B1 notes

Biology notes: B1

B1.1 Cell structures

Animal, plant and prokaryotic cells

Eukaryotic (plant and animal) cells - 10-100μm

Prokaryotic (bacteria) cells - 1-10μm

Structures found in plant and animal cells:

Nucleus - contains the genetic material, which codes for a particular protein; enclosed in a nuclear membrane

Cytoplasm - liquid substance in which chemical reactions occur; contains enzymes and other organelles

Cell membrane - contains receptor molecules to identify and selectively control what enters

and leaves the cell

Mitochondria - where aerobic respiration reactions occur, providing energy for the cell

Ribosomes - where protein synthesis occurs

Found only in plant cells:

Chloroplasts - where photosynthesis takes place, providing food for the plant; contains chlorophyll pigment which harvests the light needed for photosynthesis

Permanent vacuole - contains cell sap; found within the cytoplasm; improves cell’s rigidity

Cell wall - made from cellulose; provides strength to the cell

In prokaryotic cells (e.g. bacteria):

Cytoplasm

Cell membrane

Cell wall

Single circular strand of DNA - as they have no nucleus, this floats in the cytoplasm

Plasmids - small rings of DNA

Flagella - tail-like structures that allow the cell to move through liquids

Pili - tiny hair-like structures that enable the cell to attach to structures

Slime capsule - layer outside the cell wall that prevents bacteria from drying out, poisonous surfaces or helps it to stick to smooth surfaces

Microscopes

Light (optical) microscopes

The specimen is placed onto a slide, which is a thin piece of glass. Sometimes if the specimen is colourless then a stain is needed to identify the organelles. Another small piece of glass called a cover slip is placed on top to protect the lens. It is mounted onto the stage of the microscope and the lamp at the bottom of the microscope shines constant light on the slide so that the specimen can be viewed. The objective lens is found closest to the specimen and magnifies the image, which is then further magnified by the eyepiece lens.

Definitions:

● Magnification = the degree to which the size of an image is larger than the real object

○ Magnification of an object = image size/object size

○ Magnification of microscope = magnification of the eyepiece lens x magnification of the

objective lens

● Resolution = the ability to distinguish between to 2 points that are close to each other

When working with calculations in microscopy, it is common to come across very large or

small numbers. Standard form can be useful when working with these numbers.

Advantages of light microscopes:

● Relatively cheap

● Can be used in the field

● Does not require specialist training

● Can look at living specimens

Disadvantages of light microscopes:

● Low resolution (200nm) as limited by the wavelength of visible light

● Low magnification strength (x1500)

● Staining is required for some organelles to present

Electron microscope

The electron microscope uses electrons, instead of light, to form images. Electrons have a much smaller wavelength than that of light waves and so…