B1 Cell level systems two main two

two types of cell

• prokaryotic

• eukaryotic

prokaryotic cells

do not contain a nucleus

• their genetic material floats in the cytoplasm

• they are simple cells and are typically smaller than eukaryotic cells

• most have a size from 1 um to 10um.

• bacterial cells are examples of prokaryotic cells

eukaryotic cells

contain genetic material in a nucleus

• they are complex and relatively large

• sizes between 10um and 100um

• plant and animal cells are eukaryotic

what subcellular structures do eukaryotic cells contain?

all eukaryotic cells have a nucleus, cytoplasm, a cell membrane and mitochondria

nucleus

controls the activities of the cell.

• contains the organism’s genetic material, arranged as chromosomes.

  • this determines the cell’s appearance and function.

• the nucleus contains instructions to make new cells or organisms.

cytoplasm

• a ‘jellylike’ substance

• the chemical reactions that keep the cell alive happen here

cell membrane

• a selective barrier that controls which substances pass into and out of the cell

• the membrane also contains receptor molecules

mitochondrion

• where respiration happens

• special protein molecules, called enzymes, enable glucose and oxygen to react together

  • the reactions transfer vital energy to the organism

are plants and animals the same?

no.

• plants make their own food.

• they cannot move their whole body from place to place

• this means they need extra subcellular structures

the plant subcellular structures

• vacuole

• cell wall

• chloroplast

vauole

• full of cell sap; a watery solution of sugar and salts

• helps to keep the cell rigid, supporting the plant and keeping it upright

cell wall

• surrounds the cell

• it is made of a tough fibre called cellulose

• makes the wall rigid and supports the cell

chloroplasts

• contains green chlorophyll, used to transfer energy from the sun to the plant as light, which is used in photosynthesis

• are only in the green parts of the plant

what is bacteria

• the smallest living organisms

• unicellular- consist of just one cell

• every cell can carry out seven life processes; movement, reproduction, sensitivity, growth, respiration, excretion and nutrition

• around 1um in size- a powerful microscope to see them.

bacteria on an agar plate

each dot = a bacterial colony made up of millions of bacteria

examples of prokaryotes

single-celled organisms without a nucleus

• escherichia coli (e. coli): cause food poisoning

• streptococcus bacteria: which causes sore throats

• streptmyces bacteria: found in the soil. the antibiotic streptomycin come from these bacteria. it kills many disease-causing bacteria

common subcellular structures in prokaryotes

• cell wall

• genetic material- one long strand of DNA

• cell membrane

• cytoplasm

other subcellular structures in prokaryotes

some types of bacterial cell have extra subcellular structures, which are adaptations to their environment. Including:

• flagella

• pili

• slime capsule

• plasmid

flagella

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

pili

tiny ‘hairlike’ structures that enable the cell to attach to structures, such as the cells that line your digestive tract.

• also used to transfer genetic material between bacteria

slime capsule

this layer is outside the cell wall.

• it protects a bacterium from drying out and from poisonous substances

• also helps the bacteria stick to smooth surfaces

plasmid

a circular piece of DNA that is used to store extra genes

• these genes are normally not needed for the bacterium’s day to day survival, but may help in times of stress.

• for example, this is where antibiotic resistance genes are normally found

what is a light microscope

used to observe small structures in detail. The microscope passes light through an object placed on a slide on the stage, then through two glass lenses- objective and the eyepiece. the lenses magnify the object, so when you view it through the eyepiece, you can see it in more detail

how can you observe cells through a microscope?

1. move the stage to its lowest position

2. select the objective lens with the lowest magnification

3. place the slide, which has cells on it, on the stage

4. raise the stage to its highest position, taking care that the slide does not touch the lens

5. lower the stage slowly using the coarse focus knob until you see your objects (it will normally be blurred)

6. turn the fine focus knob slowly until your object comes into clear focus

7. to see the cells in greater detail, switch to a higher magnification objective lens without moving the stage. use the fine focus knob to bring the objects into clear focus again

total magnification

eyepiece lens magnification x objective lens magnification

why stain cells before looking at them?

many cells are colourless

• scientists often stain them to make them easier to observe. Some stains colour the whole cell, and others highlight specific subcellular structures

common stains & why

methylene blue- makes it easier to see the nucleus of an animal cell

iodine- makes it easier to see plant cell nuclei

crystal violet- stains bacterial cell walls

how to apply a stain

1. place the cells on a glass slide

2. add one drop of stain

3. place coverslip on top

4. tap the coverslip gently with a pencil to remove air bubbles

limit on microscopes

the resolution of a microscope is defined as the smallest distance between two points that can be seen as separate entities.

• you cannot see any smaller than 0.2um (2 × 10^-7m) with a light microscope

what is an electron microscope

a microscope which uses electrons instead of light to produce an image. They were developed in the 1930s to allow scientists to see in greater detail than ever before

• the greater resolution is achieved by using high-energy electrons as the light source

two types of electron microscope

1. transmission electron microscopes

2. scanning electron microscopes

transmission electron microscopes

produce the most magnified images. They work in a similar way to a light microscope.

• a beam of electrons passes through a very thin slice of the sample. The beam is focused to produce and imagery

scanning electron microscopes

produce a three-dimensional image of a surface.

• they send a beam of electrons across the surface of a specimen

• the reflected electrons are collected to produce and image

advantages of a light microscope

• cheap to buy and operate

• small and portable

• simple to prepare a sample

• natural colour of sample is seen unless staining used

• specimens can be living or dead

disadvantage of a light microscope

resolution up to 0.2 um (2 × 10^-7m)

disadvantages of electron microscopes

• expensive to buy

• large and difficult to move

• sample preparation is complex

• black and white images produced- however, colour can be added to image

• specimens are dead

advantage of electron microscopy

• more detailed

• resolution up to 0.1 um

  (1 ×10^-10m)

seeing further

developmnet of electron microscopy has allowed scientists to see the detail within subcellular structures, such as chloroplasts

what does DNA look like?

there is DNA in the nucleus of every one of the human body’s cells. long molecules of it exists as a chromosome.

• most people have 46 chromosomes in each of your cells, except the gametes

• chickens have 78 chromosome.

• you inherit half your chromosomes from your mother, and half from your father

chromosome

a long molecule of DNA

DNA 2

almost everyone’s DNA is unique. The only organisms that share identical DNA are identical twins and clones

clones

organisms identical to their parents

how is DNA arranged?

into sections. Short sections of D

DNA that code for a characteristic, such as eye colour, are called genes.

• the code that a gene contains causes specific proteins to be made. The particular proteins determine the cell’s function

• for example the protein haemoglobin found in RBCs binds to oxygen allowing RBCs to transport it around the body

gene

DNA that codes for a characteristic

what controls how an organism functions?

the combination of genes in an organism.

• for example, your genes determine your blood group, or whether u have freckles or dimples

what is the structure of DNA?

→ made up of two strands

• these strands join together by bases nucleotides.

• the strands then twist together. This forms a shape known as a double helix

why is DNA a polymer?

as it is joined together by a chain of monomers- nucleotides

nucleotides

each nucleotide is made of a sugar (deoxyribose), a phosphate group, and a base.

the four different types of nucleotide bases

• cytosine

• adesine

• guanine

• thymine

how do the bases in DNA bond?

a base from one strand bonds with a base on the other strand. this forms a base pair. they always bond together in the same formation- complementary base pairings:

• A-T

• C-G

how is a copy of DNA made?

DNA is unable to leave the nucleus of cells as it is too big. Instead a copy of the DNA is made called mRNA (messenger RNA)- a single strand of DNA

how is mRNA produced?

in a process called transcription.

1. the DNA around a gene unzips so that both strands are separated. One of the DNA strands acts as a template.

2. complementary bases attach to the strand being copied. for example C-G, BUT in mRNA, A-U (uracil) instead as there is no thymine.

3. when complete, the strand of mRNA detaches itself from the DNA template. The DNA zips back up

4. mRNA is small enough to move out of the nucleus. It travels to subcellular structures called ribosomes in the cytoplasm- this is where proteins are made.

what is protein made from?

they are made from amino acids. different amino acids join togther to former different proteins

how is protein synthesised?

by a process called translation.

1. the mRNA attaches to a ribosome

2. the ribosome reads the nucleotides on the mRNA in groups of three. These groups are called base triplets/ codons.

3. each triplet codes for a specific amino acid.

4. the ribosome continues to read the triplet code, adding more and more amino acids

5. the amino acids join together in a chain. This is a protein

what does the sequence of amino acids determine?

how a protein will fold. each type of protein has a specific shape, which is important for protein function. Many types of proteins are produced, including enzymes and hormones.

what are enzymes?

made up of protein.

• they are biological catalysts- they speed up reactions without being used up themselves. once a reaction is finished, they can be used to catalyse the same type of reaction again.

what do enzymes do within the body?

involved in many reactions in the body

• they build larger molecules from small ones, such as in protein synthesis

• break down large molecules into smaller ones, such as in digestion

what do enzymes look like?

made up of long chains of amino acids. These are folded together to form a specific shape

• the shape of one part of the enzyme is particularly important; here, molecules of other substances bind to the enzyme (substrate). this is the active site.

what happens when a substrate binds to an enzyme?

an enzyme-substrate complex is formed.

• the reaction happens quickly, and the products are released from the enzyme. they can then catalyse a reaction that produces a larger molecule from small substrates, or break larger molecules apart into smaller pieces

steps of an enzyme building a large molecule from smaller ones

1. two separate substrate moles bind to the enzyme’s active site (they want to create a large molecule)

2. the bond forms between the two substrate molecules on the enzyme’s active site

3. reaction happens and a larger molecule (the product molecule) id formed, and the enzyme is free and ready to catalyse another reaction.

steps of an enzyme breaking down large molecules into smaller ones

1. the large substrate molecule dits into the active site

2. enzyme-substrate complex is formed→ bond forms

3. reaction happens

4. two smaller substrate moles are produced.

5. enzyme is free and ready to catalyse another reaction

what factors affect enzymes?

the rate of an enzyme-catalysed reaction depends on a number of factors:

• temperature

• pH

(for example)

• it is also affected by the concentrations of the enzyme and substrate.

• an enzyme works best in optimum conditions

how does temperature affect enzyme-controlled reactions?

at higher temperatures, the enzyme and substrate molecules move faster and collide often. In general, the higher the temperature the temperature

what happens if the temperature becomes too high?

the amino acid chains in the protein start to unravel, changing the shape of the active site. the enzyme is now denatured, and the substrtae can no longer bind, so the rate of reaction decreases. once all the enzyme molecules are denatured, the reaction stops.

is it reversible?

most denatured enzymes cannot return to their original shape. the change is irreversible

how does pH affect enzyme-controlled reactions?

each enzymes has its optimum pH. a change in pH affects the interactions between amino acids in a chain. This may make the enzyme unfold, changing the shape of the shape of the active site. the enzyme is denatured.

what other factors affect enzyme-controlled reactions?

the higher the substrate concentration, the fast the rate of reaction. however, at a certain substrate concentration, all the enzyme molecules are bound to substrate molecules.

• the rate of the reaction is at its maximum. any further increase in the number of substrate molecules will not increase the rate of reaction as there are no enzymes for them to bind to.

• same is true for enzymes concentration. in general, the higher the enzyme concentration is, the faster the rate of reaction, but this is limited by substrate concentration. if no new substrate molecules are added, the reaction will stop.

why do you need food?

for energy.

the amount and type of food you eat greatly affects your health.

• foods rich in carbohydrates and fats provide you with energy to move and to stay alive.

protein rich foods

used for growth and repair of body tissues.

vitamins and minerals

you need small amounts of these to remain healthy

what is the metabollic rate?

the more active you are, the more energy you need. Chemical reactions in your cells transfer energy from its chemical stores in food. the speed at which this happens is your metabollic rate.

• the higher your metabollic rate = the more food you need to eat

what are carbohydrates

some carbohydrates are polymers.- they are made from smaller carbohydrate molecules, such as sugars.

• starch is an example of a carbohydrate polymer. it is synthesised from glucose monomers. plants often convert glucose to starch. starch is a chemical energy store

what do carbohydrase enzymes do?

break down carbohydrates.

• starch is broken down by amylase

what are proteins?

also polymers, formed from amino acids.

• there are about 20 different amino acids. The order in which the amino acids are joined determines the protein that is synthesised.

what does protease do?

protease enzymes break down proteins into amino acids.

what are lipids?

the fats and oils that you eat.

• as well as being a good store of energy, some animals use them for insulation and buoyancy.

• lipids are synthesised from three fatty acid molecules and a glycerol molecule

what lipase enzymes do?

break down lipids into fatty acids and glycerol

what happens after food molecules are fully digested?

when food molecules are fully digested (into soluble glucose, amino acids, fatty acids and glycerol), they are absorbed into your bloodstream, and then travel to the cells that need them.

word endings

ose- sugars

ase- enzymes

why do we need energy?

our bodies continually transfers energy. energy is transferred so that we can move, grow, and keep warm. Even when asleep, energy is transferred for the body to functions, in activities such as keeping our hearts beating.

aerobic respiration

your energy comes from chemical stores in the food you eat. to transfer this energy, glucose reacts with oxygen in a series of chemical reactions called aerobic respiration

aerobic respiration equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

what does the reaction do?

transfers energy from its chemical energy store in glucose to another chemical energy store for all processes in the cell. this energy store is called ATP

ATP

is used by all living organisms.

• the products carbon dioxide and water are also produced during aerobic respiration

what is ATP produced during respiration used for?

to synthesise larger molecules from smaller ones to make new cell material. Plants make amino acids from sugars, nitrates and other nutrients. in turn, the amino acids form proteins

reason 2?

for movement

animals use ATP to contract muscle cells enabling the organism to move

reason 3?

to stay warm

when an animal’s surrounding are colder than they are, they increase their rate of respiration. this transfers more energy by heating, so that they can keep their body at a constant temperature

where does respiration occur?

aerobic respiration occurs all the time in plant and animal cells. This provides the organism with a constant supply of energy.

• respiration takes place inside the mitochondria of a cell.

• each chemical reaction that takes place during respiration is controlled by a specific enzyme

inside a mitochondrion..

a folded inner membrane gives a large surface area where the enzymes that control cellular respiration are found.

number of mitochondria in cells

most cells contain mitochondria, but different cells contain different numbers of them. The number of mitochondria in a cell tells you how active the cell is.

• muscle cells transfer a lot of energy, so they contains larger numbers of mitochondria.

• liver cells have many mitochondria since they carry out many reactions.

why is respiration an exothermic reaction?

because during the process of respiration, energy is transferred to the surroundings by heating.

energy use during strenuous exercise

during exercise, your muscles need to transfer more energy than normal when they contract. your heart and breathing rate increase to provide your cells with enough glucose and oxygen for respiration to increase. however, during strenuous exercise, your heart rate cannot increase fast enough to meet the demand.

anaerobic respiration

when your body isn’t respiring enough during exercise, your body starts to transfer energy from its chemical store in glucose by anaerobic respiration.

• this does not need oxygen.

• it allows the body to transfer extra energy for short periods of time.

equation for anaerobic respiration

glucose → lactic acid

• in this reaction, glucose is not completely broken down, instead poisonous lactic acid is produced.

why doe we respire aerobically normally?

1. as it produces more ATP molecules per glucose molecule than anaerobic respiration- it has a greater yield, as glucose molecule is fully broken down.

2. lactic acid produced from anaerobic respiration causes cramp. when lactic acid builds up in cells, it causes pain and the muscles stop contracting- this is fatigue

oxygen debt

when you have finished exercising, you keep breathing heavily- the extra oxygen you inhale reacts with the lactic acid, breaking it down. The oxygen needed for this process is called oxygen debt.

do other organisms respire anaerobically?

yes, when they need to transfer energy really quickly- for example when a prey is being chased by a predator, both prey and predator are likely to respire anaerobically.

anaerobic respiration in plants

when no oxygen is available, for example in the roots of plants in waterlogged soils

plant anaerobic respiration

in plants and microorganisms, anaerobic respiration produces ethanol and carbon dioxide.

• this is fermentation.

chemical equation for anaerobic respiration in plants

C₆H₁₂O₆ →2C₂H₅OH + 2CO₂

• the reaction transfers energy from its store in glucose.