Chapter 2 – THE CELL AND ITS COMPONENTS (Taken from

(Taken from Biology 12, MHR)
Animals, plants, fungi, and protists are composed of eukaryotic cells, which have DNA, a
cell membrane, and cytoplasm.
Cytoplasm consists of organelles, the cytosol, and molecules and ions dissolved or
suspended in the cytosol.
See handout for diagrams of Animal Cell and Plant Cell.
Nucleus (Control Center)
 The nucleus controls the cell’s activities and contains DNA, which carries the genetic
information and instructions for making proteins.
 DNA combines with proteins to form thread-like chromatin. During cell division, the
chromatin condenses to form chromosomes.
 The nucleolus is a small, dense region inside the nucleus where proteins and ribosomal
RNA join to form the subunits of ribosomes.
 The nucleus is surrounded by the nuclear envelope, a double membrane consisting of
two phospholipid bilayers, which separates the nucleus from the rest of the cell.
 Nuclear pore complexes in the nuclear envelope consist of thousands of proteins that
form openings.
 The nuclear pore complexes allow water and small ions to move in and out of the
nucleus, but selectively control the passage of larger molecules such as RNA.
 See Figure 2.4, pg 60
Endoplasmic Reticulum
 The nuclear envelope is continuous with a complex of membrane-bound tubules and sacs
known as endoplasmic reticulum (ER).
 There are two types of ER: rough endoplasmic reticulum and smooth endoplasmic
 Rough ER is studded with ribosomes on its surface and is involved with protein
 Proteins that are part of membranes or intended for export from the cell are assembled
by rough ER ribosomes.
 Proteins that function in the cytosol are made by free-floating ribosomes in the
 The smooth ER has no ribosomes on its surface and is involved with the synthesis of
steroids and lipids.
 The smooth ER also forms transport vesicles, which transport proteins to the Golgi body.
 See Figure 2.5, pg 61
Golgi Apparatus (Golgi Body)
 Golgi apparatus (Golgi body) is a stack of curved membrane sacs that packages,
processes, sorts, and distributes proteins, lipids, and other substances within the cell.
 It acts like a “post office” for the cell.
 Golgi apparatus also manufactures macromolecules, particularly carbohydrates.
 In plant cells, the Golgi apparatus synthesizes pectin which is a structural polysaccharide
in cell walls.
 In animal cells, the Golgi apparatus produces lysosomes, which are membrane-bound
vesicles containing digestive enzymes.
 These enzyme help to break down macromolecules (carbohydrates, lipids, proteins) into
smaller molecules. They also break down old organelles, bacteria, and foreign
Endomembrane System
 The endomembrane system consists of a series of cellular structures that are
interconnected: the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus,
and different types of vesicles.
 The organelles of this system all work together in the synthesis, modification, and
transportation of proteins and lipids.
 See Figure 2.6, pg 61
 Peroxisomes are membrane-bound sacs containing oxidative enzymes that catalyze
redox reactions.
 These enzymes break down excess fatty acids and hydrogen peroxide, and participate in
the synthesis of bile acids and cholesterol.
 All peroxisomes contain the enzyme catalase which breaks down hydrogen peroxide
into water and oxygen gas.
 A vesicle is a membrane-bound sac used for transport and storage of substances in the
 Vesicles form by pinching off from the cell membranes and organelle membranes.
 Animal cells contain many small vesicles.
 Plant cells contain a single large central vesicle called a vacuole.
 The vacuole stores water, ions, sugars, amino acids, and macromolecules. It also
contains enzymes that break down macromolecules and cell wastes.
Chloroplasts (Plant cells)
 Chloroplasts are organelles with a double membrane that are filled with a fluid called
stroma and grana, which are stacks of chloroplyll-containing thylakoids.
 Chlorophyll, a photosynthetic pigment located inside the membranes of the thylokoids,
absorbs light energy from the Sun as part of the reaction that converts carbon dioxide
and water into energy-rich molecules such as glucose.
 See Figure 2.9, pg 64
 Both plant and animal cells have mitochondria that carry out cellular respiration.
 Mitochondria break down high-energy organic molecules (glucose) to convert stored
energy into usable energy.
 Mitochondria are composed of a fluid-filled matrix and folds of inner membrane called
 See Figure 2.10, pg 65
Cell Wall and Cytoskeleton
 A cell wall is a rigid layer surrounding plant, algae, fungal, bacterial and some archaea
 It is composed of proteins and / or carbohydrates and it provides structural support and
protection for the cell.
 Cytoskeleton is a network of protein fibres that extends throughout the cytosol,
providing structure, shape, support, and motility
 See Table 2.1, pg 66
Cell Membrane
 The activities of the living cell depend on the ability of its membrane to:
transport raw materials into the cell
transport manufactured products and wastes out of the cell
prevent the entry of unwanted matter into the cell
prevent the escape of the matter needed to perform the cellular functions
 The cell membrane acts as a barrier and regulates the movement of molecules and
ions into and out of the cell.
 In the modern fluid mosaic model, the basic framework of a cell membrane is a
phospholipid bilayer into which proteins are inserted.
 These proteins may be bound on the surface to other proteins or to lipids, including
glycoproteins and glycolipids.
 Glycoproteins and glycolipids are proteins and lipids covalently bonded to
 Integral proteins are embedded in the membrane.
 Peripheral proteins are located on the outer surface of the lipid bilayer.
 See Figure 2.12, pg 69
Function of Proteins in a Phospholipid Bilayer
 Functions of membrane proteins include:
Stabilizing the membrane
Transport of substances across the cell membrane
Signal reception and signal transduction
Cell-to-cell recognition (carbohydrate chain)
Reaction catalysis (enzymes in membrane carry out chemical reactions)
 Proteins embedded in membrane serve different functions:
Channel Proteins - form small openings for molecules to diffuse through
Carrier Proteins- binding site on protein surface "grabs" certain molecules and
pulls them into the cell
Receptor Proteins - molecular triggers that set off cell responses (such as release
of hormones or opening of channel proteins)
Cell Recognition Proteins - ID tags, to identify cells to the body's immune system
Enzymatic Proteins - carry out metabolic reactions
pg 71 #1 - 13
 The cell membrane is able to control the movement of substances into and out of the cell
because it is semi-permeable.
 This means that the cell membrane allows certain substances to pass through while
preventing other substances to pass through.
Concentration Gradient
 Concentration gradient is a difference in concentration between one side of a membrane
and the other.
 Passive transport is the movement of ions or molecules across a cell membrane from a
region of higher concentration to a region of lower concentration, without the input of
 There are three forms of passive transport: diffusion, osmosis, and facilitated diffusion.
 In passive transport, the molecules (ions) are moving along the concentration gradient
(from high to low) therefore, no energy is required.
 Diffusion is the net movement of ions or molecules from an area of higher
concentration to an area of lower concentration.
 Brownian motion of molecules and ions in the cytoplasm and extracellular fluid is
responsible for diffusion.
 Diffusion will continue until there is an equilibrium (equal concentration of substances
on either side of the cell membrane).
 Oxygen and carbon dioxide easily cross the cell membrane by diffusion because it works
well for small molecules over short distances.
 See Figure 2.13, pg 72 and Figure 2.14, pg 73
 Diffusion will continue until there is an equilibrium (equal concentration of substances on
either side of the cell membrane).
 Oxygen and carbon dioxide easily cross the cell membrane by diffusion because it works
well for small molecules over short distances.
 It is important for a cell to have a large surface area compared to its volume so that
more materials can diffuse in and out.
Factors Affecting Rate of Diffusion
 Molecule size  rate of diffusion decreases with molecule size
 Molecule polarity  non-polar molecules diffuse faster than polar molecules of the
same size
 Molecule or ion charge  charged molecules and ions cannot diffuse across a cell
 Temperature  increasing temperature increases rate of diffusion (molecules have
more energy and speed)
 Pressure  increasing pressure increases rate of diffusion
 Osmosis is the movement of water from an area of higher concentration to an area of
lower concentration, across a semi-permeable membrane.
 The cell membrane is impermeable to solutes. This means that only water is able to
pass through the membrane.
 So, if the solute concentrations are different on either side of the membrane, then water
will move either in or out of cell.
 It is the concentration of solutes in a solution that determines its osmotic pressure.
 iso = “equal”
 hypo = “less than”
 hyper = “more than”
Isotonic Conditions
 water concentration inside the cell equals the water concentration outside of the cell
 equal amounts water move in and out of the cell
 size of the cell remains the same
Hypotonic Conditions
 water concentration outside the cell is greater than inside the cell
 water moves into the cell
 size of cell increases
 Lysis may occur (cell is destroyed because it bursts)
Hypertonic Conditions
 water concentration inside the cell is greater than outside the cell
 water moves out of the cell
 size of cell decreases (shrivels)
 Plasmolysis may occur (too much water leaves the cell)
 See Figure 2.15, pg 73
Facilitated Diffusion
 Facilitated diffusion is the transport of ions or molecules across a membrane by
means of a membrane protein along the concentration gradient for that ion or molecule.
 There are two types of membrane proteins: channel proteins and carrier proteins.
Channel Proteins
 form highly specific channels
 permit the passage of ions or polar molecules
 some channels remain open all the time
 other have gates that open or close in response to a variety of signals (hormones,
electric charge, pressure, or light).
Carrier Proteins
 bind to specific molecules, transport them across the membrane, and release them on
the other side
 change shape while transporting molecules
 allow the passage of larger molecules (glucose, amino acids)
 has a slower diffusion rate than channel proteins
 See Figure 2.16, pg 75
 Active transport is the transport of a solute across a membrane against its gradient
(from lower concentration to higher concentration).
 requires energy  usually from ATP
 ATP (adenosine triphosphate) is the main source of energy in the cell.
Primary Active Transport
 uses ATP directly to move molecules or ions across a membrane
 An example is the sodium-potassium pump.
 With the sodium-potassium pump, sodium is moved out of cell and potassium is moved
into the cell (both against their concentration gradient) therefore ATP is needed.
 See Figure 2.18, pg 76
Secondary Active Transport
 uses an electrochemical gradient as a source of energy to transport molecules or ions
across a cell membrane
 Electrochemical gradient is a combination of concentration gradient and electrical
potential across a membrane.
 The electrochemical gradient created by primary active transport via an ion pump is used
by a different protein to transport other molecules across a cell membrane.
 This method of transport is found in bacteria and plant cells.
 An example is the hydrogen-sucrose pump.
 See Figure 2.19, pg 77
 Membrane-assisted transport is a transport method used to move materials that are too
large to cross the cell membrane through a channel or carrier protein.
 It requires energy.
 There are two forms of membrane-assisted transport: endocytosis and exocytosis.
 when the cell membrane folds inward, engulfing a small amount of matter from the
extracellular fluid bringing it into the cell and forming a vesicle
 There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-assisted
 See Figure 2.20, pg 78
Phagocytosis (“Cell-Eating”)
 A cell engulfs a large particle (bacteria, bits of organic matter) along with some of the
liquid surrounding it.
 occurs only in specialized cells (single-celled amoeba, bacteria-eating cells of our immune
system – macrophages) and only when they encounter something “suitable for engulfing.
Pinocytosis (“Cell-Drinking”)
 A cell engulfs a liquid and the small particles dissolved or suspended in it.
 Occurs in most eukaryotic cells most of the time.
Receptor-Mediated Endocytosis
 Receptor proteins in the cell membrane attach to specific molecules outside the cell.
 The cell membrane folds inward to form a vesicle that is coated with clathrin, a protein
that forms a cage around a vesicle.
 Exocytosis is a transport method in which a vacuole fuses with the cell membrane and
releases its contents outside the cell into the extracellular environment.
 Exocytosis is important in cells that specialize in the secretion of various cell products
such as hormones, neurotransmitters, and digestive enzymes.
 Example: specialized cells in the human pancreas secrete the hormone insulin by means
of exocytosis.
 See Figure 2.21, pg 79
 See Table 2.2, pg 79 for a Summary of Mechanisms for Transport of Substances Across a
Cell Membrane.
 pg 81 #1 – 13
 Chapter 2 Review (pg 89 #1-26, 28, 29, 31, 38-41, 45, 47-49, 52-54, 57, 59,
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