AS revision: F211, Cells

Biology: F211

Cells

Magnification: the degree to which the size of an image is larger than the object itself. Image size/actual size.

Resolution: the degree to which it is possible to distinguish between two objects that are very close together. The higher the resolution, the greater the detail you see. 

Advantages of a LIGHT MICROSCOPE:

Magnification: can be up to x1500

Resolution: can only go up to 200 nm between two points, anything closer will be seen as a single object. 

Specimens: large variety can be viewed under the microscope

Preparation

Staining

• coloured stains: allows specimens to be seen, some bind to specific cell structures, gentian violet, or acetic orcein are dyes. 

Sectioning 

• embedded within wax
• so that they can be cut without distorting the specimen

CELL SIZE AND MAGNIFICATION

ELECTRON MICROSCOPES

• have a higher resolution
• electron wavelength = 0.004 nm
• can distinguish between objects of 0.2nm apart

Transmission Electron Microscope (TEM)	Scanning Electron Microscope (SEM)
electron beam passes through a thin sample	beam does not pass through the sample, but directly onto the sample
electrons pass through sample less easily if the area is denser, creating a contrast in the image	electrons bounce off the specimen
final image is 2D	final image is 3D
Magnification = x 500,000	Magnification = x 100,000

ADVANTAGES AND LIMITATIONS OF ELECTRON MICROSCOPES

ADVANTAGES	LIMITATIONS
resolution is 0.1nm, more than light microscopes	must be done in a vacuum, as molecules in the air may disrupt electron beaming
very detailed images, eg. organelles	very expensive equipment
SEM is able to produce detailed 3D images of surfaces	requires highly trained staff in order to operate the microscopes.

Bacterial cell walls: peptidoglycan 

Fungi cell walls: Chitin

CELLS AND LIVING PROCESSES

Ultrastructure: detail inside cells, revealed by the electron microscope

Division of Labour: the idea that organelles all have a role that is specific within the cell

Cytoskeleton

• protein fibres within the cells
• keeps the cell shape stable
• move organelles within the cell

alternative fibres: microtubules

• made from tubulin
• move chromosomes during mitosis or cell division
• move vesicles form the endoplasmic reticulum to the golgi apparatus
• ATP is required to move these processes

Flagella and Cilia

• hair like extensions
• flagella/undulipodia are longer than cilia
• 9 microtubules with 2 in the middle
• use ATP to move

Vesicles and Vacuoles

• vesicles are membrane bound sacs in cells, carry many substances within the cell
• vacuole -> mantains cell stability 

Plant Cell Walls

• made of cellulose
• held rigid by the pressure of the fluid inside the cell (turgor pressure)
• supports the cell

ORGANELLES -  STRUCTURE AND FUNCTION

	STRUCTURE	FUNCTION
NUCLEUS	largest organelle, surrounded by a nuclear envelope, nucleolus inside the nucleus	houses all the DNA, genetic material, also synthesises the production of ribosomes, and RNA. during cell division, chromatin condenses into visible chromosomes
ER	membrane bound sacs called cisternae, RER is studded with ribosomes and SER are not	transports proteins that are made to the ribosomes, some will be secreted from the cell, smooth ER synthesises lipids
GOLGI APPARATUS	membrane bound flattened sacs	further modification of proteins, packages the proteins into vesicles to be transported.
MITOCHONDRIA	inner membrane is folded to form cristae, central part is the matrix	ATP is produced during respiration, it is the universal energy carrier.
CHLOROPLASTS	thylakoid stacks are called granum, stroma is the space around it. chlorophyll molecules are found on the membranes of the thylakoid	site of photosynthesis and the light dependent and independent reactions.
LYSOSOMES	spherical sacs with a single membrane	contain powerful digestive enzymes, to break down molecules
RIBOSOMES	tiny organelles, some are in the cytoplasm and some are bound to the ER, consists of two subunits	site of protein synthesis and translation, act as an assembly line where mRNA is assembled to form amino acids.
CENTRIOLES	small tubes of protein fibres, pair next to the nucleus in animal cells.	centrioles take part in cell division in the formation of spindle fibres. They move the chromosomes during cell division.

ORGANELLES AT WORK: Division of Labour

 BIOLOGICAL MEMBRANES - FLUID BOUNDARIES

Membrane functions: 

•  separate organelle contents from the cytoplasm
• separate cell contents from the outside environment
• cell recognition and signalling
• regulates the transport of materials in and out of the cell 

 membranes are permeable to water molecules because they can diffuse through the bilayer. 

partially permeable membranes are permeable to water and some solutes. 

FLUID MOSAIC MODEL 

Channel Proteins:

• Transports what comes in and out of the cell membrane
• Big ions, Na+ and Ca+ 
• Aquaporins (posh name for a protein, good for the water to go through)
• Many proteins use active transport: From a Low Concentration - High Concentration 
• Active transport requires the use of ATP
• Endocytosis and Exocytosis, for White Blood Cells: Phagocytosis

CROSSING MEMBRANES

All biological molecules are composed mainly of lipid and protein molecules

THREE TYPES OF LIPIDS IN CELL MEMBRANES:

1. Phospholipids - most abundant
2. Cholesterol - stability
3. Glycolipids - found on the external surface

All lipids have a hydroPHILIC and hydroPHOBIC

Phospholipids: Phosphate group head & fatty acid tail x2 [POLAR MOLECULE]

The more fluid a membrane the more likely it is able to change its shape:

This depends on double bonds: C=C MAKING IT UNSATURATED, this means it can make another bond.

The phospholipids form a bilayer that separates the water from the inside contents of the cell. 

Temperature & Membranes

>  0ºC

Phospholipids have little energy

packed closely, very rigid. proteins denature.

ice crystals might form ad pierce the membrane. Highly permeable membrane

0ºC - 45ºC

The phospholipids can move around and aren't packed as tightly together. 

Membrane is partially permeable - phospholipids move more - increases the permeability of the membrane

45ºC <

Phospholipid bilayer melts - membrane becomes permeable and water inside the cell expands putting more pressure on the membrane. Proteins also begin to denature and cannot control what enters and leaves the cell. Increasing permeability of the membrane even further. 

OSMOSIS:

Is the diffusion of water - net movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane. 

Key Words:

• The movement of water down a water potential gradient. Across a partially permeable membrane, higher area of water potential to an area of lower water potential. 

Pure Water: 0Ψ/0 kPa (Kilo Pascal) - indicates the pressure of water on the cell membrane.

By adding solutes (eg.salt & sugar), making it less concentrated makes the water potential go down and more negative. 

CELLS REACTIONS TO OSMOSIS

Tonoplast: Membrane surrounding the cytoplasm

Water has to cross through the: Cell Membrane -> Cytoplasm -> Tonoplast

Isotonic: Solution that matches the water potential inside the cells.

Hypotonic: any solution that has a lower osmotic pressure than another solution. In the biological fields, this generally refers to a solution that has less solute and more water than another solution.

Hypertonic: A hypertonic solution is a particular type of solution that has a greater concentration of solutes on the outside of a cell when compared with the inside of a cell.

Water moves into a plant cell by osmosis, and becomes turgid in a plant cell, (expands) Cell membrane expands out to cell wall. 

When cells are placed in a solution of low water potential, it becomes flaccid/plasmolysed, and cytoplasm shrinks away from the cell wall. - Wilting and drooping of the plant. 

Passive Processes

1. Simple diffusion
2. Facilitated diffusion using channel proteins
3. Facilitated diffusion using carrier proteins

1. Simple Diffusion

diffusion depends entirely on the kinetic energy from the ions

They move down a concentration gradient, from a higher to lower concentration of ions

Affected by:

• temperature (increase = more kinetic energy)
• concentration gradient (steep concentration gradient, for increased rate of diffusion)
• stirring/moving (movement of molecules and thus the rate of diffusion)
• surface area (greater area to diffuse across)
• distance/thickness (short diffusion distance increases rate of diffusion)
• size of molecule (smaller molecules diffuse more quickly than larger ones)

Facilitated diffusion:

• Large molecules and ions cannot diffuse into the the phospholipid bilayer directly
• diffuses through a carrier protein or channel proteins
• moves particles down a concentration gradient, from higher to lower
• passive process
• doesn’t use ATP

Carrier Proteins:

Shaped so that specific molecules can fit into them, when it fits. The protein changes shape to allow the molecule through to the other side of the membrane.

Channel Proteins: 

Pores in the membrane, shaped to allow only one type of int through. Some are gated so they can open and close, gated sodium channel proteins help with the working of the nervous system.

Passive Transport: Diffusion across a membrane without the use of ATP.

• requires no ATP
• down a concentration gradient 

Facilitated diffusion: Down a concentration gradient charged or hydrophilic molecules or ions via channel or carrier proteins. Is also a passive process that requires no ATP.

Diffusion: Down a concentration gradient, lipid soluble or very small molecules through lipid bilayer.

Osmosis: Down a water potential gradient through bilayer or protein pores. 

ACTIVE PROCESSES

(refers to the movement of molecules or ions across the membrane which uses ATP to drive protein pumps within the membrane)

Bulk transport of endocytosis and exocytosis

(Bulk transport of materials via vesicles that can fuse with or break from the cell surface membrane)

• Requires no ATP
• Moves large substances in and out
• can move solids or liquids

eg. White blood cells engulfing.

 

Endocytosis: bulk transport of materials in to the cell and has three processes

1. phagocytosis (white blood cell)
2. pinocytosis (a cell is engulfing a lot of fluid)
1. receptor-mediated endocytosis (take on a particular compound if its recognisable)

Exocytosis: the bulk transport of material out of the cell, the reverse of endocytosis.

Diffusion: 

What affects the Rate of Diffusion

• Surface area 
• Diffusion Distance
• Concentration gradient

The rate of diffusion is proportional to: 

     surface area x concentration difference

length of diffusion pathway

The thinner the membrane, the shorter the diffusion distance and rate of diffusion is faster.

7-8nm, is a cell membrane width.

Aquatic animals may rely heavily on cholesterol, in order to keep the plasma membrane stable. 

What affects the permeability of the cell membrane?

• Heat
• Ethanol
• pH (Concentration of hydrogen ions)

MITOSIS

• Process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus

important for:

• growth
• repair
• replacement
• asexual reproduction

Mitosis happens in four stages:

• Prophase

Chromosomes, shorten and condense so that they become supercoiled and are now visible using a light microscope. As they do so, it consists of a pair of sister chromatids, the nuclear envelope breaks down and the centriole divides into two. this forms a spindle. 

• Metaphase

chromosomes all line up on the equator or bivalent. where it is attached to a spindle fibre at the centromere. 

• Anaphase

sister chromatids are pulled apart at the centromere as the spindle fibres shorten and pull them across opposite poles.

• Telophase

the sister chromatids reach the poles of the cell, and a new nuclear envelope reforms around them, the chromosomes all have a full set of DNA identical to the parent cell. The splitting is called cytokinesis.