Cell structure

Microscopes

Light → up to 0.2 nanometres, magnification is x1000, uses range of light to make objects visible, cells can be alive, has lower magnification and resolution

Transmission electron → uses a beam of electrons, much shorter wavelength then light, magnify x500000, good resolving power, whole system must be in vacuum so specimen is dead, must be thin, is a time-consuming complex process that can result in alterations (artefacts), only 2D

Scanning electron → electrons passed across surface of the specimen and scattered electrons form a 3D image, resolving power is lower than TEM as less complex so less artefacts

Magnification = image size/actual size

magnification → how much bigger the image size is compared to the actual size

resolution → ability to distinguish between two points that are close together

Cell fractionation

tissue is place in cold isotonic buffer solution

cold → reduces enzyme activity that could hydrolyse organelles

Isotonic → prevent organelles bursting or shrinking as a result of osmosis

Buffer → so PH doesn’t fluctuate

cells broken up and their organelles are separated out

cells are homogenised (placed into machine that breaks open cell to release its organelles0 and then centrifuged)

Ultracentrifugation

Cellular fragments are placed into centrifuge and spun at a low speed where the heavy/dense organelles are forced to the bottom where they form a pellet

The supernatant (remaining liquid) is removed and transferred into another time that is spun faster than before where the next densest organelles form the pellet.

as it’s repeated the speed increases

Nucleus → 1000, Mitochondria → 3500, Lysosomes → 16500, Ribosomes → 100000 (speed of revolutions per min)

Eukaryotic cell structure (nucleus)

Nuclear envelope → double membrane that surrounds nucleus

pores → allows diffusion of large molecules out of nucleus

Nucleoplasm → granular, jelly like material

Nucleolus → site of rRNA production and ribosomal synthesis, site of DNA replication and transcription (mRNA), contains genetic code for each cell

Eukaryotic cell structure (Mitochondria)

Double membrane → controls entry and exit of materials and is folded to form cristae

Cristae → extensions of the inner membrane, increases surface area for attachment of enzymes and proteins involved in respiration

Matrix → semi-rigid material containing lipids, proteins, ribosomes, DNA and enzymes involved in respiration

Site of aerobic respiration and production of ATP from carbohydrates which are then used for metabolic processes and DNA to code for the enzymes used in respiration

More active tissues are made from cells with greater numbers

Eukaryotic cell structure (Ribosomes)

Tiny granules that either float free in cytoplasm or are attached to the endoplasmic reticulum

site of polypeptide synthesis

made up of proteins and DNA, are not surrounded by a membrane

site where proteins are made

more found in eukaryotic then prokaryotic

Endoplasmic reticulum

Rough → membrane encloses a network of flattened tubes called cisternae, surface is covered in ribosomes

→ protein synthesis occurs here, folds and processes proteins that have been made at the ribosomes, provides pathway for the transport of materials

Smooth → system of cisternae, no ribosomes, synthesises and processes lipids and carbohydrates

Cell surface membrane

is found on the surface of animal’s cells and inside the cell wall of other types of cells

made mainly from lipids and proteins

molecules are imbedded within and attach to the outside

regulates the movement of substances into and out of cells

has receptor molecules allowing it to respond to chemicals

Golgi apparatus and vesicles

apparatus → a stack of membranes that make up cisternae, secretory vesicles pinch off cisternae

→ adds carbohydrates to proteins to form glycoproteins, produces secretory enzymes, secretes carbohydrates, transports modifies and stores lipids, forms lysosomes

vesicle → small lipid filled sack in the cytoplasm, surrounded by a membrane and produced by apparatus

→ stores lipids and proteins made by apparatus and transports them out of the cell via cell surface membrane

lysosomes

round organelle surrounded by a membrane with no clear internal structure

→ contains hydrolytic enzymes that hydrolyse phagocytic cells and completely breaks down dead cells (autolysis)

→ exocytosis - releases enzymes to outside of cell to destroy materials

→ digests worn out organelles for reuse of materials

Chloroplast

→ surrounded by double membrane and contains membranes (thylakoid membranes which stack to form grana)

→ grana are linked by lamellae (thin flat pieces of thylakoid membrane)

→ fluid filled stroma contains enzymes for photosynthesis

→ site where photosynthesis takes place - some parts happen in thylakoid membrane other parts in the stroma

Cell wall and vacuole

rigid structure

in plants and algae - made of microfibrils of cellulose, in fungi is made from chitin

→ has thin layer called middle lamella which marks the boundary between adjacent cell walls and cements adjacent cells together

→ provides mechanical support to prevent cell from bursting, allows water to pass along and contributes to the movement of water through the plant

vacuole → filled with fluid and is surrounded by a single membrane (tonoplast)

→ helps maintain pressure inside cell to keep it turgid, temporary store for amino acids and sugars, pigments coloured petals to attract pollinating insects, is involved in the isolation of unwanted chemicals inside cell.

Prokaryotic cell

Slime capsule → prevents cell from drying out and being engulfed

cell wall → physical barrier, protects against mechanical damage

DNA → free floating, processes genetic info

Flagellum → propels prokaryotic and facilitates directional movement

Plasmid → extra DNA floating free, codes for antibiotic resistance, can be transferred from 1 prokaryote to the other

ribosome → floats free in cytoplasm, site of polypeptide synthesis

Virus

Matrix → layer of proteins

attachment proteins → used to identify and attach to host cells

capsid → protein layer, encloses 2 single strands of RNA and enzymes

Genetic material → contains the genetic material needed to reproduce

lipid envelope → not present in all viruses

Reverse transcriptase → enzyme that catalyses production of DNA from RNA

Mitosis

Used for cell growth differentiation and repair

before mitosis is one chromatid and after is two sister chromatids

mitotic index → number of cells in a mitotic stage/number of cells visible in view x 100

Interphase → cell prepares to divide, DNA is unravelled and replicated to double its genetic content, organelles are also replicated, ATP content increases to provide energy needed for cell division

Stages

Prophase → chromosomes condense and get shorter and fatter, centrioles move to opposite ends forming spindle fibres across it, nuclear envelope breaks down and chromosomes lie free in cytoplasm

Metaphase → chromosomes line up in middle of the cell and attach to the spindle fibres by their centromere

Anaphase → centromeres divide separating each pair of sister chromatids, spindle fibres contract pulling chromatids to opposite poles, centromeres fuse making chromatids appear v shaped

Telophase → chromosomes reach opposite poles and uncoil and become longer so are now chromosomes again, nuclear envelope forms around each group of chromosomes so now have 2 nuclei

cytokinesis → division of the cytoplasm, starts in anaphase and ends in telophase

Cell cycle

Gap phase 1 → cell grows and new organelles and proteins are made

synthesis → cell replicates all its DNA so ready to divide by mitosis

Gap phase 2 → cell keeps growing and proteins needed for cell division are made

Mitosis