Cells Flashcards
(27 cards)
Cell theory
- All living components are composed of one or more cells
- Cell is most basic unit of structure in all organisms
- All cells arise from pre-existing cells
What are cells?
Difference between prokaryotic and eukaryotic cells
Cells are Smallest units of life capable of replicating independently
Difference
- prokaryotes have neither
a true nucleus nor membrane bound organelles, whereas eukaryotes have both.
- Eukaryotic cells are generally much larger than prokaryotic cells.
Functions of cells
- Intake of raw materials, and from these,
- Extract useful energy, and synthesise its own molecules
- Grow in an organised manner
- Reproduce after its own kind
- Respond and adapt to the external environment
Cell size lower limit and upper limit
lower limit of cell size: determined by the minimum amount of space needed to contain the essential elements of its function (DNA, enzyme molecules)
The upper limit: is determined by the surface area: volume ratio needed for exchange of materials between the cell and its environment.
As the size of the cell increases, the surface area to volume ratio decreases.
● The number of chemical exchanges that could be performed with the extracellular environment
would be inadequate to maintain the cell, because most of its cytoplasm is relatively far from the
outer membrane. Exchange with the extracellular environment is vital as substances like oxygen
and nutrients can only enter the cell, and waste products can only leave, in this fashion.
Cell size is thus kept small and hence increase in organism size is accomplished by having a
greater number of cells.
structure of eukaryotic cell
A generalised eukaryotic cell is organised in this manner:
- Eukaryotic cells have a plasma membrane on their outer surface, and extensive and elaborately
arranged internal membranes that compartmentalise the cell.
- nucleus contains the genetic material of the organism
- The contents external to the nucleus are collectively known as the cytoplasm.
- The cytoplasm consists of organelles, which are compartments that carry out various functions in the cell, and the cytosol, an aqueous matrix in which the organelles and nucleus are suspended.
- Organelles can be subdivided into membranous and non-membranous organelles.
- In addition to the internal and cell membranes, organisms such as plants and fungi have cell wall
cytoplasm structure
cytoplasm: refers to all the organelles and cytosol within the cell surface membrane, with exception of the nucleus
- cytosol is an aqueous solute rich matrix that appears transparent and lacking structure under the light microscope
- 90% water, and dissolved in it are:
1. various essential ions and soluble organic molecules such as sugars and amino acids
2. soluble proteins including enzymes
3. the cytoskeleton – a network of fine strands of globular and fibrous proteins, which provides infrastructure and support to the cell.
The cytoplasm is bounded by the cell surface membrane, which is composed of phospholipids, proteins and carbohydrates
types of organelles
membranous organelles - compartmentalised spaces within cytoplasm, surrounded by membranes structurely and biochemicaly similar to plasma membrane
non membranous organelles
what are some advantages of having membranous organelles
- membranes maintain characteristic differences between the contents of each organelle and the cytosol.
- compartmentalisation of specific reactions -> provide different local environments -> incompatible processes can occur simultaneously - presence of membranes also helps to increase membrane surface area.
- internal membranes allow for embedding of enzymes and proteins that mediate many cellular reactions.
- greater the membrane surface area, the larger the number of enzyme complexes that can be embedded -> optimal enzyme concentration for rxn to occur -> increasing efficiency of reactions
nucleus
- largest organelle in animal cell
- encloses genetic material, protects DNA from metabolically active cytoplasm
- double membrane is perforated with pores to enable exchange of substances between the nucleus and cytoplasm.
It consists of the following components:
1. Nuclear envelope - double membrane, separates the contents of the nucleus from the cytoplasm.
- Each of the two membranes is a lipid bilayer
- The outer membrane of the nucleus is continuous with the membrane of the endoplasmic reticulum and inner and outer membranes are continuous with each other
- region between them is perinuclear space, which is continuous with the ER lumen.
- perforated by nuclear pores, made up of a large protein complex, which allow macromolecules such as mRNA and rRNA to exit the nucleus, and proteins e.g. enzymes to enter and exit the nucleus.
Nucleoplasm
- aqueous matrix within the nucleus containing proteins, metabolites, ions, RNA and chromatin
- Chromatin, composed of coils of DNA wound around histones exist in two forms:
● Loosely coiled chromatin (euchromatin) which appears as light-coloured patches in the electron micrograph
● Tightly coiled chromatin (heterochromatin), which appears as dark-coloured patches in the electron micrograph
Nucleolus
- dense mass in nucleus when viewed under EM
- composed of DNA that carries and codes for rRNA genes, RNA and protein
- functions to synthesise ribosomal RNA that formes a component of ribosomes
what is an endomembrane system? components of endomembrane system?
The endomembrane system: composed of a number of inter-related membrane sacs within the cytoplasm of the cell.
(related either by direct physical continuity or by the transfer of membrane segments known as vesicles)
Function: manufacture proteins and lipids
components:
1. Rough & Smooth endoplasmic reticulum
2. Golgi apparatus
3. Lysosomes
4. Vacuoles
plasma membrane is not part of the endomembrane system, it continuously interacts with its components by receiving and producing vesicles.
endoplasmic reticulum structure and function
ER
- consists of extensive network of hollow, membranous tubules, sacs or sheets called cisternae (singular is cisterna)
- internal space of ER = cisternal space / lumen, continuous with perinuclear space
- flatter, more compact and sheet like compared to golgi
specialised structures and their functions:
1. The extensive network of cisternae increases membrane surface area for synthesis
- rER: cisternae embedded with ribosomes for synthesis of polypeptides
- sER, the membrane allows for enzymes to be embedded -> steroids and phospholipids can be synthesised
2. Hollow cisternae
- accommodate newly synthesised substances
- allow for packaging of contents into vesicles for transport to the golgi apparatus
structure, function and significance of RER
structure: sheet like appearance which appears rough due to presence of ribosomes that stud the cystolic face of rough ER
Function:
- These rough ER-bound ribosomes are sites of protein synthesis, polypeptide chain is synthesised at the bound ribosome
- The polypeptide chain then enters the ER lumen, which is the site of protein folding, through a protein channel in the rER membrane. In lumen, polypeptide chain folds into its native conformation.
- These proteins are either destined for export, or are targeted to various cellular organelles
(diff from proteins synthesized by free ribosomes in the cytosol which remain in the cytosol)
Significance:
- Cells active in protein secretion usually have abundant rough ER.
- Some proteins synthesised in the rough ER can also directly enter the membrane of the ER to form ER membrane proteins.
- Proteins that leave RER are enclosed in vesicles known as transport vesicles
smooth endoplasmic reticulum structure and function
structure: network of tubules which lack ribosomes, resulting in its smooth appearance
It functions in diverse metabolic processes listed below, which require ATP:
1. Synthesis of lipids (including oils, phospholipids and steroids like sex hormones); cells that are active in hormone secretion usually have abundant sER.
2. Metabolism of carbohydrates,
3. Detoxification of drugs and poisons
4. Storage of calcium ions for use in muscle contraction, as well as cell signalling
structure, function, significance of golgi apparatus
Structure:
- consists of a stack of flattened, membrane-bound sacs called cisternae, separates internal space from cytosol
- distinct polarity in stacks of GA -> cis face and trans face
- new cisternae constantly being formed at cis face by receiving transport vesicles from the ER -> membranes of transport vesicles from the ER fuses with the cis face membrane
and deposit their contents into Golgi cisternal space
- At trans face, membranes bud off to form secretory vesicles, which contain materials to be transported to the extracellular matrix / bud off to form lysosomes
- Between the Golgi sacs, Golgi vesicles are responsible for transferring materials between the parts of the Golgi.
- Some Golgi vesicles also bud off from the trans face to transport substances to other organelles in the cell.
Function:
- GA is site of modification and packaging of ER products
- some modifications include glycosylation (addition of sugar groups) and trimming (removal of excess monomers)
- Different Golgi cisternae contain different enzymes for modification -> ER products are progressively modified as they move through the stacks of the Golgi complex
from the cis face to the trans face
- The processed and packaged contents are then passed on to other components of the cell by vesicles that bud off the GA complex.
Significance:
- cells that are active in any form of secretion usually have abundant GA (abundant flattened cisternae provide increased surface area for vesicle reception and budding, multiple cisternae also allow for different modification processes to occur simultaneously)
lysosomes
structure:
- membranous organelle that appears homogenously electron-dense under the EM
- It contains hydrolytic enzymes (proteases, nucleases, lipases and acid phosphatases) that can digest most biological macromolecules, Enzyme contents are synthesised on rER and transported to GA for further processing
- Due to the acidic (pH 5) nature of lysosome contents + hydrolytic activity of enclosed enzymes, lysosomal contents must be prevented from spilling into the cytoplasm under normal
cell conditions - segregation of contents within the membrane -> optimal pH for hydrolytic reactions + protects cellular contents from hydrolysis.
3 Functions of lysosome:
1. Digestion of materials taken into cells
- Food particles are engulfed by endocytosis to form food
vacuoles, which fuse with lysosomes to form endosomes -> enzymes digest endosome’s contents -> contents end up in the cytosol, used as food for the cell
- defence mechanism against bacteria in certain cell types. result of fusion is known as a phagocytic vacuole.
- Autophagy of worn-out organelles
- Unwanted structures within the cell are enclosed by a membrane of unknown origin, forming vesicle.
- This vesicle fuses with the lysosome to form an autophagic vacuole. - Autolysis
- apoptosis: process where cells self-destruct in a controlled manner when programmed to
- It is a normal event in development and differentiation (eg. destruction of tadpole tail to become a frog) + can also occur when a cell senses that it has become a threat to its environment
- For autolysis to occur, there must be a mass release of lysosomal contents in the whole cell.
vacuole
- present in animal and plant cells, but serve varying functions
- Vacuole membranes in both cell types originate from the ER and GA, vacuoles are considered part of the endomembrane system.
animal cells:
- vacuoles are small, mobile organelles that serve to house and transport substances (food vacuoles, phagocytic vacuoles etc)
plant cells
- large central vacuole surrounded by a single membrane known as a tonoplast
- solution within the tonoplast is known as cell sap, and differs in composition from the cytoplasm.
-vacuole -> versatile component, can regulate the substances it concentrates.
Functions of plant vacuole:
1. Storage of organic compounds (e.g. proteins) and inorganic ions (e.g. K+ and Cl-)
2. Disposal site for toxic metabolic by-products
3. Contains pigments (e.g. red pigments that colour petals)
4. Plant protection: vacuole accumulates compounds that are toxic or unpalatable to consumers
5. Cell growth and elongation as water accumulates in the vacuole -> allowing plant cells to increase in size with little increase in cytoplasm synthesis and without sacrificing surface area to volume ratio, as cytoplasmic contents are pushed to the periphery of the cell.
similarity between chloroplast and mitochondria
- both have double membranes
- both have own DNA and potein synthesis machinery
- involved in energy transduction
mitochondria
Mitochondria (singular: mitochondrion)
- sites of aerobic cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats and other metabolic fuels in the presence of oxygen
- number of mitochondria per cell corresponds to level of metabolic activity.
- usually elongated or spherical, smaller than chloroplast
- They are enclosed in an envelope of two membranes, each with a unique collection of
proteins and enzymes.
- The narrow, fluid-filled space between the two
membranes is known as the inter-membrane space (or peri-mitochondrial space).
structure: The outer membrane is smooth, inner membrane is highly convoluted, with infoldings known as cristae
Function: increase the surface area for the attachment of various enzyme systems involved in cellular respiration.
structure: The compartment enclosed by the inner membrane is known as the mitochondrial matrix, which houses enzymes, circular DNA, RNA and ribosomes (site of kerbs cycle)
Function: allows mitochondria to synthsise own proteins.
The compartmentalisation of the mitochondrial matrix is also necessary for a proton gradient across the inner mitochondrial membrane to be set up due to the inner mitochondrial membrane being impermeable to protons (proton grad necessary for ATP synthesis)
chloroplasts
Structure of chloroplasts
- lens shaped
- only organelles other than the nucleus visible under the light microscope.
- Each chloroplast is surrounded by a double
membrane called the chloroplast envelope
Function:
- sites of photosynthesis
- They convert solar energy to chemical energy by absorbing sunlight, and use it to drive the synthesis of organic compounds from carbon dioxide and water
Stroma:
- a semi-fluid compartment enclosed by inner membrane
- contains circular DNA that enables the synthesis of chloroplast proteins.
- Sugars synthesised by the chloroplasts are stored as starch grains in the stroma.
- contains enzymes required for the light-
independent reactions (Calvin cycle) of photosynthesis.
Thylakoids
- The chloroplasts have a third set of membranes within the stroma known as thylakoids
- thylakoids enclose the thylakoid lumen.
- The thylakoid and the thylakoid lumen together form a flattened sac called thylakoid disc -> Some thylakoid discs are stacked up to form granum
- intergranal lamellae: sheet like thylakoids, connecting stacks of grana
function:
- site for light-dependent reactions of photosynthesis
- The thylakoids allow for increased surface area for attachment of chlorophyll and other photosynthetic pigments.
- Compartmentalisation of thylakoid lumen is also necessary for the setting up of proton gradient across the thylakoid membranes due to the thylakoid membrane being impermeable to protons.
- Chloroplasts carry out photosynthesis to synthesise organic compounds. Chloroplast absorb light to synthesise ATP and NADPH (during light dependent reaction)
which are used in Calvin Cycle.
ribosome
function: site of protein synthesis in eukaryotes and prokaryotes
two subunits -> large ribosomal subunit, small ribosomal subunit, both made up of proteins and rRNA
location:
1. attached to the rough endoplasmic
reticulum (80S) -> bound ribosomes
rER bound ribosomes synthesise proteins that are
- designed for export
- insertion into membrane
- targeted to various membrane bound organelles
2. free in the cytosol (80S) (free ribosomes) -> synthesise proteins that remain in cell and function within cytosol or within the nucleus
3. in the mitochondrial matrix (70S) and
4. in the chloroplast stroma (70S)
cytoskeleton
cytoskeleton structure
- cytosol of eukaryotes is permeated by an intricate three-dimensional array of interconnected filaments and tubules known as the cytoskeleton.
- organelles are bound to skeleton, and are positioned and actively moved by it.
- cytoskeleton plays a major role in organizing the structures and activities of the cell
- constructed from microtubules, microfilaments, intemediate filaments
functions of the cytoskeleton:
(a) Giving mechanical support to the cell and maintaining cell shape
(b) Allowing for anchorage and directs the movements of organelles and molecules within
the cell
(c) Providing cell motility (cilia and flagella)
functions of microtubules, microfilaments, intermediate filaments
Microtubules are used in structural support. Specialised arrangements of microtubules are
used for varying functions. These arrangements of microtubules arise from microtubule
organizing centres (MTOCs), which give anchorage and orientation to the microtubule
assembly. Microtubules are also involved in transportation of molecules from one site in the cell
to another.
● Microfilaments are involved in the movement of cells, e.g. amoeboid crawling, and changes in
cell shape.
● Intermediate filaments may stabilise organelles, like the nucleus, or they may be involved in
specialised cell junctions.
centrioles
- found in animal cells but are absent in plant cells.
- located near the nucleus, in a region known as the centrosome that contains specialised proteins required for microtubule assembly.
- Centrioles are found in pairs at right angles to each other, each member of the pair consisting
of nine triplets of microtubules arranged in a ring
function: - Before cell division, each centriole replicates itself and move to opposite poles of the cell.
- The centrosome act as MTOC (microtubule organising centre) for the formation of spindle
fibres that play a role in nuclear division.
plant cell wall
structure
- relatively rigid and inflexible structure consisting mainly of the structural polysaccharide cellulose
- It is secreted by the plant cell from within, and supports and defines the shape of plant tissues.
- The plant cell wall is freely permeable to all but very large molecules.
- The region between the cell walls of adjacent cells is separated by the middle lamella -> region is rich in polysaccharides such as pectin that helps the cells adhere together.
Its functions include:
● Protecting the cells from both mechanical injury and invasion
● Cellulose’s high tensile strength enables the cell to withstand the hydrostatic pressure
exerted by uptake of water by the cell and enables plant cells to prevent excessive uptake of water.
in animals, extracellular matrix helps to support plants and bind them
