Section 2: Cells

What is the function of the cell membrane?

regulates the movement of substances into and out of the cell

What is the function of the nucleus?

contains the organisms genetic material (DNA) and controls cell activities, site of DNA replication and transcription( Production of mRNA and nucleolus is within the structure producing rRNA)

What is the function of the mitochondria?

Site of aerobic respiration and where ATP is produced

What is the function of chloroplasts?

Site of photosynthesis

What is the function of the Golgi Apparatus?

modification and packaging of proteins

What is the function of lysosomes?

-break down materials and waste
-hydrolyse pathogens

-host defense (destruction of bacteria by white blood cells)

What is the function of ribosomes?

site of protein synthesis

What is the function of rough endoplasmic reticulum?

Synthesis and packaging of proteins

What is the function of the smooth endoplasmic reticulum?

lipid synthesis

What is the function of the cell wall?

A rigid, permeable layer that supports the cell and provides mechanical strength against osmotic pressure and to the plant as a whole

What is the function of the cell vacuole?

1) Helps to maintain pressure inside the cell and keep the cell rigid. This stops plants wilting.

2) Also involved in the isolation of unwanted chemicals inside the cell.

3) pigments to colour petals and attract pollinating insects

What is the structure of the nucleus?

1) nuclear envelope - double membrane continuous with ER, controls entry and exit of cell

2) nuclear pores - allow passage of larger molecules (mRNA) out of nucleus

3) chromosomes - consist of linear, protein-bound DNA

4) nucleolus - manufactures ribosomal RNA and assembles ribosomes

5) nucleoplasm - fluid like substance in which nucleolus and chromatin are suspended

What is the structure of the cell surface membrane?

Phospholipid bilayer

- Phospholipids

- Proteins - carrier/channel proteins, receptors

- Cholesterol - reduce fluidity

- Glycolipids and Glycoproteins - recognition sites

What is the structure of mitochondria?

1) Double membrane where the inner membrane is highly folded into extensions called cristae

2) Cristae to increase inner surface area for the attachment of enzymes for protein synthesis

3) Matrix makes up the remainder, contains many enzymes and even the mitochondria's own DNA and ribosomes

What is the structure of the chloroplasts?

1) Chloroplast envelope - double membrane

2) Grana - stacks of many disk-like thylakoids which contain chlorophyll, joined together by intergranal lamella, where 1st stage of photosynthesis takes place

3) Stroma - fluid filled matrix for 2'nd stage

How are chloroplasts adapted to their function?

1) Granal membranes - Provide a large surface area for attachment of chlorophyll and enzymes that carry out the first stage of photosynthesis

2) Fluid of stroma - Possesses all the enzymes needed to make sugars* in the second stage of photosynthesis

3) Contain both DNA and ribosomes - So they can quickly and easily manufacture some of the proteins needed for photosynthesis*

What is the structure of Golgi Apparatus?

A stack of membranes which make up flattened sacs known as cisternae, surrounded by hollow structures knows as vesicles.

What is the function of Golgi Vesicles?

Store and transport modified proteins and lipids from the Golgi apparatus to target cells

What is the structure of Lysosomes?

a membranous sac containing hydrolytic enzymes, isolating them from the rest of the cell

What is the structure of a ribosome?

large and small subunit each containing RNA and protein

What is the Structure of the Rough Endoplasmic Reticulum?

A system of folded membranes (cisternae) enclosing a fluid filled space covered in ribosomes

What is the structure of the Smooth Endoplasmic Reticulum?

A system of membranes enclosing a fluid filled space lacking ribosomes

What is the structure of the cell wall?

Consists of cellulose microfibrils embedded in a matrix

- middle lamella marks boundary between adjacent cells

Substance for plant and algae cell walls?

Cellulose

Substance for bacterial cell walls?

Murein

Substance for fungal cell walls?

Chitin

What is the structure of the cell vacuole?

contains cell sap and has a surrounding membrane called the tonoplast

What is cell specialisation?

The process in which cells develop in different ways to perform particular roles

- Each specialised cell has evolved more or fewer of certain organelles and structures to suit the role it carries out*

How does a cell become specialised?

All cells contain the genes needed for it to specialise into a specific cell yet only some of the genes are expressed (switched on) in any one cell, at any one time. Different genes are expressed in different specialised cells

What is a tissue?

A tissue is a group of similar cells that work together to carry out a particular function

What is an organ?

A group of different tissues that work together to perform a variety of functions, with one predominant major function

What is an organ system?

An organ system is a group of organs working together to carry out a particular function

What is a eukaryotic cell?

Cell with a nucleus and membrane bound organelles

What is a prokaryotic cell?

cell that does not have a nucleus or any membrane bound organelles

Structure of a bacterial cell

- circular strand of DNA

- plasmids

- 70S ribosomes in the cytoplasm of the cell.

- surrounded by a plasma membrane,

- a cell wall

- a slime capsule.

they sometimes have a flagellum

Function of bacterial cell wall

physical barrier that excludes certain substances and protects against mechanical damage and osmotic lysis

Function of bacterial capsule

protects cell from other cells (phagocytosis) and helps groups of bacteria stick together to perform a specific function

Function of the circular DNA strand

possesses the genetic info for the replication of cells

Function of plasmids

possesses genes that may aid survival of bacteria e.g. antibiotic resistance, which can be shared between bacteria via bacterial conjugation

Differences between prokaryotic cells and eukaryotic cells

1) Prokaryotic cells have no true nucleus with membrane

2) Prokaryotic cells may contain plasmids and capsule

3) Prokaryotic cells have no membrane bound organelles

4) Prokaryotic cells have much smaller ribosomes

5) Prokaryotic cells have murein cell walls

What is a virus?

a tiny, acellular, non-living particle that enters and reproduces inside a living cell

Structure of viruses

1) genetic material (nucleic acids)

2) reverse transcriptase enzyme

3) lipid envelope

4) attachment proteins (identification and attachment to host cells

5) capsid ( protein coat that encloses genetic info)

Why do electron microscopes have higher resolving power than light microscopes?

Electrons have a smaller wavelength than visible light, leading to higher resolution.

What is resolution?

The minimum distance apart that two objects can be in order for them to appear as separate items

What is magnification?

how many times larger an image is than when compared to the actual object

What is cell fractionation?

The process where cells are broken up and the different organelles they contain are separated out.

What must the conditions be before cell fractionation?

1) cold - reduces enzyme activity that may break down organelles

2) isotonic - to prevent organelles shrinking or bursting via osmotic loss or gain of water

3) buffered - to maintain a stable pH as changes in pH may denature proteins

What are the 2 stages of cell fractionation?

Homogenisation and ultracentrifugation

What is homogenisation?

Breaking up of cells in a blender. This releases the organelles from the cell and the resultant fluid is known as the homogenate.

The homogenate is then filtered to remove any unbroken cells and large debris

What is ultracentrifugation?

The process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge.

Ultracentrifugation process

1) tube of filtrate is placed in centrifuge and spun at low speed

2) heaviest organelles (nuclei) are forced to the bottom of the tube where they form a pellet

3) the supernatant is removed

4) supernatant transferred and spun at a higher speed

5) next heaviest organelles (chloroplasts/mitochondria) form pellet

How are electron microscopes focussed?

electromagnets

Limitations of electron microscopes

1) whole system must be in vacuum - specimen must be dead

2) complex staining process and black and white image

3) extremely thin specimen (TEM ONLY)

4) increased artefacts due to rigorous preparation process

Why can't the high resolving power of a TEM always be achieved?

1) difficulties preparing specimen limit the resolution that can be achieved

2) a higher energy electron beam is required which may destroy specimen

How to measure specimen in light microscopes?

eyepiece graticule

How to calibrate eyepiece graticule?

1) Use stage micrometer.

2) Focus microscope so that both eyepiece and micrometer are in clear view

3) Count how many stage micrometer units make up the whole length of the eyepiece graticule. (for stage micrometer each division is usually 10 micrometre)

4) calculate equivalence of ONE eyepiece graticule unit.

What is mitosis?

cell division in which a eukaryotic cell divides to produce 2 identical daughter cells, each with exact copies of DNA

What happens in interphase?

G1 - cell growth

S - DNA replication

G2 - preparation for mitosis

What happens in prophase?

1) a cell's genetic material condenses (chromosomes become visible)

2) centrioles move to opposite poles of cell

3) spindle fibres starts to form and attach centrioles to centromeres, drawing chromosome to equator

4) nuclear envelope breaks down and nucleolus disappears

What happens in metaphase?

1) chromosomes are observed as 2 sister chromatids joined by centromere

2) chromosomes are pulled along spindle apparatus to arrange themselves across equator of cell

What happens in anaphase?

1) centromeres divide into 2

2) spindle fibres pull individual chromatids which we now call chromosomes

What happens in telophase?

1) chromosomes reach their poles and become longer thinner, forming chromatin

2) spindle fibres disintegrate

3) nuclear envelope and nucleolus re-form

What is cytokinesis?

the cytoplasmic division of a cell at the end of mitosis, bringing about the separation into two daughter cells.

Cell division in prokaryotic cells

binary fission

Binary Fission process

1) circular DNA molecule replicates and attaches to cell membrane

2) plasmids replicate

3) cell membrane elongates and grows between DNA molecules, pinching inward to split the cytoplasm in 2

4) new cell wall forms and 2 identical daughter cells form - identical DNA but variable plasmid quantities

Replication of viruses

1) attaching to host cell with attachment proteins

2) inject nucleic acid into host

3) host cells begins to produce new viral components to be assembled into new viruses

What are 3 stages of cell cycle?

1) Interphase

2) Mitosis

3) Cytokinesis

What is cancer?

uncontrolled cell division by mitosis often due to damage of genes that regulate cell cycle

Why is mitosis important?

1. Growth

2. Repair

3. Reproduction

How do cancer drugs work?

1) preventing DNA from replicating

2) inhibiting the metaphase stage by interfering with spindle formation

However this also impacts other fast dividing cells e.g. hair producing cells

Functions of phospholipids in cell membranes

1) allow non-polar (lipid soluble) substances to move in and out of the cell

2) prevent polar (water soluble) substances entering and leaving cell

3) make membrane flexible and self sealing

What is the fluid mosaic model?

describes the arrangement of the molecules that make up a cell membrane

- fluid as phospholipids molecules can move relative to each other making it flexible

- mosaic as proteins are embedded into the bilayer with varying size and shape

Function of glyolipids and glycoproteins

recognition sites and helps cells attach to each other to form tissues

Function of cholersterol in plasma membrane

1) reduce lateral movement of other molecules, less flexible

2) make membrane less fluid at high temperatures

3) prevent leakage of water and dissolved ions as cholesterol is very hydrophobic

Functions of protein in plasma membrane

1) provide structural support

2) act as channel proteins

3) allow active transport via carrier proteins

4) cell surface receptors

5) receptors

6) helps cells adhere together

Define simple diffusion

The net movement of particles from an area of high concentration to low concentration, down a concentration gradient until particles are evenly distributed

What are limitations to simple diffusion across cell membrane?

particles that are large and/or polar have difficulty diffusing across cell membrane

Define facilitated diffusion

facilitated diffusion is the net movement of particles down their concentration gradient with the help of carrier molecules and channel proteins

What is a channel protein?

An intrinsic protein that forms a water-filled pore in the membrane, allowing specific polar molecules to diffuse through - when certain ions bind to the protein it will open

What is a carrier protein?

A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport it requires ATP.

Define osmosis

the passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane

Define active transport

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Describe the active transport of a single molecule/ion

1) carrier proteins bind to molecule

2) molecule binds to receptor sites

3) on the inside of cell, ATP binds to carrier protein causing it to split to ADP and Pi, protein molecule changes shape and opens to the interior

4) molecule is released into the cell

5) phosphate molecule is released and protein takes original shape - phosphate recombined with ADP in respiration to form ATP

Define co-transport and absorption of glucose in the ileum

1) sodium ions are actively transported out of the epithelial cells into the blood via a sodium-potassium pump protein carrier molecule

2) this maintains a much lower concentration of sodium ions in the epithelial cells compared to the lumen of the intestine

3) sodium ions diffuse down this concentration gradient into the epithelial cells through a co-transport protein - as sodium ions diffuse, they carry either amino acid or glucose molecules with them

4) the glucose or amino acids can then diffuse into the blood stream using a uniporter via facilitated diffusion

3 ways the rate of movement across membranes can be increased

1) increasing/maintaining concentration gradient

2) increasing surface area

3) increasing density of protein channels and carrier proteins

Define co-transport

a method of membrane transport where 2 substances are both transported across a membrane at the same time either in the same or opposite directions

What is immunity?

the ability of an organism to resist infection from invading disease causing microorganisms

- this may involve an immune response through the activation of lymphocytes

Types of lymphocytes

1) T lymphocyte cells

2) B lymphocyte cells

What are T lymphocytes?

associated with cell-mediated immunity and mature in the thymus gland

what are B lumphocytes?

associated with humoral immunity and mature in the bone marrow

What is an antigen?

specific molecules on a cells surface that identify it

- a foreign protein that stimulates an immune response

What do antigens allow the immune system to identify?

- pathogens

- cells from other organisms of the same species

- abnormal body cells

- toxins

Why does the immune system have a lag between exposure and response?

The specific lymphocyte to the pathogens antigen is stimulated to divide and build up its numbers to a level where it can be effective in destroying it

What are the 2 defence mechanisms?

non specific and specific

What is a non specific defence mechanism?

Response is immediate and the same for all pathogens e.g. stomach acid and skin barrier

What is a specific defence mechanism?

Response is slower and specific to each pathogen eg T and B lymphocytes

What is antigen variability?

the antigens on a pathogen develop variation that can lead to new strains arising in a population

Effect of antigen variability on disease and disease prevention

it is easier to develop vaccines and immunity to pathogens with little or no antigen variability

Phagocytosis process

1) chemical products from the pathogen act as attractants for the phagocyte to move towards along a concentration gradient

2) the phagocyte attaches to the surface of pathogen

3) phagocyte engulfs the pathogen to form a vesicle known as a phagosome

4) lysosomes move towards the phagosome and fuse with it forming a phagolysosome

5) enzymes called lysozymes are present within lysosome and destroy the ingested bacteria by hydrolysis of cell walls

6) small soluble products of breakdown are absorbed into the cytoplasm of phagocyte

T lymphocytes response to foreign antigen

1) pathogens invade body abs are ingested by phagocytes

2) phagocyte inserts pathogens antigens into its cell surface membrane to become an antigen-presenting cell

3) receptors on a specific helper T cell bind to these antigens

4) this attachment activated the helper T cell to divide by mitosis rapidly to form clones

5) these clones

- develop into memory cells for faster future response

- stimulate phagocytes to engulf more pathogens

- stimulates B cells to divide and secrete antibody

- activate cytotoxic T cells