2.1.1 Cell structure: Subcellular structures Flashcards
Nucleus
Largest organelle (approx 5 µm)
Function: Site of DNA replication and transcription (where mRNA is made– site of ribosome synthesis)
Also contains the genetic code for each cell in the form of DNA molecules – DNA controls metabolic activities of the cell – directs the synthesis of all proteins
Nuclear Envelope
Double membrane and contains nuclear pores to allow substances to leave the cell (mRNA/ribosomes) or to allow nutrients and hormones in.
* Outer membrane connects to the Rough Endoplasmic Reticulum.
* Nucleoplasm – granular, jelly like material
* Nucleolus – responsible for ribosome production – composed of proteins and RNA – RNA is used to produce ribosomal RNA (rRNA) which is then combined with proteins to form the ribosomes necessary for protein synthesis (ALSO darker area inside the nucleus)
* DNA associates with proteins called histones to form a complex called chromatin. Chromatin coils and condenses to form chromosomes
Mitochondria
- Double membrane bound organelle – inner membrane folds inwards forming cristae
- Fluid centre is called the matrix contains enzymes needed for aerobic respiration. Also produces ATP which is needed to release the energy produced
- Contain its own DNA called mitochondrial (mt)DNA and – allows mitochondria to produce their own enzymes and reproduce themselves – found in the matrix
- Also contain its own ribosomes
- More active cells have more mitochondria
The Cytoskeleton
A network of fibres found within the cytoplasm all over a cell, necessary for shape and stability
The three components of the cytoskeleton
Microfilaments - Contractile fibres formed by the protein actin, enable cell movement and also cell contraction during cytokinesis
Microtubules – Globular tubulin proteins polymerise to form tubes that are responsible for the shape of the cell structure - Act as tracks aiding transport of substances including vesicles around the cell
Intermediate fibres - Provide mechanical strength to cell and help maintain cell integrity
Centrioles
Composed of microtubules (so part of the cytoskeleton), non-membrane bound
Occur in pairs and lie 90 degrees to one another
Two associated centrioles form a centrosome - involved in the production of spindle fibres and organisation of chromosomes in cell division
Cilia
Hair-like projections made from microtubules (allows for the movement of substances over the cell surface)
Stationary, used in sensory organs such as nose
Mobile – they beat rhythmically moving fluids adjacent to the cell, such as mucus in the trachea or egg from ovary to uterus
Each cilium contains two central microtubules (black circles) surrounded by 9 pairs of microtubules arranged like a wheel – pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion
Flagella
Found in specialised cells (e.g. sperm) Similar in structure to cilia, made of longer microtubules
Purpose is to enable cell mobility, rotating to propel cell through fluids and sometimes used as a sensory organelle detecting chemical changes in the cells environment
Difference between Cilia and Flagella
Flagella are longer than cilia, but cilia are usually present in much greater numbers
Both flagella and cilia have a 9 + 2 arrangement of microtubules. This arrangement refers to the 9 fused pairs of microtubules on the outside of a cylinder, and the 2 unfused microtubules in the center
(Both composed of centrioles, not present in all eukaryotic cells)
Microvilli
Found in specialised animal cells
Cell membrane projections
Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances
Plasma Membrane
Found in all cells - Phospholipid bilayer – molecules embed within and attached on the outside (proteins, carbohydrates, cholesterol)
Made of hydrophilic heads and hydrophobic fatty acid tails
Controls the entrance and exit of molecules
Selectively permeable, has receptors for cell recognition
Controls exchange from cell and environment, fluid to allow bulk transport
Endoplasmic Reticulum
A network of membrane enclosing cisternae (flattened sacs) and is connected with the nuclear envelope
Both have folded membrane called cisternae (rough have ribosomes on the cisternae) and both are connected to the nuclear envelope
Secretory cells, which release hormones or enzymes, have rougher endoplasmic reticulum than cells that do not release proteins
Rough Endoplasmic Reticulum
Surface covered in ribosomes - rough appearance
Involved in the synthesis and transport of proteins
Fold proteins for synthesis and transport newly synthesized proteins into vesicles to be transported to others locations
There is a greater quantity in cells that actively make or secrete proteins.
(has ribosomes bound to the surface and is responsible for the synthesis and transport in proteins)
Smooth Endoplasmic Reticulum
No bound ribosomes = smooth appearance
Involved in the production, processing and storage of lipids, carbohydrates and steroids
Contains enzymes for the detoxification of drugs and other poisons
Substances such as barbiturates and alcohol induce proliferation of smooth ER and its enzymes therefore increasing tolerance
synthesis and storing of lipids and carbohydrates)
Golgi Apparatu
- Role of modifying proteins and packaging them into vesicles (may be secretory vesicles if the proteins are destined to leave the cell OR lysosome, which stay in the cell)
- Similar in structure to the smooth endoplasmic reticulum
- Compact structure formed of folded membranes making cisternae – secretary vesicles pinch off from the cisternae
- Does not contain ribosomes
- They are constantly forming on one side (forming face) and budding off as secretory vesicles on the other (maturing face)
Function of Golgi Apparatus
- Transport, modify and store lipids - Modifies proteins and lipids before packaging them into vesicles –vesicles transport the proteins and lipids to their required destination
- Form lysosome - Proteins that go through the Golgi apparatus are usually exported, out into lysosomes or delivered to membrane bound organelles
- Finished products released into vesicles – which will fuse with the cell membrane and releases contents to be transported to another cell
Vesicle
Membranous sac with transport and storage roles in cells
- Consists of a single membrane with fluid inside
Lysozome
Specialised forms of vesicles that contain hydrolytic / digestive enzymes
- They break down waste material / old organelles in cells (enzymes that break biological molecules down - cells (autolysis))
- Important in the immune system, breaking down pathogens ingested by phagocytic cells
Ribosome
- Ribosomes can be free floating in the cytoplasm or attached to the endoplasmic reticulum
- Are not surrounded by a membrane
- They are constructed of RNA molecules made in the nucleolus of the cell and are the site of protein synthesis
- Very small and made up of two sub-units of protein and rRNA (produced by the nucleolus) They consist of one large and one small sub unit (total size 22nm)
- 80S ribosomes – large ribosome - found in eukaryotic cells (25nm)
- 70S ribosomes – smaller ribosome – found in in prokaryotic cells // mitochondria and chloroplasts
Chloroplast
Plant Cells Only
- Surrounded by a double membrane and contains their own DNA and ribosomes = able to make their own proteins
- Site of photosynthesis - contains chlorophyll
- Fluid enclosed is known as the stoma (contains enzymes for photosynthesis)
- Thylakoids are flattened sacs containing chlorophyll – thylakoids stack forming grana – grana are connected by membranes called lamellae
Vacuole
Plant Cell Only
- Membrane bound sac containing cell sap (membrane of vacuole is called the tonoplast and is selectively permeable)
- Reservoir of salts / sugars etc in the cell
- Helps maintains turgor of the cell by pushing cell contents against the cell wall
- Vacuoles occasionally appear in animal cells but they are small and transient
Cellulose Cell Wall
Plant Cell Only
(cell wall surrounding the cell membrane)
Made of cellulose, a complex carbohydrate
- Freely permeable so substances can pass into and out of the cell
- Gives cell strength / support, helps to maintain shape and rigidity (contents of the cell push against the wall) so make it rigid
- Prevents cell bursting (when water enters by osmosis), freely permeable
- Acts as a defence mechanism because it protects the contents of the cell against invading pathogens
