Biology (Cambridge 0610 IGCSE) Flashcards

Question

Protein synthesis

Answer 1

Protein synthesis: code is copied by mRNA (transcription), taken by ribosomes which assemble amino acids into polypetides then proteins (translation) that are then folded into shape. Mutations can result in wrong proteins being synthesised.

Answer 2

Characteristics are determined by the type and quatity of proteins synthesised. Some are controlled by one gene; however most are a result of two or more genes interacting. Different versions of the same gene are called ALLELES. DOMINANT Alleles e.g 'B' are expressed even when the genotype contains a RECESSIVE Allele e.g 'b'. There must be no dominant allele present in order for a recessive allele to be expressed in the phenotype. BB and bb are HOMOZYGOUS alleles, where as Bb is HETEROZYGOUS.

Answer 3

Organims are considered to be of the same species if they can produce fertile offspring.

Answer 4

Breeding organims that have desired traits/characteristics to produce offspring in which they are more pronounced.

Answer 5

The insertion of a gene into an organism's genome so it synthesises a specific protein to acheive a desired trait/characterisitic EXAMPLES: - Insulin-producing bacteria - Creating GM (genetically modified) crops e.g golden rice which produces vitamins A - disease-resisting crops

Answer 6

1. Desired gene is cut from another organism's DNA using an enzyme (Restriction enzymes) 2. Gene inserted into a vector e.g bacteria, plasmid or virus. by pasting the cut DNA fragment into the vector using enzymes (Ligase enzymes.) 3. Vectors inserts gene into cells of another organisms in development. 4. Organism develops with desired trait. due to every cell synthesising that specific protein.

Answer 7

LINNAEUS'S TAXONOMY - KINGDOM - KING - PHYLLUM - PHILLIP - CLASS - CAME - ORDER - OVER - FAMILY - FOR - GENUS - GOOD - SPECIES - SOUP Binomial name = GENUS + SPECIES DOMAIN is a level above KINGDOM which consists of: - ARCHEA (Primitive bacteria) - BACTERIA (true bacteria) - EUKARYOTA (everything else - DNA contained in a nucleus).

Answer 8

Diffusion is the movement of particles from an area of HIGH CONCENTRATION to that of a LOW CONCENTRATION ("down a concentration GRADIENT"). This is PASSIVE as it requires no energy. The rate can be increased by increasing: - Surface area - The difference in concentration - Temperature.

Answer 9

Osmosis is the diffusion of WATER across a SEMI-PERMEABLE MEMBRANE to balance the concentrations of soluable inside and outside a cell. Water must move, as larger molecules cannont fit through the holes in the membrane. Water moves IN if the concentration is higher OUTSIDE

Answer 10

- Weigh, and place identical cylinders from the same vegatable (usually potatoes) in sugar solutions of varying concentrations - After a set time, remove excess water (from both the cylinder and the potato) and reweigh, calculate: % change in mass = final mass - intial mass ÷ initial mass x 100.

Answer 11

Active transport is the movement of particles through a membrane via CARRIER PROTEINS. This requires energy, and so can move them AGAINST the concentration gradient.

Answer 12

LEAF: - Photosynthesis, gas exchange - Water evaporates out Xylem: - Continuous tubes that carry water & dissolved mineral ions upwards: TRANSPIRATION (unidirectional) rate increased with increased AIR MOVEMENT or TEMPERATURE & decreased with HUMIDITY Roots: - Water enter through osmosis, minerals ions through active transport. Flower: - Reproductive organs. Phloem: - Tubes of cells that carry sugars (e.g sucrose) and other nutrients to where needed: TRANSLOCATION - bidirectional.

Answer 13

(in decending order of the leaf structure) WAXY CUTICLE - Waterproof to stop water loss from to of leaf. UPPER EPIDERMIS - Transparant to let light through. PALISADE MESOPHYLL - Most photosynthesis takes place here (most chloroplasts) SPONGEY MESOPHYLL - Gaps to facilitate gas exchange (large surface area) VEING/VASCULAR BUNDLE - Xylem & Phloem LOWER EPIDERMIS - Bottom of leaf: STOMATA - Holes in lower epidermis to allow gases in/out, including water GUARD CELLS - chage size to control rate of gases entering/exiting through stomata.

Answer 14

Nitrates are needed for protein synthesis. Deficiency: stunted growth. Chlorosis: Magnesium deficiency: yellow leaves, stunted growth as less chlorophyll can be produced.

Answer 15

Water + carbon dioxide -> glucose + oxygen 6H₂O + 6CO₂ -> C₆H₁₂O₆ + 6O₂ GLUCOSE is used for: - Respiration - Making STARCH/FAT to store energy - Making CELLULOSE - Making AMINO ACIDS for proteins RATES INCREASED BY: - Increasing temperature - Increasing light intensity - Increasing CO₂ concentration (Any one of these can be a limiting factor)

Answer 16

Cellulose is a strong carbohydrate found in the cell walls of leaf cells. It provides structure and support to keep the leaf firm and functional.

Answer 17

DOUBLE CIRCULATORY SYSTEM: blood enters the heart twice every time it is pumped around body. DEOXYGENATED blood enters in the right side of the heart, and it enters through the VENA CAVA (the main vein) into the RIGHT ATRIUM. The valve (between the right atrium and right ventricle) prevents backflow, preventing the deoxygenated blood going back into the body. The heart muscles contract and it goes through the PULMONARY ARTERY (to the lungs) to be oxygenated, which then comes back to the heart through the PULMONARY VEIN (from lungs) into the LEFT ATRIUM, then into the LEFT VENTRICLE and finally out to the body through the AORTA -The left side has thicker walls due to the higher pressure needed to pump blood to the body-

Answer 18

ARTERY - Carries blood away from heart - Thick walls & thin lumen (hole in the middle) to withstand high pressure VEIN - Carries blood back to the heart - Thin walls & wide lumen, valves to prevent backflow (can be replaced with artifical ones if faulty) CAPILLARY - One cell thick to allow fast diffusion between blood and cells. CORONARY ARTERY - supplies blood to the heart itself. RENAL ARTERY - It supplies the kidneys with blood rich in oxygen and waste products like urea and excess salts. - The kidneys filter this blood, removing wastes to form urine. - Each kidney has its own renal artery. - Renal artery = brings unfiltered, oxygenated blood to the kidneys. Blood carries RBC, WBC & platelets (clotting) everything except oxygen is dissolved in the PLASMA.

Answer 19

when the artery supplying blood to the heart becomes blocked.

Answer 20

TRACHEA – Carries air from the nose/throat to the lungs. – Has rings of cartilage to keep it open. BRONCHI (singular BRONCHUS) – Two tubes branching from the trachea into each lung. – Carry air deeper into the lungs. BRONCHIOLES – Smaller branches of the bronchi. – Distribute air throughout the lungs. ALVEOLI – Tiny air sacs where gas exchange happens. – Oxygen enters the blood, carbon dioxide leaves. DIAPHRAGM – A muscle under the lungs. – Helps in breathing by moving up and down to change lung pressure. INTERCOSTAL MUSCLES – Found between the ribs. – Help expand and contract the chest during breathing.

Answer 21

- large surface area - rich blood supply - Thin walls (1 cell thick) = Short diffusion distance

Answer 22

OXYGEN diffuses into bloodstream and binds to the HAEMOGLOBIN in RED BLOOD CELLS to be transported.

Answer 23

- AEROBIC RESPIRATION: glucose + oxygen -> water + carbon dioxide C₆H₁₂O₆ + 6O₂ -> 6H₂O + 6CO₂ - ANAEROBIC RESPIRATION: glucose -> lactic acid Occurs during intense exercise: oxygen debt. Lactic acid is later broken back down into glucose in the LIVER using oxygen

Answer 24

The balanced chemical equation for anaerobic respiration in yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ The word equation for anaerobic respiration in yeast: glucose → alcohol + carbon dioxide

Answer 25

- Heart & breathing rate increase to increased blood flow to supply more oxygen & glucose to enable muscles to contract.

Answer 26

- chemical reactions - warmth - movement

Answer 27

The sum of all chemical reactions in a cell/otganism e.g. - Glucose -> starch, glycogen & cellulose - fatty acids -> lipids (fats) - Respiration - Breakdown of excess proteins -> urea.

Answer 28

Show the direction of BIOMASS transfer between organisms. PRODUCERS - Any organism that uses SUNLIGHT to produce BIOMASS (e.g plants, algae) PRIMARY CONSUMER - Eats producers, herbivore/omnivore SECONDARY CONSUMERS - Eats primary consumers carnivore/omnivore (Keeps going: TERTIARY, QUATERNARY) Organism exist in TROPHIC LEVELS (shown above).

Answer 29

HERBIVORES - Definition: Animals that only eat plants. -Examples: Cows, rabbits, giraffes. - Role: They are primary consumers in the food chain — they eat producers (plants). CARNIVORES - Definition: Animals that only eat other animals (meat). - Examples: Lions, eagles, sharks. - Role: They can be secondary or tertiary consumers — they eat herbivores or other carnivores. DECOMPOSERS - Definition: Organisms that break down dead plants, animals, and waste. - Examples: Fungi, bacteria, some worms. - Role: They recycle nutrients back into the soil, helping plants grow again. They are essential for the nutrient cycle.

Answer 30

Energy loss at each level – Only about 10% of energy is passed on from one trophic level to the next. (through things like respiration, or not being able to eat bones/hair). – By the time you reach the 5th consumer, very little energy is left to support it. Less food available – Each level supports fewer organisms because of the energy loss. – It's hard for a 5 or 4th-level consumer to find enough food to survive. Shorter food chains are more stable – Most ecosystems can only support 3–4 levels of consumers. – Longer chains are unstable and less efficient.

Answer 31

The atmosphere is 78% nitrogen. it can be used to make nitrates which are crucial for protein synthesis in organisms. 1. Nitrogen converted into nitrates by NITROGEN FIXING BACTERIA (FIXATION) 2. Other bacteria & fungi convert protein into ammonia during decomposition (AMMONIFICATION) 3. Ammonia converted into nitrates by NITRIFYING BACTERIA (NITRIFICIATION).

Answer 32

1. Photosynthesis: Plants absorb carbon dioxide (CO₂) from the air and use it to make carbohydrates during photosynthesis. This stores carbon in plant biomass. 2. Respiration: Plants, animals, and decomposers release CO₂ back into the air through respiration as they break down carbohydrates for energy. 3. Feeding: When animals eat plants (or other animals), carbon in the form of organic compounds is transferred through the food chain. 4. Decomposition: When organisms die, decomposers (like bacteria and fungi) break down their bodies. This releases CO₂ back into the air through respiration. 5. Formation of Fossil Fuels: Some dead organisms are buried in conditions without oxygen. Over millions of years, their carbon-rich remains form fossil fuels like coal, oil, and natural gas. 6. Combustion: Burning fossil fuels releases stored carbon back into the atmosphere as CO₂

Answer 33

Eutrophication is the process where water bodies become overly enriched with nutrients (like nitrates and phosphates), often from fertilizers. This leads to excessive growth of algae and other problems. Steps of Eutrophication: 1. Excess nutrients enter water (e.g., from fertilizer runoff). 2. Algae grow rapidly (algal bloom), blocking sunlight. 3. Aquatic plants die due to lack of light. 4. Decomposers break down dead plants, using up oxygen. 5. Oxygen levels drop (hypoxia). 6. Aquatic animals die due to lack of oxygen.

Answer 34

Assimilation is the process where digested food molecules are absorbed into body cells and used for energy, growth, and repair. For example, glucose is used in respiration to release energy, and amino acids are used to build proteins. This occurs after digestion and absorption, with the liver and body cells playing key roles.

Answer 35

STEM cells can DIFFERENTIATE or ‘specialise’ to preform specific FUNCTIONS. They are found in animal EMBRYOS and plant MERISTEMS. All cells in the body start the same. These are called UNSPECIALISED CELLS. As the organism develops, genes in the cells switch on or off. This causes the cells to change shape and develop special structures to do a specific job. This process is called DIFFERENTIATION. For example: • A cell that becomes long and thin with connections becomes a nerve cell. • A cell that has no nucleus and lots of haemoglobin becomes a red blood cell.

Answer 36

Magnification = image size —————- Object (cell) size Convert μm to mm by dividing by 1000

Answer 37

- STARCH: turns IODINE from ORANGE to BLACK - PROTEIN: turns BIURET’S REAGENT from BLUE to PURPLE - SUGARS: turn BENEDICT’S SOLUTION from BLUE to GREEN/YELLOW to ORANGE to BRICK RED.

Answer 38

- PATHOGEN: a micro-organism that causes disease: BACTERIA, VIRUSES, FUNGI or PROTOCISTS - VIRUSES: some infect and reproduce in other cells by replicating their DNA and protein coat (lytic pathway). While others (phages) reproduce by inserting DNA into cells which amuses them to make more copies (lysogenic pathway). Cells bursts relapsing them. E.g HIV: destroys WBC, leads to AIDS. STI/STD (sexually transmitted) - BACTERIA releases TOXINS into your body that damage cells. E.g CHOLERA: causes diarrhoea. Vector: water - FUNGI damage cells. Fungi can grow on or inside the body. They release enzymes to break down and damage cells. They may also produce toxins (poisons) that harm the host’s tissues. Fungal pathogens damage cells by digesting them for nutrients. - PROTISTS Protists often enter the body through insect bites or contaminated water. Once inside, they invade cells and multiply, damaging or destroying them. Some also release toxins that harm tissues. Protists can live inside cells, using them for food and reproduction.

Answer 39

Skin stops, mucus in nose/trachea traps, acid & enzymes kill pathogens. Plants: cell wall, waxy cuticle & bark are barriers: antibacterial chemicals: poison/thorns to deter other organism. - LYMPHOCYTES (a type of white blood cell) produce ANTITOXINS to neutralise toxins made by pathogens, and ANTIBODIES that bind to the ANTIGEN of a pathogen, but only if they fit. These stop viruses from infecting cells, and cause them to clump together. Once the correct antibody is found/made, T cells store it in your lymph nodes, ready for next time (IMMUNITY). - PHAGOCYTES (another white blood cell) will then INGEST these pathogens. A VACCINE introduces a DEAD/INERT version of a virus into your body, so you can gain immunity. Without you becoming ill.

Answer 40

ORGANELLES: sub cellular structures. - EUKARYOTIC CELLS. Have a nucleus where there DNA is found. They also have thing like: Cell membrane (semi-permeable, controls what enters & leaves). Plant cells have Cell walls (made out of cellulose) they also have chloroplasts (made from chlorophyll and used in photosynthesis). Both cells contain Ribosomes (used in protein synthesis) and mitrchondria (power house, respiration) e.g ANIMAL and PLANT - PROKARYOTIC CELLS. Don’t have a nucleus, instead its in the cytoplasm, or much organelle at all.

Answer 41

Mitochondria are tiny parts inside cells. They are often called the “powerhouses” of the cell. Function: They release energy by a process called aerobic respiration (using oxygen). Role: The energy released is used for important cell activities, like: • movement • growth • repair • keeping warm (in animals)

Answer 42

https://images.app.goo.gl/ma3bCKjNaPYTQevr6 1. Flower structure Insect-pollinated flowers have parts like: • Sepals (protect the bud) • Petals (bright to attract insects) • Stamens (male parts: filament + anther) • Carpels (female parts: stigma, style, ovary with ovules) Wind-pollinated flowers have small petals and long stamens/stigmas to catch or release pollen. 2. Pollination Pollination is when pollen moves from the anther (male) to the stigma (female). • Self-pollination: Pollen from the same plant. • Cross-pollination: Pollen from another plant of the same species. 3. Fertilisation Fertilisation happens when the pollen nucleus joins with the ovule nucleus inside the ovary. 4. Pollen tube (Supplement) The pollen grows a tube down the style to reach the ovule. The nucleus travels through it to cause fertilisation. 5. Germination A seed needs: • Water • Oxygen • Warm temperature to grow into a new plant. Features in: Insect-Pollinated Flowers Wind-Pollinated Flowers - PETALS INSECTS: Large, bright, scented WIND: Small, dull, no scent - NECTAR INSECTS: Present WIND: Absent - POLLEN INSECTS: Sticky, fewer grains WIND: Light, lots of grains - ANTHERS INSECTS: Inside flower WIND: Hang outside - STIGMA INSECTS: Sticky, inside flower WIND: Large, feathery, outside flower

Answer 43

1. Types of Teeth Human teeth are classified based on their shape and function. There are four main types: • Incisors: These are the sharp, chisel-shaped teeth at the front of the mouth. They are used for cutting food. • Canines: Pointed teeth next to the incisors, used for tearing food. • Premolars: These are flatter, with two or more cusps, and are used for crushing and grinding food. • Molars: Located at the back of the mouth, they are broad and flat, used for grinding food into smaller particles for digestion. 2. Structure of a Tooth A tooth is made up of several parts, each with a specific function: • Enamel: The hard, white outer layer of the tooth. It is the hardest substance in the human body and protects the tooth from decay. • Dentin: The layer beneath the enamel, which makes up the bulk of the tooth. It is harder than bone but softer than enamel. • Pulp: The innermost part of the tooth, containing nerves and blood vessels. • Cement: A substance covering the root of the tooth, helping anchor it to the jawbone. • Root: The part of the tooth embedded in the gum and jawbone. • Crown: The visible part of the tooth above the gum line. 3. Functions of Teeth The primary function of teeth is to break down food into smaller pieces, increasing the surface area for enzymes to act on during digestion. This process is called mechanical digestion. Teeth also play a role in speech and facial appearance. 5. Tooth Decay and Oral Hygiene Tooth decay (dental caries) is caused by bacteria that feed on sugars in the mouth, producing acids that erode the enamel. Good oral hygiene, such as brushing and flossing, helps prevent tooth decay. Additionally, limiting sugary food and drinks, regular dental check-ups, and fluoride use can help maintain dental health.

Answer 44

Enzymes are special PROTEINS that act as BIOLOGICAL CATALYSTS, often breaking down molecules into shorter ones (polymers into monomers). They are SPECIFIC so only break down SUBSTRATES that fit in their ACTIVE SITE (‘lock and key’ principle). ACTIVITY (rate of reaction) increases with temperature until the enzyme DENATURES (active site changes shape). The same is true for too high pH or low pH. OPTIMUM: condition for max rate/activity. EXAMPLES: - CARBOHYDRASES break down CARBOHYDRATES into simple sugars. (E.g AMYLASE breaks down STARCH into GLUCOSE) - PROTEASE break down PROTEINS into AMINO ACIDS - LIPASE break down LIPIDS into GLYCEROL & FATTY ACIDS.

Answer 45

Single Circulatory System (e.g., Fish): • Blood passes through the heart once in each complete circuit of the body. • Blood flows: Heart → Gills → Body → Heart • Lower pressure after the gills, so blood flows more slowly to the body. • Less efficient for delivering oxygen quickly. Double Circulatory System (e.g., Mammals, Humans): • Blood passes through the heart twice in each complete circuit. • Two loops: 1. Heart → Lungs → Heart (Pulmonary circulation) 2. Heart → Body → Heart (Systemic circulation) • Maintains high pressure to the body and lower pressure to the lungs. • More efficient, supports higher activity levels and warm-blooded metabolism.

Answer 46

Hydrophytes (plants adapted to live in water, e.g., water lilies): Adaptations: • Large air spaces in leaves and stems for buoyancy and gas exchange. • Stomata on upper leaf surface (not submerged) to allow gas exchange. • Thin or no cuticle since water loss is not a problem. • Reduced roots (water absorbed directly from surroundings). • Leaves are broad and flat to float and maximize light absorption. Xerophytes (plants adapted to dry environments, e.g., cacti): Adaptations: • Thick, waxy cuticle to reduce water loss. • Small or spiny leaves to reduce surface area and transpiration. • Stomata sunken in pits or only open at night (CAM photosynthesis) to reduce water loss. • Deep or widespread roots to access water from a large area. • Water storage tissues (succulent stems or leaves).

Answer 47

1. Balanced Diet A balanced diet is one that provides all the essential nutrients—carbohydrates, fats, proteins, vitamins, minerals, fiber, and water—in the right proportions to maintain health, energy, and growth. 2. Principal Dietary Sources and Importance: (a) Carbohydrates • Sources: Bread, rice, pasta, potatoes, and fruits. • Importance: Carbohydrates are the primary energy source for the body. They are broken down into glucose, which is used in respiration to release energy. (b) Fats and Oils • Sources: Butter, oils, nuts, fish, avocados. • Importance: Fats provide long-term energy, help store vitamins (A, D, E, K), and are important for cell membrane structure and insulation. (c) Proteins • Sources: Meat, fish, eggs, beans, lentils, nuts. • Importance: Proteins are essential for growth, repair, and maintenance of body tissues. They also help make enzymes, hormones, and antibodies. (d) Vitamins: • Vitamin C • Sources: Citrus fruits, strawberries, tomatoes, broccoli. • Importance: Vitamin C is necessary for collagen production, wound healing, and immune system support. • Vitamin D • Sources: Fatty fish, eggs, fortified dairy, sunlight. • Importance: Vitamin D helps the body absorb calcium for bone health and immune function. (e) Mineral Ions: • Calcium • Sources: Dairy products, green leafy vegetables, fortified foods. • Importance: Calcium is essential for strong bones and teeth, and is involved in muscle contraction and nerve transmission. • Iron • Sources: Red meat, beans, lentils, spinach, fortified cereals. • Importance: Iron is vital for the production of hemoglobin, which carries oxygen in the blood. A deficiency can cause anemia. (f) Fibre (Roughage) • Sources: Whole grains, vegetables, fruits, legumes. • Importance: Fibre aids in digestion, helps prevent constipation, and may reduce the risk of bowel diseases. (g) Water • Sources: Water, fruits, vegetables, soups. • Importance: Water is essential for maintaining hydration, temperature regulation, digestion, and transport of nutrients and waste. 3. Causes of Scurvy and Rickets: Scurvy • Cause: Deficiency of Vitamin C. • Symptoms: Weakness, bleeding gums, joint pain, and poor wound healing. Rickets • Cause: Deficiency of Vitamin D or calcium. • Symptoms: Soft, weak bones, leading to deformities such as bowed legs and stunted growth in children.

Answer 48

- DIGESTION: The process by which we break down the large food molecules we eat into much smaller molecules. E.g Starch -> glucose or Protein -> amino acids - ABSORPTION: The process by which we absorb these small molecules, e.g Vitamins or minerals into the bloodstream.

Answer 49

1. MOUTH: Two important organs: TEETH: - to physically break down food by chewing. Increases the surface area of the food (much easier for enzymes to break it down) and makes the food way easier to swallow. SALIVARY GLANDS: Releases saliva (makes it wet/moist and easier to swallow), contains the enzyme AMYLASE (digests starch into maltose) 2. OESOPHAGUS (gullet) 3. STOMACH: - Contracts muscular walls (which push the food around and mixes it up) - Produces PEPSIN (a PROTEASE ENZYME) - Produces HYDROCHLORIC ACID, which kills any bacteria eaten, and provides the right pH for PEPSIN. 4.1 PANCREAS: around the same time food is passed to the small intestine; the pancreas releases pancreatic juices into the small intestine 4.2 GALLBLADDER: releases bile into the small intestine 4.3 SMALL INTESTINE: Its where most of the digestion takes place; releases digestive enzymes (Carbohydrase, lipase or protease, which break down the nutrients) Where the nutrients are absorbed from the intestine into the blood, with the help of VILLI (or singular VILLUS) IMPORTANT FEATRUES 1. Huge surface area for diffusion 2. Single layer of surface cells (a short distance for diffusion to take place) 3. Have a really good blood supply (maintains the concentration gradient for the absorption of nutrients) 4. Villi have small finger like projections called microvilli which further enhances surface area. 5. LARGE INTESTINE: absorbs most of the excess water, leaves behind a relatively dry mixture (Faeces), which are stored in the RECTUM, and when released it travels through the ANUS.

Answer 50

A liquid mixture full of digestive enzymes to help the small intestine digest/break down the food. Contains enzymes like: - AMYLASE - PROTEASE - LIPASE

Answer 51

1. Neutralizes the acid from the stomach (bile is alkaline) 2. Emulsifying LIPIDS (gives the lipids a large surface area) (basically helps break the big blobs of fats and oil). 3. Bile is actually made in the LIVER, however it is stored in the GALLBLADDER until it is needed to be released.

Answer 52

– Found in the roots of plants – Have long extensions to increase surface area for absorbing water and minerals from the soil

Answer 53

– Found in the trachea and bronchi – Have tiny hair-like structures (cilia) that move mucus and trapped dust/microbes out of the airways

Answer 54

– Found throughout the body – Long and thin, adapted to conduct electrical impulses quickly

Answer 55

– Involved in reproduction – Sperm: tail for swimming, enzymes (acrosome) to penetrate egg – Egg: large with nutrient-rich cytoplasm for embryo development