Cells

Define the term eukaryotic cell

DNA is contained in a nucleus, contains membrane-bound specialised organelles

Define the terms prokaryotic cell

DNA is free in cytoplasm no organelles

State the relationship between a system and specialised cells

Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems

Describe the structure of the cell-surface membrane

Fluid mosaic phospholipid bilayer with extrinsic and intrinsic proteins embedded

Describe the function of the cell-surface membrane

* Isolates cytoplasm from extracellular environment
* Selectively permeable to regulate transport of substances
* Involved in cell signalling / cell recognition

Explain the role of cholesterol

Steroid molecule connects phospholipids and reduces fluidity

Explain the role of glycoproteins

Cell signalling, cell recognition and binding cells together

Explain the role of glycolipids

Cell signalling and cell recognition

Describe the structure of the nucleus

* Surrounded by nuclear envelope, a semi-permeable double membrane
* Nuclear pores allow substances to enter/exit
* Dense nucleolus made of RNA and proteins assembles ribosomes

Describe the function of the nucleus

* Contains DNA coiled around chromatin into chromosomes
* Controls cellular processes: gene expression determines specialisation and site of mRNA transcription, mitosis, semiconservative replication

Describe the structure of a mitochondrion

* Surrounded by double membrane folded inner membrane forms cristae
* Fluid matrix: contains mitochondrial DNA, repiratory enzymes, lipids, proteins

Describe the structure of a chloroplast

* Vesicular plastid with double membrane
* Thylakoids: flattened discs stack to form grana contain photosystems with chlorophyll
* Intergranal lamellae: tubes attach thylakoids in adjacent grana
* Stroma: fluid-filled matrix

State the function of mitochondria

Site of aerobic respiration to produce ATP

State the function of chloroplasts

Site of photosynthesis to convert solar energy to chemical energy

Describe the structure of the golgi body

Planar stack of membrane bound flattened sacs cis face aligns with rER, molecules processed in cisternae vesicles bud off trans face via exocytosis

Describe the function of the golgi body

* Modifies and packages proteins for export
* Synthesises glycoproteins

Describe the structure of a lysosome

Sac surrounded by single membrane embedded H+ pump maintains acidic conditions contains digestive hydrolase enzymes glycoprotein coat protects cell interior

Describe the function of a lysosome

* Digests contents of phagosome
* Exocytosis of digestive enzymes

Describe the structure of a ribosome

Formed of protein and RNA and found free in cytoplasm or attached to ER

Describe the function of a ribosome

Site of protein synthesis via translation

Describe the structure and function of the endoplasmic reticulum (ER)

Cisternae: network of tubules & flattened sacs extends from cell membrane through cytoplasm and connects to nuclear envelope

Describe the structure of the cell wall

* Bacteria → murein
* Plants → cellulose

State the functions of the cell wall

* Mechanical strength and support
* Physical barrier against pathogens
* Part of apoplast pathway to enable easy diffusion of water

Describe the structure of the vacuole in plants

Surrounded by single membrane: tonoplast contains cell sap: mineral ions, water, enzymes, soluble pigments

Describe the function of the cell vacuole

* Controls tugor pressure
* Absorbs and hydrolyses potentially harmful substances to detoxify cytoplasm

Explain some common cell adaptations

* Folded membrane or microvilli increase surface area for diffusion
* Many mitochondria = large amount of ATP for active transport
* Walls one cell thick to reduce distance of diffusion pathway

State the role of plasmids in prokaroytoes

* Small ring of DNA that carriers non-essential genes
* Can be exchanged between bacterial cells via conjugation

State the role of flagella in prokaryotes

Rotating tail propels organism

State the role of capsule in prokaryotes

* Prevents desiccation
* Acts as food reserve
* Provides mechanical protection against phagocytosis and external chemicals
* Sticks cells together

Compare eukaryotic and prokaryotic cells

Both have, cell membrane, cytoplasm and ribosomes

Why are viruses referred to as particles and no cells?

Acellular and non-living, no cytoplasm and cannot self-reproduce

Describe the structure of a viral protein

* Linear DNA or RNA
* Surrounded by capsid

Describe the structure of an enveloped virus

* Simple virus surrounded by matrix protein
* Matrix protein surrounded by envelope
* Attachment proteins on surface

State the role of attachment proteins on viral particles

Enable viral particle to bind to complementary sites on host cells

How does a virus replicate itself?

1. Virus attachment proteins on its surface binds to complementary receptor proteins on host cell.
2. Virus injects its DNA or RNA into host cell
3. Host cell uses its nucleic acid and ribosomes to produce new viral particles

Describe how optical microscopes work

1. Lenses focus rays of light and magnify the view of a thin slice of specimen
2. Different structures absorb different amounts and wavelengths of light
3. Reflected light is transmitted to the observer via the objective lens and eyepiece

Outline how a student could prepare a tissue for an optical microscope

1. Obtain thin section of tissue
2. Place plant tissue in a drop of water
3. Stain tissue on a slide to make structures visible
4. Add coverslip using mounted needle to avoid trapping air bubbles

Advantages of using an optical microscop

* Colour image
* Living structures
* Affordable apparatus

Suggest disadvantages of using an optical microscope

* 2D image
* Lower resolution than electron microscopes

Describe how a transmission electron microscope works

1. Pass a high energy beam of electrons through thin slice of specimen
2. More dense structures appear darker since they absorb more electrons
3. Focus image onto fluorescent screen or photographic plate using magnetic lenses

Suggest advantages of using a TEM

* Electrons have shorter wavelength than light = high resolution
* high magnification

Suggest disadvantages of using a TEM

* 2D image
* No living structures
* No colour image
* Artefacts

Describe how a scanning electron microscope works

1. Focus a beam of electrons onto a specimen’s surface using electromagnetic lenses
2. Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate

Suggest advantages of using an SEM

* 3D image
* Higher resolution as electrons have shorter wavelength than light

Suggest disadvantages of using an SEM

* Requires a vacuum
* No colour image
* Only shows outer surface

Define magnification

Factor by which the image is larger than the actual specimen

Define resolution

Smallest separation distance at which 2 separate structures can be distinguished from one another

Explain how to use an eyepiece graticule and stage micrometre to measure the size of a structure

1. Place micrometre on stage to calibrate eyepiece graticule
2. Line up scales on graticule and micrometre, count how many graticule divisions are in 100um on the micrometre
3. Length of 1 eyepiece division = 100um / number of divisions
4. Use calibrated values to calculate actual length of structures

What is the equation for actual size?

actual size = image size / magnification

Outline what happens during cell fractionation and ultracentrifugation

1. Blend and homogenize tissue to break open cells and release organelles
2. Filter homogenate to remove debris
3. Perform differential centrifugation
4. Spin homogenate in centrifuge
5. Most dense organelles in the mixture form a pellet
6. Filter off the supernatant and spin again at a higher speed

Explain why fractionated cells are kept in a cold, buffered, isotonic solution

* Cold → slow enzyme action
* Buffered → maintain constant pH
* Isotonic → prevent osmosis

State the stages of the cell cycle

1. Interphase
2. Mitosis or meiosis
3. Cytokinesis

Outline what happens during interphase

1. G1: cell synthesises proteins for replication and cell size doubles
2. S: DNA replicates
3. G2: Organelles divide

State the purpose of mitosis

* Growth
* Cell replacement / tissue repair
* Asexual reproduction

What does mitosis produce

2 genetically identical daughter cells

Name the stages of mitosis

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Outline what happens during prophase

1. Chromosomes condense becoming visible
2. Centrioles move to opposite poles of cell and mitotic spindle fibres form
3. Nuclear envelope and nucleolus break down

Outline what happens during metaphase

Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres

Outline what happens during anaphase

1. Spindle fibres contract and centromeres divide
2. Sister chromatids separate into two distinct chromosomes and are pulled to opposite poles
3. Spindle fibres break down

Outline what happens during telophase

1. Chromosomes decondense, becoming invisible again
2. New nuclear envelope form around each set of chromosome

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Explain the procedure for a root tip squash

1. Prepare temporary mount of root tissue
2. Focus an optical microscope on the slide, count total number of cells in the field of view and number of cells in a stage of mitosis
3. Calculate mitotic index

Explain how to prepare a temporary root tip mount

1. Place root in hydrochloric acid to halt cell division
2. Stain root tip with a dye that binds to chromosomes
3. Macerate tissue in water
4. Press down coverslip and obtain a single layer of cells

Why is only the root tip used when calculating a mitotic index?

* Meristematic cells at root tip are actively undergoing mitosis

What are tumour suppressor genes?

Genes that code for proteins to trigger death

What are proto-oncogenes?

Genes that code for proteins to stimulate cell cycle to progress from one stage to the next

How can mutation to tumour suppressor genes cause cancer?

No production of a protein to slow the cell cycle → uncontrolled cell division

How can mutation to proto-oncogenes cause cancer?

Form permanently activated oncogenes

Suggest how cancer treatments control the rate of cell division

* Prevent DNA replication
* Disrupt spindle formation

How do prokaryotic cells replicate?

1. Binary Fission
2. DNA loop replicates
3. Cell elongates, separating 2 DNA loops
4. Cell membrane contracts and septum forms
5. Cell splits into 2 identical daughter cells

Describe the fluid mosaic model of membranes

Phospholipid bilayer in which individual phospholipids can move so flexible shape with extrinsic and intrinsic proteins

Explain the role of cholestrol

Steroid molecule in some plasma membranes, connects phospholipids and reduces fluidity

Explain the role of extrinsic proteins in membranes

* Binding sites / receptors
* Antigens
* Bind cells together
* Involved in cell signalling

Explain the function of intrinsic proteins in membranes

* Electron carriers
* Channel proteins
* Carrier proteins

Explain the functions of membranes within cells

* Provide internal transport system
* Regulate passage of molecules
* Provide reaction surface

How does temperature affect the membrane permeability?

Denatures membrane proteins and molecules have more kinetic energy and move further apart

How does pH affect the membrane permeability?

Changes tertiary structure of membrane proteins

Define osmosis

Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential

How does osmosis affect plant cells?

* Osmosis into cell = cell turgid
* Osmosis out of cell = cell flacid

Define simple diffusion

* Passive process which requires no energy
* Net movement of small, lipid-soluble molecules down a concentration gradient

Define facilitated diffusion

* Passive process
* Specific channels or carrier proteins transport large polar molecules down concentration gradient

Explain how channel proteins work

Hydrophilic channels bind to specific ions = one side of the protein closes and the other opens

Explain how carrier proteins work

Binds to complementary molecule = conformational change releases molecule on other side of membrane

Name 5 factors which affect that rate of diffusion

* Temperature
* Diffusion distance
* Surface area
* Size of molecule
* Difference in concentration

State Fick’s Law

Surface area x difference in concentration / diffusion distance

Define active transport

Movement of molecules against a concentration gradient involving ATP and a carrier protein

Define co-transport

Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient

Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine

1. Na+ actively transported out of epithelial cells and into blood stream
2. Na+ concentration lower in epithelial cells than lumen of gut
3. Transport of glucose / amino acids from lumen to epithelial cells is coupled to facilitated diffusion of Na+ down electrochemical gradient

What is an antigen?

Cell surface molecule which stimulate an immune response

How does phagocytosis destroy pathogens?

1. Phagocyte moves towards pathogen via chemotaxis
2. Phagocyte engulfs pathogen via endocytosis to form a phagosome
3. Phagosome fuses lysosome
4. Lysozymes digest pathogen

Name 2 types of specific immune response

* Cell mediated
* Humoral

Outline the process of the cell-mediated response

1. Complementary T helper lymphocytes bind to foreign antigen
2. Release cytokines that stimulate


1. Clonal expansion of complementary T helper cells → become memory cells or trigger humoral response
2. Clonal expansion of cytotoxic T cells

Outline the process of the humoral response

1. Complementary T helper lymphocytes bind to foreign antigen on antigen presenting cells
2. Release cytokines that stimulate clonal expansion of complementary B lymphocytes
3. B cells differentiate into plasma cells
4. Plasma cells secrete antibodies with complementary variable region to antigen

How do antibodies lead to the destruction of a pathogen?

Formation of antigen-antibody complex results in agglutination, which enhances phagocytosis

What are monoclonal antibodies?

Antibodies produced from a single clone of B cells

What are memory cells?

* Specialised T helper and B cells produced from primary immune response
* Remain in low levels in the blood
* Can divide rapidly by mitosis if organism encounters the same pathogen again

Describe the secondary immune response compared to the primary

* Faster rate of antibody production
* Shorter time lag between exposure and antibody production
* Higher concentration of antibodies
* Antibody level remains higher after the secondary response
* Pathogen usually destroyed before any symptoms

What causes antigen variability?

1. Random genetic mutation changes DNA base sequence
2. Results in different sequence of codons on mRNA
3. Different primary structure of antigen, different tertiary structure
4. Different shape of antigen

Explain how antigen variability affects the incidence of disease

* Memory cells no longer complementary to antigen
* Many varieties of a pathogen = difficult to develop vaccine

Explain the principles of vaccination

1. Vaccine contains dead / inactive form of a pathogen or antigen
2. Triggers primary immune response
3. Memory cells are produced and remain in the bloodstream, so secondary response is rapid
4. Pathogen destroyed before symptoms

What is herd immunity?

Vaccinating large proportion of population reduces available carriers of the pathogen, protects individuals who are not immune