BIOLOGI SEL: PENDAHULUAN BISEL07-SITH/ITB-MIT/IR 1

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BISEL07-SITH/ITB-MIT/IR 1

BIOLOGI SEL:

PENDAHULUAN

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Sejarah perkembangan

• Robert Hooke : sel mati : sel dari gabus

• Anton van Leeuwenhoek : sel hidup

• Matthias Schleiden : sel pada tumbuhan

• Theodor Schwann (1839): Teori sel

– Semua organisma terdiri dari satu atau lebih sel

– Sel : unit struktural hidup

• Schleiden & Schwann : sel dapat berasal dari materi-materi

nonselular

• Rudolf Virchow (1855) : sel

berasal dari pembelahan sel yang sudah ada sebelumnya

• Penggunaan sel dalam penelitian in vitro : HeLa (sel kanker

manusia) – George Gey (1951)

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Karakteristik sel

• Sel sangat kompleks

– Molekul-molekul sederhana –

kompleks Æ organel Æ sel

misalnya

C, H, O, N, S, P Æ asam amino Æ

protein Æ misalnya salah satu komponen dalam mitokondria yang merupakan organel dari sel

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Karakteristik sel

• Sel memiliki informasi genetik

– Gen : blueprint untuk struktur sel, seluruh aktivitas dan fungsi sel

• Sel dapat ber-reproduksi

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Karakteristik sel

• Sel memperoleh dan menggunakan energi

• Sel melakukan

metabolisme sel

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Karakteristik sel

• Terdapat suatu aktivitas mekanis dalam sel yang dinamis

– Misalnya perubahan bentuk sel akibat aksi dari protein-protein dalam sitoplasma

• Sel dapat memberi respons terhadap suatu stimulus – Reseptor hormon, reseptor faktor tumbuh, reseptor

matriks ekstraselular, atau reseptor lainnya (G)

– Respons : misalnya metabolisme sel, proliferasi sel atau gerakan sel

Istirahat teraktivasi retraksi

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Karakteristik sel

• Sel mampu mengatur diri sendiri (self regulation)

– Misalnya pengaturan siklus sel

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Prokaryot -Eukaryot

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Persamaan

antara eukaryot dengan prokaryot:

• konstruksi membran plasma sama

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Persamaan

antara eukaryot dengan prokaryot

• informasi genetik dikode oleh DNA, dengan kode genetic yang identik

• mekanisme transkripsi dan translasi

Eukaryotes Prokaryotes

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• reaksi metabolisme

• apparatus yang sama untuk konversi energi kimiawi

– prokaryot Æmembran plasma – eukaryot Æ membran mitokondria

Persamaan antara eukaryot

dengan prokaryot:

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• mekanisme fotosintesis yang sama (tumbuhan – sianobakteri)

• mekanisme sintesa dan penyisipan protein membran

• konstruksi proteosom yang sama (archaebacteria dengan eukaryot)

Persamaan antara eukaryot

dengan prokaryot:

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Perbedaan antara organisme prokaryot dengan eukaryot

Prokaryot Eukaryot

Organisme Bakteri, cyanobakteri

Protista, jamur,

tumbuhan dan hewan Ukuran sel Umumnya 1-10

μm Umumnya 5-100 μm

Metabolisme Anaerobic atau aerobik

Aerobik

Organel Sedikit Mitokondria, kloroplas, retikulum endoplasma, dll

Inti Tidak ada Ada

DNA DNA sirkular

dalam sitoplasma

DNA linier dan sangat panjang, memiliki daerah yang dikode (ekson) dan tidak dikode /intron (sangat banyak); berada dalam inti

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Perbedaan antara organisme prokaryot dengan eukaryot

RNA dan protein RNA dan protein disintesis pada ruang yang sama

RNA disintesis dan diproses di inti Protein disintesis di sitoplasma Sitoplasma Tidak mengandung sitoskeleton,

tidak ada aliran sitoplasma dalam sel, tidak ada endositosis dan eksositosis

Dalam sitoplasma terdapat sitoskeleton : filamen-filamen protein, ada aliran sitoplasma dalam sel, ada endositosis dan eksositosis

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Perbedaan antara organisme prokaryot dengan eukaryot

Pembelahan sel Kromosom ditarik dengan cara pelekatan pada membran plasma

Kromosom ditarik apparatus mitosis (komponen sitoskeleton)

Organisasi sel Umumnya uniselular Umumnya multiselular, dan terjadi proses diferensiasi / spesialisasi sel

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Virus

– membawa

informasi genetic berupa rantai

tunggal atau ganda RNA atau DNA

– Materi genetiknya mengkode :

• Protein kapsul / kapsid

– aktif jika berada

pada sel hidup

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BISEL07-SITH/ITB-MIT/IR

20 Bioenergetika

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• Cell metabolism can be compared to an elaborate road map of the thousands of chemical reactions that occur in the cell

It is an intricate

network of metabolic pathways

The Chemistry of Life: A network

of metabolic pathways

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• Catabolic pathways: They

release energy by breaking down complex molecules to simpler

compounds

– A major catabolic pathway found in a cell is respiration which breaks down sugar glucose and other

fuels into carbon dioxide and water with release of energy

C₆H₁₂O₆ + 6O₂ Æ 6CO₂ + 6H₂O + Energy

• Anabolic pathways: Build

complex molecules from simpler ones by consuming energy

e.g. Photosynthesis in plants 6CO₂ + 6H₂O + Light energy Æ C₆H₁₂O₆ + 6O₂ + 6H₂O

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• Organisms Transform Energy:

– Energy: The capacity to do work

• Kinetic energy: The energy of motion possessed by all moving objects e.g. water gushing through a dam turns turbines

• Potential energy: Energy that matter possesses because of its location or structure

• Bioenergetics – The study of how organisms manage their energy resources

– to maintain its high level of activity, a cell must acquire & expend energy

Water behind dams has potential energy because of altitude

Chemical energy stored in molecules as a result of the arrangement of the atoms in these molecules

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Conversion of Energy from one form to the other:

• Thermodynamics - study of the changes in energy that

accompany events in the Universe

• Two laws of

Thermodynamics

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The First Law of Thermodynamics

• energy can be neither created nor destroyed (Law of Conservation of Energy); total energy in Universe remains constant (regardless of transduction process)

– Energy can, however, be transduced - burning fuel, polysaccharide breakdown, photosynthesis

• Several organism communities are independent of photosynthesis – communities residing in hydrothermal vents on ocean floor; depends on energy obtained by bacterial chemosynthesis

• Some animals (fireflies, luminous fish) convert chemical energy back into light

• ΔE = Q – W, where Q = heat energy & W = work energy

Reactions that result in heat lost to the environment are called exothermic;

those that result in heat gained from the environment are called endothermic

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Couple of terms

• System: Is used to denote the matter under study and refer to the rest of the universe- everything outside the systems the

surroundings

1. Closed system: e.g. a liquid in a thermos bottle is isolated from its surroundings

2. Open system: Energy (&often matter) can be transferred between the system and its

surroundings e.g. organisms

• Entropy: A measure of disorder or randomness

• Free energy: Is the portion of a system’s energy that can perform work when

temperature is uniform through out the system

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The Second Law of Thermodynamics

• Every energy transfer or transformation increases the entropy of the universe (no machine is 100% efficient which would be necessary)

• Some energy is inevitably lost as machine works (same is true of living organism)

• car

chemical energy (gasoline) Æ converted to kinetic energy + the disorder of its

surroundings will increase in the form of heat and small molecules that are the breakdown products of gasoline

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• Together the 1st & 2nd laws of thermodynamics show that the energy of the universe is constant, but that entropy continues to increase toward a maximum

• Gibbs combined concepts inherent in 1st & 2nd Laws to get equation: ΔH = ΔG + TΔS

where:

1. ΔG is the change in free energy (the change during a process in energy available to do work)

2. ΔH - change in enthalpy (total energy content of system; equivalent to ΔE for our purposes)

3. T - absolute temperature (°K; °K = °C + 273) 4. ΔS - change in entropy of system

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• Rearrange to ΔG = ΔH - TΔS - can predict direction in which process will proceed & the extent to which the process will occur

1. ΔG size shows the maximum amount of energy that can be passed on for use in another process

2. Spontaneous process has -ΔG (exergonic) & proceeds toward state of lower free energy; such a process is

thermodynamically favored

3. Non-spontaneous process, +ΔG (endergonic); cannot

occur spontaneously; it is thermodynamically unfavorable;

make it go by coupling to high -ΔG (energy-releasing)

reaction

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• An important renewable high energy compound that powers cellular work

• ATP hydrolysis is used to drive most cellular endergonic processes A. ATP is used for diverse processes because its terminal phosphate

group can be transferred to a variety of different types of molecules (amino acids, lipids, sugars, & proteins)

B. In most coupled reactions, phosphate group is transferred in initial step from ATP to one of above acceptors & is subsequently removed in

second step

ATP:

Adenosine Triphosphate

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Enzymes: Biocatalysts

• A catalyst is a chemical agent that changes the rate of reaction without being consumed by the reaction

• An enzyme is a catalytic protein

– Enzymes are substrate-specific (key-lock relationship) – Enzymes are sensitive to temperature, pH and to some

chemicals

• Some Enzymes need co-factors/coenzymes to function

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Enzymes:

Biocatalysts

• Substrates can

compete with other substrates to bind on the same

position of the same enzyme Î interrupt the

reaction

• Enzymes can be inhibited by the addition of

inhibitors

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Enzymes: Biocatalysts

• Feed back inhibition of

enzymes: Feed inhibition is the switching off of a metabolic

pathway by its end product

which acts as an inhibitor of an enzyme within the pathway

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• ATP formed 2 ways in cell:

– oxidative phosphorylation Æ inner membrane of mitochondria

– substrate-level phosphorylation

• Oxidative phosphorylation -

dehydrogenases move 2 electrons &

proton to NAD+ to make NADH

1. High energy NADH donates electrons to other molecules at electron transport (ET) chain

2. Because NADH transfers electrons so readily, it is said to have high electron transfer potential

3. As electron travels down ET system, it loses energy used to make ATP & is added to O₂ to make H₂O

• Substrate-level phosphorylation - phosphate group moved from a

substrate to ADP Æ ATP

1. ATP formation is not that endergonic, formation of other molecules is more endergonic

2. Such molecules can donate their phosphates to ADP to make ATP