Biology Paper 2 - 2.1 Cell Structure

Structure: Nucleus surrounded by double membrane (nuclear envelope). Pores in the envelope. Nucleolus has no membrane. Contains RNA. Chromatin is genetic material consisting of DNA wound around histone proteins. Cell not dividing = spread out. Dividing = condenses and coils tightly into chromosomes. Most of organism’s genome.
Function: Nuclear envelope separates nucleus from everything else. Dissolved substances and ribosomes can pass through where the outer and inner membrane have fused. Pores enable mRNA to leave and other substances to enter. Ribosomes are made in the nucleus. Chromosomes contain the organism’s genes.

Structure: System of membranes, containing fluid-filled cavities (cisternae) continuous with the nuclear membrane. Coated with ribosomes.
Function: Intracellular transport system - cisternae form channels for transporting substances. Large SA for ribosomes. Proteins made here pass through the cisternae to the Golgi apparatus.

Structure: System of membranes, containing fluid-filled cavities (cisternae) continuous with the nuclear membrane. No ribosomes.
Function: Contains enzymes that catalyse lipid metabolism reactions, e.g. synthesis of steroid hormones. Involved with absorption, synthesis and transport of lipids (from the gut).

Structure: A stack of membrane-bound flattened sacs. Secretory vesicles bring materials to and from it.
Function: Proteins are modified by adding lipid molecules to make lipoproteins for example. Proteins packaged into vesicles then stored in cell or moved to the plasma membrane.

Structure: 2-5µm long and are spherical, rod-shaped or branched. Surrounded by two membranes with a fluid-filled space in between. Inner membrane folded into cristae. Inner part is a fluid-filled matrix.
Function: Site of ATP production. Self-replicating. Abundant in cells where much metabolic activity takes place, e.g. liver cells.

Structure: 4-10µm long. Only in plant cells and in some protoctists. Surrounded by double membrane or envelope. Continuous inner membrane with stacks of thylakoids (contain chlorophyll). Each stack is called a granum. Fluid-filled matrix is called the stroma. Contain loops of DNA and starch grains.

Structure: Surrounded by a membrane called the tonoplast, and contains fluid.
Function: Only plant cells have a large permanent vacuole. Filled with water and solutes. Maintains cell stability by pushing against cell wall when full. By all cells being turgid, it supports the plant.

Structure: Small bags formed from Golgi apparatus. Surrounded by single membrane. Contain powerful hydrolytic (digestive) enzymes. Abundant in phagocytic cells (e.g. neutrophils), that ingest and digest invading pathogens.
Function: Keep hydrolytic enzymes separate from everything else. Can engulf old cell organelles and foreign matter, digest them, then return them to the cell for reuse.

Structure: Protrusions from cell. Surrounded by cell surface membrane. Contains microtubules. Formed from centrioles.
Function: Epithelial cells have hundreds of them that waft the band of mucus. Contains receptors to allow cell to detect signals. Only cell in humans that has an undulipodium (long cilium) is a spermatozoon.

The number of times larger an image appears compared to the actual size of the object.

The ability to distinguish between two points.

The clarity of the image.

Inexpensive, easy to use, portable, can view in colour, can view live specimens, can view larger organelles with staining (e.g. nucleus), used in schools, hospitals and research labs.

x1500 (some can magnify up to x2000).

Can only resolve an image larger than the wavelength of visible light - 400-700nm, can only resolve structures that are 200nm apart, cannot see structure

Uses laser light to scan an object point by point, computer assembles points into an image, high resolution and contrast, depth selectivity allows focus on structures within specimen, can view living specimens, used in medical profession, e.g. observations of the eye.

Uses beam of electrons, wavelength 0.004nm, electron fired from cathode and focused by magnets onto screen or photographic plate, high resolution and magnification.

TEM: Specimen must be dehydrated to fix, stained with metal salts (which may be hazardous to user), beam of electrons passed through specimen and focused on scren or photographic plate, 2D black and white image, magnification x 20-50 million.
SEM: Specimen placed in a vacuum, coated with fine metal film, electrons bounced off surface of specimen and focused onto a screen, 3D black and white image (false colour can be added), magnification x 15-200000.
TEM and SEM: Specimen is dead, very expensive, require training and high level of skill.

Actual size = image size divided by magnification

Magnification = image size divided by actual size

Image size = actual size x magnification

Total magnification is the eyepiece x the objective.

Stains bind to specific cell structures. Allow visulisation and identification.

Acetic orcein, DNA chromosomes, dark red.
Eosin, cytoplasm, pink.
Sudan red, lipids, orange/red.
Iodine (potassium iodide), cellulose in plant cell walls (yellow), starch granules (blue black - violet under light microscope).

Preparation may distort specimen - slicing, dehydration, staining. Specimen may be sectioned at different planes - transverse or longitudinal. Image is only 2D. Sectioning of specimen may exclude some structures.

A transparent ruler. It fits in the eyepiece or on the microscope stage. Scale is arbitrary. Divisions are calibrated according to the microscope being used.

A transparent ruler. It fits in the eyepiece or on the microscope stage. Scale is arbitrary. Divisions are calibrated according to the microscope being used.

Nucleus surrounded by a nuclear envelope, contains DNA wound and organised into linear chromosomes. Other membrane bound organelles.

Ultrastructure within a cell that carries out a specific function. Not all organelles are membrane bound.

Gene transcribed into mRNA in nucleus. mRNA passes out of nucleus via pores. mRNA translated into polypeptide at ribosomes. Polypeptide enters RER and passes through cisternae. Protein pinches off into vesicles. Vesicle fuses with Golgi, releasing protein. Protein modified in Golgi, pinched off into vesicle. Vesicle moved to cell surface plasma membrane and fuses. Protein released from cell.

Prokaryote: Usually smaller, less well developed cytoskeleton, no centrioles, no nucleus, naked DNA, no histones, DNA held in loop, additional genetic information in plasmids, no membrane bound organelles, cell wall contains peptidoglycan, may have a waxy capsule surrounding cell wall, may have flagella for movement, may have pili for adhesion to other bacterial cells.
Eukaryote: Usually larger, cytoskeleton, centrioles, nucleus, DNA wrapped around histones, DNA organised into linear chromosomes, membrane bound organelles, plant cell wall contains cellulose, fungi cell wall contains chitin, sperm have undulipodia for movement.
Both: Plasma membrane, ribosomes, cytoplasm.

Binary fission - not mitosis as chromosomes are not linear. Looped DNA and plasmids replicated before division. Produces two identical copies.