eCells

What are cells?

-​ Basic structure of all living things

Unicellular: Organisms that consist of only one cell that has to perform all functions
Multicellular: Consists of many different types of cells, each with a specific structure and function, that
work together to ensure effective functioning

Prokaryotes: Organisms which are composed of prokaryotic cells

●​ Lack membrane-bound organelles and have no nucleus
●​ Cell range in diameter from 0.1 to 5.0 µm
●​ Unicellular (made from one cell)
●​ There are two groups of prokaryotes:
○​ Bacteria
○​ archaea

Eukaryotes: Organisms which are composed of eukaryotic cells

●​ Have membrane bound nucleus containing the genetic material of the cell
●​ Internal structures are membrane bound (called organelles)
●​ Cell range from 10 to 100 µm
●​ Can be unicellular or multicellular and can be divided into 4 main groups
○​ Protists
○​ Fungi
○​ Plantae
○​ Animalia

^ Both prokaryotes and eukaryotes share a:

-​ Cell membrane
-​ Cytoplasm
-​ Ribosomes
-​ Genetic material

Summary table:
Microscopes
An effective microscope needs to do 2 things:
1.​ Give an enlarged view of an object ( called magnification)
2.​ Make the detail appear clear, giving a precise (not fuzzy) outline of all the parts of the object (called
resolution)

Compound light microscope

-​ Once cells are prepared and mounted on a glass slide they can be stained
-​ Advantage:
-​ both living and non-living specimens can be viewed
-​ Specimens can be viewed in colour
-​ Cannot magnify larger than x1500

Fluorescence microscope
-​ Similar to light microscope
-​ Specimen is labelled with a fluorescent substance that will attack to the structure of interest
-​ Then it is illuminated with a high-intensity light source causing only the structure of interest to emit
light
-​ Structure is separated from the environment
-​ Advantage:
-​ Allows structures and materials inside a cell that is usually too small to view with a normal
light microscope

Electron microscope
-​ Uses beams of electrons (instead of light) and electromagnets (instead of lenses) to view a
specimen on a screen
-​ Have a greater magnification and resolution than a compound light microscope
-​ There are 2 types
-​ Transmission electron microscope (1933)
-​ Scanning electron microscope (1955)
Transmission electron microscope
-​ Electrons are transmitted through the specimen that has been stained with heavy metals, producing
a 2D image
-​ Can be magnified up to 1.5 million times and a resolution of 2nm
-​ Structures at a subcellular level can be viewed (can see organelles) within
a specimen

Scanning electron Microscope

-​ Bombards solid specimens with a beam of electrons that scans the
surface of a sample (coated with metal)
-​ Causes secondary electrons to be emitted from the surface layers of the
specimen
-​ Shows a 3D image
-​ Only shows the outside and does not show organelles

Advantages:
-​ Can be seen at a cellular and subcellular level
-​ Seen in more detail
Disadvantages:
-​ Need to be placed in a vacuum as air interferes with the beams of electrons
-​ Living species can’t be viewed
-​ Large and very expensive
-​ Very complicated to prepare
 Skills when using light microscopes:

Cell Structure & function Function

Nucleus Image

●​ Large oval structure in the ●​ Controls all cell activities

cytoplasm ●​ Pores regulate the passage of substances
●​ Surrounded by a double between the nucleus and cytoplasm
nuclear membrane pierced by ●​ Stores DNA: hereditary information of an
tiny pores (semi-permeable) organism that gets passed from one generation
to the next
●​ Contains a nucleolus which makes ribosomes

Endoplasmic reticulum

●​ Outer nuclear membrane is ●​ A pathway between nucleus and cell

usually continuous with a environment
network of flattened, ○​ Allows transport of many substances
interconnected membranes ●​ Two types:
●​ Folding of the membranes ○​ Rough ER: ribosomes attached which
increases the surface area fold and processes proteins
allowing more efficient ○​ Smooth ER: has no ribosomes and is
transport and processing of the site of lipid production
proteins and lipids

Ribosomes
 ●​ Found free in the cytoplasm or ●​ Site of protein synthesis
attached to the rough ER ●​ Newly synthesised proteins are passed from
ribosomes to the ER where protein is folded

Golgi apparatus

●​ Made of flat membranes ●​ Process, package and sort cell products (e.g.
which are arranged in stacks, proteins)
each membrane has a curved
shape, where vesicles can be
seen budding off.

Lysosomes

-​ Formed in the golgi body ●​ ‘Lysis’ - to break apart

-​ membrane -bound vesicle ●​ Enzymes break down compounds when they
containing digestive enzymes are no longer needed or when they are passed
-​ Found in animals and their ‘use-by-date’
sometimes plants ●​ Allows the body to recycle materials

Mitochondria

-​ Oval shaped with a double -​ Site of aerobic respiration;obtains energy from

membrane organic compounds
-​ Inner membrane is highly -​ Contains DNA
folded (called cristae) to
increase the surface area of
chemical reactions

Vacuole

●​ Membrane-bound fluid-filled ●​ Store substances, water and nutrients

vesicles ●​ Involved in intracellular digestion and release
●​ In animals they are small cellular waste products
●​ In plants they are big ●​ In plants they play a role in turgor pressure and
help maintain shape of the cell

Chloroplasts

●​ Double membrane with stacks ●​ Site for photosynthesis in plants

of thylakoids containing ●​ Contains DNA
chlorophyll to trap sunlight
Cell wall

●​ External structure surrounding ●​ Helps maintain cell structure

the cell membrane ●​ Provides the cell with protection
●​ Only found in plants

Cell membrane
●​ Semi-permeable ●​ Holds the cell contents in place
●​ Lipid bilayer ●​ Controls the exchange and movement of
substances into and out of the cell
●​ It is selectively permeable

Cell measurements

Scaled drawings:

Terminology Description Diagram

Impermeable If a barrier is impermeable it
does not allow any molecules to
pass through it. Has no pores for
the molecules to pass
 Permeable It will allow all the molecules to
pass through it and it probably
has large spaces or pores

Selectively permeable Some barriers are selectively

permeable - they only allow
certain molecules to pass
through and others are held
back. May have small pores

Fluid mosaic model

The structure of the selectively permeable cell membrane is described as a ‘fluid mosaic’.

Lipid component:
-​ Is the ‘fluid’ part (not rigid) and is composed of two layers of
phospholipids forming a

-​ It’s made up of the

●​ Phosphate group on the head makes this end hydrophilic (can absorb or dissolve water)
○​ Facing outwards
●​ Fatty acid tail which is hydrophobic (water avoiding or can’t dissolve water)
○​ Facing inwards
●​ Protein molecules
○​ Some penetrate all the way through the bilayer forming
channels
○​ Partly embedded in the membrane
○​ Some are fixed in place
●​ Cholesterol
○​ Found between the phospholipid molecules which gives
the cell membrane stability
●​ Carbohydrates
○​ Linked to protruding proteins (glycoproteins) or to lipids
(glycolipids)
○​ They recognise and act as an adhesive between cells
○​ Recognise antibodies, hormones and viruses

Cell functions
Solutions
A solution is formed when a solute (i.e. sugar) dissolves in a solvent (i.e. water). The
amount of solute dissolved in a given quantity of solvent determines the concentration of
the solution.

●​ Concentrated solution: contains a large amount of solute in relation to the amount of solvent.
●​ Dilute solution: contains a small amount of solute in relation to the amount of solvent.

The movement of materials into and out of cells takes place either:
1.​ Passively; requires no energy input, usually because the molecules are polar and can
dissolve in the cytoplasm and pass through the membrane. The molecules are moving with
the concentration gradient.
2.​ Actively; requires energy input, usually because the molecules are large or nonpolar, and
cannot easily dissolve in the cytoplasm nor pass through the membrane. Active transport
also transports substances against the concentration gradient. (More on this after the
depth study)

Diffusion
Diffusion is the movement of any molecules (except water) from a region of high concentration to
a region of low concentration to the substances, until equilibrium is reached.
-​ Equilibrium is reached when there is no net movement of molecules in either direction;
the molecules move equally in each direction.
-​ This process does not require energy (passive).

Diffusion is a slow process. The rate is affected by:

•​ Concentration gradient (see next slide)
•​ Particle size: the smaller the particles, the faster the rate of diffusion
•​ Temperature: the higher the temperature, the higher the rate of diffusion. Increasing
temperature causes the molecules to move faster.

Osmosis
Osmosis is a special type of diffusion.
It is the movement of water molecules from a region of high water concentration to a region of
low water concentration through a selectively permeable membrane. This process does not
require energy (passive).

*HAS TO BE MOVEMENT OF WATER ACROSS A SEMIPERMEABLE MEMBRANE * SUPER

IMPORTANT

1.​ Water moves into and out of the cell membrane via the process of osmosis. Water does
not move directly through the lipid bilayer (remember those hydrophobic tails!), but
through a special protein channel called aquaporins (water pores).
 *Hypotonic - high water
concentration outside of cell

Increase in mass: Solution is hypotonic

Decrease in mass: Solution is hypertonic
No change: solution is isotonic

Exchange of materials
What affects the exchange of materials:
-​ Characteristics of materials
-​ Chemical factors:
-​ Uncharged particles easily diffuse through the cell membrane because it is easily
dissolved
-​ Charged particles require protein channels to travel through
-​ Water is not lipid soluble and moves into and out of the cell through aquaporins
-​ Physical factors
-​ Size and shape affect the movement of substances across the cell membrane
-​ Small molecules diffuse easily
-​ Large molecules need protein channels (think glucose is too big so it needs to go
through the channels)
-​ Very large is transported in by endocytosis and out by exocytosis
-​ Concentration gradient
-​ Cells need a high concentration gradient so it can diffuse rapidly

Facilitated diffusion
Large molecules and charged particles can’t diffuse through the cell membrane. They are transported into
or out of the cell via proteins. There are 2 types:
1.​ Channel protein
 a.​ Small ions can go through
b.​ Acts like a gate, allowing specific ions to enter or exit
2.​ Carrier proteins
a.​ Bind to molecules on one side, then changes shape to release the
molecules on the other side of the cell membrane

Active transport
Movement of molecules from an area of low concentration to an area of high concentration
-​ Requires a carrier protein that spans from the membrane using ATP.
Transports of large molecules
Particles too large to move through the cell membrane, can be actively transported by:
-​ Endocytosis
-​ Cell is changed shape to surround and engulf it
-​ Solids: Phagocytosis
-​ Liquid: pinocytosis
-​ Exocytosis
-​ Waste removed by exocytosis. Fuses with it and then releases the contents into
extracellular fluid.

-​
Cells
If a cell is too big → it is not able to maintain its homeostasis and it will
die.
Cells must maintain a large surface area (SA) to volume (V) ratio to
maintain good health.
→ if the cell grows too big it divides (cell division) into two cells so it can
maintain a large surface area to volume ratio.
Cell requirements
All organisms on Earth use matter (organic and inorganic compounds) to produce the energy required for all
biological processes.
→ Organic compounds are synthesised by autotrophs and are made up of carbon, oxygen and hydrogen
atoms. These include:
-​ Carbohydrates
-​ Lipids
-​ Proteins
-​ Nucleic acids
→ Inorganic compounds are part of the non-living world and do not contain carbon and hydrogen in long
chains. These include:
-​ Water
-​ Mineral salts
-​ Gases
Carbohydrates

Carbohydrates are important:

Energy sources; glucose is a monosaccharide and used for quick
energy and starch is a polysaccharide and is used for stored energy
Structural components; cellulose is a polysaccharide and is used
for strength and support in plant cell walls

Lipids
Lipids are composed of a glycerol head and a fatty acid tail.
(normally in triglycerides)
In cells, lipids have two main functions:
-​ Energy storage (fats and oils)
-​ Structural component of membranes
Proteins
Amino acids are the building blocks of proteins. Amino acids join
together by peptide bonds to form a polypeptide. One or more polypeptide chains twists together in a
specific shape to form a protein.
Functions of proteins include:
- Form structural components in cells, cell membranes and tissues
Some have a functional role in
- Enzymes control metabolic processes
- Hormones control functioning of cells

Nucleic acids

Nucleic acids carry the genetic information of cells. There are two types
of nucleic acids:
→ Deoxyribonucleic acid (DNA) is a double-stranded molecule that
stores the information that controls the cell.
E.g. nuclear DNA, mitochondrial DNA, viral DNA

→ Ribonucleic acid (RNA) is a single-stranded molecule that assists in

the manufacture of proteins.
E.g. rRNA, mRNA, tRNA (involved in protein synthesis).
→ Many other types including regulatory RNAs, parasitic RNAs
Inorganic compounds

Inorganic nutrient Uses in cell activities

Water -​ The transport medium in cells and organisms

-​ A solvent for many molecules inside cells

Mineral salts- chlorides, -​ Assist in chemical reactions

nitrates, phosphates -​ Used in synthesis of many macromolecules and body tissues
(i.e. calcium for bones, iron for blood)
-​ Sodium ions (Na+) and chloride ions (Cl-) assist in balance in
cells and are essential for cell membrane functioning and for
nerve and muscle cells.

Gases Carbon dioxide:

-​ Used in the process of photosynthesis
-​ Released as a by-product of cellular respiration
Oxygen
-​ Used in the process of cellular respiration
-​ Released as a by-product of photosynthesis

Removal of Cellular wastes

Not all the products that are produced from chemical reactions are required by the organism. These
products are called ‘wastes'.
 → they must be removed so they do not accumulate and interfere with cell function. The removal
of wastes from the cell is called ‘excretion’

Oxygen and carbon dioxide are waste products from the chemical reactions of photosynthesis and cellular
respiration, respectively.
→ These gases can easily be removed from the cell by diffusion through the cell membrane.
Excess water that is not required by the cell moves out of the cell by osmosis.

Lysosomes containing digestive enzymes are responsible for breaking down and removing old cells and
other cellular wastes.
-​ Any of the wastes that cannot be eliminated by lysosomes, for example hormones and enzymes, are
packaged and removed by exocytosis.
Metabolism
Metabolism is the total sum of all chemical reactions occurring within a living organism. Each step of a
metabolic pathway in cells is catalysed by enzymes.

Metabolism is divided into two:

1.​ Anabolic
2.​ Catabolic
Anabolic reactions
Anabolic reactions involve building up large organic compounds from simpler molecules. These reactions
require the input of energy:

E.g.
-​ Making a protein from amino acids (protein synthesis)
-​ Making a polysaccharide (i.e. starch) from monosaccharide units (i.e. glucose)

Catabolic reactions
Catabolic reactions involve breaking down complex organic compounds to simpler ones. These reactions
give out energy:
E.g.
-​ Breaking down of glucose for energy
-​ Digestion of food

Enzymes
Enzymes are proteins.
A protein is a long chain of amino acids joined by peptide bonds (called a polypeptide), folded into a 3-
dimensional shape (their effective functioning relies on their shape).
Enzymes are biological catalysts → able to control the rate of a chemical reaction.
An animal cell may contain up to 4000 different types of enzymes, each catalysing a different chemical
reaction.
Some enzymes are common to many types of cells carrying out particular functions.
-​ Every type of enzyme has a specific shape as it is made up of a specific pattern of amino acids.
Enzymes have active sites that are usually composed of three or
four amino acids.
-​ The active sites are the areas that substrates (the molecule
on which an enzyme acts on) will bind to and catalyse
chemical reactions.
-​ When an enzyme binds to a substrate it makes a new
molecule called the enzyme substrate complex.
Role of enzymes
Enzymes are able to speed up reactions without a change in temperature.
In chemical reactions that occur in the non-living world → heat could provide the necessary activation
energy for a chemical reaction
-​ But in the living world → heat burns tissue.

For a chemical reaction to begin, activation energy is necessary.

The role of an enzyme is to lower the activation energy needed to start a reaction (by bringing specific
molecules together, rather than relying on them colliding randomly), so that the reaction can proceed
quickly without a change in temperature.

Lowering of activation energy

For a chemical reaction to take place, the molecules involved need to collide at the correct orientation and
with the right amount of energy → called activation energy

a) Without catalyst, activation energy must be

supplied for reaction

b) catalysts accelerate specific reactions by

lowering amount of activation energy needed.

Enzyme Models
1.​ Lock and Key Model
2.​ Induced-fit model
Lock and Key model
It was thought that an enzyme’s active site is rigid →
substrate is reciprocally shaped for the active site.
Once the enzyme-substrate complex is formed, the close
proximity of the molecules allows reaction to be rapidly
catalysed and products of reaction are released.
Induced-fit model
The substrate plays a role in determining the final shape
of the enzyme-substrate complex + active site is more
flexible than once thought.
The substrate enters and binds to enzyme → shaping active site and properly aligning enzyme for the
reaction to take place, reaction will not be catalysed unless other substrates are properly able to shape the
enzyme.
Coenzymes
Many enzymes cannot function without a coenzyme (also known as a cofactor). A coenzyme is a
non-protein group such as a vitamin or a metal ion (e.g. zinc, copper or iron) that binds with the protein
part and helps to form the active site. It increases the size of the enzyme molecule and its active site,
increasing the chance of collision. It can be easily separated from the protein part of the enzyme, but its
presence cannot function without the cofactor.

A functional enzyme may therefore consist of protein only, or it may be in the form of an enzyme–cofactor
complex (where the enzyme part of the complex is a protein).

Characteristics
They are affected by:
●​ Temperature
○​ Normally in most living things they function around 37%
○​ Below this the rate of reaction decreases
○​ At temperatures above 60 degrees they stop functioning
altogether
■​ They change shape (called denaturing)
○​ Excessive cold causes it to slow down or stop functioning
■​ NOT permanent

●​ pH
○​ Has a region of pH optimal stability
○​ Varies from each enzyme depending on the reaction catalysed by the enzyme
○​ Shape of the pH graph is usually symmetrical on either side
■​ At optimum pH the enzymes reaction rate is maximum
■​ Too low the enzyme changes shape no longer fitting
■​ Can be denatured at an extreme pH
 ●​ Substrate concentration
○​ They only act on one type of substrate
○​ Each enzyme catalyses one particular chemical reaction
○​ As substrate concentration increases, the rate of reaction will increase
because more enzymes are available to catalyse the reaction
○​ At the highest rate it will plateau because no more enzymes are able to
catalyse the reaction
○​ Maximum rate is called saturation point

Denaturing
-​ Renders it useless
-​ Substrate can’t bind with the active site
-​ Can’t function properly to catalyse reactions
-​ Once denature the change is permanent

Module 2
Module 2:
Organisation of living things
Organisms can exist as:
-​ A single cell (unicellular)
-​ Single cells working together (colonial)
-​ Organism made up of many cells (multicellular)
Unicellular Organisms
Unicellular organisms can be:
-​ Prokaryotic: bacteria and archaea
-​ The lack of organelles and simple structure in prokaryotes limits the number of metabolic
reactions that can occur at any particular time.
-​ Eukaryotic: protists, unicellular algae and unicellular fungi.

They can carry out all necessary life processes in a more efficient manner using specialised organelles.
-​ They have a short lifespan due to energetically expensive workload for one cell.

●​ Are always directly exposed to the external environment.

●​ Their microscopic size means they have a large surface-area-to-volume-ratio (SA:V).
●​ A large SA:V enables all required substances (such as nutrients, gases and water) to move across the
cell membrane via diffusion.
 ●​ Wastes can also be efficiently removed through the cell membrane.
●​ Reproduce mostly asexually.

Colonial Organisms
Colonial organisms are made up of a group of identical single-celled organisms collectively called a colony.
-​ They are eukaryotes and are usually macroscopic in size.
-​ All individuals in the colony can carry out each function necessary for life.
-​ Some colonial organisms contain cells that have specialised functions that are coordinated with
other cells in the colony.
-​ They have a long lifespan, as work and energy costs are shared by cells.
-​ They reproduce mostly asexually, but sexual reproduction is present in some species.
Examples: include coral, jellyfish and Volvox.

Volvox
A Volvox colony is a hollow sphere made up of 500-60,000 algae cells. The colony is only one cell thick.

Each algae cell within the colony:

-​ Has two flagella which allows them to swim in a coordinated fashion.
-​ Are connected by strands of cytoplasm.
-​ Has a red ‘eyespot’ that can detect light. The cells at one end of the
colony have more developed eyespots which allow the colony to swim
towards the light.
-​ Contain chloroplasts.

Multicellular organisms
-​ Multicellular organisms are made up of many different types of cells, where similar cells are
grouped together for the efficient functioning of the organism.
-​ Specialised cells cannot live independently of each other, unlike unicellular and colonial
organisms.
-​ Multicellular organisms are large in size, therefore their total surface-area-to volume-ratio (SA:V) is
smaller.
-​ They cannot rely on passive transport.
-​ Multicellular organisms are made of many small cells. Each cell has its own large SA:V which
increases the efficiency of diffusion and osmosis in individual cells.

Epithelial tissue:
-​ Covers body surfaces, protects organs and forms glands.
-​ Densely packed aiding in being a barrier to injury and infection
-​ Found in skin and the surface of organs of the digestive and
respiratory systems

Connective tissue:
-​ Provides support and ensures that different parts of the body
are bound together and protects against damage
-​ Consists of en extracellular matrix with cells scattered
through it
 -​ Matrix is made of the protein fibre, collagen, for strength, elastin for flexibility

Nervous tissue:
-​ Comprises of the brain, spinal cord and peripheral nerves
-​ Must be highly specialised for communication between all parts of the body

Muscle tissue:
-​ Muscle tissues contain muscle cells called muscle fibres that are highly specialised for contractions
-​ Convert chemical energy (ATP) to mechanical energy for movement
-​ Found in
-​ Skeletal muscle: attached to bones and causes movement
-​ Cardiac muscle: present in the heart
-​ Smooth muscle: contractions push substances through
specialised organs such as the gastrointestinal tract

Specialised Cells
The structure and arrangements of these cells in the organism are closely related to their specific function.
These specialised cells work together to ensure the most efficient functioning of the organism.

How do cells become specialised?

●​ ‘Stem cells’ are undifferentiated cells which have all the information within them to form an adult
organism.
●​ When stem cells differentiate they become specialised as different types of cells. They
develop suitable structural features that allow them to carry out their specific functions.
●​ Once they have become specialised to form a particular type of cell, differentiated cells lose
their capacity to develop into other types of cells.

Cell differentiation and specialisation

All cells in an organism contain the same genetic material, but when the cell differentiates, it doesn’t use all
of this information.
●​ Different cells develop as a result of only certain parts of the genetic material being ‘switched on’.

Cell differentiation: the process that a stem cell goes through to become specialised.

Cell specialisation: refers to the particular function that a cell has.

NOT IN EXAM BUT NOTES FOR LATER
Plants
Most autotrophic organisms are plants and can be divided into two groups:
1.​ Vascular plants: have a transport system e.g. ferns
2.​ Non-vascular plants: have no transport system + uses diffusion and osmosis through the
surfaces of the plant e.g. mosses

Vascular plants
Have specialised cells grouped together in tissues → forms organs e.g. roots, stems, flowers and
seeds - these organs are part of three systems:
1.​ The root system
2.​ The shoot system (stems & leaves)
3.​ The vascular system (xylem & phloem)

1.​ The root system

Roots have two main functions:
-​ Anchoring the plant
-​ Absorbing water and inorganic nutrients
Roots have large surface area → allows water
and inorganic mineral salts to be absorbed
efficiently
Achieved by:
Extensive branching + presence of root hairs

Movement of substances:
Water + inorganic minerals are absorbed from soil → root system
Water: moves into roots via osmosis
Mineral ions: move into roots via diffusion
If movement of diffusion is too slow or concentration gradient is not high enough → facilitated
diffusion and active transport may be involved

2.​ The Shoot System: Stems

Stems have two main functions:
-​ Provide structural support
-​ Provide transport pathway between the roots and
leaves
Three main types of tissue in the stem:
1.​ Dermal tissue: the outer layer
2.​ Vascular tissue: consists of xylem and phloem
3.​ Ground tissue: all parts of the stem that is not dermal
tissue or vascular

2. The Shoot System: Leaves

Main function of a leaf is to produce glucose by absorbing sunlight and carbon dioxide during
photosynthesis.
Leaves are thin and flat to ensure: have enough surface area to absorb sunlight + no internal cell is
too far from the surface to receive light
Leaves are made up of three main parts:
Epidermis - mesophyll - vascular bundles
Leaf Structure
Epidermis: layer of cells covering the entire leaf
1.​ Upper epidermis: secretes a waterproof, waxy layer (the cuticle) → protects the leaf
and prevents excess water loss
- is transparent, allows sunlight to reach the photosynthetic cells below
2.​ Lower epidermis: contains stomata where gas exchange occurs
Mesophyll: middle layers of leaf : contains chloroplasts for photosynthesis
1.​ Palisade cells: tightly packed together + contain many chloroplasts
-​ Main site of photosynthesis
2.​ Spongy cells: loosely packed together, with air spaces between them to allow gas
exchange
-​ Contain fewer chloroplasts