OCR Biology A- Foundations in Biology- Cell Structure

Magnification: The degree to which the image is larger than the object itself

Resolution: The degree to which it is possible to distinguish between two objects, the higher the resolution, the greater the detail.

Maginification= maginifying power of the objective lens x magnifying power of the eyepiece lens

Types of microscopes:

• Optical
• Laser scanning microscope
• Transmission electron microscope
• Scanning electron microscope

Light Microscope:

Pros:

• Easy to use
• Portable
• Inexpensive compared to electron microscopes
• Can look at live samples
• No distortion- simple preparation

Cons:

• Limited resolution of x1500
• The depth of field is restiriced
• Cannot view viruses or atoms
• Resoultion is 250nm

Scanning electron microscope/ Transmission:

Pros:

• Large depth of field allows more of a specimen to be in focus at one time
• Use electromagnets so researcher has much more control in the degree of magnification
• Maginfication x500000/x250000
• Resolution of 0.20nm

Cons:

• Unable to produce colour
• Only in 2D
• Expensive
• Takes up a lot of space
• Needs training to use it
• Both have to be dead
• Requires to be cut/coated with metal/stained- distortion/inaccurate images

Staining:

• Stains are coloured chemicals that bind to molecules in or on the specimen- easy to see

Observing prepared specimens:

• Dehydrating the specimens
• embedding them in wax to prevent distortion during slicing 
• using a special instrument to make very thin slices called sections- stained and mounted in a special chemical to preserve them

Calculations involving magnification:

Magnification= image size/actual size

IAM

Measure the widest part in the diagram

All animal, plant, fungal and some protoctist cells are are eukaryotic. This means they have:

• A nucles surrounded by a nuclear envelope and containing DNA organised and wound into linear chromosomes
• An area inside the nucleus called the nucleolus, containing RNA, where chromosomes unwind; the nucleolus is also involved in making ribosomes
• Jelly-like cytoplasm in which the organelles are suspended
• A cytoskeleton- a network of protein filaments (actin or mincrotubles) within the cytoplasm that move organelles within the cell; allow some cells to move; and allow contrctions of the cell
• A plasma membrane /cell surface membrane
• Membrane-bound organelles, other than the nucleus, such as mitochondria, the Golgi apparatus and endoplasmic reticulum
• Small vesicles
• Ribosomes, which are organelles without membranes, where proteins are assemble

Organelles: each have specific functions and therefore this provides division of labour- every cell can carry out its function properly. Most are membrane bound as it keeps it seperate from the rest of the cell so that it is a discrete compartment.

Structure: 

It is surrounded by the nuclear envelope (a double membrane) and there are pores (gaps) in the the envelope. The nucleolus does not have a membrane and contains RNA. The nucleoplasm contains the chromatin.  The outer membrane is continuos with rough endoplasmic reticulum membranes.

Function:

• Nuclear envelope- To enclose and protect DNA
• Nuclear pores- allow entry of substances such as nucleotides for DNA repilication and exit of moleucles such as mRNA for protein synthesis- diffusion/active transport
• Nucleolus is where ribosomes are made
• Stores genetic material of the organism
• Transmits genetic information
• Provides the instructions for protein synthesis
• Control centre of the cell

Structure:

They are spherical, rod shaped or branched, and are 2-5 micrometres long. They are surrounded by two membranes with a fluid-filled space between them. The inner membrane is highly folded into cristae. The inner part of the mitochrondrion is the fluid-filled matrix.

Function:

• Cristae- greatly increases surface area area for the attachment of enzymes and co-enzymes involved in the electron transfer chain 
• Site of ATP production during aerobic respiration
• They are self replicating so more can be made if cells energy needs increase
• Abundant in cells that are metabolically active

Structure:

RER- a system of hollow tubes and sacs, containing fluid filled cavities  that are continous with the nuclear membrane. It is covered with ribosomes and consists of an interconnected system of flattened sacs. 

SER- Lacks ribosomes- system of interconnected tubules.

Function:

• Intracellular transport systems system- cisternae form channels for transporting substances from one area of a cell to another
• Provides a large area for ribosomes, which assemble amino acids into proteins. These proteins then actively pass through the membrane into the cisternae and transported to the Golgi apparatus for modification and packaging
• SER- contains enzymes that catalyse reactions involved with lipid and carbohydrate metabolism- site of lipid and carbohydrate metabolism
• it synthesises triglycerides, cholesterol and phospholipids which become part of the cell surface membrane and is also involved in the modification of substances such as steroid hormones which will then be secreted

Structure:

It consists of a stack of membrane-bound flattened sacs. Secretory vesicles bring materials to and from the Golgi apparatus- invaginate and fuse to form vesicles. It is connected to RER

Function:

• Proteins are brought from RER
• Modification- adding sugar molecules, lipid molecules and being folded into 3D shape
• Packaged into vesicles and then stored in the cell or moved to the cell surface membrane, either to be incorporated or exported outside the cell

Structure:

Consist of two sub units  both made of rRNA . The rRNA part is formed in the nucleus and moves out of the nucleus via the pores. The protein part is then assembled in the cytoplasm. Ribosomes may occur in dense clusters in the cytoplasm where they are known as polysomes or may occur on the membranes of the endoplasmic reticulum

Function:

• Ribosomes provide: (i) Binding sites for the binding of mRNA which allows translation of the DNA code. (ii) Two sites for the binding of 2 tRNA molecules. (iii) The enzymes necessary for (i) and (ii). Ribosomes recognise the initiation and termination codons on mRNA. Ribosomes are capable of moving along the mRNA strand. This allows decoding of the mRNA and synthesis of a polypeptide chain.
• Move along molecules of RNA and read the nucleotide code to produce proteins

Structure: they are large organelles- 4-10 micrometres long.. They are found only in plant cells and in some protoctists. They are surrounded by a double membrane or envelope. The inner membrane is continuous with stacks of flattened membrane sacs called thykaloids (piles of plates), which contain chlorophyll- grana. The fluid filled matrix is called the stroma. They contain loops of DNA and starch grains

Function:

• The site of photosynthesis
• The first stage of photosynthesis, when light energy is trapped by chlorophyll and used to make ATP, to make carbohydrates, occurs in the grana. Water is also split to supply hydroge ions
• The second stage, when hydrogen reduces carbon dioxide, using energy from ATP, to make carbohydrates, occurs in  the stroma. 
• Grana allows huge surface area for or the assembly of chlorophyll molecules for light absorption
• Starch grains- carbohydrate store
• DNA-  All involved in nucleic acid and protein synthesis

Structure:

Large fluid-filled sac and is surrounded by a membrane called the tonoplast

Function:

• Storage site
• Provides support for plant cells by creating pressure potential through osmosis- cell trgid when pushed against the wall

Structure:

They are small bags formed from the golgi apparatus. Each is surrounded by a single membrane.. They contain powerful hydrolytic enzymes. 

Function:

• When released, these enzymes can break down old organelles, storage molecules or, indeed, the whole cell, when it dies
• Keep seperate from the rest of the cell
• Engulf foreign matter
• Reuse by returning old matter

Structure:

They are protusions from the cell and are surrounded by the plasma membrane. Each contains microtubules. They are formed from centrioles. 

Function:

• Epithelial cells lining airways each have cilia that beat and move mucus
• Nearly all cells types in the vody have one cililum that acts as an atenna. It contains receptors and allows the cell to detect signals about ita immediate environment
• The only type of human cell is a spermatazoon. The undulipodium enables the spermatazoon to move

Structure:

Consist of two bundles microtubles at right angles to each other. The microtubules are made of tubulin protien sub units and are arranged to form a cylinder.

Function:

• Used to grow spindle fibres for nuclear division
• Chromosomes attach to the middle part of the of the spindle and motor proteins walk along the tubulin threads pulling the chromosomes to the opposite end of the cell
• Involved in the formation of cilia and undulipodia:
• Before cilia- centrioles multply and line up beneath the cell surface membrane- microtubules then sprout outwards from each centriole, forming a cilium or undlipodium

Structure:

A network of protein structures within the cytoplasm. It consists of:

• rod-like microfilaments  made of subunits of the protein actin
• intermediate filaments about 10nm in diameter, made of protein sub units called tubulin
• Cytoskeletal motor proteins, myosins, kinesins and dyenins, are molecular motors. They are also enyzmes and have a site that binds to and allows hydrolysis of ATP as their energy source

Functions:

• Support and mechanical strength
• Keep the cells shape stable
• Allow cell movement
• Intermediate filaments- anchor the nucleus with the cytoplasm- extend between cells in some tissues, between special junctions, enabling cell-cell signaling and allowing cells to adhere to a basement membrane, therefore stabilising tissues.

Structure:

Outside of the plasma membrane.It is made from bundles of cellulose fibres.

Structure:

• Provide strength and support
• Maintain the cell's shape
• Contribute to the strength and support of the whole plant
• Permeable and allow solutions to pass through
• Plasmodesmata

Explain how organelles work together to produce and secrete proteins:

The organelles within eukaryotic cells work closely together. For example, in the production of a secretary protein such as a digestive enzyme:

1. The genetic code for the protein lies in the chromatin granules in the nucleoplasm of the nucleus.

2. This code, now in the form of mRNA, moves through the nuclear pores.

3. The mRNA attaches itself to ribosomes on the rough endoplasmic reticulum which is continuous with the outer membrane of the nuclear envelope.

4. The code is translated into a polypeptide chain.

5. The polypeptide pass into the lumen of the endoplasmic reticulum.

6. The polypeptide is transferred to the golgi body and packaged in a vesicle.

7. The vesicle merges with the cell surface membranes and the protein is released

They are similar to eukaryotic cells that they have:

• Plasma membrane
• Cytoplasm
• Ribosomes
• DNA and RNA

They are different:

• Smaller
• Less developed cytoskeleton
• No nucleus
• No membrane bound organelles
• Cell wall= peptidoglycan
• Smaller ribosomes 
• Naked DNA- loop

Some have:

Capsule, small loops of DNA (plasmid), flagella, pili- host cells, passage of of plasmid DNA from one cell to another