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glycolipid
glycoprotein
phospholipid
protein channel
protein
cholesterol
Cell Membrane
Structure:
Function:
The cell membrane consists of a phospholipid
bilayer that is approximately 7nm thick. The
hydrophilic heads of the phospholipids form
the inner and outer surface of the membrane
and the hydrophobic tails form the inside of the
membrane. This means that the surface of the
membrane can interact with water inside and
outside the cell, but water-soluble substances
cannot diffuse through the hydrophobic core.
Cell membranes are physical barriers. They
prevent the movement of foreign bodies and
other molecules into and out of the cell. Cell
membranes are partially permeable: they
allow some molecules to pass through but
not others. Substances can be transported
across the cell membrane by diffusion,
osmosis and active transport.
The structure of the cell membrane is described
by the fluid mosaic model. The cell membrane
is made up of many structures that are
constantly moving within the bilayer (they are
fluid). Cholesterol molecules are embedded
between the phospholipids to prevent too
much movement.
Channel proteins and carrier proteins are found
within the bilayer. These allow large molecules
and ions to be transported across the cell
membrane, making the cell membrane
partially permeable.
Receptor proteins, glycoproteins and
glycolipids are scattered throughout the
membrane. Glycoproteins are proteins with
a carbohydrate attached to them, while
glycolipids are lipids with a carbohydrate
attached to them.
Membranes around organelles such as
mitochondria physically separate the
cytoplasm of the cell and the organelle.
Glycoproteins and glycolipids have several
functions in the human body, including:
• responding to insulin in liver cells,
resulting in the cell absorbing
glucose from the bloodstream;
• establishing blood type;
• immune responses;
• responding to neurotransmitters involved
in nervous responses.
stroma
ribosomes
granum
(plural: grana)
double
membrane
lamella
(plural: lamellae)
thylakoid
Chloroplast
Structure:
Function:
Chloroplasts have a double membrane that
surrounds the gel-like stroma. Within the
stroma are fluid-filled sacs called thylakoids.
These thylakoids are stacked up to form grana.
Thin pieces of thylakoid membrane called
lamellae link the grana together.
In plant and algal cells, photosynthesis
takes place in the chloroplasts via a series
of light-dependent and light-independent
reactions. The light-dependent reactions
occur within the thylakoid membranes,
whereas the light-independent reactions
take place within the stroma.
The thylakoid membranes contain
photosynthetic pigments that absorb light
energy for photosynthesis.
A by-product of this process is oxygen,
a molecule which many organisms have
evolved to rely upon.
ribosomes
Rough Endoplasmic Reticulum (RER)
Structure:
Function:
The rough endoplasmic reticulum (RER) is a
network of channel-like structures filled with
fluid. They are linked to the nuclear membrane
by small tubules.
The RER works in conjunction with the
attached ribosomes to process and produce
the 3D structures of proteins. It is also the
site of glycoprotein synthesis. Cells that make
lots of protein typically contain extremely
high amounts of RER.
The RER has ribosomes attached along the
outer surface. It has a large surface area to
increase the rate of protein synthesis.
fluid-filled
channels
Smooth Endoplasmic Reticulum (SER)
Structure:
Function:
The structure of the smooth endoplasmic
reticulum (SER) is very similar to the rough
endoplasmic reticulum (RER), but there are no
ribosomes along the outer surface. The SER
is typically attached to RER and linked to the
nuclear membrane.
The SER is responsible for storing,
synthesising and processing lipids,
phospholipids and cholesterol. Within
skeletal muscle cells the SER stores other
substances such as calcium ions.
The SER has a large surface area to
increase the rate of synthesis of lipids and
other molecules.
Within some endocrine glands, the SER
contains enzymes that detoxify harmful
substances. For example, in liver cells,
the SER is involved in breaking
down carcinogens.
vesicle
Golgi apparatus
membrane
Golgi Complex
Structure:
Function:
The Golgi complex is composed of a Golgi
apparatus and several small vesicles.
The Golgi apparatus is responsible for
processing and packaging lipids and
proteins. It also produces lysosomes.
The Golgi apparatus is a membrane-bound
channel-like structure filled with fluid. The
vesicles are detached fluid-filled pockets found
at the edges of the complex.
The lipids and proteins produced by the
Golgi apparatus are stored by the Golgi
vesicles. The vesicles can transport these
substances out of the cell through the
cell membrane.
membrane
lysozymes
Lysosome
Structure:
Function:
Lysosomes are types of Golgi vesicle with no
obvious internal structure. They are surrounded
by a membrane and contain digestive enzymes
called lysozymes. The pH inside the lysosome
is acidic compared to the alkaline cytoplasm
of the cell. This acidic pH is necessary for
lysozymes to function.
Lysozymes are responsible for digesting
invading cells, old, unwanted parts of cells and
complex biomolecules.
The membrane ensures that the lysozymes are
kept separate from the cytoplasm of the cell to
prevent self-digestion.
outer
membrane
crista
(plural: cristae)
matrix
inner membrane
Mitochondrion
Structure:
Function:
Mitochondria are oval-shaped structures with
an outer membrane and inner membrane.
The inner membrane is highly folded, forming
finger-like structures called cristae. Cristae
increase the surface area available for chemical
reactions to take place.
Mitochondria are the site of aerobic respiration,
which produces adenosine triphosphate (ATP),
a molecule that is essential for cellular activity.
Cells that require large amounts of energy
contain many mitochondria.
The matrix is formed from the folding of the
inner cell membrane and contains some of the
enzymes involved in aerobic respiration.
nucleolus
chromatin
nuclear
envelope
nuclear pore
Nucleus
Structure:
Function:
The nucleus is encased within a double
membrane called a nuclear envelope. The
nuclear envelope has spaces within it called
nuclear pores. These nuclear pores allow
substances such as RNA to move between the
cytoplasm of the cell and the nucleus.
The nucleus controls the functions of the cell. It
does this by controlling DNA transcription.
DNA associates with proteins called histones.
This complex coils tightly to form chromosomes
which are found in the nucleus. DNA is
too large to fit through the nuclear pores,
preventing it from leaving the nucleus.
Chromatin, a substance consisting of DNA and
protein, is scattered throughout the nucleus.
The nucleolus is a structure consisting of DNA,
RNA and proteins. Some cells contain more
than one nucleolus.
The functions of the nucleus include controlling
gene expression, protein synthesis and storing
DNA (hereditary material).
Protein synthesis and ribosome production
occur in the nucleolus.
cell sap
tonoplast
Permanent Cell Vacuole
Structure:
Function:
Permanent cell vacuoles are permanent
pockets of cell sap, a solution of sugars, salts
and water, surrounded by a membrane called
the tonoplast.
The permanent cell vacuole maintains osmotic
pressure inside the cell. This ensures that
plant cells remain turgid, which stops the plant
wilting. Cell vacuoles are also important for
storing unwanted chemicals that are discarded
by the cell.
cell membrane
cell wall
cytoplasm
Cell Wall
Structure:
Function:
Cell walls are rigid structures surrounding the
cell membrane of plant, algal and fungal cells.
In plants and algae, the cell wall is composed
mainly of cellulose. However, in fungal cells, it is
composed of chitin.
Cell walls are key structures in plant, algal and
fungal cells. They help maintain cell shape by
providing structural support.
Cell walls also provide the cell with protection
against invading pathogens.
small subunit
large subunit
Ribosome
Structure:
Function:
Ribosomes are very small structures composed
of a large subunit and a small subunit. They
consist of ribosomal proteins and ribosomal
RNA (rRNA). A single eukaryotic cell can
contain over 10 million ribosomes.
Ribosomes are responsible for synthesising
proteins from messenger RNA (mRNA) during
translation. In this process, the codons in mRNA
are used to produce a specific sequence of
amino acids. This amino acid chain is then
folded into a protein.
Ribosomes are often associated with another
organelle called the rough endoplasmic
reticulum, but otherwise float freely within the
cytoplasm. Ribosomes are not surrounded by a
separate membrane.