B2

B2: Cells

• • • Cell structure and organisation
      • Cell structure
        • Cells vary in size & shape -> depends of function
        • All cells: membrane & cytoplasm
        • Some cells: nucleus
      • Cytoplasm
        • Thick liquid

90% water

Dissolved sugar & salt molecules

Larger molecules: fat & protein (ex. Enzymes)

Enzymes: some attached to membrane systems of cells, other float freely

Use: build cell structures (ex. Membrane)

Contains many particles

Food reserves

Oil droplets

Starch

Organelles

Specific function

Many chemical reactions

Keep cell alive

Provide energy

Make necessary substances

• • • • Cell membrane
        • Thin layer of cytoplasm
        • Around cell
        • Controls substance leaving & entering

Leave: waste products

Enter: food, water, oxygen

Kept out: harmful substances

• • • • • Maintains structure & chemical reactions of cytoplasm
      • Nucleus (plural: nuclei)
        • Most cells: 1 nucleus
        • Round
        • Enclosed in membrane
        • Embedded in cytoplasm
        • Function:

Controls type & quantity of enzymes produced in cytoplasm

Regulates chemical changes in cells

Decides type of cell it becomes

Blood cell, liver cell, muscle cell, nerve cell

Controls cell division

Contains chromosomes

• • • • Plant cells different to animal cells
        • Have cell wall

Outside of cell membrane

Has cellulose & other compounds

Non-living

Water & dissolved substances can pass

Not selective like cell membrane

• • • • Vacuole
        • Large
        • Fluid filled
        • Contains cell sap

Watery solution

Sugars, salts & sometimes pigments

• • • • • Pushes cytoplasm aside

Forms thin lining in cell wall

Pressure makes cell firm

• • • • • In animal cell: sometimes small vacuole -> particular job & not permanent
      • Chloroplast
        • Organelle with chlorophyll

• • • Levels of organisation
      • Specialisation of cells
        • Most cells become specialised

Do one particular job

Develop distinct shape

Special chemical changes take place in cytoplasm

• • • • • Change in shape + chemical reactions allow cell to do specific function
        • Ciliated cells

Where: lining of nose + windpipe

Function:

Cilia (tiny, hairlike projections) continuously flicker

Created stream of mucus

Carries dust + bacteria through bronchi + trachea, away from lungs

• • • • • Root hair cell

Function: absorb water + mineral salts from roots

Where: plant roots

Adaptation:

Hairlike projections between soil → offers large surface area

Cell membrane controls what goes in + out of cell

• • • • • Palisade mesophyll cells

Function: Light absorption + photosynthesis

Where: underneath upper epidermis of plant leaves

Adaptation:

Large surface area → absorbs more light

Many chloroplast → more photosynthesis

• • • • • Red blood cells

Function: Carry oxygen from lungs → cells

Where: circulatory system

Adaptations

Contains haemoglobin

No nucleus → more room to carry oxygen

Biconcave shape → larger surface area

Thin cell membrane → oxygen goes in + out easily

• • • • • Sperm cells

Function: carry genetic info to ovum for reproduction

Where: male

Adaptations

Nucleus contains genetic info (from father)

Long tail: for swimming

Acrosome: contains enzymes which eat through egg cells protective membrane

Mitochondria: Provides energy for swimming

• • • • • Egg cell

Function:

Be fertilised by sperm cell

Support growing baby until attached to womb

Where: Female ovaries

Adaptations

Nucleus has genetic information from mother

Large energy supply: sustain baby growth

Many mitochondria → supply energy for growth

• • • • Tissues:
        • Made up of hundreds of cells
        • Often single type
        • Cells in tissue have similar structure + function
        • Gives tissue specific function
        • Ex in animal: bone, nerve, muscle
        • Ex in plant: epidermis, xylem, pith
      • Organs:
        • Several tissues grouped together
        • Perform specific function
        • Ex in animal: stomach, lungs, intestine, brain
        • Ex in plant: root, stem, leaves
      • Organ Systems:
        • Group of organs work together to perform function
        • Ex in animal: circulatory system (heart + blood vessels), nervous system (brain, spinal cord, nerves)
        • Ex in plant: Shoot system (stem, leaves, buds)
      • Organisms:
        • Organs + systems working together
        • Produce independent plant + animal
    • Size of specimens
      • Calculating magnification
        • Lens marked x10 → image = 10x bigger than actual size
        • Ex. two lenses (x10 + x40) → total magnification: 10 x 40 = 400
        • Magnification = Observed size of the image (or drawing)/ actual size of the specimen
        • Units must be same
        • Actual size of the specimen = observed size of image or drawing/ magnification
    • Movement in + out of cells
      • Cells need food
        • oxidised or used to built cell structure
      • Need salt + water
        • used in chemical reactions
      • Need to get rid of carbon substances (ex. carbon dioxide)
        • Otherwise poisons cell
    • Diffusion
      • Diffusion: net movement of particles from region of higher concentration → region of lower concentration down a concentration gradient, as a result of random movement
      • Gas, liquid, dissolved substances constantly move + spread out to fill available space
      • Diffusion causes molecules to moved from higher → lower concentration until concentration = balanced
      • In cells small molecules (ex. water, carbon dioxide, oxygen) can pass through cell membrane easily
        • Therefore equal concentration in + out of cell
      • Aerobic respiration (cell uses oxygen) → oxygen concentration inside cell: decreases
        • Causes oxygen diffuse into cell
      • Tissue respiration: carbon dioxide concentration in cell increases
        • Carbon dioxide diffusion out of cell
    • Rates of diffusion
      • Molecules + ions in liquids + gases: move randomly using kinetic energy
      • Rate of diffusion depends on:
        • Temperature
        • Distance it has to diffuse
        • Difference in concentration inside + outside cell
        • Size of molecules/ ions
        • Surface area across which diffusion is occurring
      • Surface area:
        • Rate of diffusion into cell depends on cell’s surface areas
        • Microvilli ('free' surface membrane formed into hundreds of tiny projections): increase absorbing surface

Found in: kidney + intestine

• • • • • Shape of cell affects surface area
      • Temperature
        • Increase temperature → increase kinetic energy → molecules/ ions move faster → speeds up diffusion
      • Concentration gradient
        • Greater difference in concentration → faster it diffuses
        • Difference is called: concentration gradient/ diffusion gradient
      • Distance
        • All cell membranes = similar thickness
        • Plant cells vary in thickness + permeability
        • Thicker wall → slower rate of diffusion
        • Increased distance → slows down diffusion rate
    • Osmosis
      • Definition: net movement of water from region of higher water potential (a dilute solution) → region of lower water potential (a concentrated solution) through a partially permeable membrane
      • If dilute solution is separated from concentrated solution by partially permeable membrane, water diffuses across membrane from dilute → concentrated solution
      • Partially permeable membrane allows water to pass through more rapidly than dissolved substances
      • Diffusion gradient favours passage of water from dilute → concentrated solution
      • In living cells:
        • Cell membrane = partially permeable
        • Cytoplasm + vacuole have dissolved substances
        • Water usually diffuses into cell by osmosis if surrounded by weak solution (ex. fresh water)
        • If cell = surrounded by stronger solution (ex. sea water) → cell lose water by osmosis
      • Animal cells
        • Cell membrane = partially permeable → most substances dissolve in cytoplasm

Even if concentration of some substances inside cell = higher → may not be able to diffuse out of cell

• • • • • Water molecules move in + out of cell

If more on outside → will move in faster than out

Liquid outside cell does not have to be 100% water

Concentration of water outside has to be higher than inside

Water will diffuse by osmosis

Water in cell → makes it swell up → cell may burst

If cells surrounded by solution with lower concentration → water goes out of cell → cell shrinks

• • • • Plant cells
        • Cytoplasm + cell sap have salts, sugars + proteins

Reduce concentration of free water in cell

• • • • • Cell wall = freely permeable to water + dissolved substances, but cell membrane of cytoplasm = partially permeable
        • If plant cell = surrounded by solution with higher water concentration → water goes into cell → vacuole expands → presses on cytoplasm + cell wall (can’t stretch) → inflow of water = resisted by inelastic cell wall
        • When plant cell absorbed maximum amount of water by osmosis → become rigid because of water pressure outwards on cell wall

Stems + leaves = supported

• • • • • Cell loses water → no water pressure pressing outwards against cell wall

Stems + leaves = not supported

Plant becomes limp + wilts

• • • • How it works
        • Substance (ex. sugar) dissolves in water → sugar attracts some water molecules → stop them moving freely
        • Reduces concentration of water
      • Water potential
        • Water potential of solution: measure of whether it is likely to gain or lose water from other solution
        • Dilute solution (high proportion of free water molecules): higher water potential than concentrated solution

Water will flow from dilute → concentrated solution (high potential → low potential)

• • • • • Pure water: highest possible water potential

Water molecules will flow from any other solution

• • • • • When adjacent cells have sap with different water potentials → creates water potential gradient

Water flows from cell with higher water potential (more dilute solution) → cell with lower water potential (more concentrated solution)

• • • • Important of water potential + osmosis in uptake of water by plants
        • Plant cell with vacuole pushing out on cell wall = turgid

Vacuole exerts turgor pressure on inelastic cell wall

If all cells in leaf + stem = turgid → stem = firm + upright + leaves = straight

• • • • • If vacuole loses water → cells lose turgor → become flaccid

Leaves → limp

Stem → droop

Plant → wilts

• • • • Importance of water potential + osmosis in animal cells + tissue
        • Fluid which bathes cells in animals: needs same water potential as cell
        • Prevents net flow of water in + out of cell
        • If bathing fluid = higher water potential (weaker concentration) than cells → water moves into cell by osmosis → cell swells up
        • Animal cells don’t have cell wall + membrane = weak → water continues to enter → cell bursts (process = haemolysis in red blood cells)
        • During physical activity: body sweats to maintain temperature

If you don’t drink → body becomes dehydrated

Water loss from blood → plasma becomes more concentrated (water potantial decreases)

Water is drawn out of red blood cells → cells become plasmolysed

Surface area = reduced

Less effective in carrying oxygen

Shape = crenated

People use sport drink

Are isotonic (same water potential as body fluids)

Contain: water, salts, glucose

Replace lost water + salts

Provide energy