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A Level Biology > Topic 2 > Flashcards

Topic 2 Flashcards

1
Q

What is the function of the cell-surface membrane in eukaryotic cells?

A

The cell-surface membrane, also known as the plasma membrane,

controls the movement of substances in and out of the cell,

provides structural support,

and facilitates communication with other cells.

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2
Q

What is contained within the nucleus of a eukaryotic cell?

A

The nucleus contains chromosomes (which are protein-bound, linear DNA)

and one or more nucleoli.

The nucleoli are involved in ribosome production.

nucleur envelope

nucleur pores

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3
Q

structure and function of the mitrochondria

A

double membrane, inner fold is called cristae and contain the matrix

Mitochondria are the powerhouses of the cell,

generating ATP through cellular respiration,

which provides energy for various cellular processes.

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4
Q

What is the structure and function of chloroplasts and where are they found?

A

double membrane, contin thylakoid membranes which are stacked to form grana

grana linked together by lamallae

Chloroplasts are found in plants and algae and are responsible for photosynthesis,

converting light energy into chemical energy stored in glucose..

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5
Q

What is the structure and function of the Golgi apparatus and Golgi vesicles?

A

group of fluid filled flattened sacs

vesicles are often seen at the edges

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Golgi vesicles transport these modified molecules..

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6
Q

What do lysosomes do in eukaryotic cells?

A

Lysosomes are membrane-bound organelles that contain hydrolytic enzymes to break down waste materials, cellular debris, and foreign substances.

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7
Q

What is the structure and function of ribosomes in a eukaryotic cell?

A

small organell made o RNA and protiens

Ribosomes are responsible for protein synthesis by translating messenger RNA (mRNA) into polypeptide chains.

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8
Q

What is the difference between rough and smooth endoplasmic reticulum?

A

(both are made of a system of fluid filled membranes)

Rough endoplasmic reticulum (RER) has ribosomes attached and is involved in protein synthesis and modification,

while smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid synthesis and detoxification processes.

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9
Q

What is the structure and function of the cell vacuole in plant cells?

A

rigid structure made of cellulose in plants and chitin in fungi

The cell vacuole stores nutrients, waste products, and helps maintain turgor pressure, which keeps the plant cell rigid.

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10
Q

How do specialized cells contribute to the organization of complex multicellular organisms?

A

Specialized cells perform specific functions, and these cells are organized into tissues.

Tissues form organs, and organs are grouped into systems to carry out complex functions within the organism.

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11
Q

What are the main differences between prokaryotic and eukaryotic cells in terms of size and organelles?

A

Prokaryotic cells are much smaller than eukaryotic cells.

They lack membrane-bound organelles,

while eukaryotic cells have them.

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12
Q

What is a key feature of prokaryotic cell cytoplasm?

A

Prokaryotic cell cytoplasm lacks membrane-bound organelles,

unlike eukaryotic cells which have various organelles.

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13
Q

How do ribosomes in prokaryotic cells differ from those in eukaryotic cells?

A

Prokaryotic cells have smaller ribosomes compared to eukaryotic cells.

70s ribosomes in prokaryotes and 80s ribosomes in eukaryotes

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14
Q

What type of genetic material is found in prokaryotic cells?

A

Prokaryotic cells have a single circular DNA molecule that is free in the cytoplasm and not associated with proteins,

unlike the linear DNA found in eukaryotic cells.

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15
Q

What is the composition of the cell wall in prokaryotic cells?

A

The cell wall of prokaryotic cells contains murein, a glycoprotein,

which is different from the cell walls found in eukaryotic cells.

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16
Q

What additional features might be present in prokaryotic cells?

A

Prokaryotic cells may have one or more plasmids,

a capsule surrounding the cell, and one or more flagella.

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17
Q

What are viruses and the structure of viruses

A

viruses are acellular and non-living. They consist of

genetic material,

a capsid (protein coat), and often an attachment protein,

but do not have cellular structures or metabolic processes.

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18
Q

What are the principles and limitations of optical microscopes?

A

Principles: Optical microscopes use visible light and lenses to magnify objects, typically up to 1,000x.

Limitations: Limited resolution (~200 nm), cannot view sub-cellular structures clearly, lower magnification compared to electron microscopes.

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19
Q

What are the principles and limitations of transmission electron microscopes (TEM)?

A

Principles: TEMs use a beam of electrons transmitted through a specimen to form an image. They offer high magnification (up to 2 millionx) and high resolution (up to 0.1 nm).

Limitations: Specimens must be very thin and placed in a vacuum, complex preparation, and potential artifacts.

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20
Q

What are the principles and limitations of scanning electron microscopes (SEM)?

A

Principles: SEMs use a beam of electrons that scans the surface of a specimen, producing a detailed 3D image of the surface.

Limitations: Lower resolution compared to TEM (~10 nm), specimens must be coated with a conductive material, and cannot view internal structures.

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21
Q

How do you measure the size of an object viewed with an optical microscope?

A

Use a stage micrometer to calibrate the eyepiece graticule, then measure the object with the calibrated graticule.

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22
Q

What is the difference between magnification and resolution?

A

Magnification: How much larger an image appears compared to the actual object.

Resolution: The ability to distinguish two adjacent points as separate; a measure of image clarity.

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23
Q

What is the formula for magnification?

A

Magnification = Size of Image / Size of Real Object

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24
Q

How can iodine in potassium iodide solution be used in microscopy?

A

It can stain starch grains in plant cells, making them visible under a microscope.

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25
Q

outline the processes involved in cell fractionation

A

HOMOGENIATION - breaking up the cells in a blender which breaks plasma membrane and releases organnels into solution

FILTRATION - solution is filtered through gauze to get rid of big bits

ULTRACENTRIFUGATION - cell fragments put into tube and pun at increasing speeds after one is at the bottom

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26
Q

Do all cells in multicellular organisms retain the ability to divide?

A

No, not all cells in multicellular organisms retain the ability to divide.

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27
Q

what is the order of organneles that reaches the bottom

A

nuclei
mitrochondria
lysosomes
endoplasmitic reticulum
ribosomes

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28
Q

What is the cell cycle in eukaryotic cells that retain the ability to divide?

A

The cell cycle includes a series of stages that eukaryotic cells go through to divide and produce new cells.

It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

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29
Q

When does DNA replication occur during the cell cycle?

A

DNA replication occurs during the interphase of the cell cycle, specifically in the S (synthesis) phase.

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30
Q

What happens during mitosis?

A

During mitosis, a eukaryotic cell divides to produce two daughter cells,

each with identical copies of DNA that were produced by the parent cell during DNA replication.

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31
Q

Describe the behavior of chromosomes during interphase.

A

During interphase, chromosomes are not visible as distinct entities.

The DNA is in the form of loosely packed chromatin,

which allows for DNA replication and gene expression.

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32
Q

What happens during prophase of mitosis?

A

in prophase, chromosomes condense and become visible as distinct structures.

The nuclear membrane begins to disintegrate, and the spindle apparatus starts to form.

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33
Q

Describe the events of metaphase.

A

During metaphase, chromosomes align in the middle of the cell.

Spindle fibers attach to the centromeres of the chromosomes

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34
Q

What occurs during anaphase?

A

In anaphase, the centromeres split, and the spindle fibers pull the sister chromatids apart toward opposite poles of the cell.

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35
Q

What happens during telophase?

A

During telophase, the separated chromatids,

now individual chromosomes, arrive at the poles.

The nuclear membrane re-forms around each set of chromosomes, which de-condense back into chromatin

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36
Q

What is the role of spindle fibers during mitosis?

A

Spindle fibers, attached to centromeres, are crucial for the separation of chromatids.

They ensure that each daughter cell receives an identical set of chromosomes.

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37
Q

What is cytokinesis and when does it occur?

A

Cytokinesis is the division of the cytoplasm that usually occurs after mitosis, resulting in two new cells.

It typically starts during anaphase and is completed after telophase.

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38
Q

what happens when cells divide uncontrollably?

A

uncontrolled cell division can lead to the formation of tumours and cancers

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39
Q

What is the basic structure of all cell membranes in eukaryotes?

A

The basic structure of all cell membranes,

including cell-surface membranes and the membranes around cell organelles, is the same.

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40
Q

What are the key components of the fluid-mosaic model of membrane structure?

A

The key components are phospholipids, proteins, glycoproteins, and glycolipids.

Cholesterol may also be present to restrict the movement of other molecules.

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41
Q

What is simple diffusion and what limitations does it involve?

A

Simple diffusion is the movement of molecules across the phospholipid bilayer without the aid of proteins.

It is limited by the nature of the phospholipid bilayer.

42
Q

How does facilitated diffusion differ from simple diffusion?

A

Facilitated diffusion involves the use of carrier proteins and channel proteins to help molecules cross the membrane.

43
Q

What is osmosis and how is it explained?

A

Osmosis is the movement of water across a membrane, down a gradient explained in terms of water potential.

44
Q

What is active transport and what is its key requirement?

A

Active transport is the movement of molecules across a membrane against their concentration gradient, involving carrier proteins and requiring the hydrolysis of ATP.

45
Q

What is co-transport and where can it be illustrated in the human body?

A

Co-transport is the simultaneous transport of two substances across a membrane.

It is illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum.

46
Q

how is glucose absorbed into the blood

A

sodium ions are actively transported out of the ileum epithelium cells into the blood by the sodium-potassium pump

this causes the sodium ions to diffuse from the lumen of the ileum into the epithiliel cell down their conc gradient via co-transporter portions

the co-transporter protiens carries glucose into the cell with sodium increasing its concentration in the cell

glucose then diffuses out of the epitheili cell into the blood via protien channels

47
Q

How can cells be adapted for rapid transport across membranes?

A

Cells can increase the surface area of their membranes or increase the number of protein channels and carrier molecules to enhance rapid transport.

48
Q

Molecules on cell surfaces

A

Each type of cell has specific molecules on its surface that identify it.

These molecules, which include proteins, enable the immune system to identify:

  • Pathogens
  • Cells from other organisms of the same species
  • Abnormal body cells
    Toxins
49
Q

Definition of antigen

A

An antigen is any substance that the immune system can recognize and respond to.

Typically, antigens are foreign molecules that trigger an immune response.

50
Q

Effect of antigen variability on disease

A

antigens on surface of virus change regularly

memory cells will not recognice these antigens/

so vaccines need to be updated

51
Q

Phagocytosis of pathogens

A

Phagocytosis is the process by which phagocytes (a type of white blood cell) engulf and digest pathogens.

Forming a vesicle and fuses with lysosomes.

The ingested pathogens are then destroyed by enzymes called lysozymes.

52
Q

Cellular response to a foreign antigen

A

phagocytes engulf and digest pathogens and present antigens on surface

helper T cell binds to antigens which activates it

release chemicals that activate b - cells

53
Q

Role of antigen-presenting cells (APCs)

A

Antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells process and present antigens on their surface to T cells, initiating the cellular immune response.

54
Q

Role of helper T cells (TH cells)

A

Helper T cells (TH cells) play a critical role in the immune response by stimulating:

Cytotoxic T cells (TC cells) to kill infected cells

B cells to produce antibodies

Phagocytes to engulf pathogens

55
Q

Humoral response to a foreign antigen

A

The response of B lymphocytes to a foreign antigen involves clonal selection and the production of monoclonal antibodies.

B cells differentiate into plasma cells that produce antibodies and memory cells that provide long-term immunity.

56
Q

Definition of antibody

A

An antibody is a protein produced by B cells that specifically binds to a particular antigen,

aiding in its neutralization or destruction.

57
Q

Antibody structure

A

Antibodies are Y-shaped molecules consisting of four polypeptide chains: two heavy chains and two light chains.

Each antibody has a variable region that binds to a specific antigen and a constant region that determines the antibody’s class and function

58
Q

Antigen-antibody complex formation

A

The formation of an antigen-antibody complex can lead to the destruction of the antigen through:

Agglutination: Clumping of pathogens, making them easier to phagocytose.

Phagocytosis: Engulfing and digesting the clumped pathogens by phagocytes.

59
Q

Role of plasma cells and memory cells

A

Plasma cells are B cells that produce and secrete large quantities of antibodies during the primary immune response.

Memory cells are long-lived B cells that provide a rapid and robust response upon subsequent exposure to the same antigen, facilitating the secondary immune response.

60
Q

Use of vaccines

A

Vaccines stimulate the immune system to produce a protective response against specific pathogens,

providing immunity to individuals and contributing to herd immunity,

which helps protect the entire population by reducing the spread of disease.

61
Q

Concept of herd immunity

A

Herd immunity occurs when a significant portion of a population becomes immune to a disease,

thereby providing indirect protection to those who are not immune,

by reducing the overall occurrence of the disease.

62
Q

Differences between active and passive immunity

A

Active immunity: The body produces its own antibodies in response to an infection or vaccination.
It provides long-term protection.

memory cells produces

takes while for protection to develop
requires exposure to antigen

Passive immunity: The body receives antibodies from another source,
such as maternal antibodies or antibody injections.

It provides short-term protection.

no memory cells

protection is intermediate
no exposure to antigen

63
Q

Structure of HIV

A

The human immunodeficiency virus (HIV) has a complex structure that includes:

An outer lipid envelope with embedded glycoproteins (gp120 and gp41)

An inner protein capsid containing two single strands of RNA

Enzymes like reverse transcriptase, integrase, and protease

64
Q

Replication of HIV in helper T cells

A

HIV replication in helper T cells involves several steps:

Attachment and fusion: HIV binds to receptors and co-receptors on helper T cells.

Reverse transcription: Viral RNA is reverse-transcribed into DNA by reverse transcriptase.

Integration: Viral DNA integrates into the host cell’s DNA
Transcription and translation: The host cell machinery produces viral RNA and proteins.

Assembly and budding: New viral particles are assembled and bud off from the host cell, destroying it in the process.

65
Q

How HIV causes the symptoms of AIDS

A

HIV causes AIDS by progressively depleting helper T cells weakening the immune system.

This leads to increased susceptibility to opportunistic infections and certain cancers, which are the primary symptoms of AIDS.

66
Q

Why antibiotics are ineffective against viruses

A

Antibiotics target specific bacterial structures or functions (like cell wall synthesis or protein synthesis) that viruses do not possess.

Viruses use host cell machinery for replication, making antibiotics ineffective against them.

67
Q

Use of monoclonal antibodies in targeting medication

A

Monoclonal antibodies can be designed to specifically target certain cell types.

By attaching a therapeutic drug to these antibodies, the medication is delivered directly to the targeted cells,

enhancing treatment efficacy and reducing side effects.

68
Q

Use of monoclonal antibodies in medical diagnosis

A

Monoclonal antibodies are used in various diagnostic tests to detect the presence of specific antigens.

Examples include pregnancy tests and tests for certain infections or diseases,

where antibodies bind to and indicate the presence of the target antigen.

69
Q

Ethical issues associated with vaccines and monoclonal antibodies

A

Ethical issues include:

Informed consent and transparency in clinical trials

Accessibility and distribution, ensuring equitable access for all populations

Potential side effects and long-term health impacts

The use of animals in research and production

70
Q

Use of antibodies in the ELISA test

A

the ELISA (Enzyme-Linked Immunosorbent Assay) test uses antibodies to detect the presence of specific antigens in a sample. It involves:

Binding of the antigen to a surface

Addition of a specific antibody linked
to an enzyme

Addition of a substrate that the enzyme

can convert to a detectable signal (usually a color change)

71
Q

how do specialized cells adapt to increase the rate of transport across their internal and external membranes?

A

specialized cells may have increased surface area,

a higher number of channel or carrier proteins,

and maintain steep gradients of concentration or water potential to facilitate faster transport.

72
Q

How does surface area affect the rate of movement across cell membranes?

A

An increased surface area allows for more space for molecules to pass through the membrane,

thereby increasing the rate of transport.

73
Q

How do the number of channel or carrier proteins influence the rate of transport across membranes?

A

A higher number of channel or carrier proteins provides more pathways for molecules to cross the membrane,

enhancing the rate of facilitated diffusion and active transport.

74
Q

How does the gradient of concentration affect the rate of movement across cell membranes?

A

A steeper concentration gradient increases the rate of diffusion,

as molecules move more rapidly from areas of higher concentration to areas of lower concentration.

75
Q

How does the gradient of water potential affect the rate of osmosis across cell membranes?

A

A steeper water potential gradient results in a faster rate of osmosis,

as water moves from areas of higher water potential (less solute) to areas of lower water potential (more solute).

76
Q

Contrast how an optical microscope and a transmission electron microscope work

and contrast the limitations of their use when studying cells.

A
  1. TEM use electrons and optical use light;
  2. TEM allows a greater resolution;
  3. (So with TEM) smaller organelles/named cell structure can be observed
    OR
    greater detail in organelles/named cell structure can be observed;
  4. TEM view only dead/dehydrated specimens and optical (can) view live specimens;
  5. TEM does not show colour and optical (can);
  6. TEM requires thinner specimens;
  7. TEM requires a more complex/time consuming preparation;
  8. TEM focuses using magnets and optical uses (glass) lenses;
77
Q

Describe how phagocytosis of a virus leads to presentation of its antigens. (3)

A
  1. Phagosome/vesicle fuses with lysosome;
  2. (Virus) destroyed by lysozymes/hydrolytic enzymes;
  3. Peptides/antigen (from virus) are displayed on the cell membrane;
78
Q

Describe how presentation of a virus antigen leads to the secretion of an antibody against a virus antigen. (3)

A
  1. Helper T cell/TH cell binds to the antigen (on the antigen-presenting cell/phagocyte);
  2. This helper T/TH cell stimulates a specific B cell;
  3. B cell clones
    OR
    B cell divides by mitosis;
  4. (Forms) plasma cells that release antibodies;
79
Q

Describe how bacteria divide. (2)

A
  1. Binary fission;
  2. Replication of (circular) DNA;
  3. Division of cytoplasm to produce 2 daughter cells;
  4. Each with single copy of (circular) DNA;
80
Q

How can high absorption of salt from the diet lead to a build-up of tissue fluid?

A
  1. (Higher salt) results in lower water potential of tissue fluid;
  2. (So) less water returns to capillary by osmosis (at venule end);
    OR
  3. (Higher salt) results in higher blood pressure/volume;
  4. (So) more fluid pushed/forced out (at arteriole end) of capillary;
81
Q

Describe the role of a ribosome in the production of a polypeptide. Do not include
transcription in your answer.

A
  1. mRNA binds to ribosome;
  2. Idea of two codons/binding sites;
  3. (Allows) tRNA with anticodons to bind/associate;
  4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);
  5. Moves along (mRNA to the next codon)/translocation described;
82
Q

Cells with BrdU in their DNA are detected using an anti-BrdU antibody with an enzyme attached.

Use your knowledge of the ELISA test to suggest and explain how the scientists identified the cells that have BrdU in their DNA.
[3 marks]

A
  1. Add antibody (anti-BrdU with enzyme attached) to cells/DNA
    OR
    Add cells/DNA to antibody (anti-BrdU with enzyme attached);
  2. Wash (cells/DNA) to remove excess/unattached antibody
    OR
    Wash (immobilised antibody) to remove
    excess/unattached cells/DNA;
  3. Add substrate to cause colour change;
83
Q

Describe the structure of the human immunodeficiency virus (HIV).

A
  1. RNA (as genetic material);
  2. Reverse transcriptase;
  3. (Protein) capsomeres/capsid; 4. (Phospho)lipid (viral) envelope
    OR
    Envelope made of membrane; 5. Attachment proteins;
84
Q

Describe and explain two features you would expect to find in a cell specialised for
absorption. (2)

A

. Folded membrane/microvilli so large surface area (for absorption);

  1. Large number of co-transport/carrier/channel proteins so fast rate (of absorption)
    OR
    Large number of co-transport/carrier proteins
    for active transport
    OR
    Large number of co-transport/carrier/channel proteins for facilitated diffusion;
  2. Large number of mitochondria so make (more) ATP (by respiration)
    OR
    Large number of mitochondria for aerobic respiration
    OR
    Large number of mitochondria to release energy for active transport;
  3. Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption);
85
Q

Name and describe five ways substances can move across the cell-surface membrane into a cell. (5)

A
  1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
  2. Facilitated diffusion down a concentration gradient via protein carrier/channel;
  3. Osmosis of water down a water potential gradient;
  4. Active transport against a concentration gradient via protein carrier using ATP;
  5. Co-transport of 2 different substances using a carrier protein;
86
Q

Use your knowledge of phagocytosis to describe how an ADC enters and kills the tumour cell.
[3 marks]

A
  1. Cell ingests/engulfs the antibody/ADC
    OR
    Cell membrane surrounds the antibody/ADC (to take it inside the cell);
  2. Lysosomes fuse with vesicle/phagosome (containing ADC);
  3. Lysozymes breakdown/digest the antibody/ADC to release the drug;
87
Q

Describe how a sample of chloroplasts could be isolated from leaves. [4 marks]

A
  1. Break open cells/tissue and filter OR
    Grind/blend cells/tissue/leaves and filter;
  2. In cold, same water potential/concentration, pH
    controlled solution;
  3. Centrifuge/spin and remove nuclei/cell debris;
  4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;
88
Q

Describe viral replication.
[3 marks]

A
  1. Attachment proteins attach to receptors;
  2. (Viral) nucleic acid enters cell;
  3. Nucleic acid replicated in cell
    OR
    Reverse transcriptase makes DNA from RNA;
  4. Cell produces (viral) protein/capsid/enzymes;
  5. Virus assembled and released (from cell);
89
Q

Explain how the use of antibiotics has led to antibiotic-resistant strains of bacteria becoming a common cause of infection acquired when in hospital. (3)

A
  1. (Some bacteria have) alleles for resistance;
  2. (Exposure to) antibiotics is the selection pressure
    OR
    Non-resistant bacteria die
    OR
    Resistant bacteria survive/reproduce;
  3. More antibiotics used in hospital (compared with elsewhere)
    OR
    Patients have weakened immune systems
    OR
    (So) high frequency of resistance allele (in bacterial population);
90
Q

Describe the structure and function of the nucleus.
[4 marks]

A

STRUCTURE

  1. Nuclear envelope and pores OR
    Double membrane and pores;
  2. Chromosomes/chromatin
    OR
    DNA with histones;
  3. Nucleolus/nucleoli;

FUNCTION

  1. (Holds/stores) genetic information/material for polypeptides (production)
    OR
    (Is) code for polypeptides;
  2. DNA replication (occurs);
  3. Production of mRNA/tRNA
    OR
    Transcription (occurs);
  4. Production of rRNA/ribosomes;
91
Q

Give one reason why antibiotics are not effective against viruses.
[1 mark]

A

Do not have bacterial structures/enzymes
OR
Do not have metabolic processes
OR
Do not have a cell wall/murein;

92
Q

Explain why viruses are described as acellular and non-living. (2)

A
  1. (Acellular) no cell(-surface) membrane
    OR
    Not made of cells;
  2. (Non-living) have no metabolism/metabolic reactions;
    OR
    Cannot (independently) move/respire/replicate/ excrete
    OR
    (Have) no nutrition
93
Q

Give the three structural features found in all virus particles and describe the function
of one of these features. (2)

A
  1. Genetic material, capsid and attachment protein;
  2. Genetic material codes for (viral) protein
    OR
    Capsid protects the genetic material/RNA/DNA
    OR
    Attachment protein bind to receptors (on cell);
94
Q

Describe how the student could use an eyepiece graticule to determine the mean
diameter of stomata. (3)

A
  1. Measure (each stoma) using eyepiece graticule;
  2. Calibrate eyepiece graticule against stage micrometer / ruler / graph paper;
  3. Take a number of measurements (to calculate a mean);
95
Q

Describe how the scientists could obtain data to produce a calibration curve and how they would use the calibration curve to find the concentration of protein in a
sample of blood plasma.
[3 marks]

A
  1. Produce known concentrations of protein;
  2. Measure absorbance of each concentration
    OR
    Measure each concentration with colorimeter;
  3. Plot a graph of absorbance on y-axis against concentration (on x-axis) and draw curve;
  4. Use absorbance of sample to find protein concentration from curve;
96
Q

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5)

A

COMPARISONS

1.Nucleotide structure is identical;

2.Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);

3.DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;

CONTRASTS

  1. Eukaryotic DNA is longer;
  2. Eukaryotic DNA contain introns, prokaryotic
    DNA does not;
  3. Eukaryotic DNA is linear, prokaryotic DNA is
    circular;
  4. Eukaryotic DNA is associated with / bound to
    protein / histones, prokaryotic DNA is not;
97
Q

A student used a dilution series to investigate the number of cells present in a liquid culture of bacteria.
Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000
dilution of the original liquid culture of bacteria. (3)

A
  1. Add 1 part (bacteria) culture to 9 parts (sterile) liquid (to make 101 dilution);
  2. Mix (well);
  3. Repeat using 9 parts fresh (sterile) liquid and 1
    part of 101 and 102 dilutions to make 103 dilution;
    OR
    Add 1 part 101 (suspension) to 99 parts (sterile) liquid (to make 103 dilution
98
Q

Explain why the student:
1. used only the first 5 mm from the tip of an onion root.
[1 mark]

A
  1. Where dividing cells are found / mitosis occurs;
    OR
    No dividing cells / mitosis in tissue further away / more than 5mm from tip;
    OR
    To get (soft) tissue that will squash;
    OR
    Length that will fit under cover slip;
99
Q

explain why the student pressed down firmly on the cover slip.

A

Single / thin layer of cells / spread out cells so light passes through (making cells / nuclei visible);

100
Q

Give two similarities in the movement of substances by diffusion and by osmosis.
[2 marks]

A
  1. (Movement) down a gradient / from high concentration to low concentration;
  2. Passive / not active processes;
    OR
    Do not use energy from respiration / from ATP / from metabolism;
    OR
    Use energy from the solution;
101
Q

The movement of substances across cell membranes is affected by membrane
structure. Describe how.
[5 marks]

A
  1. Phospholipid (bilayer) allows movement/diffusion of non- polar/lipid-soluble substances;
  2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR
    (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
  3. Carrier proteins allow active transport;
  4. Channel/carrier proteins allow facilitated diffusion/co-transport;
  5. Shape/charge of channel / carrier determines which substances move;
  6. Number of channels/carriers determines how much movement;
  7. Membrane surface area determines how much diffusion/movement;
  8. Cholesterol affects fluidity/rigidity/permeability;
102
Q
A

A Level Biology (12 decks)

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