Cell Structure

Eukaryotic cells

Cells that contain membrane bound-organelles and a nucleus containing genetic material.

Prokaryotic cells

Cells that do not contain membrane bound-organelles or a nucleus.

Genetic information in prokaryotic cells

Stored free within the cytoplasm as chromosomal DNA and plasmid DNA.

Plasmids

Small, circular loops of DNA found free in the cytoplasm, carrying genes that provide genetic advantages such as antibiotic resistance.

Components of plant and animal cells

Nucleus, Cytoplasm, Cell membrane, Mitochondria, Ribosomes.

Additional components found in plant cells (difference)

Chloroplasts, Permanent vacuole, Cell wall.

Function of the nucleus

Contains DNA and controls cellular activities.

Structure of the cytoplasm

Fluid component of the cell that contains organelles, enzymes, and dissolved ions and nutrients.

Function of the cytoplasm

Site of cellular reactions and transport medium.

Function of the cell membrane

Controls the entry and exit of materials into and out of the cell.

Function of the mitochondria

Site of later stages of aerobic respiration in which ATP is produced.

Function of the ribosomes

Joins amino acids in a specific order during translation for the synthesis of proteins.

What is plant cell wall made of?

Cellulose.

Function of the plant cell wall

Provides structure and support to the plant cell.

Permanent vacuole contents

Cell sap (a solution of salts, sugars and organic acids).

Function of the permanent vacuole

Supports the cell, maintaining its turgidity.

Function of chloroplasts

Site of photosynthesis.

Adaptation of sperm cells in animals

- Haploid nucleus contains genetic information

- Tail enables movement

- High amounts of mitochondria to provide energy for tail movement

Adaptation of nerve cells in animals

- Long axon allows electrical impulses to be transmitted all over the body

- Dendrites connect to and receive impulses from other nerve cells, muscles, and glands

- Myelin sheath insulates the axon and speeds up transmission of impulses.

Adaptation of muscle cells in animals

- Arrangement of protein filaments allows them to slide over each other to produce muscle contraction

- Mitochondria provide energy for muscle contraction

- Merged cells in skeletal muscle allow muscle fibre contraction in unison.

Adaptation of root hair cells in plants

- Large surface area to absorb nutrients and water

- Thin walls that do not restrict water absorption.

Adaptation of xylem cells in plants

- No upper or lower margins between cells provide a continuous route for water to flow

- Thick, woody side walls strengthen their structure and prevent collapse.

Adaptation of phloem cells in plants

- Sieve plates let dissolved amino acids and sugars be transported up and down the stem

- Companion cells provide energy needed for active transport of substances along the phloem.

Cell differentiation

The process by which cells become specialised.

Importance of cell differentiation

Allows production of different tissues and organs that perform various vital functions in the human body.

Purpose of cell division in mature animals

Repair and replacement of cells.

Magnification

The number of times bigger an image appears compared to the size of the real object.

Resolution

The smallest distance between two objects that can be distinguished.

Advantages of light microscopes

Inexpensive, Easy to use, Portable, Observe both dead and living specimens.

Disadvantage of light microscopes

Limited resolution.

Advantage of electron microscopes

Greater magnification and resolution.

How electron microscopes have enabled scientists to understand cells

Allow small sub-cellular structures (e.g. mitochondria, ribosomes) to be observed in detail. Enable scientists to develop more accurate explanations about how cell structure relates to function.

Disadvantages of electron microscopes

Expensive, Large so less portable, Require training to use, Only dead specimens can be observed.

How magnification can be calculated

magnification = size of image / size of real object.

How bacteria multiply

Binary fission (simple cell division).

How often bacteria multiply

Once every 20 minutes if enough nutrients are available and the temperature is suitable.

Ways to grow bacteria

Nutrient broth solution, Colonies on an agar gel plate.

Nutrients in nutrient broth solution

All nutrients required for bacteria to grow including nitrogen for protein synthesis, carbohydrates for energy and other minerals.

Preparation of an uncontaminated culture using aseptic technique

1. Use pre-sterilised plastic Petri dishes or sterilise glass Petri dishes and agar gel before using with an autoclave.

2. Pour the sterile agar gel into the Petri dish and allow time to set.

3. Sterilise the inoculating loop by passing it through a Bunsen burner flame.

4. Dip the inoculating loop into the solution of microorganisms and make streaks with the loop on the surface of the agar.

5. Put the lid on the Petri dish and secure it with tape. Label accordingly then turn and store upside down.

6. Incubate the culture at 25oC in school laboratories.

Sterilisation of Petri dishes, culture media and innoculating loops

To kill any bacteria already present.

Securing Petri dish lid with tape

Stops bacteria in the air contaminating the culture. The lid is not fully sealed to prevent the growth of anaerobic bacteria in a lack of oxygen. Upside down to prevent condensation from forming and dripping down onto the colonies.

Incubation temperature for cultures

Cultures are incubated at 25oC in school laboratories because harmful pathogens are less likely to grow at this temperature.

Formula for cross-sectional area of a bacterial colony

πr² = 3.14, where r = radius (diameter/2).

Calculating number of bacteria from mean division time

1. Calculate the number of times the bacteria will divide in the given time period from the mean division time.

2. Use the following equation: Number of bacteria in population at end of time period = number of bacteria at the beginning of the time period x 2^(number of divisions in the time period).