2. Sejarah Sel
◼ Antoni van Leewenhoek (1665)
membuat dan menggunakan mikroskop,
menyebut sel sebagai satuan kehidupan.
Antoni van Leewenhoek
adalah orang yang pertama
kali yang melihat sel tunggal
dan mengamati darah, cairan
mani, feses, dan email gigi
3. Sejarah Sel
◼ Robert Hooke (1666)
melihat rongga kosong pada sayatan jaringan gabus
tumbuhan kemudian dinamakan cellula
Mikroskop rancangan Hooke
yang digunakan untuk
mengamati sel tumbuhan
4. ◼ Schleiden & Schwann (1838 & 1839)
Teori sel: semua mh terdiri dari sel-sel, sel = unit struktural terkecil
dari semua mh.
Max Schultze dan Thomas Huxley
sel merupakan satu kesatuan fungsional kehidupan.
◼ Johannes Evangelista Purkinje (1839)
mengenalkan istilah protoplasma (zat yg pertama kali dibentuk,
tersusun dari nukleus dan sitoplasma [lebih cair])
Sejarah Sel
5. Rudolf Virchow (1858)
setiap sel yg ada berasal dari sel yg sebelumnya →
sel merupakan kesatuan pertumbuhan → sel merupakan
kesatuan heriditas dari makhluk hidup.
Walther Flemming dan Eduard Strasburger. Mereka menemukan
bahwa sel berkembang biak dengan cara membelah diri.
Sehingga dikatakan bahwa sel merupakan kesatuan reproduksi
dari makhluk hidup.
10. Fungsi sel tumbuhan :
• sebagai penyusun tubuh tumbuhan
• penggerak seluruh aktivitas di dalam tubuh tumbuhan,
berkaitan dengan proses pertumbuhan dan perkembangan
pada tumbuhan,
• pembawa sifat genetik pada tumbuhan, dan berkaitan
dengan reproduksi tumbuhan.
Setiap bagian sel tumbuhan memiliki fungsinya mesing-masing
yang berkaitan dengan fungsi sel secara keseluruhan.
11. Bakteri hewan tumbuhan
Struktur eksterior:
Dinding sel
membran sel
flagela
+ (protein)
+
+
-
+
+
+ (selulosa)
+
-
Struktur interior:
RE
ribosom
mikrotubul
sentriol
Aparatus golgi
+
+
-
-
-
+
+
+
+
+
+
+
+
-
+
Organel lain:
Nukleus
Mitokondria
kloroplas
kromosom
Lisosom
Vakuola
-
-
-
Single circle
-
-
+
+
-
Multiple, DNA,
protein komplek
+
+
-/kecil
+
+
+
multiple, DNA,
protein komplek
+/sferosom
-
+/besar
Perbandingan sel bakteri, hewan dan tumbuhan
12. Perbandingan organisme prokariot dan eukariot
Prokariot Eukariot
Organisme Bakteri, sianobakteri Protista, fungi, tumbuhan,
hewan
Ukuran sel 1-10 µm 10-100 µm
metabolisme Anaerobik/aerobik Aerobik
organel Sedikit/tidak ada Ada
DNA Bentuk sirkular terdapat
bebas di sitoplasma
Linier, panjang, dikemas
dalam kromosom, terletak di
nukleus
RNA &protein Disintesis di sitoplasma Disintesis di nukleus, protein
disintesis di sitoplasma
sitoplasma Tidak ada skeleton, tdk
ada aliran sitoplasma, tdk
terjadi endositosis atau
eksositosis
Punya skeleton
Punya aliran sitoplasma
Terjadi endositosis &
eksositosis
Pembelahan sel Pembelahan biner Dengan mitosis &meiosis
Organisasi seluler Umumnya uniseluler multiselular
13. Struktur Sel Prokariotik
Semua sel prokariotik memiliki membran plasma (ingat membran plasma,
bukan membran inti), nukleoid (berupa DNA atau RNA),
dan sitoplasma yang mengandung ribosom. Sel prokariotik tidak
mempunyai membran inti, maka bahan inti yang berada dalam sel
mengadakan kontak langsung dengan protoplasma. Ciri lain dari sel
prokariotik adalah tidak memiliki sistem endomembran (membran dalam),
seperti retikulum endoplasma dan kompleks Golgi. Selain itu, sel
prokariotik juga tidak memiliki mitokondria dan kloroplas,
yaitu mesosom dan kromatofor. Contoh sel prokariotik adalah bakteri dan
alga hijau biru.
14. Pengertian Sel Tumbuhan dan Fungsinya
Secara umum struktur sel tumbuhan dapat di bagi menjadi 3 bagian yaitu inti sel,
sitoplasma, dan membran sel. Setiap bagian sel tumbuhan memiliki fungsi yang
berbeda-beda. Berikut penjelasan lebih lanjut mengenai bagian-bagian sel
tumbuhan.
15. Inti sel atau nukleus merupakan organel sel yang berada di dalam sel eukariotik. Sel
tumbuhan termasuk kelompok sel eukariotik. Di dalam inti sel banyak mengandung materi
genetik yang berbentuk DNA (deoxyribonucleic acid). Di dalam DNA tersimpan informasi
genetik yang berbentuk polinukleotida.
Fungsi inti sel adalah untuk mengontrol segala aktivitas sel berdasarkan informasi genetik
yang dibawa oleh DNA. Informasi genetik ini nantinya akan diturunkan ke generasi
selanjuntya. Sehingga bisa dianggap bahwa nukleus merupakan organel terpenting dalam
sebuah sel.
16. Struktur inti sel tumbuhan tersusun dari membran nukleus, nukleoplasma,
kromosom, dan nukleolus.
1. Membran Nukleus atau Selaput Inti (Karioteka)
17. Jika dilihat menggunakan mikrospkop elektron, membran nukleus
terdiri dari 2 selaput yaitu selaput luar dan selaput dalam. Selaput
luar berhubungan langsung dengan sitoplasma sehingga banyak
ditempeli oleh ribosom.
Di antara dua selaput tersebut terdapat celah sempit yang disebut
perinukleus atau intermembran space. Selain itu membran nukleus
juga memiki pori-pori sehingga memungkinkan terjadinya interaksi
antara nukleoplasma (cairan di dalam nukleus) dan sitoplasma.
Fungsi membran nukleus adalah sebagai pintu yang menghubungkan
nukleoplasma dan sitoplasma. Dikarenakan terdapat pori-pori pada
membran nukleus. Dengan adanya pori-pori ini memungkinkan
molekul RNA pada nukleoplasma bisa keluar ke sitoplasma.
18. 2. Nukleoplasma (Kariolimfa)
Nukleoplasma adalah cairan kental yang mengisi bagian dalam nukleus. Cairan ini
tersusun atas asam nukleat (DNA dan RNA), protein , dan mineral garam. DNA dan
RNA merupakan materi pembawa sifat genetik yang banyak terkandung dalam
nukleoplasma. Apabila kedua materi genetik in bergabung dengan protein maka
disebut sebagai nukleoprotein.
Fungsi nukleoplasma adalah sebagai suspensi (zat cair yang mengandung zat padat)
bagi organel sel yang ada di dalam nukelus. Selain itu, nukleoplasma juga berfungsi
untuk mempertahankan bentuk nukleus dan sebagai media transportasi zat-zat
yang dibutuhkan oleh nukleus.
19. 3. Kromatin dan Kromosom
Kromatin jika diamati menggunakan mikroskop elektron terlihat seperti butiran-butiran
yang tersebar di dalam nukleus. Ketika dalam proses pemebelahan kromatin tidak
terlihat, namun hanya terlihat benang-benang kromosom. Sebutan kormosom digunakan
untuk menunjukkan kromatin yang berubah menjadi benang-benang halus ketika sel
sedang membelah diri.
Fungsi kromatin adalah sebagai pembawa informasi genetik yang berguna untuk
mengendalikan seluruh aktivitas sel. Hal ini disebabakan karena kromatin tersusun dari
DNA (16%), RNA (12%), dan nukleoprotein (72%). Sebelum dikeluarkan ke sitoplasma
informasi pada DNA disalin dan membentuk RNA. Jadi RNA adalah salinan dari DNA dan
berfungsi menyalurkan informasi genetik.
20. 4. Nukleolus (Anak Inti)
Nukleolus atau anak inti adalah sebuah organel sel yang terletak di dalam nukelus dan
berukuran lebih besar dari kromatin. Komposisi nuklelolus sebagian besar terdiri dari
benag-benang halus DNA.
Fungsi nukleolus adalah sebagai tempat berlangsungnya sintesis RNA. Di dalam
nukleolus informasi genetik yang dibawa oleh DNA diuraikan sehingga menghasilkan
rRNA. Molekul rRNA nantinya akan berfungsi sebagai penyusun organel ribosom di
dalam sitoplasma.
22. Sitoplasma adalah cairan kental seperti gel yanng mengisi rongga di dalam
sel. Di dalam sitoplasma terkandung banyak air dengan persentase sebesar
80%. Sitoplasma biasanya tidak berwarna atau bening. Selain itu di
dalamnya juga banyak terkandung garam sehingga dapat dengan baik
menghantarkan arus listrik.
Fungsi sitoplasma
• media suspensi bagi partikel-partikel kecil dan organel-organel sel.
• meyalurkan dan melarutkan zat-zat makanan yang dibutuhkan organel-
organel sel untuk melakukan aktivitas.
• mengatur posisi kloroplas. Sitoplasma akan membantu kloroplas
berpindah ke bagian yang terkena sinar matahari lebih banyak. Sehingga
bisa memaksimalkan proses fotosintesis.
Secara garis besar sitoplasma bisa dibagi menjadi 3 bagian yaitu sitosol,
organel-organel sel dan inklusi sitoplasma.
23. Sitosol
Sitosol merupakan cairan kental yang terdiri dari air, garam dan senyawa-
senyawa organik. Sitosol sebagian besar terdiri dari air sebanyak 70% dan berisi
campuran benang-benang sitoskleton (kerangka sel), senyawa organik dan
anorganik.
Fungsi sitosol adalah sebagai sumber bahan makanan bagi sel dan organel-
organel sel. Hal ini karena sitosol juga mengandung senyawa organik seperti
garam, protein, asam lemak. Sitosol juga berfungsi sebagai tempat terjadinya
proses metabolisme seperti sintesis protein dan asam lemak.
25. • Di dalam sitoplasma terdapat berbagai macam organel sel.
• Setiap organel orgenel sel memiliki bentuk, kerakteriktik dan fungsinya
masing-masing.
• Jumlah organel sel di sitoplasma lebih banyak daripada yang ada dalam
nukleus.
• Organel sel pada tumbuhan terdiri dari ribosom, lisosom, retikulum
endoplasma, mitokondria, badan golgi, kloroplas, vakuola, mikrotubulus,
mikrofilamen, dan peroksisom.
Fungsi organel-organel sel secara keseluruhan adalah untuk mendukung
aktivitas sel, karena jika ada satu organel yang tidak berfungsi maka sel tidak
bisa bekerja dengan baik.
26. Ribosom adalah organel sel tumbuhan yang mengandung protein (40%) dan
asam ribonukleat atau RNA (60%). Terdapat 2 jenis ribosom yaitu ribosom
terikat dan ribosom bebas. Ribosom terikat biasanya bergabung dengan
retikulum endoplasma.
Fungsi ribosom adalah sebagai tempat terjadinya sintesis protein.
1. Ribosom
27. 2. Retikulum endoplasma (RE)
Retikulum endoplasma adalah organel sel yang berbentuk seperti ruangan labirin,
dinding atau membran pada RE terlihat berliku-liku sperti labirin. Terdapat dua jenis
retikulum endoplasma dalam sel tumbuhan yaitu RE kasar dan halus. Pada RE kasal
permukaannya di tempeli oleh butiran-butiran ribosom. Sedangakan pada RE halsu
tidak ditempeli ribosom.
Fungsi retikulum endoplasma adalah sebagai jalur yang menghubungkan nukleus
dan sitoplasma.
28. 3. Mitokondria
Mitokondria adalah organel sel yang berbentuk bulat lonjong seperti sosis. Berdasarkan
hasil pengamatan menggunakan mikroskop elektron organel sel ini memiliki dua
bagian yaitu membran luar dan membran dalam. Di antara keduannya terdapat sebuah
celah sempit yang disebut intermembran space. Membran dalam memiliki bentuk
berbelit-belit seperti labirin yang disebut sebagai kristae.
Fungsi mitokondria adalah untuk menghasilkan energi. Mitokondria pada tumbuhan
berfungsi untuk merubah okesigen dan zat gula menjadi karbondioksida dan energi
melalui proses repirasi selular. Karena fungsinya ini mitokondria juag dijuluki sebagai the
power house of cell atau gudang penghasil energi pada sel.
29. 4. Badan golgi (Aparatus Golgi atau Diktiosom)
Badan golgi atau aparatus golgi adalah organel sel yang berbentuk kantung
tipis tersusun secara berlapis-lapis. Bentuk badan golgi hampir mirip
seperti bentuk retikulum endoplasma, hanya saja terdapat lapisan
membran pada kantungnya.
Fungsi badan golgi adalah sebagai alat sekresi pada sel. Di dalamnya
terjadi proses perubahan dari enzim yang tidak aktif menjadi enzim aktif,
Selain itu, badan golgi juga berfungsi sebagai tempat penyimpanan
sekunder protein dan zat-zat lainnya yang berasal dari retikulum
endoplasma.
30. 5. Kloroplas (Plastida)
Kloroplas adalah organel sel yang hanya terdapat pada sel tumbuhan. Kloroplas
berbentuk bulat lonjong dan berwarna hijau. Mungkin bentuknya terlihat seperti
kacang hijau. Namun juga ada kloroplas yang berbentuk pipih atau bulat seperti
telur.
Seperti halnya mitokondria, kloroplas juga memiliki 2 membran yaitu membran luar
dan dalam. Pada bagian dalam membran dalam terdapat stroma dan tilakoid.
Stroma adalah cairan yang mengisi rongga di dalam kloroplas dan tilakoid tersusun
dari kantung kecil yang ditumpuk secara vertikal di dalam kloroplas.
31. 6. Vakuola (Rongga Sel)
Vakuola atau rongga sel adalah organel terbesar yang dapat dijumpai pada sel
tumbuhan. Vakuola berbentuk sperti karung yang didalamnya terdapat cairan yang
mengadung senyawa organik dan anorganik. Vakuola memiliki lapisan membran yang
disebut sebagai tonoplas.
Fungsi vakuola adalah sebagai tempat menyimpan zat-zat makanan seperti protein
dan zat gula. Di dalamnya juga tersimpan pigmen daun, buah, dan daun. Selain itu
vakuola juga berfungsi untuk mengatur tekanan di dalam sel, meenstabilkan tingkat
nilai PH dan mengisolasi zat sisa-sisa metabolisme sel.
32. 7. Mikrotubulus
Mikrotubulus adalah organel sel berbentuk tabung panjang dan tidak bercabang.
Organel sel ini mengandung molekul-molekul protein yang tersusun secara
melingkar seperti pegas sehingga berbentu seperti tabung panjang berongga.
Organel ini memiliki sifat kaku sehingga bentuknya tidak berubah-ubah.
Fungsi mikrotubulus adalah sebagai media transportasi zat, menjaga tekanan di
dalam sel, dan membantu replikasi kromosom.
33. 8. Mikrofilamen
Mikrofilamen adalah organel sel yang termasuk sebagai sitskeleton yang
berbentuk tabung panjang padat. Organel sel ini tersusun atas benag-benag yang
terbuat dari kumpula molekul protein dan aktin. Mkrofilamen biasanya ditemukan
di dekat membran sel.
Fungsi mikrofilamen adalah sebagai kerangka yang mempertahankan bentuk sel
agar tidak berubah-ubah.
34. 9. Peroksisom (Badan Mikro)
Peroksisom adalah organe sel berukuran kecil yang dilapisi oelh membran tunggal.
Peroksisom biasanya berinteraksi dengan retikulm endoplasma dan mengandung
sejumlah enzim. Setidaknya terdapat 40 enzim yang dilapisi oleh membran lipid
(lemak) ganda.
Fungsi Peroksisom :
• menguraikan hidrogen peroksida melalui proses fotorespirasi.
• mengubah racun menjadi air dan oksigen serta mengubah asam lemak menjadi
zat gula.
35. Inklusi Sitoplasma
Inklusi sitoplasma adalah zat-zat berukuran kecil yang terdapat di dalam
sitoplasma. Zat-zat inklusi tidak aakan larut di dalam sitoplasma. Terdapat banyak
zat inklusi seperti kalsium okslata dan silikon dioksida pada sel tumbuhan. Selain
itu juga granul. Inkulis juga dapat berbentuk butiran-butiran lipid yang tersusun
atas campuran lemak dan protein.
Fungsi zat-zat inklusi beraneka ragam tergantung jenis dan karakteristik zatnya.
Sebagai contoh granula berfungsi sebagai tempat penyimpanan amilum, glikogen,
dan polihidroksibutirat. Sedangakn butiran lipid berfungsi untuk menyimpan
cadangan makanan bagi sel tumbuhan.
37. Membran sel adalah lapisan terluar yang menyelubungi seluruh badan sel. Membran sel
tersusun atas fosfolipid dan protein. Sehingga menyebabkan membran sel memiliki sifat
selektif permeabel. Sifat ini menunjukkan bawha membran sel hanya bisa dilalui oleh zat-
zat atau ion-ion tertentu saja. Beberapa zat-zat tersebut adalah asam amino, glukosa, dan
gliserol.
Fungsi utama membran sel :
• melindungi bagian dalam sel
• membatasi sel dengan lingkuan diluar sel.
• sifat selektif permeabel → mengatur keluar dan masuknya suatu zat yang menuju ke
dalam atau keluar meninggalkan sel. Sehingga zat-zat berbahaya dari luar sel tidak dapat
masuk ke dalam sel.
38. CELL MEMBRANE FUNCTION
The cell membrane (plasma membrane) is a thin semi-permeable membrane that
surrounds the cytoplasm of a cell.
• Its function is to protect the integrity of the interior of the cell by allowing certain
substances into the cell, while keeping other substances out.
• It also serves as a base of attachment for the cytoskeleton in some organisms and
the cell wall in others.
• Thus the cell membrane also serves to help support the cell and help maintain its
shape.
• Another function of the membrane is to regulate cell growth through the balance
of endocytosis and exocytosis.
• In endocytosis, lipids and proteins are removed from the cell membrane as
substances are internalized.
• In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane
increasing cell size. Animal cells, plant cells, prokaryotic cells, and fungal cells have
plasma membranes. Internal organelles are also encased by membranes.
39. CELL MEMBRANE STRUCTURE
The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the
membrane’s location and role in the body, lipids can make up anywhere from 20 to 80
percent of the membrane, with the remainder being proteins. While lipids help to give
membranes their flexibility, proteins monitor and maintain the cell's chemical climate and
assist in the transfer of molecules across the membrane.
CELL MEMBRANE LIPIDS
Phospholipids are a major component of cell membranes. Phospholipids form a lipid bilayer
in which their hydrophillic (attracted to water) head areas spontaneously arrange to face the
aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail
areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable,
allowing only certain molecules to diffuse across the membrane.
Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are
selectively dispersed between membrane phospholipids. This helps to keep cell membranes
from becoming stiff by preventing phospholipids from being too closely packed together.
Cholesterol is not found in the membranes of plant cells.
Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain
attached to them. They help the cell to recognize other cells of the body.
40. CELL MEMBRANE PROTEINS
The cell membrane contains two types of associated proteins.
Peripheral membrane proteins are exterior to and connected to the membrane by
interactions with other proteins.
Integral membrane proteins are inserted into the membrane and most pass through the
membrane. Portions of these transmembrane proteins are exposed on both sides of the
membrane. Cell membrane proteins have a number of different functions.
Structural proteins help to give the cell support and shape.
Cell membrane receptor proteins help cells communicate with their external
environment through the use of hormones, neurotransmitters, and other signaling
molecules.
Transport proteins, such as globular proteins, transport molecules across cell
membranes through facilitated diffusion.
Glycoproteins have a carbohydrate chain attached to them. They are embedded in the
cell membrane and help in cell to cell communications and molecule transport across
the membrane.
41. Starch grains of potato cells stored in
amyloplasts. Micro Discovery/Corbis
Documentary/Getty Images
An amyloplast is an organelle found in plant cells. Amyloplasts
are plastids which function to produce and store starch within internal
membrane compartments. They are commonly found in vegetative plant
tissues such as tubers (potatoes) and bulbs. Amyloplasts are also thought to
be involved in gravity sensing and helping plant roots to grow in a
downward direction. Amyloplasts are derived from a group of plastids
known as leucoplasts.
Leucoplasts have no pigmentation and therefore appear colorless.
There are several types of plastids found in plant cells.
42. TYPES OF PLASTIDS
Plastids are organelles that function primarily in nutrient synthesis and storage
of biological molecules.
While there are different types of plastids specialized to fill specific roles, plastids share
some common characteristics.
They are located in the cell cytoplasm and are surrounded by a double lipid membrane.
Plastids also have their own DNA and can replicate independently from the rest of the cell.
Some plastids contain pigments and are colorful, while others lack pigments and are
colorless.
Plastids develop from immature, undifferentiated cells called
proplastids. Proplastids mature into four types of specialized plastids: chloroplasts,
chromoplasts, gerontoplasts, and leucoplasts.
43. Chloroplasts - green plastids responsible for photosynthesis and energy production
through glucose synthesis. They contain chlorophyll, a green pigment that absorbs
light energy. Chloroplasts are commonly found in specialized cells called guard
cells located in plant leaves and stems. Guard cells open and close tiny pores
called stomata to allow for gas exchange required for photosynthesis.
Chromoplasts - colorful plastids responsible for cartenoid pigment production and
storage. Carotenoids produce red, yellow, and orange pigments. Chromoplasts are
primarily located in ripened fruit, flowers, roots, and leaves of angiosperms. They
are responsible for tissue coloration in plants, which serves to attract pollinators.
Some chloroplasts found in unripened fruit convert to chromoplasts as the fruit
matures. This change of color from green to a carotenoid color indicates that the
fruit is ripe. Leaf color change in fall is due to loss of the green pigment chlorophyll,
which reveals the underlying carotenoid coloration of the leaves. Amyloplasts can
also be converted to chromoplasts by first transitioning to amylochromoplasts
(plastids containing starch and carotenoids) and then to chromoplasts.
44. Gerontoplasts - plastids that develop from the degradation of chloroplasts, which
occurs when plant cells die. In the process, chlorophyll is broken down in chloroplasts
leaving only cartotenoid pigments in the resulting gerontoplast cells.
Leucoplasts - plastids that lack color and function to store nutrients. They are
typically found in tissues that don't undergo photosynthesis, such as roots and seeds.
45. LEUCOPLASTS
Types of leucoplasts include:
Amyloplasts - leucoplasts that convert glucose to starch for storage. The starch is stored as
granules in amyloplasts of tubers, seeds, stems, and fruit. The dense starch grains cause
amyloplasts to sediment in plant tissue in response to gravity. This induces growth in a
downward direction. Amyloplasts also synthesize transitory starch. This type of starch is
stored temporarily in chloroplasts to be broken down and used for energy at night
when photosynthesis does not occur. Transitory starch is found primarily in tissues where
photosynthesis occurs, such as leaves.
Elaioplasts - leucoplasts that synthesize fatty acids and store oils in lipid-filled
microcompartments called plastoglobuli. They are important to the proper development
of pollen grains.
Etioplasts - light-deprived chloroplasts that do not contain chlorophyll, but have the
precursor pigment for chlorophyll production. Once exposed to light, chlorophyll
production occurs and etioplasts are converted to chloroplasts.
Proteinoplasts - also called aleuroplasts, these leucoplasts store protein and are often
found in seeds.
46. AMYLOPLAST DEVELOPMENT
Amyloplasts are responsible for all starch synthesis in plants. They are found in
plant parenchyma tissue, which composes the outer and inner layers of stems and roots,
the middle layer of leaves, and the soft tissue in fruits.
Amyloplasts develop from proplastids and divide by the process of binary fission. Maturing
amyloplasts develop internal membranes which create compartments for the storage of
starch.
Starch is a polymer of glucose that exists in two forms: amylopectin and amylose.
Starch granules are composed of both amylopectin and amylose molecules arranged in a
highly organized fashion. The size and number of starch grains contained within
amyloplasts varies based on the plant species. Some contain a single spherical shaped
grain, while others contain multiple small grains. The size of the amyloplast itself depends
on the amount of starch being stored.
References:
Sean E. Weise, Klaas J. van Wijk, Thomas D. Sharkey; The role of transitory starch in C3, CAM, and C4 metabolism and opportunities for
engineering leaf starch accumulation. J Exp Bot 2011; 62 (9): 3109-3118. doi: 10.1093/jxb/err035
H. T. Horner, R. A. Healy, G. Ren, D. Fritz, A. Klyne, C. Seames, R. W. Thornburg; Amyloplast to chromoplast conversion in developing
ornamental tobacco floral nectaries provides sugar for nectar and antioxidants for protection. Am. J. Bot. January 2007 vol. 94 no. 1 12-
24. doi: 10.3732/ajb.94.1.12
47. By Mariana Ruiz LadyofHats, labels by Dake modified by smartse
[Public domain], via Wikimedia Commons
VACUOLE ORGANELLE
A vacuole is a cell organelle found in a number of different cell types. Vacuoles are fluid-
filled, enclosed structures that are separated from the cytoplasm by a single membrane.
They are found mostly in plant cells and fungi. However, some protists, animal cells,
and bacteria also contain vacuoles. Vacuoles are responsible for a wide variety of
important functions in a cell including nutrient storage, detoxification, and waste
exportation.
PLANT CELL VACUOLE
A plant cell vacuole is surrounded by
a single membrane called the
tonoplast. Vacuoles are formed when
vesicles, released by the endoplasmic
reticulum and Golgi complex, merge
together. Newly developing plant
cells typically contain a number of
smaller vacuoles. As the cell matures,
a large central vacuole forms from
the fusion of smaller vacuoles. The
central vacuole can occupy up to 90%
of the cell's volume.
48. VACUOLE FUNCTION
Plant cell vacuoles perform a number of functions in a cell
including:
Turgor pressure control - turgor pressure is the force exerted
against the cell wall as the contents of the cell push the plasma
membrane against the cell wall. The water filled central vacuole
exerts pressure on the cell wall to help plant structures remain rigid
and erect.
Growth - the central vacuole aids in cell elongation by absorbing
water and exerting turgor pressure on the cell wall. This growth is
aided by the release of certain proteins that reduce cell wall rigidity.
Storage - vacuoles store important minerals, water, nutrients, ions,
waste products, small molecules, enzymes, and plant pigments.
49. Molecule degradation - the internal acidic environment of a vacuole
aids in the degradation of larger molecules sent to the vacuole for
destruction. The tonoplast helps to create this acidic environment by
transporting hydrogen ions from the cytoplasm into the vacuole. The
low pH environment activates enzymes, which degrade biological
polymers.
Detoxification - vacuoles remove potentially toxic substances from
the cytosol, such as excess heavy metals and herbicides.
Protection - some vacuoles store and release chemicals that are
poisonous or taste bad to deter predators from consuming the plant.
Seed germination - vacuoles are a source of nutrients for seeds
during germination. They store the necessary carbohydrates, proteins,
and fats needed for growth.
VACUOLE FUNCTION
50. • Plant vacuoles function similarly in plants as lysosomes in animal
cells.
• Lysosomes are membranous sacs of enzymes that digest cellular
macromolecules.
• Vacuoles and lysosomes also participate in programmed cell
death. Programmed cell death in plants occurs by a process
called autolysis (auto-lysis). Plant autolysis is a naturally
occurring process in which a plant cell is destroyed by its own
enzymes. In an ordered series of events, the vacuole tonoplast
ruptures releasing its contents into the cell cytoplasm. Digestive
enzymes from the vacuole then degrade the entire cell.
51. This is a 3D computer graphic model of a ribosome. Ribosomes are
composed of protein and RNA. They consist of subunits that fit
together and work as one to translate mRNA (messenger RNA) into a
polypeptide chain during protein synthesis (translation). Credit: Callista
Images/Cultura/Getty Images by Regina Bailey
Updated February 06, 2017
There are two major types of cells: prokaryotic and eukaryotic cells. Ribosomes are cell
organelles that consist of RNA and proteins. They are responsible for assembling the
proteins of the cell. Depending on the protein production level of a particular cell,
ribosomes may number in the millions.
DISTINGUISHING CHARACTERISTICS
Ribosomes are typically composed of two subunits: a large subunit and a small
subunit.
Ribosomal subunits are synthesized in the nucleolus and cross over the nuclear
membrane to the cytoplasm through nuclear pores. These two subunits join together
when the ribosome attaches to messenger RNA (mRNA) during protein synthesis.
Ribosomes along with another RNA molecule, transfer RNA (tRNA), help to translate
the protein-coding genes in mRNA into proteins. Ribosomes link amino acids together
to form polypeptide chains, which are further modified before becoming
functional proteins.
Ribosomes
52. LOCATION IN THE CELL:
There are two places that ribosomes usually exist within a eukaryotic cell: suspended in
the cytosol and bound to the endoplasmic reticulum. These ribosomes are called free
ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form
aggregates called polysomes or polyribosomes during protein synthesis. Polyribosomes
are clusters of ribosomes that attach to a mRNA molecule during protein synthesis.
This allows for multiple copies of a protein to be synthesized at once from a single mRNA
molecule.
Free ribosomes usually make proteins that will function in the cytosol (fluid component
of the cytoplasm), while bound ribosomes usually make proteins that are exported from
the cell or included in the cell's membranes.
53. The following are examples of structures and organelles that can be found in typical
plant cells:
Cell (Plasma) Membrane - a thin, semi-permeable membrane that surrounds the
cytoplasm of a cell, enclosing its contents.
Cell Wall - outer covering of the cell that protects the plant cell and gives it shape.
Chloroplast - the sites of photosynthesis in a plant cell. They contain chlorophyll, a
green pigment that absorbs energy from sunlight.
Cytoplasm - gel-like substance within the cell membrane containing water, enzymes,
salts, organelles, and various organic molecules.
Cytoskeleton - a network of fibers throughout the cytoplasm that helps the cell
maintain its shape and gives support to the cell.
Endoplasmic Reticulum (ER) - extensive network of membranes composed of both
regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). The ER
synthesizes proteins and lipids.
54. Golgi Complex - responsible for manufacturing, storing and shipping certain cellular products
including proteins.
Microtubules - hollow rods that function primarily to help support and shape the cell. They
are important for chromosome movement in mitosis and meiosis, as well as cytosol
movement within a cell.
Mitochondria - these organelles generates energy for the cell by converting glucose (produced
by photosynthesis) and oxygen to ATP. This process is known as respiration.
Nucleus - membrane bound structure that contains the cell's hereditary information (DNA).
Nucleolus - structure within the nucleus that helps in the synthesis of ribosomes.
Nucleopore - tiny hole within the nuclear membrane that allows nucleic
acids and proteins to move into and out of the nucleus.
Peroxisomes - tiny structures bound by a single membrane that contain enzymes that produce
hydrogen peroxide as a by-product. These structures are involved in plant processes such
as photorespiration.
55. Plasmodesmata - pores or channels between plant cell walls that
allow molecules and communication signals to pass between
individual plant cells.
Ribosomes - consisting of RNA and proteins, ribosomes are
responsible for protein assembly. They can be found either
attached to the rough ER or free in the cytoplasm.
Vacuole - structure in a plant cell that provides support and
participates in a variety of cellular functions including storage,
detoxification, protection, and growth. When a plant cell matures,
it typically contains one large liquid-filled vacuole.
56. PLANT CELL WALL STRUCTURE
The plant cell wall is multi-layered and consists of up to three sections. From the outermost
layer of the cell wall, these layers are identified as the middle lamella, primary cell wall, and
secondary cell wall. While all plant cells have a middle lamella and primary cell wall, not all
have a secondary cell wall.
Middle lamella - outer cell wall layer that contains polysaccharides called pectins. Pectins aid
in cell adhesion by helping the cell walls of adjacent cells to bind to one another.
Primary cell wall - layer formed between the middle lamella and plasma membrane in
growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-
like matrix of hemicellulose fibers and pectin polysaccharides. The primary cell wall provides
the strength and flexibility needed to allow for cell growth.
Secondary cell wall - layer formed between the primary cell wall and plasma membrane in
some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken
to form a secondary cell wall. This rigid layer strengthens and supports the cell. In addition to
cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the
cell wall and aids in water conductivity in plant vascular tissue cells.
57. The cell wall is the rigid, semi-permeable protective layer in some cell types. This outer
covering is positioned next to the cell membrane (plasma membrane) in most plant
cells, fungi, bacteria, algae, and some archaea. Animal cells however, do not have a cell
wall. The cell wall conducts many important functions in a cell including protection,
structure, and support. Cell wall composition varies depending on the organism. In plants,
the cell wall is composed mainly of strong fibers of the carbohydrate polymer cellulose.
Cellulose is the major component of cotton fiber and wood and is used in paper
production.
58. PLANT CELL WALL FUNCTION
A major role of the cell wall is to form a framework for the cell to prevent over
expansion. Cellulose fibers, structural proteins, and other polysaccharides help to
maintain the shape and form of the cell. Additional functions of the cell wall include:
Support - the cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
Withstand turgor pressure - turgor pressure is the force exerted against the cell wall as
the contents of the cell push the plasma membrane against the cell wall. This pressure
helps a plant to remain rigid and erect, but can also cause a cell to rupture.
Regulate growth - sends signals for the cell to enter the cell cycle in order to divide
and grow.
Regulate diffusion - the cell wall is porous allowing some substances,
including proteins, to pass into the cell while keeping other substances out.
Communication - cells communicate with one another via plasmodesmata (pores or
channels between plant cell walls that allow molecules and communication signals to
pass between individual plant cells).
Protection - provides a barrier to protect against plant viruses and other pathogens. It
also helps to prevent water loss.
Storage - stores carbohydrates for use in plant growth, especially in seeds.
59. WHAT ARE CHLOROPLASTS?
Photosynthesis occurs in eukaryotic cell structures called chloroplasts. A chloroplast is a type
of plant cell organelle known as a plastid. Plastids assist in storing and harvesting needed
substances for energy production. A chloroplast contains a green pigment called chlorophyll,
which absorbs light energy for photosynthesis. Hence, the name chloroplast indicates that
these structures are chlorophyll-containing plastids. Like mitochondria, chloroplasts have their
own DNA, are responsible for energy production, and reproduce independently from the rest
of the cell through a division process similar to bacterial binary fission. Chloroplasts are also
responsible for producing amino acids and lipid components needed for chloroplast
membrane production. Chloroplasts can also be found in other photosynthetic organisms such
60. CHLOROPLAST: STRUCTURE
Plant chloroplasts are commonly found in guard cells located in plant leaves. Guard cells
surround tiny pores called stomata, opening and closing them to allow for gas exchange
required for photosynthesis. Chloroplasts and other plastids develop from cells called
proplastids. Proplastids are immature, undifferentiated cells that develop into different
types of plastids. A proplastid that develops into a chloroplast, only does so in the
presence of light. Chloroplasts contain several different structures, each having specialized
functions. Chloroplast structures include:
Membrane Envelope - contains inner and outer lipid bilayer membranes that act as
protective coverings and keep chloroplast structures enclosed. The inner membrane
separates the stroma from the intermembrane space and regulates the passage of
molecules into and out of the chloroplast.
Intermembrane Space - space between the outer membrane and inner membrane.
61. Thylakoid System - internal membrane system consisting of flattened sac-like
membrane structures called thylakoids that serve as the sites of conversion of light
energy to chemical energy.
Thylakoid Lumen - compartment within each thylakoid.
Grana (singular granum) - dense layered stacks of thylakoid sacs (10 to 20) that serve
as the sites of conversion of light energy to chemical energy.
Stroma - dense fluid within the chloroplast that lies inside the envelope but outside
the thylakoid membrane. This is the site of conversion of carbon dioxide
to carbohydrates (sugar).
Chlorophyll - a green photosynthetic pigment within the chloroplast grana that
absorbs light energy.
62. CHLOROPLAST: PHOTOSYNTHESIS
In photosynthesis, the sun's solar energy is converted to chemical energy. The chemical
energy is stored in the form of glucose (sugar).
Carbon dioxide, water, and sunlight are used to produce glucose, oxygen, and water.
Photosynthesis occurs in two stages. These stages are known as the light reaction stage
and the dark reaction stage.
The light reaction stage takes place in the presence of light and occurs within the
chloroplast grana. The primary pigment used to convert light energy into chemical
energy is chlorophyll a.
Other pigments involved in light absorption include chlorophyll b, xanthophyll, and
carotene. In the light reaction stage, sunlight is converted to chemical energy in the form
of ATP (free energy containing molecule) and NADPH (high energy electron carrying
molecule). Both ATP and NADPH are used in the dark reaction stage to produce sugar.
The dark reaction stage is also known as the carbon fixation stage or the Calvin cycle.
Dark reactions occur in the stroma. The stroma contains enzymes which facilitate a
series of reactions that use ATP, NADPH, and carbon dioxide to produce sugar. The sugar
can be stored in the form of starch, used during respiration, or used in the production of
cellulose.
63. THYLAKOID DEFINITION
A thylakoid is a sheet-like membrane-bound structure that is the site of
the light-dependent photosynthesis reactions in chloroplasts and cyanobacteria.
It is the site that contains the chlorophyll used to absorb light and use it for
biochemical reactions. The word thylakoid is from the Green word thylakos,
which means pouch or sac. With the -oid ending, "thylakoid" means "pouch-like".
Also Known As: Thylakoids may also be called lamellae, although this term may be
used to refer to the portion of a thylakoid that connects grana.
THYLAKOID STRUCTURE
In chloroplasts, thylakoids are embedded in the stroma (interior portion of a
chloroplast).
The stroma contains ribosomes, enzymes, and chloroplast DNA. The thylakoid consists
of the thylakoid membrane and the enclosed region called the thylakoid lumen.
A stack of thylakoids forms a group of coin-like structures called a granum.
A chloroplast contains several of these structures, collectively known as grana.
Higher plants have specially organized thylakoids in which each chloroplast has
10-100 grana that are connected to each other by stroma thylakoids.
The stroma thylakoids may be thought of as tunnels that connect the grana.
The grana thylakoids and stroma thylakoids contain different proteins.
64. ROLE OF THE THYLAKOID IN PHOTOSYNTHESIS
Reactions performed in the thylakoid include water photolysis, the electron
transport chain, and ATP synthesis.
Photosynthetic pigments (e.g., chlorophyll) are embedded into the thylakoid
membrane, making it the site of the light-dependent reactions in
photosynthesis. The stacked coil shape of the grana gives the chloroplast a
high surface area to volume ratio, aiding the efficiency of photosynthesis.
The thylakoid lumen is used for photophosphorylation during
photosynthesis.
The light-dependent reactions in the membrane pump protons into the lumen,
lowering its pH to 4. In contrast, the pH of the stroma is 8.
The first step is water photolysis, which occurs on the lumen site of the thylakoid
membrane. Energy from light is used to reduce or split water. This reaction
produces electrons that are needed for the electron transport chains, protons that
are pumped into the lumen to produce a proton gradient, and oxygen. Although
oxygen is needed for cellular respiration, the gas produced by this reaction is
returned to the atmosphere.
65. The electrons from photolysis go to the photosystems of the electron transport chains.
The photosystems contain an antenna complex that uses chlorophyll and related pigments
to collect light at various wavelengths. Photosystem I uses light to reduce NADP+ to
produce NADPH and H+. Photosystem II uses light to oxidize water to produce molecular
oxygen (O2), electrons (e-), and protons (H+). The electrons reduce NADP+ to NADPH. In
both systems.
ATP is produced from both Photosystem I and Photosystem II. Thylakoids synthesize ATP
using an ATP synthase enzyme that is similar to mitochondrial ATPase. The enzyme is
integrated into the thylakoid membrane.
The CF1-portion of the synthase molecule extended into the stroma, where ATP supports
the light-independent photosynthesis reactions.
The lumen of the thylakoid contains proteins used for protein processing, photosynthesis,
metabolism, redox reactions, and defense. The protein plastocyanin is an electron
transport protein that transports electrons from the cytochrome proteins to Photosystem
I. Cytochrome b6f complex is a portion of the electron transport chain that couples proton
pumping into the thylakoid lumen with electron transfer. The cytochrome complex is
located between Photosystem I and Photosystem II.
66. This is the chlorophyll B molecule. Chlorophyll is used for photosynthesis. The molecule
features a magnesium atom at the center of the chlorin pigment. LAGUNA DESIGN /
Getty Images
67. CHLOROPHYLL DEFINITION
Chlorophyll is the name given to a group of green pigment molecules found in plants,
algae, and cyanobacteria. The two most common types of chlorophyll are chlorophyll
a, which is a blue-black ester with the chemical formula C55H72MgN4O5, and
chlorophyll b, which is a dark green ester with the formula C55H70MgN4O6. Other
forms of chlorophyll include chlorophyll c1, c2, d, and f.
The forms of chlorophyll have different side chains and chemical bonds, but all are
characterized by a chlorin pigment ring containing a magnesium ion at its center.
The word "chlorophyll" comes from the Greek words chloros, which means "green",
and phyllon, which means "leaf". Joseph Bienaimé Caventou and Pierre Joseph
Pelletier first isolated and named the molecule in 1817.
Chlorophyll is an essential pigment molecule for photosynthesis, the chemical
process plants use to absorb and use energy from light. It's also used as a food
coloring (E140) and as a deodorizing agent. As a food coloring, chlorophyll is used
to add a green color to pasta, the spirit absinthe, and other foods and beverages. As
a waxy organic compound, chlorophyll is not soluble in water. It is mixed with a
small amount of oil when it's used in food.
Also Known As: The alternate spelling for chlorophyll is chlorophyl.
68. ROLE OF CHLOROPHYLL IN PHOTOSYNTHESIS
The overall balanced equation for photosynthesis is:
6 CO2 + 6 H2O → C6H12O6 + 6 O2
where carbon dioxide and water react to produce glucose and oxygen.
However, the overall reaction doesn't indicate the complexity of the
chemical reactions or the molecules that are involved.
Plants and other photosynthetic organisms use chlorophyll to absorb light
(usually solar energy) and convert it into chemical energy.
Chlorophyll strongly absorbs blue light and also some red light. It poorly
absorbs green (reflects it), which is why chlorophyll-rich leaves and
algae appear green.
In plants, chlorophyll surrounds photosystems in the thylakoid membrane
of organelles called chloroplasts, which are concentrated in the leaves of
plants. Chlorophyll absorbs light and uses resonance energy transfer to
energize reaction centers in photosystem I and photosystem II. This
happens when energy from a photon (light) removes an electron from
chlorophyll in reaction center P680 of photosystem II. The high energy
electron enters an electron transport chain. P700 of photosystem I works
with photosystem II, although the source of electrons in this chlorophyll
molecule can vary.
69. Electrons that enter the electron transport chain are used to pump hydrogen ions (H+)
across the thylakoid membrane of the chloroplast. The chemiosmotic potential is used to
produce the energy molecule ATP and to reduce NADP+ to NADPH. NADPH, in turn, is used
to reduce carbon dioxide (CO2) into sugars, such as glucose.
OTHER PIGMENTS AND PHOTOSYNTHESIS
Chlorophyll is the most widely recognized molecule used to collect light for photosynthesis,
but it's not the only pigment that serves this function.
Chlorophyll belongs to a larger class of molecules called anthocyanins. Some anthocyanins
function in conjunction with chlorophyll, while others absorb light independently or at a
different point of an organism's life cycle. These molecules may protect plants by changing
their coloring to make them less attractive as food and less visible to pests. Other
anthocyanins absorb light in the green portion of the spectrum, extending the range of light
a plant can use.
