secondary lysosomes is involved in the normal repair of cells.
-
Residual bodies
The undigested substances are retained within the vacuoles as residues. Lysosomes containing such residues are called residual bodies.
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- Functions of lysosomes
(All functions of lysosomes are related to their digestive enzyme content.)
i. digestion of endocytic (phagocytic / pinocytic) materials
- to form secondary lysosome or food vacuole.
- The products of digestion are absorbed and assimilated by the cytoplasm of the cell leaving
undigested remains be egested by the cell by exocytosis.
e.g. WBC ingests bacteria or other harmful materials; protozoa ingests food.
ii. facilitate secretion of some hormones
- digest the inactive thyroid hormone, thyroglobulin, to active form, thyroxine in thyroid
gland cell before the hormone is secreted to the blood circulation.
iii. autophagy
- to destroy redundant, worn-out or ageing cell organelles
iv. release of enzymes outside the cell ( extocytosis )
- lysosomes facilitate the release of cellular enzyme content.
v. autolysis
- i.e. self-destruction of an old, injured or dead cell by release of the contents of lysosomes
within the cell.
(in such circumstances, lysosomes have sometimes been called "suicide bags".)
- Lysosome are abundant in :
i. epithelial cells of absorptive, secretory, and excretory organ ( liver, kidney etc.)
ii. phagocytic cells (bone marrow, spleen & liver ) and leukocytes ( esp. granulocytes) .
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4. Vacuoles
- fluid (cell sap)-filled sacs bounded by Tonoplast (a single membrane)
- a concentrated solution of minerals salts, sugars , organic acids, oxygen, carbon dioxide, pigments
and some waste and "secondary" products of metabolism.
- single large one in plant cells but small and many in number in newly divided plant cells and
animal cells (e.g. food vacuoles, phagocytic vacuoles and contractile vacuoles)
- Functions of vacuoles
i. Development of turgor pressure
- solutes inside cause water enters the cell sap by osmosis,
- builds up turgor pressure within the cell
- cytoplasm is pushed by the enlarged vacuole against the cell wall
 - cell elongation
- turgidity provides support in young stems and herbs..
ii. Deposition of pigments
- e.g. anthocyanins (which are of various colours: red, blue and purple) and other related
compounds which are shades of yellow and ivory.
 give rises colours in flowers, fruits, buds and leaves to attract insects and birds for
pollination and dispersal.
iii. Autolysis of aged plant cell
- that contain hydrolytic enzymes and act as lysosomes during life. when the tonoplast loses
its partial partialmeability, the enzymes escape causing autolysis.
iv. Deposition of wastes
the wastes may accumulate in the vacuoles of leaf cells and are removed when the leaves fall.
v. Temporary food and chemical reserves
- e.g. sucrose, mineral etc. can be temporary stored as in vacuoles.
vi. Osmoregulation
e.g contractile vacuoles in some single-celled (unicellular) organisms, such as amoeba and
paramecium.
High temperaturea and organic solvents e.g. alcohols, denature membrane proteins and
increase fluidity of membrane lipids. Organic solvents at high concentrations can also
dissolve lipids. Acetone, alcohol and chloroform are organic solvents that severely destroy
membranes.
Living beetroot cells, contains a red pigment called anthocyanin in the large central
vacuoles, are suitable materials for experiments to demonstrate the effects of high
temperature and chemicals on the permeability of membranes in cell.
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5. Mitochondrion
- Spherical or sausage in shape
- Structure of Mitochondrion
i. surrounded by 2 unit membranes:
- the outer membrane - smooth, serves protection
- the inner membrane - highly folded to 'cristae' where respiratory enzyme and electron
carriers located
- greatly increase the surface area for efficient enzymatic reactions
(oxidative phosphorylation)
- with stalked elementary particle on it, contain enzymes for
phosphorylation of ADP (ATP generation)
ii. matrix- a semi-rigid material
- contains all soluble enzymes for Krebs cycle and oxidation of fatty acids
- contains circular DNA, smaller ribosomes and other RNAs for synthesis of
protein of mitochondrion
iii. intercristal space - fluid-filled cavity between inner and outer membrane
- no special function about it is reported
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- Function of Mitochondrion
i. energy metabolism and aerobic respiration
- terminal catabolism of carbohydrates, lipids and proteins (Note that the preliminary degradation
of these compounds occurs in the cytoplasm.)
-- Krebs cycle (in matrix) and oxidative phosphorylation (on cristae of inner membrane )
-- fatty acid oxidation in matrix
 produce chemical energy (ATP)
ii. heat production
- energy dissipated as heat instead of being converted into ATP
 maintain body temperature
Thus mitochondria are present in great number in the cells that require a lot of energy, e.g. in the
muscle cells (for muscle contraction), cells in the wall of the kidney tubule (for active transport)
and the sperms (for locomotion).
-
Reproduction of Mitochondrion
i. Mitochondrial DNA
- carries information for structural protein of inner membrane only
ii. Nuclear DNA
- catalytic proteins and structural proteins of mitochondria are under nuclear control and synthesis in
cytoplasm
-
Evolution of mitochondrion -- the Endosymbiont Theory
It states that mitochondria originated from the symbiosis of a prokaryotic organism (bacteria-like organisms)
with a host cell which was anaerobic.
evidence:
i. circular mitochondrial DNA (often seen in today bacteria)
ii. mitochondrial ribosomes smaller than cytoplasmic ribosomes (but equivalent in size to bacterial ribosomes)
iii. movement of mitochondrial similar to that of some bacteria
iv. both mitochondrial and bacterial protein synthesis mechanism can be inhibited by an antibiotic,
chloramphencol, which plays no action on cytoplasmic protein synthesis.
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6. Plastids (plant cells only)
- small bodies of variable sizes and shape ( round, oval or disc like particles )
- surrounded by two unit membranes
- located in nearly all plant cells
- responsible for carbohydrates metabolism
e.g. amyloplasts
in some seeds
store starch, lipidoplasts store lipids in oily nuts and sunflower seeds, and proteoplasts store protein
Chloroplasts
- the most found plastids in plant cells
- biconvex in shape
- contain the photosynthetic pigments, esp. chlorophyll.
- Structure of the chloroplasts
i. Envelope (double outer unit membrane )
ii. Stroma
- a homogenous watery matrix contains:
-- enzymes for the carbon dioxide fixation processes in photosynthesis.
-- circular DNA, RNA and smaller ribosomes
-- oil droplets, starch grains and end-products of photosynthesis.
iii. Granum and Intergranal lamella
- scattered within the stroma
- each granum consists of membrane-bounded disc-shaped vesicle (Thylakoids)
arranged like a pile of coins
- thylakoid membrane contain all of the energy-generating systems e.g. photo-pigments,
electron transport chain, and ATP synthetase
- The grana are connected with each other by Intergranal lamella. Both are the sites for
light reaction of photosynthesis.
(At the granal region, the 2 unit membranes are slightly further apart enclosing another thin membrane in
between.
Numerous chlorophyll granules called quantasomes are attached to the surface of the membranes. )
 -- hold the chlorophyll molecules in a suitable position for trapping maximum amount
of light.
-- The stacking arrangement increases the surface area for the attachment of pigment
molecules on the membranes.
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Photosynthetic pigments
Possible origin of chloroplasts ( endosymbiont theory ):
The granum contains DNA, ribsomes suggests that the chloroplasts are also a kind of semiautonomous organelles. The chloroplasts might be the "residue" of primitive aglae once
lived symbiotically in the cells of non-green organisms.
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- Functions of chloroplasts
- mainly for photosynthesis:
- light energy is absorbed and transformed into chemical energy
- oxygen is then released
- the sites for light reaction are thylakoids and intergranal lamella
- Dark reaction (carbon dioxide and water are used as raw material to form organic
material) take place at the stroma
ii. Synthesis of fatty acids
- in stroma, carbohydrates are converted to fatty acids in the aid of ATP and NADPH.
iii. Reduction of nitrite (NO) to Ammonia (NH)
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Conpartmentalization
Most intracellular organelles i.e. mitochondria. lysosomes are bounded with membranes. Within the
membrane-bound compartments, different intracellular pH, different enzyme systems can be isolated
from the other organelles.
The main significance of the "compartmentalization" ( separate various organelles by forming different
compartment in the cell ) is to enable the cell carry out different metabolic activities at the same time.
V. Non-membranous organelles
1. Ribosomes
- site of proteins synthesis
 holding the various molecules involved in the correct position.
- may attach to surface ER or free in cytoplasm
- when occur in clusters, called polysomes or polyribosomes
- spherical, consists of 2 subunits composed of ribosomal RNA (rRNA) and proteins
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2. Microtubules
- they are very fine, slender, hollow unbranched tubes
- found in all eukaryotic cells
- either free in the cytoplasm or forming part of centrioles, cilia and flagella
- they are tubules 25 nm in diameter, several micrometers long
-
chemical composition of the microtubules:
- main component is two similar proteins, alpha- and beta-tubulin
- the assembly of tubulin dimer gives rise of formation of microtubules
-
Microtubule-organising centres ( MTOCs )
- the structures to initiate the polymerization of the tubulin subunits to microtubules
- e.g. centrioles (not in plants, plants may contain smaller MTOCs)
-
Functions of microtubules:
i. Mechanical function
- they form a kind of "cytoskeleton"
- the shape of the cell and cell processes ( microvillus, cilia and flagella ) is dependent on microtubles.
ii. Involved in formation of centrioles, basal bodies of cilia & flagella
- centrioles:
-- contraction of the spindle fibres
-- movement of chromosomes and centrioles
- flagellum
-- for locomotion (algae, protozoa and sperm cells)
- cilium
-- locomotion by
--- beating in aquatic medium (e.g. Paramecium—a protozoa)
--- by gliding on smooth surfaces (e.g. cilia found on the underside of planarian.)
-- drawing material forward (e.g. ciliated epithelium of trachea)
-- feeding (e.g. Paramecium beat the cilium coordinately to create currents in order to feed preys )
-- material flow
They beat to create current for material flow
e.g. for the circulation of cerebrospinal fluid
iii. Intracellular transport
- microtubules have also been implicated in the movements of other cell organelles such as Golgi vesicles.
iv. act as the framework for cellulose cell wall to laid down
(for reference)
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Centrioles
- may be found lying near the nucleus in non-dividing cells
- usually 2 centrioles lying perpendicular to each other
- each centriole contains nine identical triplets of microtubules
( reference: )
centrosome (or centrosphere)
- the deeply stained region where the centrioles
found
- as MTOCs ( microtubule-organising centres ) during the cell division
* during cell division, centrioles duplicate themselves. From each centriole there extend
spindles fibre to attach the centromeres of the chromosomes and the spindle fibres appear
to "pull" the chromosomes to the poles
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Basal body
- found at the base of all cilia ( / cilium -- shorter but numerous) and flagella ( / flagellum -- hair like, longer but
fewer in number)
- identical in structure to the centriole
- essential for formation of cilia and flagella (both being a bundle of eleven hollow fibrils enclosed within a
membrame in "9+2 pattern")
- usually located at cell surfaces and are anchored at a kineosomes
- Both are originated from basal bodies in the 9+2 pattern :
9 groups of peripheral microtubules: double
2 central microtubules : single
(reference)
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3. Microfilaments
-
microfilaments are very fine protein filaments (actin)
about 5-7 nm in diameter
they are thought to be important in the cytoplasmic cleavage during cell division.
-
three differences between microtubules and microfilaments :
Microtubules
Microtubules
large (25 nm)
small (5-7 nm)
composed of tubulin
consist of actin
for - ciliary movement
- separation of chromosomes
... etc
for - cytokinesis and
- muscle contraction.
(reference)
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VI.
P. 30
Cell wall
---->
- surround and protect the plasma membrane
- as a framework provides mechanical support for plant tissues
- Chemical composition:
- composed principally of i. cellulose
ii. hemicellulose
iii. pectin
- other substances may be found in cell wall
e.g. lignin, cutin, suberin
- adjacent cell walls are cemented together with pectin. ( this layer is called Middle
Lamella ) .
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- Structure of cell wall:
- cell walls are complex and highly differentiated in some sequences, as shown in below
diagram.
i. Middle Lamella
- the first structure of cell wall formed after cell division
- a thin layer of calcium pectate ( a kind of pectin )
- represents a "sticky substances" between and to cement two adjoining cells.
ii. Primary cell wall
- thin and non-living layer secreted by the protoplast
- major composition is cellulose
- tough and slightly elastic  allow considerable stretching during cell growth
- the only cell wall in young undifferentiated cells.
iii. Secondary cell wall
- between primary cell wall and plasma membrane
- composed mainly of the cellulose
- but other substances (e.g. lignin in xylem and sclerenchyma cells, or suberin in cork
tissues) maybe incorporated
- lignin may produce a rigid structure of the cell wall.
- suberin provides water-proof property.
*
in different layers of a cell wall, cellulose orientated at different angle.
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iv. Pits and Plasmodesmata
- Pit - some disrupted places on secondary cell wall
- where adjacent cells are separated only by a primary wall
- Plasmodesmata - fine cytoplasmic strands (about 1 um)
- passing through the pit
- and connecting the cytoplasm of adjacent cells
---> facilitate movement of materials between the plant cells.
* Symplast (sym = together) :
as the cytoplasm of the living cells in a plant is interconnected via the plasmodesmata into a
single continuous unit, the entire continuous network of cell cytoplasms is called symplast.
Apoplast
On the other hand, the continuous system of cell walls and intercellular spaces and vessels
lumens in plant tissues is called apoplast.
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- Functions of cell wall
i. Mechanical support (機械性支持)
- mechanical strength and skeletal support
- provided for individual cells and for the plant as a whole
- extensive lignification (木質化) increase strength in some walls.
ii. Develop turgidity (硬漲度) to give rise support
- contributes to the support of all plants
- main source of support in herbaceous plants (草本植物) and organs.
iii. As pathway of water transport intercellularly
a) The system of interconnected cell walls ( apoplasm 非原質體) is a major
pathway of movement for water and dissolved mineral salts.
b) The plasmodesmata (胞間連絲) allow the protoplasm (原生質) of the cells to
link in a system called the symplasm (共質體)
iv. Against desiccation (抗乾化)
- cell walls develop a coating of waxy cutin 角質( cuticle 角質層on epidermal cells)
or impregnated with a suberin (木栓質) layer in cork cells
---> reduce water loss and risk of infection (感染的危機).
v. Food storage (食物貯存)
- some cell walls are modified as food reserves
- e.g. storage of hemicellulose (半纖維素) in some seeds (種子).
vi. Regulate direction for water transport (調節水份運輸方向)
- the cell walls of root endodermal cells (根部內皮細胞的細胞壁) are penetrated
with suberin (木栓質) forming a barrier (屏障) ( Casparian strip 凱氏帶 / 嘉氏
帶 ) on the radial and transverse walls (輻射壁及橫壁)
---> ensures (確保) all the water and solutes (溶質) entering the stele (中柱) pass
through the cytoplasm of the endodermal cells (內皮細胞的細胞質).
vii. Forming rigid tube of long distance material transport
(形成物質長途運輸的硬管)
- The walls of xylem (木質部), and sieve tubes of phloem (韌皮部篩管) are adapted
(適應) for long-distance translocation (長途輸導) of materials since the thick and
rigid cell wall (厚且硬的細胞壁) can resist (抵抗) the force (力) either from
"pulling (拉) " or "pushing (推)".
viii. For protection of internal structure of plant cells
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Summary of Membrane System & Organelles of a Cell
(a)
Meaning of Organelle, Cell, Tissue and Organ
Organelle : a discrete and specialized part of a cell having specific functions.
(e.g. chloroplast)
Cell
:
smallest unit of living organism, consisting of a piece of nucleated cytoplasm
surrounded by a membrane (and a cell wall in plant); existing singly or in group, and
containing various organelles.
(e.g. epidermal cell / guard cell in the leaf)
Tissue :
association of usually one kind of cells, bound together by middle lamella in plant or
intercellular matrix in animal, that perform a common function.
(e.g. epidermis / palisade tissue / spongy tissue)
Organ :
group of tissues integrated together to perform a specific function for an organism.
(i.e. leaf)
Organelles
 Cell  Tissue  Organ  System  Organism
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Summary of Membrane Systems & Organelles of a Cell
STRUCTURE
DESCRIPTION
FUNCTIONS
MEMBRANE SYSTEMS
plasma
membrane
unit membrane with fluid mosaic
structure
selective barrier, retains cell contents
rough ER
intracellular membrane system
encrusted with ribosomes
synthesis of proteins for export,
intracellular transport
smooth ER
intracellular membrane system
without ribosomes
synthesis of lipids including steroids
Golgi body
specialised smooth ER forming a
stack of disc-shaped cavities
(cisternae)
synthesis of glucoproteins,
polysaccharides and hormones,
production of lysosomes
Nuclear
membrane
paired membranes surrounding
nucleus
regulates exchange between nucleus and
cytoplasm, some protein synthesis
nuclear envelope
ribosomes
small particles with complex
structure
Site for synthesis of proteins
lysosomes
spherical vesicles containing
enzymes
intracellular digestion
nucleus
major cell organelle containing
chromatin
regulates cell activities, carries
hereditary information
nucleolus
specialised region of nucleus, not
surrounded by membranes
synthesis of RNA and ribosomes
mitochondrion
cell organelle with inner and outer
membranes carrying enzymes for
respiration
production of ATP
chloroplast
cell organelle containing
chlorophyll and enzymes for
photosynthesis
production of carbohydrates from simple
raw materials (CO2 and H2O)
microtubules
protein tubes
intracellular support 'cytoskeleton'
centrioles
rod-like structures containing
microtubules
cell division in animal cells
cilia and flagella
fine hairs projecting from cell
surface
9+2 arrangement of microtubules
cell locomotion, transport of
extracellular materials
cellulose cell
wall
layered structure of cellulose
microfibrils found in plants
support, protection
vacuoles
variable
granules
variable
storage of food, chemicals and waste
materials
storage : starch grains in plants,
glycogen in animals
ORGANELLES
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