Download Cells - Revision World

Core Principles
Contents
Biological Molecules
 Cells
 Cell transport
 Organisms exchange materials with their environment
 Enzymes
 Digestion

Biological Molecules

Life on Earth relies on water. At least 80% organism
mass is water. Other chemicals needed are split into 4
groups:
- proteins
- nucleic acids
93% of dry mass of living organisms
(remaining 7% from vitamins)
- carbohydrates
- lipids
Group name
monomers
polymers
% dry mass
Proteins
amino acids
polypeptides
50
nucleic acids
nucleotides
polynucleotides
18
carbohydrates monosaccharides polysaccharides
15
Group name
lipids
components
largest unit
fatty acids + glycerol Triglycerides
% dry mass
10
Cells

Biggest division between cells:
- prokaryote (no nucleus)
- eukaryote (nucleus)
Common features:
Prokaryotic Cells
Mesosome
Capsule
Cell wall
Plasma
membrane
Nucleoid
– plasma membrane infolding to aid
compartmentalisation
– sticky outer protective layer
– rigid structure to help maintain shape
– separates cell from environment
– where DNA is
found
Eukaryotic Cells: Plants
Specific to plant cells:
- Vacuole
- Chloroplast
- Plasmodesma
- Cell wall
Eukaryotic Cells: Animals
Specific to animal cells:
- Centriole
Similarities & Differences
Prokaryotic Cells
Eukaryotic cells
small cells (< 5 mm)
larger cells (> 10 mm)
always unicellular
often multicellular
no nucleus or any membrane-bound
organelles, such as mitochondria
always have nucleus and other
membrane-bound organelles
DNA is circular, without proteins
DNA is linear and associated with
proteins to form chromatin
ribosomes are small (70S)
ribosomes are large (80S)
no cytoskeleton
always has a cytoskeleton
motility by rigid rotating flagellum
made of flagellin
motility by flexible waving
undulipodium, made of tubulin
cell division is by binary fission
cell division is by mitosis or meiosis
reproduction is always asexual
reproduction is asexual or sexual
huge variety of metabolic pathways
common metabolic pathways
Cell Membrane
The most important organelle in an organism’s cell
 Controls how substances move in and out of the cell
 5 methods for movement in and out of cell:

1)
2)
3)
4)
5)
lipid diffusion
osmosis
passive transport
active transport
vesicles
Lipid Diffusion
Carried out by very small molecules – H2O, O2, CO2
 No energy is involved
 Substances move down concentration gradient
 Cannot be controlled by cell

Lipid bilayer
Osmosis
Diffusion of water (abundant in cells) across a membrane
 Higher concentration of solution = less water molecules
 Net flow of water diffuses down concentration gradient
 Water moves from dilute to concentrated solution

Solution Types
lower concentration than
cell
equal concentration to
cell
higher concentration
than cell
Passive Transport




Transport of substances across a membrane by a transmembrane protein molecule
Transport proteins are often specific to one molecule
No energy is involved
Substances move down concentration gradient
Passive Transport

Channel proteins:
- form channel in membrane which is filled with water
- ions diffuse across the membrane
- many channels can be opened or closed

Carrier proteins:
- flip around so that only one side of membrane is open
- high concentration: substance binds
- low concentration: substance is released
Active Transport



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Protein pump molecule “pumps” substances across
membrane
Molecule of substance binds to protein and is then released
on opposite face
Each molecule fits a particular protein pump
Energy is required; substances move up concentration
gradient
Vesicles



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Move large molecules in and out of cells
e.g. proteins, polysaccharides, nucleotides
In to cell: endocytosis
Out of cell: exocytosis
Cell Transport Summary
Method
Energy
Required?
Molecules
Concentration
Gradient
Lipid
Diffusion
No
Very small
Down
Osmosis
No
Water
Down
Passive
Transport
No
Small
Down
Active
Transport
Yes
Small
Up
Vesicles
Yes
Large
Down
Diffusion

Organisms exchange materials with their environment:
- food
- waste
diffuse between organism and environment
- gases
- heat
Rate of Diffusion

surface area x concentration difference
distance
surface area : volume ratio
(volume increases much more than surface area, so as
organisms get bigger it becomes harder to exchange)
Gas Exchange

Takes place at a respiratory surface

This surface must have:
- large surface area
- thin permeable surface
- moist exchange surface

Next slide shows requirements for plants, fish and humans
Gas Exchange in Plants
Gases enter leaf through stomata under leaf
 Guard cells enclose stomata and swell to reduce water loss
 Gases diffuse through and into mesophyll cells
 Diffusion rate increases during day due to photosynthesis:
- oxygen concentration increases
- carbon dioxide concentration decreases
- relative concentration gradients increase… diffusion rate
increases



Palisade mesophyll cells are packed with chloroplasts
Palisade cells packed together for maximum absorption
Gas Exchange in Fish

The water surrounding a fish contains a small percentage of
dissolved oxygen. The fish uses a special gas exchange organ
(gills). It has a large surface area and a short distance for gas
exchange to the blood capillaries. Water flows over the
filaments, and oxygen can diffuse down its concentration
gradient the short distance between water and blood.
Carbon dioxide diffuses the
opposite way down its
concentration gradient

Approximately 80% of the
dissolved oxygen in water
is extracted by the gills
Gas Exchange in Humans
- Humans have approximately 600
million alveoli
- walls are two cells thick (easy for
oxygen and carbon dioxide to diffuse)
- constantly moistened by water
- contain phagocyte cells to kill
bacteria
Inhalation and Exhalation
Diaphragm contracts, pressure
decreases. Air rushes into lungs
Diaphragm relaxes, pressure
increases. Air rushes out of lungs
Enzymes

Enzymes are…
- biological catalysts
- proteins
- suited to one specific reaction only
- affected by PH
- affected by temperature
- affected by lack of metal ions (often found in vitamins)
- able to lower the activation energy barrier of a reaction
Each active site has a particular shape
Each substrate is designed for an enzyme
They fit together like jigsaw pieces
Factors Affecting Enzymes
Digestion

Digestion consists of:
- Ingestion (taking large pieces of food into the body)
- Digestion (breaking down of food)
- Absorption (movement of soluble products into cells)
- Assimilation (using absorbed materials)
- Egestion (getting rid of undigested material from a cavity)

There are several parts to the digestive system:
Digestion
1) Mouth

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Teeth and tongue break up food  form a bolus
Saliva dissolves soluble substances
Mucus lubricates
Lysozymes kill bacteria
Amylase digests starch
Bolus is swallowed down pharynx
Food does not enter lungs as epiglottis closes off trachea
Digestion
2) Oesophagus



Tube that connects mouth to gut
Can be closed (it is soft)
Bolus is forced by ‘peristalsis’ by circular contraction of
thick muscles
3) Stomach

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
Food is stored here for a few hours
Muscle churns food, which is released into the s.intestine
by the sphincter
Gastric juice (pH1) secretes mucus, pepsin and rennin
Digestion
4) Small Intestine contains:
- Duodenum (30cm): most digestion occurs here by
pancreatic juice which contains many enzymes. Bile is
released into here to aid digestion (bile salts) and neutralise
stomach acid (sodium hydrogen carbonate) for efficient
pancreatic enzymes
- Jejunum (2m) and ileum (4m): internal surface area is
huge and contains folds that comprise villi and microvilli.
Villi contain many cells, whilst microvilli are small subcellular structures. Food is forced by peristalsis
Digestion
5) Large Intestine contains:
- Food can remain here for 36 hours
- Many glands secrete mucus
- Produces faeces (cellulose, cholesterol, bile, mucus,
mucosa cells, bacteria and water)
- Sphincter is an involuntary muscle that we learn to control
Summary
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Biological molecules: proteins, carbohydrates, nucleic acid,
lipids
Cells: prokaryotic and eukaryotic
Cell transport: Lipid diffusion, passive & active transport,
osmosis and vesicles
Organisms exchange materials with their environment and
this becomes harder as the size of the organism increases
Enzymes are vital for life and are easily affected if their
conditions are changed
Digestion occurs in the mouth, oesophagus, stomach, small
intestine and large intestine