Cell structure (Eukaryotes)
The cell is the basic unit of function and structure in living organisms. Structures within a cell with a specific function are called
ORGANELLES.
Cell membrane
Structure: The cell surface membrane is made up of two
layers of phospholipids* and is embedded with proteins, such
as receptors on the outer surface.
Function: The cell membrane separates the contents of the
cell from its external environment and regulates the
movement of substances in and out of the cell. The cell membrane is selectively permeable as it allows only certain
substances to pass through. (Most organelles inside the cell are surrounded by their own membrane; enabling separate
compartments to be formed where specialized reactions take place e.g. mitochondria.)
*Phospholipids
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Protein channels and protein carriers
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Nucleus (Plural: nuclei)
Structure: The nucleus is a large, round organelle surrounded by
a double membrane which is called the nuclear envelope. The
nuclear envelope contains large pores, called nuclear pores,
which allow the exchange of materials between the nucleus and
the cytoplasm. DNA is found in the nucleus. Usually, DNA is in
the form of chromatin which are strands of DNA and protein that appear threadlike. Just before and during the division
of the cell, chromatin condenses to form chromosomes. The nucleus also contains a dark region called the nucleolus,
which produces ribosomes.
Function: The nucleus contains the cell’s DNA and controls all the activities of the cell.
Nucleolus
Structure: A round, dense structure made of protein and RNA and found within the nucleus.
Function: Synthesize and assemble ribosomes.
Mitochondrion (plural: mitochondria)
Structure: Mitochondria are oval in shape and have a double membrane. The inner membrane of the mitochondrion is
highly folded into cristae, which increases surface area. The matrix is a viscous liquid which contains enzymes.
Function: Some of the reactions of aerobic respiration occur in the mitochondria and they are the “power houses” of the
cell, producing energy that the cell can use (ATP).
Ribosomes
Structure: Ribosomes consist of two subunits, one large and one small.
The subunits are made up of protein and ribosomal RNA. They can be found floating free or
attached to the rough endoplasmic reticulum.
Function: Ribosomes are the site of protein synthesis.
Endoplasmic reticulum (ER)
Structure: A large network of membranes. There are two types, the rough endoplasmic reticulum (RER), which is
covered with ribosomes, and the smooth endoplasmic reticulum (SER), which does not have ribosomes.
Function:
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Rough endoplasmic reticulum – Sometimes modifies proteins synthesized on the ribosomes; transports the
proteins synthesized at the ribosomes
Smooth endoplasmic reticulum- Produces and transports lipids; transports proteins and other materials for
export from the cell; contains enzymes that carry out different functions like detoxifying drugs.
Golgi apparatus (also known as Golgi body)
Structure: A stack of flattened, membrane bound sacks (like a stack of pancakes) with smaller
vesicles around it
Function:
1) Transports and chemically modifies the substances in it e.g. proteins
2) Produces lysosomes
3) Cells involved with secretion contain large amounts of Golgi apparatus e.g. cells of the pancreas which secrete
digestive enzymes
Lysosome
Structure: A sac consisting of a single membrane which contains digestive enzymes. The enzymes have to be kept apart
from the rest of the cell or they would destroy it. The contents are acidic and the enzymes have a low optimum pH.
Function: The lysosome breaks down unwanted molecules and structures inside the cell and break down lipids,
carbohydrates and proteins from food into smaller molecules that the cell can use.
Vacuole
Structure: Vacuoles are found in all plant and fungi cells and some protist, animal and bacterial cells. Vacuoles are
membrane bound containers filled with water and other molecules. They may contain solids which have been engulfed.
Their shape and size depend on the needs of the cell.
Function: The function and significance of vacuoles depends on the type of cell and include: isolating materials that
might be harmful to the cell, containing waste products and getting them out of the cell by exocytosis, containing water
in plant cells, maintaining internal hydrostatic pressure or turgor within the cell, and allowing plants to support
structures such as leaves and flowers due to the pressure of the central vacuole.
Cytoskeleton
Structure: The cytoskeleton is a series of intercellular proteins. Cytoskeleton has three main structural components:
microfilaments, intermediate filaments, and microtubules.
Function: Help a cell with shape, support, and movement.
Microfilaments:
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Microtubules
Structure: Made of proteins known as tubulins.
Function: Perform different functions in different cells:
- Critical to maintaining shape of some cells
- Form centrioles (animal cells only)
- Build projections from cell’s surface such as flagella and cilia that enable some cells to “swim”
- Some cells have them arranged so that they can be used to produce controlled movements of the cell.
Centrioles
Structure: A centriole is a small set of microtubules arranged in a specific way, found in the cytoplasm of animal cells.
When two centrioles are found next to each other, they are usually at right angles. The centrioles are found in pairs.
Function: When it is time for cell division, centrioles replicate and move towards the poles (opposite ends) of the cell to
take part in mitosis (cell division).
Cytoplasm
The cytoplasm is everything inside the cell membrane except the nucleus.
Cytosol
The cytosol is the fluid in which the organelles are found. It is made mostly of water.
THE FOLLOWING STRUCTURES ARE FOUND IN PLANT, FUNGI OR PROTISTA CELLS ONLY
Cell wall (All plants, all Fungi, some Protista)
Structure: A rigid layer that surrounds the cell membrane (outside). In plants, it is made of cellulose; in fungi, chitin and
in protists, many different substances, depending on the protist.
Function: The cell wall provides support and stops the cell from bursting. It is fully permeable though, enabling water
and other substances to pass through it.
Chloroplast (Plants)
Structure: Chloroplasts have a double membrane. The inner membranes of
chloroplasts are called thylakoids, which are where one of the reactions of
photosynthesis takes place. The thylakoids often form coin-like stacks called grana
(singular granum). Chloroplasts are green as they contain the pigment chlorophyll.
Function: Photosynthesis takes place in the chloroplast.
Large Central Vacuole (Plants)
Structure: A large sac in the cytoplasm, surrounded by a single membrane which
contains a fluid called cell sap. Cell sap is water with dissolved substances, such as
minerals and ions.
Function: The vacuole is important in maintaining the cell in a turgid state, which
provides support to the plant.
Double Membrane bound organelles: mitochondria, chloroplasts, endoplasmic reticulum, Golgi body, and
nucleus.