cell structure

Microscopy
Light microscope – light rays pass through the specimen on a slide and are focused on an objective lense and an eyepiece lens, this produces a magnified image of the specimen on the retina of the eye; max resolution is 200 nm. Affordable, easy to use, lightweight and small; lower magnification.

Electron microscope – thin specimen in a vacuum allows beams of electron through it, electrons are focused on a screen or onto a photographic film where the image is magnified. Powerful magnification, specimen used is dead, cannot observe mitosis.
Both use a form of radiation.
Magnification – the number of times greater that an image is than the actual object
Size of object + magnification = size of image
Resolution – the ability of the microscope to distinguish two objects separate from another, the smaller the object that can be seen, the higher the resolution. Resolution determined by the wavelengths of the ray being used to view the specimen. Wavelength of beam of electrons is much smaller than of light, therefore an electron microscope has higher resolution than a light microscope. If two points cannot be resolved, they will be seen as one.

1 mm = 1000 μm = 1 000 000 nm

Calibration – a comparison of the grid or scale on the eyepiece reticle with the scale markings on a stage micrometer.

Animal and plant cells

• Cell surface membrane – controls what enters and leaves the cell, partially permeable, has three layers so it is described to have a trilaminar appearance.

• Nucleus – surrounded by two membranes names nuclear envelope; outer membrane continuous with the endoplasmic reticulum. Nuclear envelope has small pores called nuclear pores which allow and control exchange between nucleus and cytoplasm such as ribosomes, nucleotides, ATP. In the nucleus , chromosomes are in loosely coiled state known as chromatin. Chromosomes contain DNA which is organised in genes which control activity of cell and inheritance therefore nucleus controls cell activity. In the nucleus, the nucleolus makes ribosomes using info in its DNA.

• Cytoplasm – aqueous material holds internal components in place and protects from damage.
• Endoplasmic reticulum – extensive system of membranes running through cytoplasm made of flattened compartments called sacs. It is continuous with the outer membrane of the nuclear envelope. Two types – rough and smooth endoplasmic reticulum. Rer is covered in ribosomes. Proteins made by ribosomes enter the sacs and are modified on their journey after which they are exported from cell via  the Golgi body vesicles. Ser lacks ribosomes, makes lipids and steroids such as cholesterol and the reproductive hormones oestrogen and testosterone.

• Golgi body – sack of flattened sacs together with the associated vesicles; constantly formed at one end from vesicles which bud off from the endoplasmic reticulum. It collects processes and sorts molecules ready for transport in golgi vesicles either to parts of cell or out of it (secretion). Adds sugar to proteins to make glycoproteins, convert sugars into cell wall, produces lysosomes.

• Ribosomes – small structures made of RNA and protein, free in cytoplasm or attached to RER. Membranes enclose small spaces called cisternae. Proteins are made on the ribosome by linking together amino acids.

• Lysosomes – spherical sacs surrounded by single membrane , contain digestive enzymes that must be kept from cell to prevent damage. Responsible for breakdown of unwanted structures such as old organelles. In wbc they are used to digest bacteria.Sperm head contains lysosome to cross pathway.

• Mitochondria – surrounded by two membranes, finger like folds inside called cristae which project into the inner matrix. Intermembrane – space between two membranes. Outer membrane called porin which forms  aqueous channels allowing access for small water soluble molecules into the intermembrane space. Inner membrane controls what ions and molecules enter matrix. Carries out aerobic respiration and synthesizes lipids. During respiration energy is released and transferred to ATP. Cristae provide power to generate ATP, folding in cristea increases surface area, more efficient.

Endosymbiotic theory
Mitochondria and chloroplast contain ribosomes (70s)  which are slightly smaller than those in the cytoplasm (80s).  Mitochondria and chloroplasts also contain circular DNA molecules like those found in bacteria. Mitochondria and chloroplasts are ancient bacteria which now live in larger cells. Symbiotic is an organisms that lives in a mutually beneficial relationship with another.

Microvilli
Finger like extensions on the cell surface membrane. They increase the surface area. Found in gut for absorption and reabsorption in the kidneys.
Microtubules and microtubule organising centers
Microtubules – long, rigid, hollow tubes found in cytoplasm. Together with filaments they make the cytoskeleton, an essential structural component of cell which helps to determine shape. Microtubules are made from proteins called tubulins, an alpha and a beta tubulin to form dimers; these are joined to form long protofilaments. Thirteen of them line up in a ring to form a cylinder, this cylinder is a microtubule. They are used for mechanical support and movement of vesicles or other organelles. Membrane bound organelles are held in place by the cytoskeleton. During nuclear division, spindle is made of microtubules. Assembly of microtubules is controlled by microtubule organising center.

Centrioles and centrosomes region
Centriole – hollow cylinder forms from a ring of microtubules, each contains nine triplets of microtubules.

Plant cells

• Chloroplasts – have an elongated shape, bigger than mitochondria. Surrounded by two membranes forming a chloroplast envelope. They replicate themselves independently from cell division by dividing in two. Main function is to carry photosynthesis, light energy absorbed by pigment chlorophyll. Energy is used to split water into hydrogen and oxygen. Stroma – their background material. It contains many paired membranes – thylakoids which are fluid filled sacs. In places, these form stacks called grana. Grana contains chlorophyll which captures the energy from sunlight. First reaction in photosynthesis is the light dependent, takes place in membranes. Second stage, light independent converts carbon dioxide to sugars. the Calvin cycle takes place in stroma. Sugars made may be stored in form of starch grains in stroma. Lipid droplets stored are used for making membranes.

• Vacuoles – membrane bound structures of enclosed compartments that are filled with cell sap and surrounded by a tonoplast.

• Cell wall – consists of a lipid bilayer with embedded proteins, the basic function is to protect the cell from its surroundings. The cell membrane controls the movement of substances in and out of cells and organelles. In this way, it is selectively permeable to ions and organic molecules.

Prokaryotes and Eukaryotes
Prokaryotes – lack nucleus; smaller in volume; DNA lies free in cytoplasm.
Eukaryotes – have nucleus; DNA inside nucleus; ex animals, plants, fungi; unicellular – protoctist

Viruses
Tiny organisms, smaller than bacteria. Viruses do not have a cell structure, not surrounded by a partially permeable membrane. They are simple structures that consist of:
– self replicating molecule of DNA or RNA which acts as genetic code
– a protective coat of protein molecules
Has a symmetrical shape. Protein coat – capsid, made up of separate protein molecules, each called a capsomere. They are parasitic because they can only reproduce by infecting and taking over living cells. The virus DNA or RNA takes over the protein synthesizing machinery of the host cell which helps making new virus particles.