Cell Structure Flashcards
(27 cards)
Plasma Membrane
Cells are enclosed by a distinct plasma membrane, which shares features with the cytomembrane system that compartmentalizes the cytoplasm and surrounds the nucleus. All membranes are composed of
lipids (mainly phospholipids, cholesterol and glycolipids) and proteins.
Plasma Membrane Lipib Bilayer
Plasma membrane lipids form a lipid bilayer, a layer two molecules thick. The hydrophobic ends of each
lipid molecule face the interior of the membrane and the hydrophilic
ends face outwards
Cytoplasm
The cytoplasm consists of the cytosol, a gel-like material enclosed by
the cell or plasma membrane. The cytosol is made up of colloidal proteins such as enzymes, carbohydrates and small protein molecules, together with ribosomes and ribonucleic acids. The cytoplasm contains
two cytomembrane systems, the endoplasmic reticulum and Golgi apparatus, as well as membrane-bound organelles (lysosomes, peroxisomes and mitochondria), membrane-free inclusions (lipid droplets,
glycogen and pigments) and the cytoskeleton.
Nucleus in relation to the cytoplasm
The nuclear contents, the nucleoplasm, are separated from the cytoplasm by the nuclear
envelope.
Endoplasmic Reticulum
The endoplasmic reticulum is a system of interconnecting membranelined channels within the cytoplasm. These channels take various forms, including cisternae (flattened sacs), tubules and vesicles.
The membranes divide the cytoplasm into two major compartments.
The intramembranous compartment, or cisternal space, is where secretory products are stored or transported to the Golgi complex and cell
exterior. The cisternal space is continuous with the perinuclear space
rER VS sER
rough or granular endoplasmic reticulum (RER), has ribosomes attached to its
outer, cytosolic surface, and
smooth or agranular endoplasmic reticulum (SER), lacks ribosomes.
Function of endoplasmic reticulum
The functions of the endoplasmic reticulum vary greatly and include: the synthesis, folding and transport
of proteins; synthesis and transport of phospholipids and steroids; and storage of calcium within the cisternal space and regulated release into
the cytoplasm
Smooth endoplasmic reticulum
The smooth endoplasmic reticulum is associated with carbohydrate metabolism and many other metabolic processes, including detoxification and synthesis of lipids, cholesterol and steroids. The membranes of the smooth endoplasmic reticulum serve as surfaces for the attachment of many enzyme systems, e.g. the enzyme cytochrome P450, which is involved in important detoxification mechanisms and is thus accessible to its substrates, which are generally lipophilic.
Rough endoplasmic reticulum
The rough endoplasmic reticulum is a site of protein synthesis; its
cytosolic surface is studded with ribosomes. Ribosomes only
bind to the endoplasmic reticulum when proteins targeted for secretion
begin to be synthesized.
Ribosomes and protein synthesis
Ribosomes are macromolecular machines that catalyse the synthesis of
proteins from amino acids; synthesis and assembly into subunits takes
place in the nucleolus and includes the association of ribosomal RNA
(rRNA) with ribosomal proteins translocated from their site of synthesis
in the cytoplasm. The individual subunits are then transported into the
cytoplasm, where they remain separate from each other when not
actively synthesizing proteins.
The Golgi apparatus function
The Golgi apparatus has several functions: it links anterograde and retrograde protein and
lipid flow in the secretory pathway; it is the site where protein and lipid
glycosylation occurs; and it provides membrane platforms to which
signalling and sorting proteins bind.
Exocytic (secretory) pathway
Secreted proteins lipids glycoproteins small molecules such as amines and other cellular products destined for export from the cell are transported to the plasma membrane in small vesicles released from the
trans-face of the Golgi apparatus. This pathway either is constitutive in which transport and secretion occur more or less continuously as with immunoglobulins produced by plasma cells or it is regulated by external signals as in the control of salivary secretion by autonomic neural stimulation. In regulated secretion the secretory product is stored temporarily in membrane-bound secretory granules or vesicles. Exocytosis is achieved by fusion of the secretory vesicular membrane with the plasma membrane and release of the vesicle contents into the extracellular domain
Endocytic (internalization) pathway
The endocytic pathway begins at the plasma membrane and ends in lysosomes involved in the degradation of the endocytic cargo through the enzymatic activity of lysosomal hydrolases. Endocytic cargo is internalized from the plasma membrane to early endosomes and then to late endosomes. Late endosomes transport their cargo to lysosomes, where the cargo material is degraded following fusion and mixing of contents of endosomes and lysosomes.
Lysosomes
Lysosomes are membrane-bound organelles 80–800 nm in diameter often with complex inclusions of material undergoing hydrolysis (secondary lysosomes).
Mitochondria
Mitochondria are the principal source of chemical energy in most cells. Mitochondria are the site of the citric acid (Krebs’) cycle and the electron transport (cytochrome) pathway by which complex organic molecules are finally oxidized to carbon dioxide and water. This process provides the energy to drive the production of ATP from adenosine diphosphate (ADP) and inorganic phosphate (oxidative phosphorylation). The various enzymes of the citric acid cycle are located in the mitochondrial matrix whereas those of the cytochrome system and
oxidative phosphorylation are localized chiefly in the inner mitochondrial membrane
Transport across cell membranes
Lipid bilayers are increasingly impermeable to molecules as they
increase in size or hydrophobicity. Transport mechanisms are therefore
required to carry essential polar molecules including ions nutrientsnucleotides and metabolites of various kinds across the plasma membrane and into or out of membrane-bound intracellular compartments.
Transport is facilitated by a variety of membrane transport proteinseach with specificity for a particular class of molecule e.g. sugars. Transport proteins fall mainly into two major classes: channel proteins and
carrier proteins.
Channel proteins
Channel proteins form aqueous pores in the membrane which open and close under the regulation of intracellular signals e.g. G-proteinsto allow the flux of solutes (usually inorganic ions) of specific size and
charge. Transport through ion channels is always passive and ion flow through an open channel depends only on the ion concentration gradient and its electronic charge and the potential difference across the membrane. These factors combine to produce an electrochemical gradient which governs ion flux. Channel proteins are utilized most effectively by the excitable plasma membranes of nerve cells where the resting membrane potential can change transiently from about −80 mV (negative inside the cell) to +40 mV (positive inside the cell) when
stimulated by a neurotransmitter (as a result of the opening and subsequent closure of channels selectively permeable to sodium and potassium).
Carrier proteins
Carrier proteins bind their specific solutes such as amino acids and transport them across the membrane through a series of conformational changes. This latter process is slower than ion transport through membrane channels. Transport by carrier proteins can occur either passively by simple diffusion or actively against the electrochemical gradient of the solute. Active transport must therefore be coupled to a source of energy such as ATP generation or energy released by the coordinate movement of an ion down its electrochemical gradient. Linked transport can be in the same direction as the solute in which case the carrier protein is described as a symporter or in the opposite direction when the carrier acts as an antiporter.
Nucleus
The nucleus is generally the largest intracellular structure and is usually spherical or ellipsoid in shape with a diameter of 3–10 µm. Conventional histological stains such as haematoxylin or toluidine blue detect the acidic components (phosphate groups) of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) in cells and
tissue sections.
Nuclear envelope
The nucleus is surrounded by the nuclear envelope which consists of
an inner nuclear membrane (INM) and an outer nuclear membrane
(ONM) separated by a 40–50 nm perinuclear space that is spanned by
nuclear pore complexes (NPCs).
Chromatin
DNA is organized within the nucleus in a DNA–protein complex known
as chromatin. The protein constituents of chromatin are the histones
and the non-histone proteins. Non-histone proteins are an extremely
heterogeneous group that includes structural proteins DNA and RNA
polymerases and gene regulatory proteins.
Nucleolus
Nucleoli are a prominent feature of an interphase nucleus.
They are the site of most of the synthesis of ribosomal RNA (rRNA) and
assembly of ribosome subunits. Nucleoli organize at the end of mitosis
Mitosis phases
- Prophase
- Metaphase
- Anaphase
- Telophase
