Cells and Organelles Flashcards
Cell membrane
hold cellular contents and are
mainly composed of phospholipids and proteins,
with small amounts of cholesterol.
Phospholipids
- glycerol backbone, one
phosphate group (hydrophilic), and two fatty
acid tails (hydrophobic).
-Amphipathic - both polar and
nonpolar parts, allowing them to form a lipid
bilayer in an aqueous environment.
Cholesterol
- four fused hydrocarbon
rings; precursor to steroid hormones; amphipathic and helps regulate
membrane fluidity.
Membrane proteins
- integral or
peripheral membrane proteins.
Integral (transmembrane) proteins
- traverse the
entire bilayer - amphipathic. Their nonpolar parts lie in the middle of the bilayer while
their polar ends extend out into the aqueous
environment on the inside and outside of the cell. - Usually assist in cell signaling or transport.
Peripheral membrane proteins
- found on the
outside of the bilayer, and they are generally
hydrophilic.
Functions of Peripheral Membrane Proteins
Receptor - trigger secondary responses within the cell for signaling. If a receptor protein transmits a signal all the way through the lipid bilayer, it is considered an integral protein. Drugs that bind to receptors can either be agonists
or antagonists. Agonists are molecules that bind to receptors and functionally activate a target, while Antagonists bind and prevent other molecules
from binding, inhibiting production of a response.
Adhesion - attaches cells to other things (eg. other cells) and act as anchors for the cytoskeleton.
Cellular recognition - proteins which have carbohydrate chains (glycoproteins). Used by cells to recognize other cells.
Fluid mosaic model
- describes how the
components that make up the cell membrane can
move freely within the membrane (“fluid”).
Furthermore, the cell membrane contains many
different kinds of structures (“mosaic”).
How is fluidity of cell membrane affected?
● Temperature - ↑ temperatures increase
fluidity while ↓ temperatures decrease it.
● Cholesterol - holds membrane together at
high temperatures and keeps membrane fluid
at low temperatures.
● Degrees of unsaturation - saturated fatty
acids pack more tightly than unsaturated fatty
acids, which have double bonds that may
introduce kinks. Trans-unsaturated fatty acids
pack more tightly than cis-unsaturated fatty
acids (which have a more severe kink).
Regulation of substances across cell membrane
1) Simple Diffusion
2) Facilitated Transport
3) Active Transport
Simple Diffusion
- flow of small, uncharged,
nonpolar substances (eg. O2 and CO2) across the cell membrane down their concentration gradient (high to low) without using energy.
e.g.: Osmosis is a type of simple diffusion that
involves water molecules (water is polar, but is small enough to cross the
membrane).
Facilitated transport
- integral proteins allow
larger, hydrophilic molecules to cross the cell
membrane. - uniporters, symporters, antiporters, channel proteins, carrier proteins, passive diffusion
Uniporter vs Symporter vs Antiporters
- uniporters (single
substance, single direction), - symporters (two substances, same direction),
- antiporters (two substances, opposite directions).
Channel Proteins vs Carrier Proteins
- channel proteins (open tunnels that face both sides
of bilayer) - carrier proteins (bind to a
molecule on one side and change shape to bring it to the other side).
Passive diffusion
- a type of facilitated
transport that is performed by channel proteins, bringing molecules down their
concentration gradient without energy
use (similar to simple diffusion, but a
protein channel is used).
e.g.: porins for hydrophilic molecules and ion
channels for ions.
Active transport
- substances travel against
their concentration gradient and require the
consumption of energy by carrier proteins.
Primary active transport
uses ATP hydrolysis to pump molecules against their concentration gradient. For example, the sodium-potassium (Na+/K+) pump
establishes membrane potential (discussed
in later chapters).
Secondary active transport
uses free energy released when other molecules
flow down their concentration gradient (gradient established by primary active transport) to pump the molecule of interest
across the membrane.
Cytosis
- transport mech; exocytosis = the bulk transport of large,
hydrophilic molecules across the cell membrane and requires energy (active transport mechanism).
Endocytosis
- involves the cell membrane wrapping around an extracellular substance, internalizing into the cell via a vesicle or vacuole.
-Different forms endocytosis:
1) Phagocytosis
2) Pinocytosis
3) Receptor- mediated endocytosis
Phagocytosis
- cellular eating around solid
objects.
Pinocytosis
- cellular drinking around
dissolved materials (liquids).
Receptor-mediated endocytosis
- requires
the binding of dissolved molecules to
peripheral membrane receptor proteins,
which initiates endocytosis.
Clathrin
a protein that aids in receptor
mediated endocytosis by forming a pit in the
membrane that pinches off as a coated vesicle.
This is known as clathrin mediated endocytosis.
Exocytosis
is the opposite of endocytosis, in which
material is released to the extracellular
environment through vesicle secretion.
Organelles
are cellular compartments enclosed by phospholipid bilayers (membrane bound). They are located within the cytosol (aqueous intracellular fluid) and help make up the
cytoplasm (cytosol + organelles).
How is Eukaryotes different from Prokaryotes in terms of organelles?
Only eukaryotic cells contain membrane-bound
organelles. Prokaryotes do not, but they have
other adaptations, such as keeping their genetic
material in a region called the nucleoid
Nucleus
primarily functions to protect and house DNA. DNA replication and transcription
(DNA → mRNA) occurs here.
Parts of Nucleus
● Nucleoplasm - is the cytoplasm of the
nucleus.
● The nuclear envelope - the membrane of the
nucleus. It contains two phospholipid bilayers
(one inner, one outer) with a perinuclear
space in the middle.
● Nuclear pores - holes in the nuclear envelope that allow molecules to travel in and
out of the nucleus.
● The nuclear lamina - provides structural
support to the nucleus, as well as regulating
DNA and cell division.
● The nucleolus - is a dense area that is responsible for making rRNA, and producing
ribosomal subunits (rRNA + proteins).
Ribosomes
not considered to be organelles; they work as small factories that carry out
translation (mRNA → protein). They are composed of ribosomal subunits.
Eukaryote Ribosomal Units
(60S and 40S)
assemble in the nucleoplasm and are then exported from the nucleus to form the complete ribosome in the cytosol (80S). (S does not refer to mass, but to sedimentation characteristics)
Prokaryote Ribosomal Units
(50S and 30S)
assemble in the nucleoid and form the complete
ribosome in the cytosol (70S).
Free-floating ribosomes
make proteins that
function in the cytosol while ribosomes embedded
in the rough endoplasmic reticulum (rough ER)
make proteins that are sent out of the cell or to the
cell membrane.
rough endoplasmic reticulum (rough ER)
continuous with the outer membrane of the
nuclear envelope and is “rough” because it has
ribosomes embedded in it. Proteins synthesized by
the embedded ribosomes are sent into the lumen
(inside of the rough ER) for modifications (eg.
glycosylation). Afterwards, they are either sent out
of the cell or become part of the cell membrane.
smooth endoplasmic reticulum (smooth ER)
is not continuous with other membranes. Its main
function is to synthesize lipids, produce steroid
hormones, and detoxify cells.
Golgi apparatus
stores, modifies, and exports
substances that will be secreted from the cell. It is
made up of cisternae (flattened sacs) that modify
and package substances. Vesicles come from the
ER and reach the cis face (side closest to ER) of the
Golgi apparatus. Vesicles leave the Golgi apparatus
from the trans face (side closest to cell
membrane).
-The Golgi apparatus has a significant role in the
endomembrane system: it receives vesicles from
the ER on the cis face that empty proteins and
lipids into the lumen of the Golgi. These
proteins/lipids undergo modifications and are then
sorted, tagged, packaged, and distributed as
secretory products.
Lysosomes
are membrane-bound organelles that
break down substances (through hydrolysis)
taken in through endocytosis. Lysosomes contain
acidic digestive enzymes that function at a low
pH. They also carry out autophagy (the
breakdown of the cell’s own machinery for
recycling) and apoptosis (programmed cell death).
Proteasomes
are similar in function to lysosomes. These are protein complexes that degrade unneeded or damaged proteins by proteolysis. Such proteins have a ubiquitin molecule attached, tagging these proteins for degradation.
Types of Vacuoles
1) Transport vacuoles
2) Food Vacuoles
3) Central Vacuoles
4) Storage Vacuoles
5) Contractile Vacuoles
Transport vacuoles
transport materials
between organelles.
Food vacuoles
- temporarily hold endocytosed
food, and later fuse with lysosomes.
Central vacuoles
- very large in plants and
have a specialized membrane called the
tonoplast (helps maintain cell rigidity by
exerting turgor). Function in storage and
material breakdown.
Storage vacuoles
- store starches, pigments,
and toxic substances.
Contractile vacuoles
- found in single-celled
organisms and works to actively pump out
excess water.
endomembrane system
- composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. It is a group of
organelles and membranes that work together to modify, package, and transport proteins and lipids that are entering or exiting a cell. The
components of the endomembrane system include the nucleus, rough and smooth ERs, Golgi apparatus, lysosomes, vacuoles, and cell membrane.
Peroxisomes
perform hydrolysis, break down stored fatty acids, and help with detoxification. These processes generate hydrogen peroxide, which is toxic since it can produce reactive oxygen species (ROS). ROS damage cells through free radicals. Peroxisomes contain an enzyme called catalase, which quickly breaks down hydrogen peroxide into water and oxygen.
Mitochondria
the powerhouses of the cell,
producing ATP for energy use through cellular
respiration (chapter 3). Mitochondrial inheritance is
maternal, meaning all mitochondrial DNA in
humans originates from the mother.
Chloroplasts
- found in plants and some
protists. They carry out photosynthesis - a type of plastid. Plastids are double-membraned organelles found exclusively
within plant cells and algae, that function in
photosynthesis and storage of metabolites.
Centrosomes
are organelles found in animal cells
containing a pair of centrioles. They act as
microtubule organizing centers (MTOCs) during
cell division
cytoskeleton
provides structure and function within the cytoplasm.
Microfilaments
are the smallest structure of the cytoskeleton, and are composed of a double helix
made of two actin filaments. They are mainly
involved in cell movement and can quickly
assemble and disassemble.
Functions of Microfilaments
- Cleavage furrow - during cell division, actin
microfilaments form contractile rings that split
the cell. - Cyclosis (cytoplasmic streaming) - the flow
(or stirring) of the cytoplasm inside the cell. It is
driven by forces via actin (microfilaments) and
myosin movement, in a manner similar to
muscle contraction. - Muscle contraction - actin microfilaments
have directionality, allowing myosin motor
proteins to pull on them for muscle
contraction.
Intermediate filaments
between microfilaments and microtubules in size.
- more stable than microfilaments and mainly help with structural support.
e.g.: keratin is
an important intermediate filament protein in skin,
hair, and nails.
e.g.: Lamins are a type of intermediate filament which helps make up the nuclear lamina, a network of fibrous intermediate filaments that
supports the nucleus.
Microtubules
largest in size and give
structural integrity to cells. They are hollow and
have walls made of tubulin protein dimers.
Microtubules also have functions in cell division,
cilia, and flagella.
Kinesin and dynein
motor proteins that
intracellulary transport cargo along microtubules.
Microtubule Organizing Centers (MTOCs)
present in eukaryotic cells and help organize
microtubule extension.
Centrioles
hollow cylinders made of nine
triplets of microtubules (9x3 array). Centrosomes
contain a pair of centrioles oriented at 90 degree
angles to one-another. They replicate during the S
phase of the cell cycle so that each daughter cell
after cell division has one centrosome.
Cilia
small hair-like projections found only in
eukaryotes. They line the outside of eukaryotic
cells and function in locomotion of either the cell
itself or fluids. There are two types:
1) Motile Cilia
2) Non-motile Cilia
Motile cilia
Help the cell or fluids move
around
Non-motile cilia
- Act as cellular antennas
that receive signals from neighboring cells
and environment.
Structure of Cilia
cilia have nine doublets of
microtubules (made of tubulin) with two singles in
the center, forming a 9 + 2 array. They are
produced by a basal body, which is initially formed
by the mother centriole (older centriole after S
phase replication).
Flagella
longer hair-like structures found in both prokaryotes and eukaryotes. Like cilia, flagella
also function in locomotion of the cell or fluids.
Eukaryote Flagella vs Prokaryote Flagella
1) Eukaryotic flagella are composed of polymers
of tubulin with the same 9+2 array as cilia. Prokaryotic flagella are composed of polymers of flagellin and do not have this 9+2 array
(they are not microtubules).
2) Eukaryotic flagella move in a bending motion, while prokaryotic flagella move in a
rotary motion.
3) Eukaryotic flagella: composed of tubulin dimers, larger and more complex, ATP driven, bending motion, complex sliding filament system
Prokaryote flagella: composed of flagellin, smaller and simpler structure, proton driven, rotary motion, rotary motor
extracellular matrix (ECM)
provides
extracellular mechanical support for cells.
Components:
1 ) Proteoglycans
2) Collagen
3) Integrin
4) Fibronectin
5) Laminin
Proteoglycan
- a type of glycoprotein that has a high proportion of carbohydrates.
Collagen
- the most common structural protein; organized into collagen fibrils (fibers of
glycosylated collagen secreted by fibroblasts).
Integrin
- a transmembrane protein that facilitates ECM adhesion and signals to cells
how to respond to the extracellular environment (growth, apoptosis, etc.).
Fibronectin
- a protein that connects integrin to ECM and helps with signal transduction.
Laminin
- behaves similarly to fibronectin. Influences cell differentiation, adhesion, and
movement. It is a major component of the
basal lamina (a layer of the ECM secreted by
epithelial cells).
Cell walls
carbohydrate-based structures that
act like a substitute ECM because they provide
structural support to cells that either do not have
ECM, or have a minimal ECM. They are present in
plants (cellulose), fungi (chitin), bacteria
(peptidoglycan), and archaea.
Peptidoglycan
is a polysaccharide with peptide chains. This is the primary component of bacterial cell walls. The cell wall of archaea is also made of polysaccharides, but does not contain peptidoglycan.
glycocalyx
a glycolipid/glycoprotein coat
found mainly on bacterial and animal epithelial
cells. It helps with adhesion, protection, and cell
recognition.
Cell-matrix junctions (connect ECM →
cytoskeleton):
1) Focal Adhesion
2) Hemidesmosomes
Focal adhesions
- ECM connects via integrins
to actin microfilaments inside the cell.
Hemidesmosomes
- ECM connects via
integrins to intermediate filaments inside the
cell.
Cell-cell junctions (connect adjacent cells):
1) Tight Junction
2) Desmosomes
3) Adherens Junctions
4) Gap Junctions
Tight junctions
- form water-tight seals
between cells to ensure substances pass
through cells and not between them.
Desmosomes
- provide support against
mechanical stress. Connects neighboring cells
via intermediate filaments.
Adherens junctions
- similar in structure and
function to desmosomes, but connects neighboring cells via actin microfilaments.
Gap junctions
- allow passage of ions and
small molecules between cells. Formed from
transmembrane proteins known as connexins.
Gap junctions are only present in animal cells.
Plants have a few unique cell junctions such as:
1) Middle Lamella
2) Plasmodesmata
Middle lamella
- sticky cement similar in
function to tight junctions.
Plasmodesmata
- tunnels with tubes between
plant cells. Allows cytosol fluids to freely travel
between plant cells.
Types of Cellular Tonicity and Cell Circulation:
1) Isotonic
2) Hypertonic
3) Hypotonic
Isotonic solutions
have the same solute
concentration as the cells placed in them.
Hypertonic solutions
have a higher solute
concentration than the cells placed in them,
causing water to leave the cell (cell shrivels).
Hypotonic solutions
have a lower solute
concentration than the cells placed in them,
causing water to enter the cell (cell swells up).
Lysis is the bursting of a cell when too much water
enters. Plasmolysis is the process by which a cell’s
cytoplasm shrinks away from the cell wall due to
water loss from being in a hypertonic solution.
Types of Cellular Adaptations cells undergo to ensure survival include:
1) Atrophy
2) Hypertrophy
3) Hyperplasia
4) Metaplasia
5) Dysplasia
Atrophy
- decrease in cell size due to
reduced metabolic activity
Hypertrophy
- increase in cell size due to
increased metabolic activity
Hyperplasia
- increase in the number of
cells in an organ or tissue that appear
normal under a microscope, often seen in
the beginning of cancer.
Metaplasia
- a somatic cell undergoing
transformation into another specialized
type of somatic cell
Dysplasia
- development of phenotypically
abnormal cells in a tissue that can lead to
cancerous growth.
