Biology Topic 2 - Cells

What are the distinguishing features of eukaryotic cells

1. Cytoplasm containing membrane-bound organelles

2. DNA Enclosed in a nucleus

Describe the general structure of eukaryotic cells

Describe the structure of the cell-surface membrane

Describe the function of the cell-surface membrane

1. Selectively permeable → enables control of passage of substances in/out the cell

2. Receptors on surface → allows cell recognition

Describe the structure of the nucleus

Describe the function of the nucleus

1. Stores genetic information which codes for polypeptides

2. Site of DNA Replication

3. Site of transcription, producing mRNA

4. Nucleolus makes ribosomes / rRNA

Describe the structure of a ribosome

1. Made of ribosomal RNA and protein (2 subunits)

2. Not a membrane-bound organelle

Describe the function of a ribosome

Site of protein synthesis (translation)

Describe the structure of rough endoplasmic reticulum and smooth endoplasmic reticulum

*ribosomes on the surface of the RER

Describe the function of rough endoplasmic reticulum

1. Ribosomes on surface synthesise proteins

2. Proteins processed/folded/transported inside

3. Proteins packaged into vesicles for transport (e.g. to Golgi apparatus)

Describe the function of smooth endoplasmic reticulum

1. Synthesises and processes lipids (e.g. cholesterol and steroid hormones)

Describe the structure of the Golgi apparatus and Golgi vesicles

Describe the function of the Golgi apparatus

1. Modifies proteins and lipids (e.g. adds carbohydrates → glycoproteins or glycolipids)

2. Packages proteins/lipids into Golgi vesicles

3. Produces lysosomes (a type of Golgi vesicle)

Describe the function of Golgi vesicles

1. Transport proteins/lipids to their required destination (e.g. move to and fuse with the cell-surface membrane)

Describe the structure of lysosomes

Describe the function of lysosomes

1. Release hydrolytic enzymes (lysozymes)

2. To hydrolyse pathogens or worn-out cell components

Describe the structure of mitochondria

Describe the function of mitochondria

1. Site of aerobic respiration

2. To produce ATP for energy release (e.g. for protein synthesis / vesicle movement / active transport)

Describe the structure of chloroplasts in plants and algae

Describe the function of chloroplasts in plants and algae

1. Absorbs light energy for photosynthesis

2. To produce organic substances eg. carbohydrates / lipids

Describe the structure of the cell wall in plants, algae and fungi

• Plants/algae: mainly cellulose (a polysaccharide)

• Fungi: chitin (a nitrogen-containing polysaccharide)

Describe the function of the cell wall in plants, algae and fungi

1. Provides mechanical strength to cell

2. So prevents cell changing shape or bursting under pressure due to osmosis

Describe the structure of the cell vacuole in plants

Describe the function of the cell vacuole in plants

1. Maintains turgor pressure in cell → stopping plant wilting)

2. Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals

How are eukaryotic cells organised in complex multicellular organisms

1. Cells become specialised for specific functions

2. Tissue → group of specialised cells with similar structure working together for a specific function

3. Organ → aggregation of tissues performing specific functions

4. Organ system → group of organs working together for specific functions

What are the distinguishing features of prokaryotic cells

1. Cytoplasm lacking membrane-bound organelles

2. So genetic material is not enclosed in a nucleus

3. e.g. bacteria and archaea (always unicellular)

Describe the general structure of prokaryotic cells

Compare and contrast the structure of eukaryotic and prokaryotic cells

Explain why viruses are described as acellular and non-living

1. Acellular → not made of cells (e.g. no cell membrane / cytoplasm / organelles)

2. Non-living → have no metabolism, cannot independently move / respire / replicate / excrete

Describe the general structure of a virus particle

1. Nucleic acids surrounded by a capsid (protein coat)

2. Attachment proteins to attach to specific host cells

3. No membrane bound organelle

4. Some also surrounded by a lipid envelope eg. HIV

Describe magnification

How many times larger the image is than the real object (I = A × M)

Describe resolution

minimum distance apart two objects can be to be seen as separate

Compare optical microscopes, transmission electron microscopes and scanning electron microscopes

How do you measure the size of an object under an optical microscope

1. Line up eyepiece graticule scale with stage micrometre scale

2. Calibrate: use stage micrometre to find size of one graticule division

3. Remove micrometre; count graticule divisions across object

4. Size = number of divisions × size of one division

5. Recalibrate at each magnification

Describe the basic steps in cell fractionation and ultracentrifugation

1. Homogenise tissue (blender) to break open cells and release organelles

2. Place in cold, isotonic, buffered solution

3. Filter to remove large debris (whole cells, connective tissue)

4. Ultracentrifuge at increasing speeds: remove pellet of heaviest organelles, respin supernatant at higher speed, repeat

Why is the homogenate kept cold, isotonic and buffered

• Cold → reduces enzyme activity so organelles aren’t broken down

• Isotonic → no osmosis, so organelles don’t burst

• Buffered → constant pH so enzymes don’t denature

In what order are organelles separated by ultracentrifugation?

(*heaviest to lightest) Nuclei → chloroplasts → mitochondria → lysosomes → endoplasmic reticulum → ribosomes

What are the 3 general stages of the cell cycle in eukaryotic cells

1. Interphase

2. Mitosis

3. Cytokinesis

Describe the S phase in interphase

DNA replicates semi-conservatively → 2 chromatids (identical copies) join at a centromere

Describe the G1 and G2 phases in interphase

Number of organelles & volume of cytoplasm increases → high rates of protein synthesis