Cell Theory
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4 basic concepts of cell theory are:
Cells are the units of structure (building blocks) of all animals and plants.
Cells are the smallest unit of function in all animals and plants.
Cells originate only from pre-existing cells by cell division.
All cells maintain homeostasis
Cell Structure, Cell Transport and Mitosis
Cell – Cell-membrane, Cytoplasm and Nucleus
Cytoplasm – Cytosol and Cell Organelles
Nucleus – Nuclear Envelope, Nucleoplasm and Chromatin (DNA + Histones)
Fig 3-2
Table 3-1 represents the summary of structure and function of various parts of a representative cell. IMPORTANT
Cell Membrane
All cells are covered with a thin covering of a double layer of Phospholipids and associated Proteins present here
and there.
Each phospholipid has a polar (hydrophilic) head and non-polar (hydrophobic) tails. In the double layer the tails
face each other forming a hydrophobic barrier which keeps water dissolved contents inside.
Proteins may be Intrinsic – embedded in the lipid double layer and Extrinsic associated outside the lipid double
layer. Fig 3-3 and Table 3-2
Cell Membrane and Transport
In bacteria, plants, and fungi cell membrane is surrounded by a non-living cell wall. But cell membrane is the real
boundary. Most cell walls are permeable.
Semi-permeable: Cell membranes allow some materials to pass through them and prevent others from doing so.
Regulators like hormones can change permeability of a cell membrane.
Transport across membrane can be Passive or Active.
Passive Transport
Active Transport
From high to low concentration
Independent of conc. Gradient
Energy (ATP) not needed
ATP needed
Mostly Carrier Protein not needed
Carrier Protein needed
Diffusion, Osmosis, Facilitated Diffusion
Primary & Secondary Active T.
Bulk Transport
Passive Transport
Passive Transport includes Diffusion, Osmosis and Facilitated Diffusion.
Diffusion: All fluids (liquids + gases) move from area of higher concentration to area of lower concentration
(concentration gradient). This movement of substances is called Diffusion. It can be through cell membranes. For
example, spreading of fragrance, dissolving of ink drop in water, movement of O2 and CO2 between lungs and
blood
Osmosis is always the net movement of water through cell membrane from its higher concentration (dilute
solution) to its lower concentration (concentrated solution) when the 2 solutions are separated by
semi-permeable membrane. For example absorption of water by roots of plants.
Facilitated diffusion is faster than normal diffusion but needs a carrier protein though no ATP needed. For
example, absorption of glucose and amino acids in intestine
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Diffusion
Movement of solute
Membrane not needed
High conclow conc
Fig 3.4
Osmosis
Movement of water
Semi-permeable memb needed
Low conchigh conc of solution
Fig 3.6
Active Transport
Only active transport can operate against concentration gradient.
It is the fastest mode of transport. Fig 3-9, Na-K pump
It always consumes ATP directly or indirectly.
It always needs one or more transport proteins.
For example absorption of minerals by plant roots, absorption of nutrients when their concentration is already
higher inside the cells.
Vesicular Transport
Large molecules like proteins cannot transport through membrane by passive or active transport discussed so far.
These are packed into membrane bound sacs and transported across cell membrane.
Endocytosis is the bulk transport into the cell. If solid material including prey is brought in as Food Vacuole, the
process is called Phagocytosis. Fig 3-11. For example, white blood cells eating bacteria. When cell brings in liquid
bound in sac the process is called Pinocytosis. Fig 3-10
Exocytosis: When the cells releases solid or liquid in sacs the process is called exocytosis. For example Amoeba
throws excess water outside to maintain required concentration (osmoregulation).
IMPORTANT Table 3-3 depicts summary of mechanisms of membrane transport.
Recap-1 Chapter-3
The liquid part of cytoplasm is --------------Structures formed of membranes and molecules, present in cytoplasm, doing a special function, are ------------------------2 halves of cell membrane are held together by -------------- bonds
Cell membrane is fluid due to --------------- molecules
Membrane bound --------- molecules regulate membrane transport
Movement of solute from high to low concentration is ------------Net Movement of water across cell membrane (from low concentration solution to high concentration solution) is
-------------------- ---------------- can operate against concentration gradient and needs both a carrier protein and ATP.
--------------------- moves along the concentration gradient and needs a carrier protein but no ATP.
Bulk movement of solids into the cell in a vesicle is --------- and of liquids is ------------------. (also called vesicular
transport)
Cytoplasm
Cytoplasm is the living fluid part between cell membrane and nucleus. It has special structures called Cell
Organelles in it.
Cytosol is the matrix (formless) part of cytoplasm formed of water having dissolved or suspended substances in it.
Cell Organelles are organ like each performing specific function/s but formed of molecules and membranes only
(sub- cellular).
Double Membrane bound Organelles: Mitochondria, Chloroplasts, Endoplasmic Reticulum, Golgi Body, and
Nucleus.
Single Membrane bound Organelles: Lysosomes, Peroxisomes, Vacuoles
Organelles lacking any membrane: Ribosomes, Centrioles, Nucleolus
IMPORTANT: Table 3.1
Nucleus and Ribosomes 1
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Genetic Control of the Cell
Nucleus: is the most distinct structure inside cell visible with light microscope. It has inside it DNA having all the
information needed to form and run the cell. The segments of DNA are called Genes. Fig 3-16
Nuclear Envelope: is formed of 2 membranes with a gap between them. It has a large number of Nuclear Pores
usually bound by a nuclear complex. The pores are large enough to allow RNA and proteins to pass through.
Nucleoplasm: is the matrix (formless) of nucleus and has a different composition than Cytosol.
Chromatin fibers: are very long molecules of DNA associated with proteins (Histones and other nuclear proteins).
Each chromatin fiber, at the time of cell division, organizes into Chromosomes. Fig 3-17
Nucleolus: is present in the nucleus when the cell is not dividing. No membrane bounds it. It assembles both units
of Ribosomes.
Nucleus and Ribosomes 2
Transcription
Translation
DNA -------------------m-RNA -----------------------Protein
A gene (DNA) produces m-RNA by transcription. The m-RNA comes out of nuclear pores into the cytoplasm. Here
Ribosomes attach one by one to m-RNA and synthesize a protein. t-RNA’s bring amino acids from cytosol to
m-RNA – ribosome complex. Fig 3-18, 19
Each ribosome has 2 sub-units larger (60S) and smaller (40S).
r-RNA and ribosomal proteins form the ribosomal subunits.
No membrane covers the ribosomal subunits. Ribosomal subunits join only around m-RNA for protein synthesis
otherwise remain separate.
At the time of cell division one DNA divides and produces 2 DNA molecules by replication. DNA replication leads to
produce sperms and eggs (gametes).
m-RNA = messenger RNA
Replication
t-RNA = transfer RNA
DNA ----------------DNA + DNA
r-RNA = ribosomal RNA
The Endomembrane System 1
Manufacturing and Distributing Cellular Products
Endoplasmic Reticulum, Golgi Apparatus, Lysosomes and Vacuoles collectively form Endomembrane System.
Endoplasmic Reticulum: is a system of double membranes in the form of tubes and sacs throughout cytoplasm (in
between cell membrane and nuclear envelope). Main manufacturing facility.
Functions include synthesis of proteins and lipids including steroids, detoxification, and cellular transportation. It
transports materials inside the cell by transport vesicles.
RER
1 Rough ER, ribosomes fixed
2 Flat sacs
3 Synthesis of memb proteins
4 Synthesis of secretory proteins
SER
1 Smooth ER, no ribosomes
2 Tubular
3 Lipids, steroids
4 Detoxification of drugs
The Endomembrane System 2
Golgi Apparatus = Golgi Body: is a stacks of flattened sacs called cisternae. A cell may have from a few to a few
hundred of Golgi stacks. Golgi Apparatus receives transport vesicles from ER on one side, modifies received
chemicals, can store them and packs them in secretory vesicles and releases them on shipping side.
Lysosomes: are single memb bound organelles rich in digestive enzymes, help in breakdown of large molecules
like proteins, polysaccharides, lipids and nucleic acids. Lysosomes provide a safe place for digestion of large
molecules without damaging molecules of the cell. A Lysosome joins a food vacuole to digest the materials inside
vacuole. Lysosomes are absent in most plant cells.
Vacuoles: are membrane bound sacs and pinch off from ER, Golgi Apparatus and cell membrane. Functions
include endocytosis, exocytosis, maintain turgor pressure in a plant cell, trasnportation.
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Mitochondria
Energy Conversion
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Mitochondria (sing. Mitochondrion): are the powerhouses of cells and the site for cellular respiration. Respiration
is oxidation of food and chief source of energy for the cell.
These are bound with double membrane, outer smooth and inner folded. Mitochondria have enzymes for
breakdown glucose derivatives, fatty acids and amino acids.
Mitochondria have Electron-Transport-System that generates ATP molecules by using the energy contained in H’s
produced during breakdown of glucose.
Mitochondria are found in plants, animals, fungi and protists.
The Cytoskeleton
Cell Shape and Movement
Maintaining Cell Shape: The shape of the cell is maintained by Intermediate Filaments, the thick ropes of twisted
protein fibers, Microtubules, the hollow organelles and Microfilaments, the solid thinner organelles. These
maintain the shape of cells and keep in position the nucleus. The framework of support is highly dynamic that can
organize and dismantle really fast.
Transport of organelles and molecules: Microtubules are the freeways used by organelles like lysosomes to move
from one part of cell to another. Microfilaments are the roads/streets used by smaller things.
Genes – DNA – Chromatin fiber – Chromosomes
Fig. 3-17
Genes, the segments of DNA, are part of chromatin fiber found in nucleus.
Chromatin fiber is formed of DNA and Histone proteins.
Most of the time the chromatin fibers exist as a diffuse network (not visible even under electron microscope).
However, when the cell starts to divide the chromatin fibers organize into compact threads called Chromosomes.
Each species has a fixed # of chromosomes – 46 in most human cells.
Cellular Reproduction
= Cell Division
Passes on Genes from Cells to Cells
Reproduction of Organisms
Chromosome – DNA – Gene
Almost all the eukaryotic genes (about 20000 in human genes) are found in the chromosomes. Some genes are
present in Mitochondria and Chloroplast.
DNA associate with 4 kinds of Histones and coil to form Nucleosome. A 5th Histone molecule, keeps the coils in
position. Nucleosomes pack and form thicker and thicker threads. The thickest threads are Chromatids. A
chromosome has 1 or 2 Chromatids in it.
A chromosome with 1 chromatid divides to form a chromosome with 2 Chromatids (sister). One chromatid is
passed on to each daughter cell.
Fig 3.17
Cell Cycle
Cell Cycle: Most cells in body divide though at different rates. There are 2 distinct phases that alternate with
each other and form a cell-cycle.
M-phase: when a cell is dividing. The daughter cells are half in size.
Interphase: Each daughter cell must grow by making new materials including proteins and DNA. Interphase is
divided into 3 sub-phases : G1, S and G2.
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S-phase occurs in the middle part of Interphase and DNA replication takes place. DNA and chromosomes are
doubled.
G1 and G2 are growth phases of cell with synthesis of proteins and ribosome. G1 takes place before S-phase. But
G2 occurs after the S-phase.
Fig 3-20
Mitosis – interphase
The cell division of growth and maintainance
Mitosis is the division of growth and replacement of lost or damaged cells.
It is equational division. 2n  2n or 1n 1n
Fig 3-22 depicts mitosis (division of nucleus) and Cytokinesis (division of cytoplasm).
Interphase near its end has inside cytoplasm 2 centrosomes, each with a pair of centrioles. These initiate the
organization of spindle fibers. The chromosomes are double with 2 sister chromatids joined only at centromeres
but still indistinct.
Mitosis has 4 distinct phases Prophase, Metaphase, Anaphase and Telophase. Memory aid: P-MAT
Mitosis – Prophase
pro = first
Prophase: is the phase that prepares the cell for mitosis.
Centrosomes start moving to opposite ends and spindle formation starts.
Chromosomes coil and pack into thick threads and get distinct.
In late prophase nuclear envelope degenerates and chromosomes are released in cytoplasm.
Spindle fibers either join a spindle fiber from the opposite centrosome or connect to the centromere of a
chromosome.
Mitosis – Metaphase
meta = after
Metaphase: The spindle is fully formed now.
The chromosome pack further and get most distinct.
Chromosomes arrange on an imaginary disc = equatorial plate at the middle. The centromeres of chromosomes lie
at the plate.
Each centromere is joined through spindle fibers to both centrosomes.
Mitosis – Anaphase
ana = apart
Anaphase: is the movement of young chromosomes from the middle towards respective poles (centrosomes).
It starts suddenly when the centromeres divide. Each chromosome is formed only of 1 chromatid.
The motor proteins at centromeres move the chromosomes on the microtubules of spindle fibers.
Telophase
telo = end
Telophase begins when the 2 groups of chromosomes reach the poles.
This phase is the reverse of prophase.
Chromosomes unpack to diffuse network.
Nuclear envelope is reorganized from Endoplasmic Reticulum.
Spindle fibers disappear.
One nucleus is completely divided into 2 genetically similar daughter nuclei.
Cytokinesis
kinesis = motion
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Cytokinesis takes place along Telophase.
In an animal cell cleavage furrow appears at the middle and divides the cytoplasm into 2 equal halves, each with a
nucleus.
In a plant cell a cell-plate is formed at the middle. Golgi apparatus provides most of the materials packed in
vesicles.
Cell plate starts at the center and proceeds towards parent cell wall.
Cell plate joins with the parental cell wall to complete the Cytokinesis.
Most plant cells lack centrioles in them and centrosomes organize spindle formation.
Cell Diversity and Differentiation
Somatic Cells
All have same genes
Some genes inactivate during development
Cells thus become functionally specialized
Specialized cells form distinct tissues
Tissue cells become differentiated
The Cell Life Cycle
Cell Division and Cancer
Abnormal cell growth
Tumors (also called, neoplasm)
• Benign (Encapsulated)
• Malignant:
1. Invasion 2. Metastasis
Cancer—Disease that results from a malignant tumor
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Recap-2 Chapter-3
Smooth E.R. synthesizes ---------------Rough E.R. synthesizes ----------------Golgi Apparatus is the --------- ------------ of the cell and produces secretory vesicles.
ATP’s are synthesized in the --------- chamber of --------------.
Synthesis of DNA takes place during ----------- of cell cycle.
When cell is not dividing DNA occurs in the form of -----------When the cell is dividing the DNA occurs in the form of ------------During ------------ chromosomes arrange on mid-plate in mitosis
Division of cytoplasm after mitosis is called ---------------Divided chromosomes move to opposite poles during ------------Spindle is formed, chromosomes appear randomly arranged and nuclear envelope breaks during ------------Nuclear envelope appears, chromosomes unpack during ------------
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